MERCURY

Mercury's year (the time it takes to orbit the Sun) takes 88 Earth days. So a year on Mercury is less than a quarter of a year on Earth. Mercury spins slowly. Each day (the time it takes to spin around once) is equal to 59 Earth days. There are fewer than two days in the Mercury year. The long periods facing away from the Sun explain why it gets so cold at night.

Mariner 10 mapped the surface of Mercury in detail. The probe discovered that the planet has no atmosphere surrounding it and no water, so nothing could possibly live there. Its bareen, rocky surface is covered with sharpedged craters.

Closest to the Sun is a small rocky planet called Mercury. It orbits at a distance from the Sun of about 58 million km. Mercury is not much larger than our Moon and has a similar surface, pock marked with craters left behind as rocks and comets crashed into the planet early in the history of the Solar System. The Sun blasts Mercury with heat and light and other forms of radiation, making it a very inhospitable place.