GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 110-5
Presentation Time: 9:10 AM


HIESINGER, Harald1, MARCHI, Simone2, SCHMEDEMANN, Nico3, SCHENK, Paul M.4, PASCKERT, Jan Hendrik1, NEESEMANN, Adrian3, O'BRIEN, David5, KNEISSL, Thomas3, ERMAKOV, Anton I.6, FU, Roger R.7, BLAND, Michael8, NATHUES, Andreas9, PLATZ, Thomas10, WILLIAMS, David A.11, JAUMANN, Ralf12, CASTILLO-ROGEZ, Julie C.13, RUESCH, Ottaviano14, SCHMIDT, Britney E.15, PARK, Ryan13, PREUSKER, Frank16, BUCZKOWSKI, D.L.17, RUSSELL, Christopher T.18 and RAYMOND, Carol A.13, (1)Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Str. 10, Münster, 48149, Germany, (2)Southwest Research Institute, Boulder, CO 80302, (3)Freie Universität Berlin, Berlin, 12249, Germany, (4)Lunar and Planetary Institute, Universities Space Research Association, 3600 Bay Area Boulevard, Houston, TX 77058, (5)Planetary Science Institute, Tucson, AZ 85719, (6)Massachusetts Institute of Technology, Cambridge, MA 02139, (7)Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Building 54-724, Cambridge, MA 02139, (8)Astrogeology Science Center, United States Geological Survey, 2255 N. Gemini Dr., Flagstaff, AZ 86001, (9)Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, Goettingen, 37077, Germany, (10)Max Planck Institute for Solar System Research, Göttingen, 37077, Germany, (11)School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, (12)German Aerospace Center (DLR), Institute of Planetary ResearchGerman Aerospace Center (DLR), Berlin, Germany, (13)Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, (14)Goddard Space Flight Center, NASA, Greenbelt, MD 20771, (15)Georgia Institute of Technology, Atlanta, GA 30332, (16)German Aerospace Center (DLR), Institute of Planetary Research, Rutherfordstr. 2, Berlin, 12489, Germany, (17)Space Departrment, Johns Hopkins Applied Physics Laboratory, 11100 Johns Hopkins Rd, Laurel, MD 20723, (18)Earth and Space Sciences, University of California, Los Angeles, 595 Charles Young Drive East, Los Angeles, CA 90095,

Dawn is the first spacecraft to closely observe the surface of Ceres with a Visible and Infrared Spectrometer (VIR), a Gamma Ray and Neutron Detector (GRaND), and a Framing Camera (FC) from an orbit as low as 850 km in radius. FC images (137 m/pixel) reveal that craters larger than 300 km are absent and that the northern cerean hemisphere is more heavily cratered than the southern hemisphere. We identified craters with bowl-shapes, polygonal shapes, terraces, central peaks, smooth floors, and flow-like features that we interpret as evidence for mobilization of crustal material in the presence of ice. Craters larger than 40 km exhibit central pits (preferentially in craters > 75 km), possible pitted floors, and floor fractures. Cerean craters are deeper than similar-sized fresh lunar craters, and modestly shallower than fresh craters on Tethys. They are much deeper than the completely flattened craters predicted for a pure outer ice layer. Dawn observations of preserved and relaxed craters along with Ceres’ shape are also inconsistent with a pure rocky outer shell. The transition from bowl-shaped simple craters to modified complex craters (8-10 km) and the transition to complex craters with central peaks (~25 km) are most consistent with an ice-bearing upper crust or a material mixture of similar viscosity. Deriving absolute model ages (AMAs) from the observed crater size-frequency distributions requires a production function (PF) and a chronology function (CF). A lunar-derived model is adapted to impact conditions on Ceres, in a manner similar to adjustments for Mars and Mercury. An asteroid-derived model alternatively uses a PF by scaling the directly observed asteroid size-frequency distribution from the main belt and extended to sizes smaller than 5 km by a collisional model. We will present AMAs derived from both models.