Mercury’s dark secret revealed

Graphite keeps inner planet dim

Crater on Mercury

DARK PAST  Akutagawa crater on Mercury is one of three craters where the MESSENGER spacecraft found deposits of carbon dug up from the planet’s original crust.

NASA, JHUAPL, Carnegie Institution of Washington

Ever since Mariner 10 flew by Mercury in 1974 and 1975, researchers have known that the planet was darker than the moon. But they didn’t know why.

Some of the moon’s darkness comes from its iron-rich minerals, but those are lacking on Mercury. Scientists suspected that graphite might color Mercury, but they had no proof — until now. NASA’s MESSENGER spacecraft, which spent four years orbiting Mercury before intentionally crashing last April (SN Online: 4/30/15), got close enough to obtain neutron spectroscopy data from a few dark patches in and around craters. Those patches are slathered with carbon, probably from a subsurface primordial crust, researchers report online March 7 in Nature Geoscience.

The crater carbon was probably churned up from a layer of graphite. A steady rain of meteorites and volcanic activity mixed this graphite into Mercury’s topsoil, darkening the entire planet, suggest Patrick Peplowski, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., and colleagues. 

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