Experimental Study of the Impact Disruption of a Porous, Inhomogeneous Target

Abstract

Measurements of the densities of interplanetary dust particles and unweathered stone meteorites indicate that both have significant porosity on the microscopic scale. In addition, the chondritic stone meteorites are generally inhomogeneous, typically consisting of strong, millimeter-size, olivine chondrules embedded in a weaker, fine-grained matrix. Since target porosity is known to influence energy partitioning in cratering and disruption, we have begun a series of experiments to study the impact disruption of inhomogeneous assemblages of two materials of different strengths and which exhibit significant porosity. Experiments were performed on three ∼300-g targets of porphyritic olivine basalt, consisting of millimeter-size olivine phenocrysts in a fine-grained vesicular matrix (simulating a stone meteorite). Using the NASA Ames Vertical Gun, each target was impacted by a 1/4-in. diameter aluminum sphere at a speed of ∼5 km s −1. To avoid measuring the secondary effects of fragmentation caused by material impacting on the walls of the gun chamber, we monitored primary debris using passive detectors. We measured the size-frequency distribution of the small fragments using thin foils. Most foils showed only small depressions, sometimes containing fragments of debris, indicating relatively low velocity debris. One foil showed ∼300 puncture holes from high-speed particles, presumably a localized jet or cone of target or projectile ejecta. The size-frequency distribution was quite steep down to the ∼10- to 20-μm limit where particle size was comparable to foil thickness. Aerogel cells were employed to capture dust-size primary debris. Using an in situ chemical analysis technique, we distinguished matrix from olivine and determined that fragments<100μm in size were matrix while the majority of the largest fragments (>200 μm in size) were olivine. We also collected the debris from the floor of the gun chamber. The largest fragments (significantly bigger than individual olivine phenocrysts) were representative of bulk target material. In the millimeter-size range we found a large number of isolated olivine crystals, indicating the target experienced preferential failure along the phenocryst-matrix boundaries. All three shots showed distinct changes in the slopes of the mass-frequency distribution near 0.4 g, the size of typical olivine phenocrysts. This suggests that the mechanical failure of the material was affected by the presence of the phenocrysts. If our results are directly applicable to chondritic meteorites, then impact cratering and disruption of chondritic asteroids may overproduce olivine-rich material from chondrules in the millimeter-size range and olivine might be underrepresented at smaller sizes in the primary debris.