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D.J. Scheeres (U. of Michigan), S.J. Ostro (JPL/Caltech)
Regolith on binary asteroids exists in a complex dynamical environment formed by the asteroid pair and their shapes, orbits and spin states. Unlike the surfaces of uniformly rotating, solitary asteroids, binary asteroid surfaces are subject to forces that vary in time, which can lead to the flow of regolith in the direction of an energetically relaxed surface. Similar conclusions hold if the asteroid is a rubble-pile aggregate.
Regolith on a primary spinning near the limit for disruption will preferentially flow toward the equator and the long ends of the body (Guibout and Scheeres, Cel. Mech. 87: 263-290, 2003). If the primary is at or near the disruption limit the migrated particles will fly off the surface into orbit or have a strongly reduced surface force. The presence of the secondary ensures that lofted particles will re-impact on the primary and, importantly, provides a relatively large periodic "shaking" that is maximized along the equator. Regolith is forced to find configurations at rest relative to the periodic forcing. As the system evolves due to internal tidal interactions, regolith distribution on the surface may vary as well.
For binary systems with a relatively large, close secondary this should result in a pronounced equatorial bulge (as hinted by preliminary radar-derived modeling of 1999 KW4; Ostro et al., in prep.). For a binary with a large, regolith-covered primary and a relatively small, distant secondary, this should force the primary towards a figure of equilibrium (Farinella et al., Icarus 46:114-123, 1981; Weidenschilling, Icarus 46:124-126, 1981). We note that a recent study (Hestroffer, BAAS 36(2), 35th AAS-DDA Meeting, Abstract 7.10, 2004) found that large main belt asteroids with small co-orbitals are remarkably close to the Jacobi ellipsoid defined for their angular momentum.
The authors acknowledge support from the NASA PG&G program.
The author(s) of this abstract have provided an email address for comments about the abstract: scheeres@umich.edu
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.