How can clouds form in Jupiter's atmosphere of Hydrogen and Helium?

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Here is a graphic of cloud layers of Jupiter (source: Wikipedia):

There are three distinct cloud layers of ammonia, ammonium hydrosulfide, and water. The temperature and pressure conditions seem to be surprisingly earth-like; temperatures between 200 and 300 K, pressures about 1 to 10 atm, gravity around 1.3g.

Clouds (of water) form on earth because they solar energy causes them to evaporate from a solid surface, rise a few km, then condense to form water droplets (or solid crystal snow). But Jupiter does not have a solid surface, or nearly as much solar energy as the Earth.

All three of the cloud forming compounds should be liquid at the conditions of their cloud layer. Given the density of those liquids (between 0.7 and 1.2 g/cm$$^3$$) and the density of the bulk of the hydrogen and helium atmosphere, how do the clouds not fall as precipitation into Jupiter's interior and never re-emerge?

First, it's a great question. Mostly the answer is straight forward, so I can answer it, but it's still a great question.

and I'll add a similar, but slightly more detailed picture to the one you posted.

Source

You're right that there is a clear difference between Earth's surface where liquid water can exist, evaporate, make clouds, rain and repeat. Earth's water cycle in theory, could go on indefinitely so long as Earth's atmosphere and the solar input are maintained (and lost hydrogen is replaced), but it's a circular system that only needs solar input.

Jupiter is different because over time, the heavier gases in Jupiter probably will sink deeper towards the center and Jupiter's cloud forming gases should decrease given enough time. Some of Jupiter's "rain" probably does fall too deep in its swirling mix of gases, and leaves Jupiter's cloud cycle permanently, similar to water seeping underground and leaving Earth's water-cloud cycle. So, in 100 billion or a trillion years or so, Jupiter could lose its clouds and cloud forming gases in its upper atmosphere for the reasons you suspect.

The reason this hasn't happened yet is simply mixing. While density of gas tends towards layers of increasing density, the internal heat inside Jupiter also wants to even out, so there's enormous convection going on pretty much all the way through the planet. This keeps some heavier gases in Jupiter's upper atmosphere. Jupiter's much too turbulent to have just hydrogen and helium in its upper atmosphere.

So, once we begin with the observation that Jupiter's upper atmosphere is (about) 90% hydrogen, 9% helium, 1% other gases and the mixing maintains the 1% of other, after that it's just cloud physics.

Clouds look like puffy collections of water vapor (tiny droplets of ice or water, as water vapor is actually transparent). They look like objects with shapes, but that's not entirely accurate. If you are up close to a cloud (flying in a plane for example), the clear edges disappear. A cloud isn't an object so much, it's a visible phase change.

Atmosphere on Earth is about 78% nitrogen, 21% oxygen, 0.9% argon and (usually not listed because it's so variable), about 0.4% water vapor on average, as high as 1% with high temperature and high humidity and close to 0% in cold temperatures or dry-deserts. When you take warm surface air that's 0.6-0.8% water vapor, and that air rises (as hot air does), it's the phase change that creates the clouds. The cloud forms in the hot rising air as it cools. There's some electrostatic attraction, but mostly it's just a block of similar air undergoing cooling and the cloud looks like it has solid edges, but it doesn't.

The same exact thing happens on Jupiter, different gases phase change at different temperatures/pressures, but the process is the same. And, just as on Earth, once the droplets or "icelets" form, they are denser and they begin to fall, but falling droplets are very small so they fall very slowly and for the most part, they are falling through rising atmosphere. Also, as they are a phase change, new cloud is being formed and old cloud is being disbursed or returned to gas all the time, kind of like sea ice. The clouds have the appearance of semi-permanence, but clouds are dynamic.

If my explanation doesn't work for you, here's an explanation on clouds and how they really aren't bound together even though they look that way.

But that's the gist of it, mixing keeps the upper atmosphere of Jupiter from being pure hydrogen and helium (or pure hydrogen), and after that, the cloud formation is pretty much the same as on Earth, just without a surface. Some of the heavier gases probably do get lost from the cycle, but the loss is slow enough that Jupiter still has some heavy clouds forming gases in its upper atmosphere and will for probably billions of years to come.

The greater density variation between H/He and other gases likely plays a role in how the clouds behave, as the density variation is greater, but the wind speeds are also higher on Jupiter. All that's really needed is mixing. After that, with gases that can become liquid or ice under temperature/pressure variations, the phase changes create the clouds.

