A Thermal Model for the Differentiation of Asteroid 4 Vesta, Based on Radiogenic Heating

Abstract

A finite element code has been developed to model the thermal history of Asteroid 4 Vesta. This is the first attempt to model the thermal history of a differentiated asteroid through core and crust formation and subsequent cooling until geochemical closure is attained. The results of the simulation are consistent with chronological measurements and other constraints provided by cumulate and noncumulate eucrites believed to have been derived from Vesta. The work solves two major problems with the hypothesis of heating by decay of26Al, an extinct radionuclide, postulated to be a plausible heat source in the early Solar System. First, the model demonstrates that it is possible to keep the mantle of Vesta hot for ∼100 Ma, thereby explaining the observed difference in ages between cumulate and noncumulate eucrites. Second, the simulation offers a possible explanation of why detectable excesses of26Mg (the decay product of26Al) are not observed in noncumulate eucrites. The simulation draws a model chronology of Vesta and predicts times (relative to CAI formation) for accretion at 2.85 Myr, core formation at 4.58 Myr, crust formation at 6.58 Myr, and geochemical closure at ∼100 Myr for a H-chondrite asteroidal bulk composition. Decay of60Fe is found to cause no perceptible difference in the thermal history of Vesta, even when sequestered into a central core. Although chondritic xenoliths have not been described in HED igneous lithologies, the thermal model suggests the possibility that a veneer of unmelted near-surface material should remain.