Cosmic Quest - TCM: Planet Mercury More Information

MERCURY

Mercury is the closest planet to the Sun. It has a diameter of 3,032 miles (4,879 kilometers), about two-fifths of Earth's diameter. Mercury is smaller in diameter than Jupiter's moons, Ganymede and Titan.

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). Mercury was given two names by the Greeks: Apollo for its apparition as a morning star and Hermes as an evening star.

To date 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 the Hubble Space Telescope). But In 2004, the United States launched the Messenger probe to Mercury which was scheduled to flyby Mercury twice in 2008 and once in 2009 before entering into orbit around the planet in 2011. With it completing its flybys and preparing to orbit the planet now, Messenger has returned some very detailed images and is preparing to begin mapping Mercury's surface. It will also study Mercury's composition, interior structure, and magnetic field upon entering orbit.

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. It is now known that Mercury rotates three times in two of its years.

Temperature changes on Mercury are the most extreme in the solar system ranging from -300°F to 800°F. 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.

Mercury's interior is mostly a large iron core whose radius is 2,800 to 3,000 miles. The silicate outer shell (the same as Earth's mantle and crust) is only 750 to 900 miles 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. Unlike the Earth and Venus whose atmospheres are stable, Mercury's atmosphere is constantly being replaced.

The surface of Mercury exhibits enormous canyons, some up to hundreds of miles in length and as much as 4 ½ miles high. One of the largest features on Mercury's surface is the Caloris Basin; it is about 900 miles in diameter. It is similar to the large basins (maria) on the Moon. 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.

In addition to the heavily cratered terrain, Mercury also has regions of relatively smooth plains. Perhaps from the result of ancient volcanic activity. 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.