Mercury, the small planet that’s closest to the sun, may have once had a surface made of graphite, the material used in making pencil lead.
This theory would explain why the surface of Mercury is unusually dark, scientists say. Earlier studies of the planet’s surface befuddled scientists because it contained low traces iron and titanium, the substances that darken the Moon.
Carbon, likely in the form of graphite, would be required to darken Mercury's surface sufficiently.
, Johns Hopkins University
