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Diameter: 4,878 km
Distance from Sun: 57,910,000 km
Surface area: 74,797,000 km²
Length of day: 58d 15h 30m
Gravity: 3.7 m/s²
Circumference: 15,329.1 km
Mass: 328.5E21 kg (0.055 Earth mass)
Length of Year: 87.97 Earth Days
Surface temp: -173 to 427 C

Mercury is the closest planet to the Sun and the  eighth largest. Mercury is slightly smaller in diameter than the moons Ganymede and Titan but  more than twice as massive.

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 sometimes given separate names for its apparitions  as a morning star and 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.

Since it is closer to the Sun than the Earth, the illumination of Mercury’s  disk varies when viewed with a telescope from our perspective.  Galileo’s  telescope was too small to see Mercury’s phases but he did see the phases of Venus.

Mercury has been now been visited by two spacecraft,  Mariner  10 and  MESSENGER.  Marriner 10 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).  MESSENGER  was launched by NASA in 2004 and will orbit Mercury starting in 2011 after  several flybys.  Its first flyby in Jan 2008 provided new high quality images of some of the terrain not seen by Mariner 10.

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 position of the perihelion 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)  slightly closer to  the Sun than Mercury might  account for the discrepancy.  But despite  much effort, no such  planet was found.

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).

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.

Mercury craters

Mercury craters

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 gm/cm3 vs 3.34).  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 is probably  molten.

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.  Thus in contrast to the Earth  and Venus whose atmospheres are stable, Mercury’s atmosphere is constantly being  replenished.

Southwest Mercury

Southwest Mercury

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.  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).

Caloris Basin, Mercury

Caloris Basin

One of the largest features on  Mercury’s surface is the Caloris Basin (right); it 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.

Western Caloris Basin, Mercury

Weird terrain opposite Caloris Basin

That impact was probably also responsible  for the odd terrain on the exact  opposite side of the planet (left).

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.

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.

Mercury has a small magnetic field whose strength is about 1% of Earth’s.

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.



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