DPS Pasadena Meeting 2000, 23-27 October 2000
Session 3. NEAR at Eros
Oral, Chairs: L. McFadden, D. Britt, Monday, 2000/10/23, 10:45am-12:15pm, C106

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[3.09] Craters, Boulders, and Impact History of Eros

C. R. Chapman, W. Merline, B. Bierhaus (SwRI), P. Thomas, J. Joseph, J. Veverka, J. Bell (Cornell), N. Izenberg, S. Murchie, L. Prockter, A. Cheng (JHU/APL), L. McFadden (U Maryland), M. Robinson (NWU)

We report size distributions for Eros craters D>15 m and boulders D>10 m. Although not saturated with giant craters like Mathilde, Eros (like Ida) exhibits empirical saturation for 200m < D <1 km. Craters D<100 m are progressively depleted (differential power-law exponent b~-1) relative to the equilibrium distribution for a lunar-like crater production function. Boulders, however, have an extremely steep distribution (b~-6). At D<20 m, boulders exceed craters. Eros has ~1 million boulders with D>8 m. Eros' fine-scale attributes may not be inconsistent with Ida (minimal resolved boulder D>~45 m [1]).

Eros has <~100 Myr lifetime against solar/planetary impact or ejection, so it was cratered mostly while in the asteroid belt. We hypothesize that unexpected attributes of Eros reflect a 100x decrease in cratering rate during the last tens of Myr; orbital simulations [2] suggest Eros had ~50% chance of such an impact hiatus since decoupling from the main belt. Such a hiatus might, in several ways, facilitate the preferential degradation/destruction of small craters on Eros, best explained by a non- equilibrium burst (relative to cratering rate) of non-impact-related topographic degradational processes. Instead of an episode of enhanced endogenic activity (the usual explanation for a planet), normally minor, unrecognizable endogenic processes on asteroids (e.g. thermal creep) might be manifest after a long cratering hiatus. Perhaps long-term relaxation from effects of a major impact (e.g. the "saddle" or Psyche crater) persisted into the hypothesized hiatus, including possible reaccretion of transient orbiting debris/boulders but without competition from continuing hypervelocity impacts. Eros' spectral homogeneity (compared with otherwise similar Ida, which has patches of less-weathered ejecta from recent impacts) may also be explained by the hiatus: space weathering may have matured since the last big impact irregularly blanketed Eros' surface.

[1] P. Lee et al. (1996) Icarus 120, 87. [2] P. Michel et al. (1998), Astron. J. 116, 2023.

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