The effect of multiple particle sizes on cooling rates of chondrules produced in large-scale shocks in the solar nebula

摘要

Chondrules represent one of the best probes of the physical conditions and processes acting in the early solar nebula. Proposed chondrule formation models are assessed based on their ability to match the meteoritic evidence, especially experimental constraints on their thermal histories. The model most consistent with chondrule thermal histories is passage through shock waves in the solar nebula. Existing models of heating by shocks generally yield a good first-order approximation to inferred chondrule cooling rates. However, they predict prolonged heating in the preshock region, which would cause volatile loss and isotopic fractionation, which are not observed. These models have typically included particles of a single (large) size, i.e., chondrule precursors, or at most, large particles accompanied by micron-sized grains. The size distribution of solids present during chondrule formation controls the opacity of the affected region, and significantly affects the thermal histories of chondrules. Micron-sized grains evaporate too quickly to prevent excessive heating of chondrule precursors. However, isolated grains in chondrule-forming regions would rapidly coagulate into fractal aggregates. Preshock heating by infrared radiation from the shock front would cause these aggregates to melt and collapse into intermediate-sized (tens of microns) particles. We show that inclusion of such particles yields chondrule cooling rates consistent with petrologic and isotopic constraints.