Abstract
Contact.The NASA New Frontiers asteroid sample return mission Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) has provided a large amount of data on the asteroid (101955) Bennu, including high-quality spectra obtained by the OSIRIS-REx Visible and InfraRed Spectrometer (OVIRS).Aims.To better constrain the surface properties and compositional variations of Bennu, we studied the visible and near-infrared spectral behavior across the asteroid surface by means of a statistical analysis aiming to distinguish spectrally distinct groups, if present.Methods.We applied theG-mode multivariate statistical analysis to the near-infrared OVIRS spectra to obtain an automatic statistical clustering at different confidence levels.Results.The statistical analysis highlights spectral variations on the surface of Bennu. Five distinct spectral groups are identified at a 2σconfidence level. At a higher confidence level of 3σ, no grouping is observed.Conclusions.The results at a 2σconfidence level distinguish a dominant spectral behavior group (group 1, background) and four small groups showing spectral slope variations, associated with areas with different surface properties. The background group contains most of the analyzed data, which implies a globally homogeneous surface at the spectral and spatial resolution of the data. The small groups with redder spectra are concentrated around the equatorial ridge and are associated with morphological surface features such as specific craters and boulders. No significant variation is detected in the band area or depth of the 2.74μm band, which is associated with hydrated phyllosilicate content. The spectral slope variations are interpreted as a consequence of different regolith particle sizes, and/or porosity, and/or space weathering, that is, the presence of more or less fresh material. The OSIRIS-REx mission primary sampling site, Nightingale, and a boulder known as the Roc, are redder than the background surface.
Highlights
The NASA asteroid sample return mission Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) was launched on September 8, 2016, and arrived at the near-Earth asteroid (101955) Bennu on December 3, 2018
The evidence on both asteroids that different spectral groups are localized on the equator might be a sign of different resurfacing directions and possibly different effects of space weathering that is correlated with the initial albedo and composition (Lantz et al 2017)
From the results of the multivariate statistical analysis of OSIRIS-REx Visible and InfraRed Spectrometer (OVIRS) spectral data of Bennu, we confirm small spectral variations evident in the detection of statistically different spectral groups, whose number increases as the confidence level decreases
Summary
The NASA asteroid sample return mission Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) was launched on September 8, 2016, and arrived at the near-Earth asteroid (101955) Bennu on December 3, 2018. OVIRS acquired reflectance spectra from 0.4 μm to 4.3 μm, showing blue spectra (negative spectral slopes with respect to the Sun), with a deep absorption band of hydrated phyllosilicates at 2.74 ± 0.01 μm typical of CI and CM meteorites (Hamilton et al 2019). The analogy with these meteorites is confirmed by OTES, which operates from 5.7 to 100 μm at lower spatial resolution (Hamilton et al 2019). We investigate the possible spectroscopic heterogeneity on the surface of Bennu, as we have done previously for asteroid (162173) Ryugu using data obtained by the JAXA Hayabusa mission (Barucci et al 2019)
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