Effects of propane on the three-phase vapor-liquid-liquid equilibrium of nitrogen/methane/ethane at temperatures relevant to Titan
Abstract
Titan is the only extraterrestrial body in the Solar System that has liquid phase on its surface. The liquid consists of mostly methane in lakes near the north pole and becomes richer with ethane at lower latitudes. In a recent model with nine chemical species (Tan et al., Icarus 2015, 250, 64), calculations using equation of state gave the composition of Titan's liquid as a function of latitude.
Upon reasonable simplification, Titan's liquid can be modeled as a ternary mixture of nitrogen/methane/ethane in two-phase vapor-liquid equilibrium with the atmosphere. In this model, all species with abundance smaller than that of ethane are lumped together with nitrogen (Tan & Kargel, Fluid Phase Equilib. 2018, 458, 153). The observation of "magic island" in Ligeia Mare in 2013-2014 brings in a hypothesis of bubbling in the deep of the lake due to the three-phase vapor-liquid-liquid (VLL) equilibria. Using the simplified ternary model, it was shown that the VLL equilibrium does occur but not at suitable conditions to support the hypothesis. Among the three binary combinations in ternary nitrogen/methane/ethane, only nitrogen/ethane shows three-phase VLL equilibrium. The presence of ethane in nitrogen-rich environment is the main cause of the VLL equilibrium. Propane, which is the next abundant species after ethane in Titan's fluid, also shows VLL equilibrium with nitrogen. Therefore, a consequent question is, what are the effects of propane on the VLL equilibrium if propane is accounted for in the calculations? Can the presence of propane facilitate conditions for bubbling? Both ethane and propane constitute small abundance in Titan's system. However, ethane has proved to be important to provide accurate two-phase equilibrium. Propane, on the other hand, is potentially important to describe more accurate three-phase equilibrium, the investigation of which will be discussed in this work. The justification of such investigation includes the lower triple point of propane than that of ethane (85.5 K vs 90.3 K, respectively) and the lower temperature range of VLL equilibrium of binary nitrogen/propane compared to that of nitrogen/ethane (T < 126.6 K vs T < 132.4 K, respectively). For the first time, the phase behavior of four-component mixture nitrogen/methane/ethane/propane is investigated to answer the above questions.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.P23D3542T
- Keywords:
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- 5210 Planetary atmospheres;
- clouds;
- and hazes;
- PLANETARY SCIENCES: ASTROBIOLOGY;
- 6207 Comparative planetology;
- PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS;
- 6281 Titan;
- PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS;
- 5405 Atmospheres;
- PLANETARY SCIENCES: SOLID SURFACE PLANETS