In situ evidence of thermally induced rock breakdown widespread on Bennu's surface.

Rock breakdown due to diurnal thermal cycling has been hypothesized to drive boulder degradation and regolith production on airless bodies. Numerous studies have invoked its importance in driving landscape evolution, yet morphological features produced by thermal fracture processes have never been definitively observed on an airless body, or any surface where other weathering mechanisms may be ruled out. The Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) mission provides an opportunity to search for evidence of thermal breakdown and assess its significance on asteroid surfaces. Here we show boulder morphologies observed on Bennu that are consistent with terrestrial observations and models of fatigue-driven exfoliation and demonstrate how crack propagation via thermal stress can lead to their development. The rate and expression of this process will vary with asteroid composition and location, influencing how different bodies evolve and their apparent relative surface ages from space weathering and cratering records.

Geophysical imaging reveals topographic stress control of bedrock weathering.

Bedrock fracture systems facilitate weathering, allowing fresh mineral surfaces to interact with corrosive waters and biota from Earth's surface, while simultaneously promoting drainage of chemically equilibrated fluids. We show that topographic perturbations to regional stress fields explain bedrock fracture distributions, as revealed by seismic velocity and electrical resistivity surveys from three landscapes. The base of the fracture-rich zone mirrors surface topography where the ratio of horizontal compressive tectonic stresses to near-surface gravitational stresses is relatively large, and it parallels the surface topography where the ratio is relatively small. Three-dimensional stress calculations predict these results, suggesting that tectonic stresses interact with topography to influence bedrock disaggregation, groundwater flow, chemical weathering, and the depth of the "critical zone" in which many biogeochemical processes occur.