Abstract
Abstract On Titan, methane (CH4) and ethane (C2H6) are the dominant species found in the lakes and seas. In this study, we have combined laboratory work and modeling to refine the methane–ethane binary phase diagram at low temperatures and probe how the molecules interact at these conditions. We used visual inspection for the liquidus and Raman spectroscopy for the solidus. Through these methods, we determined a eutectic point of 71.15 ± 0.5 K at a composition of 0.644 ± 0.018 methane–0.356 ± 0.018 ethane mole fraction from the liquidus data. Using the solidus data, we found a eutectic isotherm temperature of 72.2 K with a standard deviation of 0.4 K. In addition to mapping the binary system, we looked at the solid–solid transitions of pure ethane and found that, when cooling, the transition of solid I–III occurred at 89.45 ± 0.2 K. The warming sequence showed transitions of solid III–II occurring at 89.85 ± 0.2 K and solid II–I at 89.65 ± 0.2 K. Ideal predictions were compared with molecular dynamics simulations to reveal that the methane–ethane system behaves almost ideally, and the largest deviations occur as the mixing ratio approaches the eutectic composition.
Highlights
Titan, the largest satellite of Saturn, is host to a diverse range of surface and atmospheric processes akin to those found on Earth
The conditions found on Titan are near its triple point of 90.67 K and 0.117 bar (Fray and Schmitt, 2009), as Titan’s surface temperature ranges between 89 and 94 K (Jennings et al, 2019), and has a surface pressure of ~1.5 bar (Lindal et al, 1983; Fulchignoni et al, 2005)
The lakes themselves are predominantly composed of methane (CH4) and ethane (C2H6), with ethane being introduced via atmospheric methane photochemistry (Hörst, 2017; Magee et al, 2009)
Summary
The largest satellite of Saturn, is host to a diverse range of surface and atmospheric processes akin to those found on Earth. The conditions found on Titan are near its triple point of 90.67 K and 0.117 bar (Fray and Schmitt, 2009), as Titan’s surface temperature ranges between 89 and 94 K (Jennings et al, 2019), and has a surface pressure of ~1.5 bar (Lindal et al, 1983; Fulchignoni et al, 2005) These conditions allow for methane to function to water on Earth (Hörst, 2017). Due to the significance of methane and ethane on Titan, we mapped the binary phase diagram at low temperatures using the Astrophysical Materials Laboratory at Northern Arizona University (Grundy et al, 2011; Tegler et al, 2010, 2019) This included identifying the eutectic point and liquidus and solidus curves using visual inspection and Raman spectroscopy. The comparisons show the methane-ethane system to be denser than ideal predictions nearing the eutectic point and in comparison to a pure methane environment
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