Brown, R. H. et al. The identification of liquid ethane in Titan's Ontario Lacus. Nature 454, 607-610

Department of Planetary Sciences, University of Arizona, Tucson, Arizona 85721, USA.
Nature (Impact Factor: 41.46). 08/2008; 454(7204):607-10. DOI: 10.1038/nature07100
Source: PubMed


Titan was once thought to have global oceans of light hydrocarbons on its surface, but after 40 close flybys of Titan by the Cassini spacecraft, it has become clear that no such oceans exist. There are, however, features similar to terrestrial lakes and seas, and widespread evidence for fluvial erosion, presumably driven by precipitation of liquid methane from Titan's dense, nitrogen-dominated atmosphere. Here we report infrared spectroscopic data, obtained by the Visual and Infrared Mapping Spectrometer (VIMS) on board the Cassini spacecraft, that strongly indicate that ethane, probably in liquid solution with methane, nitrogen and other low-molecular-mass hydrocarbons, is contained within Titan's Ontario Lacus.

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    • "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. "
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    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.
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    • "A specular reflection observed in VIMS data also indicates surface liquids (Stephan et al., 2010; Soderblom et al., 2012). Although ethane has been detected as a component of the liquid (Brown et al., 2008), the composition remains largely uncertain (Cordier et al., 2012). The most recent radar analyses indicate that the lakes have a very smooth surface (Zebker et al., 2014) and are remarkably transparent (Mastrogiuseppe et al., 2014), suggesting that they are mostly composed of methane. "
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    • "In addition to its thick atmosphere, Titan's substantial reservoirs of liquids on its surface make it attractive from an astrobiological perspective (Stofan et al., 2007; Hayes et al, 2011). These ponds are mostly made of liquid ethane (Brown et al., 2008) and they feed a weather cycle with evaporation of surface liquids, condensation into clouds, and precipitation (Griffith et al., 2000). Ultimately, the moon's non-synchronous rotation period with respect to Saturn as well as its substantial orbital eccentricity of 0.0288 (Sohl et al., 1995; Tobie et al., 2005) point towards the presence of an internal ocean, the composition and depth of which is unknown (Lorenz et al., 2008; Norman, 2011). "
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