Compositional variability on the surface of 1 Ceres revealed through GRaND measurements of high‐energy gamma rays |
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| Authors: | David J Lawrence Patrick N Peplowski Andrew W Beck William C Feldman Thomas H Prettyman Chris T Russell Michael J Toplis Jack T Wilson Eleonora Ammannito Julie C Castillo‐Rogez M C DeSanctis Scott C Mest Adrian Neesemann |
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| Institution: | 1. Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA;2. Planetary Science Institute, Tucson, Arizona, USA;3. University of California Los Angeles, Los Angeles, California, USA;4. L'Institut de Recherche en Astrophysique et Planétologie, Toulouse, France;5. Agenzia Spaziale Italiana, Roma, Italy;6. Istituto di Astrofisica e Planetologia Spaziali, Istituto Nazionale di Astrofisica, Roma, Italy;7. Jet Propulsion Laboratory, Pasadena, California, USA;8. Institute of Geological Sciences, Freie Universit?t Berlin, Berlin, Germany |
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| Abstract: | High‐energy gamma rays (HEGRs) from Ceres’s surface were measured using Dawn's Gamma Ray and Neutron Detector (GRaND). Models of cosmic‐ray‐initiated gamma ray production predict that the HEGR flux will inversely vary with single‐layer hydrogen concentrations for Ceres‐like compositions. The measured data confirm this prediction. The hydrogen‐induced variations in HEGR rates were decoupled from the measurements by detrending the HEGR data with Ceres single‐layer hydrogen concentrations determined by GRaND neutron measurements. Models indicate that hydrogen‐detrended HEGR counting rates correlate with water‐free average atomic mass, which is denoted as <A>*. HEGR variations across Ceres’s surface are consistent with <A>* variations of ±0.5 atomic mass units. Chemical variations in the CM and CI chondrites, our closest analogs to Ceres’s surface, suggest that <A>* variations on Ceres are primarily driven by variations in the concentration of Fe, although other elements such as Mg and S could contribute. Dawn observations have shown that Ceres’s interior structure and surface composition have been modified by some combination of physical (i.e., ice‐rock fractionation) and/or chemical (i.e., alteration) processes that has led to variations in bulk surface chemistry. Locations of the highest inferred <A>* values, and thus possibly the highest Fe and least altered materials, tend to be younger, less cratered surfaces that are broadly associated with the impact ejecta of Ceres’s largest craters. |
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