Mercury

The Winged Messenger

Mercury Facts

• Mercury is the closest planet to the Sun and the eighth largest:
  • average distance from Sun: 57,910,000 km (0.38 AU)
  • diameter: 4,878 km
  • mass: 3.30e23 kg
  Mercury is smaller in diameter than Ganymede and Titan but more massive.

• In Roman mythology Mercury is the god of commerce 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 apparition 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.

• Mercury has been visited by only one spacecraft, Mariner 10. It flew by three times in 1973 and 1974. Only 45% of the surface was mapped (picture 1, above).

• 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 be captured in a ratio other than 1:1.

• 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 is in many ways similar to the Moon: its surface is heavily cratered (picture 5) and very old; it has no atmosphere; it exhibits 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 (analagous to Earth's mantle and crust) is only 500 to 600 km thick. At least some of the core is probably molten.

• The surface of Mercury exhibits enormous escarpments, some up to hundreds of kilometers in length and as much as three kilometers high (picture 9). 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).

• One of the largest features on Mercury's surface is the Caloris Basin (picture 2); 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. That impact was probably also responsible for the odd terrain on the exact opposite side of the planet (picture 4).

• 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.

• There is no evidence of recent volcanism on Mercury.

• 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 sometimes visible with binoculars or even the naked eye, but it is always very near the Sun and difficult to see in the twilight sky. Mike Harvey's planet finder charts show the current position of Mercury (and the other planets) in the sky.

Pictures

1. (above) Mosiac of photos taken from Mariner 10 in 1974 16k jpg; 110k gif

2. Caloris Basin 319k gif; 238k jpg; 108k jpg

3. Southwest Mercury 31k jpg; 1000k gif; 488k gif

4. Hills of Mercury 127k jpg; 185k jpg; 614k gif

5. Mercury Close Up 31k jpg

6. another mosaic 24k jpg
7. yet another mosaic 44k jpg

8. Mosaic of the Discovery quadrangle of Mercury 73k jpg
9. Close up of Discovery Rupes 34k jpg
10. south polar mosaic 335k gif
11. (low res) images from the 0.5 m Swedish Vacuum Solar Telescope html
12. ... more Mercury images

More about Mercury

• from ASU
• from LANL
• from JPL
• from RPIF
• from RGO
• from NSSDC
• from NASA Spacelink
• Mercury Nomenclature Table

Open Issues

• Mercury's density (5.43 gm/cm3) is nearly as high as Earth's. Yet in most other respects it more closely resembles the Moon. Did it lose its light rocks in some early catastrophic impact?
• What processes produced Mercury's smooth plains?
• Are there any surprises on the other 1/2 of the surface we've not seen? Low resolution radar images obtained from Earth show no surprises, but you never know.
• Recently a proposal for two new Mercury missions has been put forward to fly in 1999.