Composition and structure of the shallow subsurface of Ceres revealed by crater morphology

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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Figure 1: The apparent depths of Ceres’ largest craters.
Figure 2: Comparison of the observed depths of Ceres’ largest craters with the depths expected for an ice-rich subsurface composition as a function of latitude.
Figure 3: The subsurface viscosity required to preserve crater topography on Ceres.
Figure 4: The topography of craters on Ceres that exhibit possible evidence for viscous relaxation.

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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

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.

Correspondence to Michael T. Bland.

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Bland, M., Raymond, C., Schenk, P. et al. Composition and structure of the shallow subsurface of Ceres revealed by crater morphology. Nature Geosci 9, 538–542 (2016). https://doi.org/10.1038/ngeo2743

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