Excavation of the Subsurface by Hypervelocity Impacts: Insight from Experiments and Lessons from Missions

摘要

Impact-induced excavation is one of the few methods available that allows remote measurement of planetary subsurfaces. During a hypervelocity impact event on a planetary body, strong Shockwaves and subsequent rarefaction waves set material in motion, excavating and ejecting a portion of the target up and out of the crater to expose fresh material from below. While fresh ejecta patterns and steep slopes of craters from natural impacts have long been used to infer properties of the subsurface, recent planned artificial impacts, such as the Deep Impact and LCROSS missions, have demonstrated the utility in observing and measuring the ejecta during the impact event. Measurements from instruments capturing the in-flight ejecta dynamics, such as cameras, spectrometers, and thermal radiometers, can expose volatiles hidden in the subsurface, characterize material properties, and constrain the provenance of the excavated material. In order to interpret the measurements, the mass-velocity distribution of ejected material must be understood. In this study, we present new experimental measurements of the ejecta distribution and crater growth from early- to main-stage flow for hypervelocity impacts at the NASA Ames Vertical Gun Range. These new data are used to estimate and compare the ejecta yield from LCROSS and a hypothetical cubesat impact mission, and demonstrate the ability to characterize the subsurface by analysis of the ejected material.