Warhol crater, one of 23 recently named craters on Mercury
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As pop art icon Andy Warhol said, “In the future everyone will be famous for fifteen minutes,” and so here’s an image of the crater on Mercury that now bears his name, set up in the style of one of his multicolored silkscreens.
Warhol is one of 23 craters on Mercury to be recently approved for names by the International Astronomical Union (IAU), joining other notable artists, authors and musicians like Gustav Holst, Rene Magritte and Dr. Seuss who now have craters named in their honor on the first rock from the Sun.
95 km (59 miles) in diameter, Warhol crater features a large, elongated central peak, stepped walls and many of the curious erosions known as hollows.
The original image, seen at top left, was acquired by NASA’s MESSENGER spacecraft on October 21, 2011, using its Wide-Angle Camera Mercury Dual Imaging System (MDIS) instrument.
With the new list of 23 named craters, there are now 76 officially (and artistically) titled craters on Mercury since MESSENGER’s first pass of the planet in January 2008.
See the original release by the MESSENGER mission team here.
“I’m bored with that line. I never use it anymore. My new line is “In 15 minutes everybody will be famous.”
– Andy Warhol (1928 – 1987)
Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
Alan Shepard on board the deck of the USS Champlain after recovery of Freedom 7. Credit: NASA
51 years ago today, on May 5, 1961, NASA launched the Mercury-Redstone 3 rocket carrying Alan B. Shepard, Jr. aboard the Freedom 7 capsule. Shepard successfully became America’s first man in space, making a brief but historic suborbital test flight that propelled American astronauts into the space race of the 1960s.
The video above is made from photographs taken by a film camera mounted to the Freedom 7 spacecraft and scanned by archivists at Johnson Space Center. It shows the view from Freedom 7 as the Redstone rocket launched it into space, getting an amazing view of Earth’s limb and the blackness beyond before falling back to splash down in the Atlantic.
The video is made from the entire film reel, so at the end there’s also some shots of a light experiment inside the spacecraft. (View the individual scans at ASU’s March to the Moon website here.)
What’s amazing to realize is that, at this point in time, the space surrounding our planet was a very empty place. This was a time before communication and weather satellites, before GPS, before Space Station and space shuttles — and space junk — and student-made weather balloon videos. Just 51 years ago low-Earth orbit was a new frontier, and guys like Shepard (and Gagarin and Glenn, etc.) were blazing the path for everyone that followed.
Even though images of Earth from space are still amazing to look at today, seeing these photos reminds us of a time when it was all just so very new.
Read more about Shepard and the MR-3 launch here.
Images and video: NASA/JSC/Arizona State University
MESSENGER image of Mercury, acquired with its Wide Angle Camera on March 21, 2012.
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The subtle yet surprisingly varied colors of Mercury are revealed in the latest images from NASA’s MESSENGER spacecraft, now in its extended mission and second year in orbit.
The image above, a composite of Wide Angle Camera images acquired in 996, 748 and 433 nanometers for red, green and blue, shows a semi-lit limb of Mercury with the bright rayed crater Debussy visible at left. (The image has been rotated 180 degrees from the original, and color saturation was boosted by 25%.)
Named for the French composer Claude Debussy of “Claire de Lune” fame, the crater itself is approximately 50 miles (80 km) wide. It was first detected by ground-based radar telescopes in 1969 as a bright spot.
Now, 43 years later, we have a spacecraft in orbit sending back images like this. Amazing.
The various colors seen across Mercury are due to different mineral compositions of the geologic regions. The exact compositions are not yet known, and the current puzzle that researchers are trying to solve with MESSENGER is to figure out what materials make up Mercury’s complex, multi-hued surface. That will also give a clue as to what’s inside the planet and how it evolved… as well as how it is currently evolving today.
The image below is from MESSENGER’s Visual and Infrared Spectrograph (VIRS) and shows a map of Mercury’s surface, with RGB colors corresponding to different mineralogical compositions.
Sinusoidal equal area projection map of Mercury from MESSENGER's VIRS instrument.
Younger surface materials that are brighter at visible wavelengths and less affected by space weathering show up in reds, yellows and greens. Materials that may have relatively higher iron contents show up in blue.
These are Mercury’s “other colors”… maybe not what we would see with our own eyes, but beautiful nonetheless to planetary scientists!
See the above image on the MESSENGER website here.
Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
Until relatively recently, Mercury was one of the most poorly understood planets in the inner solar system. The MESSENGER mission to Mercury, is changing all of the that. New results from the Mercury Laser Altimeter (MLA) and gravity measurements are showing us that the planet closest to our sun is thin skinned and wrinkled, which is very different from what we originally thought.
The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft was launched back in 2004. It took a long time getting to its destination, completing 3 flybys of Mercury before finally entering orbit a little over a year ago. Currently, the spacecraft is in a highly eccentric polar orbit, approaching the planet much closer in the north than in the south. This allows the northern hemisphere to be probed and imaged at enviably high resolutions, but leaves the southern hemisphere poorly understood.
Even so, the data returned from MESSENGER is showing us some quite unanticipated findings. Two papers from the MESSENGER team, published in today’s issue of Science, are showing some surprising results from the laser altimeter and gravity experiments.
Using NASA’s Deep Space Network, Earth-based radio tracking of MESSENGER has allowed minute changes in the spacecraft’s orbit to be monitored and recorded. From this, Dr. Maria Zuber of MIT and her team calculated a model of Mercury’s gravity. Meanwhile, the on-board laser altimeter has provided invaluable topographic information. Combined together, these data have allowed the MESSENGER team to glean a great deal of information about the planet’s interior workings.
One of the most striking findings is that the iron-rich core of Mercury is very large. A combination of measurements and models suggest that the core has both a solid interior portion and a liquid outer portion. And while it is not certain how much of the core is solid and how much is liquid, it is clear that the total core has a radius of about 2030 km. This is a huge core, representing 83% of Mercury’s 2440 km radius!
The internal structure of Mercury is very different from that of the Earth. The core is a much larger part of the whole planet in Mercury and it also has a solid iron-sulfur cover. As a result, the mantle and crust on Mercury are much thinner than on the Earth. Credit: Case Western Reserve University
Furthermore, these calculations suggest that the layer above the core is much denser than previously expected. Results from MESSENGER’s X-Ray spectrometer indicate that the crust, and by extension the mantle, are too low in iron to explain this high density. Dr. Zuber’s team think that the only way to explain this discrepancy is by the presence of a solid iron-sulfur layer just above the core. Such a layer could be anywhere from 20 to 200 km thick, leaving only a very thin crust and mantle at the top. This kind of interior structure is completely different from what was originally suggested for Mercury, and it is nothing like what we have seen in the other planets!
This striking fact may help explain some unexpected altimeter results, which show that Mercury’s topography has less variation than other planets. The total difference between the highest and lowest elevations on Mercury is only 9.85 km. Meanwhile, the Moon has a total difference of 19.9 km between its highest and lowest points, and on Mars this difference is 30 km. Dr. Zuber and her team speculate that the presence of the core so close to the surface could keep the mantle hot, allowing topographic features to relax. In such a scenario, the lithosphere under tall impact-formed mountains would sink down into a mushy mantle that cannot support their weight. Conversely, the thin lithosphere under impact basins would rebound upwards, taking part of the mobile mantle with it.
In fact, the gravity data shows evidence of exactly this kind of process, in the form of “mascons”. These mass concentrations form when large imacts make the local crust very thin, allowing denser mantle material to rise closer to the surface as the lithosphere rebounds from the impact event. Mascons are well known from studies on the Moon and Mars, and now MESSENGER’s gravity data has revealed three such mascons on Mercury, located in the Caloris, Sobkou, and Budh basins.
The elliptical polar orbit of the MESSENGER spacecraft means that measurements at the North Pole of Mercury are much better than those at the South Pole, or even at the equator. This is evident in the better spatial resolution that can be seen at the high latitudes in this elevation map of the northern hemisphere. Major impact structures are identified by black circles. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
Interestingly enough, the mascons in Sobkou and Budh basins are not immediately obvious. They only show up when the effects of a regional topographic high are adjusted for. This topographic feature is a large quasi-linear rise that extends over half the circumference of Mercury in the mid-latitudes. The rise even passes through the northern part Caloris basin (which is large enough that its mascon is not overwhelmed by the rise). Studies of this rise by the MESSENGER team suggest that it is relatively young, having formed well after the formation of the basins, after the volcanic flooding of their interiors and exteriors, and even after some of the later impact craters that cover the flooded surfaces.
