The Role of the Icy Shell in the Thermal Evolution of Ceres

Abstract:

Ceres shape and crater morphology are consistent with a layer of low-density material that appears to be more dense and viscous than pure water ice, over an even more dense and viscous core. In order to understand the evolution of Ceres, we conduct a series of numerical experiments designed to understand the evolution of temperature and flow within a spherical body with a soft outer shell over a nearly rigid core using 3D spherical code CitcomS. In these experiments the sphere is heated from within using chondritic abundances of radiogenic elements. We study the impact of surface temperature, outer shell thickness, as well as the density and rheology of the softer outer shell and stiffer core on the thermal and dynamical evolution of the interior of the body, including both the soft shell and stiff core. For the outer shells with a thickness less than 10% of the radius of the body and a surface temperature at or below 90 K, the entire body remains in a conductive state and the temperature of the soft outer shell never exceeds the melting temperature of pure water ice throughout the history of the solar system. However for a range of outer shell thickness and surface temperatures, we find that within the first Gyr of evolution a degree-1 (i.e. single hemisphere) mode of convection encompassing both the stiff core and soft outer shell overtakes shorter-wavelength convective flow occurring in the softer outer shell. When this happens the body dramatically cools over a time interval of less than 100 Myrs and the internal temperature remains asymmetric throughout the subsequent evolution of the body.