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

Abstract

AbstractModels 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.