P41F-04
Subsurface carbon-bearing material on Mercury revealed by the MESSENGER Gamma-Ray and Neutron Spectrometer
Thursday, 17 December 2015: 08:45
2007 (Moscone West)
Patrick N Peplowski1, Rachel L Klima2, David J Lawrence3, Carolyn M Ernst4, Brett Wilcox Denevi4, John O Goldsten5, Scott L Murchie1, Larry R Nittler6 and Sean C Solomon7, (1)Applied Physics Laboratory Johns Hopkins, Laurel, MD, United States, (2)JHU Applied Physics Lab, Laurel, MD, United States, (3)Johns Hopkins University, Baltimore, MD, United States, (4)The Johns Hopkins University Applied Physics Laboratory, Laurel, MD, United States, (5)Johns Hopkins Univ/APL, Laurel, MD, United States, (6)Carnegie Inst Washington, Washington, DC, United States, (7)Columbia University of New York, Palisades, NY, United States
Abstract:
Mercury’s surface is markedly darker than is predicted from its major element composition. The major spectral unit lowest in reflectance, known as low-reflectance material (LRM), is typically seen in material excavated by impact craters. This observation has been taken as evidence that Mercury’s darkening phase is endogenous. The MESSENGER Gamma-Ray and Neutron Spectrometer (GRNS) acquired spatially resolved measurements of three distinct LRM deposits during the low-altitude campaign that was conducted near the end of MESSENGER’s orbital mission. The GRNS data reveal increases in thermal neutron count rates that are spatially correlated with the LRM deposits. The only element consistent with the neutron measurements and with the spectral reflectance of LRM at visible to near-infrared wavelengths is graphitic carbon, at an abundance that is 1–3 wt% higher than in surrounding non-LRM material. We infer that C is the primary darkening agent on Mercury, and that the LRM sampled C-bearing material within the crust. This interpretation supports the hypothesis that a graphite floatation crust formed on Mercury from an early global magma ocean, and we propose that its impact-gardened remains persist beneath the volcanic plains units that comprise the planet’s present upper crust. The distribution of LRM on Mercury’s surface requires numerous, discontinuous LRM source regions, as would be expected for the remains of the primordial crust given the disruptive effects of the late heavy bombardment and eons of intrusive magmatism.
