Momentum transfer in hypervelocity cratering of meteorites and meteorite analogs: Implications for asteroid deflection

Asteroid porosity ranges from 0 to >50%, with most >20%, and some asteroids exhibit a water feature in their reflection spectra. Porosity and hydration are expected to influence the momentum transferred in hypervelocity collisions. We conducted a series of measurements of the post-impact momentum, characterized by a factor β, the ratio of the total linear momentum acquired by the target to the momentum of the impactor. We measured β for anhydrous meteorites, samples of their asteroidal parent bodies, spanning a wide range of porosities: 7 samples of the CV3 carbonaceous chondrite Northwest Africa (NWA) 4502 (2.1% porosity), 7 samples of the ordinary chondrite NWA 869 (6.4% porosity), and 4 samples of the ordinary chondrite Saratov (15.6% porosity), as well as 2 samples of terrestrial pumice (80% porosity). We also measured hydrous meteorite analog targets, including 2 samples of terrestrial serpentine (17.9% porosity) and 4 samples of terrestrial montmorillonite (51.5% porosity), the two clay minerals that dominate the composition of the hydrous CI carbonaceous chondrite meteorites, as well as 4 samples of hydrous meteorite analog material prepared by powdering and hydrating an anhydrous carbonaceous chondrite. We found that for both anhydrous and hydrous samples β decreased with increasing porosity, consistent with hydrocode modeling. The β for each target type was >2 demonstrating that crater ejecta makes a significant contribution to recoil in hypervelocity collisions.. The β values we measured for the anhydrous samples are larger, with β = 3.55 for NWA 4502, 2.69 for NWA 869, 2.10 for Saratov, and 2.15 for pumice, than results from hydrocode modeling for 10 km/s impacts into relatively strong, porous rock targets. The momentum enhancement by ejecta (β - 1) for the moderate porosity (17.9%) hydrous serpentine targets (β = 4.70), the highly porous (51.55% porosity) hydrous montmorillonite targets (β = 2.79), and the intermediate porosity (~26%) CI-analogs (β = 2.99) are much larger than β value for anhydrous targets of similar porosity, indicating jetting of water vapor could significantly affect deflection of hydrous asteroids and comets in natural or human-induced collisions.