A new look at the planet Mercury has shown astronomers something they hadn't seen before; Mercury's hidden side, which looks remarkably like Earth's moon.

This new view was obtained by NASA's Messenger probe, which passed by Mercury earlier this month. Before Messenger, the best images were those obtained by Mariner 10, which passed by Mercury in 1974 and 1975. Messenger's mission includes three flybys of the planet, and an orbital mapping of Mercury, which is scheduled to begin in 2011 and should last a year.

Messenger's images show a cratered surface, just like our moon, including the double-ringed, 125-mile-wide crater Vivaldi, and Caloris, an 800-mile-wide impact basin which is also one of the newest and largest craters in our solar system.

About Mercury

Mercury is the fastest planet in the solar system, sprinting around the Sun once every 87.97 days. Humans have known about Mercury since at least the third millennium B.C. The Greeks knew the planet by two names: Apollo, when it appeared as a morning star, and Hermes, or the messenger of the gods, when it appeared as an evening star.

One of Mercury’s most noticeable features is its seemingly unpredictable orbit, which takes it as close as 29 million miles to the sun, and as far away from it as 43 million miles. This was a source of frustration among early astronomers, because what they observed did not correspond to what they predicted with their astronomical models. Mercury’s perihelion, or the point in its orbit in which it is closest to the sun, shifts slightly each time, a behavior that cannot be attributed to the gravitational pull of nearby planets. This could not be explained with Newtonian mechanics, the dominant theory of the time. It was not until Einstein’s Theory of Relativity accurately predicted Mercury’s movement that astronomers had a way to understand and forecast the planet’s strange behavior.

Mercury rotates once every 58.65 days, or three times during two of its years, leaving one side exposed to light for long periods of time, and the other side dark. Because of this, the temperature differences between the sunlit side and the dark side are extreme. The side facing the sun reaches temperatures of around 810°F, and the dark side can reach about -290°F, making the planet’s temperature variations the most drastic in the solar system.