Planetary scientists have long been baffled by Mercury's unusually dark surface - until now. Scientists from the John Hopkins University Applied Physics Laboratory discovered evidence that this unusual physical characteristic is due to carbon, which sheds light on the nature of the planet's original crust.
The team claims that the mystery was only complicated further by previous measurements gathered from Mercury's surface that revealed low levels of iron and titanium, both important 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," said Patrick Peplowski, lead author of the paper. "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."
Scientists believe that the darkening agent is in Mercury's low-reflectance material (LRM), which is typically gathered from the depth's of the planet through impact cratering.
The team analyzed MESSENGER Neutron Spectrometer measurements of LRM and surrounding materials, revealing a spatial correlation between LRM and low-energy neutrons. These increases would require that the LRM have higher concentrations of an element that does not absorb neutrons effectively. Currently, carbon is the only known darkening agent that possesses a low efficiency for neutron absorption.
In addition to solving the mystery of Mercury's dark surface, the results also shed light on what may be the ancient remains of the planet's oldest crust. Just like Earth's moon and other inner planets, scientists believe that Mercury had a global magma ocean in its early years when it's surface temperature was extremely hot.
"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 co-author Rachel Klima. "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."
Peplowski added: "If we've really identified the remains of Mercury's original crust, then understanding its properties provides a means for understanding Mercury's earliest history."
The findings were published in the Mar. 7 issue of Nature Geoscience.
