The solar system’s innermost planet, Mercury, may get its dark color from a coating of carbon deposited by micrometeorites, a new study suggests (Nat. Geosci. 2015, DOI: 10.1038/ngeo2397). Mercury’s surface contains little iron, which typically darkens solar system bodies. So scientists have been unsure why Earth’s iron-rich moon is lighter. Megan Bruck Syal of Lawrence Livermore National Laboratory and colleagues note that inner planets are more likely to be struck by carbon-rich comets than are bodies farther from the sun. Such impacts would transform carbon into heat-stable forms such as graphite and nanodiamonds. The group calculated the amount of carbon that might be delivered at high speeds to Mercury, in the form of micrometeorites derived from disintegrated comets. They found that Mercury’s surface carbon should be 50 times as dense as the moon’s. The researchers also simulated the phenomenon in the lab, where they mixed rock samples with carbon-based compounds and bombarded the mixture with Pyrex pellets. The resulting melted material contained opaque carbon particles, which lowered the sample’s reflectance to a level comparable with Mercury’s surface.