Radiative transfer modeling of MESSENGER VIRS spectra: Detection and mapping of submicroscopic iron and carbon

Abstract

We model the spectral effects of submicroscopic Fe and submicroscopic C particles within a transparent mineral host in order to investigate whether such materials could reproduce the major spectral characteristics of Mercury's reflectance spectra (i.e., low reflectance relative to the Moon, a lack of Fe absorption features in the near infrared, and an increasing continuum slope between visible and near-infrared wavelengths). By using the radiative transfer technique to model the VIRS (Visible and Infrared Spectrograph) spectral dataset obtained from the MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) mission, we found that our spectral models based on nanophase and microphase Fe and C consistently fit the VIRS data (except of regions of Mercury's low-reflectance material). Our models show that the mean global submicroscopic Fe abundance is 2.5wt%, which exceeds the total Fe abundances obtained from the MESSENGER Gamma-Ray Spectrometer (GRS) and the X-Ray Spectrometer (XRS) and the global average submicroscopic C abundance is 1.9wt%, which is within the three-standard-deviation level of the MESSENGER GRS C measurements for Mercury's northern hemisphere. We also produced nanophase and microphase Fe and C abundance maps that show: (1) submicroscopic C is present at percent levels across the surface, (2) the spatial variations of submicroscopic particle abundances are correlated to the maximum surface temperature, (3) lower concentrations of nanophase Fe in fresh craters and their ejecta, (4) the submicroscopic Fe abundance is lower in the northern volcanic plains and Caloris basin than the global average, (5) the submicroscopic C and Fe abundances are very low around NE Rachmaninoff, a pyroclastic deposit, and (6) the submicroscopic particle abundances vary between low-reflectance material (LRM) deposits. In correlating these maps to geology and surface temperatures, we concluded that Ostwald ripening is responsible for the longitudinal and latitudinal variations and that the C is endogenic in origin.