Impact Velocity as a Source of Variations in Crater Depth on Mercury

High-resolution images and altimetry of 115 craters obtained by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission have been analyzed to further the understanding of the formation and subsequent modification on impact craters on Mercury. Measurements derived from altimetry include crater depth (d), rim height, central peak/ring height, and crater wall/terrace width. Images have been used to derive crater diameter (D) and to assess crater degradation state as well as the nature of any observed impact melt. Analyses of the measurements tie these geometrical parameters to the crater characteristics seen in images. An important first observation is that the freshest craters on Mercury are not necessarily the deepest, as is typically inferred in most studies of d/D ratios. Frequently, the heights of the central peak are also smaller. A good example is provided by a comparison of two peak-ring craters of similar size on Mercury: Atget (D = 102 km) and Hokusai (D = 93 km). Hokusai is extremely fresh and has few superposed craters, broadly extensive craters rays, and distal secondaries. Hokusai also possesses substantial amounts of visible impact melt and evidence for a rampart-like structure reminiscent of ejecta observed on Mars. Although not markedly altered by subsequent impacts, Atget crater is slightly older and more degraded. Atget ejecta show no obvious evidence for melt ponds in the ejecta as seen at Hokusai. The younger Hokusai has a smaller d/D value of 0.022 (d = 2.1 km) than Atget's ratio of 0.032 (d = 3.3 km). In other examples, shallower fresh craters also display a central structure with less relief than comparable but less fresh craters of similar size. The Hokusai-Atget comparison suggests that a shallower depth may be associated with a greater volume of impact melt, but calculations indicate that this increased melt volume is probably not directly responsible for the lesser depth. The observed variation in impact melt volume, as well as the difference in crater depth, could alternatively be the result of differences in impact angle, as has been proposed for craters on the Moon, but no oblique impact-angle morphologies are apparent in the data associated with these and other examples. Structural effects that might be associated with the presence of different central structures, such as a central peak ring, are unlikely to be the cause of the observed depth difference because both Atget and Hokusai have peak rings. Target variations are also unlikely to be important, because both craters are in similar volcanic terrain. The observed greater melt generated at Hokusai than at Atget is best explained by the broad range of impact velocities expected on Mercury, which can range from 15 to 75 km/s. A larger impact velocity will not only generate more impact melt volume, an indicator of higher impact velocity, but from laboratory data a higher velocity will also produce shallower transient craters that can form shallower final craters.