Mercury

Mercury is the planet closest to the Sun and the smallest of the eight planets in the solar system. It is a rocky planet with a heavily cratered surface, similar to that of the Moon. Because it is so close to the Sun, when viewed from Earth it appears mostly only during twilight, on the eastern or western horizon.

Mercury's orbit around the Sun takes approximately 88 Earth days, the shortest orbital period of all planets. Its orbit is elliptical and highly eccentric, which means the distance between Mercury and the Sun varies considerably.

Orbital resonance

Mercury rotates on its axis in a 3:2 resonance with the Sun: it completes three rotations on its axis for every two orbits around the Sun. As a result, a Mercurial day is extremely long relative to its year. Its axial tilt is very small, meaning it has no seasons like those on Earth.

Atmosphere and temperature

Mercury has no substantial atmosphere. It has only an extremely thin layer called an exosphere, composed of atoms including hydrogen, helium, oxygen, sodium, and potassium, which come from the solar wind and from atoms knocked loose from the surface by impacts. Because the atmosphere is so thin, there is no temperature regulation: the side facing the Sun grows intensely hot, while the night side cools dramatically. The craters in the polar regions, which are permanently in shadow, are thought to contain water ice.

Internal structure

Relative to its size, Mercury has an unusually large iron core, making it a very dense planet for its volume. It also possesses a weak magnetic field generated from within—a feature not seen in other rocky planets aside from Earth.

Space exploration

The most notable spacecraft to have visited Mercury are Mariner 10, which made three close passes in the 1970s, and MESSENGER, which orbited Mercury for several years and produced a complete map of the surface. The BepiColombo mission, a joint project of the European and Japanese space agencies, is designed to conduct detailed study of Mercury.