Measuring Exsolution Rates of Gases in a Laboratory Analog for Enceladus Plume Formation

Introduction:  Enceladus’s erupting plume likely originates from a subsurface ocean, and thus represents an avenue for revealing the ocean composition. The plume composition was measured by two mass spectrometers on Cassini during flythroughs of the plume. These measurements currently provide our best means of estimating Enceladus’s ocean chemistry and the moon’s potential to host life. The Cosmic Dust Analyzer (CDA; Srama et al. 2004) measured the composition of ice grains in the plume and Saturn’s E ring, finding a variety salts (including biologically useful phosphate) and organic molecules that could be hydrothermal, primordial, or possibly biological in origin (e.g., Postberg et al. 2018, 2023; Khawaja et al. 2019). The Ion and Neutral Mass Spectrometer (INMS; Waite et al. 2006) analyzed the gases in the plume and detected CO_2, NH₃, H₂, CH₄ and HCN, which suggests conditions in the ocean are likely favorable for chemotrophy (e.g., methanogenesis) and possibly prebiotic chemistry (e.g., Waite et al. 2017, Peter et al. 2023).

Motivation: However, the relative abundances of key molecules in the plume may be quite different in the ocean due to fractionating processes during eruption (Fifer et al. 2022). The effects of fractionation are important when considering how the plume gas represents (or misrepresents) the abundances of gases in the ocean. For instance, condensation of water vapor onto the icy walls of the tiger stripe fissures can cause gases like CO₂ to have much higher abundances (relative to water) in the plume than in the ocean (Glein et al. 2015; Glein & Waite 2020; Fifer et al. 2022). In a competing fractionation, the differential exsolution of gases from Enceladus’s ocean will tend to enrich water vapor in the plume relative to other gases (Fifer et al. 2022). While CDA in situ measurements suggest that the ocean’s pH is ~8.5 – 10.5 (e.g., Postberg et al. 2009, Postberg et al. 2023), studies to account for fractionation and estimate ocean gas content and pH from the plume measurements have produced a wide range of possible ocean compositions, with pH ~6–13 (e.g., Marion et al. 2012; Glein et al. 2015). Thus, quantifying fractionation in the plume gas during eruption can better determine the ocean composition.

Here, we used laboratory experiments to constrain a key fractionation process: the exsolution of gases at the liquid-gas interface.

Methods:  In a stainless steel vessel at 0°C, we added pure water or saline solutions and degassed them under vacuum. We then introduced a single gas (e.g., CO₂) and allowed it to dissolve to equilibrium. We monitored the headspace pressure and partially evacuated the headspace gas, driving gas exsolution in an analogous process to how the plumes may form from a water-filled fissure on Enceladus. We can calculate a mass transfer coefficient associated with exsolution by monitoring the increasing headspace pressure during exsolution and deriving the concentration remaining in solution. We also used a stir bar to investigate the effects of stirring or mixing on exsolution.

Results:  We find a positive linear correlation between stir rate and mass transfer coefficient for CO₂ (Figure 1) consistent with previous experiments investigating gas transfer in water (Nishimura et al. 1991). Notably, our mass transfer coefficients are comparable to those derived for ocean-atmosphere exchange on Earth (Broecker & Peng 1982). In trials using a 0.2 NaCl solution, we found a reduction in the mass transfer coefficient of CO₂ by ~25% compared to pure water, which is larger than in previous studies (~10%) for CO₂ diffusion in NaCl solutions (Zhang et al. 2015).

Figure 1: Mass transfer coefficient for CO₂ exsolution from pure water as a function of stir rate in solution.

Conclusions: We find that the mass transfer coefficient of CO₂ strongly depends on the rate of stirring. For Enceladus’s plume formation, this means that an observed flux of erupting gas could originate from either a well-mixed ocean with low gas concentrations or a poorly-mixed ocean with higher gas concentrations. Therefore, it is important to quantify the degree of mixing in the surface ocean where the plume gas is likely sourced.

 

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