Several studies have suggested that target heterogeneity and slopes affect the dynamics of impact cratering and the resulting final crater morphology. In this proposal we plan to explore both effects.…

This is a $431,549.49 grant awarded by the National Aeronautics and Space Administration (NASA) to The Johns Hopkins University to conduct research on how target heterogeneities and surface slopes influence the crater formation process. The research will involve laboratory experiments, numerical modeling, and planetary observations to gain insights into how these factors affect shock propagation, ejecta excavation, crater growth, and final crater morphology. This work is relevant to understanding how impacts modify asteroid and planetary surfaces, and will help inform the connection between crater size and projectile dimensions, which is critical for planetary surface age dating. The grant is part of NASA's Solar System Workings research program and does not have a set-aside designation.

Several studies have suggested that target heterogeneity and slopes affect the dynamics of impact cratering and the resulting final crater morphology. In this proposal we plan to explore both effects. Here target heterogeneity means any pre-existing target structure caused by the presence of joints, fractures, blocks, or grains. Several studies show that such pre-existing target structures can influence the crater growth rate and ejecta velocity. There are two possible contributors: (1) as target grain size becomes small enough, cohesive forces between the grains strengthen the target affecting crater excavation; and (2), as target grain size approaches the projectile size, complex interactions develop between the individual grains and the impact generated shock because the latter's width is similar in size to the average dimension of the target grains. IT is this latter set of processes that we plan to explore relative to what is expected from a point source approximation employed in the cratering scaling rules. Surface slopes also affect the cratering process, especially during crater modification. Preliminary studies indicate that asymmetric craters form on slopes, and that their morphology and topography change with surface slope angle, crater size, and possibly target type (i.e., whether formed in the lunar mare or highlands). Craters on 433 eros, for example, do not show flat floors, but rather possess accumulations of dark debris near the lower crater rims. Many of these craters also possess shorter rims on their down slope sides. Similar observations are seen on lutetia, vesta, and the moon. These observations indicate that IT may be possible to gain quantitative inferences on terrain types, in particular target strength, by measuring both crater growth and final crater morphology as a function of slope, crater size (or impact energy), and pre-impact target strength (or target type). The objectives of this proposal are to gain additional insights on how target heterogeneities and slope influence the cratering process. When considering target heterogeneity, we focus on the interplay between initial projectile-target coupling effects and target heterogeneities, and investigate how this interplay affects shock propagation, ejecta excavation, crater growth, cratering efficiency, and crater shape. When considering target slope effects, we explore the interplay between target strength characteristics, impact energy, and slope angle. In both these investigations, we will undertake laboratory experiments, complimented with some numerical modeling, and planetary observations. To achieve these objectives, we propose two main tasks: (task 1) investigations of crater growth and ejecta excavation targets with well-characterized heterogeneity both in the laboratory (JHU/APL and NASA ames) and with numerical models. (task 2) investigations of impact cratering with target slope angle in the laboratory (JHU/APL). This proposal is relevant to the NASA solar system workings call in that IT addresses how pre-existing physical target heterogeneities and slopes might influence our understanding of the cratering process on asteroids and planets. This effort should therefore inform further our understanding of how impacts modify asteroid and planetary surfaces, and may influence the connection between crater size and projectile dimension, which are critical to understanding age relationships on any planetary surface including eros, mars, earth and the moon.