THE STOCHASTIC EVOLUTION OF ASTEROIDAL REGOLITHS AND THE ORIGIN OF BRECCIATED AND GAS-RICH METEORITES

Abstract

A model is constructed which views regolith evolution on asteroids as a stochastic process. Average values are shown to be poor descriptors of regolith depth. Large deviations from the average are expected to occur due both to variations in the depth over the surface of a body and to stochastic fluctuations in the variables which determine regolith depth, e.g. the number of craters produced on an asteroid. The utility of the average depth is not significantly increased by avoiding large craters of thick ejecta deposits; a procedure adopted in previous regolith studies. The statistical uncertainty associated with regolith depth severely limits the power of regolith models in predicting parent-body size for brecciated meteorites. Virtually any rocky asteroid larger than 100-200 km in diameter could have produced the abundance of brecciated material observed in the achondritic meteorites. Bodies which are composed of weaker materials and which have diameters greater than 20 km could have produced the abundance of breccias observed in the chondrites. A Monte Carlo algorithm is used to simulate the random walks and corresponding changed-particle irradiation histories of grains in regoliths. On rocky asteroids, only about 20% of the grains are exposed to solar cosmic ray ions. These grains typically spend a few thousand years in the upper 100 microns of the regolith and acquire particle track densities of 10⁷-10⁹/cm² at their surfaces. Only about 5% of the grains acquire track densities greater than 10⁸/cm². Grains which reach the surface are exposed to galactic cosmic rays for roughly 10⁶y. Weak asteroids with diameters less than a few tens of kilometers have very immature regoliths because of short collisional lifetimes and the ejection of heavily irradiated grains to space. Only a few percent of the grains are exposed at the surface and these acquire track densities of 10⁷-10⁸/cm². Exposure times and the fraction of grains irradiated should increase for larger weak bodies due to longer collisional lifetimes. These results, which are based on present-day conditions in the asteroid belt, agree well with irradiation features observed in gas-rich meteorites; an origin during epochs of early solar system evolution is not required.