Mercury
Mercury
Mercury is the closest planet to the Sun, and the second smallest in the Solar System. Most of the scientific findings about Mercury comes from the Mariner 10 spacecraft which was launched on November 3, 1973. It flew past the planet on March 29, 1974 at a distance of 705 km (438 miles) from the surface.
The pictures returned from the Mariner 10 spacecraft showed a world that resembles the Moon. It is pocked with craters, contains hugh multi-ring basins, and many lava flows. The craters range in size from 100 m to 1,300 km (328 ft - 807 miles). They are in various stages of preservation. Some are young with sharp rims and bright rays extending from them. Others are highly degraded, with rims that have been smoothed from the bombardment of meteorites. The largest crater on Mercury is the Caloris basin.
Mercury's thin atmosphere consist of trace amounts of hydrogen and helium. Since the atmosphere is so slight, the sky would appear pitch black (except for the sun, stars, and other planets, when visible), even during the day. Also, there is no "greenhouse effect" on Mercury. When the sun sets, the temperature drops very quickly since the atmosphere does not help retain the heat.