Kinetic effect on the freezing of ammonium-sodium-carbonate-chloride brines and implications for the origin of Ceres’ bright spots

Abstract Prominent bright spots containing natrite (Na 2 CO 3 ) and smaller amounts of NH 4 Cl or NH 4 HCO 3 observed in Ceres' Occator crater have been proposed to originate from an internal reservoir, where subsurface brines freeze upon reaching the surface. Our group has previously examined the freezing of putative subsurface brines containing ammonium, sodium, carbonate, and chloride ions at a cooling rate of 30 K/min (Vu et al., 2017), and found support for this hypothesis. The present study investigates this further by examining the formation of minerals on Ceres’ surface upon kinetic vs thermodynamic freezing. Using Infrared and Raman spectroscopies, flash frozen brines were found to form predominantly ammonium chloride and ammonium bicarbonate, even in sodium-dominated solutions. In contrast, hydrohalite (NaCl·2H 2 O) only formed when sodium and/or chloride were present in excess in the brine solutions. Additionally, the effect of vacuum exposure on these frozen brines was further analyzed to simulate the surface conditions of Ceres. It was observed that the vacuum exposure did not affect speciation, although it did clarify spectral features and confirm the presence of the aforementioned species. Our results provide evidence that (i) ammonium-bearing materials are kinetically-favored products under fast-freezing conditions with these brine mixtures, and (ii) the bright spots on Ceres's surface likely originate from subsurface liquids, mobilized either by a cryovolcanic process or by the Occator-forming impact, and not by direct excavation.