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takyon [soylentnews.org] writes:

The planet Mercury's unusual dark coloration may be due to graphite on the surface [bbc.com], exposed by impact craters:

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. [...] They found that the darkest "stuff" on Mercury had a carbon-rich composition and that it was associated with large impact craters.

According to the team, this association is consistent with the dark material coming from deeper within the planet and being exposed when space rocks gouged it out. Like Earth's Moon and the other inner planets, Mercury likely had a global magma ocean when it was young and the surface was very hot. "As this magma ocean cooled and minerals began to crystallise, minerals that solidified would all sink with the exception of graphite, which would have been buoyant and would have accumulated as the original crust of Mercury," said Rachel Klima, also from APL.

Info about the Gamma Ray and Neutron Spectrometer [messenger-education.org] (GRNS).

Remote sensing evidence for an ancient carbon-bearing crust on Mercury [nature.com] (DOI: 10.1038/ngeo2669)

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