New Illumination and Temperature Constraints of Mercury's Volatile Polar Deposits

Images from the Mercury Dual Imaging System (MDIS) aboard the MErcury Surface, Space ENvironment, GEochemistry, and Ranging mission reveal low-reflectance polar deposits that are interpreted to be lag deposits of organic-rich, volatile material. Interpretation of these highest-resolution images of Mercury's polar deposits has been limited by the available topography models, so local high-resolution (125 m pixel^-1) digital elevation models (DEMs) were made using a combination of data from the Mercury Laser Altimeter (MLA) and from shape-from-shading techniques using MDIS images. Local DEMs were made for eight of Mercury's north polar craters; these DEMs were then used to create high-resolution simulated image, illumination, and thermal models. The simulated images reveal that the pixel brightness variations imaged within Mercury's low-reflectance deposits are consistent with scattered light reflecting off of topography and do not need to be explained by volatile compositional differences as previously suggested. The illumination and thermal models show that these low-reflectance polar deposits extend beyond the permanently shadowed region, more than 1.0 km in some locations, and correspond to a maximum surface temperature of greater than 250 K but less than 350 K. The low-reflectance boundaries of all eight polar deposits studied here show a close correspondence with the surface stability boundary of coronene (C₂₄H₁₂). While coronene should only be viewed as a proxy for the myriad volatile compounds that may exist in Mercury's polar deposits, coronene's surface stability boundary supports the idea that Mercury's low-reflectance polar deposits are composed of macromolecular organic compounds, consistent with the hypotheses of exogenous transport and in situ production.