Mercury’s surface is unusually dark, an observation that until recently had planetary scientists mystified. But in a new study published today in Nature Geoscience, a team of researchers provides evidence that the darkening agent is carbon, a finding that offers important clues to the nature of the planet’s original crust.
Patrick Peplowski, a research scientist at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland, and lead author of the paper, explains that earlier measurements of the chemistry of Mercury’s surface only added to this mystery because they indicated that Mercury’s surface has low abundances of iron and titanium, important constituents of the most common darkening agents on the Moon and other silicate bodies.
“A process of elimination led prior researchers to suggest that carbon may be the unidentified darkening agent, but we lacked proof,” he said. “Spectral modeling of MESSENGER color imaging data suggested that weight-percent levels of carbon, likely in the form of graphite, would be required to darken Mercury’s surface sufficiently. This level is unusually high, given that carbon is found at typical concentrations of only ~100 parts per million on the Moon, Earth and Mars.”
Whatever the darkening agent, the scientists surmised that it was most concentrated in Mercury’s low-reflectance material (LRM), which generally appears as deposits excavated from depth by impact cratering. The researchers examined MESSENGER Neutron Spectrometer measurements of LRM and surrounding materials, and they found that increases in low-energy neutrons are spatially correlated with LRM. Such increases require that the LRM have higher concentrations of an element that is inefficient at absorbing neutrons. Carbon is the only darkening agent suggested for Mercury that is also an inefficient neutron absorber.
These measurements were possible only late in MESSENGER’s second extended mission, when the spacecraft regularly passed within tens of kilometers of Mercury’s surface — a necessary condition to resolve LRM deposits with the Neutron Spectrometer. Prior measurements acquired at altitudes greater than 200 kilometers couldn’t resolve such deposits. The data used to identify carbon included measurements taken just days before MESSENGER impacted Mercury in April 2015. … (MESSENGER)