Why is Mercury's Density So Low?

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I know the title sounds odd. You might be thinking "Doesn't Mercury have the highest uncompressed density of any terrestrial planet? Much higher than a planet its size normally should have?" Here's the thing, though: Iron has an uncompressed density of $$7.874 mathrm{g/cm^3}$$ and silicon has an uncompressed density of $$2.329 mathrm{g/cm^3}$$. I know the distinction for planet composition is usually metals-silicates, not iron-silicon, but iron and silicon comprise the vast majority of those two categories.

Mercury is said to be 70% metals 30% silicates with an uncompressed density of $$5.4 mathrm{g/cm^3}$$.1 However, if you run the numbers, 70 % iron and 30 % silicon gives you an uncompressed density of $$6.21 mathrm{g/cm^3}$$, or over $$0.8 mathrm{g/cm^3}$$ higher. My question is, why is Mercury's actual density lower than my proposed figure? I know that there are other materials in Mercury, like nickel, sulfur, etc., but can these things really make up the difference?

1Wikipedia and NASA

The actual density depends on the mineralogy, we don't have a crystalline iron core and silicon crust. You do have a lot of oxygen available, too when you look at the overall elementary abundance. So the abundant materials are Fayalite ($${ m Fe_2SiO_4}$$), Olivine ($${ m (Fe,Mg)_2SiO_4}$$), Fosterite ($${ m Mg_2SiO_4}$$) etc. which make up most of the crust. These have a bulk density between $${ m 3g/cm^3}$$ and $${ m 4.5g/cm^3}$$ at normal pressure and of course a somewhat higher density under pressure.

In the core we are talking about some iron nickel sulfide alloys which have a density of less than pure iron, too (typically $${ m FeS}$$ is around $${ m 4.8g/cm^3}$$ at norm pressure). So conversely knowing the total mass from celestial mechanics and satellite date, the volume from imaging and the surface composition from spectroscopy we can actually make density estimates on the core's composition (with some further guesstimates on the equations of state on the alloys in question).

Add to these figures that we will have in the interior some high-pressure phases which have somewhat higher density, the estimated mean density of $${ m 5.4g/cm^3}$$ seems to fit quite well.

Also mind, whether a source talks about weight% (thus referencing the weight contribution of single elements. One iron atom weighs nearly 4x as much as one oxygen and twice that of magnesium) or whether the source talks about number fractions (thus counting actual atoms, disregarding weight). For one single molecule of Olivine we have $${ m Fe:Mg:Si:O}$$ in the weight ratio 56:24:28:64 while we have the number ratio of 1:1:1:4.

Geology of Mercury

The geology of Mercury is the scientific study of the surface, crust, and interior of the planet Mercury. It emphasizes the composition, structure, history, and physical processes that shape the planet. It is analogous to the field of terrestrial geology. In planetary science, the term geology is used in its broadest sense to mean the study of the solid parts of planets and moons. The term incorporates aspects of geophysics, geochemistry, mineralogy, geodesy, and cartography. [1]

Historically, Mercury has been the least understood of all the terrestrial planets in the Solar System. This stems largely from its proximity to the Sun which makes reaching it with spacecraft technically challenging and Earth-based observations difficult. For decades, the principal source of geologic information about Mercury came from the 2,700 images taken by the Mariner 10 spacecraft during three flybys of the planet from 1974 to 1975. These images covered about 45% of the planet’s surface, but many of them were unsuitable for detailed geologic investigation because of high sun angles which made it hard to determine surface morphology and topography. [2] This dearth of information was greatly alleviated by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft which between 2008 and 2015 collected over 291,000 images [3] covering the entire planet, along with a wealth of other scientific data. The European Space Agency’s (ESA’s) BepiColumbo spacecraft, scheduled to go into orbit around Mercury in 2025, is expected to help answer many of the remaining questions about Mercury’s geology.

Mercury's surface is dominated by impact craters, basaltic rock and smooth plains, many of them a result of flood volcanism, similar in some respects to the lunar maria, [4] [5] and locally by pyroclastic deposits. [6] Other notable features include vents which appear to be the source of magma-carved valleys, often-grouped irregular-shaped depressions termed "hollows" that are believed to be the result of collapsed magma chambers, [7] scarps indicative of thrust faulting and mineral deposits (possibly ice) inside craters at the poles. Long thought to be geologically inactive, new evidence suggests there may still be some level of activity. [8] [9]

Mercury's density implies a solid iron-rich core that accounts for about 60% of its volume (75% of its radius). [10] Mercury's magnetic equator is shifted nearly 20% of the planet's radius towards the north, the largest ratio of all planets. [11] This shift suggests there being one or more iron-rich molten layers surrounding the core producing a dynamo effect similar to that of Earth. Additionally, the offset magnetic dipole may result in uneven surface weathering by the solar wind, knocking more surface particles up into the southern exosphere and transporting them for deposit in the north. Scientists are gathering telemetry to determine if such is the case. [11]

After having completed the first solar day of its mission in September 2011, more than 99% of Mercury's surface had been mapped by NASA's MESSENGER probe in both color and monochrome with such detail that scientists' understanding of Mercury's geology has significantly surpassed the level achieved following the Mariner 10 flybys of the 1970s. [7]

INTERSTELLAR MEDIUM

The air we breathe has a density of approximately 10 19 molecules per cubic centimeter. (One cubic centimeter = 1 milliliter = 1/1000 liter).

By contrast, the lowest density regions of interstellar space contains approximately 0.1 atoms per cubic centimeter.

The remaining 1% of the interstellar medium consists of dust. That's right, dust -- like the stuff that accumulates on your bookshelves and under your bed.

• Composition: carbon, metals, silicates, and ice
• Size of grains: 500 nanometers or less in diameter (1 nanometer = 1 billionth of a meter)
• Number density of grains: 1 per million cubic meters

(2) Interstellar gas consists of cool clouds embedded in hot intercloud gas,

The densest nebulae can have densities of 10,000 molecules per cubic centimeter (or sometimes even more). The coolest nebulae can have temperatures of T = 10 Kelvin (or even less). A temperature of 10 Kelvin is colder than midnight on Pluto.

The other half of the interstellar gas is spread over the remaining 98 percent of the galaxy's volume. The lowest density gas has a density of 0.1 atoms per cubic centimeter (or less). The hottest interstellar gas has a temperature of 8000 Kelvin (or more). (The Solar System, by the way, seems to be located within a large, low-density bubble within the interstellar medium.)

(3) The interstellar medium emits, absorbs, and reflects radiation.

Sometimes we know the interstellar medium is there because it emits light. An emission nebula is a hot, ionized cloud, surrounding a hot, luminous star (of spectral type O' or B', thus possessing a surface temperature of tens of thousands of degrees). The gas in the emission nebula is heated by ultraviolet light from the star, and thus, like all hot, low-density gas, produces an emission line spectrum. [Example: the Orion Nebula, 450 parsecs away from us, in the constellation Orion, is an emission nebula.]

Sometimes we know the interstellar medium is there because it absorbs light. A dark nebula is a cold, dense cloud, containing a high concentration of dust. A dark nebula is dusty enough to be opaque at visible wavelengths. Thus, a nearby dark nebula blocks our view of more distant stars, making it look as if there were a hole in the heavens'' - a dark spot with no stars. The dust in a dark nebula, heated by starlight, re-radiates the light at infrared wavelengths. Thus, a `dark nebula', though dark at visible wavelengths, is luminous at infrared wavelengths. [Example: Barnard 86, 1700 parsecs away from us, in the constellation Sagittarius, is a dark nebula.]

Sometimes we know the interstellar medium is there because it scatters light. A reflection nebula is a dusty cloud surrounding a star. The dusty cloud is visible because the dust reflects starlight. The scattered starlight is always very blue, even if the star itself is red. Why is this? The individual dust grains, which are comparable in size to the wavelengths of visible light, are more efficient at scattering blue light than red light. A reflection nebula is blue because we are seeing scattered starlight. Stars seen through a dusty cloud are red because we are seeing the light left over after all the blue light has scattered away. [Example: the Pleiades, 117 parsecs away from us, in the constellation Taurus, are in the midst of a reflection nebula.]

Why Is Mercury a Liquid?

Mercury is the only metal that is a liquid at normal temperatures and pressure. Why is mercury a liquid? What makes this element so special? Basically, it's because mercury is bad at sharing—electrons, that is.

