Progressive failure of rocks on asteroids and comets: review of recent studies and space mission’s results

I will present novel discoveries from theoretical works, laboratory experiments, and observations from planetary space missions that, in the last fifteen years, considerably improved our understanding of progressive failure of rocks on the surface of solar system small bodies. In particular, several studies evidenced the role of thermal fatigue cracking due to the mechanical stresses generated by temperature cycling between day and night in causing progressive cracking and failure of surface rocks.
 I will review how the thermal cracking driven by temperature changes and assisted by volatile sublimation actively shape the landscape of cometary nuclei (1, 2), drive cometary activity (3), boulder breakdown (4), evolution of boulder size distribution (5, 6), and surface erosion. Several theoretical studies (7–10) based on simulations of the stress fields within rocks on asteroids that experience temperature variations of many tens of degrees between day and night (7, 11) showed that subcritical crack propagation produce rock damage. Supporting evidence came from laboratory experiments (7, 12, 13), indicating the importance of the material microscopic structure (14). Images from space missions revealed asteroid surface features that could be interpreted as the result of the progressive rock cracking and failure due to thermal fatigue, such as the presence of boulders that appear to erode in place (15), exfoliation fractures that can be modelled as the results of crack propagation due to diurnal temperature cycling (16), and preferential alignment of fractures on boulders (17). Preferential meridional crack orientation is known, from study of Earth and Mars (18) to be consistent with cracking induced by diurnal temperature variations. I will discuss also the time-scales of progressive failure of rocks on the surface of small bodies, which appears to be much faster (17) compared to the typical lifetimes of these objects. REFERENCES: 1 N. Attree et al., A&A. 610, A76 (2018). 2 A.-T. Auger et al., Icarus. 301, 173–188 (2018). 3 V. Alí-Lagoa, M. Delbo’, G. Libourel, ApJ. 810, L22 (2015). 4 M. R. El-Maarry et al., Geophys. Res. Lett. 42, 5170–5178 (2015). 5 M. Pajola et al., Nat Astron. 1, 0092 (2017). 6 J.-B. Vincent et al., A&A. 587, A14 (2016). 7 M. Delbo et al., Nature. 508, 233–236 (2014). 8 C. El Mir, K. T. Ramesh, M. Delbo, Icarus. 333, 356–370 (2019). 9 J. L. Molaro, S. Byrne, J.-L. Le, Icarus. 294, 247–261 (2017). 10 B. Ravaji, V. Alí-Lagoa, M. Delbo, J. W. Wilkerson, Journal of Geophysical Research: Planets. 124, 3304–3328 (2019). 11 B. Rozitis et al., Sci. Adv. 6, eabc3699 (2020). 12 K. Hazeli, C. El Mir, S. Papanikolaou, M. Delbo, K. T. Ramesh, Icarus. 304, 172–182 (2018). 13 G. Libourel et al., Monthly Notices of the Royal Astronomical Society. 500, 1905–1920 (2020). 14 M. Patzek, O. Rüsch, J. L. Molaro, JGR Planets, in press, doi:10.1029/2023JE007944. 15 A. J. Dombard, O. S. Barnouin, L. M. Prockter, P. C. Thomas, Icarus. 210, 713–721 (2010). 16 J. L. Molaro et al., Nat Commun. 11, 2913 (2020). 17 M. Delbo et al., Nat. Geosci. 15, 453–457 (2022). 18 M.-C. Eppes, A. Willis, J. Molaro, S. Abernathy, B. Zhou, Nat Commun. 6, 6712 (2015).