CRYOCHEM modeling of Titan’s liquid: the effects of hydrogen cyanide (HCN)

Abstract:

It is widely known that Titan is the only body in the Solar System, other than Earth, that has an abundant liquid phase on its surface. Its liquid composition has been derived from thermodynamic models that assume equilibrium between liquid and the atmosphere. Efforts to obtain composition data of Titan’s lakes have been made, in particular using bathymetry and microwave absorption analysis of Cassini fly-by data, the initial semi-quantitative findings of which include the dominant fraction of methane in liquids of northern lakes (Mastrogiuseppe et al., GRL 2013, 41, 1432). These efforts can constrain the composition of each component in the liquids of both northern lakes and Ontario Lacus in the southern hemisphere.
A molecular-based thermodynamic model for cryogenic chemical systems, referred to as CRYOCHEM, was shown to reproduce the vertical composition profile of Titan’s atmospheric methane measured by the Huygens probe (Tan et al., Icarus 2013, 222, 53). It was also used to calculate the liquid composition in equilibrium with the atmosphere. The results revealed exotic behavior of liquid density with respect to changes of temperature and pressure (Tan et al., Icarus 2015, 250, 64). Within a temperature range of 3.7 K between equatorial and Polar regions, the liquid composition changes from ethane-rich in the equator to methane-rich at polar latitudes, thus consistent with the bathymetry and microwave absorption analysis. This consistency will have to be tested quantitatively when the analysis is completed and gives us tighter compositional constraints.
CRYOCHEM is currently enhanced by including HCN, the only nitrile that has an amount comparable to the heavy hydrocarbons already accounted for in the model. The reason for initially omitting HCN was the inability of the old version of CRYOCHEM to deal with electrically polar molecules such as HCN, which has a strong dipole moment. Its present inclusion brings the model fluids closer to the actual condition on Titan and enhances the versatility of CRYOCHEM to simulate Titan geochemistry.
We will discuss the effects of the inclusion of HCN on the calculated composition of Titan’s surface liquids and the calculation accuracy to anticipate the crucial comparison with the compositional constraints resulting from bathymetry and microwave absorption analysis.