Collisional History of Gaspra

Interpretation of the impact record on Asteroid 951 Gaspra requires understanding of the effects of collisions on a target body of Gaspra's size and shape. Recent hydrocode models show that major impacts on Gaspra may leave craters larger than previously thought possible and that they can create substantial regolith and produce global jolting capable of erasing many smaller craters. A Gaspra-size body has a mean lifetime of about 10⁹ years and is likely to have several impact craters of 4 km diameter or larger, which is consistent with the number of observed concavities on Gaspra. The steep size distribution of small craters on Gaspra implies an even steeper distribution for small asteroids, and the relative paucity of kilometer-size craters seems to require a twice-in-a-Gaspra-lifetime impact to have occurred about 50 myr ago. All of these considerations consistently point to a scenario in which several of the very large concavities (>4 km diameter) on Gaspra are probably impact craters.