Mercury

Mercury is the closest planet to the sun. Its distance from the sun varies throughout its orbit, from 46 million km away at perihelion to 70 million km away at aphelion. It is the second smallest planet in the solar system, yet it has a very high density in comparison to its size. While its density is about the same as Earth's, its gravity is less. This means that Mercury has a greater proportion of metals than Earth does. The surface of Mercury is very similar to that of the moon. Mercury has no significant atmosphere, which means that there is no insulation and temperatures vary widely with incredibly high day temperatures to very low night temperatures. Mercury also has a magnetic field that is similar to Earth's but much weaker. It is however strong enough to create a magnetosphere.

Mercury rotates three times in two of its years which makes it unique because all of the other bodies have a 1:1 orbital/rotational ratio. Another interesting feature of Mercury is that there is indications that the surface area contracted at an early point in its history.The surface may have contracted by as much as .1% which means that the radius decreased by 1 km. This surface compression was probably a result of cooling of either the core, the crust, or both. Evidence for surface compression are scarps, which are long vertical walls running across a flat plain.

moons: None