Sen—Mercury is a tortured small rock, subjected to violently hot and unimaginably cold temperatures, a planet battered and scared by meteorites.
Mercury is the closest planet to the Sun. It orbits the Sun very quickly and takes just 88 Earth days to orbit the Sun once, travelling at an orbital speed of 170,000 kilometres per hour (105,000 mph).
Mercury is the smallest planet in the solar system - about 4,900 kilometres (3,000 miles) across.
Although Mercury orbits the Sun quickly, a day on Mercury is 58.6 days. This slow rotation means that daytime and night time on Mercury each last nearly 29 days.
Night time is freezing cold with a surface temperature as low as -173 degrees Celsius. During day time (the side of the planet facing the Sun) the temperature is extremely hot with temperatures reaching 427 degrees Celsius. Mercury has the largest temperature 'swing' between night and day of any of the eight planets. The reason it is so cold at night is because Mercury has no atmosphere to trap heat - so all the heat absorbed during the day radiates quickly into space leaving the surface in the dark very cold indeed.
Mercury has no atmosphere and cannot support any form of life that we know of due to its extreme temperatures and lack of water and atmosphere. Mercury had an atmosphere when it was formed but lost it because its gravity was not strong enough to hang on to it. (Atmospheres are stopped from disappearing off into space because the gravitational forces of the planet hold on to it). Mercury's weak gravity (due to its small mass) would also have had a harder job trying to hold on to its gas shell because of Mercury's proximity to the Sun. The extreme high temperatures burning down on Mercury meant that the gas molecules that formed its atmosphere would have moved quickly (temperature being a measure of the speed with which things move), making it even harder for the weak gravitational force to hold on to its protective gas layer. The atmosphere therefore drifted off into space a long time ago, leaving Mercury exposed to the solar winds and impacts for meteorites and asteroids.
Without an atmosphere Mercury is defenceless against the impact of meteorites because there is nothing to cushion the blows and slow down the incoming rocks, meaning Mercury's surface has been scarred badly and is covered by many craters.