ARTICLES https://doi.org/10.1038/s41550-019-0722-2 Evidence for widespread hydrated minerals on asteroid (101955) Bennu V. E. Hamilton 1*, A. A. Simon2, P. R. Christensen3, D. C. Reuter2, B. E. Clark4, M. A. Barucci 5, N. E. Bowles6, W. V. Boynton7, J. R. Brucato8, E. A. Cloutis9, H. C. Connolly Jr10, K. L. Donaldson Hanna6, J. P. Emery 11, H. L. Enos7, S. Fornasier5, C. W. Haberle3, R. D. Hanna 12, E. S. Howell7, H. H. Kaplan1, L. P. Keller13, C. Lantz14, J.-Y. Li15, L. F. Lim2, T. J. McCoy16, F. Merlin5, M. C. Nolan 7, A. Praet5, B. Rozitis17, S. A. Sandford18, D. L. Schrader 19, C. A. Thomas20, X.-D. Zou15, D. S. Lauretta7 and the OSIRIS-REx Team21 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 aque- ously 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. he Origins, Spectral Interpretation, Resource Identification, observations (with the final ground truth coming from measure- and Security-Regolith Explorer (OSIRIS-REx) mission began ments of the returned sample). Tits Approach phase to asteroid (101955) Bennu in August On five days between 2 and 9 November 2018, both spectrom- 2018. Before and just after arrival at Bennu on 3 December, the eters obtained whole-disk (sub-FOV) spectra of Bennu for 4.5 h, OSIRIS-REx Visible and InfraRed Spectrometer (OVIRS) and which is just over one full rotation period (~4.3 h). In December Thermal Emission Spectrometer (OTES) collected hyperspectral 2018, both instruments collected spatially resolved spectra of Bennu data of this B-type asteroid, which is thought to be related to the as ‘ride-along’ observations during imaging activities optimized for carbonaceous chondrite meteorites1. The OVIRS instrument2 is a the PolyCam and MapCam imagers6. hyperspectral, point spectrometer that measures the reflected and emitted energy of Bennu across the spectral region from 0.4 to Visible and near-infrared spectral characteristics 4.3 µm (25,000 to 2,300 cm–1) with a circular, 4 mrad field of view The ground-based, composite (0.4 to 2.4 µm) reflectance spec- (FOV). The OTES instrument3, the first thermal infrared spec- trum of Bennu shows a spectrally ‘blue’ (negative) continuum trometer to visit an asteroid, is a hyperspectral, point spectrometer slope across the visible and near-infrared, characteristic of B-type that measures the emitted radiance of Bennu across the spectral asteroids1. Clark et al.1 did not find strong spectral absorptions in region from ~1,750 to 100 cm–1 (~5.71 to 100 µm) with a circular, the Bennu telescopic data, and they identified CI- and CM-type 8 mrad FOV. The primary role of visible-to-infrared spectroscopy carbonaceous chondrites as the most likely spectral matches, on the OSIRIS-REx mission is to characterize the mineralogy and with a preference for a CM1-like composition. (Please note that chemistry of Bennu and aid in sample site selection4. The OTES throughout this Article we follow the standard convention of pet- radiance data also are used in conjunction with thermophysical rologic types for chondrites, such as CI1 and CM2, first introduced models to determine properties of the surface, such as particle size by Van Schmus and Wood7.) Thus, Bennu was predicted to have and roughness, and to study the Yarkovsky effect5. The mineralogy hydrated minerals, but no spectral features attributable to hydra- and chemistry of the surface of Bennu provide information about tion were observed. The average OVIRS disk-integrated spectrum the geological processes that have affected the asteroid, the poten- of Bennu compares very well with the telescopic data at these tial for resource extraction and the accuracy of telescopic spectral wavelengths, also having a negative slope and no clear absorption 1Southwest Research Institute, Boulder, CO, USA. 2NASA Goddard Space Flight Center, Greenbelt, MD, USA. 3School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA. 4Department of Physics and Astronomy, Ithaca College, Ithaca, NY, USA. 5LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Univ. Paris Diderot, Sorbonne Paris Cité, Meudon, France. 6Atmospheric, Oceanic and Planetary Physics, University of Oxford, Oxford, UK. 7Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA. 8INAF–Astrophysical Observatory of Arcetri, Florence, Italy. 9Department of Geography, University of Winnipeg, Winnipeg, Manitoba, Canada. 