Experimental constraints on the composition and dynamics of Titan's polar lakes

doi:10.1016/j.epsl.2014.11.023

Earth and Planetary Science Letters

Volume 410, 15 January 2015, Pages 75-83

https://doi.org/10.1016/j.epsl.2014.11.023 Get rights and content

Highlights

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Evaporation rates of C₂H₆–CH₄ mixtures were experimentally measured.
•
Experiments performed at simulated Titan surface conditions
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Evaporation is negligible for C₂H₆ and increases linearly with CH₄ concentration.
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Constraints on the composition of Ontario Lacus are provided.
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Our results along with observations can be used to estimate polar lake composition.

Abstract

Titan's polar lakes are thought to be predominantly composed of liquid ethane and methane; however, little is known on the ratio of these hydrocarbons in the lakes, and the stability and dynamics of these mixtures. Here we provide the first experimental constraints under Titan surface conditions of liquid hydrocarbon mixture evaporation. Our results are relevant to Titan's polar temperatures and pressures (∼92 K and 1.5 bar), and cover a wide range of methane–ethane compositions. We show that evaporation is negligible for pure ethane, but increases nearly linearly with increasing methane concentration. Early dissolution of N₂ results in ternary mixtures evaporating, which is modeled by a ‘hybrid’ thermodynamic equilibrium approach combining Perturbed-Chain Statistical Associating Fluid Theory with a diffusion and buoyancy-driven mass flux model. The approach follows the experimental evaporation rate measurements presented in this study, and allows for the calculation of the corresponding liquid methane–ethane–nitrogen ratios. Such results along with Cassini inferred lake evaporation rates can be used to estimate the composition of Titan's polar liquids, and may have implications on their origin. Our results suggest that Ontario Lacus is predominantly composed of ethane (>50–80 mol%), indicating it may be a residual lake following extensive seasonal methane evaporation, and/or might be in contact with a subsurface liquid reservoir.

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 $(3.1 \pm 0.6) \times 10^{- 4} kg m^{- 2} s^{- 1}$ from experimental simulations for pure CH₄ in a N₂ atmosphere, with a gravity-corrected value for Titan of $(1.6 \pm 0.3) \times 10^{- 4} kg s^{- 1} m^{- 2}$ for ∼94 K, 1.5 bar, and $\sim 2 \times 10^{- 2}$ 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: $J = 0.17 D_{{CH}_{4} / N_{2}} Δ η {(\frac{\frac{Δ ρ}{ρ_{surf}} g}{ν^{2}})}^{\frac{1}{3}}$ where $D_{{CH}_{4} / N_{2}}$ is the diffusion coefficient of CH₄ gas in nitrogen (Poling et al., 2007), $Δ η = ρ_{{CH}_{4}, surf} - ρ_{{CH}_{4}, atm}$ 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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Citation Excerpt :
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Experimental constraints on the composition and dynamics of Titan's polar lakes

Highlights

Abstract

Introduction

Section snippets

Laboratory simulations

Experimental evaporation rates

Theoretical approach

Conclusions

Acknowledgments

Icarus

Icarus

Planet. Space Sci.

Geochim. Cosmochim. Acta

Planet. Space Sci.

Icarus

Icarus

Icarus

Icarus

Icarus

Icarus

Icarus

Icarus

Adv. Space Res.

Planet. Space Sci.

An asymmetric distribution of lakes on Titan as a possible consequence of orbital forcing

Nat. Geosci.

The solubility of nitrogen and air in liquids

J. Phys. Chem. Ref. Data

The identification of liquid ethane in Titan's Ontario Lacus

Nature

Vapor–liquid equilibriums in the nitrogen–methane–ethane system

Chem. Eng. Prog. Symp. Ser.

Erratum: “An estimate of the chemical composition of Titan's lakes” [Astrophys. J. Lett. 707 (2009) L128]

Astrophys. J. Lett.