| Autor: |
Molaro, J. L., Hergenrother, C. W., Chesley, S. R., Walsh, K. J., Hanna, R. D., Haberle, C. W., Schwartz, S. R., Ballouz, R.‐L., Bottke, W. F., Campins, H. J., Lauretta, D. S. |
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| Zdroj: |
Journal of Geophysical Research. Planets; Aug2020, Vol. 125 Issue 8, p1-24, 24p |
| Abstrakt: |
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. Plain Language Summary: Soon after its rendezvous with the asteroid Bennu, the OSIRIS‐REx spacecraft observed the asteroid to be ejecting tiny particles of material. Bennu is a rubble‐pile asteroid covered in boulders of varying size. Many of these boulders show evidence of exfoliation, a process where thin layers of material are shed from their surfaces. Exfoliation is one consequence of thermal fatigue, which is the slow and progressive lengthening of cracks caused by the daily variation in boulder temperature from exposure to the Sun. Here we explore how thermal fatigue may cause the degradation and fracturing of boulders on Bennu and how the specific process of exfoliation could lead to the ejection of particles from the asteroid surface. We develop a model to predict the timing, number, and speeds of particles that may be ejected during exfoliation events and compare our results to the spacecraft observations of the ejection events from Bennu's surface. Our results suggest that particles ejected from boulder surfaces during exfoliation can have speeds up to ~2 m/s and are most likely occur when Bennu is closest to the Sun and during the late afternoon, consistent with spacecraft observations. Key Points: We simulated stress fields in boulders to assess the nature and efficacy of thermal breakdown on Bennu, including by exfoliationOur model predicts that exfoliation is capable of ejecting centimeter‐scale particles from the asteroid at speeds of meters per secondThis mechanism is consistent with observations of particle ejection at Bennu and is a viable explanation for Bennu's activity [ABSTRACT FROM AUTHOR] |
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