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 core^{2, 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.

1. Center for Radiophysics and Space Research, Cornell University, Ithaca, New York 14853, USA
2. Department of Space Studies, SwRI, 1050 Walnut Street, Boulder, Colorado 80302, USA
3. Department of Astronomy, University of Maryland, College Park, Maryland 20742, USA
4. IGPP & ESS, University of California Los Angeles, Los Angeles, California 90095, USA
5. Planetary Science Institute, 1700 East Fort Lowell, Tucson, Arizona 85719-2395, USA

Correspondence to: P. C. Thomas¹ Correspondence and requests for materials should be addressed to P.C.T. (Email: thomas@baritone.astro.cornell.edu).

Received 22 April 2005; Accepted 10 June 2005

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