Ejecta Reaccretion on Rapidly Rotating Asteroids: Implications for 243 Ida and 433 Eros

The morphology of crater ejecta blankets and the distribution of impact-derived regolith on asteroids such as 243 Ida and 433 Eros may vary markedly with surface location, due to their low gravity, nonspherical shape and rapid rotation. We have modelled the processes of ejecta reaccretion and regolith redistribution for the specific cases of Ida and Eros with a 3-dimensional numerical simulation which tracks the orbits of particles launched from the surface of a rotating triaxial ellipsoid. The results depend primarily upon the initial launch velocities: at low ejection speeds (V $<<$ Vesc), craters form well-defined ejecta blankets which are asymmetric in morphology between leading and trailing rotational surfaces. Ejecta blankets located near the equator have sharp boundaries in the direction of rotation, at a threshold beyond which ejecta is catapulted into orbit by the rotational velocity. The net effect of cratering at low ejecta launch velocities is to produce a thick regolith which is evenly distributed across the surface of the asteroid. No clearly defined ejecta blankets are formed when ejecta are launched at higher initial velocities (V $\sim$ Vesc). Most of the ejecta escape, while the fraction that is retained is preferentially derived from the rotational trailing surfaces. These particles spend a significant time in temporary orbit around the asteroid, in comparison to the asteroid's rotation period, and tend to be swept up onto the rotational leading surfaces upon reimpact. The net effect of impact cratering with high ejecta launch velocities is to produce a thinner and less uniform soil cover, with concentrations on the asteroids' rotational leading surfaces.