Impact-produced seismic shaking and regolith growth on asteroids 433 Eros, 2867 Steins, and 25143 Itokawa

摘要

Of the several near-Earth and Main Belt asteroids visited by spacecraft to date, three display a paucity of small craters and an enhanced number of smoothed and degraded craters: 433 Eros, which has a deficit of craters less than or similar to 100 m in diameter; 2867 Steins, which has a deficit of craters less than or similar to 500 m in diameter; and 25143 Itokawa, which has a deficit of craters less than or similar to 100 m in diameter. The purpose of this work was to investigate and model topographic modification and crater erasure due to impact-induced seismic shaking, as well as impact-driven regolith production and loss, on these asteroid surfaces. To perform this study, we utilized the numerical, three-dimensional, Cratered Terrain Evolution Model (CTEM) initially presented in J. E. Richardson, Icarus 204 (2009), which received a small-body (SB) specific update for this work. SBCTEM simulations of the surface of 433 Eros correctly reproduce its observed cratering record (for craters up to similar to 3 km in diameter) at a minimum Main Belt exposure age of 225 +/- 75 Myr using a 'very weak rock' target strength of 0.5-5.0 MPa, and producing a mean regolith depth of 80 +/- 20 m, which agrees with published estimates of an actual regolith layer "tens of meters" in depth. SBCTEM simulations of the surface of 2867 Steins correctly reproduce its heavily softened cratering record at a minimum Main Belt exposure age of 175 +/- 25 Myr using a 'weak rock' target strength of 2.5-5.0 MPa. Extending this simulation produces a mean regolith depth of 155 +/- 5 m at a minimum Main Belt exposure age of 475 +/- 25 Myr, in good agreement with the 145 +/- 35 m actual mean regolith depth estimated from two chains of drainage-pit craters on the surface, with both crater density and regolith depth in an equilibrium state. SBCTEM simulations of the surface of 25143 Itokawa correctly reproduce its extremely sparse cratering record at a minimum Main Belt exposure age of 20 +/- 5 Myr, using a 'soft soil' target strength of 10-20 kPa, and producing a mean regolith overturn depth of 5.0 +/- 1.0 m, given that Itokawa is composed entirely of regolith and boulders. These simulations demonstrate the efficacy of impact-induced seismic shaking to erase small craters on the surface of asteroids less than or similar to 25 km mean diameter, correctly reproducing their observed cratering records while using a Main Belt impactor population and standard crater scaling relationships. These simulations also show that the observed regolith layer on asteroids in the 5-50 km size range can be generated entirely through impact cratering processes, and indicate that regolith depth can be used as a additional constraint on the surface age of a given asteroid.