Experimental constraints on the composition and dynamics of Titan's polar lakes
Introduction
One of the landmark discoveries of the Cassini–Huygens mission to date is the existence of stable liquid bodies on Titan's surface. Cassini Synthetic Aperture Radar (SAR) images showed several lake-like features in the north polar region of Titan (Stofan et al., 2007). RADAR microwave radiometry provided further evidence, suggesting these radar-dark features are in fact liquid filled basins with a dielectric constant consistent with an ethane–methane mixture (Janssen et al., 2009). Thus far, hundreds of these hydrocarbon lakes and seas have been identified, mainly confined to the colder and presumably more humid polar regions, with more observed lakes in the north (Aharonson et al., 2009). There is evidence for tropical liquids as well, possibly supplied by occasional heavy rainfall events (Turtle et al., 2011) and/or underground aquifers (Griffith et al., 2012).
While the lakes are thought to be dominated by ethane and methane, there is little direct evidence on the exact amount of these components in the liquid phase. Brown et al. (2008) reported on spectral features observed by the Visible and Infrared Mapping Spectrometer (VIMS) in Ontario Lacus that were interpreted as liquid ethane. Alternatively, Moriconi et al. (2010) suggest the same absorption feature might be in the region surrounding the lake, and could be associated with damp sediments of ethane, propane, methane and possibly other minor hydrocarbons, indicative of retreat due to evaporation. While the presence of ethane in Ontario Lacus does not rule out the presence of methane in the lake, direct surface detection of liquid methane is essentially impossible due to the strong atmospheric absorption of methane. There is, though, a variety of thermodynamic and geochemical models aimed at determining lake composition. Cordier et al. (2009) considered the lakes as non-ideal solutions in thermodynamic equilibrium with the atmosphere and calculated the ethane and methane mole percent to be 76–79% and 6–11%, respectively (Cordier et al., 2013b). Their model based on Regular Solution Theory predicts negligible amounts (∼0.4–0.6%) of dissolved nitrogen in the mixture. Glein and Shock (2013) estimate 15.5% ethane, 68.1% methane, and 14.8% N2 in their modified van Laar model, while Tan et al. (2013) calculate 53.2-8.3% C2H6, 31.8–68.4% CH4, and 6.9–22% N2 for the equator and poles, respectively. The discrepancy in the various model results may be due to the absence of extended datasets at Titan relevant cryogenic temperatures and pressures.
Because methane is thought to be the primary participant in the hydrological cycle on Titan (Lunine and Atreya, 2008), accurate evaporation rates are crucial for general circulation models, as well as to predict the stability of polar lakes. Luspay-Kuti et al. (2012) reported an average evaporation flux of from experimental simulations for pure CH4 in a N2 atmosphere, with a gravity-corrected value for Titan of for ∼94 K, 1.5 bar, and CH4 mole fraction in the simulated atmosphere. While that study focused on evaporation of methane at equatorial temperature conditions, it does not directly represent the poles in composition.
Here we present experimental measurements on the evaporation rate of two major components of the polar lakes under Titan relevant temperature and pressure conditions for a variety of methane–ethane compositions. We also propose a model to describe mixture evaporation and liquid composition, and discuss the implications of our experimental results to Titan's lakes.
Section snippets
Laboratory simulations
We used an experimental facility specifically designed for simulating Titan surface conditions (Wasiak et al., 2013). It consists of a larger, stainless steel host chamber (Andromeda), with a smaller unit (Temperature Control Box (TCB)) located inside. Temperatures relevant to Titan are reproduced via liquid nitrogen flow through coils positioned on both the inside and outside of the TCB and within the condenser, while a 1.5 bar atmosphere is maintained with pressurized N2. A schematic of the
Experimental evaporation rates
The results and details for each simulation performed are summarized in Table 1. Fig. 2, Fig. 3 show typical experimental data of mass loss over time of pure liquid ethane and ethane–methane mixtures at three different initial concentrations, as well as the corresponding temperatures in the liquid, and the gas an inch above the liquid layer (Fig. 2, Fig. 3, bottom panels). The sudden ethane mass increase at 4000 s indicates the introduction of the liquid sample from the condenser into the Titan
Theoretical approach
Previously, pure CH4 evaporation inside the Titan simulation chamber was described as primarily driven by two major effects: diffusion and buoyancy (Luspay-Kuti et al., 2012), using the equation developed by Ingersoll (1970), and modified for Titan conditions. Under these effects, the mass flux is described as: where is the diffusion coefficient of CH4 gas in nitrogen (Poling et al., 2007), is the methane concentration gradient between
Conclusions
We performed laboratory simulations under temperature and pressure conditions relevant to Titan's poles on C2H6–CH4 liquid mixtures. A linear relationship has been found between methane concentration and evaporation rate. Methane–ethane mixtures exhibit an initial N2 dissolution from the simulated atmosphere with increasing methane concentration. To account for nitrogen dissolution and calculate the ternary composition of the evaporating liquids, a thermodynamic equilibrium model that follows
Acknowledgments
This work was funded by the NASA Outer Planet Research Program #NNX10AE10G. DC acknowledges financial support from the Observatoire des Sciences de l'Univers THETA Franche-Comté-Bourgogne, France. The authors would also like to thank two anonymous reviewers for improving the quality of the paper with their valuable comments.
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2017, Geochimica et Cosmochimica ActaCitation Excerpt :Luspay-Kuti et al. (2012, 2015), estimates the lake composition of Ontario Lacus to be ∼70–30% of ethane to methane, the major lake in Titan's southern hemisphere. Whereas, northern lakes are thought to be more methane-rich reaching up to 90–10% methane to ethane concentration (Luspay-Kuit et al., 2012, 2015). Depending on the solubility rates of Titan’s solids in liquid methane and ethane, we should be able to estimate the amounts of solutes that are present on Titan.