Timescales for boulder evolution from thermal fatigue and impacts on asteroid (101955) Bennu

Recent works by the proposing team show the discovery of boulder morphologies on asteroid Bennu that are consistent with exfoliation, i.e., the flaking of thin layers or shells of material from boulder surfaces and one of the most distinctive signatures of thermal fatigue. This is the first time this mechanical weathering process has been observed on an airless body surface, though previous studies had hypothesized its importance. Further, this process is thought to contribute to the ejection of particles from Bennu’s surface that have been observed by the Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) spacecraft. Impacts forming small scale craters (“bullet holes”) are also known to cause boulder degradation on Bennu, though the relative rates between the two processes is currently unknown. Understanding the contribution of both impacts and thermal fatigue to boulder breakdown and regolith production is critical to characterizing the evolution of Bennu’s surface and boulder size-frequency distribution (SFD) and has significant implications for the broader airless body population.

The proposed work consists of four tasks: 1) For a number of study locations, we will use high-resolution images to search for boulders which show morphological evidence of both bullet holes and fatigue-driven exfoliation flakes. The SFD distribution of small craters on Bennu has already been quantified, which will allow us to place constraints on the boulder surface ages and therefore the formation timescale of their exfoliation flakes. Surface ages will be further constrained by searching spectral data for aliphatic organic signatures at each site. 2) We will perform 3D finite element modeling of the boulders with exfoliation and impact signatures in order to quantify the amount of thermally induced stress they experience. The results of these simulations, combined with the formation timescales from Task 1, will allow us to model thermally induced crack propagation and constrain the rate at which exfoliation flakes develop over time. This will provide key insight into the rate of boulder degradation relative to impact processes. 3) We will survey the location and thickness of exfoliation flakes more broadly across Bennu’s surface using images and altimeter data for our study locations. We will analyze the exfoliation and impact features with respect to boulder size, type, geographical distribution, and other attributes to understand mechanical weathering on Bennu in a more global context. 4) With the insights provided by Tasks 2 and 3, we will assess how both impact and exfoliation processes alter the size and shape of boulders over time and drive the evolution of Bennu’s surface and boulder SFD.