Mercury’s dark secret revealed | Science News

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Mystery Solved

Mercury’s dark secret revealed

Graphite keeps inner planet dim

By
11:11am, March 7, 2016
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.

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

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