Dr. Zuber and her team also identified another young topographically elevated region, the Northern Rise, located in the lowlands surrounding the North Pole. They speculate that these young rises represent a buckling of the lithosphere, which happened when the planet’s interior cooled and contracted. This interpretation is supported by the presence of lobate scarps and ridges that can be seen around the planet, and which represent faulting of the crust when it was compressed.
So, it seems that Mercury is unlike the other planets of the Solar System. It appears to have a disproportionately large core that is covered by a thin skin of mantle and lithosphere. Furthermore, this skin seems to have wrinkled like a raisin’s when the huge core of the planet shrunk as it cooled.
Sources
Gravity Field and Internal Structure of Mercury from MESSENGER, Smith et al., Science V336 (6078), 214-217, April 13 2012, DOI:10.1126/science.1218809
Topography of the Northern Hemisphere of Mercury from MESSENGER Laser Altimetry, Zuber et al., Science V336 (6078), 217-220, April 13 2012, DOI:10.1126/science.1218805
The bright object in the center of this video sequence is the planet Mercury, seen by NASA’s STEREO-B spacecraft as it was pummeled by wave after wave of solar material ejected from the Sun during the week of March 25 – April 2, 2012.
The video above was released by NASA’s Goddard Space Flight Center earlier today. The Sun is located just off-frame to the left, while Earth would be millions of miles to the right.
Proof that it’s not easy being first rock from the Sun!
Named after the 17th-century metaphysical poet, Mercury’s Donne crater was captured in this image by NASA’s MESSENGER spacecraft. The 53-mile (83-km) -wide crater features a large, rounded central peak and numerous lobate scarps lining its floor.
Lobate scarps are found all across Mercury. Visible above as arc-shaped ridges, they are most likely thrust faults resulting from surface compression and contraction.
Donne’s central peak has been well-eroded by impacts into a softly rolling mound. Central peaks are common features of larger craters, thought to be formed when the excavation of material during an impact springs the crater floor upwards — a process called “isostatic rebound”.
This image was acquired by MESSENGER’s Narrow-Angle Camera (NAC) on August 2, 2011.
On March 17 MESSENGER successfully wrapped up a year-long campaign to perform the first complete reconnaissance of the geochemistry, geophysics, geologic history, atmosphere, magnetosphere, and plasma environment of Mercury. The following day, March 18, marked the official start of its extended phase designed to build upon those discoveries.
“Six plus years of cruise operations, capped by a year of nearly flawless orbital operations, with an additional year of scientific return ahead in the harsh environment at 0.3 astronomical units (27,886,766 miles) from the Sun,” said MESSENGER Mission Systems Engineer Eric Finnegan at JHU/APL. All this “achieved with a 1,000 kg satellite, designed, built, and launched in less than four years for a total mission cost of less than $450 million.”
Well “Donne”, MESSENGER!
Read more about the MESSENGER mission’s extension here.
Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.
MESSENGER's first image from Mercury orbit, with the bright Debussy crater visible at upper right. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
[/caption] Quick Mercury Stats Mass: 0.3302 x 1024 kg Volume: 6.083 x 1010 km3 Average radius: 2439.7 km Average diameter: 4879.4 km Density: 5.427 g/cm3 Escape velocity: 4.3 km/s Surface gravity: 3.7 m/s2 Visual magnitude: -0.42 Natural satellites: 0 Rings? – No Semimajor axis: 57,910,000 km Orbit period: 87.969 days Perihelion: 46,000,000 km Aphelion: 69,820,000 km Mean orbital velocity: 47.87 km/s Maximum orbital velocity: 58.98 km/s Minimum orbital velocity: 38.86 km/s Orbit inclination: 7.00° Orbit eccentricity: 0.2056 Sidereal rotation period: 1407.6 hours Length of day: 4222.6 hours Discovery: Known since prehistoric times Minimum distance from Earth: 77,300,000 km Maximum distance from Earth: 221,900,000 km Maximum apparent diameter from Earth: 13 arc seconds Minimum apparent diameter from Earth: 4.5 arc seconds Maximum visual magnitude: -1.9
Size of Mercury
How big is Mercury? Mercury is the smallest planet in the Solar System by surface area, volume, and equatorial diameter. Surprisingly, it is also one of the most dense. It gained its ‘smallest’ title after Pluto was demoted. That is why older material refers to Mercury as the second smallest planet. The aforementioned are the three criteria that we will use to show the size of Mercury in relation to Earth.
Some scientists think that Mercury is actually shrinking. The liquid core of the planet occupies about 42% of the planet’s volume. The spin of the planet allows a small portion of the core to cool. This cooling and shrinking is thought to be evidenced by the fracturing of the planet’s surface.