Most metal atoms readily share valence electrons with other atoms. The electrons in a mercury atom are bound more tightly than usual to the nucleus. In fact, the s electrons are moving so fast and close to the nucleus that they exhibit relativistic effects, behaving as if they were more massive than slower-moving electrons. It takes very little heat to overcome the weak binding between mercury atoms. Because of the behavior of the valence electrons, mercury has a low melting point, is a poor electrical and thermal conductor, and doesn't form diatomic mercury molecules in the gas phase.

The only other element on the periodic table that is a liquid at room temperature and pressure is the halogen bromine. While mercury is the only liquid metal at room temperature, the elements gallium, cesium, and rubidium melt under slightly warmer conditions. If scientists ever synthesize a sufficient quantity of flerovium and copernicium, these elements are expected to have an even lower boiling point (and perhaps melting point) than mercury.

Density of Earth:

Earth has the highest density of any planet in the Solar System, at 5.514 g/cm 3 . This is considered the standard by which other planet’s densities are measured. In addition, the combination of Earth’s size, mass and density also results in a surface gravity of 9.8 m/s². This is also used as a the standard (one g) when measuring the surface gravity of other planets.

Like the other terrestrial planets, Earth’s interior is divided into layers which are distinguished by their chemical or physical (rheological) properties. These layers consist of a core composed of iron and nickel, an upper and lower mantle composed of viscous silicate materials, and a crust composed of solid silicate materials.

Artist’s impression of the Earth’s interior, which includes the upper and lower mantle, and the inner and outer core. Credit: Huff Post Science

However, unlike the other terrestrial planets, Earth’s core region is divided into a solid inner core and a liquid outer core. The inner core measures an estimated 1220 km and is composed of iron and nickel, while the outer core extends beyond it to a radius of about 3,400 km. The outer core also rotates in the opposite direction of the Earth’s rotation, which is believed to be the source of the Earth’s magnetosphere. Like all planet’s, this density increases the closer one gets to the core, reaching an estimated 12,600–13,000 kg/m 3 in the inner core.

MODERN EXPLORATION OF MERCURY

Although it was first observed with a telescope by Galileo, this planet is not easily viewed with ground-based instruments, and apart from the phases of the planet as described above, little else of interest could be discerned about the surface features of the planet until the 1960s.

(At this point can I offer the hopefully needless warning about viewing Mercury through binoculars or telescopes? The proximity of the planet to the Sun makes such viewing a potentially disastrous exercise without proper filtration of the light. Catch the Sun in the field of view, and blindness may be the result).

There have been just two space missions to investigate this small world. In 1974-5 the Mariner 10 space probe made the first visit to Mercury. 40-45% of the planet was mapped during three fly-bys, revealing much data about the surface features.

Another 30 years passed before America commenced its second big mission to Mercury. The orbiting probe Messenger launched on 3rd August 2004. It reached Mercury in 2008 and over the next two years made three photographic fly-bys. Messenger finally assumed an orbit around the planet in March 2011, and began to intensively map the surface, taking more than 100,000 images. Data has also been collected by the probe about the interior structure and magnetic field and much else.

Rays of Ejected Material emanating from the Kuiper Crater on Mercury

Why is Mercury's Density So Low? - Astronomy

When men are arrived at the goal, they should not turn back. - Plutarch

If an explorer were to step onto the surface of Mercury, he would discover a world resembling lunar terrain. Mercury's rolling, dust-covered hills have been eroded from the constant bombardment of meteorites. Fault-cliffs rise for several kilometers in height and extend for hundreds of kilometers. Craters dot the surface. The explorer would notice that the Sun appears two and a half times larger than on Earth however, the sky is always black because Mercury has virtually no atmosphere to cause scattering of light. As the explorer gazes out into space, he might see two bright stars. One appearing as cream colored Venus and the other as blue colored Earth.

Until Mariner 10, little was known about Mercury because of the difficulty in observing it from Earth telescopes. At maximum elongation it is only 28 degrees from the Sun as seen from Earth. Because of this, it can only be viewed during daylight hours or just prior to sunrise or after sunset. When observed at dawn or dusk, Mercury is so low on the horizon that the light must pass through 10 times the amount of Earth's atmosphere than it would if Mercury was directly overhead.

During the 1880's, Giovanni Schiaparelli drew a sketch showing faint features on Mercury. He determined that Mercury must be tidally locked to the Sun, just as the Moon is tidally locked to Earth. In 1962, radio astronomers looked at radio emissions from Mercury and determined that the dark side was too warm to be tidally locked. It was expected to be much colder if it always faced away from the Sun. In 1965, Pettengill and Dyce determined Mercury's period of rotation to be 59 +- 5 days based upon radar observations. Later in 1971, Goldstein refined the rotation period to be 58.65 +- 0.25 days using radar observations. After close observation by the Mariner 10 spacecraft, the period was determined to be 58.646 +- 0.005 days.

Although Mercury is not tidally locked to the Sun, its rotational period is tidally coupled to its orbital period. Mercury rotates one and a half times during each orbit. Because of this 3:2 resonance, a day on Mercury (sun rise to sun rise) is 176 Earth days long as shown by the following diagram.

During Mercury's distant past, its period of rotation may have been faster. Scientists speculate that its rotation could have been as rapid as 8 hours, but over millions of years it was slowly despun by solar tides. A model of this process shows that such a despinning would take 10 9 years and would have raised the interior temperature by 100 degrees Kelvin.

Most of the scientific findings about Mercury comes from the Mariner 10 spacecraft which was launched on November 3, 1973. It flew past the planet on March 29, 1974 at a distance of 705 kilometers from the surface. On September 21, 1974 it flew past Mercury for the second time and on March 16, 1975 for the third time. During these visits, over 2,700 pictures were taken, covering 45% of Mercury's surface. Up until this time, scientists did not suspect that Mercury would have a magnetic field. They thought that because Mercury is small, its core would have solidified long ago. The presence of a magnetic field indicates that a planet has an iron core that is at least partially molten. Magnetic fields are generated from the rotation of a conductive molten core and is known as the dynamo effect.

Mariner 10 showed that Mercury has a magnetic field that is 1% as strong as Earth's. This magnet field is inclined 7 degrees to Mercury's axis of rotation and produces a magnetosphere around the planet. The source of the magnetic field is unknown. It might be produced from a partially molten iron core in the planet's interior. Another source of the field might be from remnant magnetization of iron-bearing rocks which were magnetized when the planet had a strong magnetic field during its younger years. As the planet cooled and solidified remnant magnetization was retained.

Even before Mariner 10, Mercury was known to have a high density. Its density is 5.44 g/cm 3 which is comparable to Earth's 5.52g/cm 3 density. In an uncompressed state, Mercury's density is 5.5 g/cm 3 where Earth's is only 4.0 g/cm 3 . This high density indicates that the planet is 60 to 70 percent by weight metal, and 30 percent by weight silicate. This gives a core radius of 75% of the planet radius and a core volume of 42% of the planet's volume.

Surface of Mercury

Mercury is marked with great curved cliffs or lobate scarps that were apparently formed as Mercury cooled and shrank a few kilometers in size. This shrinking produced a wrinkled crust with scarps kilometers high and hundreds of kilometers long.

The majority of Mercury's surface is covered by plains. Much of it is old and heavily cratered, but some of the plains are less heavily cratered. Scientists have classified these plains as intercrater plains and smooth plains. Intercrater plains are less saturated with craters and the craters are less than 15 kilometers in diameter. These plains were probably formed as lava flows buried the older terrain. The smooth plains are younger still with fewer craters. Smooth plains can be found around the Caloris basin. In some areas patches of smooth lava can be seen filling craters.

Mercury's history of formation is similar to that of Earth's. About 4.5 billion years ago the planets formed. This was a time of intense bombardment for the planets as they scooped up matter and debris left around from the nebula that formed them. Early during this formation, Mercury probably differentiated into a dense metallic core, and a silicate crust. After the intense bombardment period, lava flowed across the surface and covered the older crust. By this time much of the debris had been swept up and Mercury entered a lighter bombardment period. During this period the intercrater plains formed. Then Mercury cooled. Its core contracted which in turn broke the crust and produced the prominent lobate scarps. During the third stage, lava flooded the lowlands and produced the smooth plains. During the fourth stage micrometeorite bombardment created a dusty surface also known as regolith. A few larger meteorites impacted the surface and left bright rayed craters. Other than the occasional collisions of a meteorites, Mercury's surface is no longer active and remains the same as it has for millions of years.

