Investigating Clouds on Titan

Based on recent modeling, dissolved nitrogen appears to significantly reduce the evaporation rate of liquid methane on the surface of Titan. We have developed a new method for calculating the evaporation rate of liquid methane saturated with nitrogen under Titan’s 1.5 bar atmosphere. The newly derived evaporation rates were compared to evaporation rates calculated for pure methane as a function of both temperature and relative humidity. We find that the pure methane assumption significantly overestimates the methane evaporation rate from surface pools on Titan. For example, at 94 K and 60% humidity, the pure methane assumption overestimates the methane evaporation rate by more than 40%.

This previous overestimation of evaporation rates likely has a profound impact on previous interpretations of Cassini VIMS observations of Titan’s surface and of clouds in Titan’s atmosphere. For example, recent observations of northern summer clouds in Titan’s troposphere have been sparser and less frequent than predicted by general circulation models (GCMS). An overestimation of methane evaporation from Titan’s lakes and seas could explain this discrepancy between observations and existing predictions. The seasonal evolution of the methane and ethane seas depend on not just the evaporation of methane saturated with nitrogen but also on the evaporative processes associated with nitrogen-methane-ethane mixtures.

We propose to model the evaporative processes of binary mixtures of nitrogen-methane and ternary mixtures of nitrogen-methane-ethane and to then incorporate these results into a general circulation model (GCM) of Titan. We will compare the GCM predictions of cloud formation, derived using the new evaporative relationships, to Cassini VIMS observations of clouds (e.g., Brown et al. 2009, Ap.J 706; Rodriguez et al. 2011, Icarus 216; Turtle et al. 2011, GRL 38).