Astronomy

Did the Juno mission locate Jupiter at the same precision as Saturn (4 km)?

Did the Juno mission locate Jupiter at the same precision as Saturn (4 km)?


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Juno's low perijove should be great for measuring Jupiter's gravitational effect on Juno's orbit. Cassini together with VLBI has measured Saturn's location to within 4 km.

Will Was Juno be able to be measured by VLBI to determine Jupiter's location and orbit at a similar precision? At a precision useful for mapping the Solar System outside of the Kuiper Belt?


Here’s Jupiter from Juno’s Latest Flyby

Most massive planet in the solar system – twice that of all the other planets combined. This giant world formed from the same cloud of dust and gas that became our Sun and the rest of the planets. But Jupiter was the first-born of our planetary family. As the first planet, Jupiter’s massive gravitational field likely shaped the rest of the entire solar system. Jupiter could’ve played a role in where all the planets aligned in their orbits around the Sun…or didn’t as the asteroid belt is a vast region which could’ve been occupied by another planet were it not for Jupiter’s gravity. Gas giants like Jupiter can also hurl entire planets out of their solar systems, or themselves spiral into their stars. Saturn’s formation several million years later probably spared Jupiter this fate. Jupiter may also act as a “comet catcher.” Comets and asteroids which could otherwise fall toward the inner solar system and strike the rocky worlds like Earth are captured by Jupiter’s gravitational field instead and ultimately plunge into Jupiter’s clouds. But at other times in Earth’s history, Jupiter may have had the opposite effect, hurling asteroids in our direction – typically a bad thing but may have also resulted in water-rich rocks coming to Earth that led to the blue planet we know of today.

Early solar system and protoplanetary disk with a young Jupiter – c. NASA

Jupiter is a window into our own solar system’s past – a past literally enshrouded beneath Jupiter’s clouds which is why Juno, the probe currently orbiting Jupiter, is so named. Juno, Jupiter’s wife in mythology, was able to peer through a cloak of clouds Jupiter used to hide himself and his wrongful deeds. In this case, however, we are looking through Jupiter’s clouds into our own history. Juno entered orbit of Jupiter July 5 th , 2016 after travelling for nearly five years to reach the gas giant. Falling into Jupiter’s gravity well, Juno arrived at a speed of 210,000 km/h, one of the fastest speed records set by any human-made object.

Juno is in a highly eccentric 53 day orbit. During Perijove, or the closest orbital approach, Juno skims Jupiter at an altitude of 4,200 km and then sweeps outward to 8.1 million. Juno’s orbit is designed to navigate through weaker areas of Jupiter’s incredibly powerful magnetic field. Second in power only to the Sun itself, Jupiter’s magnetic field accelerates high energy particles from the Sun creating powerful bands of radiation that encircle the planet – electronics-frying radiation. In addition to its nimble navigation, Juno’s electronics are hardened against radiation with its “radiation vault” – a 1cm thick titanium shell that houses its sensitive scientific equipment. One piece of equipment which dazzles all of us back on Earth is JunoCam – an RGB colour camera taking visual images of Jupiter’s clouds as the probe buzzes the planet in just two hours each orbit spending as little time as possible in Jupiter’s radiation.

Artist’s impression of Juno at Jupiter – c. NASA

Most recently, Juno completed Perijove 29 and some of the photos were posted by “Software Engineer, planetary and climate data wrangler, and science data visualization artist” Kevin Gill. Kevin has an absolutely astonishing Flickr page where he posts images he’s processed from Juno as well as other missions like Saturn’s Cassini and the HiRISE camera orbiting Mars on the Mars Reconnaissance Orbiter.

Okay. And finally, why you came here: Behold Juno’s Perijove 29 processed by Kevin Gill (You can click each image to see their full size).

You can also follow Kevin’s work on Twitter (@kevinmgill) and Instagram (@apoapsys)

JunoCam isn’t really part of Juno’s primary scientific mission. But the camera does provide a key function – allowing Juno to bring us along for the journey. Which I think is truly spectacular. Sometimes astrophotography is thought more of as art than science. But as an astrophotographer myself, I believe these images inspire future scientists, general awareness of ongoing scientific missions, and hopefully public support for the funding of science. Speaking of which, what has our science discovered about our giantest of giant worlds?

One of the greatest mysteries of Jupiter is what lies at its heart. Juno helped settle an ongoing debate in the planetary science community about how Jupiter formed. There were two possibilities: The first is that Jupiter began as a rocky world – a core about 10 times the mass of Earth. The gravity of this core drew in surrounding hydrogen and helium until the Jupiter we know of was formed – that original rocky world buried beneath the churning maelstrom. The second possibility is that eddies in the rotating protoplanetary disk of our early solar system collapsed on themselves and Jupiter formed from them directly with no rocky core. Both theories describe different conditions at the start of our solar system. Juno revealed something stranger, not a solid core, but a “fuzzy” or “diluted” core. It appears that Jupiter did form from a rocky body, but rather than that core being situated at the centre of the planet, its is spread throughout the interior of Jupiter. The core’s dilution is likely the result of a massive planet-sized impact with Jupiter that shattered the initial core and spread it through half of Jupiter’s diameter. Imagine being present for an event like that – Jupiter swallowing a would-be planet in our solar system we’ve never known. History of our place in space revealed. We’ve also learned that Jupiter’s winds dive deep below the outer clouds, that the Great Red Spot is hundreds of kilometers deep, and we’ve seen giant cyclones at Jupiter’s North and South Poles that could swallow a country.

Jupiter South Polar Cyclones in Infrared with Size Comparison to US and Texas – JPL/NASA/Caltech

Jupiter is presently the brightest object in the night sky after sunset. If you have clear skies and can see it, look South! Remember, that bright point is a giant world hundreds the times the size of Earth, millions of kilometers away, and yet potentially one of the key factors in your existence. By Jove, that’s amazing.


YIR III: From dwarfs to giants – the missions of Dawn, Juno, and Cassini

Moving out into the asteroid belt and farther still to the gas giants, NASA’s Dawn, Juno, and Cassini missions churned away in orbit of their respective hosts – with Dawn continuing an up-close-and-personal investigation of the dwarf planet Ceres, Juno’s fantastically accurate arrival at Jupiter but frustratingly crippled start to its science mission, and the beginning of the end for the Cassini mission that’s set to end in September 2017.

Dawn – Revealing dwarf planet Ceres:

This year, Dawn marked the first anniversary of its arrival at Ceres, and Dawn’s deputy principal investigator, Carol Raymond, stated that “Ceres has defied our expectations and surprised us in many ways thanks to a year’s worth of data from Dawn. We are hard at work on the mysteries the spacecraft has presented to us.”

Among Ceres’ most enigmatic features is the mountain Ahuna Mons, which appeared as a small, bright-sided bump as seen by Dawn’s camera as early as February 2015 from a distance of 46,000 km (29,000 mi) before the spacecraft entered orbit.

As Dawn gradually lowered its orbital altitude over its first year, the shape of this mysterious feature came into focus.

From afar, Ahuna Mons looked pyramid-shaped, but upon closer inspection, it is best described as a dome with smooth, steep walls.

Dawn’s latest images of Ahuna Mons, taken 120 times closer than in February 2015, reveal a significant amount of bright material on some of the mountain’s slopes, and less on others.

“No one expected a mountain on Ceres, especially one like Ahuna Mons,” said Chris Russell, Dawn’s principal investigator. “We still do not have a satisfactory model to explain how it formed.”

But Ahuna Mons isn’t the only feature on Ceres that interests scientists.

About 670 km (420 mi) northwest of Ahuna Mons is Occator Crater – which the Hubble Space Telescope revealed to have a prominent bright patch on its surface prior to Dawn’s arrival.

Dawn’s subsequent orbital observations have revealed that there are at least 10 bright spots in this crater alone, with the brightest area on Ceres located in the center of Occator.

“Dawn began mapping Ceres at its lowest altitude in December, but it wasn’t until very recently that its orbital path allowed it to view Occator’s brightest area,” said Marc Rayman, Dawn’s chief engineer and mission director.

By late-April, Dawn returned stunning new images from its low-altitude (385 km – 240 mi) mapping orbit of the dwarf planet’s numerous bright material craters.

In particular, Dawn’s view of Haulani Crater, with a diameter of 34 km (21 mi), revealed evidence of landslides from its crater rim – indications that the crater is a relatively new formation.

“Haulani perfectly displays the properties we would expect from a fresh impact into the surface of Ceres,” said Martin Hoffmann, co-investigator on the Dawn framing camera team.

The crater’s polygonal structure is also noteworthy as most craters on planetary bodies are nearly circular, but the unique straight edges of some Cerean craters, including Haulani, are due to pre-existing stress patterns and faults beneath the surface.

Moreover, another crater, Oxo, also presents a uniqueness in that its rim is slumped – indicating an area where material has dropped below the surface – and that its crater floor contains minerals observed nowhere else on Ceres’ surface.

However, a big focused remained on Occator crater, and by mid-year, Dawn had finally returned enough information about it that scientists were gaining a better understanding of its composition.

At the end of June, NASA announced findings that Occator’s bright areas contain the highest concentration of carbonate minerals ever seen outside Earth.

“This is the first time we see this kind of material elsewhere in the solar system in such a large amount,” said Maria Cristina De Sanctis, principal investigator of Dawn’s visible and infrared mapping spectrometer.

Specifically, the dominant mineral of this bright area is sodium carbonate, a salt found on Earth in hydrothermal environments.

On Ceres, the material appears to have come from inside the dwarf planet, having been lifted to the surface by an impacting asteroid – which suggests that temperatures inside Ceres are warmer than previously believed.

More intriguingly, the results suggest that liquid water may have existed beneath the surface of Ceres in recent geologic time and that the salts could be remnants of an ocean, or localized bodies of water, that reached the surface and then froze millions of years ago.

“The minerals we’ve found at the Occator central bright area require alteration by water,” De Sanctis said. “Carbonates support the idea that Ceres had interior hydrothermal activity, which pushed these materials to the surface within Occator.”

This discovery announcement came just one day before Dawn completed its primary mission on 30 June.

At this time, Dawn had taken 69,000 images, completed 48,000 hours of ion engine thrusting, collected more than 132 GB of science data, completed 2,450 orbits of Vesta and Ceres, travelled 3.5 billion miles since launch, and explored two new worlds.

On 1 July, Dawn entered its extended mission, which will see the craft continue to operate in Ceres orbit into 2017 – at which point, due to its highly stable orbit of the dwarf planet, it will become a permanent artificial satellite of Ceres.

By the end of July, Dawn had returned information that helped scientists start to answer the question of what happened to all of Ceres large impact craters.

Presently, Ceres is covered in countless small, young craters, but none are larger than 280 km (175 mi) in diameter. To scientists, this is a rather large mystery given that the dwarf planet must have been hit by numerous large asteroids during its 4.5 billion-year lifetime.

“We concluded that a significant population of large craters on Ceres has been obliterated beyond recognition over geological time scales, which is likely the result of Ceres’ peculiar composition and internal evolution,” said Simone Marchi, a senior research scientist at the Southwest Research Institute.

Marchi and her colleagues modeled collisions of other bodies with Ceres since the dwarf planet’s formation, and these models predicted that Ceres should have up to 10 to 15 craters larger than 400 km (250 mi) in diameter, and at least 40 craters larger than 100 km (60 mi) wide.

However, Dawn has shown that Ceres has only 16 craters larger than 100 km, and none larger than 280 km across.

“Whatever the process or processes were, this obliteration of large craters must have occurred over several hundred millions of years,” Marchi said.

One potential reason for the lack of large craters could be related to Ceres’ interior structure.

Specifically, since Ceres’ upper layers contain ice and salts – which are less dense than rock – the topography could “relax,” or smooth out, more quickly if ice or salt dominates the subsurface composition.

Moreover, past hydrothermal activity, which may have influenced the rising of salts to the surface at Occator Crater could also have something to do with the erasure of craters.

If Ceres had widespread cryovolcanic activity in the past, the ejected cryogenic materials could have flowed across the surface and possibly buried pre-existing large craters.

However, its wasn’t just Ceres’ surface features that scientists learned more about this year.

In August, a careful study of minute changes in Dawn’s orbit from the first year of its orbital mission helped scientists gain a better understanding of Ceres’ gravity field – and therefore its internal composition.

“The data suggests that Ceres has a weak interior, and that water and other light materials partially separated from rock during a heating phase early in its history,” said Ryan Park, supervisor of the solar system dynamics group at JPL.

Among the things confirmed about Ceres in this data return is that Ceres has hydrostatic equilibrium – meaning its interior is weak enough that its shape is governed by how the dwarf planet rotates.

This confirmation validated one of the reasons why the International Astronomical Union classified Ceres as a dwarf planet in 2006.

Moreover, the data indicate that Ceres is differentiated – meaning it has compositionally distinct layers at different depths, with the densest layer at the core.

Scientists were also able to confirm that Ceres is much less dense than Earth, the Moon, Vesta, and other rocky bodies in our solar system.

The data also led scientists to conclude that Ceres’ weak mantle can be pushed aside by the mass of mountains and other high topography in its outermost layer – as though the high-elevation areas “float” on the material below.

Overall, by combining this new information with previous data from Dawn about Ceres’ surface composition, scientist are beginning to reconstruct Ceres’ history – in which water must have been mobile in the ancient subsurface while the interior did not heat up to the temperatures at which silicates melt and a metallic core forms.

Following this announcement, Dawn controllers began maneuvering the spacecraft into its higher, mission extension orbit in early September.

Dawn had been – for eight months – in its low-altitude science orbit, but due to its mission extension and limited supply of hydrazine for orientation operations, controllers decided to raise Dawn’s orbit for its extended mission so that the hydrazine can be used more sparingly.

“Most spacecraft wouldn’t be able to change their orbital altitude so easily. But thanks to Dawn’s uniquely capable ion propulsion system, we can maneuver the ship to get the greatest scientific return from the remaining mission,” said Marc Rayman.

The orbit raising maneuver, which began from an altitude of 385 km (240 mi), will push Dawn to 1,460 km (910 mi) above Ceres’ surface – just about the orbit in which Dawn first slid into orbit around the dwarf planet.

Also in September, Dawn scientists released information on a possible detection of a temporary atmosphere around the dwarf planet.

The surprising finding emerged after Dawn’s Gamma Ray and Neutron Detector (GRaND) observed evidence that Ceres had accelerated electrons from the solar wind to very high energies over a period of six days.

In theory, the interaction between the solar wind’s energetic particles and atmospheric molecules could explain the GRaND observations.

A temporary atmosphere would also be consistent with water vapor detections via the Herschel Space Observatory in 2012-2013.

The electrons that GRaND detected could have been produced by the solar wind hitting the water molecules that Herschel observed, but scientists are also looking into alternative explanations.

“We’re very excited to follow up on this and the other discoveries about this fascinating world,” Russell said.

Juno – Triumphant arrival, less than stellar start to science mission:

Originally, Juno was to complete two of these 53.4-day orbits before performing a perijove burn on 19 October that would have altered its orbit to the pre-mission determined 14-day science orbit.

However, just a few days before this scheduled burn, controllers noticed a performance issue with a pair of valves that are part of Juno’s fuel pressurization system.

At the time, Rick Nybakken, Juno project manager, said, “Telemetry indicates that two helium check valves that play an important role in the firing of the spacecraft’s main engine did not operate as expected during a command sequence.

“The valves should have opened in a few seconds, but it took several minutes.”

Controllers subsequently delayed the planned orbit adjustment burn to allow time to study the issue.

As of writing, controllers have still not determined a best way forward and are currently investigating the valves’ potential link to similar failures on Akatsuki and an Intelsat satellite.

For Juno, this meant more orbits of Jupiter in its longer orbit.

Currently, the spacecraft has completed just three close flybys of Jupiter – not counting the flyby that occurred on the night of its arrival.

The third flyby occurred on 11 December, with the fourth now slated for 2 February 2017.

For comparison, when the third flyby occurred on 11 December, Juno should have been gearing up for its fifth flyby.

Now, if controllers are unable or unwilling to perform the orbit adjustment burn, the effects on the science mission as well as the mission’s planned duration are somewhat unknown.

What is known is that the amount and quality of science collected during a close flyby is not affected by the prolonged orbit – which has a much greater apojove than the standard science orbit would but a nearly identical perijove.

Nonetheless, NASA has been quiet on the effect the prolonged orbit might have on the science collected at other points in the orbit and the effect the prolonged exposure to Jupiter’s harsh radiation field will have on the craft’s instruments – which now receive just under 4 times the amount of radiation exposure between scientific close flybys of Jupiter.

Moreover, Juno’s mission is slated to only last until February 2018 – at which time it is anticipated that Juno will have to be deorbited into Jupiter’s atmosphere due to system failures triggered by the intense radiation.

Additionally, Juno mission directives call for a minimum of 7 to 10 operational flybys of Jupiter to achieve minimum mission success.

Given the safe mode pre-approach profile flown in October, the first operational flyby did not occur until 11 December – though even this wasn’t a fully operational flyby as a critical instrument, the Jovian Infrared Auroral Mapper (JIRAM), was not active due to the need to upload a software patch to allow Juno’s software to process information from JIRAM.

At present, if Juno is forced to remain in its 53.4-day orbit and if it still needs to be purposefully disposed of into Jupiter’s atmosphere in February 2018, the mission stands a high chance of not actually or just barely meeting minimum mission success criteria.

Cassini – 19 years after launch, the intrepid little probe prepares for its Grand Finale exit:

The first major event for Cassini this year was a carefully choreographed observation of Enceladus as it occulted – passed in front of, as viewed from a specific location – the star Epsilon Orionis, the central star in Orion’s belt.

