Cold Compaction and Macro‐Porosity Removal in Rubble‐Pile Asteroids: 2. Applications. Issue 10 (19th October 2022)

• Record Type: Journal Article
• Authors: Zhang, Zhongtian; Bercovici, David; Elkins‐Tanton, Linda
• Is Part Of: Journal of geophysical research. Volume 127:Issue 10(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-10-19
• Subjects: Planets -- Periodicals; Geophysics -- Periodicals; 559.9
• DOI: 10.1029/2022JE007343
• Languages: English
• ISSNs: 2169-9097

Abstract

Abstract: 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. Plain Language Summary: Many asteroids are gravitationally‐bound rubble piles that are expected to contain large void fractions. Mass and volume estimates of asteroid bodies imply that (a) for asteroids of similar materials, larger bodies tend to have smaller void fractions, and (b) bodies that are suspected to be metallic appear to retain larger void fractions than stony bodies of similar sizes. The difference between stony and metallic asteroids is possibly caused by the variation in the extent of compaction under self‐gravitation. We use theoretical models for cold compaction of rubble‐pile bodies, developed in a companion paper, to evaluate this hypothesis. We suggest that (a) the density versus size relation of stony asteroids can be partly explained by a cold compaction mechanism, and (b) metallic rubble piles can retain large (over 50%) porosities. Therefore asteroids that have been classified as metallic based on spectral properties but have lower than expected density, such as (16) Psyche and (216) Kleopatra, could be entirely metallic. Key Points: We model cold compaction in stony and metallic rubble piles, under the assumption of initially narrow boulder size distributions Stony asteroid density increases with size, which can be largely explained by cold compaction under self‐gravitation Metallic rubble piles can preserve large enough porosities to explain the densities of M‐type asteroids like Psyche and Kleopatra