Structure of the principal (or sole) CM parent body

CM chondrites likely hail from a single parent asteroid, although a minority may be from a few other bodies. The principal CM parent body did not have an onion-shell structure; it lacked petrologic type 4, 5 and 6 materials. The body was not stratified with more-altered rocks at increasing depths. During accretion, the CM asteroid accumulated a moderate amount of water-ice and consisted mainly of chondrules, CAIs, amoeboid olivine aggregates, fine-grained matrix material, and grains of metallic Fe-Ni and sulfide; it was CM3.0 and fairly homogeneous. Stochastic collisions caused extensive fracturing and foliations in some regions. About 5 Ma after CAIs, collisions melted much of the accreted ice, mobilizing aqueous fluids. More-fractured regions (with prominent petrofabrics) achieved higher water/rock ratios and became more altered. A major collision disrupted the body after most alteration had occurred. Asteroidal fragments were jumbled up and gravitationally reassembled into a rubble pile. Some CM asteroidal fragments may have escaped the original body and evolved Earth-crossing orbits. Disruption-and-reassembly, along with near-surface impact gardening, mixed together CM materials with different alteration histories to form genomict breccias. Solar-wind gases were implanted into the matrix before aqueous alteration. Some anhydrous projectiles that impacted the regolith were incorporated as xenoliths.