Abstract
Abstract We present a regional photometric analysis of asteroid (101955) Bennu, using image data from the MapCam color imager of the OSIRIS-REx Camera Suite (OCAMS). This analysis follows the previously reported global photometric analysis of Bennu, which found that Bennu’s roughness was difficult to photometrically model owing to unresolved surface variation. Here we find that, even with a high-resolution shape model (20 cm per facet) and automatic image registration (<1 pixel error), Bennu remains a challenging surface to photometrically model: neither a suite of empirical photometric models nor the physically motivated Hapke model were able to eliminate the scatter in the data due to pixel-scale variations. Nonetheless, the models improved on the global analysis by identifying regional variations in Bennu’s photometric response. A linear empirical model, when compared with independent measures of surface roughness and albedo, revealed correlations between those characteristics and phase slope. A regional Hapke analysis showed the same structure in its single-scattering albedo and asymmetry factors; although the Hapke parameters were loosely constrained, complicating interpretation of their spatial variation, the regional variation in relative parameter sensitivity also correlated with shallower phase slope, higher albedo, and less macroscopic roughness.
Highlights
Our automatic registration procedure used the same photometric simulations described above as a photogrammetric reference with a series of Integrated Software for Imagers and Spectrometers 3 (ISIS3) applications. This process began with the findfeatures ISIS3 application, which used a Features from Accelerated Segment Test (FAST; Rosten & Drummond 2005) detector and a Scale Invariant Feature Transform (SIFT; Lowe 2004) extractor to identify a control network of tie points between the actual and simulated images
Neither a suite of empirical models nor a Hapke model were able to represent the variations across the disk faithfully, at high photometric angles
We conclude that the solution recommended by Golish et al (2021a)—a Robotic Lunar Observatory (ROLO) phase function paired with a Lommel–Seeliger disk function—performs as well as or better than the other empirical models tested in this regional analysis
Summary
A global photometric analysis (Golish et al 2021a) fit Bennu’s photometric response to a suite of empirical models using color images acquired by MapCam of the OSIRIS-REx Camera Suite (OCAMS; Rizk et al 2018). That work verified ground-based expectations (Hergenrother et al 2013; Takir et al 2015) of a relatively steep phase slope commensurate with Bennu’s low albedo and highly shadowed surface. Golish et al (2021a) treated Bennu as a homogenous surface, using the median reflectance and photometric angles of every image (each of which covered ∼3/4 of Bennu’s hemisphere) when fitting its photometric models. Golish et al (2021a) suggested that higher-resolution shape models, which provide a more accurate measure of the photometric angles, would be required to understand the photometric response of the surface. The spatial variation of Bennu’s photometric properties (phase slope, phase reddening, and albedo) and their correlation with other characteristics of Bennu’s surface provide a more detailed understanding of the photometric behavior of this heterogeneous body
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