"It's an oddball planet," David Blewett, with the Applied Physics Laboratory at Johns Hopkins University, told reporters at the American Association for the Advancement of Science meeting in Boston last week.
Mercury is the smallest of the solar system's eight planets (the dwarf planet Pluto is no longer is considered a full-fledged planet), but its most dense. It also sports a global magnetic field like Earth's, a feature Venus and Mars lack.
In the image, released by NASA on Friday, the giant Caloris impact basin appears near the top, filled with orange-toned volcanic plains. The small orange and tan spots are places where materials have been deposited by volcanic eruptions.
Not pictured are ice and organic matter deposits discovered inside Mercury's permanently shadowed polar craters.
MESSENGER completed its year-long primary mapping mission in March 2012 and presently is making more detailed surveys.