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Shape of (101955) Bennu indicative of a rubble pile with internal stiffness

Abstract

The shapes of asteroids reflect interplay between their interior properties and the processes responsible for their formation and evolution as they journey through the Solar System. Prior to the OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer) mission, Earth-based radar imaging gave an overview of (101955) Bennu’s shape. Here we construct a high-resolution shape model from OSIRIS-REx images. We find that Bennu’s top-like shape, considerable macroporosity and prominent surface boulders suggest that it is a rubble pile. High-standing, north–south ridges that extend from pole to pole, many long grooves and surface mass wasting indicate some low levels of internal friction and/or cohesion. Our shape model indicates that, similar to other top-shaped asteroids, Bennu formed by reaccumulation and underwent past periods of fast spin, which led to its current shape. Today, Bennu might follow a different evolutionary pathway, with an interior stiffness that permits surface cracking and mass wasting.

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Code availability

Most of the image and digital terrain analyses shown were undertaken with the JHUAPL Small Body Mapping Tool (SBMT). It is available for the analysis of a broad suite of asteroid and comet data at sbmt.jhuapl.edu. On release of the OSIRIS-REx data by the PDS, a version of SBMT with those data will be made publicly available. The spherical harmonic assessment was performed using the Spherical Harmonic Transform Library hosted at Mathworks (https://www.mathworks.com/matlabcentral/fileexchange/43856-real-complex-spherical-harmonic-transform-gaunt-coefficients-and-rotations). The SPC code used to develop the GDTM of Bennu can be made available with special permission. Please contact the corresponding author for additional information on how.

Data availability

Raw through to calibrated data sets 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 (for example, the GDTM) discussed here will be available in the PDS one year after departure from the asteroid.

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Acknowledgements

This material is based on work supported by NASA under contract NNM10AA11C issued through the New Frontiers Program. The Canadian team members were supported by the Canadian Space Agency. 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.

Author information

O.S.B. is the OSIRIS-REx Altimetry Working Group (AltWG) lead responsible for the generation of all the global and local digital terrain models produced during the OSIRIS-REx Mission. He worked with M.G.D., the instrument Principle Investigator for OLA, as well as E.E.P., R.W.G. and J.R.W., who developed and tested the SPC software, and subsequently made the GDTM. C.L.J. and M.M.A. undertook verification of the SPC GDTM and assessed the circularity of Bennu’s circumference, and J.H.R. and G.A.N. performed the spherical harmonic and Maclaurin assessment of the shape. M.E.P., as the AltWG system engineer, facilitated verification of the GDTM products and contributed to lineament assessments. H.C.M.S. provided roughness assessments of Bennu. R.T.D. measured crater shapes under the guidance of E.E.B. R.T.D. also performed the hypsometry analysis. J.A.S. is the OLA instrument scientist and helped to verify the SPC products. R.M.E., A.H.N., L.N. and C.M.E. helped to generate the final topographic products (for example, elevation, slope and radius) used in the analyses presented. C.M.E. also aided R.T.D. in the crater analysis. W.V.B. and M.C.N. oversaw the collection of the data necessary to make the presented GDTM. C.D.A., M.C.M. and E.M.M. are part of the OSIRIS-REx flight dynamics team who worked closely with the AltWG and provided an independent verification of the SPC model presented. B.R. and C.D.D. are the lead engineers for OCAMS, without which the SPC shape models would not be possible. E.R.J., K.J.W., P.M., S.R.S., R.-L.B. and E.M.M. are members of the OSIRIS-REx Regolith Working Group who either mapped, modelled, or undertook analyses that are part of the presented manuscript. D.J.S. and J.M. worked with the OSIRIS-REx flight dynamics team to generate the asteroid mass needed for the porosity discussion and provided some of the shape inferences from their modelling efforts. S.S., N.H. and S.W. are Hayabusa2 team members who provided access to Ryugu data that was used to motivate some of the discussion presented in this study. K.N.B., D.N.D. and C.A.B. provided the boulder distribution used to demonstrate the influence of the longitudinal ridges on boulder locations. D.S.L. is the OSIRIS-REx Principal Investigator. O.S.B., M.G.D., C.L.J. and M.E.P. drafted the manuscript, which was reviewed by all the authors. The entire OSIRIS-REx Team made the Bennu encounter possible.

Correspondence to O. S. Barnouin.

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Fig. 1: The GDTM of Bennu.
Fig. 2: Amplitude spectrum of a spherical harmonic expansion for the GDTM.
Fig. 3: Deviations of Bennu’s radius from that of a circle, for different latitudinal cross-sections.
Fig. 4: DTMs and images of geological features that contribute to the shape of Bennu.