The Unexpected Surface of Asteroid (101955) Bennu

NASA’S Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) spacecraft recently arrived at 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 [1]. Bennu is a low-albedo B-type asteroid [2] that has been linked to organic-rich hydrated carbonaceous chondrites [3]. Such meteorites are altered by ejection from their parent body and contaminated by atmospheric entry and terrestrial microbes. Thus, the primary mission objective is to return a sample of Bennu to Earth that is pristine, i.e., not affected by these processes [4]. The OSIRIS-REx spacecraft carries a sophisticated suite of instruments to characterize Bennu’s global properties; support selection of a sampling site; and document that site at sub-centimeter scales [5-11]. Here we consider early observations to understand how Bennu’s properties compare to pre-encounter expectations and 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 modeling of Bennu’s thermal inertia [12] and radar polarization ratios [13]—which indicated a generally smooth surface covered by centimeter-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-m-diameter patches of loose regolith with grain sizes less than 2 cm [4]. We observe only a small number of apparently hazard-free regions, on the order of 5 to 20 meters in extent, the sampling of which poses a substantial challenge to mission success.