Mercury ... metal-rich world1


Above, Mercury as imaged by Messenger in real color to the left and enhanced to the right. Below is Mercury as imaged by Mariner 10 in 1974-1976.

Introduction to Mercury

Planet Mercury is the closest planet to the sun, and most would think it to be the most hot, however that title is reserved for Venus. A complete lack of an atmosphere means that any incoming solar radiation escapes back into space at night. Several things distinguish Mercury from all other planets. In appearance, it looks like our Moon, with craters and maria. The only difference is that our Moon has larger maria. Mercury has the greatest temperature extremes of any planet, with a daytime high of 700 K, and nighttime cold of 90 K. Mercury is the ONLY planet to spin on its axis without any tilt. While you might think this would mean no seasons, Mercury's orbit is so elliptical, that "seasons" occur due to the great difference between aphelion and perihelion. Finally, Mercury is the ONLY planet tidally locked in a 3 rotation:2 revolution:1 solar day motion. You will learn more about this later. In the more recent news is the radar discovery of water ice in the polar craters!

Planetary data

Mass (kg), and mass relative to Earth

3.303x1023 kg = 0.553 earths

Equatorial diameter (km)

4879.4 km

Mean density (gm/cm3)


Acceleration of gravity (m/s2)


Velocity of escape (km/s)


Period of rotation

58.646225 days

Period of revolution

.2408467 years

Aphelion (AU)


Aphelion (km)


Perihelion (AU)


Perihelion (km)


Mean orbital distance from the sun (AU)


Mean orbital distance from the sun (km)


Orbital velocity (km/s)






Inclination to the ecliptic

7.00487 degrees

Inclination of the equator to the orbit

0.0 degrees

Number of natural satellites


Names of natural satellites

not applicable

More Information on the Planet Mercury from the Nine Planets Website

Much of the information below is direct from the Nine Planets Website. Some material has been altered by me for this course, while other items and comments are directly copied. I hope to maintain a continuous update of this material to keep up with the findings from space satellites and telescopes.

Mercury is the closest planet to the Sun and the eighth largest. Mercury is smaller in diameter than Ganymede and Titan but more massive. You are at a nice mosaic the planet, and it is quite clear that Mercury looks like the Moon in color and cratering, with the exception of large lava plains, or maria, that are some common on the Earth's Moon.


orbit: 57,910,000 km (0.38 AU) from Sun
diameter: 4,880 km
mass: 3.30x1023 kg











In Roman mythology Mercury is the god of commerce, travel and thievery, the Roman counterpart of the Greek god Hermes, the messenger of the gods. The planet probably received this name because it moves so quickly across the sky.

Mercury has been known since at least the time of the Sumerians (3rd millennium BC). It was given two names by the Greeks: Apollo for its appearance as a morning star and Hermes as an evening star. Greek astronomers knew, however, that the two names referred to the same body. Heraclitus even believed that Mercury and Venus orbit the Sun, not the Earth. Just to clarify this concept of being in the morning and evening sky, Mercury spent a few weeks as an evening star, close to the horizon, and later a few weeks as a morning sky, close to the horizon. The comment does not mean that Mercury could be seen in the morning AND evening sky in the same 24 hour period.


Mercury has been visited by only one spacecraft, Mariner 10. It flew by three times in 1974 and 1975. Only 45% of the surface was mapped (and, unfortunately, it is too close to the Sun to be safely imaged by HST). The reasons for the lack of more surface pictures is due to the unusual chance occurance that the spacecraft flew by Mercury on three separate occasions, but each time the same side of Mercury was lit by the Sun. After a third fly by, spacecraft failure cancelled communications between ground stations and the satellite. Mariner 10 may still be in its original orbit, but we are not able to communicate with it.




Mercury Messenger = Mercury Surface, Space Environment, Geochemistry, and Ranging

The Messenger spacecraft was launched August 3, 2004 and inserted into orbit March 18, 2011.


The image below is a global map of the entire surface of the planet taken over the first two years of the Messenger mission following orbital insertion. I kept this huge picture in here because it is really fun to explore. Click here to learn more about this image.

global mercury

MERCURY - Planet overview in one picture :)

mercury overview

Please note that there is some difference in the temperature data for Mercury from the most widely accepted values. The source of the image can be obtained by clicking on the image itself.


