The Seismic Effect of Impacts on Asteroid Surface Morphology: Three-

Introduction: In recent years, spacecraft observations of 'medium-sized' asteroids 951 Gaspra, 243 Ida, 253 Mathilde, and most importantly 433 Eros have shown the overriding dominance of impact processes with regard to the structure and appearance of these irregular bodies. In a previous work [1], I investigated the relatively unexplored area of seismic shaking as a cause for the downslope motion of regolith and crater degradation on this type of object. This early work showed that seismic shaking is indeed a dominant source for surface modification on small bodies, and the cause for the paucity of small craters on asteroid 433 Eros, in particular. In this current work, I have extended the theory and model development presented in [1] from a general or 'global'-scale view of seismic effects to the more regional, or local-scale view of the seismic effects of individual impacts. Two forms of numerical models were utilized: (1) a numerical 'shake-table', and (2) a three-dimensional terrain evolution model. Numerical Shake Table: To evaluate the seismic effects of an individual impact at some distance r from the impact site, a numerical shake-table was developed which takes the accelerations recorded in a series of synthetic seismograms (see [1] for a detailed description) and applies them to a hypothetical regolith layer resting on a slope of variable angle, and placed in a low, asteroid gravity field. For this numerical 'experi-ment,' I use a form of Newmark slide-block analysis, which can be applied when the regolith layer thickness under consideration is much smaller than the seismic wavelengths involved. This assumption works well for all but the highest impact seismic frequencies, which are the most quickly lost by attenuation. Under this restriction, we can approximate the motion of a mobilized regolith layer by modeling the motion of a rigid block resting on an inclined plane (for discussions of the forces involved, see [2]). I compute the accelerations imparted to the block by the asteroid's surface gravity (static loading) and seismically shaking slope (dynamic loading) to obtain an overall block (layer) displacement. The amount of downslope motion for both cohesive and non-cohesive regoliths is computed as a function of impactor size/mass, impact velocity, target surface gravity, and distance from the impact site, and general fits developed. Figure 1 shows a schematic view of this model.