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R. Greenberg, D.P. O'Brien (Lunar and Planetary Laboratory, Univ. of Arizona)
On Eros, the size distributions of craters, and of the boulders that likely were ejected from them, provide important constraints on the collisional history of the asteroid. NEAR images showed a remarkably steep size distribution of boulders, and few craters smaller than 50m, relative to extrapolation from larger-size craters. The excess of small (few-meter) boulders and paucity of small craters was attributed to removal by the Yarkovsky effect of smaller projectiles from the bombarding population (Chapman et al. Icarus 155, 104, 2002), but O’Brien and Greenberg (LPSC 2002, 2003) showed that the latter effect is unlikely to be significant. Some small craters are likely removed by seismic jolting (Greenberg et al. Icarus 107, 84, 1994; Icarus 120, 106, 1996), augmented by the steep effective gravitaional slopes on much of the surface (Robinson et al. LPSC 2003), but it remains unclear whether these erasure effects can adequately explain the crater statistics. Here we show that plausible crater formation rates over the past 50 Myr, consistent with the observed crater record (and a likely age since a resetting event), would produce ejecta blocks in numbers in good agreement with the observed size distribution of boulders. This result follows simply from a conventional model for ejecta size distributions. One implication would be that most of the (few-meter-plus) boulders lie on the surface, rather than buried in the regolith of smaller debris. The paucity of visible small craters may be (at least partly) an observational effect: The numbers of ejecta blocks just smaller than image resolution may disguise small craters. Thus, the observed size distribution of boulders might be explainable in large part as a natural consequence of cratering, and the small numbers of small craters visible in NEAR images may result from the presence of those boulders.
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.