Radiative Transfer Modeling of MESSENGER VIR Spectra of Mercury: Detection and Mapping of Submicroscopic Carbon

Abstract:

The composition of Mercury is elusive because the planet is optically dark and exhibits a relatively featureless red visible and near-infrared spectrum. Studies have invoked opaque minerals and space weathering to explain these characteristics. In this study we used radiative transfer modeling of MESSENGER VIRS data to show that carbon is required to explain the detailed shape of the spectrum of Mercury from 0.3–1.5 µm. In this model, we assume the regolith comprises a high albedo host mineral, which contains nanophase and microphase iron and carbon. We varied the abundance of nanophase and microphase iron and carbon to fit the observed spectra. We found that the nanophase and microphase iron alone inadequately fits the visible portion of the observed spectra. By adding carbon, we find that nanophase and microphase iron and nanophase amorphous carbon fit the VIRS spectra consistently. We produced a global nanophase amorphous carbon map as well as nanophase and microphase iron maps. From the carbon map (see figure), we find that nanophase amorphous carbon is sensitive to geologic features associated with space weathering, such as fresh crater rays. Thus, amorphous carbon is a new indicator for regolith maturity. We also observe that the global average submicroscopic carbon content is 1.4±0.3 wt.% C, which is consistent with MESSENGER Gamma-Ray Spectrometer measurements. Using this abundance, we can constrain the source of water and carbon on Mercury. Assuming the water ice in the poles is derived from wet impactors, both asteroids and comets, and knowing C/H ratios of those sources, the surface nanophase carbon layer can be no thicker than 10 m if all carbon is derived from an exogenous source. It is almost certain the space weathered regolith on Mercury is thicker than 10 m, so the majority of carbon on Mercury must be endogenous.

Figure: Nanophase amorphous carbon map. In this map, fresh craters and their rays are distinguishable by the lower proportion of carbon.