Mercury is darker than the moon, something that has perplexed scientists, especially because Mercury lacks iron, the darkening agent found on the moon. Now, scientists think they know the reason why: The surface of the innermost planet is enriched in carbon, in the form of graphite, aka “pencil lead.” Scientists using NASA’s MESSENGER orbiter found evidence for carbon at levels of a few percent—much higher than is typically found on Earth, the moon, and Mars. The observations came from the last days of the MESSENGER mission, just before it crashed into the surface in 2015, when the spacecraft got up close and personal to large craters (seen above) where the darkening agent is most prevalent, scientists report today in Nature Geoscience. Scientists suspect that the graphite comes from Mercury’s original crust 4.5 billion years ago, when the planet was solidifying from a ball of molten magma. Whereas most minerals crystallizing out of the magma ocean would sink, graphite would have floated to the top.