Could water exist on Mercury?

Mercury Statistics
Mass (kg)3.303e+23
Mass (Earth = 1)5.5271e-02
Mean density (gm/cm^3)5.42
Mean distance from the Sun (km)57,910,000
Mean distance from the Sun (Earth = 1)0.3871
Rotational period (days)58.6462
Orbital period (days)87.969
Mean orbital velocity (km/sec)47.88
Orbital eccentricity0.2056
Tilt of axis (degrees)0.00
Orbital inclination (degrees)7.004
Equatorial surface gravity (m/sec^2)2.78
Equatorial escape velocity (km/sec)4.25
Visual geometric albedo0.10
Magnitude (Vo)-1.9
Mean surface temperature179°C
Maximum surface temperature427°C
Minimum surface temperature-173°C
Atmospheric composition

Mercury Shows Its True Colors
MESSENGER's Wide Angle Camera (WAC), part of the Mercury Dual Imaging System (MDIS), is equipped with 11 narrow-band color filters. As the spacecraft receded from Mercury after making its closest approach on January 14, 2008, the WAC recorded a 3x3 mosaic covering part of the planet not previously seen by spacecraft. The color image shown here was generated by combining the mosaics taken through the WAC filters that transmit light at wavelengths of 1000 nanometers (infrared), 700 nanometers (far red), and 430 nanometers (violet). These three images were placed in the red, green, and blue channels, respectively, to create the visualization presented here. The human eye is sensitive only across the wavelength range from about 400 to 700 nanometers. Creating a false-color image in this way accentuates color differences on Mercury's surface that cannot be seen in black-and-white (single-color) images.

Color differences on Mercury are subtle, but they reveal important information about the nature of the planet's surface material. A number of bright spots with a bluish tinge are visible in this image. These are relatively recent impact craters. Some of the bright craters have bright streaks (called "rays" by planetary scientists) emanating from them. Bright features such as these are caused by the presence of freshly crushed rock material that was excavated and deposited during the highly energetic collision of a meteoroid with Mercury to form an impact crater. The large circular light-colored area in the upper right of the image is the interior of the Caloris basin. Mariner 10 viewed only the eastern (right) portion of this enormous impact basin, under lighting conditions that emphasized shadows and elevation differences rather than brightness and color differences. MESSENGER has revealed that Caloris is filled with smooth plains that are brighter than the surrounding terrain, hinting at a compositional contrast between these geologic units. The interior of Caloris also harbors several unusual dark-rimmed craters, which are visible in this image. The MESSENGER science team is working with the 11-color images in order to gain a better understanding of what minerals are present in these rocks of Mercury's crust. (Courtesy NASA/JHUAPL)

The Interior of Mercury
Most of what is known about the internal structure of Mercury comes from data acquired by the Mariner 10 spacecraft that flew past the planet in 1973 and 1974. Mercury is about a third of the size of Earth, yet its density is comparable to that of Earth. This indicates that Mercury has a large core roughly the size of Earth's moon or about 75% of the planet's radius. The core is likely composed of 60 to 70% iron by mass. Mariner 10's measurements of the planet reveals a dipolar magnetic field possibly produced by a partially molten core. A solid rocky mantle surrounds the core with a thin crust of about 100 kilometers. (Copyright Calvin J. Hamilton)

Caloris Basin&mdashin Color!
This false-color image of Mercury, recently published in Science magazine, shows the great Caloris impact basin, visible in this image as a large, circular, orange feature in the center of the picture. The contrast between the colors of the Caloris basin floor and those of the surrounding plains indicate that the composition of Mercury's surface is variable. Many additional geological features with intriguing color signatures can be identified in this image. For example, the bright orange spots just inside the rim of Caloris basin are thought to mark the location of volcanic features, such as the volcano shown in image PIA10942. MESSENGER Science Team members are studying these regional color variations in detail, to determine the different mineral compositions of Mercury's surface and to understand the geologic processes that have acted on it. Images taken through the 11 different WAC color filters were used to create this false-color image. The 11 different color images were compared and contrasted using statistical methods to isolate and enhance subtle color differences on Mercury's surface. (Courtesy NASA/Johns Hopkins University Applied Physics Laboratory/Arizona State)

MESSENGER Discovers Volcanoes on Mercury
As reported in the July 4, 2008 issue of Science magazine, volcanoes have been discovered on Mercury's surface from images acquired during MESSENGER's first Mercury flyby. This image shows the largest feature identified as a volcano in the upper center of the scene. The volcano has a central kidney-shaped depression, which is the vent, and a broad smooth dome surrounding the vent. The volcano is located just inside the rim of the Caloris impact basin. The rim of the basin is marked with hills and mountains, as visible in this image. The role of volcanism in Mercury's history had been previously debated, but MESSENGER's discovery of the first identified volcanoes on Mercury's surface shows that volcanism was active in the distant past on the innermost planet. (Courtesy NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)

Mercury - in Color!
One week ago, the MESSENGER spacecraft transmitted to Earth the first high-resolution image of Mercury by a spacecraft in over 30 years, since the three Mercury flybys of Mariner 10 in 1974 and 1975. MESSENGER's Wide Angle Camera (WAC), part of the Mercury Dual Imaging System (MDIS), is equipped with 11 narrow-band color filters, in contrast to the two visible-light filters and one ultraviolet filter that were on Mariner 10's vidicon camera. By combining images taken through different filters in the visible and infrared, the MESSENGER data allow Mercury to be seen in a variety of high-resolution color views not previously possible. MESSENGER's eyes can see far beyond the color range of the human eye, and the colors seen in the accompanying image are somewhat different from what a human would see.

The color image was generated by combining three separate images taken through WAC filters sensitive to light in different wavelengths filters that transmit light with wavelengths of 1000, 700, and 430 nanometers (infrared, far red, and violet, respectively) were placed in the red, green, and blue channels, respectively, to create this image. The human eye is sensitive across only the wavelength range 400 to 700 nanometers. Creating a false-color image in this way accentuates color differences on Mercury's surface that cannot be seen in the single-filter, black-and-white image released last week.

This visible-infrared image shows an incoming view of Mercury, about 80 minutes before MESSENGER's closest pass of the planet on January 14, 2008, from a distance of about 27,000 kilometers (17,000 miles). (Courtesy NASA/JHUAPL)

Mercury's Complex Cratering History
On January 14, 2008, the MESSENGER spacecraft observed about half of the hemisphere not seen by Mariner 10. These images, mosaicked together by the MESSENGER team, were taken by the Narrow Angle Camera (NAC), part of the Mercury Dual Imaging System (MDIS) instrument, about 20 minutes after MESSENGER's closest approach to Mercury (2:04 pm EST), when the spacecraft was at a distance of about 5,000 kilometers (about 3,100 miles). The image shows features as small as 400 meters (0.25 miles) in size and is about 370 kilometers (230 miles) across.

The image shows part of a large, fresh crater with secondary crater chains located near Mercury's equator on the side of the planet newly imaged by MESSENGER. Large, flat-floored craters often have terraced rims from post-impact collapse of their newly formed walls. The hundreds of secondary impactors that are excavated from the planet's surface by the incoming object create long, linear crater chains radial to the main crater. These chains, in addition to the rest of the ejecta blanket, create the complicated, hilly terrain surrounding the primary crater. By counting craters on the ejecta blanket that have formed since the impact event, the age of the crater can be estimated. This count can then be compared with a similar count for the crater floor to determine whether any material has partially filled the crater since its formation. With their large size and production of abundant secondary craters, these flat-floored craters both illuminate and confound the study of the geological history of Mercury. (Courtesy NASA/JHUAPL)

Looking Toward the South Pole of Mercury
On January 14, 2008, the MESSENGER spacecraft passed 200 kilometers (124 miles) above the surface of Mercury and snapped the first pictures of a side of Mercury not previously seen by spacecraft. This image shows that previously unseen side, with a view looking toward Mercury's south pole. The southern limb of the planet can be seen in the bottom right of the image. The bottom left of the image shows the transition from the sunlit, day side of Mercury to the dark, night side of the planet, a transition line known as the terminator. In the region near the terminator, the sun shines on the surface at a low angle, causing the rims of craters and other elevated surface features to cast long shadows, accentuating height differences in the image.

