Two boundary layers in Titan's lower troposphere inferred from a climate model

Saturn's moon Titan has a dense atmosphere, but its thermal structure is poorly known. Conflicting information has been gathered on the nature, extent and evolution of Titan's planetary boundary layer-the layer of the atmosphere that is influenced by the surface-from radio-occultation observations by the Voyager 1 spacecraft ¹ and the Cassini orbiter ², measurements by the Huygens probe ^3-5 and by dune-spacing analyses ⁶. Specifically, initial analyses of the Huygens data suggested a boundary layer of 300m depth with no diurnal evolution ⁴, incompatible with alternative estimates of 2-3km (refs 1,2,6). Here we use a three-dimensional general circulation model ⁷, albeit not explicitly simulating the methane cycle, to analyse the dynamics leading to the thermal profile of Titan's lowermost atmosphere. In our simulations, a convective boundary layer develops in the course of the day, rising to an altitude of 800m. In addition, a seasonal boundary of 2km depth is produced by the reversal of the Hadley cell at the equinox, with a dramatic impact on atmospheric circulation. We interpret fog that had been discovered at Titan's south pole earlier ⁸ as boundary layer clouds. We conclude that Titan's troposphere is well structured, featuring two boundary layers that control wind patterns, dune spacing and cloud formation at low altitudes.