The drying of Titan's dunes: Titan's methane hydrology and its impact on atmospheric circulation

Abstract

We explore the effect of a finite reservoir of methane on Titan's atmospheric circulation, precipitation patterns, and surface methane content. We develop a soil model that accounts for the methane cycle in the surface‐atmosphere system, and we implement this surface model in a two‐dimensional model of the Titan's atmosphere. Seasonal oscillations in latitude of the large‐scale circulation accomplish net drying of the low‐latitude surface by diverging methane vapor from low latitudes to higher latitudes. Simulations with an initially deep methane reservoir indicate this mechanism is able to dry ∼1.75 meters of liquid methane per Titan year from the low‐latitude surface. The existence of low‐latitude desert morphologies suggests that the system has had sufficient time to completely remove the surface methane by this mechanism. We then varied the reservoir size, focusing on initial depths of 30 meters of liquid methane or less and compared the results to available observations. The climate system has an abrupt transition to a warmer state with less precipitation and nearly global surface drying near the level at which the atmosphere can store the majority of the methane reservoir as vapor or around 6.5 meters of equivalent liquid methane for our particular choice of parameters. A comparison of our model results with Huygens' observations suggests Titan's climate mimics a state in which most of the methane inventory with direct access to the atmosphere (i.e., excluding underground sources) is stored in the atmosphere.