Properties of rubble-pile asteroid (101955) Bennu from OSIRIS-REx imaging and thermal analysis

Abstract

Establishing the abundance and physical properties of regolith and boulders on asteroids is crucial for understanding the formation and degradation mechanisms at work on their surfaces. Using images and thermal data from NASA’s Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) spacecraft, we show that asteroid (101955) Bennu’s surface is globally rough, dense with boulders, and low in albedo. The number of boulders is surprising given Bennu’s moderate thermal inertia, suggesting that simple models linking thermal inertia to particle size do not adequately capture the complexity relating these properties. At the same time, we find evidence for a wide range of particle sizes with distinct albedo characteristics. Our findings imply that ages of Bennu’s surface particles span from the disruption of the asteroid’s parent body (boulders) to recent in situ production (micrometre-scale particles).

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Fig. 1: Resolved phase curves of Bennu.
Fig. 2: Asteroid Bennu imaged by the OSIRIS-REx Camera Suite.
Fig. 3: Global cumulative size–frequency distribution of boulders.
Fig. 4: Albedo trends among Bennu’s boulders.
Fig. 5: Evidence of in situ boulder disaggregation.
Fig. 6: Dark diffuse units.

Code availability

The thermophysical analysis reported here uses a custom code that is based on the Advanced Thermophysical Model (ATPM) of refs. 46,59,60. The ISIS3 code used to generate the image processing data products is available from the US Geological Survey–Astrogeology Science Center: https://isis.astrogeology.usgs.gov/

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding authors upon reasonable request. Raw (L0) through calibrated (L2, L3) OCAMS, OVIRS and OTES data will be available via the Planetary Data System (PDS) (https://sbn.psi.edu/pds/resource/orex/). Data are released to the PDS according to the schedule provided in the OSIRIS-REx Data Management Plan found in the OSIRIS-REx PDS archive. Image mosaics and photometric models will be available in the PDS 1 year after departure from the asteroid.

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Acknowledgements

This material is based upon work supported by NASA under contract NNM10AA11C issued through the New Frontiers Program. We thank C. Ernst for providing the data used in Supplementary Fig. 4 to compare the size-frequency distribution of boulders on Bennu to other small bodies. B.R. acknowledges funding support from the Royal Astronomical Society in the form of a research fellowship. P.M. acknowledges funding support from the French space agency CNES and from Academies of Excellence: Complex systems and Space, environment, risk, and resilience, part of the IDEX JEDI of the Université Côte d’Azur. M.A.B., J.D.P.D. and S.F. also acknowledge financial support from CNES the French space agency. M.P. acknowledges funding support from the Italian Space Agency (ASI) under the ASI-INAF agreement no. 2017–37-H.0. Part of this work was performed at the Jet Propulsion Laboratory, California Institute of Technology under contract with the National Aeronautics and Space Administration. S.R.S. acknowledges support from NASA Grant no. 80NSSC18K0226 as part of the OSIRIS-REx Participating Scientist Program.

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D.N.D. leads the OSIRIS-REx Image Processing Working Group (IPWG). J.P.E. leads the OSIRIS-REx Thermal Analysis Working Group (TAWG). Both led the analysis and paper writing efforts. M.A.B., C.A.B., K.N.B., H.C., J.d.L., C.Y.D.d’A., S.F., D.R.G., C.H., E.S.H., H.H.K., T.K., L.L.C., J.-Y.L., J.L., J.L.R.G., P.M., M.N., M.P., D.C.R., B.R., P.H.S., E.T., C.A.T. and X.-D.Z. contributed to the image processing analysis of MapCam images. O.S.B., K.J.B., C.A.B., D.R.G. and C.H. contributed to the production of the global mosaic. E.A., R.-L.B., O.S.B., C.A.B., E.B.B., W.F.B., K.N.B., H.C., H.C.C.Jr, M.D., M.D., C.M.E., E.R.J., P.M., J.M., M.N., M.P., A.R., S.R.S., E.T. and K.J.W. contributed to the interpretation of the boulder size-frequency distribution. B.E.C., J.D.P.D., C.Y.D.d’A., S.F., D.R.G., C.H., E.S.H., J.-Y.L., B.R., N.S., A.A.S., P.H.S. and X.-D.Z. contributed to the interpretation of the photometric model. E.A., R.-L.B., J.L.B., O.S.B., R.P.B., W.F.B., N.B., P.R.C., B.C.C., M.D., C.M.E., V.E.H., E.S.H., E.R.J., T.K., L.F.L., J.M., M.N., M.P., D.C.R., B.R., A.R., S.R.S., M.A.S., A.A.S., E.T., C.A.T. and K.J.W. contributed to the interpretation of the thermophysical measurements. N.B., P.R.C., L.F.L., B.R. and M.A.S. contributed to the analysis or calibration of thermal data. D.N.D., C.Y.D.d’A., J.P.E., D.R.G., V.E.H., C.H., L.F.L., D.C.R., B.R. and A.A.S. contributed to the planning of observations. K.J.B., C.Y.D.d’A., D.R.G., E.S.H., B.R., N.S., P.H.S. and X.-D.Z. contributed to the calibration of image data. D.S.L. leads the mission and contributed significantly to the analysis. C.W.V.W. contributed substantially to the content and writing of the paper. The entire OSIRIS-REx Team made the encounter with Bennu possible.

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Correspondence to D. N. DellaGiustina or J. P. Emery.

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DellaGiustina, D.N., Emery, J.P., Golish, D.R. et al. Properties of rubble-pile asteroid (101955) Bennu from OSIRIS-REx imaging and thermal analysis. Nat Astron 3, 341–351 (2019). https://doi.org/10.1038/s41550-019-0731-1

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