Titan’s liquids: Exotic behavior and its implications on global fluid circulation

Abstract

Based on a validated model for cryogenic chemical systems, referred to as CRYOCHEM (“Cryogenic Chemistry Model”), surface liquids on Titan are shown to exhibit exotic behavior of density increase with temperature but decrease with pressure, unless the temperature falls below 89.8K. It is also the case for the atmospheric liquid condensates below an altitude where the liquid density is minimum. The exotic behavior is of compositional origin, which does not have an analog in the atmosphere and liquid water on Earth. As the latitudinal and seasonal variations of surface temperature are known, it is possible to map out the global liquid and vapor density variations as well as the equilibrium phase compositions, which will be useful as inputs for atmospheric general circulation models (GCMs) and investigations of Titan’s methane-equivalent of Earth’s hydrological cycle, local subsurface alkanology (equivalent to hydrology on Earth), lake convection, and clastic and chemical sedimentation in the lakes. Further, the density variations can be used to derive a general idea about global fluid circulation in the upper crust based on averaged conditions on Titan. The surface liquid should tend to flow toward the hottest spot on Titan and a return flow occurs beneath the surface, thus providing analogies with thermohaline circulation in Earth’s oceans. The vapor phase, on the other hand, has ordinary properties that make the global atmospheric circulation similar to the Hadley cell on Earth, but Titan’s cycle reaches the polar regions. The calculated compositions of surface liquids are more methane-rich than other models indicated, thus qualitatively in the right direction to satisfy polar-lake compositions deduced from loss tangents. However, quantitatively there remains a need to find yet more accurate liquid compositions and an optimum equilibrium within constraints of the atmospheric measurements.