NIR-MID Reflectance and Emissivity Study at Different Temperatures of Sodium Carbonate Minerals: Spectra Characterization and Implication for Remote Sensing Identification

Abstract

Recent satellite observations and the deconvolution of remote sensing data have shown the existence of various carbonate minerals in different solar system bodies. Emissivity, from 403 to 803 K, and reflectance spectra at 300 and 193 K of selected carbonates minerals with different ratios of Na:Ca and water amounts were respectively collected at 3–20 μm and at 1–16 μm. All reflectance spectra show absorption features in the 1.9–2.5 and 3.4–4.0 μm areas due to overtone and a combination of CO32− and fundamental vibrational models at ≈9.09, 11.35, 7.06, and 14.7 μm. The increase of the Na:Ca ratio in anhydrous samples produces a shift of the absorption features in the 3.4–4.0 μm area toward shorter wavelengths, and the peak at 3.9 μm doubles in the presence of a CO32− oxygen group shared with two cations in minerals having more complex structures. The comparison of the bands at ≈669 and ≈794 cm−1 in the emissivity spectra collected at high temperatures indicates that around 600 K, phase transitions occurred in natrite and thermonatrite. The reflectance spectra measured at 193 K reveal a fine structure compared to spectra collected at room temperature. The comparison of laboratory results with the spectrum of Ceres’s brightest crater Occator from the Dawn mission, taken as a case study, showed how the anhydrous samples, shortite and nyerereite, studied in this work can also be hypothesized for Ceres’s surface beyond that already suggested (trona, natrite, thermonatrite).