A high-resolution normal albedo map of asteroid (101955) Bennu

Abstract

We present a near-global normal albedo map of asteroid (101955) Bennu, created using images from the PolyCam imager onboard the Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) spacecraft. PolyCam acquired high-resolution images (nadir pixel scale of ~6.25 cm/pixel) of the surface from the equator to mid-latitudes (~ ± 50°) at a low phase angle (~8°). We applied specialized charge smear and radiometric correction to the data to compensate for image artifacts stemming from very short exposure times. We photogrammetrically controlled the images to shape model tiles with a 5-cm ground sample distance to register the images to each other and to ground. Variations in albedo on Bennu's globally dark surface (median albedo of 0.046 ± 0.002) are associated with clusters of dark and bright boulders, as well as a much sparser population of meter-scale boulders with very high reflectances (albedo >0.10). Accordingly, Bennu has a relatively broad albedo-frequency distribution (~25% full width at half maximum) with a long tail toward higher values. Owing to the distribution of the dark boulders and boulder clusters, the southern hemisphere of Bennu is darker than the northern hemisphere; this hemispheric dichotomy varies with longitude, resulting in a large-scale diagonal pattern in albedo across the full disk of Bennu.