A Probabilistic Approach to Determination of Ceres' Average Surface Composition From Dawn Visible‐Infrared Mapping Spectrometer and Gamma Ray and Neutron Detector Data

• Published in: Journal of geophysical research. Planets Vol. 125; no. 12
• Main Authors: Kurokawa, H., Ehlmann, B. L., De Sanctis, M. C., Lapôtre, M. G. A., Usui, T., Stein, N. T., Prettyman, T. H., Raponi, A., Ciarniello, M.
• Format: Journal Article
• Language: English
• Published: 01.12.2020
• Subjects: carbonaceous chondrites; Ceres; elemental chemistry; infrared spectra; mineralogy
• DOI: 10.1029/2020JE006606
• ISSN: 2169-9097

The Visible‐Infrared Mapping Spectrometer (VIR) on board the Dawn spacecraft revealed that aqueous secondary minerals—Mg‐phyllosilicates, NH4‐bearing phases, and Mg/Ca carbonates—are ubiquitous on Ceres. Ceres' low reflectance requires dark phases, which were assumed to be amorphous carbon and/or magnetite (∼80 wt.%). In contrast, the Gamma Ray and Neutron Detector (GRaND) constrained the abundances of C (8–14 wt.%) and Fe (15–17 wt.%). Here, we reconcile the VIR‐derived mineral composition with the GRaND‐derived elemental composition. First, we model mineral abundances from VIR data, including either meteorite‐derived insoluble organic matter (IOM), amorphous carbon, magnetite, or combination as the darkening agent and provide statistically rigorous error bars from a Bayesian algorithm combined with a radiative‐transfer model. Elemental abundances of C and Fe are much higher than is suggested by the GRaND observations for all models satisfying VIR data. We then show that radiative transfer modeling predicts higher reflectance from a carbonaceous chondrite of known composition than its measured reflectance. Consequently, our second models use multiple carbonaceous chondrite endmembers, allowing for the possibility that their specific textures or minerals other than carbon or magnetite act as darkening agents, including sulfides and tochilinite. Unmixing models with carbonaceous chondrites eliminate the discrepancy in elemental abundances of C and Fe. Ceres' average reflectance spectrum and elemental abundances are best reproduced by carbonaceous‐chondrite‐like materials (40–70 wt.%), IOM or amorphous carbon (10 wt.%), magnetite (3–8 wt.%), serpentine (10–25 wt.%), carbonates (4–12 wt.%), and NH4‐bearing phyllosilicates (1–11 wt.%). Plain Language Summary Ceres is a dwarf planet and the largest object in the main asteroid belt, consisting of ice and assemblages of hydrous minerals that incorporate water, ammonium, and carbon. An open question about Ceres is its surface composition and the nature of the materials that make its surface very dark. Whereas iron oxides and amorphous carbon have been suggested from the analyses of spectra at infrared wavelengths of light and mixture modeling using pure mineral or organic endmembers, elemental analysis independently found that C and Fe are not as abundant as posited by these analyses. Thus, another phase or set of phases must be responsible. The dark nature of Ceres is similar to the dark nature of carbonaceous chondrite meteorites, measured in laboratory. We find that the tiny nanometer‐ and micrometer‐scale of darkening agents in these meteorites is not well‐accounted for by existing physics‐based mixture models of mineral and organic endmembers. Consequently, we use a spectral unmixing model that involves minerals, organics and carbonaceous chondrite meteorites to show that Ceres' surface contains multiple darkening agents of the style found in carbonaceous‐chondrite meteorites and additional carbon, hydrous minerals, carbonates, and NH4‐bearing minerals. Key Points We derive Ceres' composition using a Bayesian Hapke radiative‐transfer model for infrared spectra, coupled with elemental constraints We reconcile compositional estimates of Ceres' mineralogy from reflectance spectra and elemental abundances from gamma ray and neutron data Ceres is modeled by a ∼50% carbonaceous chondrite‐like composition with 10% excess carbon and additional carbonates and phyllosilicates