Planetary Science

When Rubble Piles Collide

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Science  30 Jun 2000:
Vol. 288, Issue 5475, pp. 2287
DOI: 10.1126/science.288.5475.2287c

Recent observations by the Near Earth Asteroid Rendezvous (NEAR) spacecraft revealed that the asteroid Mathilde has a density close to that of water and a huge impact crater. The asteroid's low density and large craters suggest that it may in fact be a pile of porous rubble held together by gravity. Early in the life of the solar system, planet formation might have been initiated or accelerated by the accretion of multiple rubble piles within a dusty, gaseous environment.

Leinhardt et al. have begun the task of defining the crucial parameters and modeling the outcomes of different collisional scenarios in order to understand how objects like Mathilde may have formed and how planet formation may have occurred. In their simulations, they varied the sizes of the objects, their impact speeds, their spin rates and directions, and the angle of the impact. Slow, head-on collisions led to accretion while fast, high-angle collisions led to erosion. They could create contact binaries (dog bone-shaped objects) by high-angle collisions at slow-impact speeds; however, it was difficult to produce detached binaries or an appreciable amount of orbiting material. Nonetheless, their suite of simulations provides a useful map of some of the parameter space in the rough and tumble environment of rubble pile collisions.—LR

Icarus146, 133 (2000).

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