The role of regolith thickness in seismic shaking on asteroids

Near-Earth Asteroids visited by spacecraft display a depletion in the number of small craters (< 100 m). For example, the fractured monolith 433 Eros (Thomas et al., 2005), and the rubble piles 25143 Itokawa (Michel et al., 2009), 162173 Ryugu (Noguchi et al., 2021), and 101955 Bennu (Daly et al., 2020) all show a depletion in small craters. Models of the crater populations on Eros and Itokawa indicate that the depletion can be explained by seismic shaking induced by meteorite impacts (e.g., Thomas et al., 2005; Richardson et al., 2004; 2005; Michel et al., 2009). The effects of seismic activity occur in the active layer, the uppermost layer of the regolith. Previous models of seismic shaking that recreate crater populations have used a broad range of active layer depths, ranging from 0.1 m to 5 m across various models for Itokawa and Eros (Richardson et al., 2004; 2005; 2020; Michel et al 2009; Susorney et al., 2021). However, the actual regolith thickness is poorly constrained or unknown in many cases.

In this study, the uncertainty introduced into seismic shaking models from the assumed active layer thickness is investigated by comparing the relative timescales of crater relaxation (crater erasure). We use the Richardson et al., (2004) seismic shaking model, as modified by Michel et al., (2009) for Itokawa with impactor populations from O’Brien and Greenberg (2005). Our results show that decreasing the active layer depth leads to a nonlinear increase in the time to erase a crater. The total increase in time to erasure for a crater 20 m in diameter when changing from regolith depths of 5 m to 0.1 m is over three magnitudes, mostly accommodated between depths of 1 m to 0.1 m. We also investigated the relative timescales of crater erasure for craters of different sizes. Increasing the crater diameter leads to a non-linear increase in crater erasure time, with a 10³ increase in erasure time when the diameter is increased from 5 m to 100 m.

The high sensitivity of crater erasure time on active layer depth and crater size implies that care should be taken when inferring surface properties, in particular asteroid surface age, time since a resetting event, or depth/diameter comparisons between asteroids with different crater populations.

 

References

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