The accretion of bodies in the asteroid belt was halted nearly 4.6 billion years ago by the gravitational influence of the newly formed giant planet Jupiter. The asteroid belt therefore preserves a record of both this earliest epoch of Solar System formation and variation of conditions within the solar nebula. Spectral features in reflected sunlight indicate that some asteroids have experienced sufficient thermal evolution to differentiate into layered structures1. The second most massive asteroid—4 Vesta—has differentiated to a crust, mantle and core2,3. 1 Ceres, the largest and most massive asteroid, has in contrast been presumed to be homogeneous, in part because of its low density, low albedo and relatively featureless visible reflectance spectrum, similar to carbonaceous meteorites that have suffered minimal thermal processing4. Here we show that Ceres has a shape and smoothness indicative of a gravitationally relaxed object. Its shape is significantly less flattened than that expected for a homogeneous object, but is consistent with a central mass concentration indicative of differentiation. Possible interior configurations include water-ice-rich mantles over a rocky core.
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We thank B. Carcich and K. Consroe for technical assistance. J. Burns and J. Veverka provided discussions. This work was supported by NASA through the Space Telescope Science Institute, which is operated by the Association of Universities for Research and Astronomy, Inc. L.A.M., C.T.R. and M.V.S. were supported by NASA's Dawn Discovery mission. A review by S. Dermott improved the manuscript.
Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.
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Thomas, P., Parker, J., McFadden, L. et al. Differentiation of the asteroid Ceres as revealed by its shape. Nature 437, 224–226 (2005). https://doi.org/10.1038/nature03938
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