The Physical Properties of Meteorites and Interplanetaery Dust Particles: Implications for the Properties of Stone Asteroids

Introduction: Most asteroids for which porosities have been inferred have porosities ranging from 20% to >50%, with a mean of ~30% porosity [1]. Porosity significantly affects the physical properties of rocks, including the strength, thermal conductivity, seismic velocity, and dielectric permeability. Some asteroids have high enough porosities to affect their internal structure, gravitational field, response to impacts, and collisional lifetimes. This suggests modeling the behavior of asteroids using the physical properties of low-to moderate-porosity terrestrial rocks is not appropriate. Most/all stone meteorites and a significant fraction of interplanetary dust particles (IDPs) are believed to be fragments of asteroids, with some IDPs sampling comets. Thus, the physical properties of meteorites and IDPs provide indications of the properties of their parent asteroids [2]. Thermal evolution modeling indicates that, within each group (H, L, or LL) of ordinary chon-drite meteorites, increasing thermal metamorphism correlates with increasing formation depth in ~100 + km diameter parent asteroids [3], so these meteorites permit determination of properties of asteroids at depths ranging from the near surface to the deep interior. Effects of Porosity: Meteorites generally exhibit significant porosity, with three distinctly different types of porosity, cracks, gaps, and vugs, identified by computed microtomography and examination of thin sections. Specific classes of meteorites are associated with different types of asteroids based on similarity of visible and near-infrared reflection spectra, However, the mean porosity of ordinary chondrite meteorites is lower than that of the associated S-type asteroids and the mean porosity of carbonaceous chondrite meteorites is lower than that of the C-type asteroids. This indicates meteorites do not sample the entire range of porosity observed in asteroids – in particular, the highest porosi-ty material is missing in the meteorite population. Strength. Porosity reduces the compressive, tensile and sheer strengths of rocks. While the compressive strengths of the strongest ordinary chondrite meteorites are comparable to low-porosity terrestrial basalt (100-300 MPa), meteorite measurements span a much wider range. Some highly metamorphosed ordinary chon-drites, potentially sampling deep interiors of asteroids, have very low compressive strengths-20 MPa for the L5 Elenovka [4] and 6 MPa for the L6 Holbrook [5]. An alternative measure of strength is the response to hypervelocity impact, characterized by the parameter Q* D , the impactor energy per unit target mass produc