Impact-Induced Regolith Renewal on Small Bodies

Introduction

Small bodies in the Solar System (asteroids, comets, etc.) are covered by layers of regolith whose distribution and evolution record their collisional histories. Hypervelocity impacts not only produce new ejecta but also drive seismic shaking that can mobilize or erase existing regolith. For example, detailed analysis of regolith particles from Itokawa (Hayabusa) shows that meteoroid impacts formed much of the regolith, and subsequent seismic-induced grain motion steadily abrades and sorts these grains [1]. Likewise, high-resolution imaging of asteroid Eros revealed a paucity of small craters and evidence of downslope regolith flows that are well explained by impact-driven seismic shaking [2]. Laboratory experiments on granular media reinforce these ideas: even low-velocity impacts cause measurable surface grain motion that decays with distance from the crater, indicating that impacts of all sizes can induce regolith displacement [3]. Recent modeling of Eros's surface confirms that large impacts (e.g. the Shoemaker crater) excite global resurfacing because Eros's interior is highly dissipative [4]. 

 

Observational evidence underscores that regolith can be globally "refreshed" on short timescales. In situ measurements show that Itokawa's regolith has cosmogenic exposure ages < 10 Myr, much younger than the ~75 Myr age of the body itself [1], implying recent widespread resurfacing. Similarly, Bennu (OSIRIS-REx) exhibits ubiquitous mass-movement features: boulder flows and deposits (e.g. in Bralgah Crater) attest to ongoing regolith transport under impacts and spin-induced processes [5]. These studies highlight how even small bodies can rapidly reshuffle their surface. In contrast, highly spinning objects like 2016 HO3 (Kamoʻoalewa) challenge regolith stability: modeling shows that only very fine (mm–cm) grains can remain attached (with cohesion <0.2 Pa), and any disturbed regolith tends to slide and bounce until escaping [6]. Thus, a critical open question is how impact energy, material strength, and body shape govern where regolith is eroded, transported, or retained on small bodies.

 

Methods

To address this, we develop a unified theoretical and numerical framework to model regolith “refresh” driven by impacts. Our approach combines analytical scaling with high-resolution, mesh-free simulations. We use the Material Point Method (MPM) to obtain the stress wave propagation subject to impacts. MPM naturally handles large deformations and discontinuous flows without a fixed mesh, making it well-suited for simulating ejecta and landslides [7]. We will first consider idealized spherical bodies, constructing models with tunable bulk strength, gravity, and cohesion. For each simulation, a hypervelocity impact is applied (varying energy/impactor size and incidence) and the regolith response is tracked. We quantify three outcomes: (1) Escape of material into space, (2) Migration of particles (e.g. downslope transport or ballistically redeposited material), and (3) Stability (areas where regolith remains largely undisturbed). Key parameters include impact energy, target and regolith cohesion, grain size, and rotation rate.

 

Discussions

Conceptually, our model predicts how different impact regimes refresh small-body surfaces. On spherical rubble piles, we expect a critical impact energy above which most surface regolith is globally disturbed or lost (leading to a “fresh” surface and erasure of small craters). Below this threshold, impacts produce localized disturbances: for a rotating body, material tends to migrate toward low-potential regions (e.g. equator) while leaving poles relatively unchanged. Incorporating cohesion, we anticipate that cohesive regolith will resist motion up to higher impact energies, creating a dependence of surface maturity on grain binding forces.

 

By linking hypervelocity impacts to the spatial pattern of regolith erosion and deposition, it will help interpret the contrasting surface ages and properties observed on asteroids and meteorite parent bodies (e.g. Itokawa, Ryugu, Bennu). Ultimately, understanding the regolith "refresh" process is essential for using crater records to date surfaces and for predicting sample maturity for future missions.

 

References

[1] Tsuchiyama, A. et al. (2011) Sci. 333(6046).

[2] Richardson, JE. et al. (2004) Sci. 306(5701).

[3] Neiderbach, M. et al. (2023) Icar. 390.

[4] Ballouz, RL. et al. (2025) Nat Astron. 9.

[5] Tang, Y. et al. (2023) Icar. 395.

[6] Li, X. et al. (2021) Icar. 357.

[7] Yan, X. et al. (2024) EPSC2024-1111.