Consus Crater on Ceres: Ammonium-Enriched Brines in Exchange With Phyllosilicates?

Ceres is a partially differentiated dwarf planet located in the main asteroid belt. Consus crater (diameter ∼64 km) is one of the oldest impact features (∼450 Ma) on the Cerean surface that surprisingly still shows a large variety of color lithologies, including exposures of bright material, which are thought to be brine residues. Here, we present new results that help in understanding the structure and composition of the Cerean crust. These results were deduced by using newly processed Dawn Framing Camera (FC) color imagery and FC clear filter images combined with infrared spectral data of Dawn's Visible and Infrared Spectrometer (VIR). Consus exhibits a variety of color lithologies, which we describe in detail. Interestingly, we found three spectrally different types of bright material exposed by a large old crater on Consus' floor. One of these, the yellowish bright material (Nathues et al., 2023, https://www.hou.usra.edu/meetings/lpsc2023/pdf/1073.pdf) and its modification, shows spectral signatures consistent with ammonium-enriched smectites. We hypothesize that the ammonium in these smectites stems from contact with ascending brines, originating from a low-lying former brine ocean that has been enriched in ammonium during the differentiation and freezing process of the Cerean crust. This enrichment is mainly due to ammonium uptake by sheet silicates. If such an ammonium enrichment occurred over long-time scales on a global scale, this process may explain the vast presence of ammonium on the Cerean surface. Therefore, an outer solar system origin of Ceres is possibly not needed to explain the global presence of ammonium.