It's also possible that Jupiter's cloud forming gases, from time to time, are replenished by asteroid and comet impacts. Shoemaker-Levy 9 was about 5 km in diameter and a fair percentage of that was probably ammonia and water ice. That's a lot of cloud forming gas added to Jupiter's upper atmosphere. Jupiter's faint ring system, which might have been much larger millions of years ago, but since rained onto Jupiter, and eruptions from Io might also play roles in keeping Jupiter's upper atmosphere rich enough in cloud making elements like water and ammonia.

How do the clouds not fall as precipitation into Jupiter's interior and never re-emerge?

Gases in a planet's troposphere don't differentiate chemically; the turbulence driven by heating and planet rotation keep the atmosphere well-mixed. We can see this in our own atmosphere. Carbon dioxide and argon are considerably more dense than are the nitrogen and oxygen that form the bulk of the atmosphere. Yet we don't have a layer of carbon dioxide at the bottom of the atmosphere. The turbopause marks where an atmosphere shifts from being dominated by turbulent mixing to being dominated by diffusion. Chemical differentiation by atomic mass does occur above the turbopause, but even there, it's gradual.

But what about rain? The answer is simple: It evaporates. That happens here on Earth, particularly in arid regions. Clouds form, and rain falls from those clouds, but the rain sometimes evaporates before it reaches the ground. This is called virga.

Temperatures rise inside Jupiter due to compressional heating, at a rate of about 1.85 K per kilometer of increasing depth. That means the temperature reaches the water's critical temperature (647 K) about 240 kilometers below the 1 bar pressure level. So even if rain water could fall that far as rain before evaporating (which is dubious), it would cease to be a liquid.

Atmosphere of Jupiter

The atmosphere of Jupiter is the largest planetary atmosphere in the Solar System. It is mostly made of molecular hydrogen and helium in roughly solar proportions other chemical compounds are present only in small amounts and include methane, ammonia, hydrogen sulfide, and water. Although water is thought to reside deep in the atmosphere, its directly measured concentration is very low. The nitrogen, sulfur, and noble gas abundances in Jupiter's atmosphere exceed solar values by a factor of about three. [2]

The atmosphere of Jupiter lacks a clear lower boundary and gradually transitions into the liquid interior of the planet. [3] From lowest to highest, the atmospheric layers are the troposphere, stratosphere, thermosphere and exosphere. Each layer has characteristic temperature gradients. [4] The lowest layer, the troposphere, has a complicated system of clouds and hazes, comprising layers of ammonia, ammonium hydrosulfide and water. [5] The upper ammonia clouds visible at Jupiter's surface are organized in a dozen zonal bands parallel to the equator and are bounded by powerful zonal atmospheric flows (winds) known as jets. The bands alternate in color: the dark bands are called belts, while light ones are called zones. Zones, which are colder than belts, correspond to upwellings, while belts mark descending gas. [6] The zones' lighter color is believed to result from ammonia ice what gives the belts their darker colors is uncertain. [6] The origins of the banded structure and jets are not well understood, though a "shallow model" and a "deep model" exist. [7]

The Jovian atmosphere shows a wide range of active phenomena, including band instabilities, vortices (cyclones and anticyclones), storms and lightning. [8] The vortices reveal themselves as large red, white or brown spots (ovals). The largest two spots are the Great Red Spot (GRS) [9] and Oval BA, [10] which is also red. These two and most of the other large spots are anticyclonic. Smaller anticyclones tend to be white. Vortices are thought to be relatively shallow structures with depths not exceeding several hundred kilometers. Located in the southern hemisphere, the GRS is the largest known vortex in the Solar System. It could engulf two or three Earths and has existed for at least three hundred years. Oval BA, south of GRS, is a red spot a third the size of GRS that formed in 2000 from the merging of three white ovals. [11]

Jupiter has powerful storms, often accompanied by lightning strikes. The storms are a result of moist convection in the atmosphere connected to the evaporation and condensation of water. They are sites of strong upward motion of the air, which leads to the formation of bright and dense clouds. The storms form mainly in belt regions. The lightning strikes on Jupiter are hundreds of times more powerful than those seen on Earth, and are assumed to be associated with the water clouds. [12] Recent Juno observations suggest Jovian lightning strikes occur above the altitude of water clouds (3-7 bars). [13] A charge separation between falling liquid ammonia-water droplets and water ice particles may generate the higher-altitude lightning. [13] Upper-atmospheric lightning has also been observed 260 km above the 1 bar level. [14]

Experiments validate the possibility of helium rain inside Jupiter and Saturn

An international research team, including scientists from Lawrence Livermore National Laboratory, have validated a nearly 40-year-old prediction and experimentally shown that helium rain is possible inside planets such as Jupiter and Saturn (pictured). Credit: NASA/JPL/Space Science Institute.