The surface of Mercury is heavily cratered, much like the Moon, and the continued presence of those craters indicates that the planet has not been geologically active for billions of years. That knowledge is based on partial mapping of the planet(55%). It is unlikely to change even after NASA’s MESSENGER spacecraft maps the entire surface. The planet was most likely bombarded heavily by asteroids and comets during the Late Heavy Bombardment about 3.8 billion years ago. Some regions would have been filled by magma eruptions from within the planet. These created smooth plains similar to those found on the Moon. As the planet cooled and contracted cracks and ridges formed. These features can be seen on top of other features, which is a clear indication that they are more recent. Volcanic eruptions ceased on Mercury about 700-800 million years ago when the planet’s mantle had contracted enough to prevent lava flow. This WAC image showing a never-before-imaged area of Mercury’s surface was taken from an altitude of ~450 km (280 miles) above Mercury. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington Diameter of Mercury (and the Radius)
The diameter of Mercury is 4,879.4 km.
Need some way to compare that to something more familiar? The diameter of Mercury is only 38% the Earth’s diameter. In other words, you could put almost 3 Mercurys side to side to match the diameter of Earth.
In fact, there are two moons in the Solar System which actually have a larger diameter than Mercury. The largest moon in the Solar System is Jupiter’s moon Ganymede, with a diameter of 5,268 km and the second largest moon is Saturn’s moon Titan, with a diameter of 5,152 km.
The Earth’s moon is only 3,474 km, so Mercury isn’t much bigger.
If you want to calculate the radius of Mercury, you need to divide the diameter of Mercury in half. While the diameter is 4,879.4 km, the radius of Mercury is only 2,439.7 km.
Diameter of Mercury in kilometers: 4,879.4 km Diameter of Mercury in miles: 3,031.9 miles Radius of Mercury in kilometers: 2,439.7 km Radius of Mercury in miles: 1,516.0 miles
Circumference of Mercury
The circumference of Mercury is 15,329 km. In other words, if Mercury’s equator was perfectly flat, and you could drive around it in your car, your odomotor would add 15,329 km from the trip.
Most planets are oblate spheroids, so their equatorial circumference is larger than their pole to pole. The more rapidly they spin, the more the planet flattens out, so the distance from the center of the planet to its poles is shorter than the distance from the center to the equator. But Mercury rotates so slowly that its circumference is the same no matter where you measure it.
You can calculate the circumference of Mercury all by yourself, using the classic mathematical formulae to get the circumference of a circle.
Circumference = 2 x pi x radius
We know the radius of Mercury is 2,439.7 km. So if you put these numbers in: 2 x 3.1415926 x 2439.7, you get 15,329 km.
Circumference of Mercury in kilometers: 15,329 km Circumference of Mercury in miles: 9,525 miles Crescent Mercury Volume of Mercury
The volume of Mercury is 6.083 x 1010km3. That seems to be a huge number on the face of it, but Mercury is the smallest planet in the Solar System by volume (since the demotion of Pluto). It is even smaller than some of the moons in our Solar System. The Mercurian volume is only 5.4% of Earth’s and the Sun has 240.5 million times the volume of Mercury.
Over 40% of Mercury’s volume is occupied by its core, 42% to be exact. The core is about 3,600 km in diameter. That makes Mercury the second most dense planet amongst our eight. The core is molten and mainly consists of iron. The molten core is able to produce a magnetic field which helps to deflect the solar wind. The magnetic field and slight gravity of the planet allow it to hold onto a tenuous atmosphere.
It is thought that Mercury was at one time a larger planet and; therefore, had a higher volume. There is one theory to explain its current size that many scientists accept on several levels. The theory explains Mercury’s density and the high percentage of core material. The theory states that Mercury originally had a metal-silicate ratio similar to common meteorites, as is typical of rocky matter in our Solar System. At that time, the planet is thought to have had a mass approximately 2.25 times its current mass, but, early in the Solar System’s history, it was struck by a planetesimal that was about 1/6 its mass and several hundred kilometers in diameter. The impact would have stripped away much of the original crust and mantle, leaving the core as a large percentage of the planet and greatly reducing the planet’s volume as well.
