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A partially differentiated interior for (1) Ceres deduced from its gravity field and shape

Abstract

Remote observations of the asteroid (1) Ceres from ground- and space-based telescopes have provided its approximate density and shape, leading to a range of models for the interior of Ceres, from homogeneous to fully differentiated1,2,3,4,5,6. A previously missing parameter that can place a strong constraint on the interior of Ceres is its moment of inertia, which requires the measurement of its gravitational variation1,7 together with either precession rate8,9 or a validated assumption of hydrostatic equilibrium10. However, Earth-based remote observations cannot measure gravity variations and the magnitude of the precession rate is too small to be detected9. Here we report gravity and shape measurements of Ceres obtained from the Dawn spacecraft, showing that it is in hydrostatic equilibrium with its inferred normalized mean moment of inertia of 0.37. These data show that Ceres is a partially differentiated body, with a rocky core overlaid by a volatile-rich shell, as predicted in some studies1,4,6. Furthermore, we show that the gravity signal is strongly suppressed compared to that predicted by the topographic variation. This indicates that Ceres is isostatically compensated11, such that topographic highs are supported by displacement of a denser interior. In contrast to the asteroid (4) Vesta8,12, this strong compensation points to the presence of a lower-viscosity layer at depth, probably reflecting a thermal rather than compositional gradient1,4. To further investigate the interior structure, we assume a two-layer model for the interior of Ceres with a core density of 2,460–2,900 kilograms per cubic metre (that is, composed of CI and CM chondrites13), which yields an outer-shell thickness of 70–190 kilometres. The density of this outer shell is 1,680–1,950 kilograms per cubic metre, indicating a mixture of volatiles and denser materials such as silicates and salts14. Although the gravity and shape data confirm that the interior of Ceres evolved thermally1,4,6, its partially differentiated interior indicates an evolution more complex than has been envisioned for mid-sized (less than 1,000 kilometres across) ice-rich rocky bodies.

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Figure 1: Mollweide projection of topography and the Bouguer anomaly.
Figure 2: Ceres gravity and error magnitude spectra of normalized spherical harmonic coefficients7,8.
Figure 3: Ceres core and shell densities computed by numerical integration of Clairaut’s equations of hydrostatic equilibrium9.

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Acknowledgements

This research was carried out in part at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. We thank the Dawn operations team for the development, cruise, orbital insertion and operations of the Dawn spacecraft at Ceres. R.S.P. thanks W. M. Folkner, E. M. Mazarico, and M. D. Rayman for comments and suggestions. N.R. is grateful to the CNU, Section 34, for supporting a six-month full-time research project through CRCT-2015 funding delivered by the MESR and acknowledges funding from the French National Programme of Planetology (PNP). M.J.T. acknowledges funding by the CNES. Government sponsorship acknowledged. All rights reserved.

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Authors

Contributions

R.S.P., A.S.K. and A.T.V. performed data analysis and calibration. R.S.P., B.G.B., N.R., J.C.C.-R., C.A.R., A.I.E., M.T.Z., R.R.F., M.J.T., C.T.R., A.N. and F.P. contributed to the interpretation of the data. All authors contributed to the discussion of the results and to writing the paper.

Corresponding author

Correspondence to R. S. Park.

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The authors declare no competing financial interests.

Additional information

The gravity science and framing camera data is available through the PDS Small Bodies Node website (http://sbn.pds.nasa.gov/data_sb/missions/dawn).

Extended data figures and tables

Extended Data Figure 1 Range of values for the degree-2 zonal gravity coefficient for Ceres.

We assume a two-layer model for Ceres and vary the density and radius of the core by solving Clairaut’s equations of rotational equilibrium at third order9. The best-fitting solution for J2 = 264.99 × 10−4 (blue line) is compared with the direct approach of Gao and Stevenson27 (red line), which generally shows a good agreement except at high core-density values. The white area at the upper right corresponds to solutions with shell density below 900 kg m−3.

Extended Data Table 1 Gravity spherical harmonic coefficients of Ceres7
Extended Data Table 2 Various best-fitting ellipsoid estimates of the shape of Ceres with respect to the centre of mass

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Park, R., Konopliv, A., Bills, B. et al. A partially differentiated interior for (1) Ceres deduced from its gravity field and shape. Nature 537, 515–517 (2016). https://doi.org/10.1038/nature18955

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