Hypervelocity cratering and disruption of the Northwest Africa 869 ordinary chondrite meteorite: Implications for crater production, catastrophic disruption, momentum transfer and dust production on asteroids

Abstract

Most asteroids for which porosities have been inferred have porosities from 20% to more than 50%, comparable to the range measured on chondritic meteorites. To investigate the effects of target porosity on cratering, impact disruption, momentum transfer and dust production we performed a series of hypervelocity impact experiments on targets of Northwest Africa (NWA) 869, a brecciated L3-6 ordinary chondrite, impacted by 1/16″ to ¼” Al-spheres at speeds ranging from 4.08 to 5.74 km/s at the NASA Ames Vertical Gun Range. These samples had a mean porosity of ∼6.4% and a mean unconfined compressive strength of ∼87 MPa. Ten hypervelocity disruptions demonstrated that these NWA 869 targets are significantly more resistant to disruption, i.e., they require more impactor kinetic energy per unit target mass to produce an equivalent disruption, than non-porous terrestrial basalt targets. The threshold collisional specific energy, Q*D, for these NWA 869 targets is ∼1795 J/kg, more than twice the literature value for non-porous terrestrial basalt, but significantly lower than the ∼2380 J/kg value we measured previously for highly-porous terrestrial pumice. Volatile-rich matrix was overrepresented in the ≤30 μm fraction of the dust while dust >30 μm in size was dominated by chondrule material in the disruption events. We measured the post-impact momentum of seven NWA 869 cratering events and found a mean momentum transfer of 2.7 times the momentum of the projectile, showing that the recoil from the crater ejecta exceeded the direct momentum transferred by absorption of the projectile by a factor of ∼1.7. We observed a two-stage development of the crater ejecta, with an initial release of high-speed, fine-grained material in a cone with an apex angle near 90°, followed by larger, lower-speed material directed nearly opposite the direction of the impactor.