Nearly 40 years ago, scientists first predicted the existence of helium rain inside planets composed primarily of hydrogen and helium, such as Jupiter and Saturn. However, achieving the experimental conditions necessary to validate this hypothesis hasn't been possible—until now.

In a paper published today by Nature, scientists reveal experimental evidence to support this long-standing prediction, showing that helium rain is possible over a range of pressure and temperature conditions that mirror those expected to occur inside these planets.

"We discovered that helium rain is real, and can occur both in Jupiter and Saturn," said Marius Millot, a physicist at Lawrence Livermore National Laboratory (LLNL) and co-author on the publication. "This is important to help planetary scientists decipher how these planets formed and evolved, which is critical to understanding how the solar system formed."

"Jupiter is especially interesting because it's thought to have helped protect the inner-planet region where Earth formed," added Raymond Jeanloz, co-author and professor of earth and planetary science and astronomy at the University of California, Berkeley. "We may be here because of Jupiter."

The international research team, which included scientists from LLNL, the French Alternative Energies and Atomic Energy Commission, the University of Rochester and the University of California, Berkeley, conducted their experiments at the University of Rochester's Laboratory for Laser Energetics (LLE).

"Coupling static compression and laser-driven shocks is key to allow us to reach the conditions comparable to the interior of Jupiter and Saturn, but it is very challenging," Millot said. "We really had to work on the technique to obtain convincing evidence. It took many years and lots of creativity from the team."

The team used diamond anvil cells to compress a mixture of hydrogen and helium to 4 gigapascals, (GPa approximately 40,000 times Earth's atmosphere). Then, the scientists used 12 giant beams of LLE's Omega Laser to launch strong shock waves to further compress the sample to final pressures of 60-180 GPa and heat it to several thousand degrees. A similar approach was key to the discovery of superionic water ice.

Using a series of ultrafast diagnostic tools, the team measured the shock velocity, the optical reflectivity of the shock-compressed sample and its thermal emission, finding that the reflectivity of the sample did not increase smoothly with increasing shock pressure, as in most samples the researchers studied with similar measurements. Instead, they found discontinuities in the observed reflectivity signal, which indicate that the electrical conductivity of the sample was changing abruptly, a signature of the helium and hydrogen mixture separating. In a paper published in 2011, LLNL scientists Sebastien Hamel, Miguel Morales and Eric Schwegler suggested using changes in the optical reflectivity as a probe for the demixing process.

"Our experiments reveal experimental evidence for a long-standing prediction: There is a range of pressures and temperatures at which this mixture becomes unstable and demixes," Millot said. "This transition occurs at pressure and temperature conditions close to that needed to transform hydrogen into a metallic fluid, and the intuitive picture is that the hydrogen metallization triggers the demixing."

Numerically simulating this demixing process is challenging because of subtle quantum effects. These experiments provide a critical benchmark for theory and numerical simulations. Looking ahead, the team will continue to refine the measurement and extend it to other compositions in the continued pursuit of improving our understanding of materials at extreme conditions.

Jupiter

Jupiter, the most massive planet in our solar system — with dozens of moons and an enormous magnetic field — forms a kind of miniature solar system. Jupiter does resemble a star in composition, but it did not grow big enough to ignite. The planet’s swirling cloud stripes are punctuated by massive storms such as the Great Red Spot, which has raged for hundreds of years.

Jupiter’s appearance is a tapestry of beautiful colors and atmospheric features. Most visible clouds are composed of ammonia. Water vapor exists deep below and can sometimes be seen through clear spots in the clouds. The planet’s “stripes” are dark belts and light zones created by strong east-west winds in Jupiter’s upper atmosphere.

On 7 January 1610, using his primitive telescope, astronomer Galileo Galilei saw four small “stars” near Jupiter. He had discovered Jupiter’s four largest moons, now called Io, Europa, Ganymede, and Callisto. These four moons are known today as the Galilean satellites.

