MERCURY'S PEAK-RING BASIN POPULATION AND THE FORMATION OF PEAK RINGS: OBSERVATIONS FROM MESSENGER FLYBY AND ORBITAL DATA

Analyses of the transition with increasing feature size from complex craters to multi-ring basins on the terrestrial planets have been important for understanding the processes controlling the final morphology of large impact structures. Particularly important are peak-ring basins, which have two rings—a rim crest and an interior ring of peaks— and protobasins, which exhibit a rim crest and both an interior central peak and peak ring. Observations of flyby images from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft confirm that Mercury has the largest population and surface density of peak-ring basins and protobasins in the inner Solar System. Mercury thus serves as an important laboratory for understanding the formation of peak rings on planetary bodies. From MESSENGER flyby data, 74 peak-ring basins and 32 protobasins have been cataloged on Mercury, a 150% and 100% increase from previous catalogs due to increased image coverage. Measurements of the rim-crest diameters of peak-ring basins range from 84 to 320 km (geometric mean of 180 km); the rim-crest diameters of protobasins range from 75 to 172 km (geometric mean of 102 km). Logarithmic plots of peak-ring versus rim-crest diameter reveal similar power-law trends for peak-ring basins and protobasins with diameters ≥ 90 km. Comparisons of these power laws with predictions of a nested melt-cavity model for peak-ring formation suggest that increasing volumes and depths of impact melting with increasing basin size may be important in the development of peak rings. Mercury also has a higher density of peak-ring basins and lower peak-ring basin onset diameter than on the Moon, which may be explained by the higher impact velocity and corresponding higher production of impact melt on Mercury. Orbital data from MESSENGER now enable detailed geological analyses of basins on Mercury. Many peak-ring basins (e.g., Mozart, Raditladi, Rachmaninoff, Vivaldi) have been infilled by smooth plains materials and have small melt ponds on the rim and within the continuous ejecta deposit. Ongoing analysis of the occurrence, volumes, and relative ages of these smooth materials within peak-ring basins and protobasins permit an evaluation of the importance of impact melt production in modifying the interior morphology of basins on Mercury.