CURRENT IMPACT CRATERING OF THE MOON AND MARS

The discovery of small (<50 m) impact craters forming on Mars (Malin et al., 2006, Science 314) provided new data constraining the primary cratering rate. There are now more than 90 new impact sites documented by before-and-after images of Mars by multiple spacecraft. More than half of these sites consist of crater clusters, and the dispersion of the craters constrains the properties of the bolides (Ivanov et al., 2009, LPSC 40). The new impact sites are primarily discovered over dusty regions of Mars because the impact event disturbs the dust and darkens an area much larger than the craters and ejecta, allowing them to be discovered in medium-resolution images covering large areas (especially MRO/CTX). They are then imaged by MRO/HiRISE at ~30 cm/pixel to identify and measure the craters. However, these data do not provide a good measure of the size-frequency distribution (SFD) of the bolides because the largest events are preferentially discovered. The LRO Camera (LROC) began re-imaging the Moon at sub-meter scales in the summer of 2009, and the LROC team is searching for new impact craters over regions imaged at better than ~2 m equivalent scale in newly scanned Apollo Panoramic and Metric camera images acquired forty years ago (see http://apollo.sese.asu.edu/). In contrast to the martian data, the lunar data will provide a better measure of the SFD of the bolides from ~10 to 100 m diameter. This SFD can be compared to various candidate production functions, extrapolated to smaller sizes to determine engineering requirements for lunar structures, and extrapolated to larger sizes to address whether or not the current cratering rate is typical of post-heavy-bombardment geologic time. These data will also calibrate the Moon/Mars cratering ratio, useful for extension of the calibrated lunar chronology to Mars and providing a constraint on models for the orbital evolution of planet-crossing asteroids. Accurate determination of the current cratering rate will also inform expectations for future seismic experiments on the Moon and Mars.