Volume of Mercury in cubic kilometers: 6.083 x 1010km3
Mass of Mercury
The mass of Mercury is only 5.5% of the Earth’s; the actual value is 3.30 x 1023 kg. Since Mercury is the smallest planet in the Solar System, you would expect this relatively small mass. On the other hand, Mercury is the second most dense planet in our Solar System (after Earth). Given its size, the density comes largely from its core, estimated at almost half the planet’s volume.
The planet’s mass is comprised of materials that are 70% metallic and 30% silicate. There are several theories to explain why the planet is so dense and the abundance of metallic material. The most widely held theory holds that the high core percentage is the result of an impact. In this theory the planet originally had a metal-silicate ratio similar to the chondrite meteorites common in the Universe and around 2.25 times its current mass. Early in the history of our Solar System, Mercury was struck by a planetesimal sized impactor that was about 1/6 of its hypothesized mass and hundreds of km in diameter. An impact of that magnitude would strip away much of the crust and mantle, leaving behind a large core. Scientists believe that a similar incident created our moon. An additional theory says that the planet formed before the Sun’s energy had stabilized. The planet would have had much more mass in this theory as well, but the temperatures created by the protosun would have been as high as 10,000 K and the majority of the surface rock could have been vaporized. The rock vapor could have then been carried away by the solar wind.
Mass of Mercury in kg: 0.3302 x 1024 kg Mass of Mercury in pounds: 7.2796639 x 1023 pounds Mass of Mercury in tonnes: 3.30200 x 1020 tonnes Mass of Mercury in tons: 3.63983195 x 1020 Artist's concept of MESSENGER in orbit around Mercury. Courtesy of NASA Gravity on Mercury
Gravity on Mercury is 38% of the gravity here on Earth. A man weighing 980 Newtons on Earth (about 220 pounds), would only weigh about 372 Newtons (83.6 pounds) landing on the planet’s surface. Mercury is only slightly bigger than our moon, so you might expect its gravity to be similar to the Moon’s at 16% of Earth’s. The big difference Mercury’s higher density – it’s the second densest planet in the Solar System. In fact, if Mercury were the same size as Earth, it would be even more dense than our own planet.
It’s important to clarify the difference between mass and weight. Mass measures how much stuff something contains. So if you have 100 kg of mass on Earth, you will have the same amount on Mars, or intergalactic space. Weight, however, is the force of gravity you feel. While bathroom scales measure pounds or kilograms, they should really be measuring newtons, which is a measure of weight.
Take your current weight in either pounds or kilograms and then multiply it by 0.38 with a calculator. For example, if you weigh 150 pounds, you’d weigh 57 pounds on Mercury. If you weigh 68 kilograms on the bathroom scale, your weight on Mercury would be 25.8 kg.
You can also turn this number around to figure out how much stronger you would be. For example, how high you could jump, or how much weight you could lift. The current world record for the high jump is 2.43 meters. Divide 2.43 by 0.38, and you get the world’s high jump record if it were done on Mercury. In this case, it would be 6.4 meters.
In order to escape the gravity of Mercury, you would need to be traveling 4.3 kilometers/second, or about 15,480 kilometers per hour. Compare this to Earth, where the escape velocity of our planet is 11.2 kilometers per second. If you compare the ratio between our two planets, you get 38%.
Surface gravity of Mercury: 3.7 m/s2 Escape velocity of Mercury: 4.3 kilometers/second
Density of Mercury
The density of Mercury is the second highest in the Solar System. Earth is the only planet that is more dense. It is 5.427 g/cm3 compared to Earth’s 5.515 g/cm3. If gravitational compression were to be removed from the equation, Mercury would be more dense. The high density of the planet is attributed to its large percentage of core. The core constitutes 42% of Mercury’s overall volume.
Mercury is a terrestrial planet like Earth, one of only four in our Solar System. Mercury is about 70% metallic material and 30% silicates. Add the density of Mercury and scientists can infer details of its internal structure. While the Earth’s high density mainly results from gravitational compression at the core, Mercury is much smaller and is not so tightly compressed internally. These facts have allowed NASA scientists and others to surmise that its core must be large and contain overwhelming amounts of iron. Planetary geologists estimate that the planet’s molten core accounts for about 42% of its volume. On Earth that percentage is 17. Interior of Mercury
That leaves a silicate mantle that is only 500–700 km thick. Data from Mariner 10 led scientists to believe that the crust is even thinner, at a mere 100–300 km. This surrounds a core that has a higher iron content than any other planet in the Solar System. So, what caused this disproportionate amount of core material? Most scientists accept the theory that Mercury had a metal-silicate ratio similar to common chondrite meteorites several billion years ago. They also believe that it had a mass of about 2.25 times its current; however, Mercury may have been impacted by a planetesimal 1/6 that mass and hundreds of km in diameter. The impact would have stripped away much of the original crust and mantle, leaving the core as a major percentage of the planet.
