MERCURY

 Named after the Roman Messenger God, Mercury is the innermost and second smallest planet in the solar system. It is also one of the four terrestrial planets of the solar family. It has no known satellites. Mercury has the most eccentric and inclined (7 degrees) orbit of any planet except Pluto. Mercury’s rotation period is 58.646 Earth days and its orbital period is 87.969 Earth days. Because Mercury is so close to the Sun, has no insulating atmosphere, and has such a long solar day, it experiences the greatest range (day to night) in surface temperatures (635 K) of any planet or satellite in the solar system. Its maximum surface temperature reaches about 725 K at perihelion on the equator, but drops to about 90 K at night.

To date, Mariner 10 is the only space craft that has explored the planet. It flew past Mercury on 29 March and 21 September 1974, and on 16 March 1975. Because of its close proximity to the Sun, missions to Mercury are extremely difficult; more so than to the outer solar system.

Mercury’s diameter is only 4878 km, but it has a relatively large mass of 3.301 × 1023 kg. Because of its large mass in relation to its volume, Mercury has an exceptionally high mean density of 5.44 g cm−3. The manner in which the planet reflects light is very similar to the way light is reflected by the Moon. Mercury is covered with a regolith consisting of fragmented material derived from the impact of meteoroids over billions of years. Mercury’s surface is heavily cratered with smooth planes filling and surrounding large impact basins. One of the largest and most prominent features on Mercury's surface is the Caloris Basin; it is about 1300 km in diameter. It is supposed to be similar to the large basins (Maria) on the Moon. Like the lunar basins, it was probably due to a very large asteroid impact early in the history of the solar system. That impact was possibly also responsible for the odd terrain on the exact opposite side of the planet.

In general, the surface of Mercury can be classified into four major terrains: (1) heavily cratered regions, (2) intercrater plains, (3) smooth plains, and (4) hilly and lineated terrain. Other relatively minor units have been observed, such as ejecta deposits exterior to the Caloris basin and other large impact craters. Another peculiarity of Mercury is it’s has a large iron core, which generates a magnetic field about 1% as strong as that of the Earth. It is also interesting to note that 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.