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Evidence for widespread hydrated minerals on asteroid (101955) Bennu

Abstract

Early spectral data from the Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) mission reveal evidence for abundant hydrated minerals on the surface of near-Earth asteroid (101955) Bennu in the form of a near-infrared absorption near 2.7 µm and thermal infrared spectral features that are most similar to those of aqueously altered CM-type carbonaceous chondrites. We observe these spectral features across the surface of Bennu, and there is no evidence of substantial rotational variability at the spatial scales of tens to hundreds of metres observed to date. In the visible and near-infrared (0.4 to 2.4 µm) Bennu’s spectrum appears featureless and with a blue (negative) slope, confirming previous ground-based observations. Bennu may represent a class of objects that could have brought volatiles and organic chemistry to Earth.

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Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request. Raw and calibrated spectral data will be available via the Planetary Data System (PDS) (https://sbn.psi.edu/pds/resource/orex). Data are delivered to the PDS according to the OSIRIS-REx Data Management Plan available in the OSIRIS-REx PDS archive. Higher-level products will be available in the PDS one year after departure from the asteroid. Laboratory spectral data are deposited in the spectral library hosted by Arizona State University (http://speclib.mars.asu.edu/).

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Acknowledgements

This material is based on work supported by NASA under Contract NNM10AA11C issued through the New Frontiers Program. T. Burbine, F. S. Anderson and J. Joseph provided considerable assistance with early software development for the spectral analysis working group. H. Campins, R. Binzel and E. Dotto participated in discussions of space weathering and the spectral results. C. Wolner provided helpful copyediting support. The J-Asteroid software tool and development team at ASU enabled visualization of the spectral data that was critical to the analysis. The authors also extend their gratitude to the following people without whom this work would not have been possible: the instrument teams at NASA Goddard Spaceflight Center (GSFC) and Arizona State University; the spacecraft teams at GSFC, KinetX and Lockheed Martin; the science planning and operations teams at the University of Arizona; and the Science Processing and Operations Center staff at the University of Arizona. INAF is supported by Italian Space Agency agreement no. 2017-37-H.0. The French co-authors acknowledge support from CNES. B.R. acknowledges the support of the Royal Astronomical Society in the form of a research fellowship.

Author information

V.E.H. is the spectral analysis working group lead, the OTES deputy instrument scientist, and wrote this manuscript. A.A.S. is the spectral analysis working group deputy, the OVIRS deputy instrument scientist, and led the calibration of the OVIRS data and production of the disk-integrated average spectrum. P.R.C. is the OTES instrument scientist and led the calibration of OTES data. D.C.R. is the OVIRS instrument scientist. B.E.C. is the OSIRIS-REx Mission Asteroid Scientist. M.A.B., H.H.K., R.D.H. and A.P. contributed to the analysis of the OVIRS 2.7 µm band. N.E.B. hosts the laboratory that made the simulated asteroid environment spectral measurements. W.V.B. is the mission instrument scientist and contributed to ensuring the mission plan enables the instruments to meet their observation requirements. J.R.B., E.A.C., S.F., C.L., J.-Y.L., F.M., S.A.S., C.A.T. and X.-D.Z. contributed to the development of science pipeline software. H.C.C. Jr is the mission sample scientist and helped guide the selection and acquisition of the meteorite samples used in this work. K.L.D.H. measured the samples shown in Fig. 4b. J.P.E. and B.R. contributed to the subtraction of thermal emission from OVIRS spectra. H.L.E. is the deputy principal investigator for the OSIRIS-REx mission. C.W.H. contributed to the data processing and analysis of OTES spectra. E.S.H. contributed to the development of science pipeline software and provided manual processing of some of the data shown in this manuscript. L.P.K. and T.J.M. helped guide the selection and acquisition of the meteorite samples used in this work. L.F.L. contributed to extensive discussions about the laboratory measurements. M.C.N. is the science team chief and contributed the resampled solar spectrum used in the calibration of OVIRS data. D.L.S. contributed to the preparation and characterization of meteorite samples used in this work. D.S.L. is the OSIRIS-REx principal investigator and the entire OSIRIS-REx Team made the Bennu encounter possible.

Competing interests

The authors declare no competing interests.

Correspondence to V. E. Hamilton.

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Fig. 1: Average whole-disk, full-rotation OVIRS spectrum of Bennu compared with the ground-based spectrum.
Fig. 2: Average DOY 306 OVIRS spectrum between 2.3 and 3.5 µm compared with spectra of example carbonaceous chondrites.
Fig. 3: Average OTES spectrum of Bennu between 1,500 and 200 cm–1.
Fig. 4: Average OTES spectrum of Bennu compared with spectra of whole-rock and fine-particulate carbonaceous chondrite meteorites.
Fig. 5: Average OTES Bennu spectrum compared with a spectrum of pure, fine-particulate (<90 µm) magnetite.