Previous Cassini observations of Enceladus saw its polar eruptions spraying three times as much icy dust into space when the moon neared aposaturnium – farthest point in its elliptical orbit around Saturn.

But scientists hadn’t had an opportunity to see if the gas part of the eruptions – which account for the majority of the plume’s mass – also increased at this time.

They got that chance on 11 March… and the results were surprising.

During a carefully planned observation, Cassini set its gaze on Epsilon Orionis, and at the appointed time, Enceladus – roughly at aposaturnium – and its erupting plume glided in front of the star.

Cassini’s Ultraviolet Imaging Spectrometer (UVIS) measured how water vapor in the plume dimmed Epsilon Orionis’ ultraviolet light, thereby revealing how much gas the plume contained.

Since lots of extra icy dust appears at this point in Enceladus’ orbit, scientists expected to measure a lot more gas in the plume.

But instead of the expected large increase in gas output, UVIS only saw a bump of 20% in the total amount of gas.

“We went after the most obvious explanation first, but the data told us we needed to look deeper,” said Cassini scientist Candy Hansen.

This led Hansen and her colleagues to focus on one of Enceladus’ ejecta-spewing jets that was discovered to be four times more active than anticipated – producing 8% of the occultation-observed plume’s total gas instead of just 2% as predicted.

Thus, the occultation observation revealed that at least some of the narrow jets that erupt from the moon’s surface blast with increased fury when the moon is at aposaturnium – but why the gas in the plume was so much less than anticipated is still a mystery.

However, the new observations provide helpful insights on what could be going on with the underground plumbing – cracks and fissures through which water from the moon’s potentially habitable subsurface ocean makes its way into space.

“We had thought the amount of water vapor in the overall plume, across the whole south polar area, was being strongly affected by tidal forces from Saturn. Instead we find that the small-scale jets are what’s changing,” said Larry Esposito, UVIS team lead

Following this observation, the first major scientific release from previously gathered Cassini data occurred in April when scientists announced confirmation of a 2014 discovery that the Ligeia Mare sea on Titan was composed primarily of liquid methane.

“Before Cassini, we expected to find that Ligeia Mare would be mostly made up of ethane, which is produced in abundance in the atmosphere when sunlight breaks methane molecules apart,” said Alice Le Gall, a Cassini radar team associate.

“Instead, this sea is predominantly made of pure methane.”

The confirmation of the 2014 discovery came from data collected with Cassini’s radar during flybys of Titan between 2007 and 2015.

But with this discovery – as do most things in science – came a host of new questions, one being how the methane in the lake is replenished.

“Either Ligeia Mare is replenished by fresh methane rainfall or something is removing ethane from it,” said Le Gall. “It is possible that the ethane ends up in the undersea crust, or that it somehow flows into the adjacent sea, Kraken Mare. But that will require further investigation.”

The same data also revealed that the shoreline of Ligeia Mare may be porous and flooded with liquid hydrocarbons.

This hypothesis comes from Cassini data that did not show any significant difference between the sea’s temperature and that of the shore throughout the local winter to spring timeframe.

Scientists had expected that – like on Earth – the surrounding solid terrains would warm more rapidly than the sea.

“It’s a marvelous feat of exploration that we’re doing extraterrestrial oceanography on an alien moon,” said Steve Wall, deputy lead of the Cassini radar team at JPL. “Titan just won’t stop surprising us.”

And the moon did not stop surprising throughout the year.

On 25 July, Cassini performed its 122nd close flyby of Titan, observing the moon’s long, linear dunes, thought to be comprised of grains derived from hydrocarbons that have settled out of Titan’s atmosphere.

Cassini has shown that dunes of this sort encircle most of Titan’s equator, and scientists use the dunes to learn about winds, the sands the dunes are composed of, and topographical changes.

The flyby also allowed Cassini to investigate a mysterious region known as Xanadu Annex.

The main Xanadu region was first imaged in 1994 by the Hubble Space Telescope and was the first surface feature recognized on Titan.

The new Cassini data revealed that the Xanadu Annex was composed of the same type of mountainous terrains observed in Xanadu.

Overall, though, Xanadu and its Annex remain something of a mystery as mountainous terrain elsewhere on Titan appears in small, isolated patches. But Xanadu covers a large area.

“These mountainous areas appear to be the oldest terrains on Titan, probably remnants of the icy crust before it was covered by organic sediments from the atmosphere,” said Rosaly Lopes, a Cassini radar team member at JPL.

Nonetheless, the observation of Xanadu and southern hemisphere terrain during Titan flyby 122 marked the final observation of southern hemisphere targets for Cassini.

In August, when Cassini flew by Titan again, it focused – as will the final three remaining flybys – on the northern hemisphere lake region.

During the August flyby, Cassini discovered deep, steep-sided canyons on Titan flooded with liquid hydrocarbons.

The discovery marked the first direct evidence of the presence of liquid-filled channels on Titan, as well as the first observation of canyons hundreds of meters deep.

Then, when Cassini returned to Titan on 30 November, the flyby was carefully choreographed to gently nudge Cassini into the initial phase of its penultimate orbit.

The nudge from Titan’s gravity altered Cassini’s orbit just enough to send the probe arcing high over and under Saturn’s poles and amazingly close to the outer-most edges of Saturn’s rings.

The new orbit resulted in an orbital period of seven days, and Cassini will perform 20 of these Ring-Grazing Orbits.

As Cassini moved toward completion of the first of these orbits and its first ring-graze, the craft fired its main engine for 6 seconds at 07:09 EST on 4 December – about one hour prior to the first ring-graze.

The engine burn completed Cassini’s orbit alteration to place the craft into the proper position for its penultimate 20-orbit mission.

At 08:09 EST that same day, Cassini grazed through the faint, outer-most F-ring of Saturn.

A few hours after the ring-plane crossing, Cassini began a complete scan across the rings with its radio science experiment to study the rings’ structure in great detail.

“It’s taken years of planning, but now that we’re finally here, the whole Cassini team is excited to begin studying the data that come from these ring-grazing orbits,” said Linda Spilker, Cassini project scientist at JPL.

“This is a remarkable time in what’s already been a thrilling journey.”

On 11 December, Cassini completed its second ring-graze dive, returning spectacular, up-close images of what is arguably the most majestic structure in the solar system.

But as amazing and scientifically rich as the images are, they are a reminder that all good things must come to an end.

“This is it, the beginning of the end of our historic exploration of Saturn. Let these images – and those to come – remind you that we’ve lived a bold and daring adventure around the solar system’s most magnificent planet,” said Carolyn Porco, Cassini imaging team lead at the Space Science Institute.

The current series of Ring-Grazing Orbits will set Cassini up for one last flyby of Titan on 22 April 2017 – a flyby that will mark the commencement of Cassini’s Grand Finale.

The final encounter with Titan, designed to cause Cassini’s orbit to jump over Saturn’s rings, will begin the 22 orbit Grand Finale sequence that will take Cassini into the 2,400 km (1,500 mi) gap between Saturn and the inner-most of its rings.

These 22 orbits will culminate on 15 September 2017 at 08:07 EDT (12:07 UTC) when Cassini plunges into Saturn’s atmosphere – transmitting back as much data as it can until Cassini bids its farewell.

(Part 4 – Pluto – of NASASpaceflight.com’s 5-part Year In Review will be published in the coming days)

(Images: NASA and L2 Artist Nathan Koga. The full gallery of Nathan’s L2 images can be *found here*)


Key Terms

Auroras are the emission of different types of light (infrared, visible, ultraviolet, or x-ray) from the upper atmosphere of a planet that is caused by electrically charged particles striking atoms of gases in the atmosphere from above. On Earth, we often call auroras the northern and southern lights. Jupiter has auroras that are many times more brilliant and powerful than Earth&rsquos. Auroras are a consequence of a planet having a magnetic field, and Jupiter&rsquos are influenced greatly by the planet&rsquos fast rotation.

A spacecraft&rsquos orientation, meaning the direction in which it is pointed. Juno has an attitude control system that consists of four sets of thrusters that allow the spacecraft to change its orientation in space.

Particles that make up atoms, such as electrons, neutrons, and protons.

The layer of gases surrounding a planet or moon. Jupiter&rsquos atmosphere is mostly hydrogen and helium gas.

The Juno spacecraft was launched aboard an Atlas V 551 rocket on Aug. 5, 2011. Atlas refers to a family of expendable launch vehicles that has played a major role in U.S. space history and dates back to the 1950s. The Atlas V (in service since 2002) uses a core booster zero-to-five, strap-on, solid rocket boosters (SRBs) a Centaur upper stage and one of several payload fairings.

Large, high-pressure systems in planetary atmospheres are often called anticyclones. An anticyclone exists where cooler (and therefore heavier) air at higher altitudes sinks in spiraling motions to reach lower altitudes. The circular motion of winds in large air masses like cyclones and anticyclones results from a planet&rsquos rotation. On Earth and Jupiter (and other planets that rotate in the same direction), anticyclones rotate clockwise in the northern hemisphere and counter-clockwise in the southern hemisphere. Jupiter&rsquos Great Red Spot is a large example of an anticylonic feature.

A chemical compound composed of the elements oxygen, hydrogen, nitrogen, and chlorine that is used in solid-rocket motors, like the solid-rocket boosters on Juno&rsquos Atlas V launch vehicle. Ammonium perchlorate is a powerful oxidizer, which means it allows rocket fuel to burn very quickly and efficiently, producing a great deal of thrust.

Ammonium hydrosulfide is a chemical compound made from the elements nitrogen, hydrogen and sulfur with the chemical formula NH4SH. On Jupiter, below an upper layer of clouds made of ammonia ice crystals, there is thought to be a layer of ammonium hydrosulfide clouds.

The bright, usually white, uppermost layer of clouds on Jupiter is thought to consist of ammonia ice. On Earth, white wispy cirrus clouds are made of ice crystals. On Jupiter, the same sort of clouds can form, but the crystals are made of ammonia (NH3) instead of water (H20). Scientists think it is possible that the formation of ammonia cirrus clouds at high altitude may have caused one of Jupiter&rsquos belts (the darker southern equatorial belt) to seemingly disappear in 2010.

Ammonia is a strong-smelling chemical compound made from the elements nitrogen and hydrogen. It has the chemical formula NH3. Along with water and methane, ammonia is one of the chief compounds that make up the ices found in the outer solar system. The bright, usually white, uppermost layer of clouds on Jupiter is thought to consist of ammonia ice. On earth, ammonia is a common ingredient in products like window cleaner and smelling salts.

Juno has three low-gain antennas, one located on its forward deck (F-LGA), and two located on its aft deck (A-LGA and T-LGA). [The forward deck is where the large, saucer-shaped high-gain antenna (HGA) is mounted the aft deck is where the main engine is located.] The aft LGA is used during the period in 2013 when the spacecraft is inside of Earth&rsquos orbit and the angle formed by the sun, Juno, and Earth is greater than 110 degrees.

The Advanced Stellar Compass, or ASC, is used to provide accurate information about how the Juno spacecraft is oriented in space. It does this by comparing an onboard star map with images of the sky taken by two CCD cameras. The ASC was built by the Technical University of Denmark.

Biomarkers, or biosignatures, are evidence for life on a planetary body. Ideally, a biomarker would provide a chemical signature indicating that a distinct biological process is occurring on a world. For example, such a signature might be recognizable in the spectrum of a planet&rsquos light.

A bar is a unit for measuring atmospheric pressure. One bar (also called one atmosphere) of pressure is roughly equal to the air pressure on Earth at sea level. The pressure at the center of Jupiter is estimated to be roughly 50 million bars.

Within an atmosphere, convection is a type of circulation in which warm air from deeper in the atmosphere rises while cooler air from higher altitudes sinks. Jupiter is still hot on the inside from its formation 4.6 billion years ago, and convection is the main way the heat is able to get out from the planet&rsquos interior. Convection can also take place within liquids (for example a pot of soup or hot caramel boiling on a stove top) and in plasmas (as on the Sun).

Liquid, gas or plasma material in which electrons are able to flow, thus making the fluid able to conduct electricity. When an electrically conductive fluid circulates, it generates a magnetic field. Earth&rsquos outer core is an ocean of hot, liquid iron in constant motion its circulation generates our planet&rsquos magnetic field. Within Jupiter, scientists think its hydrogen is compressed by gravity into a liquid form that can conduct electricity thus it is a conducting fluid that generates the planet&rsquos magnetic field.

The Common Core Booster is the large, first stage of the Atlas V rocket that launched Juno into space.

A comet is an like icy clod of dirt made of frozen gases, rock, and dust, leftovers from the early formation of our solar system. Many comet-like bodies may have collided and stuck together to form the ancient core from which the giant planet Jupiter grew. In size, comets are often roughly the size of a small town. When a comet&rsquos orbit brings it close to the Sun, it heats up and spews dust and gases into a giant glowing head larger than most planets. The dust and gases form a tail that stretches away from the Sun for millions of kilometers.

The visible tops of clouds in a planet&rsquos atmosphere. When we look at Jupiter in visible light (for example, with our eyes through a telescope), we are only able to see the tops of the clouds. The clouds are not flat, but towering, three-dimensional structures. In some places the tops of the visible clouds are at lower altitudes, allowing us to see deeper into the planet&rsquos atmosphere. Juno will pass over Jupiter&rsquos uppermost cloud tops at an altitude of only 3,100 miles (5000 kilometers) every 11 days.

Broad, parallel bands of clouds are Jupiter&rsquos most distinguishing feature. Winds in adjacent bands generally flow in opposite directions, either east-to-west or west-to-east. The bands are the result of the planet&rsquos fast rotation combined with convection in which warm air from deeper in the atmosphere rises and cooler air from higher altitudes sinks.

Electrons or atomic nuclei (called ions) that have an electric charge.

The Centaur rocket served as the upper stage of Juno&rsquos launch vehicle. This upper stage also gave the spacecraft its initial spin. The Centaur was put into a slightly different orbit around the Sun so it would not follow Juno. Centaur has a long history of launching U.S. spacecraft, and dates back to the 1950s.

Cassini is the first spacecraft to orbit the planet Saturn. Launched in 1997 and arriving at its destination in 2004, Cassini has since carried out an in-depth, multi-year tour of Saturn, its rings, and moons. Cassini flew past Jupiter on the way to Saturn, picking up a gravity assist as Juno will do at Earth. In addition, Cassini will transfer to a nearly polar orbit at the end of its mission. This will allow it to get very close to Saturn and conduct similar observations of that planet at the same time Juno is orbiting Jupiter. Cassini is slated to plunge into Saturn in 2017, about one month before Juno is de-orbited into Jupiter.

Located on the eastern central coast of Florida, Cape Canaveral is the historic site of most launches in the U.S. space program. Juno was launched from Cape Canaveral Air Force Station in Aug. 2011. NASA&rsquos Kennedy Space Center is located directly adjacent to Cape Canaveral Air Force Station.

A common example of a Doppler shift is the change in the pitch of a train&rsquos whistle as it approaches and then recedes. When the train is moving toward you, the pitch gets higher (the wavelength of the sound waves it produces are getting shorter) once the train passes the pitch gets lower (the wavelength of the sound waves it produces are getting longer). The Doppler effect applies to light waves and radio waves as well as sound waves. When an object (Juno, for example, emitting its radio signal) is moving toward Earth, its signal is shifted to shorter wavelengths. When it moves away from Earth, the signal is shifted to longer wavelengths. This effect can be measured very precisely to detect small changes in the motions of objects (including spacecraft).

Some spacecraft that land on planets or other bodies carry a camera called a descent camera (or descent imager) to take photographs of the terrain as they approach landing sites. NASA&rsquos Mars Science Laboratory rover, scheduled to land on Mars in late 2012, will carry the Mars Descent Imager or (MARDI). Juno&rsquos visible light camera, JunoCam, was adapted from the design of MARDI.

The NASA Deep Space Network, or DSN, is a global network of giant radio antennas that supports space missions. The DSN consists of three facilities spaced approximately 120 degrees apart around the world, in the U.S. (California), Spain, and Australia. This strategic placement permits constant observation of spacecraft as Earth rotates and helps to make the DSN the largest and most sensitive scientific telecommunications system in the world. The DSN is managed and operated by NASA&rsquos Jet Propulsion Laboratory (JPL).

The use of a spacecraft&rsquos propulsion system to change its orbital path, or trajectory, when it is far from Earth. Juno performs two large deep-space maneuvers, or DSMs, using its main engine about a year after launch. The DSMs occur a bit beyond the orbit of Mars, when Juno is at the farthest point in its orbit around the Sun. This is when the spacecraft is moving the slowest, making it less costly in terms of rocket propellant required to change Juno&rsquos orbit. The DSMs will direct Juno back toward the Earth for its gravity-assist flyby in October 2013.

Toward the equator of a planet. The opposite direction would be poleward, or toward the pole or poles. The equator is an imaginary line that encircles a planet&rsquos &lsquomiddle&rsquo (it is perpendicular to the planet&rsquos rotational axis).

Enrichment refers to the chemical composition of a planet or other body in our solar system having a greater abundance of certain elements than that of the sun. Most relevant to Juno, most elements (other than hydrogen and helium) measured by the Galileo atmospheric probe were found to be enriched by two to four times, compared to the sun. How these materials came to be enriched in Jupiter is a great mystery that Juno will help us understand.

To protect its most sensitive and vital electronic components from the searing radiation that encircles Jupiter, the Juno spacecraft carries a radiation-shielded electronics vault. The vault is made of half-inch thick titanium and has a volume about the size of a car&rsquos trunk.

