The planet Mercury may once have been covered in graphite, the same material used in pencil lead, according to new research.
Mercury's surface is very dark, and now scientists think they know why.
Information sent from Nasa's Mercury Messenger spacecraft suggests the dark areas are caused by graphite.
Scientists at Johns Hopkins University in the US think Mercury was covered in graphite when the planet first formed.
But volcanoes and huge landscape changes over billions of years had hidden the graphite from view.