Spectral analysis of craters on (101955) Bennu

J.D.P. Deshapriya, M.A. Barucci, E.B. Bierhaus, S. Fornasier, P.H. Hasselmann, F. Merlin, B.E. Clark, A. Praet, M. Fulchignoni, A.A. Simon, Victoria E. Hamilton, E.A. Cloutis, C. Lantz, X.D. Zou, J.-Y. Li, D.C. Reuter, J.R. Brucato, G. Poggiali, R.T. Daly, D. Trang, S. Ferrone, D.N. DellaGiustina, D.S. Lauretta

Available online 13 December 2020



• The shortward shift of the 2.7-μm band inside craters indicates the presence of freshly excavated sub-surface material, which accordingly has undergone less alteration due to space weathering than the average Bennu surface.
• A scan-dependentant anti-correlation between REFF0.55 μm and spectral slopes is observed for spectra within three craters, with spectra becoming redder and darker towards the center of the given crater. We propose that this anti-correlation is a proxy for the presence of fine particulates inside a crater.
• Spectral heterogeneity inside the most prominent equatorial crater on Bennu (ID 6) implies within-crater physical or compositional differences of the material present. We propose that spectral heterogeneity of a crater can be used as a tracer of mass movement.
• Younger craters are brighter and have deeper 2.7-μm hydration bands and redder spectral slopes. As they age, they become darker with shallower bands and bluer spectral slopes as a consequence of space weathering processes. The craters presumed to be younger based on the shortward shift of their 2.7-μm band (less space weathered) generally corroborate this spectral trend.
• REFF0.55 μm values and the 2.7-μm band depths of craters are positively correlated, meaning that brighter craters are more hydrated.
• Nightingale (Crater ID 2), the primary sample site of the OSIRIS-REx mission, is the reddest and the darkest crater. In the REFF0.55 μm and 2.7-μm band depth space, it is the most distant outlier. Nightingale is thus spectrally distinct from the other studied craters.”

“Using data acquired by the OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer) mission, we investigate spectral properties of craters on the near-Earth asteroid (101955) Bennu. We compare Bennu’s craters with its global average by means of four spectral parameters: (a) minimum position of the band at 2.7 μm, (b) depth of the hydrated phyllosilicate absorption band at 2.7 μm, (c) normalized spectral slope from 0.55 to 2.0 μm, and (d) reflectance factor at 0.55 μm. We examine 45 craters using spectral data obtained under various observing conditions. For 20 craters, we find a shortward shift of the 2.7-μm band minimum relative to the global 2.7-μm band minimum, which we attribute to the presence of relatively fresh (less space-weathered) material excavated from the sub-surface by crater-forming impacts. For three craters, we find an anti-correlation between spectral slopes and reflectance factor for a series of spectra acquired during a specific scan, where we observe that spectra become redder and darker towards the center of the crater. We attribute this to the presence of fine-particulate regolith. Localized spectral heterogeneities are apparent inside a prominent equatorial crater on Bennu, which is one of the asteroid’s oldest geological features. We propose that such local spectral heterogeneities could be used as a tracer of mass movement on Bennu. We show that younger craters are redder, brighter, and have deeper 2.7-μm bands. Comparing global average spectral values of Bennu and crater frequency distributions as a function of the chosen spectral parameters, we find that craters evolve to assume the global average spectral properties of Bennu. A positive correlation identified between the reflectance factor and 2.7-μm band depth suggests that brighter craters tend to be more hydrated. Finally, we put into context, the results from the Small Carry-on Impactor experiment by the Hayabusa2 spacecraft, which created an artificial crater on the near-Earth asteroid (162173) Ryugu.”