Juno will fly past Earth, performing a gravity assist maneuver, once on its way to Jupiter, about two years after launch. Other spacecraft have previously used Earth flybys to reach their destinations as well. The Earth flyby will provide Juno about half of the change in velocity required to reach Jupiter the other half was provided by its launch vehicle.

Third planet from the Sun. A world of rock and metal with oceans of liquid water on its surface. Our home planet. The only planet known to harbor life.

Juno has thee low-gain antennas, one located on its forward deck (F-LGA), and two located on its aft deck (A-LGA and T-LGA). [The forward deck is where the large, saucer-shaped high-gain antenna (HGA) is mounted the aft deck is where the main engine is located.] The forward LGA points in the same direction as the medium- and high-gain antennas, but can send and receive radio signals in a much broader beam.

The two Flux Gate Magnetometers (FGM) are part of Juno&rsquos magnetometer (MAG) experiment and will measure the strength and direction of Jupiter&rsquos magnetic field.

Objects made of mostly hydrogen and helium gas, having much more mass than Jupiter, but much less mass than the sun are sometimes called &lsquofailed stars.&rsquo Below a certain amount of mass, perhaps 60 times the amount of material in Jupiter, objects do not achieve pressures and temperatures high enough within their cores that can sustain the nuclear fusion reactions that cause stars to shine. Thus their &lsquofailure&rsquo in this context refers to having insufficient mass to allow for fusion. They are also sometimes referred to as sub-stellar objects or brown dwarfs.

Gyroscopes are devices on a spacecraft that sense the slightest change in the rate of rotation in one or multiple directions. They are reference devices that keep the spacecraft stable. Reference gyros measure the forces acting on them. In deep space, the force will be proportional to the ship&rsquos velocity (speed). In a spacecraft, gyroscopes tell the onboard computer when the craft has changed its orientation in space. The computer then sends the information to the spacecraft&rsquos stabilization device, which can make corrections.

A giant, oval-shaped feature in Jupiter&rsquos atmosphere that is more than twice as wide as Earth. It has been present for at least 250 years and could be much older. Scientists do not understand very well what causes the spot&rsquos reddish color how deeply it is rooted to the interior is also unknown.

All objects with mass have gravity, which is a force of attraction between the object and all others in the universe. The more mass, or material, an object has, the more powerful its gravity. The gravity field of an object (Jupiter, for example) is the three-dimensional region of space around it in which the force of its gravity has a measurable influence. Unless an object is perfectly spherical, its gravity field will have variations in strength at different places that are related to how material within it is arranged.

To save on rocket fuel and/or launch vehicle size, spacecraft traveling to other worlds sometimes make use of maneuvers in space called gravity assists. These maneuvers use the gravity of one massive body (a planet or moon) to change the spacecraft&rsquos orbit. The approaching spacecraft actually steals a tiny amount of momentum from the body it flies past, adding that momentum to its own. Juno&rsquos October 2013 Earth flyby is an example of a gravity assist maneuver. The Cassini spacecraft uses gravity assist flybys of Saturn&rsquos moon Titan to adjust its orbital path and reach various destinations in the Saturn system.

One of two types of orbits Juno will use during its science mission. During a Gravity Science pass, Juno&rsquos high-gain antenna will be pointed at Earth so that it can receive and retransmit a precise radio signal. Variations in Jupiter&rsquos gravity field will affect the spacecraft&rsquos motion, speeding it up in some places and slowing it down in others. These changes in motion due to the shape of gravity field will be imprinted upon the radio signal as a Doppler shift that can be measured and made a 3D map of the planet&rsquos gravity.

The Juno Gravity Science experiment will enable Juno to measure Jupiter&rsquos gravitational field and reveal the planet&rsquos deep internal structure. Instead of a dedicated instrument mounted on the spacecraft, the gravity science experiment uses radio signals sent between the spacecraft and the antennas of the Deep Space Network.

Jupiter&rsquos gossamer rings are diffuse rings of fine dust particles outside the planet&rsquos main ring. Observations by NASA&rsquos Galileo spacecraft indicated that the rings coincide with the orbits of two small moons, Amalthea and Thebe. Jupiter&rsquos rings are formed from dust particles hurled up by micro-meteor impacts on Jupiter&rsquos small inner moons and captured into orbit. If the impacts on the moons were any larger, then the larger dust thrown up would be pulled back down to the moon&rsquos surface by gravity. The rings must constantly be replenished with new dust from the moons to exist.

NASA&rsquos Galileo mission sent the first spacecraft to orbit Jupiter. Galileo was launched from the Space Shuttle Atlantis in 1989 and arrived at Jupiter in 1995. The mission was designed to be an extensive up-close survey of the Jovian system &ndash the planet, its rings, and especially its moons. In 2003, after nearly eight years in orbit in which the spacecraft provided a treasure trove of amazing discoveries, Galileo was purposely deorbited (meaning a controlled crash) into the atmosphere of Jupiter.

The Galileo Probe was an atmospheric entry probe carried by NASA&rsquos Galileo spacecraft. The probe separated from Galileo shortly before arrival at Jupiter and went on to parachute into the planet&rsquos clouds. The probe survived for about one hour, reaching a depth of about 120 miles (200 km) below the cloud tops, where the pressure was about 24 times that at sea level on Earth.

Galileo Galilei is perhaps the best known astronomer in history. He is usually credited as the first scientist to use the telescope (in 1610, at the time a cutting edge military technology) to view the heavens. His most famous discoveries include the four large moons of Jupiter, mountains and craters on the Moon, sunspots, the phases of Venus, and the rings of Saturn. Galileo lived in Italy from 1564 to 1642 and is also notable for his work in mathematics and physics. A Lego® minifigure of Galileo is mounted onboard the Juno spacecraft, along with a plaque that celebrates the great astronomer.

An enormous collection of stars, usually numbering in the billions to hundreds of billions. Our sun and its family of planets (our solar system) is just one of perhaps 400 billion stars in the Milky Way Galaxy. Galaxies come in a variety of shapes and sizes. Ours is shaped like a spiral disk with a bright bulge in the center and is so vast that it takes light (the fastest thing we know of) 100,000 years to cross from one side to the other.

Hydrogen is a lightweight gas, the first element on the periodic table, and the most abundant element in the universe. Jupiter and Saturn, the gas-giant planets, are composed mainly of hydrogen and helium, much like stars. As a lightweight gas, it takes a massive body with strong gravity to hold onto hydrogen in its atmosphere and keep it from escaping to space.

Hydrazine is a chemical used as fuel in many spacecraft, including Juno, for maneuvering in space. Hydrazine is sometimes used by itself (as a monopropellant) in maneuvering thrusters, and sometimes with liquid oxygen (combined to form a bi-propellant) in rocket engines. Hydrazine is a hazardous chemical and is treated with great care by technicians when fueling a spacecraft in preparation for launch.

The Hubble Space Telescope (launched 1990) and designed to be serviceable by astronauts, is one of NASA&rsquos most successful and long-lasting science missions. Located above the Earth&rsquos atmosphere, which distorts and blocks the light that reaches our planet, it gives a view of the universe that typically far surpasses that of ground-based telescopes. Famous for its dazzling pictures of galaxies and nebulae, the Hubble Space Telescope has also been an important tool for planet hunters.

Hotspots refer to places in Jupiter&rsquos atmosphere that appear bright when viewed in infrared light that represents heat (thermal infrared at a wavelength of 5 microns). These features are places in which the uppermost cloud layers have cleared or descended, allowing heat from deeper in the planet to escape directly to space. In visible light images, the hotspots look dark and grayish or slightly bluish, allowing a glimpse slightly deeper into Jupiter&rsquos murky depths. Thanks to the Galileo probe, which is thought to have parachuted into a hotspot, we think these areas are dryer, like deserts in the atmosphere with little water vapor.

The High-Gain Antenna (HGA) is a 2.5-meter (8-foot) wide, saucer shaped radio antenna that serves as Juno&rsquos main communications link with Earth. It will be the primary antenna used during Juno&rsquos time at Jupiter, both for sending science data back to Earth and for transmitting information about the health of the spacecraft. The HGA has the strongest signal of the spacecraft&rsquos five antennas, which enables Juno to transmit data at a much higher rate than the others. The HGA is covered with insulating blankets for protection from heat produced by the sun&rsquos harsh light while Juno is in the inner solar system. In addition to its communications role, the HGA also functions as part of Juno&rsquos Gravity Science system. The antenna requires extremely accurate pointing because it sends and receives radio signals in the form of a tight beam.

Helium is a lightweight gas, the second element on the periodic table, and the second most abundant element in the universe. Jupiter and Saturn are gas-giant planets, composed mostly of hydrogen and helium, much like stars. Since helium is a lightweight gas, it takes a massive body with strong gravity to hold onto helium in its atmosphere and keep it from escaping to space.

The halo is part of Jupiter&rsquos faint and dusty ring system. First seen by the Galileo spacecraft, it is a broad, faint torus (or donut-shaped region) of material about 6,000 miles thick and extending halfway from the main ring down to the planet&rsquos cloud tops. Halo particles are very small - perhaps 100 times smaller than the width of a human hair. Particles this small are believed to survive only for years, and so must somehow be replenished to Jupiter&rsquos ancient ring system. One possible explanation for this unusual halo is that electromagnetic fields around Jupiter gently push small charged particles out of the ring plane.

Ions are atoms that have lost or gained electrons, giving them a positive or negative electric charge. Electrically charged atomic particles (both ions and electrons) feel the force of magnetic fields and move in response. The magnetosphere of Jupiter is filled with ions and electrons.

Another name for infrared light.

Light that is &ldquoinfrared&rdquo features a longer wavelength than human eyes can detect (between red light and microwave radiation in the electromagnetic spectrum). Some of Jupiter&rsquos auroral emission is in the infrared, and the JIRAM instrument will observe these auroras. Infrared light is also emitted from the warmer depths of the planet. JIRAM will observe the heat from within with cooler clouds silhouetted against the warm interior, and detect the chemical fingerprints of gases in the atmosphere.

Jupiter orbit insertion refers to the period of time during which the Juno spacecraft will arrive at the planet Jupiter and be captured by its gravity. The JOI maneuver is accomplished by approaching Jupiter over the north pole and then firing the spacecraft&rsquos main engine for about 30 minutes. This is a critical event in the mission that slows Juno enough to become bound to Jupiter, like an artificial satellite.

The fifth planet outward from the sun, Jupiter is the exploration target of the Juno mission. Jupiter has more than twice the mass of all the other planets in the solar system combined. A gas-giant planet, Jupiter is composed like a star. It did not, however, grow big enough to ignite the core nuclear fusion that makes stars shine. With an enormous magnetic field, the planet has a kind of miniature solar system with dozens moons. Its swirling cloud stripes are punctuated by massive storms such as the Great Red Spot, which has raged for hundreds of years.

JunoCam is a color, visible-light camera designed to capture remarkable pictures of Jupiter&rsquos clouds. The camera&rsquos main purpose is for education and public outreach still, scientists will be very interested in its images.

The Jupiter Energetic Particle Detector Instrument (JEDI) will measure the energetic particles that stream through space, and study how they interact with Jupiter&rsquos magnetic field.

The Jovian Infrared Auroral Mapper (JIRAM) will study Jupiter&rsquos atmosphere in and around the auroras, learning more about the interactions of the auroras, the magnetic field, and the magnetosphere. JIRAM will be able to probe the atmosphere down to 50 to 70 kilometers (30 to 45 miles) below the cloud tops, where the pressure is five to seven times greater than at Earth&rsquos sea level.

The Jovian Auroral Distributions Experiment (JADE) will work with some of Juno&rsquos other instruments to identify the particles and processes that produce Jupiter&rsquos stunning auroras. JADE will also help create a three-dimensional map of the planet&rsquos magnetosphere.

Jet streams are like flowing rivers of air. As motions in an atmosphere, jet streams carry high-altitude clouds rapidly eastward or westward. Jupiter has very prominent bands in its atmosphere that are driven by east- and west-flowing jets. On Earth, jet streams can reach speeds approaching 200 miles per hour on Jupiter they have been measured to travel at around 400 miles per hour.

The Kuiper Belt is a disc-shaped region of icy objects beyond the orbit of Neptune &ndash billions of kilometers from our sun. Pluto and Eris are the best known of these icy worlds. There may be hundreds more of these ice dwarfs out there. The Kuiper Belt and even more distant Oort Cloud are believed to be the home of most comets that orbit our sun.

NASA&rsquos main launch operations center, located on Florida&rsquos eastern central coast, Kennedy Space Center (KSC) has been the point of origin for most of NASA&rsquos planetary exploration missions and all of its manned launches. The Juno spacecraft was launched from KSC on Aug. 5, 2011.

The Kelvin temperature scale measures an object&rsquos temperature above absolute zero, the theoretical coldest possible temperature. On the Kelvin scale the freezing point of water is 273 Kelvin (0 degrees Celsius, 32 degrees Fahrenheit). It is often used in astronomy and other sciences.

Ka-band is a new radio frequency developed to enhance performance in spacecraft communications by using significantly less power. The long-time standard in deep-space communication is a section of the radio frequency spectrum known as X-band. Operating four times higher than X-band (32 gigahertz compared to 8 gigahertz), Ka-band frequency enables transmission of much higher data rates using less power. Juno makes use of both the X- and Ka-bands for its gravity science investigation.

Juno has three low-gain antennas, one located on its forward deck (F-LGA), and two located on its aft deck (A-LGA and T-LGA). Its high-gain and medium-gain antennas send and receive radio signals in much narrower, or more tightly focused, beams. LGAs were designed with a sufficiently broad beam width so that even when Juno is pointed at the sun and not earth, the LGA can still receive radio signals from the earth. The LGA also helps mission controllers to command the Juno spacecraft.

The company that built the Juno spacecraft under contract with NASA. Juno was built at Lockheed Martin Space Systems near Denver, Colo.

Oxygen (like other gases) can become a liquid when cooled sufficiently or placed enough pressure. Liquid oxygen is used by spacecraft and rockets in propellants it reacts with fuel (causing the fuel to burn), which creates thrust.

A form of liquid hydrogen thought to exist in the inner half of Jupiter, where pressures are so great that the electrons are squeezed off the hydrogen atoms, allowing the fluid to conduct electricity. Since being good conductors of electricity is a property usually ascribed to metals, this strange material is called liquid metallic hydrogen.

Hydrogen gas can become a liquid when it is cooled sufficiently or placed under enough pressure. The pressure within Jupiter&rsquos gaseous hydrogen atmosphere creates sufficient pressure to force the hydrogen to liquefy.

Hydrogen and helium are light gases, meaning they do not have much mass. It takes a planet with powerful gravity to hold onto such light gases.

Another name for the rocket used to launch a spacecraft. There are many different types and configurations of launch vehicle. Juno&rsquos launch vehicle was the Atlas V 551.

Juno will use its Microwave Radiometer (MWR) instrument to probe deep into Jupiter&rsquos atmosphere, learning about its structure and chemical composition. To see what&rsquos under the planet&rsquos cloud tops, the MWR will measure the microwave radiation emitted from inside the planet. This radiation also carries imprinted upon it information about the amount of water in the atmosphere &ndash a key piece of missing information about Jupiter.

Microwaves are a type of electromagnetic radiation that has a longer wavelength than visible light and infrared light. Microwaves can be used to study the Universe, communicate with satellites in space, and cook foods. On Earth, microwaves are good for transmitting information because microwave energy can penetrate haze, light rain and snow, clouds, and smoke. For the same reason, microwaves coming from inside Jupiter provide a great way to see what&rsquos going on deep below the planet&rsquos cloud tops.

A micron, or micrometer, is a millionth of a meter, or a thousandth of a millimeter. As a reference, the diameter of a human hair is about 100 micrometers. Wavelengths of infrared radiation are typically expressed in micrometers. A thousandth of a micrometer is called a nanometer.

A meteoroid is usually a fragment of space debris consisting of rock, ice and/or metal. It can be very large with a mass of several hundred tons, or it can be very small. A micrometeoroid is a very small meteoroid &ndash a particle smaller than a grain of sand. Micrometeors are a hazard for spacecraft because their impacts can cause damage. Spacecraft get some protection from these impacts thanks to the multi-layered blankets that cover most of their external surfaces.

Methane, chemical formula CH4, is a compound containing the elements carbon and hydrogen. Along with water and ammonia, methane is one of the chief compounds that make up the ices found in the outer solar system.

The Medium-Gain Antenna (MGA) is one of two small antennas located next to Juno&rsquos High-Gain Antenna. It has wider field of view than the HGA, which means it doesn&rsquot have to be pointed quite as precisely at the Earth to transmit and receive signals, but its signal is not as powerful. The MGA will be used during the cruise phase of the mission at times when it would be difficult to keep the high-gain antenna locked onto Earth while pointing the solar rays more or less toward the sun. If the spacecraft enters safe mode due to a problem, it will switch to the MGA in most cases.

Fourth planet from the sun. A rocky planet similar to Earth, but about half its size. It is thought that because it is smaller than Earth, Mars cooled much more quickly, allowing its inner core to solidify. The lack of a churning liquid metal core shut down the Red Planet&rsquos magnetic field and allowed much of its atmosphere to be stripped away by the solar wind.

The main ring is about km 4000 miles wide and has an abrupt outer boundary about km 80,000 miles from the center of the planet. The main ring encompasses the orbits of two small moons, Adrastea and Metis, which may act as the source for the dust that makes up most of the ring. At its inner edge, the main ring merges gradually into the vertically thick, but more diffuse, ring region called the halo.

