Graphite keeps inner planet dim
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