This image was acquired about 98 minutes after MESSENGER's closest approach to Mercury, when the spacecraft was at a distance of about 33,000 kilometers (21,000 miles). (Courtesy NASA/JHUAPL)

MESSENGER Looks to the North
As MESSENGER sped by Mercury on January 14, 2008, the Narrow Angle Camera (NAC) of the Mercury Dual Imaging System (MDIS) captured this shot looking toward Mercury's north pole. The surface shown in this image is from the side of Mercury not previously seen by spacecraft. The top right of this image shows the limb of the planet, which transitions into the terminator (the line between the sunlit, day side and the dark, night side) on the top left of the image. Near the terminator, the Sun illuminates surface features at a low angle, casting long shadows and causing height differences of the surface to appear more prominent in this region.

It is interesting to compare MESSENGER's view to the north with the image looking toward the south pole, released on January 21. Comparing these two images, it can be seen that the terrain near the south pole is more heavily cratered while some of the region near the north pole shows less cratered, smooth plains material, consistent with the general observations of the poles made by Mariner 10. MESSENGER acquired over 1200 images of Mercury's surface during its flyby, and the MESSENGER team is busy examining all of those images in detail, to understand the geologic history of the planet as a whole, from pole to pole. (Courtesy NASA/JHUAPL)

Mariner 10 Outgoing Color Image of Mercury
This mosaic of Mercury was created from more than 140 images taken by the Mariner 10 spacecraft as it flew past the innermost planet on March 29, 1974. Mariner 10's trajectory brought the spacecraft across the dark hemisphere of Mercury. The images were acquired after the spacecraft exited Mercury's shadow. The color data is from more distant global views. (Copyright Ted Stryk)

MESSENGER Views Mercury's Horizon
As the MESSENGER spacecraft drew closer to Mercury for its historic first flyby, the spacecraft's Narrow Angle Camera (NAC) on the Mercury Dual Imaging System (MDIS) acquired an image mosaic of the sunlit portion of the planet. This image is one of those mosaic frames and was acquired on January 14, 2008, 18:10 UTC, when the spacecraft was about 18,000 kilometers (11,000 miles) from the surface of Mercury, about 55 minutes before MESSENGER's closest approach to the planet.

The image shows a variety of surface textures, including smooth plains at the center of the image, many impact craters (some with central peaks), and rough material that appears to have been ejected from the large crater to the lower right. This large 200-kilometer-wide (about 120 miles) crater was seen in less detail by Mariner 10 more than three decades ago and was named Sholem Aleichem for the Yiddish writer. In this MESSENGER image, it can be seen that the plains deposits filling the crater's interior have been deformed by linear ridges. The shadowed area on the right of the image is the day-night boundary, known as the terminator. Altogether, MESSENGER acquired over 1200 images of Mercury, which the science team members are now examining in detail to learn about the history and evolution of the innermost planet. (Courtesy NASA/JHUAPL)

"The Spider" - Radial Troughs within Caloris
The Narrow Angle Camera of the Mercury Dual Imaging System (MDIS) on the MESSENGER spacecraft obtained high-resolution images of the floor of the Caloris basin on January 14, 2008. Near the center of the basin, an area unseen by Mariner 10, this remarkable feature - nicknamed "the spider" by the science team - was revealed. A set of troughs radiates outward in a geometry unlike anything seen by Mariner 10. The radial troughs are interpreted to be the result of extension (breaking apart) of the floor materials that filled the Caloris basin after its formation. Other troughs near the center form a polygonal pattern. This type of polygonal pattern of troughs is also seen along the interior margin of the Caloris basin. An impact crater about 40 km (

25 miles) in diameter appears to be centered on "the spider." The straight-line segments of the crater walls may have been influenced by preexisting extensional troughs, but some of the troughs may have formed at the time that the crater was excavated. (Courtesy NASA/JHUAPL)

MESSENGER Reveals Mercury's Geological History
Shortly following MESSENGER's closest approach to Mercury on January 14, 2008, the spacecraft's Narrow Angle Camera (NAC) on the Mercury Dual Imaging System (MDIS) instrument acquired this image as part of a mosaic that covers much of the sunlit portion of the hemisphere not viewed by Mariner 10. Images such as this one can be read in terms of a sequence of geological events and provide insight into the relative timing of processes that have acted on Mercury's surface in the past.

The double-ringed crater pictured in the lower left of this image appears to be filled with smooth plains material, perhaps volcanic in nature. This crater was subsequently disrupted by the formation of a prominent scarp (cliff), the surface expression of a major crustal fault system, that runs alongside part of its northern rim and may have led to the uplift seen across a portion of the crater's floor. A smaller crater in the lower right of the image has also been cut by the scarp, showing that the fault beneath the scarp was active after both of these craters had formed. The MESSENGER team is working to combine inferences about the timing of events gained from this image with similar information from the hundreds of other images acquired by MESSENGER to extend and refine the geological history of Mercury previously defined on the basis only of Mariner 10 images. (Courtesy NASA/JHUAPL)

Ridges and Cliffs on Mercury's Surface
A complex history of geological evolution is recorded in this frame from the Narrow Angle Camera (NAC), part of the Mercury Dual Imaging System (MDIS) instrument, taken during MESSENGER's close flyby of Mercury on January 14, 2008. Part of an old, large crater occupies most of the lower left portion of the frame. An arrangement of ridges and cliffs in the shape of a "Y" crosses the crater's floor. The shadows defining the ridges are cast on the floor of the crater by the Sun shining from the right, indicating a descending stair-step of plains. The main, right-hand branch of the "Y" crosses the crater floor, the crater rim, and continues off the top edge of the picture it appears to be a classic "lobate scarp" (irregularly shaped cliff) common in all areas of Mercury imaged so far. These lobate scarps were formed during a period when Mercury's crust was contracting as the planet cooled. In contrast, the branch of the Y to the left ends at the crater rim and is restricted to the floor of the crater. Both it and the lighter-colored ridge that extends downward from it resemble "wrinkle ridges" that are common on the large volcanic plains, or "maria," on the Moon. The MESSENGER science team is studying what features like these reveal about the interior cooling history of Mercury.

Ghostly remnants of a few craters are seen on the right side of this image, possibly indicating that once-pristine, bowl-shaped craters (like those on the large crater's floor) have been subsequently flooded by volcanism or some other plains-forming process. (Courtesy NASA/JHUAPL)

Detailed Close-up of Mercury's Previously Unseen Surface
This scene was imaged by MESSENGER's Narrow Angle Camera (NAC) on the Mercury Dual Imaging System (MDIS) during the spacecraft's flyby of Mercury on January 14, 2008. The scene is part of a mosaic that covers a portion of the hemisphere not viewed by Mariner 10 during any of its three flybys (1974-1975). The surface of Mercury is revealed at a resolution of about 250 meters/pixel (about 820 feet/pixel). For this image, the Sun is illuminating the scene from the top and north is to the left.

The outer diameter of the large double ring crater at the center of the scene is about 260 km (about 160 miles). The crater appears to be filled with smooth plains material that may be volcanic in nature. Multiple chains of smaller secondary craters are also seen extending radially outward from the double ring crater. Double or multiple rings form in craters with very large diameters, often referred to as impact basins. On Mercury, double ring basins begin to form when the crater diameter exceeds about 200 km (about 125 miles) at such an onset diameter the inner rings are typically low, partial, or discontinuous. The transition diameter at which craters begin to form rings is not the same on all bodies and, although it depends primarily on the surface gravity of the planet or moon, the transition diameter can also reveal important information about the physical characteristics of surface materials. Studying impact craters, such as this one, in the more than 1200 images returned from this flyby will provide clues to the physical properties of Mercury's surface and its geological history. (Courtesy NASA/JHUAPL)

Hills of Mercury
"Weird terrain" best describes this hilly, lineated region of Mercury. This area is at the antipodal point from the large Caloris basin. The shock wave produced by the Caloris impact was reflected and focused to this antipodal point, thus jumbling the crust and breaking it into a series of complex blocks. The area covered is about 100 kilometers (62 miles) on a side. (Copyright Calvin J. Hamilton FDS 27370)

Caloris Basin Floor
This image is a high resolution view of the Caloris Basin shown in the previous image. It shows ridges and fractures that increase in size towards the center of the basin (upper left). (Copyright Calvin J. Hamilton FDS 126)

