Shock compression and adiabatic release of lunar fines from Apollo 17

Abstract

Shock compression and adiabatic release data on lunar fines (sample, 70051), with an initial density of 1.80 g/cm^3, are obtained in the 20-125 kbar range. The sample contains approximately 60% pyroxenes, 30% plagioclase and 10% brown glass and opaques, and has an estimated intrinsic density of 3 .10 ± 0.10 g/cm^3. Irreversible compaction, and presumably, induration to a post-shock, zero-pressure density of 3.06 ± 0.01 g/cm^3 occurs upon shock compression to 20 kbar (or lower) and subsequent adiabatic release. Above this pressure level the Hugoniot data are closely predicted, at a given volume, by adding the thermal pressure calculated from the Mie-Gruneisen equation (Gruneisen's ratio 0.8) to the pressure along the theoretical Hugoniot for intrinsic crystal density material. For shock compression to pressures of between 20 and 125 kbar, the calculated post shock temperature varies from 215±15 to 770±155°C. When taken with the determined post-shock density variation from 3.1 to 2.6 g/cm^3, this result implies that increasing quantities of glassy material is produced via solid-state reaction and possibly melting over this pressure range. The onset of post-shock thermal melting is calculated to occur upon impact of iron or stony meteoroids traveling at a minimum of 2.5 and 3 km/sec, respectively. This implies that appreciable solid-state transformation of plagioclase, and melting of previously lightly shocked surficial rubble will result only from primary impacts. However, the pressures required for irreversible compaction imply that agglutinates will form in the regolith upon impact of secondary objects (ejecta) at speeds on the order of only 1 km/sec.