NIR reflectance spectroscopy of hydrated and anhydrous sodium carbonates at different temperatures

Abstract

Recent space-based observations have revealed or suggested the existence of various types of carbonate salts in several Solar System bodies, such as Mars, Ceres, Enceladus, and Europa. Natrite is the main component of the crater Occator's faculae observed in detail by the Dawn spacecraft on the dwarf planet Ceres. Sodium carbonates are thought to form as precipitates in brines, originating in aqueous environments in the subsurface of Ceres and icy bodies. Here we report about near-infrared (0.8-4.2 μm) reflectance spectroscopic investigations on three compounds, namely natrite (anhydrous Na<SUB>2</SUB>CO<SUB>3</SUB>), monohydrated sodium carbonate (Na<SUB>2</SUB>CO<SUB>3</SUB>∙H<SUB>2</SUB>O, thermonatrite) and decahydrate sodium carbonate (Na<SUB>2</SUB>CO<SUB>3</SUB>∙10H<SUB>2</SUB>O, natron). Spectral measurements have been carried out in the overall temperature range 93-279 K, representative of planetary surfaces. The analysis of diagnostic spectral signatures shows different temperature-dependent trends for several band parameters, as well as different behavior as a function of the grain size for a given temperature. While spectra of natrite are characterized by several CO<SUB>3</SUB> absorptions, broad and strong absorption features due to H<SUB>2</SUB>O dominate the spectra of heavily hydrated natron. The intermediate sample (monohydrated) shows multiple bands due to the overlap of CO<SUB>3</SUB> and H<SUB>2</SUB>O vibrational modes. Our temperature-dependent laboratory spectra are compared with Dawn-VIR spectra of Ceres and with Galileo-NIMS spectra of Europa.