On Titan, methane (CH4) and ethane (C2H6) are the dominant species found in the polar lakes and seas. In this study, we have combined molecular dynamics simulations with laboratory work to both create a binary phase diagram at cryogenic temperatures and gain a better understanding of how the system interacts at a molecular level.
The molecular dynamics (MD) simulations reveal that the methane-ethane system deviates from ideality as the mixing ratio approaches the eutectic point, indicating that the methane-ethane interactions are stronger than the self-interactions of either molecule at the eutectic. Identifying the deviation from ideality was accomplished through comparing the MD simulations to experimental data, focusing on excess volume, density, and temperature vs. mixing ratio of the liquidus line and eutectic point.
From the laboratory work, we have found that Raman spectroscopy is a reliable means of detecting the liquidus, solidus, and solvus lines, allowing for full characterization of the phase diagram. The liquidus is defined as the first point in which ice forms on cooling or when the last ice disappears on warming. The solidus marks the last point in which liquid occurs on cooling the sample. Lastly, the solvus differentiates between a fully homogeneous system and a structure where one species dominates the crystal structure, with the other as only a minority contaminant.
The temperatures and mixing ratios at which the solidus and solvus lines occur are mostly below 90 K. This means they likely have a larger impact on processes taking place at deeper parts of the lakes and seas as well as for portions of the atmosphere, as opposed to on the surface. In general, this diagram is not only useful in terms of understanding the methane-ethane system itself, but also as a foundation for creating more complex systems that better exemplify the compositions of Titan’s various lakes and seas.