Phase Diagram for the Methane-Ethane System and its Implications for Titan's Lakes

TL;DR

This study refines the methane-ethane phase diagram at low temperatures relevant to Titan's lakes, combining laboratory experiments and modeling to understand molecular interactions and solid-solid transitions.

Contribution

It provides the first detailed low-temperature phase diagram for methane-ethane, including eutectic points and solid-solid transition data, with implications for Titan's lake composition.

Findings

Eutectic point at 71.15 K with 0.644 methane mole fraction.

Solidus temperature at 72.2 K for the eutectic composition.

Solid-solid transition of ethane occurs near 89.45 K.

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$\pm$0.5 K at a composition of 0.644$\pm$0.018 methane - 0.356$\pm$0.018 ethane mole fraction from the liquidus data. Using the solidus data, we found a eutectic isotherm temperature of 72.2 K with a standard deviation of 0.4 K. In addition to mapping the binary system, we looked at the solid-solid transitions of pure ethane and found that, when cooling, the transition of solid I-III occurred at 89.45$\pm$0.2 K. The warming sequence showed transitions of solid III-II occurring at 89.85$\pm$0.2 K and solid II-I at 89.65$\pm$0.2 K. Ideal predictions were compared to 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.