Mercury's mysterious surface darkness has baffled scientists: Though it's far closer to the sun than our moon, the latter reflects more sunlight. Last year, it was proposed that the root cause is carbon. As Patrick Peplowski of the Johns Hopkins University Applied Physics Laboratory tells the Christian Science Monitor, "When you think of charcoal or you think of pencil lead, it's really dark. So if there was a little bit of carbon mixed in" with the surface elements, that would explain the darkening. But the solar system's planets don't have much carbon in their crusts, meaning Mercury would have to be an anomaly. And as Peplowski and his team determined after reviewing data taken during the 2011-2015 MESSENGER mission, it is.
His team surmises that when Mercury was but a young planet 4.6 billion years ago, its hot surface was likely covered in an ocean of molten lava, which when cooled slowly over time would see its minerals solidify and sink, per a press release. But graphite (a crystalline form of carbon) is so light it would remain buoyant and harden into the planet's original crust. As the millennia passed, a new crust would have covered that one courtesy of volcanic activity, and "we propose that the heavily disrupted remnants of this ancient layer persist beneath the present upper crust," they write in Nature Geoscience. Further, whenever a meteor hit the planet hard enough to expose that ancient layer, what Universe Today terms the "peculiar" dark spots also visible today would have occurred. (NASA's impact on Mercury can literally be measured as a 52-foot-wide crater.)