[M-IS17] Aqua planetology

[MIS17-P05] Formation of bright materials on Ceres with explosive cryovolcanic eruption of deep subsurface liquid brines.

Masahiro Yoda¹, *Yasuhito Sekine¹, Takazo Shibuya², Sakiko Kikuchi², Shuya Tan¹ (1.Earth-Life Science Insitute, Tokyo Institute of Technology, 2.JAMSTEC)

Keywords:Ceres, carbonate, cryovolcano

Ceres is a dwarf planet in the asteroid belt, which have been attracting attention due to the possibility of liquid eruption from the interior. The surface of Ceres is typically composed of dark components, such as organic matter, and several secondary minerals (e.g., De Sanctis et al., 2015). In addition to the secondary minerals, the bright materials mainly composed of Na₂CO₃ with small amounts of NH₄-bearing salts have been observed on geologically young units of Ceres (e.g., De Sanctis et al., 2016). Gravity and geomorphic data strongly suggest that the liquid brine source that erupted on the surface was originated from deep oceanic water of Ceres (Raymond et al., 2020). On the other hand, the presence of Na₂CO₃ in the bright materials requires highly alkaline brines (e.g., pH > 11), although water chemistry of the deep oceanic water of Ceres was estimated to be moderately alkaline (e.g., 9 < pH <10.5) (e.g., Castillo-Rogez et al., 2018). Thereby, there was a contradiction between geophysical (gravity and geomorphology) evidence and geochemical predictions for formation of the bright materials on Ceres. One potential issue with the contradiction is that effects of decompression of liquid brines upon eruption on salt formation are unclear due to the lack of investigations by laboratory experiments.
Here, we examined the salt formation upon decompression of volatile-containing liquid brines based on laboratory experiments. Our experimental results suggested that alkalinization of liquid brines due to consequent degassing of dissolved CO₂ upon rapid decompression can explain the formation of Na₂CO₃ from moderately alkaline brines (pH down to 10.0). Small amounts of NH₄-bearing salts could be formed with Na₂CO₃ if the brine samples contain the highly levels of NH₃ (e.g., ΣNH₃/ΣCO₂ > 1). Based on our results, we constrained the possible chemical compositions of liquid brine source of the bright materials on Ceres. We also evaluated the chemical evolution of deep oceanic water of Ceres using thermodynamic equilibrium models. The comparison of experimental results and model evaluations suggest that if the subsurface liquid brines originate from deep oceanic water generated by water-rock interactions with water-to-rock ratios less than ~1, Na₂CO₃ with small amounts of NH₄-bearing salts in the bright materials can be explained with its explosive cryovolcanic eruption. Given the young surface age of the bright materials, our results support that Ceres may have maintained the deep subsurface liquid ocean until today.