Astronomers have long struggled to explain Mercury's darkness, but new research suggests carbon is responsible for the planet's limited reflectivity.

Iron-rich minerals limit the amount of sunlight reflected by the moon, but Mercury is mostly without iron. Newly analyzed spectrometer data from NASA's Messenger probe instead reveals a high concentration of carbon on Mercury's surface.

When researchers at Johns Hopkins University and the Carnegie Institution for Science compared a map of carbon-rich deposits with a map of Mercury's reflectivity, they found a strong correlation between carbon and darkness.

"We used Messenger's Neutron Spectrometer to spatially resolve the distribution of carbon and found that it is correlated with the darkest material on Mercury, and this material most likely originated deep in the crust," Larry Nittler, Carnegie scientist and deputy principal investigator of the Messenger mission, said in a news release. "Moreover, we used both neutrons and X-rays to confirm that the dark material is not enriched in iron, in contrast to the Moon where iron-rich minerals darken the surface."

Models suggest that when Mercury was young it was so hot that the entire surface was one big ocean of magma. As the molten rock cooled, most minerals sank to the bottom. Graphite, being more buoyant, was left to solidify at the top, forming Mercury's first crust.

"The finding of abundant carbon on the surface suggests that we may be seeing remnants of Mercury's original ancient crust mixed into the volcanic rocks and impact ejecta that form the surface we see today," Nittler explained.

The new research was published this week in the journal Nature Geoscience.