Titan’s atmosphere and surface liquid: New calculation using Statistical Associating Fluid Theory

Abstract The application of PC-SAFT equation of state (EOS) in analyzing the in situ measurement of atmospheric data by Huygens probe reveals new insights into Titan’s atmosphere and surface liquids. The EOS offers the most reliable and accurate calculations in fluid phase equilibria at the cryogenic conditions encountered in Titan and other extra-terrestrial bodies. This paper and a succeeding one pertaining to solid phases are foundational introductions to a new thermodynamics tool (new to planetary science) and will open the way for many diverse planetological applications, but here we limit applications to Titan. Titan’s lower stratosphere and lower troposphere are modeled as a well-mixed chemical solution with fixed overall composition of nine components. Using this model in the lower stratosphere, the dew point, below which condensation occurs, is calculated to be at an altitude of 65.3 km ( T  = 91.3 K, P  = 0.031 bar). The first drop of liquid at this point is almost pure propane, which would form a haze (not a dense cloud) due to the minor abundance of this species. Using this model in the lower troposphere, the atmospheric methane mole fractions measured by Huygens probe is well predicted up to an altitude of 29 km, thus validates the model and the EOS. The surface liquid, which is assumed to be in thermodynamic equilibrium with the ground-level atmosphere, is dominated by C 2 H 6 , CH 4 , C 3 H 8 , and N 2 with mole percents of 53%, 32%, 7%, and 7%, respectively, at a density of 614 kg/m 3 in the equator. The effects of the temperature on the surface liquid composition are also discussed. Despite the small surface temperature difference between equatorial and polar regions (3.7 K), the composition of liquid in polar regions is very different from that in the equator: 68% CH 4 , 22% N 2 , and 8% C 2 H 6 with the amount of liquid nine times larger than that in the equator at a 10%-smaller density of 551 kg/m 3 . The system is accurately estimated using the binary of CH 4 and N 2 only at an altitude higher than 8 km up to 29 km. In this case the binary phase diagram can be applied to estimate that the methane-rich solid phase appears at an altitude of 21 km and higher.