Physics
Scientific paper
Dec 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufm.p31c..06m&link_type=abstract
American Geophysical Union, Fall Meeting 2004, abstract #P31C-06
Physics
6045 Physics And Chemistry Of Materials, 6205 Asteroids And Meteoroids, 6215 Extraterrestrial Materials, 6240 Meteorites And Tektites
Scientific paper
Chondrites are increasingly understood to be the products of uniquely nebular processes. Refractory inclusions are now thought to be mixtures of high-temperature condensates and evaporation residues. High-resolution chronometers demonstrate that the formation of refractory inclusions predated chondrules. The heating mechanism for chondrules remains controversial, but rapid melting (required for retention of moderately volatile elements) of dust aggregates by nebular shocks or stellar outflows appears plausible. After (commonly incomplete) melting, as revealed by their textures, chondrules solidified during non-linear cooling and were sorted aerodynamically, perhaps in nebular eddies. Accretion of chondrules, inclusions, and other components within several million years of their formation produced planetesimals. These bodies experienced thermal processing driven by decay of short-lived radionuclides. Increasingly sophisticated thermal evolution models of asteroids, now involving incremental accretion, can account for the metamorphic effects, peak temperatures, radio-isotope blocking ages, and cooling rates measured in chondrites. Many onion-shell asteroids were subsequently converted into rubble piles, reassembled after catastrophic collisions and modified by shock. These planetesimals were the building blocks for planets. Many constraints provided by chondrites to the study of nebular processes are attributable to this year's Whipple Award recipient, John Wood. He is responsible for a universally used chondrite classification scheme that first quantified the effects of thermal metamorphism, the first estimates of chondritic asteroid cooling rates (based on Ni diffusion profiles in metal), the proposal of a nebular shock model for chondrule formation, and constraints from chondrites on the accretion process and on planetesimal heat sources. Moreover, he was also the first to recognize the feldspathic composition of the lunar highlands (based on plagioclase grains in Apollo 11 soils) and to propose an explanation through fractional crystallization in a magma ocean. From observations of tiny samples, Wood has evoked astrophysical and geologic hypotheses that have shaped our understanding of the early solar system.
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