Mapping the change in temperature of the methane-ethane freezing points with the addition of nitrogen at Titan conditions

Titan is unique among the icy satellites in that it has a thick, opaque atmosphere and stable bodies of liquid on its surface. These two systems interact with one another via precipitation, dissolution, and evaporation. This is akin to what we see on Earth, but methane (CH₄), ethane (C₂H₆), and nitrogen (N₂) are the dominant species rather than water. The presence of three primary species, as opposed to one, creates a more complex system and may give rise to unique geochemical features within the lakes. To better understand the potential processes occurring in and around Titan’s lakes, we are mapping how the addition of N₂ to the CH₄-C₂H₆ system at constant vapor pressure affects the temperature at which ice first appears. This study is built on recent work completed in the Northern Arizona University Astrophysical Materials Lab, which focused on mapping the CH₄-C₂H₆ phase diagram at low temperatures and pressures using Raman spectroscopy [1]. We first condense a liquid CH₄-C₂H₆ hydrocarbon (HC) mixture into the cell and then introduce N₂ vapor to the sample to maintain a constant vapor pressure. To best capture effects at Titan conditions, we are running experiments at both 1.38 and 1.50 bar. Due to dissolution rates, more N₂ must be added to the sample with decreasing temperature, or in experiments with more CH₄, to maintain constant vapor pressure. While this does change the liquid composition of the sample as the experiment progresses, the total HC ratio remains the same. We use visual inspection and Raman spectroscopy to collect data and compare our results to the CH₄-C₂H₆ system using a pseudo binary phase diagram and also to a model created using CRYOCHEM 2.0. Thus far, we have found that phase changes can be sensitive to pressure, even within our range of 1.38 – 1.50 bar. We also find that at both pressures, C₂H₆-rich mixtures up to ~0.2 methane HC ratio exhibit a first ice temperature depression that follows the trend of the binary liquidus. The first ice temperature then transitions to a flat line of 81.5±0.5 K at 1.38 bar. While we are still conducting experiments at 1.50 bar, the CRYOCHEM 2.0 model indicates we should see a flat line form at ~82.5 K. [1] Engle A.E. et al. (2021) PSJ, 2, 118.