Newly discovered moons of Jupiter are reported by astronomers and acknowledged with a temporary designation by the International Astronomical Union once their orbits are confirmed, they are included in Jupiter’s large moon count. Not including the “temporary” moons, Jupiter has 50 total.

Galileo would be astonished at what we have learned about Jupiter and its moons, largely from the NASA mission named after him. Io is the most volcanically active body in our solar system. Ganymede is the largest planetary moon and the only moon in the solar system known to have its own magnetic field. A liquid ocean may lie beneath the frozen crust of Europa, and icy oceans may also lie beneath the crusts of Callisto and Ganymede. Jupiter’s appearance is a tapestry of beautiful colors and atmospheric features. Most visible clouds are composed of ammonia. Water vapor exists deep below and can sometimes be seen through clear spots in the clouds. The planet’s “stripes” are dark belts and light zones created by strong east-west winds in Jupiter’s upper atmosphere. Dynamic storm systems rage on Jupiter. The Great Red Spot, a giant spinning storm, has been observed since the 1800s. In recent years, three storms merged to form the Little Red Spot, about half the size of the Great Red Spot.

The composition of Jupiter’s atmosphere is similar to that of the sun — mostly hydrogen and helium. Deep in the atmosphere, the pressure and temperature increase, compressing the hydrogen gas into a liquid. At depths of about a third of the way down, the hydrogen becomes metallic and electrically conducting. In this metallic layer, Jupiter’s powerful magnetic field is generated by electrical currents driven by Jupiter’s fast rotation. At the center, the immense pressure may support a solid core of rock about the size of Earth.

Jupiter’s enormous magnetic field is nearly 20,000 times as powerful as Earth’s. Trapped within Jupiter’s magnetosphere (the area in which magnetic field lines encircle the planet from pole to pole) are swarms of charged particles. Jupiter’s rings and moons are embedded in an intense radiation belt of electrons and ions trapped by the magnetic field. The Jovian magnetosphere, comprising these particles and fields, balloons 1 to 3 million km (600,000 to 2 million miles) toward the sun and tapers into a windsock-shaped tail extending more than 1 billion km (600 million miles) behind Jupiter as far as Saturn’s orbit.

Discovered in 1979 by NASA’s Voyager 1 spacecraft, Jupiter’s rings were a surprise: a flattened main ring and an inner cloud-like ring, called the halo, are both composed of small, dark particles. A third ring, known as the gossamer ring because of its transparency, is actually three rings of microscopic debris from three small moons: Amalthea, Thebe and Adrastea. Data from the Galileo spacecraft indicate that Jupiter’s ring system may be formed by dust kicked up as interplanetary meteoroids smash into the giant planet’s four small inner moons. The main ring probably is composed of material from the moon Metis. Jupiter’s rings are more easily visible when backlit by the sun but have been captured by Hubble Space Telescope images.

In December 1995, NASA’s Galileo spacecraft dropped a probe into Jupiter’s atmosphere, which made the first direct measurements of the planet’s atmosphere. The spacecraft then began a multiyear study of Jupiter and the largest moons. As Galileo began its 29th orbit, the Cassini-Huygens spacecraft was nearing Jupiter for a gravity-assist maneuver on the way to Saturn. The two spacecraft made simultaneous observations of the magnetosphere, solar wind, rings, and Jupiter’s auroras.

NASA launched a mission named Juno in 2011 to conduct an in-depth study of Jupiter from a polar orbit. Juno will examine Jupiter’s chemistry, atmosphere, interior structure, and magnetosphere.

Acknowledgements

We gratefully acknowledge M. Millerioux and F. Occelli for help in the preparation of the pre-compressed targets. We thank F. Soubiran and S. Hamel for useful discussions and F. Soubiran for sharing unpublished data. We thank the OMEGA laser facility management, staff and support crew for excellent shots and diagnostic support with special thanks to C. Sorce, A. Sorce and J. Kendrick. The OMEGA Laser Facility shots were allocated under the NLUF programme. Part of this work was prepared by LLNL under contract number DE-AC52-07NA27344 with support from LLNL LDRD programme and the US Department of Energy Fusion Energy Sciences Program. This work was performed under the auspices of a cooperation agreement CEA/DAM and DOE/NNSA on fundamental sciences. Partial funding for G.W.C. and J.R.R. was provided by NSF Physics Frontier Center award PHY-2020249 and DOE NNSA award DE6NA 0003856.