Bright Rayed Craters
This image shows two prominent craters (upper right) with bright halos on Mercury. The craters are about 40 kilometers (25 miles) in diameter. The halos and rays cover other features on the surface indicating that they are some of the youngest on Mercury. (Copyright Calvin J. Hamilton FDS 275)

Large Faults on Mercury
This Mariner 10 image shows Santa Maria Rupes, the sinuous dark feature running through the crater at the center of this image. Many such features were discovered in the Mariner images of Mercury and are interpreted to be enormous thrust faults where part of the mercurian crust was pushed slightly over an adjacent part by compressional forces. The abundance and length of the thrust faults indicate that the radius of Mercury decreased by 1-2 kilometers (.6 - 1.2 miles) after the solidification and impact cratering of the surface. This volume change probably was due to the cooling of the planet, following the formation of a metallic core three-fourths the size of the planet. North is towards the top and is 200 kilometers (120 miles) across. (© Copyright 1998 by Calvin J. Hamilton FDS 27448)

This is an image of a 450 kilometer (280 mile) ridge called Antoniadi. It travels along the right edge of the image, and transects a large 80 kilometer (50 mile) crater about half way in between. It crosses smooth plains to the north and intercrater plains to the south [Strom et al., 1975]. (Copyright Calvin J. Hamilton)

Double Ring Basin
This image shows a double-ring basin which is 200 kilometers (120 miles) in diameter. The floor contains smooth plains material. The inner ring basin is at a lower elevation than the outer ring. (Copyright Calvin J. Hamilton FDS 27301)

Incoming View of Mercury
This photomosaic of Mercury was constructed from photos taken by Mariner 10 six hours before the spacecraft flew past the planet on March 29, 1974. These images were taken from a distance of 5,380,000 kilometers (3,340,000 miles). (Courtesy USGS, and NASA)

Mercury
This two image (FDS 26850, 26856) mosaic of Mercury was constructed from photos taken by Mariner 10 a few hours before the spacecraft's closest and first encounter with the planet on March 29, 1974. (Copyright Calvin J. Hamilton)

Caloris Basin
This mosaic shows the Caloris Basin (located half-way in shadow on the morning terminator). Caloris is Latin for heat and the basin is named this because it is near the subsolar point (the point closest to the sun) when Mercury is at aphelion. Caloris basin is 1,300 kilometers (800 miles) in diameter and is the largest know structure on Mercury. It was formed from an impact of a projectile with asteroid dimensions. The interior floor of the basin contains smooth plains but is highly ridged and fractured. North is towards the top of this image. (Copyright Calvin J. Hamilton FDS 188-199)

Davies, M. E., S. E. Dwornik, D. E. Gault, and R. G. Strom. Atlas of Mercury. NASA SP-423. Washington, D.C.: U.S. Government Printing Office, 1978.

Mariner 10 Preliminary Science Report. Science , 185 :141-180, 1974.

Mariner 10 Imaging Science Final Report. Journal of Geophysical Research , 80 (17):2341-2514, 1975.

Strom, Robert G. et al. "Tectonism and Volcanism on Mercury." Journal of Geophysical Research , 80 (17):2478-2507, 1975.

Trask, Newell J. and John E. Guest. "Preliminary Geologic Terrain Map of Mercury." Journal of Geophysical Research , 80 (17):2461-2477, 1975.

Trying science something something dark side..

Question: Mercury is the closest planet to our sun. So why do nights on Mercury get so extremely cold if it’s closer to the sun than earth or even Venus?

On Mercury temperatures can get as hot as 430 degrees Celsius during the day and as cold as -180 degrees Celsius at night.
Mercury is the planet in our solar system that sits closest to the sun. The distance between Mercury and the sun ranges from 46 million kilometers to 69.8 million kilometers. The earth sits at a comfy 150 million kilometers. This is one reason why it gets so hot on Mercury during the day.

The other reason is that Mercury has a very thin and unstable atmosphere. At a size about a third of the earth and with a mass (what we on earth see as ‘weight’) that is 0.05 times as much as the earth, Mercury just doesn’t have the gravity to keep gases trapped around it, creating an atmosphere. Due to the high temperature, solar winds, and the low gravity (about a third of earth’s gravity), gases keep escaping the planet, quite literally just blowing away.
Atmospheres can trap heat, that’s why it can still be nice and warm at night here on earth.
Mercury’s atmosphere is too thin, unstable and close to the sun to make any notable difference in the temperature.

Space is cold. Space is very cold. So cold in fact, that it can almost reach absolute zero, the point where molecules stop moving (and they always move). In space, the coldest temperature you can get is 2.7 Kelvin, about -270 degrees Celsius.
Sunlight reflected from other planets and moons, gases that move through space, the very thin atmosphere and the surface of Mercury itself are the main reasons that temperatures on Mercury don’t get lower than about -180 °C at night.

Astronomy of Planets

The atmospheres of Venus and Mercury, when compared to Earth, are significantly different these differences create environments that are unsuitable for life.

A Comparison of Mercury, Venus, and Earth.
Courtesy of Wikimedia/NASA/JHUAPL/JPL. Cropped version of original.

An Introduction to Venus and Mercury

Venus and Mercury, along with Mars and Earth, are considered terrestrial planets they are composed primarily of silicate rock and metals. These four planets are also considered the inner planets because they orbit close to the Sun, especially compared to the outer planets. These similarities often lead to Venus and Mercury being referred to as ‘Earth-like’. However, there are significant differences in the atmospheres of Venus and Mercury, as compared to Earth, which make them unsuitable for life.

Venus’ Extremes

Axial tilt of Venus and Earth
Copyright 2008 Calvin J. Hamilton. Cropped from original.

Venus is similar to Earth in respect to density, size, mass, and volume. However, the two planets differ when it comes to their atmosphere, which could explain why Venus has been, to our knowledge, unable to sustain life. The axial tilt of Venus is 177°, equivalently an axial tilt of 3° with the rotation backwards for comparison, Earth has an axial tilt of 23°. The axial tilt of a planet determines the duration and severity of its seasons. Earth’s axial tilt is significant, meaning that the hemisphere that is tilted towards the Sun receives more energy than the one tilted away. In contrast, Venus’ axial tilt is insignificant, meaning that no hemisphere receives more energy from the Sun than the other. The daytime and nighttime sides of Venus remain at an average of 470°C at the surface. This is interesting given that Venus rotates very slowly, about once every 243 Earth days. This leaves the nighttime side of the planet without sunlight for an extended period. However, due to extreme winds of up to 224 mph, clouds are able to circulate the planet every 4 days which distributes the heat of the planet more evenly. 1 This prevents the nighttime side of the planet from cooling too much. In addition to the winds, Venus also has a very thick atmosphere, and the greenhouse gases in it (mainly carbon dioxide) act as an insulating blanket this keeps surface temperatures roughly the same everywhere on the planet, regardless of whether it is day or night. Thus, there are no cooler spots on Venus, and life would be forced to deal with the extreme heat, without reprieve.

In 1966, Venus experienced the first impact of an artifact on the surface of another planet. That artifact was the unmanned Venera 3 atmospheric probe. Between 1966 and 1982, the former Soviet Union conducted the Venera series of missions that sent atmospheric and lander probes to Venus. The data collected during these missions helped scientists develop new and exciting theories about the planet.

The Venera Missions

Science from Venera 4 19

Carbon dioxide 90-95%
Nitrogen 7%
Molecular oxygen 0.4-0.8%
Water vapor 0.1-1.6%
Temperature 270-280°C @ point of crash
Pressure 20 kg/cm2 or 15-22 atm @ point of crash
No radiation belts, magnetic fields found

Range of surface temperature: Mercury, Venus and Earth.
Copyright 2014 Matt Doyle. Cropped from original.

