Dark Stains on Mercury Reveal Its Ancient Crust

Expanded-color image of Mercury's 52-km Degas crater, showing an abundance of low-reflectance material (LRM).

Ever since the MESSENGER spacecraft entered orbit around Mercury in 2011, and indeed even since Mariner 10‘s flyby in 1974, peculiar “dark spots” observed on the planet’s surface have intrigued scientists as to their composition and origin. Now, thanks to high-resolution spectral data acquired by MESSENGER during the last few months of its mission, researchers have confirmed that Mercury’s dark spots contain a form of carbon called graphite, excavated from the planet’s original, ancient crust.Commonly found within and around impact craters and volcanic vents, the dark spots on Mercury—also referred to as “low-reflectance material,” or LRM—were originally suspected to contain carbon delivered to the planet by comets.Data from MESSENGER’s Gamma-Ray and Neutron Spectrometer (GRNS) and X-ray instruments confirmed the LRM to contain high amounts of graphitic carbon, but likely originating from within Mercury itself. It’s thought that Mercury was once covered by a crust composed of graphite, when much of the planet was still molten.”Experiments and modeling show that as this magma ocean cooled and minerals began to crystallize, 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, co-author of a recent study on LRM and a planetary geologist at the Johns Hopkins University Applied Physics Laboratory. “We think that LRM may contain remnants of this primordial crust. If so, we may be observing the remains of Mercury’s original, 4.6-billion-year-old surface.”See more MESSENGER images of LRM on Mercury here.Although similar in visible coloration and covered in craters, cracks, and mountains, any similarities between Mercury and other smaller worlds in our Solar System—including our Moon—end there. Mercury has a formation history all it own and is compositionally unique among the planets.These —> Read More

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