Abstract
AbstractThe 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.
Highlights
The OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer) spacecraft arrived at near-Earth asteroid (NEA) (101955) Bennu in December (Lauretta et al 2019)
For our Bennu investigation, we assume that if the particle ejection events were driven by thermal fractures, they would have originated from areas of relatively high ΔT
We find that millimeter-thick layers of surface water ice are stable over ~103-year timescales within polar centimeter-scale cold traps with maximum surface temperatures below 131 K
Summary
Modeled temperatures indicate that water ice sublimation is not the process ejecting particles from the surface of Bennu. Sub-surface water ice could be stable in small regions near the poles. The diurnal temperature curve has a large amplitude at all latitudes, which supports thermal fracturing as a cause of the ejection events
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