Experimental studies of ejecta dynamics during vertical and oblique impacts

Impact cratering affects every body in our solar system. In particular, crater excavation affects the redistribution of material across planetary surfaces, the ejection of material off a planetary body, and the interpretation of impact craters and cratered surfaces throughout the solar system. The majority of investigations regarding impact crater excavation assume vertically impacting projectiles, allowing for two-dimensional symmetry in experimental design and numerical models. Statistically, however, the most common impact angle on planetary surfaces is 45 degrees above horizontal, not vertical (or 90 degrees).; This dissertation seeks to address the accuracy of the assumption that vertical impact excavation is representative of crater excavation during oblique impacts (impacts at angles shallower than 90 degrees). The effect of impact angle on crater excavation is investigated by studying ejecta dynamics during vertical and oblique experimental impacts at the NASA Ames Vertical Gun Range. A technique new to impact cratering, three-dimensional particle image velocimetry (3D PIV), is used to directly measure ejecta particle velocities and positions within the ejecta curtain in real time as the crater grows. From the measured ejecta dynamics, ejection parameters such as ejection speed, angle, position and azimuth around a flow-field center, are calculated. These ejection parameters are compared to current crater excavation models such as Maxwell's Z-Model and non-dimensional ejection-velocity scaling relationships to assess changes in the subsurface flow-field at a variety of impact angles including 90, 60, 45, 30, and 15 degrees. The migrations of subsurface flow-fields representing different regions of the ejecta curtain during oblique impacts are followed in an effort to retain the simplicity of two-dimensional excavation flow as understood from vertical impacts. It is found that ejecta dynamic asymmetries persist through the first half of crater growth and are present at impact angles as high as 45 degrees. Asymmetries in ejecta dynamics during oblique impacts are shown to be an important factor in producing asymmetric ejecta deposits at planetary scales, specifically the uprange zone of avoidance. The results from this dissertation provide a unique view of the cratering flow-field inside the target based on ballistic ejecta dynamics.