The Meter-Scale Roughness of Asteroid (101955) Bennu From the OSIRIS-REx Laser Altimeter

Abstract

Asteroid (101955) Bennu is a near-Earth, potentially hazardous, rubble pile asteroid, and was the primary target of the NASA OSIRIS-REx mission. The surface is dominated by the expression of boulders and has been heavily modified by impact cratering. Here, we analyze surface roughness, calculated using data from the OSIRIS-REx Laser Altimeter, to investigate spatial variations in boulders and finer-grained material across Bennu globally. Surface roughness is a statistical measure of change in surface height over a given baseline (horizontal spatial scale) and can be used to gain insight into the geologic processes that form and modify the surface over different scales. We calculate surface roughness at baselines of 0.20–20 m using the root-mean-square (RMS) deviation. We find that Bennu's surface roughness is self-affine over length scales between 0.2 m and $\sim$1.0 m, and between $\sim$1.0 and 20.0 m. We also find that surface roughness varies spatially and is dominated by the local size-frequency distribution of boulders. At the longest baselines, roughness is produced by the prominent equatorial ridge and the topographic relief of Bennu's largest boulders. At baselines between 0.20 and 2.0 m, the interiors of craters with diameters less than $\sim$25 m tend to be smooth compared with larger craters and the average background, supporting the presence of a finer-grained subsurface layer. Our results extend previous LiDAR-based asteroid roughness studies of (25143) Itokawa and (433) Eros to baselines more than 10 times shorter, and to an asteroid with different spectral class.