Physico-chemical evolution of the H2O-CO2-Na2CO3-NH3-CH3OH system at low temperatures and high pressures. Implications for icy ocean worlds.

There is a volatiles enrichment in planetary bodies with the distance from the Sun because the lower temperatures caused their condensation during the solar system formation. As a consequence, the role of volatiles in the geochemistry and thermal evolution became important on certain bodies of the outer solar system, such as the ocean worlds. In order to understand both, physico-chemical interactions between volatiles and salts or other minerals also present in the internal aqueous reservoirs should be characterized. In this regard, the H₂O-CO₂-Na₂CO₃-NH₃-CH₃OH system is a potential candidate to be present within some ocean worlds, such as Enceladus, Titan and Triton. Whereas carbon dioxide (CO₂) has been widely detected on icy satellites [e.g., 1-3], signatures of other volatiles such as ammonia (NH₃) and methanol (CH₃OH) are scarce and have only been found in low concentrations [4], despite their presence is theoretically expected [5-8]. A plausible explanation for the absence of these volatiles on icy moon surfaces is that they reacted at the interior with other substances, forming new compounds [9]. In addition, we included carbonates in our experiments since they are found in carbonaceous chondrites that have suffered low-temperature hydrothermal alteration, and therefore might be considered to come from the outer region of the solar system [10].

In this presentation we will show the results of the Raman spectroscopy kinetic study of the systems H₂O-CO₂-NH₃, H₂O-CO₂-NH₃-CH₃OH, H₂O-Na₂CO₃-NH₃, and H₂O-Na₂CO₃-NH₃-CH₃OH, which were exposed to temperatures down to 230 K and pressures up to 50 MPa. From the Raman spectra taken in situ throughout the experiments performed in a high-pressure cell, we were able to monitor the changes in pH of the systems by analyzing the carbonate-bicarbonate ion peak intensity ratio at each time step due to variations in aqueous chemical speciation and precipitation of solids. Our experiments also allowed us to evaluate the active role of CH₃OH in the redox equilibria of the systems over time. Finally, the solubility of the carbonates formed was also investigated first, in a CO₂ atmosphere, and then at high-pressure water.

This study is the progress of a previous work carried out by differential scanning calorimetry (DSC) [11], where we confirmed that CO₂ clathrates stabilize in coexistence with carbonates. In this work, we observed that the presence of salts and volatiles during clathrate formation affect their level of cage occupancy, resulting in less-stable partially-filled structures with low dissociation enthalpies.

This work is funded by the Spanish State Research Agency (AEI) Project No. MDM-2017-0737 Unidad de Excelencia "María de Maeztu"- Centro de Astrobiología (CSIC-INTA). We also acknowledge support from the Spanish MINECO project PID2019-107472RB-C32.

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