COMPARING CENTRAL PIT CRATERS ACROSS THE SOLAR SYSTEM: CHARACTERISTICS AND IMPLICATIONS FOR FORMATION MODELS

Craters with central depressions, either on the crater floor or atop a central rise, were first noted in the 1970’s following the Viking missions to Mars and the Voyager missions to the Jupiter system. The discovery of central pit craters on Mars, Ganymede, and Callisto led to several formation models invoking the role of target volatiles in the creation of the pits. This study is conducting the first solar system-wide comparison study of central pit craters, which have now been reported on Mercury, the Moon, Mars, Ceres, Europa, Ganymede, Callisto, Tethys, Dione, Rhea, and Pluto. The variety of target properties, surface gravities, and impactor velocities among these bodies suggest that a combination of factors is needed to produce central pit structures. Our results reveal (1) floor pit craters increase in frequency on bodies with volatile-rich crusts, (2) summit pit craters increase in frequency as target strength increases, (3) peaks on which pits occur are morphometrically identical to non-pitted peaks, (4) the frequency of central pit craters decreases with a decrease in surface gravity and a decrease in target volatiles, (5) floor pits tend to be larger relative to their parent craters than summit pits, (7) craters containing central pits occur in the same diameter ranges as craters containing central peaks on all bodies except Ganymede, (8) the pit-to-crater diameter ratio for floor pits is smaller than the peak-to-crater diameter ratio for central peaks on both Mars and Ganymede, (9) no strong correlation of central pit craters with specific terrain units is seen on any object in this analysis, (10) raised rims around floor pit craters suggest pit formation involves initial uplift followed by collapse, and (11) the presence of impact melt on the floors of both floor and summit pits on Mars indicates pits form contemporaneously with the crater. The results of this study do not support formation models invoking an explosive origin for the pit, and none of the existing formation models for central pits completely explains the observations. Instead we are developing a new formation model involving weakened subsurface layers (which may or may not contain volatiles), uplift followed by collapse, and possibly a limited impactor velocity range.