The presence of ammonium on Ceres was first speculated based on telescopic data in the 1990s. Subsequent data from Dawn unambiguously confirmed the presence on Ceres’s surface. Ammonium has been identified within near-ubiquitous dark materials, and in salts in few localized bright faculae in the interiors of craters as we describe further in this chapter.

The presence of ammonium on Ceres is significant because it implies the availability of ammonia during its evolution. More broadly, understanding the processes that led to the presence of ammonium on Ceres provides important information on the aqueous environments in the early Solar System and the origins and dynamical histories of the large outer main belt asteroids. We briefly review the significance of ammonia and then describe what was known or speculated about ammoniated species on Ceres before Dawn’s arrival. We then review findings of the Dawn mission, in particular the detection and mapping of ammoniated phases by the Visible and Infrared spectrometer (VIR): which species host ammonia/ammonium, their abundance, and spatial distribution. We then discuss the potential origins and implications of ammonia, drawing on laboratory studies and modeling efforts. Finally, we summarize the key findings and the outstanding questions that remain for future investigation.