NASA Messenger Mercury: Image

At first glance, Mercury seems remarkably similar to the Moon - a grey, airless world dominated by craters.

As the smallest of the four terrestrial planets, Mercury cooled fairly rapidly, losing heat and slowing its geological activity several billion years ago.

Since then, the Sun's tidal forces have slowed Mercury's rotation so that it rotates three times in every two 88-day orbits around the Sun.

This unique arrangement means that most parts of Mercury only see a sunrise every two years, and creates an extreme range of temperatures, varying from -190ºC (-342ºF) to 430ºC (806ºF).

Picture: Image produced by NASA/Johns Hopkins University Applied Physics Laboratory/Arizona State University/Carnegie Institution of Washington. Image reproduced courtesy of Science/AAAS