OUR SUN	MERCURY	VENUS	EARTH	MARS
JUPITER	SATURN	URANUS	NEPTUNE	PLUTO

MERCURY

         Mercury is so close to the sun that it is difficult to observe in the glare of the Sun's rays. It revolves once around the Sun every 88 days in an eccentric orbit that takes it very close to the Sun. Its period of rotation is 58.6 days. Since the planet's spin is exactly two thirds of its orbital period, the same region on Mercury returns to face the same point on the Sun every second perihelion passage. Each Mercurian day is 176 Earth days long.

         Mercury's radius is only 2,440 km, making it the smallest planet after Pluto. However, it is the second densest of the planets, hence its interior must contain a large metallic core extending to some 75 percent of the planet's radius. Surface gravity is about one third that of the Earth. The surface of Mercury is moon-like, heavily cratered and made of dark, grey rock. The planet has no real atmosphere, although extremely small amounts of helium, comparable in density to the Earth's atmosphere 200 km up, have been detected. Daytime temperatures on Mercury reach 970 degrees, dropping at night to -170 degrees.

         A distinctive class of Mercurian surface features are long scarps, often 2-3 km high and several hundred kilometres in extent. These are thought to be the result of global stresses on the planet's crust, perhaps associated with the cooling and shrinking of Mercury's presumed large metallic core or with the stresses involved in the slowing down of the planet's fast original spin rate by solar winds. While extensive areas of lava flow have been identified on the surface, no evidence of other volcanic features, such as domes or lava tubes, have been recognised.

         Mercury has a relatively weak magnetic field, about 3 percent that of the Earth, indicating, perhaps, that part of the planet's metallic core is still fluid. Mercury has no known satellites.

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