Moisture transport and the methane cycle of Titan’s lower atmosphere

Titan’s surface and lower atmosphere support a hydrologic cycle that influences various aspects of the icy moon’s appearance and evolution. Here, we review the state of knowledge around this methane cycle, focusing on its relationship to the circulation of the troposphere and to the distribution of surface liquids. Titan’s meridional circulation consists mainly of Hadley cells, with an intertropical convergence zone—in which clouds and precipitation are promoted—that oscillates with latitude seasonally. There are separate regions at the poles wherein precipitation occurs in summertime. Overall, the character of precipitation depends on the amount of liquid available to evaporate on the surface, and a realistic liquid distribution (that is, with liquids limited to polar regions) leads to highly sporadic seasonal precipitation. This also produces a latitudinal profile of near-surface humidity wherein the poles are more humid than the lower latitudes. The lower latitude humidity reflects the horizontal transport by the Hadley circulation, but is cut off from the high near-surface humidity at the poles. Polar moist convection humidifies the mid-levels and from there the low latitudes, and equatorward, downgradient transport of moisture is accomplished by traveling storm systems in the high mid-latitudes. These waves in some cases interact with the convection to communicate the effects of latent heating nearly globally. Separately, surface and subsurface hydrology are important processes that lead to the observed distribution of liquids in polar basins, and furthermore indicate the influence of a subsurface methane table interacting with the climate system. Precipitation at lower latitudes largely runs off or infiltrates into the surface; runoff at higher latitudes feeds some of the low-lying polar basins; and subsurface methane flow regulates the distribution of near-surface methane such that the seas are surface exposures of, and other polar areas sustain evaporation from, a shallow methane table. Finally, we discuss the possible long-term evolution of surface liquids, including the influence of Croll-Milankovitch cycles and their effect on atmospheric moisture transport by eddies; whether or not Titan’s surface features indicate past cycling of polar liquids, slower secular trends, or something else entirely remains unresolved.