Mercury's orbit is highly eccentric; at perihelion it is only 46 million km from the Sun but at aphelion it is 70 million. The perihelion of its orbit precesses around the Sun at a very slow rate. 19th century astronomers made very careful observations of Mercury's orbital parameters but could not adequately explain them using Newtonian mechanics. The tiny differences between the observed and predicted values were a minor but nagging problem for many decades. It was thought that another planet (sometimes called Vulcan) might exist in an orbit near Mercury's to account for the discrepancy. The real answer turned out to be much more dramatic: Einstein's General Theory of Relativity! Its correct prediction of the motions of Mercury was an important factor in the early acceptance of the theory.

Until 1962 it was thought that Mercury's "day" was the same length as its "year" so as to keep that same face to the Sun much as the Moon does to the Earth. But this was shown to be false in 1965 by doppler radar observations. It is now known that Mercury rotates three times in two of its years. Mercury is the only body in the solar system known to have an orbital/rotational resonance with a ratio other than 1:1 (though many have no resonances at all).

The image to your left shows how the Mercury day and year are related in this 3:2 ratio. Following the planet from position #1, where the blue triangle points directly at the Sun, the time at this position would be noon. As the planet is rotating counterclockwise, it is also moving around the Sun counterclockwise, and after Mercury has completed one half of a turn, it has moved to position #3, but instead of being midnight like it would be on Earth, it is still almost noon. By the time the planet has completed one full turn (or day), it has moved to position #5, and yet the Sun is not over head in the sky. When the planet is at position #7, the triangle is in total darkness, or midnight. The planet has completed one full revolution, or year, and one and a half days, but it has not yet gotten to a place where the Sun is directly overhead again. By the time Mercury has moved to position #10, it has completed two full rotations, or days, but still the Sun is not overhead. Only when Mercury is at position #15, or back to its starting position, is the Sun back in the noon position as it was when this paragraph began. This Mercury spins three times (3 days) and orbits two times (2 years) in order to complete one "solar day." You can therefore define time either by the spin, or the orbit, or by the Sun's movement.

This fact and the high eccentricity of Mercury's orbit would produce very strange effects for an observer on Mercury's surface. At some longitudes the observer would see the Sun rise and then gradually increase in apparent size as it slowly moved toward the zenith. At that point the Sun would stop, briefly reverse course, and stop again before resuming its path toward the horizon and decreasing in apparent size. All the while the stars would be moving three times faster across the sky. Observers at other points on Mercury's surface would see different but equally bizarre motions.


Temperature variations on Mercury are the most extreme in the solar system ranging from 90 K to 700 K. The temperature on Venus is slightly hotter but very stable. To put this into perspective, the night temperatures on Mercury drop to -300 F and rise during the day to 800 F. This 1100 F difference between night and day temperatures, as well as between sunlight and shadow is the greatest extreme in the solar system.


Mercury is in many ways similar to the Moon: its surface is heavily cratered and very old; it has no plate tectonics. On the other hand, Mercury is much denser than the Moon (5.43 g/cm^3 vs 3.34 g/cm^3). Mercury is the second densest major body in the solar system, after Earth. Actually Earth's density is due in part to gravitational compression; if not for this, Mercury would be denser than Earth. This indicates that Mercury's dense iron core is relatively larger than Earth's, probably comprising the majority of the planet. Mercury therefore has only a relatively thin silicate mantle and crust.





Mercury's interior is dominated by a large iron core whose radius is 1800 to 1900 km. The silicate outer shell (analogous to Earth's mantle and crust) is only 500 to 600 km thick. At least some of the core may probably be molten, for astronomers can detect a very weak magnetic field, but there is not enough internal activity to cause any form of plate tectonics that are so important to the Earth.






Mercury actually has a very thin atmosphere consisting of atoms blasted off its surface by the solar wind. Because Mercury is so hot, these atoms quickly escape into space. Mercury has so little "air" that is you were to take the amount of gases in an average room of your house and spread it out over the entirety of Mercury's surface, it would be about the right density of the atmosphere. The actual "atmosphere" is not really gaseous like ours, but made up of atoms that are excited out of the rocks by the Sun's incoming energy. Thus in contrast to the Earth and Venus whose atmospheres are stable, Mercury's atmosphere is constantly being replenished.


The surface of Mercury exhibits enormous escarpments, some up to hundreds of kilometers in length and as much as three kilometers high. Some cut thru the rings of craters and other features in such a way as to indicate that they were formed by compression. Scientists believe that the exterior of the planet cooled and hardened before the interior. When the interior finally cooled, it shrank, as do all solids relative to liquids except water. With a now smaller interior than the exterior, the crust caved in as the interior shrank, causing these scarps to form. It is estimated that the surface area of Mercury shrank by about 0.1% (or a decrease of about 1 km in the planet's radius). These scarps are one of the distinctive features of Mercury's surface.




