B.B.C. article

The planet Mercury may once have been encased in an outer shell of graphite, the same material used as pencil lead.
The surface of the innermost planet is unusually dark, and scientists now think they know why.
Scientists analysing data from Nasa's Mercury Messenger spacecraft now think this mystery darkening agent is carbon in the form of graphite.
This graphite may be a relic of the planet's primordial crust, which was later covered up by volcanism.
The findings are published in the journal Nature Geoscience.
Patrick Peplowski from the Johns Hopkins University Applied Physics Laboratory (APL) in Maryland and colleagues analysed measurements of the darkest parts of Mercury's surface taken by Messenger at the end of its mission.

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