The unexpected surface of asteroid (101955) Bennu
© 2019, The Author(s), under exclusive licence to Springer Nature Limited. 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 Earth1. Bennu is a low-albedo B-type asteroid2 that has been linked to organic-rich hydrated carbonaceous chondrites3. 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 processes4. 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 scale5–11. 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 inertia12 and radar polarization ratios13—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 centimetres4. 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.
Lauretta, D. S.; DellaGiustina, D. N.; Bennett, C. A.; Golish, D. R.; Becker, K. J.; Balram-Knutson, S. S.; Barnouin, O. S.; Becker, T. L.; Bottke, W. F.; Boynton, W. V.; Campins, H.; Clark, B. E.; Connolly, H. C.; Drouet d’Aubigny, C. Y.; Dworkin, J. P.; Emery, J. P.; Enos, H. L.; Hamilton, V. E.; Hergenrother, C. W.; Howell, E. S.; Izawa, M. R.M.; Kaplan, H. H.; Nolan, M. C.; Rizk, B.; Roper, H. L.; Scheeres, D. J.; Smith, P. H.; Walsh, K. J.; Wolner, C. W.V.; Highsmith, D. E.; Small, J.; Vokrouhlický, D.; and Bowles, N. E., "The unexpected surface of asteroid (101955) Bennu" (2019). Faculty Articles Indexed in Scopus. 189.