Mercury

It is the closest planet to the Sun and the smallest planet in the Solar System. Mercury is an extreme planet: the smallest, the second highest in the Solar System, the one with the oldest surface, the one with the largest daily variations in surface temperature, and the least explored. Mercury, like Earth, has a global internal magnetic field. Mars and Venus do not. Only less than half of the surface of Mercury had been imaged by a spacecraft until 2009.

Mercury is one of four terrestrial planets in the Solar System, and is a rocky body like Earth. Mercury consists of approximately 70% metallic and 30% silicate material. Geologists estimate that Mercury's core occupies about 42% of its volume; for Earth this proportion is 17%. Research published in 2007 suggests that Mercury has a molten core. Surrounding the core is a 500–700 km mantle consisting of silicates. Based on data from the Mariner 10 mission and Earth-based observation, Mercury's crust is believed to be 100–300 km thick.

Mercury's surface is very similar in appearance to that of the Moon, showing extensive mare-like plains and heavy cratering, indicating that it has been geologically inactive for billions of years. Prior to 2009, our knowledge of Mercury's geology had been based only on the 1975 Mariner flyby and terrestrial observations.