Mercury is the smallest planet in the Solar System and the closest to the Sun. It is a rocky, terrestrial planet with a heavily cratered surface similar to Earth's Moon. Its diameter is about 4,880 km (3,030 miles), making it considerably smaller than Earth. Mercury has a very high density, second only to Earth, largely due to its large iron-rich core which takes up about 57% of its volume. The core is thought to be partially molten and is responsible for Mercury's weak magnetic field. Mercury has a very thin atmosphere, or exosphere, made up of atoms like hydrogen, helium, oxygen, sodium, and potassium, which are continuously replenished from the surface and solar wind. Mercury's orbit is highly eccentric, and it rotates on its axis in a 3:2 spin- orbit resonance, meaning it rotates three times on its axis for every two orbits around the Sun. This results in one solar day on Mercury lasting about 176 Earth days, twice its year length of 88 Earth days. Surface temperatures vary widely, from extremely hot (up to 427°C or 800°F) on the side facing the Sun to extremely cold (around -170°C or -270°F) on the night side. Its very small axial tilt means Mercury does not experience seasons like Earth and has permanently shadowed craters at its poles that likely contain water ice despite the planet's proximity to the Sun. Mercury also has comet-like tails of sodium atoms pushed off its surface by solar wind.
