Ceres as an exciting objective for exploration

We modeled several possible thermal evolution scenarios for Ceres in an attempt to explore the nature of wet protoplanets, and to predict current day evidence that might be found by the DAWN mission. Current density for Ceres is near 2.1, suggesting water content between 17 % and 27 % by mass, depending on the density of the rocky material. This is near the 25% predicted for cold solar nebula material available to form Ceres, suggesting little loss of water over the history of Ceres. The surface reflectance spectrum is similar to that of carbonaceous chondrite material and contains absorptions from water of hydration and/or OH in altered minerals. Further, there is evidence of OH, perhaps dissociating from H₂O, escaping from Ceres. We studied several models with different heat fluxes and starting scenarios to bracket the possibilities. Short- and long-lived radioactive nuclide heating is considered. We start with a cold body composed of a uniform mixture of 74% silicates (density 3.54) and 26% water ice. Even if only long-lived radionuclide heating is assumed, the water ice in Ceres melts quickly, a water mantle forms, but the crust does not melt. The circulating warm water would alter the silicates, leading to carbonaceous chondrite-like compositions and additional heating. As heat is lost by conduction through the frozen crust, which reaches a minimum thickness of about 10 km, water begins to freeze out at the base of the crust. Solid state convection begins at about a crust thickness of 20 km and is continuous when the crust reaches a thickness of about 28 km. Ceres' water layer continues to freeze but a liquid layer may still exist at the core surface, especially if there is some anti-freeze component. Additional possible sources of heat considered, including short-lived radionuclides and (with less precision and certainty) exothermal mineralization, enhance the melting of water ice and alter the temperature profiles with depth and time but the crust and silicate core do not melt. Water and altered minerals could be entrained in the convecting ice, creating deposits near or on the surface, which might also be distributed by impacts. Melting and freezing plus mineralization would lead to several dimensional changes over time, creating topographic features, zones of weakness and perhaps disruptions and overturning in the crust. Volcanism is possible. Thus, measurements of present day compositional units and topographic features on Ceres' surface and its internal density structure, will be of considerable help in constraining the nature of Ceres' history and that of its sister protoplanets, who helped form Earth.