Physical Properties and Processing of Asteroid Regoliths and Interiors

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Thermal Cycling, Electrostatics, Meteorites, Ejecta

Scientific paper

Four aspects of the physical properties and processing of asteroid regoliths and interiors are examined: (1) impact cratering, (2) thermal cycling, (3) electrostatic processing, and (4) asteroid densities. These aspects contribute to understanding the production, emplacement, redistribution, segregation, disruption, loss, and overall state of regoliths on asteroids. Impact cratering (Chapter 2) is considered through a study of the scaling characteristics and distribution of the large blocks revealed on 243 Ida in Galileo images. The blocks are interpreted as coarse impact ejecta fragments, most of them remaining within or in the vicinity of the large impact structures from which they were excavated. Alternative origins and the probable age of the blocks are discussed. Extrapolation of ejecta scaling laws applicable to Ida lead to predicting maximum ejecta blocks sizes on other asteroids. Thermal cycling, the periodic stressing and straining of asteroid regolith materials due to insolation-induced diurnal and orbital temperature variations, is investigated as another process whereby asteroid regoliths might be disrupted (thermal weathering and disaggregation; thermal quakes), and transported (thermal creep) (Chapter 3). Thermal cycling is found to be of minor significance in the evolution of asteroid regoliths. Electrostatic processing, the levitation, transport, and/or ejection of charged dust under electrostatic fields produced on resistive surfaces by insolation-induced photoelectron emission, is proposed as a contributing mechanism whereby asteroid regoliths may be sorted, redistributed, and winnowed of their finest particle size fraction (Chapter 4). The process may help explain differences in regolith texture between asteroids and the Moon, and among various types of asteroids. Finally, asteroid and meteorite density data are reviewed and interpreted to constrain the internal structure of asteroids (Chapter 5). A relation between meteorite and asteroid densities is proposed that takes into account the strength selection undergone by meteorites and the collisional bulking experienced by their parent bodies. This relation leads to revising Ida's bulk porosity in stronger favor of an ordinary chondrite composition for this S-type asteroid. complex and dynamic than previously understood. The relation between the bulk density of these objects and their internal structure is also clarified.

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