The Seismic Effect of Impacts on Asteroid Surface Morphology: Early

Introduction: In recent years, spacecraft observations of asteroids 951 Gaspra, 243 Ida, 253 Mathilde, and 433 Eros have shown the overriding dominance of impact processes with regard to the structure and appearance of these small, irregular bodies. One currently unanswered question is: to what degree does seismic shaking from impacts affect the surface morphology of these bodies [1]? Through detailed modeling, we begin to address this question. The Impact Seismic Signal: An impact can be modeled as an impulsive point source acting on a free surface. The displacement field consists of a downward, vertical singlet component and two orthogonal, horizontal dipole components, which create an expanding, hemispherical pressure pulse within the medium. The resulting seismic waveform is shown in Fig. 1, consisting of a small P-wave displacement and a larger Raleigh wave displacement, and is quite similar to the waveform of the ‘classic’ Lamb pulse [2]. A power spectrum of the impact seismic waveform displays a broad frequency peak between 1 and 200 Hz (max. at 50-80 Hz), which agrees well with experimental (missile impact) results [3]. The seismic effects of an impact on an asteroid can be divided into two categories: short-term pulse effects and long-term reverberation effects. Seismic Pulse Effects: This is surface modification due to the passage of discrete seismic pulses over the surface of the asteroid shortly after impact. These effects are significant because only tiny accelerations are needed to overcome the resistance due to gravity on these bodies (on the order of 1/1000 Earth g). Thus, relatively small impacts can have ‘global’ seismic effects. For example, hydrocode models of an impact producing a 1 km diameter crater on a 20 km diameter asteroid show that it will generate a magnitude 4 quake, impart surface accelerations of over 10 asteroid g out to an angular radius of 50 from the impact site, and destabilize all slopes on the surface. Seismic Reverberation Effects: Due to their small volumes and potentially high seismic quality factors (Q = 2000-4000 [4]), asteroids can experience seismic reverberations lasting for several minutes following the dispersion of the initial seismic pulses. These reverberations would create additional accelerations on the surface and likely cause further surface modification. For example, numerically modeled impacts which produce craters in the 0.1-1 km range on a 20 km diameter asteroid are able to generate reverberations of better than 1 asteroid g acceleration over the entire surface, again destabilizing all slopes and rearranging loose material.