Carbon-rich material spotted by NASA’s MESSENGER spacecraft on its final orbits of Mercury may be the remnants of a primordial graphite crust, according to a study published online in Nature Geoscience this week. The exposure of this ancient crust explains why Mercury’s surface appears darker than expected.

Mercury’s surface is darker than the Moon’s, yet iron - thought to be the main darkening agent on many airless planetary bodies - is more abundant on the Moon than on Mercury. Instead, carbon has been proposed as the darkening agent on Mercury, but whether the carbon is indigenous to the planet or was delivered by carbon-rich micrometeorites has been debated.

Patrick Peplowski and colleagues analysed measurements of the darkest parts of Mercury’s surface taken by the MESSENGER spacecraft at the end of its mission. They find conclusive evidence that the composition of the darkest material on Mercury is indeed carbon-rich. The association of this carbon-rich material with large craters is consistent with an indigenous origin from deep within the crust and later exposure by impact. The authors suggest that the patches of dark material are exposed remnants of an ancient carbon crust that formed when graphite crystallised from an early magma ocean and floated to the surface. Although this primordial crust was obscured by later volcanism and other geological processes, some of this carbon-rich material would have been mixed into the overlying materials to cause a global darkening of Mercury’s surface.