Helium rain on Jupiter explains lack of neon in atmosphere

A slice through the interior of Jupiter shows the top layers that are depleted of helium and neon, the thin layer where helium drops condense and fall, and the deep interior where helium and neon again mix with metallic hydrogen. (Burkhard Militzer graphic)

(PhysOrg.com) -- On Earth, helium is a gas used to float balloons, as in the movie "Up." In the interior of Jupiter, however, conditions are so strange that, according to predictions by University of California, Berkeley, scientists, helium condenses into droplets and falls like rain.

Helium rain was earlier proposed to explain the excessive brightness of Saturn, a gas giant like Jupiter, but one-third the mass.

On Jupiter, however, UC Berkeley scientists claim that helium rain is the best way to explain the scarcity of neon in the outer layers of the planet, the solar system's largest. Neon dissolves in the helium raindrops and falls towards the deeper interior where it re-dissolves, depleting the upper layers of both elements, consistent with observations.

"Helium condenses initially as a mist in the upper layer, like a cloud, and as the droplets get larger, they fall toward the deeper interior," said UC Berkeley post-doctoral fellow Hugh Wilson, co-author of a report appearing this week in the journal Physical Review Letters. "Neon dissolves in the helium and falls with it. So our study links the observed missing neon in the atmosphere to another proposed process, helium rain."

Wilson's co-author, Burkhard Militzer, UC Berkeley assistant professor of earth and planetary science and of astronomy, noted that "rain" - the water droplets that fall on Earth - is an imperfect analogy to what happens in Jupiter's atmosphere. The helium droplets form about 10,000 to 13,000 kilometers (6,000-8,000 miles) below the tops of Jupiter's hydrogen clouds, under pressures and temperatures so high that "you can't tell if hydrogen and helium are a gas or a liquid," he said. They're all fluids, so the rain is really droplets of fluid helium mixed with neon falling through a fluid of metallic hydrogen.

The researchers' prediction will help refine models of Jupiter's interior and the interiors of other planets, according to Wilson. Modeling planetary interiors has become a hot research area since the discovery of hundreds of extrasolar planets living in extreme environments around other stars. The study will also be relevant for NASA’s Juno mission to Jupiter, which is scheduled to be launched next year.

Militzer and Wilson are among the modelers, using "density functional theory" to predict the properties of Jupiter's interior, specifically what happens to the dominant constituents - hydrogen and helium - as temperatures and pressures increase toward the center of the planet. These conditions are yet too extreme to be reproduced in the laboratory. Even experiments in diamond-anvil cells can only produce pressures at the Earth's core. In 2008, Militzer's computer simulations led to the conclusion that Jupiter's rocky core is surrounded by a thick layer of methane, water and ammonia ices that make it twice as large as earlier predictions.

The two modelers embarked on their current research because of a discovery by the Galileo probe that descended through Jupiter's atmosphere in 1995 and sent back measurements of temperature, pressure and elemental abundances until it was crushed under the weight of the atmosphere. All elements seemed to be as slightly enriched compared to the abundance on the sun - which is assumed to be similar to the elemental abundances 4.56 billion years ago when the solar system formed - except for helium and neon. Neon stood out because it was one-tenth as abundant as it is in the sun.

Their simulations showed that the only way neon could be removed from the upper atmosphere is to have it fall out with helium, since neon and helium mix easily, like alcohol and water. Militzer and Wilson's calculations suggest that at about 10,000 to 13,000 kilometers into the planet, where the temperature about 5,000 degrees Celsius and the pressure is 1 to 2 million times the atmospheric pressure on Earth, hydrogen turns into a conductive metal. Helium, not yet a metal, does not mix with metallic hydrogen, so it forms drops, like drops of oil in water.

This provided an explanation for the removal of neon from the upper atmosphere.

"As the helium and neon fall deeper into the planet, the remaining hydrogen-rich envelope is slowly depleted of both neon and helium," Militzer said. "The measured concentrations of both elements agree quantitatively with our calculations."

Saturn's helium rain was predicted because of a different observation: Saturn is warmer than it should be, based on its age and predicted rate of cooling. The falling rain releases heat that accounts for the difference.

Jupiter's temperature is in accord with models of its cooling rate and its age, and needed no hypothesis of helium rain until the discovery of neon depletion in the atmosphere. Interestingly, theoretician David Stevenson of the California Institute of Technology (Caltech) predicted neon depletion on Jupiter prior to the Galileo probe's measurements, but never published a reason for his guess.

Astronomy Picture of the Day

Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

2021 June 8
A Face in the Clouds of Jupiter from Juno
Image Credit: NASA/JPL-Caltech/SwRI/MSSS/Jason Major

Explanation: What do you see in the clouds of Jupiter? On the largest scale, circling the planet, Jupiter has alternating light zones and reddish-brown belts. Rising zone gas, mostly hydrogen and helium, usually swirls around regions of high pressure. Conversely, falling belt gas usually whirls around regions of low pressure, like cyclones and hurricanes on Earth. Belt storms can form into large and long-lasting white ovals and elongated red spots. NASA's robotic Juno spacecraft captured most of these cloud features in 2017 during perijove 6, its sixth pass over the giant planet in its looping 2-month orbit. But it is surely not these clouds themselves that draws your attention to the displayed image, but rather their arrangement. The face that stands out, nicknamed Jovey McJupiterFace, lasted perhaps a few weeks before the neighboring storm clouds rotated away. Juno has now completed 33 orbits around Jupiter and just yesterday made a close pass near Ganymede, our Solar System's largest moon.

Evidence of hydrogen-helium immiscibility at Jupiter-interior conditions

The phase behaviour of warm dense hydrogen-helium (H-He) mixtures affects our understanding of the evolution of Jupiter and Saturn and their interior structures 1,2 . For example, precipitation of He from a H-He atmosphere at about 1-10 megabar and a few thousand kelvin has been invoked to explain both the excess luminosity of Saturn 1,3 , and the depletion of He and neon (Ne) in Jupiter's atmosphere as observed by the Galileo probe 4,5 . But despite its importance, H-He phase behaviour under relevant planetary conditions remains poorly constrained because it is challenging to determine computationally and because the extremes of temperature and pressure are difficult to reach experimentally. Here we report that appropriate temperatures and pressures can be reached through laser-driven shock compression of H2-He samples that have been pre-compressed in diamond-anvil cells. This allows us to probe the properties of H-He mixtures under Jovian interior conditions, revealing a region of immiscibility along the Hugoniot. A clear discontinuous change in sample reflectivity indicates that this region ends above 150 gigapascals at 10,200 kelvin and that a more subtle reflectivity change occurs above 93 gigapascals at 4,700 kelvin. Considering pressure-temperature profiles for Jupiter, these experimental immiscibility constraints for a near-protosolar mixture suggest that H-He phase separation affects a large fraction-we estimate about 15 per cent of the radius-of Jupiter's interior. This finding provides microphysical support for Jupiter models that invoke a layered interior to explain Juno and Galileo spacecraft observations 1,4,6-8 .

How can clouds form in Jupiter's atmosphere of Hydrogen and Helium? - Astronomy

The first thing you see when you look at the atmosphere of Jupiter is the bands of dark and light clouds as well as swirls, waves and oval spots. The patterns are mezmerizing! So what causes this complex mixture of colors and shapes?

Let’s start with the composition of the atmosphere itself. Jupiter’s atmosphere is made of mostly hydrogen and helium in a mixture that closely resembles that of the sun. Gasses such as oxygen and nitrogen are present along with compounds including methane, ammonia, hydrogen sulfide and water.

The clouds visible on the surface of the atmosphere (the part that we can see) are made up of ammonia and are arranged in a dozen or so zonal bands. These bands seem to alternate between light and dark and have distinct boundries. They run parallel to the equator. The difference in colors is thought to be caused by the composition of the ammonia itself. The lighter bands or zones as they are called, have a higher concentration of ammonia ice and are opaque at high altitudes. The darker bands or belts are thinner and lie a a lower altitude. Why there is so much difference in color is not entirely known.

Jovian Clouds: Zones and Belts

The cool thing about the Jovian atmosphere is that it is a very active, violent place. The belts and zones are bounded by atmospheric jets. The wind sheer between these areas causes the beautiful swirls or vortices that dance across the surface. There are also enormous storms in the atmosphere. The best known example of a Jovian storm is the Great Red Spot. The Great Red Spot is an old storm. It has lasted for at least 180 years and possibly as long as 345 years! It is large enough to contain three planets the size of Earth. Did you know that there is also a “Little Red Spot” called Oval BA that was first seen in 2000 after the collision of three smaller white storms. Who knows, maybe Oval BA will become the next Great Red Spot…in another hundred years or so!

