Abstract

Before NASA’s Dawn mission, the dwarf planet Ceres was widely believed to contain a substantial ice-rich layer below its rocky surface. The existence of such a layer has significant implications for Ceres’s formation, evolution, and astrobiological potential. Ceres is warmer than icy worlds in the outer Solar System and, if its shallow subsurface is ice-rich, large impact craters are expected to be erased by viscous flow on short geologic timescales. Here we use digital terrain models derived from Dawn Framing Camera images to show that most of Ceres’s largest craters are several kilometres deep, and are therefore inconsistent with the existence of an ice-rich subsurface. We further show from numerical simulations that the absence of viscous relaxation over billion-year timescales implies a subsurface viscosity that is at least one thousand times greater than that of pure water ice. We conclude that Ceres’s shallow subsurface is no more than 30% to 40% ice by volume, with a mixture of rock, salts and/or clathrates accounting for the other 60% to 70%. However, several anomalously shallow craters are consistent with limited viscous relaxation and may indicate spatial variations in subsurface ice content.

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Acknowledgements

M.T.B. thanks Trent Hare for ArcGIS support. This work was supported by NASA’s Dawn Guest Investigator Program (NNH15AZ85I).

Author information

Affiliations

  1. US Geological Survey, Astrogeology Science Center, Flagstaff, Arizona 86001, USA

    • Michael T. Bland
  2. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, USA

    • Carol A. Raymond
    • , Ryan S. Park
    •  & Julie C. Castillo-Rogez
  3. Lunar and Planetary Institute, Houston, Texas 77058, USA

    • Paul M. Schenk
  4. Columbia University, New York, New York 10027, USA

    • Roger R. Fu
  5. Institute of Geological Sciences, Freie Universität Berlin, Berlin 12249, Germany

    • Thomas Kneissl
  6. Institut für Planetologie, Westfälische Wilhelms-Universität, Münster 48149, Germany

    • Jan Hendrik Pasckert
    •  & Harry Hiesinger
  7. German Aerospace Center (DLR), Berlin 12489, Germany

    • Frank Preusker
  8. Southwest Research Institute, Boulder, Colorado 80302, USA

    • Simone Marchi
  9. Virginia Institute of Technology, Blacksburg, Virginia 24061, USA

    • Scott D. King
  10. Department of Earth and Space Sciences, University of California, Los Angeles, California 90095, USA

    • Christopher T. Russell

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Contributions

M.T.B. measured all craters, designed, performed and analysed all simulations, and wrote the manuscript. P.M.S. provided comparison data for crater depths on planetary bodies and discussed the results and implications of the observations and simulations. T.K., J.H.P. and H.H. provided crater counts for Coniraya and Vinotonus and discussed implications of the results. F.P. and R.S.P. created and provided the DTMs used to measure crater depths. C.A.R., R.R.F., J.C.C.-R., S.D.K. and S.M. provided substantive discussion of the observations, simulations and implications for Ceres’ near-surface composition, and provided critical comments on the manuscript at all stages of development. C.T.R. is responsible for the Dawn mission and all data acquisition.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Michael T. Bland.

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DOI

https://doi.org/10.1038/ngeo2743