There are some visual similarities between Mercury and the Moon. Mercury is only slightly larger than the Moon and both are gray and covered with craters. NASA's MESSENGER spacecraft captured visible and infrared light images of Mercury on January 14, 2008. These observations took place about 80 minutes before the craft made its closest approach to the planet at a distance of about 17,000 miles (27,000 kilometers). MESSENGER made two more close approaches to Mercury on October 6, 2008 and September 29, 2009.

MESSENGER, whose full name is “Mercury Surface, Space Environment, Geochemistry, and Ranging”, was the first spacecraft to orbit Mercury. The main objective of the mission was to investigate the planet's geological structure, magnetic field and chemical properties. It was the first scientific mission to Mercury since the Mariner 10 flyby more than 30 years earlier.

One of MESSENGER's first discoveries was that the night side of Mercury has high concentrations of magnesium and calcium. It also detected abundant water in the outermost layer of the planet's atmosphere and discovered traces of past volcanic activity on the surface.

The spacecraft successfully completed its primary mission on March 17, 2012, acquiring nearly 100,000 images of Mercury's surface.

Image description:

Mercury looks almost like a shrinking crescent in this image. About three quarters of the planet is in the frame, only a thin slice is bright. The bright part stands out sharply against the darkness of space.

Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington