Ammoniation of Phyllosilicates, Carbonaceous Chondrite Meteorites, and Implications for the Nature of Ammoniated Materials on Ceres

Abstract

Ammonciated- and phyllosilicatebearing materials were discovered on Ceres and globally mapped with the VIR imaging spectrometer on the Dawn mission [1-3]. Key Ceres infrared spectral features are located at ~3.05 µm, indicating NH4-bearing materials, and 2.72 µm, indicating Mg-OH-bearing materials. Their co-occurrence everywhere on the surface [3] makes it likely, but not certain, that Mg phyllosilicates are the carrier of the ammonium. Readily available spectral libraries have ammoniated, Al-phyllosilicates, which have been used in radiative transfer unmixing models to estimate the composition of the Ceres surfaces [2]. Yet, more suitable may be Fe- or Mg-phyllosilicates, which are more likely from meteoritic materials. Here, we obtain, purify, and ammoniate a variety of Ceres-relevant analog materials, including both phyllosilicates and meteorites. We then evaluate (i) which phases ammoniate; (ii) which ammoniated phases exhibit absorptions with similar positions, widths, and shapes to those observed for Ceres; and (iii) if/how the NH4-related absorptions change at different temperatures under vacuum [detailed further in 4]. The overarching goals are to refine the identification of the ammoniated species on Ceres and develop a spectral library suitable for accurate, quantitative radiative transfer models that estimate Ceres' composition.

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