Revealing the Varied Sources of Faculae-Forming Brines in Occator Crater via Surface and Sub-Surface Analyses

Dawn observed enigmatic faculae (bright regions) in Occator crater, which are the remnants of brines sourced in the subsurface (Russell+, 2016; De Sanctis+, 2016; Raponi+, 2019). Using up to ~3 m/pix Dawn Framing Camera images obtained during Dawn's final extended mission, we create a new, highest resolution geologic map of Occator and its faculae. We find that the faculae were emplaced via a process we term 'brine effusion': hydrothermal deposits were emplaced ballistically and as flows, originating from numerous localized sources. The association of the faculae with fractures and the crater center (which would remain hottest for the longest time) are analogous to terrestrial/Martian hydrothermal deposits (Osinski+, 2013). A complex hydrologic plumbing system of fracture and hydrologic networks can explain the deposition of Pasola Facula (PF) on a ledge about the central pit, which contains Cerealia Facula (CF). Both CF and PF were primarily sourced in an impact-induced melt chamber, predicted to form in Occator's center (Bowling+, 2019), with some contribution from a deep, long-lived brine reservoir (Nathues+, 2019). Our mapping shows that the availability of the faculae-forming brines varied on short spatial scales, and that the system was often brine-limited. In particular, the thinner Vinalia Faculae (VF), located in the eastern crater floor, are too far from the center to be sourced in the impact-induced melt chamber. Instead, VF are sourced by the deep, long-lived brine reservoir only, which thermal modeling predicts existed before the Occator-forming impact (Castillo+, 2019). The thinner VF deposits suggest the brines took a longer and more difficult path to the surface than the CF-forming brines, which were locally fed from the shallower impact-inducted melt chamber. Moreover, evidence for more ballistic emplacement at VF suggests that the deep brine reservoir had a greater gas content than the impact melt chamber, which lost more gas prior to, or during, emplacement. For all faculae, pathways to the surface were opened by the prevalent impact-induced fracturing throughout the crater (Raymond+, submitted). Part of this work is being carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract to NASA. Government sponsorship acknowledged.