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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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