Impact Excitation of a Seismic Pulse and Vibrational Normal Modes on Asteroid Bennu and Associated Slumping of Regolith

TL;DR

This study models how energetic impacts on asteroid Bennu can excite seismic waves, potentially causing surface slumping and shape changes, especially at the equatorial ridge, through seismic reverberation and normal mode excitation.

Contribution

It introduces a new seismic model for small asteroids, demonstrating how impacts can excite normal modes and cause localized surface reshaping.

Findings

Seismic waves can be excited near the asteroid's normal modes by energetic impacts.

Seismic reverberation may cause modest elevation changes on Bennu's surface.

Impact-induced vibrations are unlikely to cause large shape alterations.

Abstract

We consider an impact on an asteroid that is energetic enough to cause resurfacing by seismic reverberation and just below the catastrophic disruption threshold, assuming that seismic waves are not rapidly attenuated. In asteroids with diameter less than 1 km we identify a regime where rare energetic impactors can excite seismic waves with frequencies near those of the asteroid's slowest normal modes. In this regime, the distribution of seismic reverberation is not evenly distributed across the body surface. With mass-spring model elastic simulations, we model impact excitation of seismic waves with a force pulse exerted on the surface and using three different asteroid shape models. The simulations exhibit antipodal focusing and normal mode excitation. If the impulse excited vibrational energy is long lasting, vibrations are highest at impact point, its antipode and at high surface elevations such as an equatorial ridge. A near equatorial impact launches a seismic impulse on a non-spherical body that can be focused on two additional points on an the equatorial ridge. We explore simple flow models for the morphology of vibration induced surface slumping. We find that the initial seismic pulse is unlikely to cause large shape changes. Long lasting seismic reverberation on Bennu caused by a near equatorial impact could have raised the height of its equatorial ridge by a few meters and raised two peaks on it, one near impact site and the other near its antipode.