Topographic and orbital forcing of Titan's hydroclimate

The cause of the hemispheric asymmetry of Titan's methane lakes and seas is the subject of ongoing debate. A leading hypothesis posits that seasonal insolation asymmetries caused by Saturn's eccentric orbit lead to differences in net precipitation over the two poles, perhaps mediated by asymmetric atmospheric transport of moisture. But topographic variations have also been proposed to contribute, albeit without considering the importance of surface hydrology. Here we present general circulation model simulations including a synchronously coupled surface and ground hydrology scheme, testing the separate and combined influences of topography and orbital forcing on Titan's hydroclimate. We find that, while topography leads to warmer polar regions relative to a flat surface which in turn enhance methane loss to the atmosphere, the overall effect on the global distribution of surface methane liquid is minor. In particular, topography does not force any notable asymmetry in the meridional circulation, nor does it affect the seasonality of the methane cycle, though it does increase the regional heterogeneity of average precipitation at mid-latitudes. We also find that Titan's atmospheric methane transport robustly responds to orbital forcing, in agreement with previous results, but this is insufficient to overcome the distribution of surface liquids dictated by surface hydrology. We conclude that Croll-Milankovitch cycles are plausible on Titan, but potentially not the dominant driver of the current distribution of liquids; relatedly, our results suggest that the volume of the large seas and lakes has not varied substantially on millennial timescales.

Automated summary

The cause of the hemispheric asymmetry of Titan's methane lakes and seas is still under debate, with seasonal insolation asymmetries and topographic variations both proposed as contributing factors. However, simulations suggest that topography has a minor effect on the global distribution of methane liquid, while orbital forcing and surface hydrology are more likely to dictate the distribution.