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Elemental composition and mineralogy of Vesta and Ceres: Distribution and origins of hydrogen-bearing species

Prettyman, T. H. and Yamashita, N. and Ammannito, E. and Ehlmann, B. L. and McSween, H. Y. and Mittlefehldt, D. W. and Marchi, S. and Schörghofer, N. and Toplis, M. J. and Li, J.-Y. and Pieters, C. M. and Castillo-Rogez, J. C. and Raymond, C. A. and Russell, C. T. (2019) Elemental composition and mineralogy of Vesta and Ceres: Distribution and origins of hydrogen-bearing species. Icarus, 318 . pp. 42-55. ISSN 0019-1035. http://resolver.caltech.edu/CaltechAUTHORS:20180622-080449020

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Abstract

Combined analyses of the surface elemental composition and mineralogy of Vesta and Ceres provide insights into their interior evolution, crustal formation, and regolith processes. Compositional data acquired by Dawn's Visible to Infrared Mapping Spectrometer (VIR) and Gamma Ray and Neutron Detector (GRaND) are sensitive to different depths and spatial scales. To compare these data sets, high-resolution maps of absorption band strengths from VIR are degraded to the broad spatial scales sampled by GRaND using a physics-based smoothing algorithm that accounts for the shape and topography of Vesta and Ceres. On Vesta, the distributions of elemental hydrogen and hydroxyl are similar, which implies that hydrogen is primarily in the form of hydroxyl, likely as phyllosilicates delivered by the infall of carbonaceous chondrite impactors. Small differences in the spatial patterns of hydroxyl and hydrogen imply that hydrogen is layered in some locations. In Vesta's dark hemisphere, hydrogen deposits are more extensive than hydroxyl, which indicates higher concentrations of hydrated minerals at depth. In contrast, the distributions of elemental hydrogen and hydrogen-bearing species (OH and NH_4^+) on Ceres are dissimilar. High concentrations of hydrogen in the Ceres’ polar regions (approaching 30 wt.% equivalent H_2O) indicate the presence of subsurface ice as predicted by ice stability theory. The concentration of iron follows a water-dilution trend when plotted as a function of regolith hydrogen content, consistent with the presence of subsurface water ice. The VIR and GRaND data jointly constrain aspects of Ceres’ surface chemistry and evolution. GRaND iron measurements place a firm upper bound on magnetite content, which supports graphitized carbon as an alternative to magnetite as a darkening agent. Lower-bounds on the concentration of carbon in carbonates implied by VIR, together with the ratio of carbonates to organics in carbonaceous chondrite meteorite analogs suggest high concentrations of carbon within Ceres’ regolith. GRaND neutron measurements permit elevated carbon concentrations, equal to or in excess of that found in CI chondrites (greater than a few wt.%). Organic matter, detected by VIR at Ernutet crater, might be widespread and may have been converted to graphite, e.g. via UV exposure, elsewhere on the surface. Furthermore, elevated concentrations of carbonaceous material can explain the difference between iron and hydrogen concentrations measured by GRaND and the CI carbonaceous chondrites, which are representative of the materials from which Ceres accreted. The elemental measurements indicate that ice and rock fractionated during Ceres’ evolution producing a crust that differs in composition from the whole body.


Item Type:Article
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https://doi.org/10.1016/j.icarus.2018.04.032DOIArticle
ORCID:
AuthorORCID
Ehlmann, B. L.0000-0002-2745-3240
Additional Information:© 2018 Elsevier B.V. Received 3 January 2018, Revised 26 April 2018, Accepted 27 April 2018, Available online 20 June 2018. This work was carried out under contract with the NASA Jet Propulsion Laboratory, which is operated by the California Institute of Technology in Pasadena, California. Partial support was provided by NASA's Solar System Exploration Research Virtual Institute's Toolbox for Research and Exploration (TREX) project. The Dawn mission is led by the University of California, Los Angeles under the auspices of the NASA Discovery Program. GRaND is operated by the Planetary Science Institute under contract with JPL. VIR is operated by the Italian National Institute for Astrophysics under contract with the Italian Space Agency. The VIR instrument was manufactured by Leonardo S.p.A. We thank reviewers Cristina De Sanctis and Tim Titus for their comments, which helped improve the manuscript. Data used in this study are available for download from the NASA Planetary Data System (https://pds-smallbodies.astro.umd.edu/data_sb/missions/dawn/).
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NASA/JPL/CaltechUNSPECIFIED
Record Number:CaltechAUTHORS:20180622-080449020
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20180622-080449020
Official Citation:T.H. Prettyman, N. Yamashita, E. Ammannito, B.L. Ehlmann, H.Y. McSween, D.W. Mittlefehldt, S. Marchi, N. Schörghofer, M.J. Toplis, J.-Y. Li, C.M. Pieters, J.C. Castillo-Rogez, C.A. Raymond, C.T. Russell, Elemental composition and mineralogy of Vesta and Ceres: Distribution and origins of hydrogen-bearing species, Icarus, Volume 318, 2019, Pages 42-55, ISSN 0019-1035, https://doi.org/10.1016/j.icarus.2018.04.032. (http://www.sciencedirect.com/science/article/pii/S0019103517308643)
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:87306
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:23 Jun 2018 15:54
Last Modified:13 Dec 2018 18:05

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