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The Paucity of Small Craters on Eros and Itokawa: You Can't Explain This One, Yarkovsky

Wednesday, March 19, 2008
O'Brien (PSI)

Images of the near-Earth asteroid Eros returned by the NEAR Shoemaker spacecraft were the first to resolve craters smaller than a few hundred meters in diameter on the surface of an asteroid. These images showed a surface significantly depleted in small (< 200 m diameter) craters relative to the nearly-saturated population of larger craters. More recently, images of the near-Earth asteroid Itokawa, taken by the Hayabusa spacecraft, have shown a population of small craters very similar to that of Eros. In both cases, the Yarkovsky effect has been invoked as a possible explanation. The Yarkovsky effect is a radiation force capable of giving significant semimajor axis mobility to asteroids smaller than ~1 km, and increases in strength with decreasing size. In this scenario, most small impactors would be rapidly removed from the main belt, and thus very few craters smaller than ~200 m in diameter would ever be formed. However, that scenario has never been explicitly modeled. I will present the results of a self-consistent collisional and dynamical evolution model for the main belt and NEAs, along with a model for the evolution of asteroid crater populations, that show that the lack of small craters on these asteroids is not due to the depletion of small impactors by the Yarkovsky effect, or any other depletion mechanism. To produce a main-belt size distribution that is suitably depleted in small impactors to match Eros and Itokawa's small crater populations requires a more extreme size-dependent removal rate than the Yarkovsky effect (and even Poynting-Robertson drag) can provide. Using such an extreme removal rate would introduce a wave into the model main-belt size distribution that propagates to large sizes, and is inconsistent with the observed main-belt population. Similarly, it would introduce a wave in the model NEA population that is inconsistent with the observed NEAs. I will discuss other more plausible explanations for the lack of small craters on Eros and Itokawa, in particular erasure by seismic shaking.

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