A Possible Brine Reservoir Beneath Occator Crater: Thermal and Compositional Evolution and Formation of the Cerealia Dome and Vinalia Faculae

The Dawn spacecraft has imaged several putative cryovolcanic features on Ceres, and several lines of evidence point to past cryovolcanic activity at Occator crater. It is therefore possible that cryovolcanism played a key role in delivering sodium carbonate- and chloride-enriched brines to Ceres' surface in recent geological times. The detection of a 200 km×200 km negative Bouguer anomaly beneath Occator suggests the presence of a low-density region beneath the crater. If this region is a residual, partially crystallized, cryomagma chamber, excess pressures caused by its gradual freezing, or stresses produced by the Occator-forming impact, could have facilitated the delivery of cryolavas to the surface in the geologically recent past. Here, the progressive solidification of a cryomagma chamber beneath Occator and implications for the delivery of cryolavas to the surface has been explored. Models for the behavior of cryolavas at Ceres' surface, and for the formation of the Cerealia Dome and Vinalia Faculae, are also presented. Minimal crystallization of a subsurface fluid reservoir located at the crust-mantle boundary could have driven cryolavas enriched in chloride salts and NH₃ to Ceres' surface. However, cryolavas enriched in sodium carbonates would have had to have existed at shallower levels in the crust in order to be delivered to the surface via pressure-driven ascent. Depending on the size of the reservoir, cryolavas could have been driven to Ceres' surface for tens to hundreds of millions of years after cooling of the cryomagma chamber commenced. The mineralogy at Occator is suggestive of subsurface cryomagma reservoirs enriched in sodium carbonate and chloride salts. Aqueous solutions enriched in chloride salts and/or ammonia would have arrived at the surface at warm enough temperatures to erupt if transported in propagating fractures that traveled at least 10^-5 m/s. Additionally, if the Cerealia Dome was formed from viscous cryolava extrusions, bulk kinematic lava viscosities may have been between 10⁶-10⁸ m²/s at the onset of relaxation. Plausible relaxation times to form the dome, which are linked to bulk cryolava rheology, are found to have ranged from 2.5 to 273 days. Moreover, the low volatile content necessary to drive explosive eruptions on Ceres supports the possibility that Cerealia and Vinalia Faculae were emplaced as a consequence of ballistic eruptions. The enigmatic geology of Occator crater is consistent with a diversity of exchange processes operating on Ceres in the geologically recent past. Further, the processes that have occurred at Occator could shed light on the changing geology associated with a relic ocean world. Future studies of Occator and Ceres should be undertaken with these results in mind.