To your left is a clost-up of this scarp.

Another nice scarp is seen in this image to your left, taken in the Discovery Quadrant of Mercury. It is rougly 500 km long.






The largest feature on Mercury's surface is the Caloris Basin (left). In fact, it is the largest impact crater in the entire solar system, and its width is about 1300 km in diameter. It is thought to be similar to the large basins (maria) on the Moon. Like the lunar basins, it was probably caused by a very large impact early in the history of the solar system. That impact was probably also responsible for the odd terrain on the exact opposite side of the planet (left). To see a larger version of this gigantic impact site, click on the image.

To get an idea of the magnitude of the impact, imagine the force of a hard thrown rock on a melon. The rock would bore a hole into the melon and put a bulge out of the opposite site. So too was the result of this rock that struck Mercury. The force of the impact was so great that it blew out a portion of the farside of the planet, cracking and breaking the surface. Indeed, images of that backside show a jumbled region of faults and ridges called the "chaotic terrain."

In addition to the heavily cratered terrain, Mercury also has regions of relatively smooth plains. Some may be the result of ancient volcanic activity but some may be the result of the deposition of ejecta from cratering impacts.

A reanalysis of the Mariner data provides some preliminary evidence of recent volcanism on Mercury. But more data will be needed for confirmation. To see a terrific collection of photographs of Mercury from the Mariner 10 mission, go to more Mercury pictures.


Amazingly, radar observations of Mercury's north pole (a region not mapped by Mariner 10) show evidence of water ice in the protected shadows of some craters. It would appear that Mercury could not support water in any form. It has very little atmosphere and is blazing hot during the day, but in 1991 scientists at Caltech bounced radio waves off Mercury and found an unusual bright return from the north pole. The apparent brightening at the north pole could be explained by ice on or just under the surface. But is it possible for Mercury to have ice? Because Mercury's rotation is almost perpendicular to its orbital plain, the north pole always sees the sun just above the horizon. The insides of craters would never be exposed to the Sun and scientists suspect that they would remain colder than -161 C. These freezing temperatures could trap water outgassed from the planet, or ices brought to the planet from cometary impacts. These ice deposits might be covered with a layer of dust and would still show bright radar returns.

Mercury has a small magnetic field whose strength is about 1% of Earth's implying that a portion of the interior may still be molten.

Mercury has no known satellites.

Mercury is often visible with binoculars or even the unaided eye, but it is always very near the Sun and difficult to see in the twilight sky. There are several Web sites that show the current position of Mercury (and the other planets) in the sky. More detailed and customized charts can be created with a planetarium program such as Starry Night.

Mercury Messenger = Mercury Surface, Space Environment, Geochemistry, and Ranging

Below are some of my favorite images from the Messenger mission. Click on the images to learn more about each one.

rings in rings

in and out of caloris

rayed horizon

spacecraft graveyard

This image above is exceptional, and click on it to learn just what you are seeing here. After you are done wondering how different spacecraft that has been lost in space winds up on the surface of Mercury and all in the same area, please look at the date the image was released to the public.

Much of the information shown above is from the Nine Planets website, and courtesy of author Bill Arnett. Please go to that site to find any updates I might not have caught.

I want to encourage you to scour these websites to learn some of the more interesting things pertaining to Mercury. These are the questions I would like you to discover answers for.

1) Give a brief description of the physical appearance of Mercury.

2) What is the planet's interior structure?

3) Is there any evidence of present or past geological activity?

4) What is the surface like, and what would it be like to be outside during a Mercury day?

5) What is the atmosphere made of, and what are the conditions in that atmosphere?

6) What is the most prominent feature on Mercury?

7) What spacecraft have visited the planet, and when?

8) What is the mythological origin of this planet's name?

9) What makes manned missions to Mercury so difficult?

10) Earth-based radar has discovered something interesing in the polar craters of Mercury. What was discovered and how might it have gotten there?

When you think you know the answers to these questions, please go to the Mercury Quiz and send your answers to me.

Sites to explore to learn more about Mercury.

Mercury from JPL

Mercury from SSE

Mercury from Nine Planets

Mercury from Solarviews

Mariner 10

Forward to Venus, back to either the Introduction to the Inner Rocky Worlds, or to Planet Introduction, or even the Syllabus.

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