Journal Article

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

Preusker, F., Raymond, C. A., Castillo-Rogez, J. C., Nathues, A., Park, R. S., Toplis, M. J., Rambaux, N., Zuber, M. T., Konopliv, A. S., Vaughan, A. T., Ermakov, A. I., Russell, C. T., Bills, B. G., Fu, R. R.

2016

Overview

Gravity and shape measurements for Ceres obtained from the Dawn spacecraft mission show that it is in hydrostatic equilibrium with its inferred normalized mean moment of inertia of 0.37, suggesting that Ceres has a rocky chondritic core overlaid by a volatile-rich icy shell. Geophysical observations of dwarf planet Ceres This paper presents geophysical observations of Ceres—the closest dwarf planet to the Sun, in an orbit between those of Mars and Jupiter—based on radio tracking and onboard image data acquired by the Dawn spacecraft. Gravity and shape measurements provide a key parameter that has been unobtainable through remote observations—the moment of inertia. Ceres is shown to be in hydrostatic equilibrium with an inferred normalized mean moment of inertia of 0.37. The Dawn spacecraft data and analysis reported here give the first constraints on the interior structure of a dwarf planet. Ceres emerges as a partially differentiated body, with a rocky core overlaid by a volatile-rich icy shell. 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 differentiated 1 , 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 variation 1 , 7 together with either precession rate 8 , 9 or a validated assumption of hydrostatic equilibrium 10 . However, Earth-based remote observations cannot measure gravity variations and the magnitude of the precession rate is too small to be detected 9 . 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 studies 1 , 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 compensated 11 , such that topographic highs are supported by displacement of a denser interior. In contrast to the asteroid (4) Vesta 8 , 12 , this strong compensation points to the presence of a lower-viscosity layer at depth, probably reflecting a thermal rather than compositional gradient 1 , 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 chondrites 13 ), 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 salts 14 . Although the gravity and shape data confirm that the interior of Ceres evolved thermally 1 , 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.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 639/33/445 / 639/33/445/123 / 639/33/445/845 / 639/33/445/848 / Asteroids / Astrophysics / Gravitational waves / Gravity / Humanities and Social Sciences / letter / multidisciplinary / Observations / Physics / Science / Silicates / Spacecraft / Topography