Mercury is the first planet from the Sun and the smallest in the Solar System. The surface of Mercury is heavily cratered, as a result of countless impact events that have accumulated over billions of years. Its largest crater, Caloris Planitia, has a diameter of 1,550 km (960 mi) and one-third the diameter of the planet (4,880 km or 3,030 mi). Mercury is classified as a terrestrial planet.
Similar to the Earth's Moon, Mercury's surface displays an expansive rupes system generated from thrust faults and bright ray systems formed by impact event remnants. By having a very small axial tilt, the planet's poles are permanently shadowed. This strongly suggests that water ice could be present in the craters. Mercury has a faint magnetic field that is strong enough to deflect solar winds, which is probably generated from the core dynamo interactions. Above the planet's surface is an extremely tenuous exosphere.
Mercury's sidereal year (88.0 Earth's day) and sidereal day (58.65 Earth's day) is in a 3:2 ratio. This phenomenon is called spin–orbit resonance and sidereal here means "relative to the stars". Combined with its orbital eccentricity, the planet surface has widely varying sunlight intensity and temperature, with the equator regions range from −170 °C (−270 °F) at night to 420 °C (790 °F) during sunlight. Mercury has no natural satellite. (Full article...)