Physics and Chemistry of the Solar System

The Atmospheres of Jupiter and Saturn: Observed Composition

We have known since the pioneering work of Rupert Wildt in the 1930s that hydrogen, methane, and ammonia are present in Jupiter's atmosphere. Since 1966, however, there have been enormous advances in spectroscopic instrumentation. These advances have greatly multiplied the number of known species and correspondingly enriched our understanding of the processes affecting atmospheric composition.

Conventional spectroscopy disperses light according to its wavelength, by either refraction through a prism or diffraction off a grating. The dispersed light is then swept across a small detector, which converts the photon beam to an electric current. This current is recorded on magnetic media, or amplified and used to drive a chart recorder. The width of the wavelength interval intercepted by the detector at any time, Δλ, is called the spectral resolution of the instrument. A typical broad-coverage spectrum, such as the entire visible or entire near infrared region, must therefore consist of approximately λ/Δλ separate measurements. Thus a spectrum of resolution λ/Δλ = 10 4 must contain 10 4 samples of the spectrum, and the detector must therefore waste 99.99% of the incident light during sampling of any single wavelength interval. To some degree, this problem may be offset by using several detectors simultaneously (called multiplexing), but it is still obvious that taking a high-resolution spectrum of a faint source will require long observing times or very large telescopes with great light-gathering power.

There are, of course, strong reasons for desiring high spectral resolution. A low-resolution spectrum that resolves only the envelope of rotation–vibration bands without resolving the individual lines is inadequate for estimating rotational temperatures. A medium-resolution spectrum that separates the lines but does not resolve individual line profiles can provide useful temperature and abundance data, but will not allow the collision-broadening of the spectral lines to be measured. Only a high-resolution spectrum with Δλ several times less than the line widths can permit full extraction of the information inherent in the spectrum. Depending on the species observed and on the wavelength region, resolutions of 10 4 to 10 6 may be needed.

The spectroscopic technique that has revolutionized planetary astronomy is interferometry, sometimes called Fourier transform spectroscopy. In this technique, a wide spectral region is admitted undispersed but well collimated into the spectrometer ( Fig. V.5 ). There the beam is passed through a diagonal half-silvered (uniformly but incompletely reflective) mirror called a beam splitter. One half of the beam is reflected off a fixed mirror and traverses a constant path, whereas the other half of the beam reflects off a moveable mirror. These two reflected beams are then recombined to interfere with each other. The resultant intensity of the combined beams, measured as a function of the path-length difference between the two beam paths, is called an interferogram. When the paths followed by the two beams differ by an integral number of wavelengths of light of a particular frequency, then that light will interfere constructively with itself and be fully represented in the observed interferogram. Other wavelengths which do not meet this criterion will interfere destructively.

Figure V.5 . Interferometric spectroscopy. a depicts an interferometer accepting two parallel beams from a telescope (T) aimed at a Planckian source and an internal calibration laser (L). The beams strike a lightly silvered beam splitter, and half of the intensity of each beam is reflected off a stationary mirror and thence to the two detectors DT and DL, separately. The other half of the beam intensity passes through the beam splitter and is reflected off a traveling mirror. The laser-source and Planckian-source interferograms are shown in b and c. The laser source is used to monitor the precise position of the moveable mirror. In practice it is sufficient to use a single detector for both beams together. The original spectrum, reconstructed by taking the Fourier transform of the sum of the two interferograms, is shown in d.

To determine the interferogram precisely, it is essential that the mirror position be monitored equally precisely. This is done by inserting a very pure single-wavelength (monochromatic) beam of light into the instrument with a small laser.

In Fig. V.5 we describe the mirror position by the variable x, which is zero when the light paths in the two arms of the interferometer are equal. When x = 0, all light of any frequency whatsoever interferes constructively with itself at the detector. This is called a “white light spike” in the interferogram. A mirror displacement of x in either direction will increase the total light path by 2x, and a monochromatic light source of wavelength λ will interfere constructively with itself and give a large signal at detector D whenever 2x is some exact integer multiple of λ: x = /2 (see Fig. V.5b ).

An interferogram for a Planck function is given in Fig. V.5c . Note that low multiples of λm show up as peaks in the interferogram, but each repetition becomes more smeared out, with the intensity eventually reaching a nearly constant level.

The original spectrum can be reconstructed from the interferogram by taking the Fourier transform of the interferogram. Because of the symmetry of the interferogram about x = 0, the Fourier series may be expressed using only cosine terms as

where the index i runs from zero to some maximum I. The larger the value of I (the number of samples in the interferogram), the higher the resolution of the spectrum. The value of x is precisely monitored by counting the laser interference fringes.