While scientists have a few facts about the density of Mercury, there is still more to be discovered. Mariner 10 send back a great deal information, but was only able to study about 44% of the planet’s surface. The MESSENGER mission is filling in some of the blanks as you are reading this article and the BepiColumbo mission will go even farther in extending our knowledge of the planet. Soon, there mat be more than theories to explain the high density of the planet.
Density of Mercury in grams per cubic centimeter: 5.427 g/cm3
Axis of Mercury
Like all of the planets in the Solar System, the axis of Mercury is tilted away from the plane of the ecliptic. In this case, Mercury’s axial tilt is 2.11 degrees.
What exactly is a planet’s axial tilt? First imagine that the Sun is a ball in the middle of a flat disk, like a record or a CD. The planets orbit around the Sun within this disk (more or less). That disk is known as the plane of the ecliptic. Each planet is also spinning on its axis as it’s orbiting around the Sun. If planet was spinning perfectly straight up and down, so that a line running through the north and south poles of the planet was perfectly parallel with the Sun’s poles, the planet would have a 0-degree axial tilt. Of course, none of the planets are like this.
So if you drew a line between Mercury’s north and south poles and compared it to an imaginary line if the Mercury had no axial tilt at all, that angle would measure 2.11 degrees. You might be surprised to know that this Mercury tilt is actually the smallest of all the planets in the Solar System. For example, the Earth’s tilt is 23.4 degrees. And Uranus is actually flipped completely over on its axis, and rotates with an axial tilt of 97.8 degrees.
Here on Earth, the axial tilt of our planet causes the seasons. When it’s summer in the northern hemisphere, the Earth’s north pole is angled towards the Sun. and then in the winter, the north pole is angled away. We get more sunlight in the summer so it’s warmer, and less in the winter.
Mercury barely experiences any seasons at all. This is because it has almost no axial tilt. Of course, it also doesn’t have much of an atmosphere to hold the Sun’s heat. Whichever side is facing the Sun is heated to 700 degrees Kelvin, and the side facing away drops to less than 100 Kelvin.
MESSENGER captured this high-resolution image of an elongated pit crater within the floor of the 355-km (220-mile) -wide crater Tolstoj on Mercury on Jan. 11, 2012. The low angle of sun illumination puts the interior of the pit crater into deep shadow, making it appear bottomless.
Pit craters are not caused by impacts, but rather by the collapse of the roof of an underground magma chamber. They are characterized by the lack of a rim or surrounding ejecta blankets, and are often not circular in shape.
Since the floor of Tolstoj crater is thought to have once been flooded by lava, a pit crater is not out of place here.
The presence of such craters on Mercury indicates past volcanic activity on Mercury contributing to the planet’s evolution.
Mercury's limb. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
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Mercury is one of the 5 planets visible with the unaided eye. Even thousands of years ago, ancient astronomers knew that the 5 wanderers were different from the other stars in the sky. The 5 planets visible with the unaided eye are Mercury, Venus, Mars, Jupiter and Saturn. They gave them distinct names, and charted their positions with incredible accuracy. It’s impossible to say “when was Mercury discovered”, since that would have been before recorded history.
But when did astronomers realize that Mercury was a planet? That happened with Copernicus developed his model of a Sun-centered Solar System, published in 1543. With the Sun at the center of the Solar System, and not the Earth, it meant that both the Earth and Mercury were planets. This discovery was confirmed when Galileo first turned his telescope on the planets and realized they matched predictions made by Copernicus. Unfortunately, Galileo’s telescope wasn’t powerful enough to reveal a disk for Mercury, but it did show how Venus went through phases like the Moon.
This model was backed up by Galileo, who pointed his first rudimentary telescope at Mercury in the 17th century. Unfortunately his telescope wasn’t powerful enough to see Mercury go through phases like he saw with Venus.
Because it’s so small and close to the Sun, Mercury was difficult to observe with ground-based telescopes. More powerful telescopes only revealed a small grey disk; they didn’t have the resolution to display features on the planet’s surface, like craters or lava fields.