A magnetosphere is the area of space around a planet that is controlled by the planet&rsquos magnetic field. Jupiter&rsquos magnetosphere is the largest structure in the solar system and is a primary target of investigation for the Juno mission.

Using its magnetometer (MAG), Juno will create an extremely accurate and detailed three-dimensional map of Jupiter&rsquos magnetic field. This unprecedented study will allow us to understand Jupiter&rsquos internal structure and how the magnetic field is generated by the dynamo action inside &ndash the churning of electrically charged material deep below the surface. MAG will also monitor the magnetic field for long-term variations, helping to determine the depth of the region where the field is generated.

A magnetic field is the invisible region of influence around a magnet in which other magnets, things made of metal, or things with an electric charge feel its magnetic force. Some objects in space, like the planets earth and Jupiter, as well as the sun, act like magnets and thus have their own magnetic fields. By studying their magnetic fields, scientists can learn what these objects are like deep inside.

Magnetic braking is a process by which a spinning object in space that has a magnetic field can have its rotation slowed as the magnetic field drags though a disk of material that surrounds the object. Young stars are thought to be affected by magnetic braking as their powerful magnetic fields drag though the disk of excited gas that surrounds them. Jupiter&rsquos rotation might have been similarly affected by this process, and studying its magnetosphere can teach us how the process affects many similarly magnetic objects in the universe.

Nitrogen is a chemical element . It is the most abundant gas in Earth&rsquos atmosphere, and is important to the chemical reactions that power life as we know it. Nitrogen is present in various forms of ices in the outer solar system, especially those involving ammonia. The atmosphere of Saturn&rsquos moon Titan is mostly nitrogen, and Neptune&rsquos moon Triton has geysers of liquid nitrogen on its surface.

NASA&rsquos New Horizons spacecraft is designed to make the first close-up study of Pluto and its moons and other icy worlds in the distant Kuiper Belt. The spacecraft has seven scientific instruments to study the atmospheres, surfaces, interiors, and intriguing environments of Pluto and its distant neighbors. New Horizons flew past Jupiter in 2007, receiving a gravitational assist from the giant planet.

Neptune, the eighth-farthest planet from the sun, is a giant planet made of mostly gas and the &lsquoices&rsquo of the outer solar system: water, ammonia, and methane. Dark, cold and whipped by supersonic winds, Neptune is more than 30 times as far from the sun as Earth. The planet takes almost 165 years to orbit our sun.

A nebula is a vast cloud of gas and dust in space, often light years wide, and usually illuminated by radiation from stars within or nearby. Nebulae are places where stars are born, as the gravitational collapse of some of their material forms new systems of stars and planets. Some nebulae are the remnants of stars that have reached the end of their lives &ndash stars that have either thrown off their outer layers or exploded as supernovae.

The Oort Cloud is a vast spherical shell (or bubble-shaped region) of icy, comet-like bodies that surround our sun and extends from about 5000 times Earth&rsquos distance from the sun to perhaps halfway to the nearest star. Most of the comets in the Oort Cloud are thought to have been sent there by close gravitational encounters with giant Jupiter, which kicked the comets into wildly long and random orbits.

Propellant refers to the material(s) used to move or propel rockets or spacecraft. A propellant can be a single material (called a monopropellant), like the hydrazine used for Juno&rsquos attitude control thrusters they can also be composed of two materials (called a bi-propellant) like the ones that that ignite in Juno&rsquos main engine to produce thrust. In either case, the continuous ejection of a stream of expanding gases in one direction causes a steady motion of the spacecraft in the opposite direction. This is the basic principle of a rocket.

An orbit that passes over the north and south poles of a planet, compared to an equatorial orbit, which passes over the planet&rsquos equator, or middle. Polar orbits are excellent for spacecraft on missions to map planets, because they cover all latitudes from one pole to the other on each orbit.

Plasma waves occur within the plasma of a magnetosphere and are created by the movement of electrically charged particles that fill the magnetosphere. They can be similar to sound waves (electrostatic waves) or they can be radio waves (electromagnetic waves). Spacecraft can detect a wide range of phenomena that occur on and around planets can create plasma waves from lightning and auroras, to particles streaming along the contours of a planet&rsquos magnetic field. For this reason, the study of plasma waves can reveal much about processes occurring in the space around a planet that would otherwise be undetectable.

Plantesimals represent one of the small bodies that formed from the original disk of gas and dust from which our solar system formed (the solar nebula). Most planetesimals stuck or collided, eventually growing into more massive objects that became the planets.

Planetary protection is a practice of protecting solar system bodies (i.e., planets, moons, comets, and asteroids) from contamination by Earth life. It also protects Earth from possible life forms that may be returned from other solar system bodies. Planetary protection is essential to: preserve our ability to study other worlds as they exist in their natural states avoid contamination that would obscure our ability to find life elsewhere &ndash if it exists and ensure that we take prudent precautions to protect Earth&rsquos biosphere in case it does.

Phosphine, a chemical compound, is composed of the elements phosphorous and hydrogen. Phosphine is found in small amounts in the atmospheres of Jupiter and Saturn. On Earth, phosphine is flammable and toxic to humans.

The fairing of a launch vehicle is the cone-shaped section at the front end (the nosecone). The payload fairing protects the spacecraft within (the rocket&rsquos payload) from weather and contamination on the ground prior to launch and from friction with the atmosphere during flight. Once the rocket has reached a height above most of Earth&rsquos atmosphere, the payload fairing splits in half and falls away, landing in the ocean.

Payload refers to the important material being carried into space by a rocket or spacecraft &ndash the things it was intended to carry to its destination. For missions like Juno that explore the solar system, their payload is the suite of scientific instruments and experiments.

Rotations per Minute (RPM) refers to the number of times a spinning object rotates in one minute. The Juno spacecraft rotates at about 2 RPM for much of its cruise to Jupiter.

Rocket fuel refers to a variety of chemical compounds that release large amounts of stored energy when burned in a rocket engine. Most rocket fuels are combined in an engine with an oxidizer, such as liquid oxygen, that allows them to burn very efficiently both within earth&rsquos atmosphere and in space where there is no atmospheric oxygen. The reaction of a fuel with an oxidizer creates an explosive chemical reaction that produces a rocket engine&rsquos thrust.

Radiometry pass refers to one of two orbit types the Juno science mission will use. To explore Jupiter&rsquos inner clouds, Juno turns on its microwave radiometer (MWR) and orients itself so that the MWR antennas point at Jupiter. Mounted on two of Juno&rsquos six sides, the MWR antennas take continuous measurements while Juno spins. These so-called radiometry passes occur during orbits 3 and 5 through 8, out of a total 33 planned orbits.

Radio frequency represents the number of radio waves passing an antenna per second. The more waves that pass by each second, the higher the frequency. One wave, or &ldquocycle,&rdquo per second is called one Hertz. The radio communications equipment used by spacecraft are tuned to send and receive specific radio frequencies in the range of billions of cycles per second (or Gigahertz). Juno carries an antenna (WAVES) sensitive to a range of radio frequencies produced by phenomena in the Jupiter system.

A radiation belt is a donut-shaped region of charged-particle radiation that surrounds a planet. Both Jupiter and Earth have radiation belts. Particles in these belts carry a great deal of energy and pose a hazard for spacecraft (and any organisms) traveling through them.

Radiation describes a variety of waves or particles that carry energy through space. Electromagnetic radiation includes visible, infrared and ultraviolet light radio waves x-rays and gamma rays. Particle radiation refers to atomic-scale particles &ndash from whole atomic nuclei to electrons, protons and neutrons &ndash that move through space, carrying energy with them. These particles can be the product of radioactive decay, in which a material emits particle radiation as it turns into another material. Particle radiation can also result from the acceleration of particles within a powerful magnetic or electric field &ndash as is the case with the charged particle radiation within Jupiter&rsquos magnetosphere.

Smaller than atoms, subatomic particles refer to typically constituent particles such as protons, neutrons electrons, or quarks that make up atoms.

A star is huge, spherical mass of mostly hydrogen gas that shines brilliantly due to nuclear fusion occurring at its core. A star represents a fine balance between the crushing weight of its outer material and the outward pressure produced by the nuclear reactions in its center. In fusion reactions, atoms of one element are squeezed together to form a heavier element, a process that releases enormous amounts of energy. Stars come in a range of sizes and colors (from white to red to yellow to blue) that depend on the amount of mass they contain and their age. More massive stars live for only a few million years, while smaller, less massive stars (like our sun) can live for billions of years.

A solid rocket booster is a type of rocket, packed with a solid form of rocket fuel, that gets attached to a launch vehicle to provide extra boost in velocity in its way to orbit. Solid rocket boosters are jettisoned a couple of minutes into the flight of a rocket.

Solar wind refers to the escape of rapidly moving atoms and ions from the sun&rsquos outer atmosphere (the corona). Blowing outward in a million mile-per-hour gale, solar wind fills the solar system and interacts with all of the planets.

A solar eclipse is observed when a planet or moon completely blocks an observer&rsquos view of the sun. The Juno spacecraft will experience only one solar eclipse during its cruise to Jupiter, for about 20 minutes during its earth flyby gravity assist maneuver in October 2013.

Single fault tolerance is a mission design principle that seeks to ensure mission success. This allows spacecraft to continue functioning in the event that one or another systems experiences a problem, or &ldquofault&rdquo. In this way, no one particular fault should result in a total failure of the spacecraft.&rdquo

Saturn is the sixth planet outward from the sun and distinguished by a distinctive ring system. All four gas-giant planets have rings &ndash made of bits of ice and rock &ndash but none are as spectacular or as complicated as Saturn&rsquos. Like the other gas giants, Saturn is a massive ball of mostly hydrogen and helium.

Safe mode is a pre-programmed condition that a spacecraft enters when encountering a problem it does not know how to correct on its own. Safe mode carries out a specific set of actions that are intended to prevent the loss of the mission. Safe mode is different for each spacecraft, but it typically involves the spacecraft reorienting itself so that a particular antenna is pointing toward earth, calling home, and waiting for mission controllers to diagnose what caused it to enter the safe condition.

Tungsten is a hard metallic element with a high melting point that is often used in fabricating spacecraft components.

A torus is a shape like a ring, but with substantial thickness. A round life preserver and a donut are both shaped like a torus.

The Toroidal Low-Gain Antenna (TLGA) fills the gap between low-gain antennas by sending its signal out of the sides of the spacecraft. This antenna comes in handy during maneuvers when the other antennas must be pointed far away from the direction of Earth. Juno will rely on its TLGA during a couple of critical moments: the Deep Space Maneuvers, which adjust the spacecraft&rsquos path on the way to Jupiter, and the Jupiter Orbit Insertion burn that will be executed upon arrival at the giant planet.

Tones refer to special sets of radio signals that some modern planetary exploration missions use to indicate specific events that have occurred onboard a spacecraft, such as an engine being fired or a parachute being deployed. The Juno tones are a collection of more than 100 radio signals the spacecraft can produce to indicate its status when it is operating autonomously and cannot send richer telemetry data. Tones will be the only indicators of spacecraft status during the two critical deep space maneuvers and Jupiter orbit insertion, plus the final main engine burn that reduces Juno&rsquos orbital period to its desired 11-day cadence.

Titanium is a strong metallic element with many uses, especially in the space and aerospace industries. The box that shields Juno&rsquos most sensitive electronics from Jupiter&rsquos searing radiation has half-inch-thick walls of titanium.

Thermal blankets are used to protect instruments and spacecraft in the space environment (helping to disperse the energy of micrometeoroid strikes) and help maintain the temperature range required to ensure a spacecraft&rsquos operational performance. The blankets are typically made of multiple, very thin layers of plastic and cloth materials. Different materials are used depending on the spacecraft&rsquos operating environment. Juno&rsquos thermal blankets are doped with the conductive metal germanium, which helps electrons in Jupiter&rsquos magnetosphere to flow over the spacecraft, rather than building up an electrical charge that could cause a dangerous discharge or spark.

A spacecraft&rsquos communications equipment is collectively referred to as its telecommunications (or telecom) subsystem. Key components of telecom include antennas and radio receivers. Telecommunications subsystem components are chosen for a particular spacecraft in response to the requirements of the mission. Anticipated maximum distances, planned frequency bands, desired data rates, available on-board electrical power (especially for a transmitter), and mass limitations, are all taken into account.

Tantalum is a strong multi-purpose metallic element. Engineers originally considered tantalum for the material to construct Juno&rsquos radiation shielding vault, but eventually chose titanium instead.

Uranus is the seventh planet outward from the sun and is especially notable for its unique tilt, which has left the planet rotating on its side. Nearly a twin in size to Neptune, Uranus has more methane in its mainly hydrogen and helium atmosphere than Jupiter or Saturn. Methane gives Uranus its bluish tint &ndash not because methane is blue, but because it scatters red light out of sunlight leaving mostly bluish light to be reflected by the planet.

The universe is a vast expanse of space that contains all of the matter and energy in existence. The universe contains all of the galaxies, stars, and planets. The exact size of the universe is unknown and may be infinite. Scientists believe the universe is still expanding outward due to a violent, powerful explosion that occurred about 13.7 billion years ago.

An ultraviolet emission refers to light with a shorter wavelength than human eyes can detect (between violet light and x-rays in the electromagnetic spectrum). Some gases in Jupiter&rsquos upper atmosphere give off, or emit, ultraviolet light when they absorb energy from the hail of charged particles raining down on the planet&rsquos polar regions from the magnetosphere. Juno carries a detector (UVS) designed to study this emission from Jupiter&rsquos auroras.

The Ultraviolet Imaging Spectrograph (UVS) will take pictures of Jupiter&rsquos auroras in ultraviolet light. Working with Juno&rsquos JADE and JEDI instruments, which measure the particles that create the auroras, UVS will help us understand the relationship between the auroras, the streaming particles and the magnetosphere as a whole.

Venus, the second planet outward from the sun is about the same size as Earth. A very dense atmosphere of carbon dioxide, with thick clouds of sulfuric acid, engulfs this rocky world. This thick blanket of atmosphere holds in heat, creating a surface temperature of nearly 900 degrees.


NASA's Voyager 1 was launched after Voyager 2, but because of a faster route, it exited the asteroid belt earlier than its twin, having overtaken Voyager 2 on Dec. 15, 1977.

It began its Jovian imaging mission in April 1978 when it was about 165 million miles (265 million kilometers) from the planet. Images sent back by January 1979 indicated that Jupiter&rsquos atmosphere was more turbulent than during the Pioneer flybys in 1973-1974.

Beginning Jan. 30, 1979, Voyager 1 took a picture every 96 seconds for a span of 100 hours to generate a color time-lapse movie to depict 10 rotations of Jupiter.

On Feb. 10, 1979, the spacecraft crossed into the Jovian moon system and in early March, it discovered a thin ring circling Jupiter (less than 19-miles or 30 kilometers-thick).

Voyager 1&rsquos closest encounter with Jupiter was at 12:05 UT March 5, 1979, at a range of about 174,000 miles (280,000 kilometers), following which it encountered several of Jupiter&rsquos moons, including Amalthea (at a 261,100-mile or 420,200-kilometer range), Io (13,050 miles or 21,000 kilometers), Europa (45,830 miles or 733,760 kilometers), Ganymede (71,280 miles or 114,710 kilometers), and Callisto (78,540 miles or 126,400 kilometers), in that order, returning spectacular photos of their terrains and opening up completely new worlds for planetary scientists.

Among the most interesting findings was on Io, where images showed a bizarre yellow, orange, and brown world with at least eight active volcanoes spewing material into space, making it one of the most (if not the most) geologically active planetary bodies in the solar system. The presence of active volcanoes suggested that the sulfur and oxygen in Jovian space may be a result of the volcanic plumes from Io which are rich in sulfur dioxide.

The spacecraft also discovered two new moons, Thebe and Metis.

Following the Jupiter encounter, Voyager 1 completed an initial course correction April 9, 1979, in preparation for its meeting with Saturn. A second correction on Oct. 10, 1979, ensured that the spacecraft would not hit Saturn&rsquos moon Titan.

Its flyby of the Saturn system in November 1979 was as spectacular as its previous encounter.

Voyager 1 found five new moons, a ring system consisting of thousands of bands, wedge-shaped transient clouds of tiny particles in the B-ring that scientists called &ldquospokes,&rdquo a new ring (the G-ring), and &ldquoshepherding&rdquo satellites on either side of the F-ring -- satellites that keep the rings well-defined.

During its flyby, the spacecraft photographed Saturn&rsquos moons Titan, Mimas, Enceladus, Tethys, Dione, and Rhea. Based on incoming data, all the moons appeared to be composed largely of water ice.

Perhaps the most interesting target was Titan, which Voyager 1 passed at 05:41 UT Nov. 12, 1979, at a range of about 2,500 miles (4,000 kilometers).

Images showed a thick atmosphere that completely hid the surface. The spacecraft found that the moon&rsquos atmosphere was composed of 90% nitrogen. Pressure and temperature at the surface was 1.6 atmospheres and minus 292 degrees Fahrenheit (minus 180 degrees Celsius), respectively.

Atmospheric data suggested that Titan might be the first body in the solar system, apart from Earth, where liquid might exist on the surface. In addition, the presence of nitrogen, methane, and more complex hydrocarbons indicated that prebiotic chemical reactions might be possible on Titan.

Voyager 1&rsquos closest approach to Saturn was at 23:46 UT Nov. 12, 1980, at a range of about 78,290 miles (126,000 kilometers).

