Cold Compaction and Macro-Porosity Removal in Rubble-Pile Asteroids: 2. Applications

Models of asteroid collisional evolution suggest that many asteroids are gravitationally-bound rubble piles. Although rubble piles may be expected to retain large void fractions, compaction may reduce the porosity. We apply models for cold compaction of rubble-pile bodies developed in a companion paper toward observations of asteroid densities. The model for chondritic boulders is applied to S-type (stony) and C-type (carbonaceous) asteroids. The relation between density and size of S-type asteroids is largely explained by cold compaction of rubble piles through fracturing of boulders, under the assumption that boulder size distributions are narrow before fracturing and fractal-like afterward. The density variation of C-type asteroids can only partly be explained by this mechanism, and the removal of micro-voids inside the boulders would be required to match observations. The model for metal boulders is applied to M-type asteroids, and the results suggest that, because of cold welding between metal boulders and the high yield strength of metal for either ductile or brittle-like deformation, metallic rubble piles can preserve large (≳50%) porosities if the boulders are ∼1 m in size. This implies that M-type asteroids such as Psyche and Kleopatra may be purely metallic, even though their densities are less than half that of iron. We also consider the hypothesis that Psyche is a primitive body of a CB chondrite-like material. Assuming that the strength of CB chondrite is controlled by a silicate matrix, we predict that the density of a Psyche-sized rubble pile of CB chondrite is higher than that of Psyche.