Hemispherical differences in the shape and topography of asteroid (101955) Bennu

Abstract

We investigate the shape of near-Earth asteroid (101955) Bennu by constructing a high-resolution (20 cm) global digital terrain model from laser altimeter data. By modeling the northern and southern hemispheres separately, we find that longitudinal ridges previously identified in the north extend into the south but are obscured there by surface material. In the south, more numerous large boulders effectively retain surface materials and imply a higher average strength at depth to support them. The north has fewer large boulders and more evidence of boulder dynamics (toppling and downslope movement) and surface flow. These factors result in Bennu’s southern hemisphere being rounder and smoother, whereas its northern hemisphere has higher slopes and a less regular shape. We infer an originally asymmetric distribution of large boulders followed by a partial disruption, leading to wedge formation in Bennu’s history.

The Morphometry of Impact Craters on Bennu

“…The primary direction of mass movement on Bennu is from the polar and midlatitude regions toward the equatorial region, here defined as ±20° latitude (Jawin et al, 2020). The equatorial ridge is the ultimate sink for downslope movement of material (Daly et al, 2020). If mass movement were a dominant factor in d/D variations, d/D would decrease from the poles toward the equator.…”

“…We emulated the preimpact and postimpact comparisons of Fujiwara et al (1993) by comparing the high‐resolution, 20‐cm GSD global OLA shape model (Daly et al, 2020) (i.e., postimpact Bennu) to a long‐wavelength approximation of the global shape (i.e., preimpact Bennu). Preimpact Bennu was approximated by decomposing the high‐resolution OLA model into spherical harmonics, and then reconstructing the global shape up to degree and order four.…”

Variations in color and reflectance on the surface of asteroid (101955) Bennu

“…Heterogeneity in boulder reflectance on Bennu may be the result of distinct rock types that fragment differently, leading to an observed difference in their size-frequency distributions (4). Different compositions of the dark versus bright boulders may account for the correlation between reflectance and thermal properties for boulders on Bennu (60). The dark boulders have lower thermal inertia, which is attributed to higher porosity (60), consistent with their rougher and more crumbly (friable) appearance (Fig.…”

“…Conversely, the apparently smoother, more consolidated and angular bright boulders (Fig. 3, A and B) tend to have higher thermal inertias, attributed to lower porosity (60). Distinct texture and porosity can follow from differing mineralogy, levels of compaction, or heating and aqueous alteration histories.…”

“…To establish statistically meaningful relationships between color, reflectance, and morphological features, we mapped ~1600 boulders and ~700 craters, then extracted their median spectra using established methods (31,4,33). We assessed spectral variations that were bounded by irregular morphological features using centimeter-scale 3D digital terrain models produced with data from the OSIRIS-REx Laser Altimeter (OLA) (34)(35)(36). We also compared our findings with near-infrared hyperspectral data acquired at lower spatial resolution using the OSIRIS-REx Visible and InfraRed Spectrometer (OVIRS) (37).…”