Abstract
A number of asteroids have been observed to have extremely low bulk densities in the 1.3 g/cm 3 range. Given reasonable compositions based on meteorite analogues, this bulk density implies bulk porosities of 40–60%, suggesting that many asteroids are rubble piles held together by self-gravity and containing significant empty space, perhaps more empty space than solid material. Simple energy balance calculations and accretional models of disrupted asteroids suggest that materials in porous asteroids may be sorted by particle size. The large irregular pieces (and larger voids/fractures) are probably located deeper inside the asteroid and the fine particle size fractions that are observed on their surfaces are restricted to the surface regolith zone. The large interior voids/fractures are preserved from infilling by the effects of friction on the smaller size fractions. Friction tends to dominate the downward pull of gravity and prevents the fine fractions from filtering into the interior of the asteroid and infilling the large fractures and voids. Friction may also play a role in allowing shattered asteroids to maintain their relief features and shape by resisting the movements of pieces within the object, in effect providing strength to non-coherent objects.
Published Version
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