Thermal Fatigue as a Driving Mechanism for Activity on Asteroid Bennu

Abstract

Many boulders on (101955) Bennu, a near‐Earth rubble pile asteroid, show signs of in situ disaggregation and exfoliation, indicating that thermal fatigue plays an important role in its landscape evolution. Observations of particle ejections from its surface also show it to be an active asteroid, though the driving mechanism of these events is yet to be determined. Exfoliation has been shown to mobilize disaggregated particles in terrestrial environments, suggesting that it may be capable of ejecting material from Bennu's surface. We investigate the nature of thermal fatigue on the asteroid, and the efficacy of fatigue‐driven exfoliation as a mechanism for generating asteroid activity, by performing finite element modeling of stress fields induced in boulders from diurnal cycling. We develop a model to predict the spacing of exfoliation fractures and the number and speed of particles that may be ejected during exfoliation events. We find that crack spacing ranges from ~1 mm to 10 cm and disaggregated particles have ejection speeds up to ~2 m/s. Exfoliation events are most likely to occur in the late afternoon. These predictions are consistent with observed ejection events at Bennu and indicate that thermal fatigue is a viable mechanism for driving asteroid activity. Crack propagation rates and ejection speeds are greatest at perihelion when the diurnal temperature variation is largest, suggesting that events should be more energetic and more frequent when closer to the Sun. Annual thermal stresses that arise in large boulders may influence the spacing of exfoliation cracks or frequency of ejection events.

Trajectory Estimation for Particles Observed in the Vicinity of (101955) Bennu

“…Our findings generally agree with the predictions of Molaro et al. (2020) regarding fatigue fracturing due to the diurnal thermal cycle, in particular in regard to the size distribution of the fractured particles, assuming our distribution of particle axis ratios. The prevalence of ejections in the evening and afternoon is also in accord, though our reduced number of ejections at polar latitudes does not match the thermal fracturing models.…”

“…For comparison, the plot also depicts the size distribution of thermally fractured fragments (red bars) as described in the text according to the exfoliation model by Molaro et al. (2020).…”

Implications for Ice Stability and Particle Ejection From High‐Resolution Temperature Modeling of Asteroid (101955) Bennu

“…For rock thermal fracture, simulations by rock thermomechanical models are required to explain how particles can be ejected via this mechanism. Such simulations are addressed in Molaro et al (2020) but here we can provide some evidence to help establish the processes involved. For instance, the late afternoon timing (between 15:22 and 18:05) of the three large particle ejection events characterized in Lauretta, Hergenrother et al (2019) is a curious feature.…”

Introduction to the Special Issue: Exploration of the Activity of Asteroid (101955) Bennu

“…Comparison of the theoretical and observed distribution of particle volume‐equivalent spherical diameters. The theoretical size distribution based on the thermal fracturing model of Molaro, Hergenrother, et al (2020) is depicted in red. The observed distribution is by Chesley et al (2020).…”