Chabot, Nancy - Inner Solar System Volatiles: Insights from Images of Mercury’s Polar Deposits

Abstract: 
Earth-based radar astronomers first discovered evidence for water ice near Mercury’s poles, and the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission has acquired additional evidence. These include maps of areas of permanent shadow, active measurements of high and low surface reflectance, thermal modeling consistent with the long-term retention of water ice, and the detection of hydrogen-rich material. More recently, using light scattered from illuminated crater walls, MESSENGER’s Mercury Dual Imaging System (MDIS) captured images of permanently shadowed and likely ice-bearing crater floors. These images reveal extensive, spatially continuous regions with distinctive reflectance properties. Within Prokofiev crater, a location where surface water ice is thermally stable, both the sunlit and permanently shadowed areas exhibit a similar cratered texture, but the shadowed area has a uniformly higher reflectance, suggesting the emplacement of water ice on the surface after the formation of even small craters on Prokofiev’s floor. In areas where water ice is stable only in the near surface, and where a surficial layer of organic-rich volatile material has been predicted, the images reveal regions with uniformly lower reflectance that extend to the edges of the shadowed areas and terminate with sharp boundaries. The sharp boundaries indicate that the volatile deposits at Mercury’s poles are geologically young, relative to the timescale for lateral impact mixing. The images of the polar deposits on Mercury contrast with images acquired by a similar approach for shadowed craters on the Moon. Though laser reflectance measurements have yielded higher reflectance values for Shackleton crater at the lunar south pole, indicative of modest amounts of water frost or a reduction in the effectiveness of space weathering, imaging of permanently shadowed lunar craters has not revealed surfaces with anomalously high- or low-reflectance similar to those found on Mercury. Understanding the different volatile inventories in the polar regions of Mercury and the Moon would provide insights into the nature and delivery of volatiles in the inner Solar System. One possibility for the contrasting observations is that Mercury’s polar deposits were recently delivered to the planet by one or several large events. Such a scenario would suggest that the Moon also may have hosted more extensive polar deposits in its past, but that most of the lunar deposits have subsequently been lost or sequestered beneath the surface. Alternatively, the fresh appearance of Mercury’s polar deposits may suggest an ongoing process that is able to restore the deposits even at present. The total amount of ice currently at Mercury’s poles is substantial, with estimates of ~10^16-10^18g, the high end of which is comparable to the volume of Lake Ontario. If delivered by a single comet, the estimated comet diameter is ~8-40 km. If Mercury’s current polar volatile inventory is the product of the most recent portion of a longer process, then a considerable mass of volatiles may have been delivered to the inner Solar System throughout its history.