CAT MOUNTAIN - A METEORITIC SAMPLE OF AN IMPACT-MELTED ASTEROID REGOLITH

Cat Mountain is a new ordinary chondrite impact melt breccia that cont ains several shocked chondrule-bearing clasts of L5 material. These cl asts are surrounded by a total impact melt of similar composition mate rial which appears to have cooled over a period of a few thousand year s, probably within a melt breccia lens in the bottom of a large (>1 km diameter) crater on an L chondrite asteroid. Noble gas isotopes indic ate that the sample was involved in at least two different impact even ts, approximately ssb and 20 Myr ago, following the 4.55 Ga accretion of primitive chondritic material. The 880 Ma event is responsible for the impact breccia texture of the sample, and the 20 Ma event reduced the sample to a meter-sized object. We also infer that another impact occurred between 880 and 20 Ma (possibly the similar to 500 Ma event r ecorded in many other L chondrites) to jettison the material from the asteroid belt into an orbit that evolved into an Earth-crossing trajec tory. The shock-metamorphic processes that occurred at 880 Ma redistri buted the opaque phases in the meteorite and altered the crystalline c haracteristics of silicate phases. This reduced the reflectance of the L5 material and decreased the amplitude of its spectral absorption fe atures. These characteristics are consistent with the spectral charact eristics of some C class. asteroids and suggest that some dark asteroi ds that appear to belong to the C class could be covered with shocked ordinary chondrite material. If one assumes that Cat Mountain came fro m the same asteroid as other L chondrites with the same cosmic ray exp osure age, then the juxtaposition of these different materials suggest s asteroids are rubble piles which are heterogeneous on a scale less t han 100 m. Furthermore, the structural integrity of Cat Mountain and o ther L chondrites suggests the strengths of asteroid rubble piles are limited by fractures and contrasting material properties and are thus inherently weak in a ram pressure regime produced when they enter a pl anetary atmosphere. However, in a regime where the asteroid is the tar get of impact fragmentation rather than the projectile, the added poro sity of a rubble pile structure will compensate for the presence of fr actures and absorb a large amount of the impact energy. In this case t he structural integrity of the asteroid may appear to be the same as a previously unshocked chondritic material.