  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.
