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The unexpected surface of asteroid (101955) Bennu

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Abstract
NASA’S Origins, Spectral Interpretation, Resource Identification and Security-Regolith Explorer (OSIRIS-REx) spacecraft recently arrived at the near-Earth asteroid (101955) Bennu, a primitive body that represents the objects that may have brought prebiotic molecules and volatiles such as water to Earth. Bennu is a low-albedo B-type asteroid that has been linked to organic-rich hydrated carbonaceous chondrites. Such meteorites are altered by ejection from their parent body and contaminated by atmospheric entry and terrestrial microbes. Therefore, the primary mission objective is to return a sample of Bennu to Earth that is pristine—that is, not affected by these processes. The OSIRIS-REx spacecraft carries a sophisticated suite of instruments to characterize Bennu’s global properties, support the selection of a sampling site and document that site at a sub-centimetre scale. Here we consider early OSIRIS-REx observations of Bennu to understand how the asteroid’s properties compare to pre-encounter expectations and to assess the prospects for sample return. The bulk composition of Bennu appears to be hydrated and volatile-rich, as expected. However, in contrast to pre-encounter modelling of Bennu’s thermal inertia and radar polarization ratios—which indicated a generally smooth surface covered by centimetre-scale particles—resolved imaging reveals an unexpected surficial diversity. The albedo, texture, particle size and roughness are beyond the spacecraft design specifications. On the basis of our pre-encounter knowledge, we developed a sampling strategy to target 50-metre-diameter patches of loose regolith with grain sizes smaller than two centimetres. We observe only a small number of apparently hazard-free regions, of the order of 5 to 20 metres in extent, the sampling of which poses a substantial challenge to mission success. ©2019
Date issued
2019-03
URI
https://hdl.handle.net/1721.1/124687
Department
Massachusetts Institute of Technology. Department of Earth, Atmospheric, and Planetary Sciences
Journal
Nature
Publisher
Springer Science and Business Media LLC
Citation
Lauretta, D.S., et al., "The unexpected surface of asteroid (101955) Bennu." Nature 568, 7750 (March 2019): p. 55-60 doi 10.1038/s41586-019-1033-6 ©2019 Author(s)
Version: Author's final manuscript
ISSN
2041-1723

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