Mercury

MERCURY IS THE NEAREST PLANET to the Sun, orbiting at an average distance of about 58 million kilometers. Because Mercury is the closest planet to the Sun, it moves faster than any other planet, travelling at an average speed of nearly 48 kilometer per second and completing an orbit in just under 88 days. Mercury is very small (only 40 per cent bigger than the Moon) and rocky. Most of the surface has been heavily cratered by the impact of meteorites, although there are also smooth, sparsely crater, measuring about 1,300 kilometers across. It is thought to have been formed when a 60-kilometers-diameter asteroid hit the planet, and is surrounded by concentric rings of mountains thrown up by the impact.

The surface also has many cliff-like ridges (called rupes) that are thought to have been formed when the hot core of the young planet cooled and shrank about four billion years ago, buckling the planet's surface in the process. The planet rotates about its axis very slowly, taking nearly 59 Earth days to complete one rotation. As a result, a solar day ( sunrise to sunrise ) on Mercury is about 176 Earth days - twice as long as the 88- day Mercurian year. Mercury has extreme surface temperature, ranging from a  maximum of 430 degree celsius on the sunlit side to - 170 degree celsius on the dark side. At nightfall, the temperature drops very quickly because the planet's atmosphere is almost non-existent. It consist only of minute amounts of helium and hydrogen captured from the solar wind, plus traces of other gases.