Observations From a Global Database of Impact Craters on Mercury With Diameters Greater than 5 km

Abstract

We created a global database of Mercurian impact craters with diameters over 5 km, excluding obvious secondary craters. For craters over 10 km in diameter, we assessed basic morphological parameters, including an evaluation of whether significant postimpact filling occurred, the nature of any central structure, a three-point assessment of degradation state, and a recording of the presence of rays and other atypical features. We used the database to evaluate the spatial patterns of different subsets of the crater population. The distribution of craters with diameters of 5 to 10 km compared to the distribution of larger craters shows many background secondaries associated with primary craters with diameters over 150 km, and their abundance would distort crater counts for surface dating. Counting only unfilled craters gives an estimate of the last possible date of endogenic geologic activity on a surface. Only a third of craters with diameters over 20 km are classified as unfilled, and the highest spatial density of unfilled craters is about half the total population at the same location. Our results support a globally rapid cessation in volcanism, with variations in the volume of late stage volcanism accounting for variations in the number of filled craters. We see regional variations of interior morphology for craters ~100 km in diameter that do not obviously correlate with other patterns derived from surface observations; our observations imply that there are variations in the mechanical properties of the upper several kilometers of crust.