The Formation of Terraces on Asteroid (101955) Bennu

The surface of the rubble-pile asteroid (101955) Bennu has been characterized in detail by the OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer) mission. By examining global and local digital terrain models, we observed that Bennu possesses terraces, that is, a series of roughly latitude-parallel, step-like slope breaks. These partially circumscribe the poles and extend east-west over several longitudinal quadrants at mid- to high (>= 30 degrees) latitudes. The terraces are subtle in amplitude, with heights ranging from 1 to 5 m. They often exhibit back-wasting that results in V-shaped scarps that open downslope in some locations. When boulders >5-10 m are absent at or near a terrace, the steeper portion (the drop) of the terrace lacks rocks, whereas the flatter portion (the bench) of the terrace has accumulations of rocks at its crest. When boulders >5-10 m are present, their steep downslope faces often make up the drop from the terrace crest, and they retain debris upslope, thereby enhancing the terrace structure. A geotechnical stability analysis indicates that Bennu's surface is likely unstable and that surface cohesion is <0.6 Pa. Bennu's terraces strongly resemble scarps generated in laboratory and numerical simulations of a cohesionless granular bed as the slope of the bed increases quasi-statically. We conclude that terraces are probably actively forming on Bennu as its surface slowly fails owing to creep induced by spin acceleration.