The advantage of an interferometer is that every photon in the entire spectral region under study is used at all times in constructing the interferogram. Thus no light is ever wasted, and even high-resolution spectrograms can be made using reasonable-sized telescopes with observing times of about 1 second per interferogram. A dispersive spectrometer on the same telescope, with a resolution of 10 6 , would need 10 6 detectors in order to operate as efficiently! This strong point of the interferometer is called the multiplex advantage. When even greater spectral resolution or sensitivity is desired, the interferogram may be scanned more slowly, and large numbers of interferograms may be added coherently (that is, they are aligned at x = 0 before adding).

Such high-resolution interferometric spectra of Jupiter and Saturn have been available for a number of years, and the information in them is immense. After methane, ammonia, and hydrogen, which had already been identified as constituents of both Jupiter and Saturn prior to the introduction of Fourier-transform spectroscopy, a number of other species have been discovered. These include ethane (C2H6), acetylene (C2H2), ethylene (C2H4), monodeuteromethane (CH3D), carbon-13 methane ( 13 CH4), HD, phosphine (PH3), water vapor (H2O), germane (GeH4), hydrogen cyanide (HCN), and carbon monoxide (CO). The 13 C: 12 C ratio has been found to be indistinguishable from the terrestrial or meteoritic value, whereas the D:H ratio is much lower than in Earth's oceans, but very similar to that believed characteristic of primitive solar nebula material. The 15 N: 14 N ratio in ammonia has been found to decrease upward in the stratosphere by about a factor of 2 relative to the ammonia isotopic composition seen in the 5-μm “hot spots.” Photolysis would destroy isotopically light ammonia slightly more readily than heavy ammonia, contrary to observation, but fractional distillation of ammonia would leave a vapor that is slightly enhanced in the lighter isotope. The other chemical species require chemical explanation of their sources, stability, and observed abundances. In addition, large numbers of other species have been sought but not found in high-resolution spectra. Their absence is a great help in that it places constraints on chemical models of the Jovian planets: a model not only must pass the test of explaining the abundances and altitude distribution of the observed species, but also must not predict abundances of other species in excess of their observational detection limits. The abundances of known species and upper limits on some other species of interest are given in Table V.1 .

Table V.1 . Abundances in the Atmospheres of Jupiter and Saturn

Abundance (Mole fraction)
SpeciesFormulaNotesJupiterSaturn
HydrogenH2 0.850.84
HeliumHe 0.150.16
MethaneCH4 1 × 10 −3 2 × 10 −3
AmmoniaNH3A8 × 10 −4 6 × 10 −4
Water vaporH2OB1 × 10 −3 2 × 10 −3 ?
Hydrogen sulfideH2SB8 × 10 −5 ?
Hydrogen deuterideHD 6 × 10 −5 6 × 10 −5
NeonNeC2.2 × 10 −5 ?
Argon 36 Ar + 38 ArD1.6 × 10 −5 ?
PhosphinePH3 1 × 10 −6 6 × 10 −6
MonodeuteromethaneCH3D 2 × 10 −8 2 × 10 −8
KryptonKrD1.5 × 10 −9 ?
Hydrogen chlorideHCl &lt 10 −9 10 −9
GermaneGe 6 × 10 −10 &lt 10 −9
XenonXeD2 × 10 −10 ?
Carbon monoxideCOE1 × 10 −10 &lt 10 −10
EthaneC2H6F4 × 10 −6 5 × 10 −6
AcetyleneC2H2F3 × 10 −8 1 × 10 −7
EthyleneC2H4F1 × 10 −9 &lt 10 −9
Hydrogen cyanideHCNF1 × 10 −9 &lt 10 −9
Methyl amineCH3NH2F&lt 10 −9 &lt 10 −6
HydrazineN2H4F&lt 10 −9 &lt 10 −9

Notes: a: lower tropospheric abundance from microwave opacity b: solar abundance assumed for Jupiter below the water and NH4SH cloud bases c: Ne:H on Jupiter was measured by the Galileo Probe as 10 times the solar Ne:H ratio d: the heavy noble gases Ar, Kr and Xe were measured by the Galileo Probe at about 2.5 × solar e: CO is probably transported up to the tropopause by turbulent mixing f: photochemical products detected in the stratosphere but of negligible abundance in the troposphere.