Fatigue-driven boulder exfoliation as a driving mechanism for activity on asteroids

Thermally driven fracture processes, such as thermal fatigue, have been hypothesized to drive rock breakdown on asteroid surfaces [e.g., 1-5]. Thermal cycling induces mechanical stresses in rocks that drive the propagation of microcracks, which may coalesce into larger-scale features over time. Images from the Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) spacecraft of the surface of asteroid Bennu provide the opportunity to search for in situ evidence of thermal breakdown over a wide range of scales. Recent works by the authors [5-7] show observations of Bennu boulder morphologies [6] consistent with terrestrial observations of fatigue-driven boulder exfoliation [e.g., 8]—i.e., the flaking of thin layers or shells of material from boulder surfaces, one of the most distinctive signatures of thermal fatigue. Here we explore how boulder exfoliation may lead to the ejection of particles observed at Bennu’s surface [7] in an analogous manner to mobilization of rock fragments during large-scale, terrestrial dome exfoliation events [9]. We have observed particle ejection events from Bennu’s surface repeatedly since first entering orbit in January 2019. Observed particles range in size from <1 to 10 cm [5], consistent with our predictions for exfoliation [7]. We quantified the available thermal strain energy in boulders beyond what is needed to propagate cracks and converted it to kinetic energy to constrain the speed of ejected particles. We find that particles may be ejected with speeds up to ~2 m/s for boulders smaller than or equal to 6 m in diameter, which is comparable to the maximum observed particle speed of 3.3 m/s [5]. These results suggest that fatigue-driven exfoliation is a viable mechanism for producing or contributing to the activity observed at Bennu.

Acknowledgements: This work was supported by NASA under Contract NNM10AA11C issued through the New Frontiers Program and NNH17ZDA001N-ORPSP through the Participating Scientist Program. We thank the OSIRIS-REx Team for their hard work in making the encounter with Bennu possible.

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