Topography, Illumination, and Thermal Models of Mercury’s Polar Deposits

Auteur

Institution

Theme

Abstract

The first evidence for water ice near Mercury’s poles was obtained three decades ago using Earth-based radar. Roughly two decades later, NASA’s MESSENGER spacecraft became the first to orbit the innermost planet, transforming our knowledge of Mercury. MESSENGER’s observations provided compelling support for the presence of water ice and other volatile compounds in Permanently Shadowed Regions (PSRs) near both of Mercury’s poles. Following the end of MESSENGER’s mission, we were funded by NASA for a three-year study to utilize the available MESSENGER data to investigate volatile polar deposits of high science interest in greater detail. Here we provide our results to date from this multi-year effort:

Polar deposits with low-reflectance surfaces: High-resolution local Digital Elevation Models (DEM) were produced for eight craters that host volatile deposits with low-reflectance surfaces, as observed by both MESSENGER’s Mercury Laser Altimeter (MLA) and Mercury Dual Imaging System (MDIS). Illumination and thermal models derived from these new DEMs indicate that the low-reflectance surfaces can extend slightly beyond the PSRs and are consistent with being due to the presence of volatile organic compounds. 

Surface water ice in Prokofiev: Prokofiev (diameter: 112 km) hosts a deposit with a high-reflectance surface in MLA and MDIS observations, interpreted to be surface water ice. Using a high-resolution local DEM produced for this crater, Prokofiev is found to be the first location identified on Mercury to host a large radar-bright region that extends for several kilometers beyond its PSR. Modeling also shows its surface ice is not pure. 

Northernmost craters: High-resolution DEMs were created for four large craters near Mercury’s north pole that host extensive radar-bright deposits. The thermal models indicate surface ice and low-reflectance volatile compounds are stable at the surface in these craters, but MESSENGER observations this close to the pole are sparse.

Lowest-latitude deposits: Radar-bright regions have been identified in small PSRs located at latitudes below 70°N. Such low latitudes may present challenging thermal environments for the presence of water ice and earlier thermal studies all but neglected these low-altitude deposits. Local, high-resolution DEMs have been constructed for these low-latitude craters, and thermal models are currently being examined.

Mercury’s south pole: The global Mercury DEM lacked the resolution needed for investigations of south polar deposits. A higher resolution DEM that covers Mercury’s south polar region from the pole to 75°S was constructed from MDIS images and used to determine the thermal environments for Mercury’s south polar deposits for the first time. 

These results give new insight into the nature and origin of Mercury’s polar volatile deposits as well as provide key products that can inform BepiColombo’s future exploration of Mercury.