Following the encounter with Saturn, Voyager 1 headed on a trajectory to escape the solar system at a speed of about 3.5 AU (325 million miles or 523 million kilometers) per year, 35 degrees out of the ecliptic plane to the north and in the general direction of the Sun&rsquos motion relative to nearby stars.

Because of the specific requirements for the Titan flyby, the spacecraft was not directed to Uranus and Neptune.

On Feb. 14, 1990, Voyager 1&rsquos cameras were pointed backward and captured about 60 images of the Sun and planets -- the first "portrait" of our solar system as seen from the outside. The images were taken when the spacecraft was about 40 AU from the Sun (3.7 billion miles or 6 billion kilometers).

A mosaic of those images became the &ldquoPale Blue Dot&rdquo image made famous by Cornell University professor and Voyager science team member Carl Sagan (1934-1996).

The image has also been called the "Solar System Family Portrait"&mdasheven though Mercury and Mars can&rsquot be seen. Mercury was too close to the Sun to be seen, and Mars was on the same side of the Sun as Voyager 1, so only its dark side faced the cameras.

These images were the last of 67,000 images taken by the two Voyager spacecraft. Their cameras were turned off to save power and memory for the interstellar mission.

All the planetary encounters finally were over in 1989 and the missions of Voyager 1 and 2 were declared part of the Voyager Interstellar Mission (VIM), which officially began Jan. 1, 1990.

The goal of the new mission is to extend NASA&rsquos exploration of the solar system beyond the neighborhood of the outer planets to the outer limits of the Sun&rsquos sphere of influence, and possibly beyond.

Specific goals include collecting data on the transition between the heliosphere&mdashthe region of space dominated by the Sun&rsquos magnetic field and solar field&mdashand the interstellar medium.

On Feb. 17, 1998, Voyager 1 became the most distant human-made object in existence when, at a distance of 69.4 AU from the Sun, it overtook Pioneer 10.

On Dec. 16, 2004, Voyager scientists announced that Voyager 1 had reported high values for the intensity for the magnetic field at a distance of 94 AU, indicating that it had reached the termination shock and had now entered the heliosheath. The spacecraft finally exited the heliosphere and began measuring the interstellar environment on Aug. 25, 2012, the first spacecraft to do so.

On Sept. 5, 2017, NASA marked the 40th anniversary of Voyager 1&rsquos launch, as it continues to communicate with NASA&rsquos Deep Space Network and to send data back from four still-functioning instruments -- the cosmic ray telescope, the low-energy charged particles experiment, the magnetometer, and the plasma waves experiment.

Each Voyager carries a message, prepared by a team headed by Carl Sagan, in the form of a 12 inch (30-centimeter) diameter gold-plated copper disc for potential extraterrestrials who might find the spacecraft.

Like the plaques on Pioneers 10 and 11, the record has symbols to show the location of Earth relative to several pulsars.

The records also contain instructions to play them using a cartridge and a needle, much like a vinyl record player.

The audio on the disc includes greetings in 55 languages, 35 sounds from life on Earth (such as whale songs, laughter, etc.), 90 minutes of generally Western music including everything from Mozart and Bach to Chuck Berry and Blind Willie Johnson. It also includes 115 images of life on Earth and recorded greetings from then U.S. President Jimmy Carter (1924&ndash ) and then-UN Secretary-General Kurt Waldheim (1918-2007).

The two Voyagers are now over 11 billion miles (18 billion kilometers) from the Sun and far from its warmth. To ensure the vintage robots continue to return the best scientific data possible, mission engineers in 2019 began implementing a new plan to manage them. The plan involves making difficult choices, particularly about instruments and thrusters on the spacecraft.

During the Jupiter leg of its journey, Voyager 1 explored the giant planet, its magnetosphere and moons in greater detail than the Pioneer spacecraft that preceded it. Voyager 1 also used Jupiter as a springboard to Saturn, using the gravity-assist technique.

Voyager 1 succeeded on all counts Jupiter, with the single exception of experiments using its photopolarimeter, which failed to operate.

Jupiter's atmosphere was found to be more active than during the visits of Pioneer 10 and 11, sparking a rethinking of the earlier atmospheric models which could not explain the new features.

The spacecraft imaged the moons Amalthea, Io, Europa, Ganymede, and Callisto, showing details of their terrain for the first time.

Possibly the most stunning of Voyager 1's discoveries was that Io has extremely active volcanoes, powered by heat generated by the stretching and relaxing the moon endures every 42 hours as its elliptical orbit brings it closer to and then farther from Jupiter. This finding revolutionized scientists' concept of the moons of the outer planets.

The spacecraft also discovered a thin ring around the planet (then making it the second planet known to have a ring), and two new moons: Thebe and Metis.

Saturn Accomplishments

Voyager 1 was the second spacecraft to visit Saturn. It explored the planet and its rings, moons, and magnetic field in greater detail than was possible for its predecessor, Pioneer 11.

Voyager 1 met all of its goals except for the experiments planned for its photopolarimeter, which failed to operate.

The spacecraft found three new moons: Prometheus and Pandora, the "shepherding" moons that keep the F ring well-defined, and Atlas which similarly shepherds the A ring.

Saturn's largest moon, Titan, was found to have a thick atmosphere which hides its surface from visible-light cameras and telescopes. Spacecraft instruments showed it to be mostly nitrogen, like Earth's atmosphere, but with a surface pressure 1.6 times as high as ours.

The spacecraft also imaged the moons Mimas, Enceladus, Tethys, Dione, and Rhea revealed the fine structures of Saturn's complex and beautiful ring system and added the G ring to the list of known rings.

Just as it used Jupiter's gravity to help it reach Saturn, Voyager 1 used a gravity assist at Saturn to alter its course and increase its speed, giving it a trajectory to take it out of the solar system.

Interstellar Accomplishments

In August 2012, Voyager 1 became the first spacecraft to cross into interstellar space.

However, if we define our solar system as the Sun and everything that primarily orbits the Sun, Voyager 1 will remain within the confines of the solar system until it emerges from the Oort cloud in another 14,000 to 28,000 years.


Contents

Jupiter is the biggest planet in the Solar System with a diameter of 142,984 km. This is eleven times bigger than the diameter of Earth. [18]

Atmosphere Edit

The atmosphere near the surface of Jupiter is about 88 to 92% hydrogen, 8 to 12% helium, and 1% other gases.

The lower atmosphere is so heated and the pressure so high that helium changes to liquid. It rains down onto the planet. [19] Based on spectroscopy, Jupiter seems to be made of the same gases as Saturn. It is different from Neptune or Uranus. These two planets have much less hydrogen and helium gas. [20]

The very high temperatures and pressures in Jupiter's core mean scientists cannot tell what materials would be there. This cannot be found out, because it is not possible to create the same amount of pressure on Earth.

Above the unknown inner core is an outer core. The outer core of Jupiter is thick, liquid hydrogen. [21] The pressure is high enough to make the hydrogen solid, but then it melts because of the heat.

The planet Jupiter is sometimes called a failed star because it is made of the same elements (hydrogen and helium) as is the Sun, but it is not large enough to have the internal pressure and temperature necessary to cause hydrogen to fuse to helium, the energy source that powers the sun and most other stars. [22]

Mass Edit

Jupiter is twice as massive as all the other planets in the Solar System put together. [12] It gives off more heat than it gets from the sun. [23] Jupiter is 11 times the width of Earth and 318 times as massive. The volume of Jupiter is 1,317 times the volume of Earth. In other words, 1,317 Earth-sized objects could fit inside it. [24]

Cloud layers Edit

Jupiter has many bands of clouds going horizontally across its surface. The light parts are zones and the darker are belts. The zones and belts often interact with each other. This causes huge storms. Wind speeds of 360 kilometres per hour (km/h) are common on Jupiter. [25] To show the difference the strongest tropical storms on Earth are about 100 km/h. [26]

Most of the clouds on Jupiter are made of ammonia. [27] There may also be clouds of water vapour like clouds on Earth. Spacecrafts such as Voyager 1 have seen lightning on the surface of the planet. Scientists think it was water vapour because lightning needs water vapour. [28] These lightning bolts have been measured as up 1,000 times as powerful as those on Earth. [28] The brown and orange colours are caused when sunlight passes through or refracts with the many gases in the atmosphere.

Great Red Spot Edit

One of the biggest features in Jupiter's atmosphere is the Great Red Spot. It is a huge storm which is bigger than the entire Earth. It is on record since at least 1831, [29] and as early as 1665. [30] [31] Images by the Hubble Space Telescope have shown as many as two smaller "red spots" right next to the Great Red Spot. [32] [33] Storms can last for hours or as long as hundreds of years in the case of the Great Red Spot. [34] [35]

Magnetic field Edit

Jupiter has a magnetic field like Earth's but 11 times stronger. [36] It also has a magnetosphere much bigger and stronger than Earth's. The field traps radiation belts much stronger than Earth's Van Allen radiation belts, strong enough to endanger any spacecraft travelling past or to Jupiter. The magnetic field is probably caused by the large amounts of liquid metallic hydrogen in the core of Jupiter. [37] The four largest moons of Jupiter and many of the smaller ones orbit or go around the planet within the magnetic field. This protects them from the solar wind. Jupiter's magnetic field is so large, it reaches the orbit of Saturn 7.7 million miles (12 million km) away. [38] The Earth's magnetosphere does not even cover its moon, less than a quarter of a million miles (400,000 km) away.

Ring system Edit

Jupiter also has a thin planetary ring system. [39] These rings are difficult to see and were not discovered until 1979 by NASA's Voyager 1 probe. [40] There are four parts to Jupiter's rings. The closest ring to Jupiter is called the Halo Ring. [41] The next ring is called the Main Ring. It is about 6,440 km (4,002 mi) wide and only 30 km (19 mi) thick. [41] The Main and Halo rings of Jupiter are made of small, dark particles. [40] The third and fourth rings, called the Gossamer Rings, are transparent (see through) and are made from microscopic debris and dust. [40] This dust probably comes from small meteors striking the surface of Jupiter's moons. The third ring is called the Amalthea Gossamer Ring, named after moon Amalthea. The outer ring, the Thebe Gossamer Ring, is named after the moon Thebe. The outer edge of this ring is about 220,000 km (136,702 mi) from Jupiter. [41]

The orbit of a planet is the time and path it takes to go around the Sun. In the amount of time it takes for Jupiter to orbit the Sun one time, the Earth orbits the Sun 11.86 times. [42] One year on Jupiter is equal to 11.86 years on Earth. The average distance between Jupiter and the Sun is 778 million kilometres. This is five times the distance between Earth and the Sun. Jupiter is not tilted on its axis as much as Earth or Mars. This causes it to have no seasons, for example summer or winter. Jupiter rotates, or spins around very quickly. [43] This causes the planet to bulge in the middle. Jupiter is the fastest spinning planet in the Solar System. [43] It completes one rotation or spin in 10 hours. [11] Because of the bulge, the length of the equator of Jupiter is much longer than the length from pole to pole. [44]

From Earth Edit

Jupiter is the third brightest object in the night sky, after the Moon and Venus. [15] Because of that, people have always been able to see it from Earth. The first person known to really study the planet was Galileo Galilei in 1610. [45] He was the first person to see Jupiter's moons Io, Europa, Ganymede and Callisto. [45] This was because he used a telescope, unlike anyone before him.

No new moons were discovered for more than two hundred years. In 1892, astronomer E.E Barnard found a new moon using his observatory in California. He called the moon Amalthea. [46] It was the last of Jupiter's 67 moons to be discovered by human observation through a telescope. [45] In 1994, bits of the comet Shoemaker Levy-9 hit Jupiter. It was the first time people saw a collision between two Solar System objects. [47]

From spacecraft Edit

Seven spacecraft have flown past Jupiter since 1973. [11] These were Pioneer 10 (1973), Pioneer 11 (1974), Voyagers 1 and 2 (1979), Ulysses (1992 and 2004), Cassini (2000) and New Horizons (2007).

The Pioneer missions were the first spacecraft to take close up pictures of Jupiter and its moons. Five years later, the two Voyager spacecraft discovered over 20 new moons. They captured photo evidence of lightning on the night side of Jupiter. [48]

The Ulysses probe was sent to study the Sun. It only went to Jupiter after it had finished its main mission. Ulysses had no cameras so it took no photographs. In 2006, the Cassini spacecraft, on its way to Saturn, took some very good, very clear pictures of the planet. Cassini also found a moon and took a picture of it but it was too far away to show the details. [49]

The Galileo mission in 1995 was the first spacecraft to go into orbit around Jupiter. It flew around the planet for seven years and studied the four biggest moons. It launched a probe into the planet to get information about Jupiter's atmosphere. The probe travelled to a depth of about 150 km before it was crushed by the weight of all the gas above it. [50] This is called pressure. The Galileo spacecraft was also crushed in 2003 when NASA steered the craft into the planet. They did this so that the craft could not crash into Europa, a moon which scientists think might have life. [50]

NASA have sent another spacecraft to Jupiter called Juno. It was launched on August 5, 2011 [51] and arrived at Jupiter on July 4, 2016. [52] NASA published some results from the Juno mission in March 2018. [53] Several other missions have been planned to send spacecraft to Jupiter's moons Europa and Callisto. One called JIMO (Jupiter Icy Moons Orbiter) was cancelled in 2006 because it cost too much money. [54]

Jupiter has 79 known moons. The four largest were seen by Galileo with his primitive telescope, and nine more can be seen from Earth with modern telescopes. The rest of the moons have been identified by spacecraft. [55] The smallest moon (S/2003 J 12) is only one kilometre across. The largest, Ganymede, has a diameter of 5,262 kilometres. It is bigger than the planet Mercury. [56] The other three Galilean moons are Io, Europa and Callisto. Because of the way they orbit Jupiter, gravity affects three of these moons greatly. The friction caused by the gravity of Europa and Ganymede pulling on Io makes it the most volcanic object in the Solar System. It has over 400 volcanoes, more than three times as many as Earth. [57]

Jupiter's large gravity has had an effect on the Solar System. Jupiter protects the inner planets from comets by pulling them towards itself. Because of this, Jupiter has the most comet impacts in the Solar System. [47]

Two groups of asteroids, called Trojan asteroids, have settled into Jupiter's orbit round the Sun. One group is called the Trojans and the other group is called the Greeks. They go around the Sun at the same time as Jupiter. [58] [59]


Juno at Jupiter: Extended Mission Flybys of Galilean Moons

The news that NASA will extend the InSight mission on Mars for two years, taking it through December of 2022, is not surprising, given the data trove the mission team has collected through operation of the mission seismometer. A live asset on Mars also deepens our knowledge of the planet’s atmosphere and magnetic field, all reasons enough for pushing for another two years. But the extension of the Juno mission to Jupiter deserves more attention than it’s getting, given that Juno’s remit will be expanded deep into the Jovian system.

Image: NASA has extended both the Juno mission at Jupiter through September 2025 and the InSight mission at Mars through December 2022. Credit: NASA/JPL-Caltech.

For those of us fascinated with the outer system, this is good news indeed. I’m looking over two documents, the first being a presentation based on a report submitted to NASA’ Outer Planets Assessment Group (thanks to Ashley Baldwin for passing this along). The OPAG document was produced by Scott Bolton (Southwest Research Institute) it gives the overview of what a mission extension could look like. Also on my desk this morning is the text of the 2020 Planetary Missions Senior Review (PMSR), outlining a set of three mission scenarios. The context of both analyses is the success of the mission in studying Jupiter’s interior structure, magnetic field and magnetosphere, not to mention the examination of its atmospheric dynamics, seen in such roiling imagery as that depicted with stunning complexity in many of the JunoCam images.

Launched in 2011 and operational at Jupiter since 2016, Juno’s prime missions were to have ended in July of this year, with the spacecraft having completed 34 polar orbits, each of 53 day duration. The OPAG report refers to the subsequent extended mission as “a full Jovian system explorer with close flybys of satellites and rings.” The extended mission is to last through September, 2025, with observations of the planet’s ring system, its large moons, and a series of targeted observations and close flybys of Ganymede, Europa and Io.

That last clause really got my attention, as I hadn’t seen it coming. Juno is in an elliptical orbit with a 53-day period whose perijove migrates northward. This bit from the Senior Review reveals in depth the interactions between the various mission scenarios and satellite flybys. The three scenarios mentioned offer alternatives given varying science and budget considerations:

The proposed Juno extended mission (EM) would take advantage of the natural northward progression of the periapsis of the spacecraft’s orbit and the consequent lowering of spacecraft altitudes over Jupiter’s high northern latitudes. The EM would run until the end of the mission, with an expected duration of approximately four years. Under the High and Medium Scenarios, propulsive maneuvers would be utilized not only to target Jupiter-crossing longitude and perijove altitude, as during the prime mission, but also to target close flybys of Ganymede, Europa, and Io. The flyby maneuvers would act to shorten the spacecraft orbital period, yielding more close passes of Jupiter within a given time interval, and increase the rate of northward movement of spacecraft perijove. Under the Low scenario for EM operation, the satellite gravity assists and close satellite flybys would not be attempted.

So mission scientists have a number of options to work with. The extended mission investigates the northern hemisphere and probes the region above Jupiter’s polar cap aurora. The northern adjustment in Juno’s orbit is what makes the satellite flybys possible and enables as well close analysis of its ring structures. The Juno team can look forward to 3D mapping of Jupiter’s polar cyclones and studies of the planet’s unusual dilute core, the latter an earlier Juno discovery revealing a core consisting of both rocky material and ice as well as hydrogen and helium.

