Characterization of NH4-montmorillonite under conditions relevant to Ceres

Abstract

Research interest in NH4-smectites in planetology is increasing after the discovery of their high abundance on the surface of Ceres. This dwarf planet is considered a relic ocean world, showing evidence of extended aqueous alteration and cryovolcanic activity that occurred during the course of its history (De Sanctis et al., 2020). Despite the position of Ceres in the asteroid belt, a variety of ammonium-rich minerals, including phyllosilicates, carbonates, and chlorides, have been reported on its surface (Raponi et al., 2019). Ammonium-rich phases are expected to be found at greater distances from the Sun. Consequently, the study of the stability of ammonium-bearing phases such as smectites under environmental conditions relevant to the interior of Ceres and its regolith can help elucidate certain ambiguities concerning the provenance of its precursor materials. In this study, smectites were characterized by Raman and infrared spectroscopy, X-ray diffraction, and scanning electron microscopy under conditions mimicking the surface environment of Ceres, that is, at low temperatures (110–145 K) and near-vacuum conditions (1–0.001 Pa). When NH4-montmorillonite was freeze-dried, different structural stabilities were observed depending on the aggregation texture of the sample. In case the clay minerals on the surface of Ceres have an endogenic origin, its chemical composition and predominant interlayer cation are expected to provide insights into the type of fluids and chemical reactions that occur at deeper layers of the icy crust. The results of this characterization study are relevant for the future selection of payloads for space missions. The strength of combining different analytical methods in characterizing and interpreting mineralogical composition has been demonstrated in this study.