Suchergebnisse

Thermal evolution modeling has yielded a variety of interior structures for Ceres, ranging from a modestly differentiated interior to more advanced evolution with a dry silicate core, a hydrated silicate mantle, and a volatile-rich crust. Here we compute the mass and hydrostatic flattening from more than one hundred billion three-layer density models for Ceres and describe the characteristics of the population of density structures that are consistent with the Dawn observations. We show that the mass and hydrostatic flattening constraints from Ceres indicate the presence of a high-density core with greater than a 1 sigma probability, but provide little constraint on the density, allowing for core compositions that range from hydrous and/or anhydrous silicates to a mixture of metal and silicates. The crustal densities are consistent with surface observations of salts, water ice, carbonates, and ammoniated clays, which indicate hydrothermal alteration, partial fractionation, and the possible settling of heavy sulfide and metallic particles, which provide a potential process for increasing mass with depth. ; NASA from the Dawn at Ceres Guest Investigator Program [NNX15AI30G]; Discovery ProgramAustralian Research Council [NNM05AA86C]; NASANational Aeronautics & Space Administration (NASA) ; We thank Walter Kiefer, Anton Ermakov, and an anonymous reviewer for their constructive comments. We thank Frederic Chambat for making the Matlab program based on Chambat et al. (2010) available. S. D. K. is supported by NASA award NNX15AI30G from the Dawn at Ceres Guest Investigator Program. We thank the Dawn team for the development, cruise, orbital insertion, and operations of the Dawn spacecraft at Ceres.; C. T. R. is supported by the Discovery Program through contract NNM05AA86C to the University of California, Los Angeles.; A portion of this work was performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. The authors have no conflict of interest to declare. ; Public domain authored by a U.S. government employee

Before NASA's Dawn mission, the dwarf planet Ceres was widely believed to contain a substantial ice-rich layer below its rocky surface. The existence of such a layer has significant implications for Ceres's formation, evolution, and astrobiological potential. Ceres is warmer than icy worlds in the outer Solar System and, if its shallow subsurface is ice-rich, large impact craters are expected to be erased by viscous flow on short geologic timescales. Here we use digital terrain models derived from Dawn Framing Camera images to show that most of Ceres's largest craters are several kilometres deep, and are therefore inconsistent with the existence of an ice-rich subsurface. We further show from numerical simulations that the absence of viscous relaxation over billion-year timescales implies a subsurface viscosity that is at least one thousand times greater than that of pure water ice. We conclude that Ceres's shallow subsurface is no more than 30% to 40% ice by volume, with a mixture of rock, salts and/or clathrates accounting for the other 60% to 70%. However, several anomalously shallow craters are consistent with limited viscous relaxation and may indicate spatial variations in subsurface ice content. ; Published (Publication status) ; Public domain authored by a U.S. government employee