CONSTRAINING THE FERROUS IRON CONTENT OF SILICATES IN MERCURY’S HIGH- AND LOW-REFLECTANCE MATERIAL

The low iron content of Mercury’s surface complicates the identification of specific minerals with near-infrared reflectance spectroscopy, because the diagnostic crystal field bands used to identify and characterize mineralogy at these wavelengths require transition metals to be structurally bound within a silicate crystal lattice. The absence of a crystal field band near 1000 nm can, however, be used to constrain the amount of iron that is hosted in the silicates in various terrains on the surface of Mercury. To accomplish this end, we examine spectra collected by the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) Visible and Infrared Spectrograph (VIRS) on the MESSENGER spacecraft. We focus on relatively fresh exposures of two terrain types: high-reflectance red plains (HRP, specifically the northern volcanic plains) and low-reflectance material (LRM). These terrains represent the brightest and darkest of the laterally extensive units (i.e., excluding hollows or dark spots) on the surface as mapped by the Mercury Dual Imaging System (MDIS). By examining relatively fresh crater ejecta and rays, we seek to mitigate some of the effects of space weathering, which alters the planet’s surface and further subdues weak spectral features. We have identified several hundred bright MASCS spectra in the high- and low-reflectance material. From these spectra, we selected those with the highest signal-to-noise ratio for more in-depth analysis. From laboratory measurements of the spectral reflectance of low-iron ferrous silicates, we modeled the spectra to determine the iron threshold at which absorption bands should be evident in the MASCS data, given the viewing geometry and signal-to-noise constraints. Initial results from fresh craters in HRP material suggest that there is approximately an order of magnitude less iron in the silicates than has been measured on the surface by MESSENGER’s X-Ray Spectrometer and Gamma-Ray Spectrometer. This result supports the suggestion that much of the iron on the surface of Mercury exists in a reduced form, either as endogenic or exogenic iron metal or bound within sulfide phases.