Asteroid Thermal Evolution with Fragmentation and Reassembly into a Rubble Pile

Abstract:

Recent geochemical observations show that the H-chondrite parent body cooled rapidly from near peak temperature. This suggests that this body experienced catastrophic fragmentation and quickly reassembled into a rubble pile asteroid that cooled slowly through low temperatures. We have developed a thermal model for asteroid fragmentation and reassembly to test if this process is consistent with both rapid cooling from high temperature and slow cooling through low temperatures. The initial body is heated by the decay of short-lived radionuclides until it is catastrophically disrupted at high temperature. The resulting fragments are described by a discrete power-law for the cumulative size. Fragments are assumed to be spherical and evolved with an analytic solution. This allows us to evolve a large number of fragments individually to estimate the energy loss during reassembly. Dynamical arguments suggest that a majority of the fragments reassemble very quickly. We assume that the largest fragments form the nucleus of the rubble pile that is surrounded by a porous mega-regolith formed by the smaller fragments. The thermal evolution of the rubble pile is controlled by the reduction in the thermal conductivity in the porous mega-regolith.

The effect of fragmentation and reassembly is to produce fast initial cooling, but to slow the cooling of the asteroid through low temperature intervals, rather than to accelerate it. The energy loss during fragmentation is typically outweighed by the reduction in the effective thermal conductivity in the rubble pile. This is largely due to the very short reassembly time, which is thought to be less than one year. Over these short time scales only the most extreme fragmentations lead to significant energy loss. Thus, our models demonstrate it is possible to have rapid cooling from high temperature during fragmentation followed by slow cooling through low temperatures in the rubble pile. This suggests that fragmentation and reassembly near peak temperature provides a suitable framework to reconcile the disparate cooling rates inferred for the H-chondrite parent body. Data from other ordinary chondrite parent bodies also show rapid high temperature cooling, followed by slow cooling, suggesting that fragmentation and reassembly may have been a common process in the early solar system.