| Abstrakt: |
The finding by the OSIRIS‐REx (Origins, Spectral Interpretation, Resource Identification, and Security‐Regolith Explorer) mission that its target (101955) Bennu is an active asteroid has raised questions as to whether the observed particle ejection events are driven by temperature. To investigate sublimation of water ice and rock thermal fracture as possible temperature‐driven causes, we modeled the global temperatures of Bennu and searched for correlations with the identified ejection points on the asteroid surface. We computed temperatures with the Advanced Thermophysical Model and the 75‐cm‐resolution global shape model of Bennu derived by the OSIRIS‐REx mission. We find that ~1,856 m2 of Bennu's polar regions have orbit‐averaged temperatures that are sufficiently cold to enable water ice, if buried within the top few meters of the surface, to remain stable over geological timescales. Millimeter thick layers of surface water ice are also stable over ~103‐year timescales within polar centimeter‐scale cold traps. However, we do not find evidence of conditions enabling ice stability in the warmer equatorial regions, where ejection events have been observed, implying that sublimation of water ice is not the cause of particle ejection. Conversely, rock thermal fracture remains a possible mechanism of particle ejection. We find high amplitudes of diurnal temperature variation, a proxy for the efficacy of thermal fracturing, at all latitudes on Bennu due to its extreme ruggedness. Therefore, if rock thermal fracture is the mechanism, particles could be ejected from any latitude, which is consistent with the continued observations of particle ejection by OSIRIS‐REx. Plain Language Summary: The OSIRIS‐REx mission discovered that particles are being ejected periodically from the surface of near‐Earth asteroid Bennu. Some hypotheses for the process (es) driving these ejection events relate to temperature. These include sublimation of volatile substances such as water (like in a comet) and thermal fracturing (cracking of rocks driven by day‐night temperature changes). To evaluate these hypotheses, we performed numerical simulations of temperatures across the surface of Bennu over its orbit. Temperatures on the majority of the surface, including at the ejection sites, are too warm for water ice to be present, even if covered by dust. We therefore conclude that sublimation of water ice is not responsible for the particle ejections. Nevertheless, portions of the polar regions are cold enough that subsurface water ice could exist. Small (centimeter‐scale) cold traps near the poles could store surface water ice for up to ~1,000 years. We find that thermal fracturing is a viable mechanism to explain the particle ejections because Bennu exhibits large day‐night differences in temperature. These large temperature differences occur even at high latitudes on the sunward facing sides of boulders. This widespread viability of thermal fracturing is consistent with the observation of particles ejecting from various latitudes on Bennu. Key Points: Modeled temperatures indicate that water ice sublimation is not the process ejecting particles from the surface of BennuSubsurface water ice however could be stable in small regions near the polesThe diurnal temperature curve has a large amplitude at all latitudes, which supports thermal fracturing as a cause of the ejection events [ABSTRACT FROM AUTHOR] |
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