Mercury

The Winged Messenger

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

        orbit:    57,910,000 km (0.38 AU) from Sun
        diameter: 4,880 km
        mass:     3.30e23 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 visited by only one spacecraft, Mariner 10. It flew by three times in 1973 and 1974. Only 45% of the surface was mapped (and, unfortunately, it is too close to the Sun to be safely imaged by HST).

   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.

   This fact and the high eccentricity of Mercury's orbit would produce very strange effects for an observer on Mercury's surface. At some places the observer would see the Sun rise and then gradually increase in apparent size as it slowly moved toward the zenith (highest point in the sky). 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 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.

   Mercury's interior is mainly a large iron core whose radius is 1800 to 1900 km. The silicate outer shell (similar 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 renewed.

   The surface of Mercury exhibits enormous escarpments, some up to hundreds of kilometers in length and as much as three kilometers high. Some cut through 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 (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. 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 dust and rocks thrown up by meteorites hitting the surface.

   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 naked eye, but it is always very near the Sun and difficult to see in the twilight sky.

• 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?
• No trace of iron has been seen in spectroscopic studies of Mercury's surface. Given its presumably large iron core this is very odd. Is Mercury much more completely differentiated than the other terrestrial planets?
• What processes produced Mercury's smooth plains?
• Are there any surprises on the other half 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. Several other proposals have been rejected for cost reasons.