It wasn’t until the early 1960s when radio astronomers started bouncing signals off the surface of Mercury that more information was finally known about the planet. These signals revealed that Mercury’s day length is about 59 days. Even more detailed observations have been made with the Arecibo telescope, mapping surface features down to a resolution of 5 km.
The most detailed observations of Mercury have come from the exploration from spacecraft sent from Earth. NASA’s Mariner 10 spacecraft swept past Mercury in 1974, capturing images from an altitude of just 327 km. It eventually mapped about half of the planet in unprecedented detail, revealing that the planet looked very similar to the Earth’s moon, with many impact craters and ancient lava fields.
If you’re wondering who discovered the element mercury, nobody knows that either. The element has been known for thousands of years, and was used by the ancient Chinese. Liquid mercury was found in Egyptian tombs closed up almost 4,000 years ago.
A High-resolution Look over Mercury's Northern Horizon. Credit: MESSENGER team
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When you look at an image of Mercury, it looks like a dry, airless world. But you might be surprised to know that Mercury does have an atmosphere. Not the kind of atmosphere that we have here on Earth, or even the thin atmosphere that surrounds Mars. But Mercury’s atmosphere is currently being studied by scientists, and the newly arrived MESSENGER spacecraft.
Mercury’s original atmosphere dissipated shortly after the planet formed 4.6 billion years ago with the rest of the Solar System. This was because of Mercury’s lower gravity, and because it’s so close to the Sun and receives the constant buffeting from its solar wind. Its current atmosphere is almost negligible.
What is Mercury’s atmosphere made of? It has a tenuous atmosphere made up of hydrogen, helium, oxygen, sodium, calcium, potassium and water vapor. Astronomers think this current atmosphere is constantly being replenished by a variety of sources: particles of the Sun’s solar wind, volcanic outgassing, radioactive decay of elements on Mercury’s surface and the dust and debris kicked up by micrometeorites constantly buffeting its surface. Without these sources of replenishment, Mercury’s atmosphere would be carried away by the the solar wind relatively quickly.
Mercury atmospheric composition:
Oxygen 42%
Sodium 29%
Hydrogen 22%
Helium 6%
Potassium 0.5%
With trace amounts of the following:
Argon, Carbon dioxide, Water, Nitrogen, Xenon, Krypton, Neon, Calcium, Magnesium
In 2008, NASA’s MESSENGER spacecraft discovered water vapor in Mercury’s atmosphere. It’s thought that this water is created when hydrogen and oxygen atoms meet in the atmosphere.
Two of those components are possible indicators of life as we know it: methane and water vapor(indirectly). Water or water ice is believed to be a necessary component for life. The presence of water vapor in the atmosphere of Mercury indicates that there is water or water ice somewhere on the planet. Evidence of water ice has been found at the poles where the bottoms of craters are never exposed to light. Sometimes, methane is a byproduct of waste from living organisms. The methane in Mercury’s atmosphere is believed to come from volcanism, geothermal processes, and hydrothermal activity. Methane is an unstable gas and requires a constant and very active source, because studies have shown that the methane is destroyed in less than on Earth year. It is thought that it originates from peroxides and perchlorates in the soil or that it condenses and evaporates seasonally from clathrates.
Despite how small the Mercurian atmosphere is, it has been broken down into four components by NASA scientists. Those components are the lower, middle, upper, and exosphere. The lower atmosphere is a warm region(around 210 K). It is warmed by the combination of airborne dust(1.5 micrometers in diameter) and heat radiated from the surface. This airborne dust gives the planet its ruddy brown appearance. The middle atmosphere contains a jetstream like Earth’s. The upper atmosphere is heated by the solar wind and the temperatures are much higher than at the surface. The higher temperatures separate the gases. The exosphere starts at about 200 km and has no clear end. It just tapers off into space. While that may sound like a lot of atmosphere separating the planet from the solar wind and ultraviolet radiation, it is not.
Helping Mercury hold on to its atmosphere is its magnetic field. While gravity helps hold the gases to the surface, the magnetic filed helps to deflect the solar wind around the planet, much like it does here on Earth. This deflection allows a smaller gravitational pull to hold some form of an atmosphere.
The atmosphere of Mercury is one of the most tenuous in the Solar System. The solar wind still blows much of it away, so sources on the planet are constantly replenishing it. Hopefully, the MESSENGER spacecraft will help to discover those sources and increase our knowledge of the innermost planet.
