- MEAN RADIUS: 2439.7 km
- MASS: 0.055 (Earth=1)
- DENSITY: 5.43 (g/cm^3)
- GRAVITY: 0.376 (Earth=1)
- ORBIT PERIOD: 87.97 (Earth days)
- ROTATION PERIOD: 58.65 (Earth days)
- SEMIMAJOR AXIS OF ORBIT: 0.387 au
- ECCENTRICITY OF ORBIT: 0.206
Named after the Roman god of commerce, travel and thievery, Mercury is the closest planet to the Sun and the eighth largest. Mercury's existence has been known of since before the third century BC. The Greeks gave it two names, Apollo for when it appeared as a morning star and Hermes when it came as an evening star.
Mercury has a large iron core which is most likely at least partially molten. The silicate outer shell is only 500 to 600 km thick. It actually has a very thin atmosphere made up of atoms blasted off its surface by the solar wind. Due to the heat of the planet, these atoms quickly escape into space. Thus unlike the Earth and Venus which have stable atmospheres, Mercury's atmosphere is constantly being replenished.
Mercury is a planet of extremes. Temperature variations swing from 90 K to 700 K. It's hotter on Venus, but with less fluctuations. Mercuryalso has a very eccentric orbit; at perihelion it is only 46 million km from the Sun but at aphelion it is 70 million.
Mercury has no known moons or satellites.
