Weak spectral features on (101995) Bennu from the OSIRIS-REx Visible and InfraRed Spectrometer

¹ Solar System Exploration Division, NASA Goddard Space Flight Center, Greenbelt, MD, USA
e-mail: amy.simon@nasa.gov
² Department of Geography, University of Winnipeg, Winnipeg, Canada
³ Southwest Research Institute, Boulder, CO, USA
⁴ Institut d’Astrophysique Spatiale, Université Paris-Saclay, CNRS, 91405 Orsay, France
⁵ Hawai’i Institute of Geophysics and Planetology, University of Hawai’i at Manoa, Honolulu, HI, USA
⁶ LESIA, Observatoire de Paris, Université PSL, CNRS, Université de Paris, Sorbonne Université, 5 place Jules Janssen, 92195 Meudon, France
⁷ Institut Universitaire de France (IUF), 1 rue Descartes, 75231 Paris Cedex 05, France
⁸ Department of Physics and Astronomy, Ithaca College, Ithaca, NY, USA
⁹ Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA

Received: 15 October 2020
Accepted: 18 November 2020

Abstract

Context. The NASA Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) mission has obtained thousands of spectra of asteroid (101955) Bennu with the OSIRIS-REx Visible and InfraRed Spectrometer.

Aims. We present a spectral search for minor absorption bands and determine compositional variations on the surface of Bennu.

Methods. Reflectance spectra with low and high spatial resolutions were analyzed for evidence of weak absorption bands. Spectra were also divided by a global average spectrum to isolate unique spectral features, and variations in the strongest band depths were mapped on a surface shape model. The global visible to near-IR spectrum of Bennu shows evidence of several weak absorption bands with depths of a few percent.

Results. Several observed bands are consistent with phyllosilicates, and their distribution correlates with the stronger 2.74-μm hydration band. A 0.55-μm band is consistent with iron oxides and is deepest in the spectrally reddest areas on Bennu. The presence of hydrated phyllosilicates and iron oxides indicates substantial aqueous alteration in Bennu’s past.

Conclusions. Bennu’s spectra are not identical to a limited set of carbonaceous chondrite spectra, possibly due to compositional properties and spatial scale differences; however, returned samples should contain a mixture of common chondrite materials.