Titan's sand seas : effects of seasonality and surface heterogeneity on dune-forming winds

Abstract

Titan is one of Saturn’s moons and the second largest moon in the solar system; 1.5 times larger than Earth’s moon and 1.8 times as massive. It is the only body in space with clear evidence of stable surface liquid. Titan’s thick atmosphere is primarily composed of nitrogen with ∼ 5% methane and 1% hydrogen, and has a surface pressure of 1.5 bar. Titan has a hydrological cycle that is similar Earth, albeit based on liquid hydrocarbons. This, combined with Titan’s complex photochemistry, allows for the deposition of organic compounds at the surface. These particles are the basis for Titan’s equatorial dune system, in part due to relatively dry conditions as a result of Titan’s general circulation, and because of the dense atmosphere and low gravity. Titan’s dunes contain organic compounds that may be conducive to early life and may be used to understand the moon’s complex methane-based hydrological cycle. Equatorial dune formation on Titan is currently poorly understood and few general circulation models (GCM) have successfully explained the conditions required to match observations. Here, I use the ROCKE-3D GCM to explore Tokano’s (2010) hypothesis that dune formation on Titan is a result of westerly equatorial equinoctial gusts. Using a combination of wind statistics, including drift potential and resultant drift direction, I show the influence of surface roughness and topography on the occurrence of dune forming winds. This study expands upon previous models in the application of topography and surface roughness heterogeneity using backscatter model approximations as a function of sand sea region and interdune fraction. Four model sensitivity tests are run with the inclusion or exclusion of sand sea surface roughness variability, topography or a combination of both. These are compared to a flat and smooth control simulation. I find the influence of topography much greater than that of variable surface roughness heterogeneity. The drift potential and resultant drift direction are highly localized and in some cases reverse the dune orientation to match Titan surface observations. The signal of seasonality is dependent on latitude and shifts the period at which the drift potential reaches its maximum and the duration of the seasonal drift potential plateau. The results of this study may be used to advise future missions in locations of interest given an enhanced understanding of the general circulation and the dominant factors controlling equatorial surface winds and as an analogue to early Earth and origins of life studies.