DPS 35th Meeting, 1-6 September 2003
Session 24. Asteroid Physical Studies II
Oral, Chairs: A. Cheng and L. Benner, Thursday, September 4, 2003, 1:30-3:00pm, DeAnza III

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[24.07] Asteroid Collisional Evolution, Internal Structure, and Regolith - Lessons from NEAR, Predictions for MUSES-C

A. Cheng (Johns Hopkins APL)

A new synthesis of asteroid collisional evolution will be attempted, motivated by the long-standing question of whether asteroids below a few hundred km size are collisional shards or strengthless rubble piles. NEAR found Eros not to be a rubble pile, but a shattered body, with a through-going fracture system, and an average of about 20 m regolith cover. Gaspra, with its faceted appearance and grooves, and Ida with its grooves, blocks and regolith, have also been interpreted as 'shards' based on limited evidence from Galileo. Mathilde, with its high porosity and giant craters, has been interpreted from the NEAR flyby as a rubble pile, but geological evidence for such a structure is lacking, while there is evidence that Mathilde is not strengthless. Nevertheless, the rubble pile hypothesis has found increasing favor. Reasons include: theoretical understanding of how asteroid static strength should decline with increasing size up to a scale ~ few hundred m beyond which strengthening results from self-gravity; how strain rate dependence of dynamic strength also affects brittle fragmentation of asteroids; the observed distribution of rotation rates versus size, which is consistent with a 'speed limit' at which some part of the interior must be in tension, and where the vast majority of asteroids above a few hundred m are below the speed limit.

This situation leaves us with perplexing questions: is Eros, the single well-observed asteroid, highly unusual because it is not a rubble pile? is Mathilde, IF it is a rubble pile, like most asteroids under 100 km? what should we expect of 1998SF36, the MUSES-C sample return target? I will argue that Eros should be the rule rather than the exception, and examine implications for asteroid collisional evolution. The boulder size distribution on Eros measures fragmentation of asteroidal material, and it is a sample of impactor size distributions in the asteroid belt. Small asteroids like 1998 SF36 are expected to be shattered objects, but not necessarily rubble piles and not necessarily strengthless. The regolith on Eros may result largely from the last giant impact, and the same may be true of 1998 SF36, in which case about a meter of regolith would be expected. This work has been supported by NASA.

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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.