The unexpected surface of asteroid (101955) Bennu

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