Mercury's mysterious 'darkness' explained
published in Nature Geoscience.
Co-author and Deputy Principal Investigator of the MESSENGER mission, Carnegie's Larry Nittler, explained: "The previous proposal of comets delivering carbon to Mercury was based on modelling and simulation. Although we had prior suggestions that carbon may be the darkening agent, we had no direct evidence. We used MESSENGER's Neutron Spectrometer to spatially resolve the distribution of carbon and found that it is correlated with the darkest material on Mercury, and this material most likely originated deep in the crust. Moreover, we used both neutrons and X-rays to confirm that the dark material is not enriched in iron, in contrast to the Moon where iron-rich minerals darken the surface."
When Mercury was very young, much of the planet was likely so hot that there was a global "ocean" of molten magma. From laboratory experiments and modeling, scientists have suggested that as this magma ocean cooled, most minerals that solidified would sink. A notable exception is graphite, which would have been buoyant and floated to form the original crust of Mercury.
"The finding of abundant carbon on the surface suggests that we may be seeing remnants of Mercury's original ancient crust mixed into the volcanic rocks and impact ejecta that form the surface we see today. This result is a testament to the phenomenal success of the MESSENGER mission and adds to a long list of ways the innermost planet differs from its planetary neighbors and provides additional clues to the origin and early evolution of the inner Solar System," concluded Nittler.
Source: Carnegie Institution [March 07, 2016]