In situ evidence of thermally induced rock breakdown widespread on Bennu\u2019s surface

J L Molaro,E R Jawin,R D Hanna,R.-L Ballouz,K J Walsh,S J Martel,C A Bennett,B Rizk,D R Golish,D S Lauretta,M Pajola,H Campins,S R Schwartz,W F Bottke,A J Ryan,C Drouet D’Aubigny,D N Dellagiustina

doi:10.1038/s41467-020-16528-7

Abstract

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.

Highlights

Rock breakdown due to diurnal thermal cycling has been hypothesized to drive boulder degradation and regolith production on airless bodies
The OSIRIS-REx Camera Suite (OCAMS)[25] has obtained images of the surface of asteroid (101955) Bennu at pixel scales down to ~1 cm/px, providing an opportunity to search over a wide range of scales for evidence of thermal breakdown occurring in situ
The interaction of thermal fatigue with rock fabrics or sheeting joints may influence the direction of crack propagation due to anisotropy in material strength or the presence of joints that are mechanically weaker than adjoining layers

Summary

Introduction

Rock breakdown due to diurnal thermal cycling has been hypothesized to drive boulder degradation and regolith production on airless bodies. The best evidence of extraterrestrial thermal fatigue is the predominant N–S trend in the orientation of boulder-scale fractures on Mars[20], which is consistent with both models[12] and terrestrial observations[3,21] It is unknown whether this reflects weathering that occurred under the current or a past Martian climate regime, and laboratory studies show that thermally driven crack propagation is harder to achieve in anhydrous and vacuum environments than in ambient atmosphere[22,23,24]. We show evidence of boulder exfoliation consistent with both terrestrial observations[2] and models[1,13] of fatigue-driven boulder degradation These findings provide substantive and compelling evidence that thermal fracturing plays an important role on airless body surfaces, which has major implications for understanding the evolution of asteroid surfaces, orbits, and populations

Methods

Results

Conclusion