Earth and Planetary Science Letters 410 (2015) 75–83 Contents lists available at ScienceDirect Earth and Planetary Science Letters www.elsevier.com/locate/epsl Experimental constraints on the composition and dynamics of Titan’s polar lakes A. Luspay-Kuti a,b,∗ , V.F. Chevrier b , D. Cordier c , E.G. Rivera-Valentin d , S. Singh b , A. Wagner b , F.C. Wasiak b a Space Science & Engineering Division, Southwest Research Institute, 6220 Culebra Rd., San Antonio, TX 78238, USA b Arkansas Center for Space and Planetary Sciences, STON F47, 346 1/2 N. Arkansas Ave., University of Arkansas, Fayetteville, AR 72701, USA c Université de Franche-Comté, Institut UTINAM, CNRS/INSU, UMR 6213, Besancon Cedex, France d Arecibo Observatory (USRA), National Astronomy and Ionosphere Center, Arecibo, PR 00612, USA a r t i c l e i n f o a b s t r a c t Article history: Received 10 December 2013 Received in revised form 4 September 2014 Accepted 4 November 2014 Available online xxxx Editor: C. Sotin Keywords: Titan evaporation rate hydrocarbon lakes experimental lake composition 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 N 2 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.  2014 Elsevier B.V. All rights reserved. 1. Introduction One of the landmark discoveries of the Cassini–Huygens mission to date is the existence of stable liquid bodies on Titan’s sur- face. 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 mix- ture (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). * Corresponding author. E-mail address: aluspaykuti@swri.edu (A. Luspay-Kuti). 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 liq- uid 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 re- treat 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 de- termining 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 per- cent to be 76–79% and 6–11%, respectively (Cordier et al., 2013b). Their model based on Regular Solution Theory predicts negligi- ble amounts (∼0.4–0.6%) of dissolved nitrogen in the mixture. http://dx.doi.org/10.1016/j.epsl.2014.11.023 0012-821X/ 2014 Elsevier B.V. All rights reserved.