Both Europa Clipper and the European Space Agency’s JUICE mission (Jupiter Icy Moons Explorer) should benefit from Juno data on the radiation environment they will operate within. At Europa, Juno will continue the search for possible plume activity while examining the ice shell and mapping surface features, while studies of Io’s magma, polar volcanoes and interactions with Jupiter’s magnetosphere will be enabled by its encounters there. At Ganymede, magnetospheric interactions and surface composition data should be produced in abundance.

In the OPAG presentation, most of the Juno flybys will be at Io, with 11 possible between mid-2022 and 2025. Two encounters are planned for Ganymede (and recall that JUICE is scheduled to orbit the huge moon), and three encounters are feasible for Europa. The actual number of flybys will, according to the Senior Review, depend upon budget choices. In that document, I find this overview of Juno’s satellite flybys:

The orbit of Juno in the EM [extended mission] would take the spacecraft through the Io and Europa plasma tori and in close proximity to Io, Europa and Ganymede. Maps of Ganymede’s surface composition would allow studies to understand the importance of radiolytic processes in surface weathering, identify changes since Voyager and Galileo, and search for new craters. Juno’s Microwave Radiometer (MWR) is particularly sensitive to the upper 10 km of Europa’s ice shell. Studies at wavelengths complementing expected results from Europa Clipper’s radar would identify regions of thick and thin ice and search for regions where shallow subsurface liquid may exist. Juno’s visible and low-light cameras would search Europa for active plumes and changes in color/albedo that may reveal eruption regions since Galileo. The fields and particles experiments would look for evidence of recent activity. Finally, the Juno EM would include a flyby of Io and search for evidence of a magma ocean.

What an interesting development Juno’s extended mission turns out to be! Continuing science operations with existing equipment far undercuts the cost of new missions while extending long-duration datasets and, in the case of Juno, enabling a set of exciting new targets. We have the option here of a series of Galilean moon flybys that were never in Juno’s original mission, observations that could inform later choices made for Europa Clipper and JUICE. All told, Juno’s unanticipated extended mission is a heartening contribution to outer system science.


Affiliations

California Institute of Technology, Pasadena, CA, USA

Cheng Li, Andrew Ingersoll & Zhimeng Zhang

Southwest Research Institute, San Antonio, TX, USA

Scott Bolton, Daniel Santos-Costa & Hunter Waite

Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA

Steven Levin, Michael Janssen, Shannon Brown, John Arballo, Virgil Adumitroaie, Samuel Gulkis, Sidharth Misra, Glenn Orton & Fabiano Oyafuso

University of Michigan, Ann Arbor, MI, USA

Cornell University, Ithaca, NY, USA

Georgia Institute of Technology, Atlanta, GA, USA

Paul Steffes & Amoree Hodges

Université Côte d’Azur, OCA, Lagrange CNRS, Nice, France

Goddard Institute for Space Studies, New York, NY, USA

Lockheed Martin, Grand Prairie, TX, USA

University of Houston, Houston, TX, USA

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Contributions

C.L. developed the inversion software and performed the data analysis. All authors discussed the results and commented on the manuscript.

Corresponding author


The Interiors of Jupiter and Saturn

Probing the interiors of the gaseous giant planets in our solar system is not an easy task. It requires a set of accurate measurements combined with theoretical models that are used to infer the planetary composition and its depth dependence. The masses of Jupiter and Saturn are 317.83 and 95.16 Earth masses (M), respectively, and since a few decades, it has been known that they mostly consist of hydrogen and helium. The mass of heavy elements (all elements heavier than helium) is not well determined, nor are their distribution within the planets. While the heavy elements are not the dominating materials inside Jupiter and Saturn, they are the key to understanding the planets’ formation and evolutionary histories.

The planetary internal structure is inferred from theoretical models that fit the available observational constraints by using theoretical equations of states (EOSs) for hydrogen, helium, their mixtures, and heavier elements (typically rocks and/or ices). However, there is no unique solution for determining the planetary structure and the results depend on the used EOSs as well as the model assumptions imposed by the modeler.

Major model assumptions that can affect the derived internal structure include the number of layers, the heat transport mechanism within the planet (and its entropy), the nature of the core (compact vs. diluted), and the location (pressure) of separation between the two envelopes. Alternative structure models assume a less distinct division between the layers and /or a non-homogenous distribution of the heavy elements. The fact that the behavior of hydrogen at high pressures and temperatures is not perfectly known and that helium may separate from hydrogen at the deep interior add sources of uncertainty to structure models. In the 21st century, with accurate measurements of the gravitational fields of Jupiter and Saturn from the Juno and Cassini missions, structure models can be further constrained. At the same time, these measurements introduce new challenges for planetary modelers.

Keywords

Subjects

Introduction

Investigating the interiors of the giant planets in the solar system goes back several decades. Jupiter and Saturn are located at radial distances of about 5.2 and 9.6 AU from the Sun and their composition is dominated by light elements, in particular, hydrogen and helium (hereafter H–He). 1 Jupiter and Saturn are massive fast rotators, and their atmospheres are characterized by impressive signatures of dynamics. These colorful atmospheres, however, represent only the “skin” of the planets and cannot reveal the secrets of their internal structures. Therefore, despite the significant progress on both the observational and theoretical fronts, Jupiter and Saturn remain mysterious planets.

Due to their large distances from the Earth and their gaseous nature, revealing information on the deep interiors of Jupiter and Saturn must be done by using indirect measurements. As more information about the planets is collected, more comprehensive theoretical structure models must be developed. However, it becomes increasingly challenging to find a self-consistent theoretical framework that meets all the observational constraints. The accurate measurements of the physical and chemical planetary properties provide new and important constraints, but they also lead to new open questions. The current situation in modeling planetary interiors can be summarized by Albert Einstein quote: “The more I learn, the more I realize how much I don’t know.” Fitting the new data requires more complex structure models and the inclusion of various physical processes and assumptions which are not well justified or completely understood.

Currently, planetary scientists still don’t have unique and self-consistent views of the interiors of Jupiter and Saturn, but there is a better understanding of the relevant physical and chemical processes that should be considered as well as the limitations of the theoretical approaches. Making progress in that direction does not only help in studying planetary interiors but also in better understanding the behavior of simple elements at high pressures and temperatures, and in putting important constraints on giant planet formation and evolution models.

In this article, the current knowledge of the internal structures of Jupiter and Saturn is summarized. Recent reviews of this topic include Militzer, Soubiran, Wahl, and Hubbard (2016), Guillot and Gautier (2014), Helled and Guillot (2018), Baraffe, Chabrier, Fortney, and Sotin (2014), Fortney and Nettelmann (2010), Fortney et al. (2016), and the references therein.

Making an Interior Model

Observational Constraints

Internal structure models are designed to fit the observed physical data of the planets, such as their masses, radii, gravitational and magnetic fields, 1-bar temperatures, atmospheric composition, and internal rotations. Key physical properties of Jupiter and Saturn are summarized in Table 1. Interestingly, the atmospheres of both Jupiter and Saturn show a depletion in helium in comparison to the helium mass fraction of the protosolar value of Yproto ∼ 0.275 as inferred from stellar evolution models for the Sun (e.g., Bahcall, Pinsonneault, & Wasserburg, 1995). The measured helium mass fractions in Jupiter and Saturn are found to be ∼ 0.238 (von Zahn, Hunten, & Lehmacher, 1998) and 0.18–0.25 (Conrath & Gautier, 2000), respectively. As discussed below, this does not imply that Jupiter and Saturn are depleted in helium globally but rather that the distribution of helium is inhomogeneous within their interiors due to the phenomenon of helium settling (see the section “Helium and Hydrogen” for details). In addition, for Jupiter, the Galileo entry probe provided abundances measurements of other components, suggesting that Jupiter’s outer envelope is enriched with heavy elements by a factor of ∼ 24 compared to protosolar abundance (e.g., Atreya, Mahaffy, Niemann, Wong, & Owen, 2003 Guillot & Gautier, 2014). Two exceptions were neon and oxygen, both found to be depleted however, neon is expected to be affected by the process of helium rain (Roustlon & Stevenson, 1995 Wilson & Militzer, 2010) and the low abundance of water is probably linked to the special location of the probe’s entry, which is known as a “dry spot” where the atmosphere is dry and does not represent the bulk of the atmosphere. Therefore, at the moment the oxygen abundance in Jupiter is still unknown.

Since giant planets consist of mostly fluid hydrogen and helium, they do not have a solid surface below the clouds like terrestrial planets. Therefore, the “surface” of the planet is defined as the location where the pressure is 1 bar, comparable to the pressure at the Earth’s surface. The temperature at this location is measured (with a small uncertainty). Then, with information of the temperature at 1 bar, the entropy of the outer envelope is determined and adiabatic models can be constructed. 2

The gravitational field of the planet, or more precisely, the total potential, which also includes the rotational term, and is given by

where (r, θ ‎, φ ‎) are spherical polar coordinates, a is the equatorial radius, and M is the total planetary mass. The potential U is represented as an expansion in Legendre polynomials (e.g., Zharkov & Trubitsyn, 1978), where typically only the even indices (i.e., P2n) are taken into account due to the primarily north–south symmetry of the two hemispheres. 3 The gravitational harmonic coefficients J2n are typically inferred from Doppler tracking data of a spacecraft orbiting or flying by the planet and are used to constrain the density profile as discussed below.

Governing Equations

The planetary interior is modeled by solving the standard structure equations, which include the mass conservation, hydrostatic balance, and thermodynamic equations as follows:

where P is the pressure, ρ ‎ is the density, m is the mass within a sphere of a radius r, and ω ‎ is the rotation rate. In order to account for rotation, the hydrostatic equation (Equation 2) includes an additional term that depends on ω ‎, which is assumed to be constant (i.e., uniform rotation), for a non-spinning planet: ω ‎ = 0. For a rapidly rotating planet, this equation is valid in the limit of a barotropic fluid and a solid-body rotation. The radius r is then considered as a mean volumetric radius. Equation 3 is the first-order expansion of the total potential U . The temperature gradient ∇T ≡ d ln T/d ln P depends on the heat transport mechanism (convection vs. conduction/radiation). Typically, the temperature gradient is taken to be the smallest among the adiabatic ∇ad, radiative/conductive ∇rad/cond gradients, since the heat transport mechanism that leads to the smallest temperature gradient is the most efficient one. In other words, the temperature gradient is taken to be ∇T = min[∇ad, ∇rad/cond]. 4 Finally, in order to solve this set of equations, the density dependence on the temperature and pressure needs to be known, that is, ρ ‎(P, T ), which is determined by the equation of state.

Table 1. Basic Observed Properties of Jupiter and Saturn

Note: Adapted from Helled and Guillot (2018), NASA(2019a), and references therein. Jupiter’s gravitational field is taken from Iess et al. (2018). The gravitational coefficients correspond to the reference equatorial radii of 71,492 km and 60,330 km for Jupiter and Saturn, respectively (see NASA [2019b]). These are theoretical values based on interior model calculations. See “Rotation Rate and Depth of Winds” and Helled et al. (2015) for discussion on Saturn’s rotation rate uncertainty.

The density profile of the planets is set to reproduce the measured gravitational moments J2n. The relation between the gravitational moments and the density profile is given by (e.g., Zharkov & Trubitsyn, 1978):

where the integration is carried out over the volume τ ‎. Traditionally, the theoretical density profile and gravitational moments were calculated by using the theory of figures (TOF) in which the harmonics are computed from a series approximation in the smallness parameter m = ω ‎2R3/GM , where R is the mean radius of the planet, typically up to an order of three or four. This was sufficient as long as no information was known about the gravitational coefficients beyond J6 and the measurements have relatively large uncertainties. An alternative method to TOF that is designed to be compatible with accurate data and provide estimates for the higher-order harmonics was developed by W. Hubbard (e.g., Hubbard, 2012, 2013). In this approach, called concentric Maclaurin spheroid (CMS), the density profile is represented by a large number of Maclaurin spheroids where a continuous density can be achieved if the number of spheroids is large enough. While the computational resources needed are large, the gravitational coefficients can be calculated to any order with an excellent precision (∼109). Discussion and comparison between the TOF and CMS methods can be found in Hubbard, Schubert, Kong, and Zhang (2014), Wisdom and Hubbard (2016), Nettelmann (2017), Debras and Chabrier (2018), and references therein.

Equation of State

In thermodynamics, the equation of state (EOS) relates the state variables such as the temperature, pressure, density, internal energy, and entropy. Since Jupiter and Saturn are mostly composed of H–He, modeling their structures relies on information of the EOS of hydrogen, helium, and their mixture. Giant planet interiors serve as natural laboratories for studying different elements at exotic conditions that do not exist on Earth. At the same time, calculating the EOS of materials in Jupiter and Saturn interior conditions is a challenging task because molecules, atoms, ions, and electrons coexist and interact, and the pressure and temperature range varies by several orders of magnitude, going up to several tens of megabars (Mbar) (i.e., 100 GPa and several 104 Kelvins). Therefore, information on the EOS at such conditions requires performing high-pressure experiments and/or solving the many-body quantum mechanical problem to produce theoretical EOS tables that cover such a large range of pressures and temperatures. Despite the challenges, there have been significant advances in high-pressure experiments and ab initio EOS calculations. The EOS of hydrogen, helium, and heavier elements is briefly described below. More information on that topic can be found in Fortney and Nettelmann (2010), Baraffe et al. (2014), Militzer et al. (2016), Guillot and Gautier (2014), Helled and Guillot (2018), and references therein.

Hydrogen

Hydrogen is the most abundant element in the universe, and yet its phase diagram is still a topic of intensive research. The behavior of hydrogen at high pressures can be investigated experimentally and theoretically. There are several types of laboratory experiments such as gas guns, convergent shock waves, and laser-induced shock compression that can probe hydrogen (or its isotope, deuterium) at Mbar pressures. Unfortunately, the available experimental data have a large range, and each experiment suffers from different limitations and systematics. Nevertheless, some progress has been made toward convergence when comparing the shock Hugoniot curve of hydrogen from different studies. Although empirical, the information from experiments is limited, and theoretical calculations are required to provide a wide-range EOS for hydrogen. On the other hand, laboratory experiment results are of primary importance since they are used to calibrate the theoretical EOS used for planetary modeling.

The most popular ab initio technique in materials science, and the most common approach to probe the EOS of hydrogen and helium at planetary conditions, is density functional theory (DFT). Although the theory is exact, all existing practical implementations rely on approximations. While DFT calculations provide a relatively accurate determination of the EOS of hydrogen in a large range of temperatures and pressures, using moderate computational resources, it shows a poor performance in assessing phase transitions (e.g., Azadi & Foulkes, 2013). An alternative approach is quantum Monte Carlo (QMC), which is a wave function-based method and can accurately solve the electronic problem (e.g., Mazzola, Helled, & Sorella, 2018 and references therein). This approach is much more computationally expensive, but is potentially one order of magnitude more accurate than DFT (e.g., Foulkes, Mitas, Needs, & Rajagopal, 2001) and therefore can closely simulate the phase transitions, where the (free) energy difference between the competing phases (at given thermodynamic conditions) can be small. At the moment, QMC calculations can be used to calibrate other existing wide-range EOS tables and are expected to play an important role in EOS calculations in the 2020s.

Figure 1 shows the phase diagram of dense hydrogen and H–He mixture. Shown is the transition between the insulating molecular and the metallic-atomic hydrogen fluid (shaded area). The location of this transition is not only important for understanding the generation of strong magnetic fields in these planets but also for determining the division of the planets to layers with different structure models (brown and mustard color lines, respectively).

There are several interesting conclusions about Jupiter and Saturn that can be made simply by looking at the hydrogen phase diagram. First, both planets lie in the regime above solid hydrogen, suggesting that they are fluid planets, as already suggested by Hubbard (1968). Second, both planets cross the critical point of hydrogen, which indicates that in the outer parts of the planets hydrogen is in the molecular form (H2) and in the metallic form in the deep interiors. Metallic hydrogen is a phase of hydrogen at high pressures/temperatures where electrons are free and hydrogen becomes an excellent conductor, like a metal. The main uncertainty concerning the hydrogen EOS is in the region of 0.5–10 Mbars (50−103 GPa), where the transition from the molecular phase to the metallic phase occurs. In fact, the metallization of hydrogen is an active area of research and the exact metallization pressure/temperature is still being debated, but it is expected to be at ∼1 Mbar for Jupiter’s conditions. 5 Finally, it is clear that Saturn’s adiabat covers lower temperatures and pressures (due to its lower mass). As a result, Saturn’s interior consists of a smaller fraction of metallic hydrogen in comparison to Jupiter, and since this regime of the EOS is less understood, there is less uncertainty in Saturn’s structure due to the hydrogen EOS (Saumon & Guillot, 2004). On the other hand, as discussed below, Saturn is more likely to be affected by the phase separation of helium.

Figure 1. Phase diagram of hydrogen. Gray area: solid phase (experimental) shaded blue: insulating molecular liquid solid blue: first-order liquid–liquid phase boundary (QMC) dashed blue: continuous liquid–liquid phase boundary (QMC) black points: possible location of the critical point (endpoint of the first-order line) solid red: first-order for the H–He (QMC) dashed red: continues liquid–liquid boundary H–He (QMC) empty triangles: static compression experiments H (Zaghoo, Salamat, & Silvera, 2016 Ohta et al., 2015) solid triangles: dynamic compression H (Knudson et al., 2015).