The Venera 4 mission was the first of Soviet Union’s many probes that successfully studied Venus’ surface and atmosphere. 2 It was the first probe to provide in-place analysis of another planet’s environment. It provided data that showed the Venusian atmosphere consists primarily of carbon dioxide, as well as nitrogen, oxygen, and water vapor. Upon first inspection, this atmospheric composition does not preclude the suitability of life. However, the Venera 4 also provided direct measurements that demonstrated the extreme heat of Venus and the tremendous density of the atmosphere. Although the capsule was designed to withstand extreme g-forces and temperatures, Venera 4 experienced a malfunction and stopped sending data before it landed on the surface. Venera 5 and 6 experienced similar fates. However, Venera 5 was able to detect a light level of 250 Watts per square meter inside the atmosphere, which is roughly one-quarter that of Earth’s average. 3 This smaller amount of light reaching the surface of Venus could hamper the productivity of photochemical processes needed for some types of life.The transmitter of the Venera 7 probe was affected by the dense atmosphere and, as a result, sent very weak signals. It wasn’t until a month later that the descent signal tapes were reviewed, and it was found that Venera 7 had transmitted information from the surface of Venus. This made it the first probe to safely reach the surface of Venus. It found that Venus has a very dense atmosphere and a pressure at the surface of 92 standard atmospheres, which is far greater than scientists had originally estimated. To give some perspective on this value, a pressure of 92 standard atmospheres is similar to being under 1000 meters of water. 4 Venera 7 also measured a surface temperature of 475 °C and surface winds of 2.5 m/s. 3

The Venera missions found that carbon dioxide, a greenhouse gas, makes up 95% of Venus’s atmosphere this is the main cause of a greenhouse effect on Venus. The Bond albedo of Venus is 0.90, so only 10% of light penetrates the atmosphere. Venus’ albedo makes it difficult for longwave radiation to penetrate its atmosphere, however, shortwave radiation is met with much less resistance. The shortwave radiation that reaches the surface of the planet heats the ground and is re-emitted as infrared radiation. The greenhouse gases in the atmosphere absorb and trap this infrared radiation. 5 Although this occurs on Earth as well, the extreme amounts of carbon dioxide in the Venusian atmosphere only let a much smaller fraction of the infrared radiation escape into space. This effect creates an extremely hot and steady surface temperature that is enough to easily melt lead, and makes the planet unsuitable for life. The melting temperatures of different proteins, an essential part of all living organisms, varies, but temperatures above 41 °C cause them to break down thus, organisms would require extreme survival mechanisms to combat the heat on Venus. 6

Venera 11 found sulfuric acid and chlorine in the cloud layers, as well as carbon monoxide at low altitudes in the atmosphere. 7 Each of these chemicals poses a danger to organic compounds and/or organic processes.

A set of images of the Venus south polar vortex in infrared light (at 3.8 microns) acquired by the Visible and Infrared Thermal Imaging Spectrometer instrument on ESA’s Venus Express spacecraft. The images show the temperature of the cloud tops at about 65km (40.4 miles) altitude.
Courtesy of ESA/VIRTIS/INAF-IASF/Obs. De Paris-LESIA

Venera 11 and 12 were equipped with the GROZA instrument, which was used to measure the sounds on Venus wind, thunder, and lightning were detected. However, Venus is the only planet whose lightning is not associated with water clouds, but clouds of sulfur dioxide and droplets of sulfuric acid. 8 Venera 11 found that lightning flashes every 25 seconds somewhere in the planet’s atmosphere, and Venera 12 identified 1,200 strikes altogether. 1 The differences in atmospheric composition, pressure, temperature, wind speeds, and source of lightning, compared to Earth, combine to create a very hostile environment, which would be unsuitable for life as we currently understand it.

Developments since Venera

Many new astronomical techniques have been developed since the Venera missions. One technique, infrared heterodyne spectroscopy, has been particularly useful in observing weather characteristics of Venus with unprecedented accuracy. This technique focuses solely on incoming light in the infrared frequency range, then mixes that light with laser light at a similar (but not the same) frequency, and generates a difference pattern between the two frequencies in the radio-frequency range. 9 This allows astronomers to take a very close look at the infrared frequencies of an object. This is akin to using a microscope to study an object only a small portion of it is visible, but at a large magnification level, which reveals very fine details of the object. Using this technique, the Heterodyne Instrument for Planetary Wind and Composition (HIPWAC) project can determine the chemical composition of planetary atmospheres, measure planetary winds, determine atmospheric profiles (i.e., how gas abundance, pressure, and temperature change with altitude), and measure photochemical processes. 9 HIPWAC was involved in the first direct measurement of sub-solar and anti-solar winds, winds that are not directly caused by solar winds, on Venus, which were measured to an accuracy of roughly 2 m/s at an altitude of 110 km. 10 Infrared spectroscopy was also used to measure and remotely monitor the abundance of sulfur dioxide below the clouds of Venus, between altitudes of 35-45 km this sulfur dioxide is a likely tracer of Venusian volcanism. The results of this new spectroscopy have been consistent with laboratory and modeling studies. 11 They are also consistent with, and often more accurate than, the findings from the Venera missions.

An Introduction to Mercury

Mercury and Earth size comparison
Courtesy of NASA (Earth) NASA/APL (from MESSENGER) (Mercury)

Mercury, like Venus, is categorized as both an inner planet and a terrestrial planet. Of the four terrestrial planets within our Solar System, Mercury orbits closest to the Sun. It has a very thin atmosphere the density and composition of its atmosphere is a result of its physical size and the strength of its magnetic field. Mercury’s radius, which can be measured using a telescope, is approximately one-third that of Earth’s, making it barely larger than our moon. As a result, Mercury’s surface area is only 15% that of Earth’s. The surface consists primarily of plains and craters from the collision of asteroids and comets. 12 The persistence of craters is evidence that the planet has a virtually nonexistent atmosphere, and remains unprotected from large or high energy impacts.

Mercury’s Extremes

NASA’s MESSENGER probe collected a massive amount of data about Mercury during its three flybys and four years in orbit of the planet. Scientists were able to combine radio tracking and topographic data to determine that Mercury has a large, metallic, partially liquid, rotating core. 13,14 Scientists were then able to model Mercury’s magnetic field using their knowledge of its core composition and dynamo theory, which says that a rotating, convecting, and electrically conducting fluid can maintain a magnetic field. A strong magnetic field is important for the development and maintenance of an atmosphere. Earth’s magnetic field is able to slow down and trap high-energy charged particles from the Sun, creating what is called the Van Allen Belt. 15 Due to Mercury’s core composition and small size, the strength of its magnetic field is more than 100 times weaker than Earth’s. 16 This is far too weak to produce a similarly protective belt, so damaging radiation from the Sun is able to reach the surface of the planet. Charged particles that reach the surface can cause surface material to be ejected high above the planet, but not high enough to escape Mercury’s gravity. This results in heavier elements (e.g. sodium and magnesium) being added to the atmosphere. 17 NASA’s MESSENGER and Mariner 10 probes provided an abundance of data that scientists could use to explain why Mercury has an atmosphere, and why the atmosphere is made up of heavier elements. Mercury’s thin atmosphere is created by the combination of solar winds, its weak – but dynamic – magnetosphere, and its gravity.

The battered surface of Mercury.
Courtesy of NASA/USGS/JHUAPL

The Mariner 10 was launched in 1973 and flew by Mercury three times. It was equipped with an onboard ultraviolet spectrometer, which was able to collect atmospheric data from airglow and occultation. Using these data, an upper bound of the atmospheric pressure at Mercury’s surface was calculated to be about 5 quadrillion times less than Earth’s. 12 Mercury’s atmosphere is so thin that it has been classified as a surface-bounded exosphere. This means that the density is so low that a particle has a higher chance of escaping into space than colliding with another particle. Its atmosphere is constantly in a state of flux it’s being stripped away by strong solar winds and replenished by a bombardment of charged particles, radioactive decay of elements, and dust from meteorite impacts.

The Mariner 10 and MESSENGER probes were used to observe ultraviolet radiation through a photometer and an imaging mass spectrometer. This provided evidence of oxygen, hydrogen, helium, potassium, water vapor, and silicon in Mercury’s atmosphere. 18 Because Mercury has such a thin atmosphere, it is unable to store and regulate incoming solar radiation therefore, the surface temperature fluctuates between 427 °C on the side that faces the Sun, and -173 °C on the night time side. 12 This could be why Mercury has been nicknamed “the planet of extremes”. These extreme temperature fluctuations, along with a weak magnetosphere and thin atmosphere, create a very hostile and inhospitable environment on Mercury.

Concluding Remarks

Studying Mercury and Venus’ atmospheres, and comparing them to Earth’s, can help us appreciate that certain atmospheric conditions are essential to making a planet hospitable for life. Venus’ atmosphere contains some components vital for life, such as carbon dioxide and nitrogen however, its extremely high density and convection currents create an environment so hot that unprotected organic materials would melt, boil, and vaporize. Similarly, Mercury’s atmosphere, or lack thereof, contains some components vital for life, such as oxygen and water vapor. However, Mercury’s extremely thin atmosphere and lack of protection from solar winds create an environment that is simply too barren and fluctuating for life to begin or survive.

