Jack B. Hartung

Catastrophic rupture of lunar rocks - A Monte Carlo simulation

A computer model based on Monte Carlo techniques was developed to simulate the destruction of lunar rocks by ‘catastrophic rupture’ due to meteoroid impact. Energies necessary to accomplish catastrophic rupture were derived from laboratory experiments. A crater-production rate derived from lunar rocks was utilized to calculate absolute time scales.Calculated median survival times for crystalline lunar rocks are 1.9, 4.6, 10.3, and 22 m.y. for rock masses of 10, 102, 103, and 104 g respectively. Corresponding times of 6, 14.5, 32, and 68 × 106 yr are required, before the probability of destruction reaches 0.99. These results are consistent with absolute exposure ages measured on returned rocks.Some results also substantiate previous conclusions reached by others: the catastrophic rupture process is significantly more effective in obliterating lunar rocks compared to mass wasting by single particle abrasion. The view is also corroborated that most rocks presently on the lunar surface are either exhumed from the regolith or fragments of much larger boulders, rather than primary ejecta excavated from pristine bedrock.

Surface features on glass spherules from the Luna 16 sample

Two ellipsoidal spherules approximately 0.5 mm in diameter were studied in detail using a scanning electron microscope. A variety of surface features were observed: vesicles, mounds, dimples, streaks, ridges, grooves, accretion phenomena, and high-speed impact craters. The diameters of 27 glass-lined pits formed by impact on one spherule range from less than 1μm to approximately 50μm. Intermediate-sized glass-lined pits surrounded by concentric fractures demonstrate the transition between larger craters that have both a pit and a spall zone and generally smaller craters that have only a pit. Assuming all craters showing evidence of impact-related melting or flow are the result of primary impacts, the differential mass spectrum of impacting meteoroids in the range 10−11 to 10−10 g is in good agreement with a spectrum based on satellite-borne particle-detecting experiments.

Micrometeorite craters and related features on lunar rock surfaces

Abstract Craters in the 0.4 mm and larger size class were observed on six Apollo 12 whole rock surfaces (12017, 12021, 12038, 12047, 12051 and 12073). Craters on crystalline surfaces are characterized by a central, glass-lined cavity, a concentric zone of shock fractured, high albedo material and a concentric spallation area. The crater geometries observed are similar to craters produced on glasses and crystalline materials in the laboratory with projectile velocities exceeding 10 km/sec. The high projectile velocities required and the presence of a distinct demarcation line between cratered and uncratered surfaces on individual rocks indicate that most of the microcraters are produced by primary cosmic particles. These discrete impact events account for most of the erosion and fragmentation of lunar surface rocks.