Helium and Hydrogen

The behavior of pure helium at the extreme conditions inside giant planet interiors is more constrained than that of hydrogen simply because helium ionization requires larger pressures and a phase transition is not expected to take place. In Figure 1, the blue and red curves correspond to the phase diagram of pure hydrogen and a H–HE mixture (red) with a protosolar value, respectively (see Mazzola et al. [2018] for details). It is clear that the presence of helium delays the dissociation (metallization) pressure compared to pure hydrogen and therefore the presence of helium cannot be neglected when estimating the metallization pressures in Jupiter’s and Saturn’s interiors.

Figure 2. Phase diagram for a H–He mixture. The orange region shows the region of the H- He separation as derived by Lorenzen et al. (2011). The red curve shows the critical temperature for the separation, according to Morales et al. (2013). Numerical and experimental results by Schouten, de Kuijper, and Michels (1991) and Loubeyre, Letoullec, and Pinceaux (1991) are also presented. The back curves show the isoentropes of Jupiter (plain) and Saturn (dashed), respectively.

In addition, the interaction between hydrogen and helium under the interior conditions of Jupiter and Saturn leads to challenges in determining the EOS. This is because helium is expected to become immiscible in hydrogen, leading to helium settling (known as “helium rain”) that results in a non-homogenous distribution of helium within the planet, where helium settles (and is therefore enriched) toward the deep interior. This phenomenon of helium rain was already predicted in the 1970s (e.g., Salpeter, 1973 Stevenson, 1975 Stevenson & Salpeter, 1977a, 1977b) and received observational support when the helium in the atmospheres of Jupiter and Saturn was found to be depleted in comparison to the protosolar value. Recently, ab initio calculations of the phase diagram have confirmed the immiscibility of helium in hydrogen (Salpeter, 1973 Lorenzen, Holst, & Redmer, 2011 Morales, Hamel, Caspersen, & Schwegler, 2013 Sch¨ottler & Redmer, 2018, and references therein). Figure 2 shows the phase diagram for a H–He mixture with a helium mole concentration slightly lower than protosolar (see Guillot & Gautier [2014] for details). The exact location in the phase diagram in which helium rain occurs is still being investigated and is of great importance for understanding the structure and evolution of both Jupiter and Saturn. What seems to be robust is that since Saturn has a smaller mass than Jupiter and therefore its internal temperatures and pressures are lower, it is located “deeper” within the phase diagram in comparison to Jupiter (see Figure 2). This means that the process of helium rain is more significant inside Saturn and has begun earlier. Jupiter’s interior has to cool for a longer time to reach the temperatures corresponding to this phase separation. This is consistent with the measurements of helium in the giant planet atmospheres where Saturn’s atmosphere is found to be more depleted in helium.

Heavy Elements

In astrophysics, heavy elements represent all the elements that are heavier than helium. Ideally, structure models should include all the possible elements when modeling the planetary interior. However, this introduces an additional complexity to the models because the ratios between the different elements within the planets have to be assumed. In addition, the details of the EOS of the heavy elements are less crucial, since the temperature dependence on the density of these elements is rather weak at giant planet interior conditions, and their contribution to the planetary density is of a second-order effect in comparison to H–He (e.g., Baraffe et al., 2008 Saumon & Guillot, 2004 Fortney et al., 2016). Often, the heavy elements in Jupiter and Saturn are represented by water and/or rock, where rocks lead to about 50% less massive cores compared to water (e.g., Fortney & Nettelmann, 2010). While the measured gravitational field is essentially blind to the innermost regions of the planet, these regions can affect the density profile indirectly via the constraints on the outer envelope (e.g., Helled, Anderson, Schubert, & Stevenson., 2011 Guillot & Gautier, 2014). Currently there is ongoing progress in ab initio calculations of the EOSs for water, ammonia, silicates, and iron as well as their miscibility in metallic hydrogen (e.g., French et al., 2009 Knudson et al., 2012 Wilson & Militzer, 2010, 2012). As such ab initio EOS calculations become available, it is desirable to include them in structure models and further investigate their effect on the inferred composition and internal structure.

Internal Structure Models

Since several decades, studies aimed to better constrain the interiors of Jupiter and Saturn (e.g., Saumon & Guillot, 2004 Militzer, Hubbard, Vorberger, Tamblyn, & Bonev, 2008, Militzer et al., 2016 Nettelmann et al., 2008 Nettelmann, Becker, Holst, & Redmer, 2012 Nettelmann, Fortney, Moore, & Mankovich, 2015 Helled & Guillot, 2013 Hubbard & Militzer, 2016 Miguel et al., 2016). Unfortunately, there is no unique solution for the internal structure of a planet. The non-unique nature of the problem is inherent because the available data are (and will remain) insufficient to uniquely infer the planetary internal structure. In addition, the inferred structure depends on the model assumptions and the EOSs used by the modeler. The main uncertainties in structure models are linked to the following assumptions and setups: (a) the number of layers, (b) the composition and distribution of heavy elements, (c) the heat transport mechanism, (d) the transition pressure of hydrogen metallization, and (e) the rotation period and the dynamical contribution of winds (e.g., differential rotation).

Typically, the interiors of Jupiter and Saturn are modeled assuming the existence of a distinct heavy-element core which is surrounded by an inner envelope of metallic hydrogen and an outer envelope of molecular hydrogen. Due to the indication of helium rain in the planets, the inner and outer envelopes are set to be helium-rich and helium-poor, respectively. For the heavy elements distribution there are two common assumptions. In the first, they are assumed to be homogeneously mixed within the two envelopes. Then, if Zin and Zout represent the heavy element mass fraction in the inner and outer envelopes, respectively, for this case Zin = Zout. In the second case, the heavy element enrichment is assumed to be higher at the metallic region (inner envelope) (i.e., Zin > Zout).

Figure 3. Sketches of the internal structures of Jupiter and Saturn as inferred from structure models. For each planet, two possible structures are shown: one consisting of distinct layers and one with a gradual distribution of heavy elements. Schematic representation of the interiors of Jupiter and Saturn. The core masses of Jupiter and Saturn are not well constrained for Saturn, the inhomogeneous region could extend down all the way to the center, resulting in a “helium core.”

In both of these cases, the heavy elements are taken to be homogeneously distributed, suggesting a homogenous composition, at least within one part of the envelope. The location where the envelope is divided into a helium-poor–helium-rich region corresponds to the pressure in which helium becomes immiscible in hydrogen. For simplicity, for the models with ZinZout the location of the heavy element discontinuity is assumed to occur at the same location.

The division of the planetary structure into three layers is not written in stone it only represents the simplest model that can be considered. It may indeed be that the discontinuity in helium and the heavy elements occurs at different pressures, and especially in the case of Saturn, that a nearly pure helium layer also exists (e.g., Fortney & Hubbard, 2003). In addition, the core itself may not be a distinct region (as well as non-existing), and the heavy elements may have a gradual distribution along the planetary interior. Even within this simple three-layer model framework, the inferred composition and core mass depend on the model assumptions. More complex models increase the range of possible solutions even further. Figure 3 shows a simple sketch of the interiors of Jupiter and Saturn. Figure 4 shows representative density and pressure profiles within the planets for standard three-layer models.

Jupiter

For Jupiter, structure models typically differ by the assumption regarding the heavy element distribution, the assumed number of layers, and the calculated entropy from EOS calculations. Saumon and Guillot (2004) explored the possible range of solutions for Jupiter using different EOSs and inferred total heavy element mass between 10 and 40 M, and core masses between 0 and 10 M. Later, when DFT EOS calculations became available, new Jupiter models were presented. The first set of

Figure 4. Representative density (left) and pressure (right) profiles of Jupiter and Saturn as a function of the planetary mean radius. The data are taken from Miguel et al. (2016) and Helled and Guillot (2013), respectively.

models was based on the entropy calculation of the Rostock H–He EOS, which was calculated using ab initio DFT (e.g., Nettelmann et al., 2008, 2012 Becker et al., 2014). In Nettelmann et al. (2008), Nettelmann, Helled, Fortney, and Redmer (2012), Becker et al. (2014), and Nettelmann (2017), the models relied on the three-layer assumption, where the interior is separated into a distinct core and two homogeneous envelopes. The helium mass fraction in the outer envelope, Yout, was set to match the Galileo entry probe value of Y = 0.238. The inner envelope helium abundance Yin is chosen to yield a bulk helium mass fraction that reproduces the protosolar value. The heavy element mass fractions Zout and Zin were chosen to match the then measured values of the low-order gravitational harmonics J2 and J4, with J4 being slightly more sensitive to Zout than J2 is, and vice versa for Zin. In the transition between the inner and outer envelopes, the pressure and temperature are assumed to change continuously, while the density and entropy have discontinuities. In these models, the transition pressure Ptrans is taken to be a free parameter, between 1 and 5 Mbar, although H–He phase diagram calculations suggest that Ptrans ∼ 1 Mbar. Higher assumed Ptrans values lead to higher envelope metallicity and smaller inferred values for the core mass. Overall, the studies of this group have confirmed the ranges derived by Saumon and Guillot (2004), where the core mass was smaller than ∼10 M, with a global enrichment of tens M of heavy elements.

A second type of Jupiter models based on Militzer’s H–He EOS calculation, also using ab initio DFT-MB (e.g., Militzer et al., 2008 Hubbard & Militzer, 2016). A comprehensive equation of state of H–He mixtures and their inferred internal energies as well as a Jupiter adiabat have been presented by Militzer and Hubbard (2013). These models lead to a significant inferred core mass for Jupiter of the order of 15–20 M with a low envelope metallicity, sometimes even less than solar.

Note that in these Jupiter models, the transition pressure is not taken as a free parameter but is set by identifying the location in which the pressure of Jupiter’s adiabat intersects with the H–He immiscibility region as derived by Morales et al. (2013 see Figure 2). It should be noted that the EOSs used by Militzer and collaborators and Nettelmann and collaborators are not very different in terms of the raw data, but in the entropy calculation, and therefore in the constructed adiabat. A hotter adiabat, as inferred by Nettelmann and collaborators, leads to a larger inferred heavy element mass (see Militzer et al. [2016] and Miguel et al. [2016] for further discussion).

The recent gravity measurements from the Juno spacecraft (Bolton et al., 2017 Iess et al., 2018) introduced new constraints on Jupiter structure models. Jupiter models that fit the Juno data have been presented by Wahl et al. (2017), Nettelmann (2017), and Guillot et al. (2018). Overall, it seems that preferable solutions are ones with cores (∼10 M) and a discontinuity of the heavy element enrichment in the envelope, with the inner helium-rich envelope consisting of a larger fraction of heavy elements than the outer helium-poor envelope (i.e., Zin > Zout). In addition, interior models of Jupiter that fit Juno data suggest that another feasible solution for Jupiter’s internal structure is the existence of a diluted/fuzzy core (e.g., Wahl et al., 2017). In this case, Jupiter’s core is no longer viewed as a pure heavy element central region with a density discontinuity at the core-envelope-boundary, but as a central region whose composition is dominated by heavy elements, which could be gradually distributed or homogeneously mixed. Such a diluted core could extend to a few 10s of percentages of the planet’s total radius and can also consist of lighter elements (H–He). While the total amount of heavy elements in the central region of diluted core models in the central region does not change much (e.g., Wahl et al., 2017, Nettelmann, 2017), the size of the core increases significantly due to the inclusion of H–He.

The existence of a diluted core or a steep heavy element gradient inside Jupiter is actually consistent with formation models of Jupiter (see “Constraints on Internal Structure and Origin” for details). Giant planet formation models in the core accretion scenario (e.g., Pollack et al., 1996) suggest that once the core mass reaches ∼ 12M, the accreted solid material (heavy elements) vaporizes and remains in the planetary envelope (e.g., Stevenson, 1982). This leads to a structure in which the deep interior is highly enriched with heavy elements, with no sharp transition between the core and the inner envelope (e.g., Helled & Stevenson, 2017, and references therein). Another explanation for a diluted core is core erosion. If the heavy elements within a compact core are miscible in metallic hydrogen (e.g., Wilson & Militzer, 2010, 2012), the presence of vast convection could mix some of the core elements in the deep interior (e.g., Guillot, Stevenson, Hubbard, & Saumon, 2004). Long-term evolution models of Jupiter with composition gradients suggest that steep composition gradients can persist up to present-day (see the section “Evolution Models”).

Finally, it is important to note that inferred core mass and total planetary enrichment do not only depend on the assumed EOS but also on the model assumptions. Jupiter’s structure models as presented by Wahl et al. (2017) show that both fuzzy and compact cores are consistent with the Juno data, with the core mass being between ∼ 1.5 and 20 M, depending on the model. A Jupiter structure model with a diluted core resembles the primordial structure derived by formation models (Stevenson, 1985 Helled & Stevenson, 2017), providing a potential link between giant planet formation models and the current-state structure of the planets. In all of these models, Jupiter’s normalized moment of inertia was found to be ∼0.264. This value is relatively well constrained, at least from the modeling point of view, due to the accurate determination of Jupiter’s gravitational field by Juno (e.g., Wahl et al., 2017). Indeed, it has been shown that there is a very strong correlation between J2 and the moment of inertia, but this is not a perfect one-to-one correspondence (e.g., Helled, Anderson, Podolak, & Schubert [2011], and references therein).

Saturn

Saturn is often considered to be a small version of Jupiter but, in fact, the two planets have significant differences. First, the relative enrichment in heavy elements is rather different, as well as the geometry, the magnetic field, the axis tilt, and the long-term evolution. Just from a simple comparison of their normalized moment of inertia values, one can conclude that Saturn is more centrally condensed compared to Jupiter. Naively, one would expect that it is easier to model Saturn’s interior, since a smaller portion of its mass sits in the region of the high uncertainty in the hydrogen EOS, but this is not the case due to the possibility of helium rain. Additional complication arises from the uncertainty in Saturn’s rotation period and shape (see Fortney et al., 2016, and references therein).

Saturn models calculated by Helled and Guillot (2013) also used the three-layer model approach. The range of the helium mass fraction in the outer envelope Yout was taken to be between 0.11 and 0.25, with a global Y = 0.265–0.275 consistent with the protosolar value. Here the EOS of H–He was set to the Saumon, Chabrier, van Horn value (SCVH), which was calculated for a large range of pressures and temperatures (Saumon et al., 1995) and has been broadly used in the astrophysics community. A range of temperatures at 1 bar was considered (130–145 K) and Ptrans was allowed to range between 1 and 4 Mbars. For the heavy element distribution, they assumed Zin = Zout. These Saturn models also accounted for the uncertainty in Saturn’s shape and rotation rate and rotation profile (see “Rotation Rate and Depth of Winds”). Saturn models were constructed for two different assumed rotation periods for both the Voyager and Cassini gravity data. For the range of different model assumptions, the derived core mass was found to range between ∼ 5 and 20 M, while the heavy element mass in the envelope was found to be between ∼ 0 and 7 M. Like for Jupiter, increasing Ptrans leads to smaller core masses and more enriched envelopes.

Finally, the Cassini gravity data reduces the inferred core mass by about 5 M. It should be noted, however, based on the recent Juno data for Jupiter as well as new studies on giant planet formation and evolution, that the assumption of Zin = Zout may be inappropriate, and a more realistic assumption is Zin > Zout. The total heavy element mass in Saturn is estimated to be ∼16–30 M, with a core mass between zero and 20 M (e.g., Saumon & Guillot, 2004 Nettelmann et al., 2012 Helled & Guillot, 2013). However (see “Internal Structure Models”), this conclusion is based on relatively simple interiors models. Figure 5 provides a schematic presentation of the two possible internal structures of Jupiter and Saturn.

Figure 5. Sketches of the internal structures of Jupiter and Saturn.

Non-Adiabatic Interiors

Standard structure models of Jupiter and Saturn assume that the dominating energy transport mechanism is convection that is, that the temperature gradient is given by the adiabatic one, apart from the (thin) outer radiative atmosphere. This assumption simplifies the calculation, since the temperature profile is then well constrained, and in addition, one can assume that the composition within the envelope(s) is homogenous. However, it is now realized that in some cases (and perhaps in most cases), a fully adiabatic model for the giant planets is too simplistic. Non-adiabatic giant planet interiors are in fact a natural outcome of their formation process where the accreted heavy elements result in a non-homogenous interior (e.g., Stevenson, 1985 Helled & Stevenson, 2017 Lozovsky, Helled, Rosenberg, & Bodenheimer, 2017). Non-adiabatic interiors can also be a result of core erosion (e.g., Guillot et al., 2004) and immiscibility of materials in metallic hydrogen (e.g., Wilson & Militzer, 2012 Soubrian & Militzer, 2016).

The existence of composition gradients can inhibit convection due to their stabilizing effect. Moderate composition gradients can be erased by overturning convection, especially at early evolution stages where convection is strong, which leads to a rapid mixing and homogenization of the planet. Otherwise, they can either lead to layered convection, a less efficient type of convection (e.g., Wood, Garaud, & Stellmach, 2013), or inhibit convection and lead to heat transport by conduction and/or radiation.

Leconte and Chabrier (2012, 2013) accounted for the possibility of double-diffusive convection in both Jupiter and Saturn interiors caused by heavy element gradients. It was shown that both Jupiter and Saturn can satisfy all the observational constraints also when assuming non-adiabatic structures with compositional gradients throughout the entire planetary interiors. Since in this scenario heat loss (cooling) is less efficient, the planetary interiors can be much hotter, and the planets can accommodate larger amounts of heavy elements. The core masses derived by these models were found to be 0–0.5 M for Jupiter and

10–21 M for Saturn. The heavy element mass in the envelope was found to be 41–63.5 M and 10–36 M for Jupiter and Saturn, respectively (see Leconte & Chabrier, 2012 for further details). Although these models could be viewed as extreme cases, since the composition gradients are assumed to persist across the entire planetary interiors, they clearly demonstrate the importance of the model assumptions and the limitation of the simple three-layer models. It is also interesting to note that although the semi-convective structure models for Jupiter and Saturn are more rich in heavy elements, the solution for Jupiter indicates the absence of a core.