References

2 D.E. Reese and P.R. Swan, Science 159, 1228 (1968).

3 B. Harvey, Astronomische Nachrichten 34, 367 (1996).

4 Wikibooks, Open Books for an Open World (2010), (https://en.wikibooks.org/wiki/Solar_System/Venus), Web. Mar. 2016

5 M. Snels, S. Stefani, D. Grassi, G. Piccioni, and A. Adriani, Planetary And Space Science 103, 347 (2014).

7 N.L. Johnson, American Astronautical Society 47, 276 (1979).

10 NASA Goddard Space Flight Centre, – Lasers And the Dynamic Mesosphere/Thermosphere of Venus, (2010), (http://ntrs.nasa.gov/search.jsp?R=20100031081), Web. Mar. 28, 2016.

11 B. Beard, C. de Bergh, F. Bruce, D. Crisp, J. Maillard, T. Owen, J.B. Pollack, and D. Grin spoon, Geophysical Research Letters 20, 1587 (1993).

14 N.L. Chabot, E.A. Wollack, R.L. Klima, and M.E. Minitti, Earth And Planetary Science Letters 390, 199 (2014).

15 P. Grego, Venus And Mercury, and How to Observe Them (Springer, New York, 2008).

18 D.J. Stevenson, Nature 485, 52 (2012).

19 B. Harvey, Russian Planetary Exploration: History, Development, Legacy, Prospects (Springer, Berlin, 2007).

20 G. Elert, Pressure On the Surface of Venus (2000), (http://hypertextbook.com/facts/2000/NangMiu.shtml), Web. Mar. 2016.

22 Pamela Elizabeth Clarke (2007), Dynamic Planet-mercury in the context of its environment. Springer.

ASTR 104 | Astronomy of Planets

The atmospheres of Venus and Mercury, when compared to Earth, are significantly different these differences create environments that are unsuitable for life.

A Comparison of Mercury, Venus, and Earth.
Courtesy of Wikimedia/NASA/JHUAPL/JPL. Cropped version of original.

An Introduction to Venus and Mercury

Venus and Mercury, along with Mars and Earth, are considered terrestrial planets they are composed primarily of silicate rock and metals. These four planets are also considered the inner planets because they orbit close to the Sun, especially compared to the outer planets. These similarities often lead to Venus and Mercury being referred to as ‘Earth-like’. However, there are significant differences in the atmospheres of Venus and Mercury, as compared to Earth, which make them unsuitable for life.

Venus’ Extremes

Axial tilt of Venus and Earth
Copyright 2008 Calvin J. Hamilton. Cropped from original.

Venus is similar to Earth in respect to density, size, mass, and volume. However, the two planets differ when it comes to their atmosphere, which could explain why Venus has been, to our knowledge, unable to sustain life. The axial tilt of Venus is 177°, equivalently an axial tilt of 3° with the rotation backwards for comparison, Earth has an axial tilt of 23°. The axial tilt of a planet determines the duration and severity of its seasons. Earth’s axial tilt is significant, meaning that the hemisphere that is tilted towards the Sun receives more energy than the one tilted away. In contrast, Venus’ axial tilt is insignificant, meaning that no hemisphere receives more energy from the Sun than the other. The daytime and nighttime sides of Venus remain at an average of 470°C at the surface. This is interesting given that Venus rotates very slowly, about once every 243 Earth days. This leaves the nighttime side of the planet without sunlight for an extended period. However, due to extreme winds of up to 224 mph, clouds are able to circulate the planet every 4 days which distributes the heat of the planet more evenly. 1 This prevents the nighttime side of the planet from cooling too much. In addition to the winds, Venus also has a very thick atmosphere, and the greenhouse gases in it (mainly carbon dioxide) act as an insulating blanket this keeps surface temperatures roughly the same everywhere on the planet, regardless of whether it is day or night. Thus, there are no cooler spots on Venus, and life would be forced to deal with the extreme heat, without reprieve.

In 1966, Venus experienced the first impact of an artifact on the surface of another planet. That artifact was the unmanned Venera 3 atmospheric probe. Between 1966 and 1982, the former Soviet Union conducted the Venera series of missions that sent atmospheric and lander probes to Venus. The data collected during these missions helped scientists develop new and exciting theories about the planet.

The Venera Missions

Science from Venera 4 19

Carbon dioxide 90-95%
Nitrogen 7%
Molecular oxygen 0.4-0.8%
Water vapor 0.1-1.6%
Temperature 270-280°C @ point of crash
Pressure 20 kg/cm2 or 15-22 atm @ point of crash
No radiation belts, magnetic fields found

Range of surface temperature: Mercury, Venus and Earth.
Copyright 2014 Matt Doyle. Cropped from original.

The Venera 4 mission was the first of Soviet Union’s many probes that successfully studied Venus’ surface and atmosphere. 2 It was the first probe to provide in-place analysis of another planet’s environment. It provided data that showed the Venusian atmosphere consists primarily of carbon dioxide, as well as nitrogen, oxygen, and water vapor. Upon first inspection, this atmospheric composition does not preclude the suitability of life. However, the Venera 4 also provided direct measurements that demonstrated the extreme heat of Venus and the tremendous density of the atmosphere. Although the capsule was designed to withstand extreme g-forces and temperatures, Venera 4 experienced a malfunction and stopped sending data before it landed on the surface. Venera 5 and 6 experienced similar fates. However, Venera 5 was able to detect a light level of 250 Watts per square meter inside the atmosphere, which is roughly one-quarter that of Earth’s average. 3 This smaller amount of light reaching the surface of Venus could hamper the productivity of photochemical processes needed for some types of life.The transmitter of the Venera 7 probe was affected by the dense atmosphere and, as a result, sent very weak signals. It wasn’t until a month later that the descent signal tapes were reviewed, and it was found that Venera 7 had transmitted information from the surface of Venus. This made it the first probe to safely reach the surface of Venus. It found that Venus has a very dense atmosphere and a pressure at the surface of 92 standard atmospheres, which is far greater than scientists had originally estimated. To give some perspective on this value, a pressure of 92 standard atmospheres is similar to being under 1000 meters of water. 4 Venera 7 also measured a surface temperature of 475 °C and surface winds of 2.5 m/s. 3

The Venera missions found that carbon dioxide, a greenhouse gas, makes up 95% of Venus’s atmosphere this is the main cause of a greenhouse effect on Venus. The Bond albedo of Venus is 0.90, so only 10% of light penetrates the atmosphere. Venus’ albedo makes it difficult for longwave radiation to penetrate its atmosphere, however, shortwave radiation is met with much less resistance. The shortwave radiation that reaches the surface of the planet heats the ground and is re-emitted as infrared radiation. The greenhouse gases in the atmosphere absorb and trap this infrared radiation. 5 Although this occurs on Earth as well, the extreme amounts of carbon dioxide in the Venusian atmosphere only let a much smaller fraction of the infrared radiation escape into space. This effect creates an extremely hot and steady surface temperature that is enough to easily melt lead, and makes the planet unsuitable for life. The melting temperatures of different proteins, an essential part of all living organisms, varies, but temperatures above 41 °C cause them to break down thus, organisms would require extreme survival mechanisms to combat the heat on Venus. 6

Venera 11 found sulfuric acid and chlorine in the cloud layers, as well as carbon monoxide at low altitudes in the atmosphere. 7 Each of these chemicals poses a danger to organic compounds and/or organic processes.

A set of images of the Venus south polar vortex in infrared light (at 3.8 microns) acquired by the Visible and Infrared Thermal Imaging Spectrometer instrument on ESA’s Venus Express spacecraft. The images show the temperature of the cloud tops at about 65km (40.4 miles) altitude.
Courtesy of ESA/VIRTIS/INAF-IASF/Obs. De Paris-LESIA

Venera 11 and 12 were equipped with the GROZA instrument, which was used to measure the sounds on Venus wind, thunder, and lightning were detected. However, Venus is the only planet whose lightning is not associated with water clouds, but clouds of sulfur dioxide and droplets of sulfuric acid. 8 Venera 11 found that lightning flashes every 25 seconds somewhere in the planet’s atmosphere, and Venera 12 identified 1,200 strikes altogether. 1 The differences in atmospheric composition, pressure, temperature, wind speeds, and source of lightning, compared to Earth, combine to create a very hostile environment, which would be unsuitable for life as we currently understand it.

