Exogenic basalt on asteroid (101955) Bennu

Abstract

When rubble-pile asteroid 2008 TC3 impacted Earth on 7 October 2008, the recovered rock fragments indicated that such asteroids can contain exogenic material1,2. However, spacecraft missions to date have only observed exogenous contamination on large, monolithic asteroids that are impervious to collisional disruption3,4. Here, we report the presence of metre-scale exogenic boulders on the surface of near-Earth asteroid (101955) Bennu—the 0.5-km-diameter, rubble-pile target of the OSIRIS-REx mission5 that has been spectroscopically linked to the CM carbonaceous chondrite meteorites6. Hyperspectral data indicate that the exogenic boulders have the same distinctive pyroxene composition as the howardite–eucrite–diogenite (HED) meteorites that come from (4) Vesta, a 525-km-diameter asteroid that has undergone differentiation and extensive igneous processing7,8,9. Delivery scenarios include the infall of Vesta fragments directly onto Bennu or indirectly onto Bennu’s parent body, where the latter’s disruption created Bennu from a mixture of endogenous and exogenic debris. Our findings demonstrate that rubble-pile asteroids can preserve evidence of inter-asteroid mixing that took place at macroscopic scales well after planetesimal formation ended. Accordingly, the presence of HED-like material on the surface of Bennu provides previously unrecognized constraints on the collisional and dynamical evolution of the inner main belt.

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Fig. 1: In OCAMS PolyCam images, six unusually bright boulders exhibit a variety of textures.
Fig. 2: Physical and spectrophotometric properties of Bennu’s bright pyroxene-bearing boulders.
Fig. 3: Bennu’s bright pyroxene-bearing boulders are spectrally similar to the HED meteorites.

Data availability

The OCAMS (MapCam and PolyCam), OLA and OVIRS data that support the findings and plots within this paper are available from the Planetary Data System (PDS) at https://sbn.psi.edu/pds/resource/orex/ocams.html, https://sbn.psi.edu/pds/resource/orex/ola.html and https://sbn.psi.edu/pds/resource/orex/ovirs.html, respectively. Data are delivered to the PDS according to the schedule in the OSIRIS-REx Data Management Plan, available in the OSIRIS-REx mission bundle at https://sbnarchive.psi.edu/pds4/orex/orex.mission/document/. Data shown in Supplementary Figs. 7 and 8 were obtained from the Minor Planet Physical Properties Catalogue (MP3C, https://mp3c.oca.eu/) of the Observatoire de la Côte d’Azur.

Code availability

The collisional analysis reported here uses a custom code that is based on established methods described in refs. 53,54,55,56,57,58). The ISIS3 code used to generate the image processing data products is a customized version of code available from the US Geological Survey–Astrogeology Science Center: https://isis.astrogeology.usgs.gov/. The MGM code used to analyse OVIRS spectral data is available from RELAB at Brown University: http://www.planetary.brown.edu/mgm/.

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Acknowledgements

This material is based upon work supported by NASA under contract NNM10AA11C issued through the New Frontiers Program. The work of C.A. was supported by the French National Research Agency under the project ‘Investissements d’Avenir’ UCAJEDI ANR-15-IDEX-01. C.A. and M.D. would like to acknowledge the French space agency CNES and support from the ANR ‘ORIGINS’ (ANR-18-CE31-0014). M.A.B. also acknowledges support from CNES the French space agency. G.P. acknowledges support from the INAF-Astrophysical Observatory of Arcetri, which is supported by Italian Space Agency agreement no. 2017-37-H.0. E.T. was supported by the JSPS core-to-core program International Planetary Network. The OLA instrument and funding for M.G.D., M.M.A.A., L.P. and J.S. is provided by the Canadian Space Agency. This research uses spectra acquired at the NASA RELAB facility at Brown University. This work also makes use of data provided by the Minor Planet Physical Properties Catalogue (MP3C) of the Observatoire de la Côte d’Azur. We thank the entire OSIRIS-REx team for making the encounter with Bennu possible.

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Contributions

D.N.D. leads the OSIRIS-REx image processing working group (IPWG) that discovered and characterized exogenic boulders on Bennu using OCAMS data. H.H.K. led the OVIRS spectral analysis that linked the exogenic boulders on Bennu to the HED meteorites. D.R.G., M.P., H.C., L.L.C., N.P., J.L.R.G., E.T., J.d.L., J.L., S.F., B.R. and M.C.N. conducted the image processing of OCAMS data. A.A.S., V.E.H., M.A.B., G.P., B.E.C., E.S.H., R.P.B. and D.C.R. conducted the spectral characterization and compositional analysis using OVIRS data. W.F.B., C.A., M.D. and K.J.W. conducted the collisional modelling. R.-L.B., R.T.D., E.R.J., T.J.M. and H.C.C. conducted an assessment of the geologic setting. M.G.D., M.M.A.A., L.P., J.S. and O.S.B. produced the OLA digital terrain models. D.S.L. is the principal investigator and leads the OSIRIS-REx mission.

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Correspondence to D. N. DellaGiustina or H. H. Kaplan.

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Extended data

Extended Data Fig. 1 Comparing high- and low-resolution OVIRS spectra of site 6.

a, The lower-resolution spectrum (magenta) of site 6 shown in the main-text as compared to three higher-resolution pyroxene spectra obtained of site 6 (teal) during a lower altitude (~1.4 km) regional flyby of Bennu by the OSIRIS-REx spacecraft. The lower-resolution spectrum (magenta) has been ratioed by Bennu’s global average spectrum to bring out the subtle pyroxene absorption features near 1 and 2 μm, whereas the high-resolution spectra do not require any ratioing to observe these absorption features. b, The band I and II centers (1 and 2 μm) calculated for the pyroxene absorption features plotted against each other, for the lower-resolution (ratioed, magenta) and higher-resolution (unratioed, teal) spectra of site 6. The spectral ratioing does not affect the band centers obtained beyond the uncertainty assigned by the fitting procedure. c, HCP% versus the ratio of the LCP to the HCP band strengths for the lower-resolution (ratioed, magenta) and higher-resolution (unratioed, teal) spectra of site 6, which again shows that the ratioing procedure does not affect the results obtained by applying the MGM to these spectra.

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Supplementary Figs. 1–9 and Tables 1–4.

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DellaGiustina, D.N., Kaplan, H.H., Simon, A.A. et al. Exogenic basalt on asteroid (101955) Bennu. Nat Astron (2020). https://doi.org/10.1038/s41550-020-1195-z

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