Evidence of a non-adiabatic interior for Saturn is also indicated from the observed frequency spectrum of its ring oscillations. Some of Saturn’s ring modes observed by the Cassini spacecraft can be attributed to oscillations within the planetary interior (Hedman & Nicholson, 2013). An analysis of the splitting of these oscillation modes suggests the existence of a thick stably stratified region above the core where gravity modes can penetrate (Fuller, 2014). Currently, this is the only proposed explanation to the unexpected splittings via interactions between f-modes propagating in the convective envelope and g-modes propagating in the stable region of the deep interior. While further investigations on this topic are required, these important observation and analysis further suggest that a fully convective structure is too simplistic for describing Saturn’s (and possibly Jupiter’s) interior.

Evolution Models

Another piece of information that can be used to constrain structure models is the planetary evolution. The idea is that the current-state structure of the planets must be consistent with the age of the solar system (

4.56×109 years) (i.e., with the planetary evolution). In fact, the simple assumption of an adiabatic structure was originated by an evolution model where it was shown that the high thermal emission of Jupiter is somewhat consistent with an interior that is convective (e.g., Hubbard, 1968 Guillot, Gautier, Chabrier, & Mosser, 1994 Fortney, Ikoma, Nettelmann, Guillot, & Marley, 2011). Evolution models with layered convection in the helium rain region of Jupiter and Saturn have recently been calculated (Nettelmann et al., 2015 Mankovich, Fortney, & Moore, 2016). In these models, the molecular envelope cools over time, but the deep interior can even heat up if the super-adiabaticity in the inhomogeneous He rain zone is strong. While it is not yet clear whether layer convection occurs in the helium mixing region, these models show that non-adiabaticity is an important aspect that should be considered when calculating the long-term evolution of gaseous planets such as Jupiter and Saturn. 6

Evolution models with primordial composition gradients for Jupiter and Saturn have also been presented (Vazan, Helled, Podolak, & Kovetz, 2016 Vazan, Helled, & Guillot, 2018). It was found that a moderate primordial heavy element gradient becomes homogenous via convective mixing after several million years and that this mixing leads to an enrichment of the planetary envelope with heavy elements. Of the other hand, if the primordial composition gradient is steep, convection in the deep interior was found to be inhibited. This affects the thermal evolution and leads to hotter interiors in comparison to the standard adiabatic case. As in the structure models with layered convection, also here the total heavy element mass in the planets is higher than in the adiabatic models and was found to be up to 40 M and 36 M for Jupiter and Saturn, respectively.

New Insights from the Juno and Cassini Missions

In July 2016 , the Juno mission began to orbit Jupiter and, among other things, has provided an accurate measurement of Jupiter’s gravitational and magnetic fields (e.g., Bolton et al., 2017 Folkner et al., 2017 Iess et al., 2018). At the same time, the Cassini spacecraft performed its last orbits having geometries similar to that of Juno, known as Cassini Grand Finale, providing similar information about Saturn’s fields (e.g., Spilker, 2012), allowing a comparative study of the solar system giant planets. Studies and investigations are still ongoing and more results are likely to appear at the time of (or after) this article, but some key conclusions and new insights on the structure of Jupiter and Saturn from these recent measurements have already been achieved.

Rotation Rate and Depth of Winds

The atmospheres of both Jupiter and Saturn have strong zonal winds, with equatorial speeds of

100 m s1 and 400 m s1, respectively. These zonal wind velocities are relative to the assumed rotation period of the planet’s deep interior (Table 1). In fact, it is not necessarily intuitive to think that giant planets rotate as solid bodies and are therefore represented by a single rotation period due to the fact that they are fluid objects and are characterized by zonal winds that hint the possibility of differential rotation (on cylinders).

Jupiter’s rotation period is assumed to be represented by the rotation period of its magnetic field which is tilted

10 ° from its spin pole and has not changed in many decades (e.g., Riddle & Warwick, 1976 Higgins, Carr, & Reyes, 1996). On the other hand, Saturn’s magnetic pole is aligned with its rotation axis. This spin-aligned configuration and the fact that the magnetic field is dipolar prevent a direct determination of the rotation rate of Saturn’s deep interior because there is no variable component of the magnetic field that is associated with the planetary rotation (e.g., Cao, Russell, Christensen, Dougherty, & Burton, 2011 Cao, Russell, Wicht, Christensen, & Dougherty, 2012). The 10 hr 39 min 22 s rotation period of Saturn, which leads to an equatorial speed of 400 m s1 , was derived from the Voyager spacecraft measurement of the periodicity in Saturn’s kilometric radiation (e.g., Ingersoll & Pollard, 1982 Dessler, 1983). In fact, measurements from the Cassini spacecraft did not only measure a different periodicity by several minutes but also showed that the period is changing with time (e.g., Gurnett et al., 2007), suggesting that the periodicity in Saturn’s kilometric radiation does not represent the rotation of the deep interior. Therefore, at the moment Saturn’s rotation period is not well constrained. Accordingly, the atmospheric zonal wind velocities with respect to the underlying rotating planet are also unknown for Saturn. Several theoretical approaches have been presented to constrain Saturn’s rotation period, and the estimated values range between

10 hr 32 min and 10 hr 45 min (e.g., Anderson & Schubert, 2007 Read, Dowling, & Schubert, 2009 Helled, Galanti, & Kaspi, 2015 Mankovich et al., 2018). While an uncertainty of about 10 minutes sounds small, it can affect the inferred internal structure of the planet and also has implications on its atmosphere dynamics.

The relation between the rotation period of Jupiter and Saturn to their zonal wind, physical shapes, and gravitational and magnetic fields has been studied for decades and is still being investigated. Nevertheless, recently substantial progress in this direction has been made thanks to the Cassini and Juno missions. Deep winds can change the planetary density profile and therefore contribute to the measured gravitational harmonics, and as a result this contribution has to be accounted for as an uncertainty in structure models, since they are hydrostatic and do not include dynamical effects. The depth of the winds can be constrained by accurate measurements of the high-order gravitational and/or the odd harmonics (e.g., Hubbard, 1999 Kaspi, Hubbard, Showman, & Flierl, 2010 Kaspi et al., 2018). A determination of the depth of the winds in Jupiter was recently possible thanks to the Juno data (Iess et al., 2018 Kaspi et al., 2018 Guillot et al., 2018). The winds were found to penetrate to depths of 2000–3000 km, suggesting that 1% of the outer planetary mass rotates differentially in patterns similar to that of the observed atmospheric winds. This depth is consistent with the one expected from ohmic dissipation constraints linked to the metallization of hydrogen (e.g., Liu, Goldreich, & Stevenson, 2008). Since the metallization in Saturn occurs at deeper regions (due to its smaller mass and resulting pressures), then by following the same argument the depth the winds in Saturn is predicted to reach deeper, down to

9000 km. These estimates correspond to depths of around 95% and 80% of the total planetary radius for Jupiter and Saturn, respectively (e.g., Cao & Stevenson, 2017).

Magnetic Fields

Both Jupiter and Saturn possess dipolar intrinsic magnetic fields. The existence and nature of the magnetic fields provide important observational constraints on their current interior structure and dynamics. The existence of an intrinsic magnetic field requires large-scale motions in a medium that is electrically conducting (e.g., Roberts & King, 2013). For Jupiter and Saturn, large-scale radial motions are caused by convective motions which also transport heat from the deep interior toward the outer regions, with the conducting material being metallic hydrogen. Indeed, a significant electrical conductivity is expected inside Jupiter and Saturn before the full metallization of hydrogen at Mbar (100 GPa) pressures (e.g., French et al., 2012). The magnitude of the electrical conductivity inside Jupiter and Saturn combined with the planetary measured magnetic field strength and surface luminosity can be used to estimate the internal ohmic dissipation and introduce additional constraints for structure models (e.g., Liu et al., 2008 Cao & Stevenson, 2017). Thus, our ability to decode the interiors of Jupiter and Saturn from the measured properties of the magnetic field is limited by our current understanding of the dynamo process.

Jupiter’s intrinsic magnetic field is strongest among all solar system planets, with surface field strength ranging from 4 Gauss to 20 Gauss (Connerney et al., 2018 Moore et al., 2018). Recent Juno observations revealed several surprising factors in the morphology of Jupiter’s magnetic field. When viewed at the dynamo surface, Jupiter’s magnetic field is characterized by an intense isolated magnetic spot near the equator with negative flux, an intense and relatively narrow band of positive flux near 45 ° latitude in the northern hemisphere, and a relatively smooth magnetic field in the southern hemisphere. The north–south dichotomy in Jupiter’s magnetic field morphology has been speculated to be due to the existence of a diluted core inside Jupiter, which either limits the dynamo action to the upper layer of Jupiter or creates two spatially separate dynamo actions inside Jupiter (Moore et al., 2018). This provides a nice link between internal structure models that are based solely on the gravity data and magnetic field measurements.

Saturn’s intrinsic magnetic field is unusually weak, with surface field strength ranging from 0.2 Gauss to 0.5 Gauss (Dougherty et al., 2005 Cao et al., 2011, 2012). Surprisingly, Saturn’s magnetic field seems to be perfectly symmetric with respect to the spin axis (Cao et al., 2011, 2012). Both the weak strength and the extreme spin axis-symmetry of the magnetic field of Saturn were attributed to helium rain, which could create a stably stratified layer atop the deep dynamo. However, whether helium rain or composition gradients inside Saturn create substantial stable stratification and whether this stratification layer is above rather than below the deep dynamo are still being investigated.

Constraints on Internal Structure and Origin

In the standard view of giant planet formation, known as core accretion (e.g., Pollack et al., 1996), a giant planet forms in three stages:

Phase 1: Primary core/heavy element accretion. During this early phase, the core accretes solids (planetesimals and/or pebbles) until it empties its gravitational dominating region (feeding zone). The mass associated with the end of this stage is known as “isolation mass” and its exact value depends on the local formation conditions. At this point, the planet is primarily composed of heavy elements with a negligible fraction of an H–He envelope.

Phase 2: Slow envelope accretion. During this phase, the solid accretion rate decreases, and the H–He accretion rate increases until the envelope accretion rate exceeds the heavy element accretion rate. The growth of the envelope enlarges the planet’s feeding zone and thus allows heavy elements to be accreted but at a slow rate.

Phase 3: Rapid gas accretion. Once the H–He mass is comparable to the heavy element mass, the gas accretion rate continuously increases and exceeds the heavy element accretion rate until the disk can no longer supply gas fast enough to maintain equilibrium and keep up with the planetary contraction, and a rapid hydrodynamic accretion of H–He initiates.

In the early core accretion simulations, for the sake of numerical simplicity, it was assumed that all the heavy elements reach the core while the envelope is composed of H–He. However, formation models that follow the heavy element distribution during the planetary formation show that once the core mass reaches a small value of

1–2 M and is surrounded by a small envelope, the solids composed of heavy elements tend to dissolve in the envelope instead of reaching the core (e.g., Lozovsky et al., 2017 Helled & Stevenson, 2017). In this case, the resulting giant planet has a small core mass and an inner envelope which is enriched with heavy elements. Interestingly, this view is consistent with the possibility of Jupiter having a diluted/fuzzy core. Although the prediction on the distribution of heavy elements corresponds to Jupiter right after its formation, evolution models confirm that in several cases such a structure can persist until present time (e.g., Vazan et al., 2016, 2018). A point that still needs to be investigated is whether the composition discontinuity in the heavy elements is caused by the formation process or a result of phase separations and core erosion that occur at later stages during the planetary long-term evolution.

Another missing piece of the puzzle in our understanding Jupiter is linked to its water abundance. The low abundance of water in Jupiter’s atmosphere measured by the Galileo probe is likely to be a result of the special entry spot, which keeps the enrichment of water in Jupiter’s atmosphere unknown. Jupiter’s water abundance is now being measured by Juno using a microwave radiometer (MWR), which probes down to pressure levels of

100 bar at radio wavelengths ranging from 1.3 to 50 cm using six separate radiometers to measure thermal emissions. The water measurement is not only important for constraining Jupiter’s origin (e.g., Helled & Lunine, 2014 and references therein) but also for further constraining structure models. First, since updated Jupiter interior models suggest that Zin > Zout, the water measurement will provide a lower bound for the total (water) enrichment within Jupiter. Second, since several of the new interior models infer a low metallicity for the outer envelope, they could be excluded. Finally, the variation of water with depth can provides information about Jupiter’s atmosphere dynamics and put constraints on the convective behavior in its upper atmosphere and also indicates the presence of a non-convective region within Jupiter’s interior. It should be kept in mind that the MWR measurement still reveals the information about a very small fraction of the planet. Nevertheless, when combined with other measurements, it will provide new insights about the most massive planet in the solar system. Future studies should explore the relations between various formation and evolution model assumptions and the inferred planetary composition and internal structure.

Conclusion

There are still many unsolved questions regarding the origins and internal structures of Jupiter and Saturn. As open questions are being solved, new questions arise and our understanding is still incomplete. Nevertheless, the early decades of the 21st century is a golden era for giant planet exploration given the ongoing Juno mission and the recent measurements from Cassini Grand Finale that are still being processed. The possibility of having similar information about Jupiter and Saturn simultaneously opens opportunities to improve our understanding of giant planets and to explore the physical and chemical processes that lead to the differences. It is now known that even within our planetary system, there are significant differences between the two giant planets, suggesting that there is no one simple way to model giant planet interiors.

The continuous theoretical efforts and the new measurements from Juno and Cassini provide data that will keep planetary modelers busy for a while. In the meantime, the knowledge of the EOS of different elements and their interaction needs to keep improving, and all the available information (gravity field, magnetic field, atmospheric composition, etc.) needs to be combined to further constrain the planetary interior. In addition, it is desirable to develop a united theoretical framework for giant planet formation, evolution, and current-state structure.

Future missions will also play an important role in better constraining the interiors of Jupiter and Saturn. The upcoming JUICE mission can reveal further information on Jupiter, and a potential Saturn probe mission will provide constraints on Saturn’s atmospheric composition and the process of helium rain. Finally, the detection and characterization of giant planets around other stars, combined with the knowledge of solar system giants, can lead to a more comprehensive understanding of gaseous planets.

Acknowledgments

The author thanks Nadine Nettelmann, Hao Cao, and Guglielmo Mazzola for their important contributions. The author also acknowledges valuable comments and support from David Stevenson, Tristan Guillot, and Allona Vazan, as well as the two anonymous referees. Finally, the author acknowledges all the Juno science team members for inspiring discussions.

References

Notes

1. 1 AU is an astronomical unit, the average distance between the Earth and the Sun.

2. In adiabatic models, the temperature profile is set by the adiabatic gradient, and the entropy is (nearly) constant within the planet (see Militzer et al., 2016 and references therein for details).

3. However, this is a simplification as odd harmonics have been measured for Jupiter with the Juno spacecraft (Iess et al., 2018). The measurement of Jupiter’s gravitational field being north–south asymmetric, has been used to reveal the planets atmospheric and interior flows (Kaspi et al., 2018).

4. The adiabatic gradient ∇ a d = ∂ l n T ∂ ln P | s , where S is the entropy, corresponds to a case in which the material is homogenous and convective. The radiative/conductive gradient is given by ∇ r a d / c o n d = 3 k L P 64 π σ T 4 G m , where κ ‎ is the Rosseland opacity which accounts for contributions from both radiation and conduction, and σ ‎ is the Stephan–Boltzmann constant. See Guillot et al. (2004), Militzer et al. (2016), and references therein for further details.

5. It should also be noted that the nature of the transition of hydrogen from “molecular” to “metallic” along Jupiter’s and Saturn’s adiabats is still debated. The transition could be a “first-order” one or a smooth one, although most studies imply that in Jupiter’s and Saturn’s interiors the transition is smooth and is a first-order transition at lower (intermediate) temperatures.

6. This would depend on the thermodynamic behavior of the H–HE mixture in the presence of a phase separation.


Finding an ingredient of plastic

In 2013, scientists discovered an essential ingredient of plastic, called polypropylene, in the atmosphere of Titan. On Earth, it is used everywhere from car bumpers to plastic containers. It can appear organically in nature, but humans typically produce it artificially from sources such as oil refining.

Polypropylene is a hydrocarbon, and consists of three carbon atoms and six hydrogen atoms. The Voyager probes did not detect polypropylene in Titan's atmosphere, but they did detect molecules from the same chemical family &mdash those that contained three carbon atoms, but with four hydrogen atoms and eight hydrogen atoms, respectively. Researchers were puzzled that the middle member of the family seemed to be absent.

"This measurement was very difficult to make because propylene's weak signature is crowded by related chemicals with much stronger signals," Michael Flasar, Goddard scientist and principal investigator for the Composite Infrared Spectrometer (CIRS) instrument that was used for the observations, said in a statement at the time. "This success boosts our confidence that we will find still more chemicals long hidden in Titan's atmosphere," Flaser said.

Life as we know it could not survive on Titan's chilly surface (it's too cold for liquid water), but it is a fascinating destination: It looks similar to rocky planets like Earth, but it has a totally different base chemistry. Its lakes and rivers are made of methane and ethane, and the wind-swept dunes on its surface are made from hydrocarbon grains (unlike sand grains on Earth, which are silicates). Researchers are interested to find out if Titan could have the ingredients to create a variation on life as we know it.