OBSERVATIONS AND EXPERIMENTAL SIMULATIONS OF LOW-ANGLE LUNAR IMPACTS

Understanding the mechanics of low-angle impacts is important because they have been featured in a variety of hypotheses (e.g., true polar wander on Mars, effects of the Chicxulub impact, production of tektites) and they alter the scaling relationship between impactor and final crater that is critical to surface dating. In 1978, Gault and Wedekind (Proc. 9th LPSC, 3843-3875) conducted a landmark series of experiments that matched observed lunar crater forms with impacts occurring at low angles with respect to horizontal. Over the subsequent decades, techniques for observing experimental impacts have dramatically improved. Improved imaging of the Moon provides details of the geology of larger low-angle impact craters and has revealed a number of smaller fresh craters, some of which have forms different from those previously discussed. In the past three years we have conducted a series of experimental low-angle impacts at the Johns Hopkins Planetary Impact Laboratory and the Ames Vertical Gun Range that take advantage of the improvements in syn- and post-impact imaging to reveal new details of how crater excavation proceeds to generate the final crater forms. We are matching these observations to case studies of well-preserved lunar craters. Here we present initial results from our analyses. One highlight of the work we will present is the newly characterized “heart-shaped” craters: downrange the crater is shallow and corresponds to the bottom of the heart (sometimes with a cusp), and an uprange “forbidden zone” of ejecta points to the top of the heart, which often has a steeper wall in the crater cavity and a flattened uprange rim. We also were able to reproduce the situation observed in Messier where ricocheted material extends in a direction a few degrees off from the long axis of the crater.