Letter to the Editor

OSIRIS-REx spectral analysis of (101955) Bennu by multivariate statistics

¹ LESIA, Observatoire de Paris, Université PSL, CNRS, Université de Paris, Sorbonne Université, 92195 Principal Cedex Meudon, France
e-mail: antonella.barucci@obspm.fr
² Institut Universitaire de France (IUF), Paris, France
³ Department of Physics and Astronomy, Ithaca College, Ithaca, NY 14850, USA
⁴ NASA Goddard Space Flight Center, Greenbelt, MD, USA
⁵ Southwest Research Institute, Boulder, CO, USA
⁶ Department of Astronomy and Planetary Science, Northern Arizona University, Flagstaff, AZ, USA
⁷ Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA
⁸ INAF Astrophysical Observatory of Arcetri, Florence, Italy
⁹ Department of Geography, University of Winnipeg, Winninpeg, Canada
¹⁰ Planetary Science Institute, Tucson, AZ, USA
¹¹ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, Nice, France
¹² University of Florence, Department of Physics and Astronomy, Firenze, Italy

Received: 10 April 2020
Accepted: 1 May 2020

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 the G-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 2σ confidence level. At a higher confidence level of 3σ, no grouping is observed.

Conclusions. The results at a 2σ confidence 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 2.74 μm 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.