Ammonium-rich bright areas on Ceres demonstrate complex chemical activity

Abstract

The dwarf planet Ceres has been proven to be a world with an extraordinary diversity of chemical species formed in aqueous environments. Dantu crater, being one of the largest and deepest impact craters on Ceres, offers valuable insights into the geological history and composition of this enigmatic world. Its particular position, within a topographic low, can be considered a window into the composition of Ceres’s subsurface. One of the intriguing aspects of Dantu is the presence of several bright areas called “faculae”. These bright materials, distributed unevenly on the Cerean surface, are believed to be residua from salty fluids, likely still circulating in the subsurface and capable to extrude onto the surface, as demonstrated by the identification of fresh hydrohalite (NaCl·2(H2O)) on the bright faculae in another exceptional crate, Occator. The detailed investigation of the Dantu’s faculae reveals the presence of at least two different “populations” of bright materials, compositionally distinct even if in close geographical proximity. These faculae appear different, white and yellow, in the color images (RGB: R = 0.917 μm, G = 0.653 μm, B = 0.438 μm) taken by the Dawn camera. The spectra show that the first population -white- is mainly composed of sodium carbonate, similar to other identified bright areas on Ceres; the second one -yellow- is likely dominated by ammonium-rich components. The spectra of the yellow faculae show specific and clear bands, associated to ammonium bearing phases, but the exact species producing such spectral features are not unambiguously identified. A potential candidate is ammonium bicarbonates, but we cannot exclude other compounds and combinations of different species. Other occurrences of these yellow faculae have been identified in a few other areas, even if less numerous and with a much smaller extent with respect to the Dantu crater. The discovery of a very bright component different from the carbonates and salts previously identified increases the variety and complexity of the salty aqueous solutions on Ceres, offering tantalizing clues about the potential for habitable environments and subsurface fluid reservoirs on this dwarf planet.