GEOLOGICAL AND SPECTRAL PROPERTIES OF THE REMBRANDT IMPACT BASIN ON MERCURY

Analyses of impact basins on Mercury are important for understanding impact processes at large scales and the relationship of extensive smooth plains deposits to basin formation and evolution. Among the youngest and largest impact basins on Mercury are Rembrandt (715 km diameter) and Caloris (1550 km diameter). From images obtained by the Mercury Dual Imaging System (MDIS) on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft, the interiors of both basins have been mapped geologically as smooth plains units, mostly interpreted as volcanic units that postdate the formation of their host basin. Less clear are the relationships between basin formation and smooth plains units exterior to the basins.

Recent progress in processing spectral reflectance data from MESSENGER’s Mercury Atmospheric and Surface Composition Spectrometer (MASCS) instrument allows us to study the spectral characteristics of local areas on Mercury in greater detail than before. Here we describe a detailed study focusing on the Rembrandt impact basin, which uses MASCS spectral data to compare with geologic units mapped using MDIS images.

Mapped high-reflectance plains within Rembrandt show good correlation with a spectral unit defined by MASCS reflectance data at 750 nm wavelength. Including shorter-wavelength data from the MASCS instrument reveals several sub-units within the interior plains units. Exterior to Rembrandt, MASCS data indicate a unit that extends much farther than the mapped low-reflectance plains.

The interior of Rembrandt shows markedly less spectral heterogeneity than the interior of Caloris. We note that within Caloris, ejecta and crater materials from a number of impact features (e.g., Apollodorus, Atget) are spectrally distinct from and cover basin fill material over a large area. Within Rembrandt such effects are less evident and the variation of spectral reflectance is much less pronounced.

The combination of spectral information from the MASCS instrument and geological information derived from MDIS data are helping to document and assess the characteristics of impact basin and smooth plains materials on Mercury and provide new insights into the processes that modified Mercury’s surface.