Developments since Venera

Many new astronomical techniques have been developed since the Venera missions. One technique, infrared heterodyne spectroscopy, has been particularly useful in observing weather characteristics of Venus with unprecedented accuracy. This technique focuses solely on incoming light in the infrared frequency range, then mixes that light with laser light at a similar (but not the same) frequency, and generates a difference pattern between the two frequencies in the radio-frequency range. 9 This allows astronomers to take a very close look at the infrared frequencies of an object. This is akin to using a microscope to study an object only a small portion of it is visible, but at a large magnification level, which reveals very fine details of the object. Using this technique, the Heterodyne Instrument for Planetary Wind and Composition (HIPWAC) project can determine the chemical composition of planetary atmospheres, measure planetary winds, determine atmospheric profiles (i.e., how gas abundance, pressure, and temperature change with altitude), and measure photochemical processes. 9 HIPWAC was involved in the first direct measurement of sub-solar and anti-solar winds, winds that are not directly caused by solar winds, on Venus, which were measured to an accuracy of roughly 2 m/s at an altitude of 110 km. 10 Infrared spectroscopy was also used to measure and remotely monitor the abundance of sulfur dioxide below the clouds of Venus, between altitudes of 35-45 km this sulfur dioxide is a likely tracer of Venusian volcanism. The results of this new spectroscopy have been consistent with laboratory and modeling studies. 11 They are also consistent with, and often more accurate than, the findings from the Venera missions.

An Introduction to Mercury

Mercury and Earth size comparison
Courtesy of NASA (Earth) NASA/APL (from MESSENGER) (Mercury)

Mercury, like Venus, is categorized as both an inner planet and a terrestrial planet. Of the four terrestrial planets within our Solar System, Mercury orbits closest to the Sun. It has a very thin atmosphere the density and composition of its atmosphere is a result of its physical size and the strength of its magnetic field. Mercury’s radius, which can be measured using a telescope, is approximately one-third that of Earth’s, making it barely larger than our moon. As a result, Mercury’s surface area is only 15% that of Earth’s. The surface consists primarily of plains and craters from the collision of asteroids and comets. 12 The persistence of craters is evidence that the planet has a virtually nonexistent atmosphere, and remains unprotected from large or high energy impacts.

Mercury’s Extremes

NASA’s MESSENGER probe collected a massive amount of data about Mercury during its three flybys and four years in orbit of the planet. Scientists were able to combine radio tracking and topographic data to determine that Mercury has a large, metallic, partially liquid, rotating core. 13,14 Scientists were then able to model Mercury’s magnetic field using their knowledge of its core composition and dynamo theory, which says that a rotating, convecting, and electrically conducting fluid can maintain a magnetic field. A strong magnetic field is important for the development and maintenance of an atmosphere. Earth’s magnetic field is able to slow down and trap high-energy charged particles from the Sun, creating what is called the Van Allen Belt. 15 Due to Mercury’s core composition and small size, the strength of its magnetic field is more than 100 times weaker than Earth’s. 16 This is far too weak to produce a similarly protective belt, so damaging radiation from the Sun is able to reach the surface of the planet. Charged particles that reach the surface can cause surface material to be ejected high above the planet, but not high enough to escape Mercury’s gravity. This results in heavier elements (e.g. sodium and magnesium) being added to the atmosphere. 17 NASA’s MESSENGER and Mariner 10 probes provided an abundance of data that scientists could use to explain why Mercury has an atmosphere, and why the atmosphere is made up of heavier elements. Mercury’s thin atmosphere is created by the combination of solar winds, its weak – but dynamic – magnetosphere, and its gravity.

The battered surface of Mercury.
Courtesy of NASA/USGS/JHUAPL

The Mariner 10 was launched in 1973 and flew by Mercury three times. It was equipped with an onboard ultraviolet spectrometer, which was able to collect atmospheric data from airglow and occultation. Using these data, an upper bound of the atmospheric pressure at Mercury’s surface was calculated to be about 5 quadrillion times less than Earth’s. 12 Mercury’s atmosphere is so thin that it has been classified as a surface-bounded exosphere. This means that the density is so low that a particle has a higher chance of escaping into space than colliding with another particle. Its atmosphere is constantly in a state of flux it’s being stripped away by strong solar winds and replenished by a bombardment of charged particles, radioactive decay of elements, and dust from meteorite impacts.

The Mariner 10 and MESSENGER probes were used to observe ultraviolet radiation through a photometer and an imaging mass spectrometer. This provided evidence of oxygen, hydrogen, helium, potassium, water vapor, and silicon in Mercury’s atmosphere. 18 Because Mercury has such a thin atmosphere, it is unable to store and regulate incoming solar radiation therefore, the surface temperature fluctuates between 427 °C on the side that faces the Sun, and -173 °C on the night time side. 12 This could be why Mercury has been nicknamed “the planet of extremes”. These extreme temperature fluctuations, along with a weak magnetosphere and thin atmosphere, create a very hostile and inhospitable environment on Mercury.

Concluding Remarks

Studying Mercury and Venus’ atmospheres, and comparing them to Earth’s, can help us appreciate that certain atmospheric conditions are essential to making a planet hospitable for life. Venus’ atmosphere contains some components vital for life, such as carbon dioxide and nitrogen however, its extremely high density and convection currents create an environment so hot that unprotected organic materials would melt, boil, and vaporize. Similarly, Mercury’s atmosphere, or lack thereof, contains some components vital for life, such as oxygen and water vapor. However, Mercury’s extremely thin atmosphere and lack of protection from solar winds create an environment that is simply too barren and fluctuating for life to begin or survive.

References

2 D.E. Reese and P.R. Swan, Science 159, 1228 (1968).

3 B. Harvey, Astronomische Nachrichten 34, 367 (1996).

4 Wikibooks, Open Books for an Open World (2010), (https://en.wikibooks.org/wiki/Solar_System/Venus), Web. Mar. 2016

5 M. Snels, S. Stefani, D. Grassi, G. Piccioni, and A. Adriani, Planetary And Space Science 103, 347 (2014).

7 N.L. Johnson, American Astronautical Society 47, 276 (1979).

10 NASA Goddard Space Flight Centre, – Lasers And the Dynamic Mesosphere/Thermosphere of Venus, (2010), (http://ntrs.nasa.gov/search.jsp?R=20100031081), Web. Mar. 28, 2016.

11 B. Beard, C. de Bergh, F. Bruce, D. Crisp, J. Maillard, T. Owen, J.B. Pollack, and D. Grin spoon, Geophysical Research Letters 20, 1587 (1993).

14 N.L. Chabot, E.A. Wollack, R.L. Klima, and M.E. Minitti, Earth And Planetary Science Letters 390, 199 (2014).

15 P. Grego, Venus And Mercury, and How to Observe Them (Springer, New York, 2008).

18 D.J. Stevenson, Nature 485, 52 (2012).

19 B. Harvey, Russian Planetary Exploration: History, Development, Legacy, Prospects (Springer, Berlin, 2007).

20 G. Elert, Pressure On the Surface of Venus (2000), (http://hypertextbook.com/facts/2000/NangMiu.shtml), Web. Mar. 2016.

22 Pamela Elizabeth Clarke (2007), Dynamic Planet-mercury in the context of its environment. Springer.

Neptune is significantly larger than Earth at 49,528 km, it is about four times Earth’s size. And with a mass of 102 x 10 24 kg (or 102,000,000,000 trillion metric tons) it is also more massive – about 17 times more to be exact. This makes Neptune the third most massive planet in the Solar System while its density is the greatest of any gas giant ( 1.638 g/cm 3 ). Combined, this works out to a “surface” gravity of 11.15 m/s 2 (1.14 g).

As you can see, the planets of the Solar System range considerably in terms of mass. But when you factor in their variations in density, you can see how a planets mass is not always proportionate to its size. In short, while some planets may be a few times larger than others, they are can have many, many times more mass.

For more information, check out Nine Planets overview of the Solar System, NASA’s Solar System Exploration, and use this site to find out what you would weigh on other planets.

Astronomy Cast has episodes on all of the planets. Here’s Episode 49: Mercury to start!