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Paper: Exploration-Based Reconstruction of Planetesimals
Monograph: 9, Protostars and Planets VII
Page: 993
Authors: Watanabe, S.; Arakawa, M.; Hirabayashi, M.; Sugita, S.; Bottke, W. F.; Michel, P.
Abstract: We glean insights into the origins of planetesimals using mission results from asteroids and comets. Planetesimals are thought to be self-gravitating bodies that bridge the size range between dust aggregates and protoplanets. Their formation is complex; small particles can accrete with one another until reaching meter-sizes, but they then become susceptible to collisional fragmentation and quickly drift toward the Sun. Direct formation of large planetesimals from dust aggregates through the streaming/gravitational instabilities may provide a solution, but testing this hypothesis requires constraints. To this end, we investigate what can be learned from asteroids and comets explored by space missions, concentrating on small bodies that likely formed from the disruption of a larger parent body. Like small planetesimals, these worlds are gravitational aggregates; they have geophysical and collisional histories that tell us about the common processes that affect small-body evolution in a low-g regime. For example, the Hayabusa2 impact experiment on the carbonaceous chondrite-like asteroid Ryugu showed that small craters follow gravity-controlled scaling laws, with the surface layer of Ryugu having very low cohesion. The returned samples from Ryugu reveal the material transfer in the early Solar System. In a second example, the OSIRIS-REx spacecraft observed numerous particle ejection events on Bennu, another carbonaceous chondrite-like asteroid, that show no signs of involving volatiles or outgassing. Furthermore, the asteroid surface offered almost no resistance to the sampling process. Some implications are that small primitive planetesimals may be easier to disrupt than previously thought, and they may fall apart if they approach the Sun. Using results from different missions, we review what multi-km and sub-km bodies tell us about planetesimals from the perspective of shape, structure, impacts, mass ejection, and other processes. We also explore how these new results fit in with current scenarios of collisional evolution and growth (or disruption) for planetesimals.
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