Mapping changes in the methane-ethane system when adding nitrogen at titan surface conditions

Introduction: From 2004 to 2017, the Cassini spacecraft explored the Saturn system and provided an in depth look at its largest moon, Titan. During the mission, the RADAR (RAdio Detection And Ranging) instrument detected lakes and seas on Titan’s polar regions. [1-2]. Since Titan’s surface temperatures range from 89-95 K and the surface pressure is ~1.47 bar [3], water is not the liquid in the lakes and seas, but rather it is a combination of methane (CH4), ethane (C2H6), and dissolved nitrogen (N2) [1]. In particular, recent analysis of RADAR data estimates mixing ratios of 69% methane, 15% ethane, and 16% nitrogen in the north polar lakes and seas [4]. Recent work in the Astrophysical Materials Laboratory mapped the methane-ethane phase diagram at low temperatures and pressures using Raman spectroscopy [5]. The work completed on the binary system shows the eutectic point to be at 71.15±0.65 K and at a composition of around 64.4±1.8% methane and 35.6±1.8% ethane, which is an approximate temperature depression of 19 K compared to the freezing points of the pure species. We also found that supercooling occurs at the liquidus on the ethane-rich side and a supercooling-like effect takes place at the solidus on the methane-rich side. It is suspected that both effects are due to the ethane solid I-III transition (skipping solid II at low pressures when cooling) and could have implications for the lakes and seas. Specifically, [6] report that the solid I-III transition is quite exothermic, which could result in heat being released into the surrounding environment when the transition occurs. With the completion of the methane-ethane work, we now aim to identify the temperature at which ice first appears when nitrogen is added to the system at 1.45 bar as compared to that of the binary system. A previous study by [7] examined the dissolution of nitrogen into methane-ethane mixtures and demonstrated that nitrogen has a higher tendency of dissolving into the hydrocarbon mixture when it is introduced into methane-rich mixtures at lower temperatures and higher pressures. The work conducted in the Astro Mat Lab builds on this by focusing on changes in the phase transitions caused by the introduction of nitrogen. Along with the experiments by [7], [8] also shows strong temperature and composition dependencies on the dissolution of nitrogen into methane-ethane mixtures. This significantly affects the liquids’ densities and since estimates show the northern lakes to be methane-rich, it could lead to lake turnover and/or stratification. We also see an added complexity to the ternary system with the presence of two liquids at certain pressures and temperatures. Modeling completed by [9] and lab work by [10] show that this two-liquid system is composed of an ethane-rich top layer and a methaneand nitrogen-rich bottom layer. Mostly, the two-liquid system appears to favor temperatures and pressures that could be found at the bottom of the seas, but this may still have repercussions for the surface since it could encourage nitrogen bubble formation and further promote lake turnover and/or stratification. This is all to say that the methane-ethane-nitrogen system found on Titan is complex and lends itself to additional investigation beyond what has already been conducted in the past. The ongoing experiments in the Astro Mat Lab look at the ternary system from a different angle, namely in identifying how nitrogen affects the binary system, as opposed to looking at all three in the scope of a ternary system. We use the [7] dissolution rates as a guide for introducing nitrogen into the methane-ethane system, with the intent of providing clarification on potential processes occurring in the lakes at Titan surface conditions.