10Department of Geology, Rowan University, Glassboro, NJ, USA. 11Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN, USA. 12Department of Geological Sciences, Jackson School of Geosciences, University of Texas, Austin, TX, USA. 13NASA Johnson Space Center, Houston, TX, USA. 14Institut d’Astrophysique Spatiale, CNRS/Université Paris Sud, Orsay, France. 15Planetary Science Institute, Tucson, AZ, USA. 16Smithsonian Institution National Museum of Natural History, Washington, DC, USA. 17School of Physical Sciences, The Open University, Milton Keynes, UK. 18NASA Ames Research Center, Moffett Field, CA, USA. 19Center for Meteorite Studies, Arizona State University, Tempe, AZ, USA. 20Department of Physics and Astronomy, Northern Arizona University, Flagstaff, AZ, USA. 78A full list of authors and their affiliations appears at the end of the paper. *e-mail: [email protected] 332 NatURE Astronomy | VOL 3 | APRIL 2019 | 332–340 | www.nature.com/natureastronomy NATURE ASTRONOMY ARTICLES 1.1 Bennu 1.0 1.0 0.8 Ivuna (CI1) 0.9 0.6 LAP 02277 (CM1) 0.8 0.4 Bennu 1 MET 00639 (CM2) Normalized reflectance Clark et al. 0.7 0.2 Cold Bokkeveld (CM2) 0.5 1.0 1.5 2.02.5 3.03.5 Normalized reflectance 0 Wavelength (µm) –0.2 Fig. 1 | Average whole-disk, full-rotation OVIRS spectrum of Bennu compared with the ground-based spectrum. The OVIRS radiance factor –0.4 2.4 2.6 2.8 3.03.2 3.4 spectrum (black) and ground-based spectrum1 (red) are normalized to a Wavelength (µm) reflectance of 1.0 at 0.55 µm. The OVIRS data were acquired on DOY 306 (2 November 2018), and the FOV was approximately 40% filled during Fig. 2 | Average DOY 306 OVIRS spectrum between 2.3 and 3.5 m these observations. µ compared with spectra of example carbonaceous chondrites. The carbonaceous chondrites were measured in vacuum after heating19 (see Methods for full meteorite names). The spectra are normalized to features (Fig. 1). There is no variation in the spectra (above the a reflectance of 1.0 at 2.4 µm and offset vertically for clarity. The dotted noise) with rotational phase. Analysis of spatially resolved data vertical line at 2.74 µm denotes the Bennu band minimum position is ongoing and will be used to confirm or refute ground-based (see Methods). observations of spectral slope changes8. A blue-sloped continuum could be explained in one or more ways; such a continuum has been observed in some CI and CM carbonaceous chondrites and, in CI meteorites, is attributed to the CM2.3 and 2.6 (where decimal values indicate relative alteration presence of fine-particulate magnetite and/or insoluble organic within type 2, with smaller values representing greater alteration). material; it is also commonly associated with larger-particle-size The band centre for ‘group 2’ meteorites ranges from 2.76 to 2.78 µm samples and possibly space weathering9–11. Lauretta et al.12 have and includes petrologic subtypes CM2.1 to 2.2, which are the most identified a candidate magnetite feature at 0.55 µm (ref. 13) in the aqueously altered petrologic type-2 meteorites. Finally, ‘group 3’ darkest materials imaged by the MapCam instrument; however, as meteorites are also CM2.1 to 2.2 but have a band centre at 2.72 µm. of yet, no such feature has been observed in OVIRS spectra that Ivuna, the only CI1 in the study, has a band centre at 2.71 µm. The would confirm this detection or its assignment to magnetite. Such a OVIRS band centre lies between groups 2 and 3 and is consistent feature may become evident in the higher-spatial-resolution OVIRS with meteorites having petrologic types of CM2.1–2.2. Meteorites data that will be collected later in the mission. Experimental space with these petrologic types are among the most aqueously altered weathering of carbonaceous materials can result in reddening or samples studied. Space weathering effects on asteroids in this spec- bluing of the spectral slope11,14,15; at present, we do not have suf- tral region do not always match predictions25 but if solar wind ficient information from OVIRS spectra to draw any conclusions irradiation is affecting this band in a manner consistent with experi- about the nature or degree of space weathering on Bennu as it relates mental data on Murchison (CM2.5), the predicted effect would be to Bennu’s spectral slope or the presence of magnetite. to shift the band centre to slightly longer wavelengths (a maximum At longer wavelengths (>2.4 µm), both disk-integrated and of 0.03 µm for Murchison) and introduce a concave shape24. As seen spatially resolved OVIRS spectra display an approximately 2.7 µm in Fig.