Angular momentum exchange between the atmosphere/hydrosphere and interior of Saturn's moon Titan and its influence on Titan's rotation
Final Report Abstract
Saturn’s moon Titan possesses different geophysical fluids (atmosphere, hydrocarbon seas) on the surface. This project aimed at comprehensively and quantitatively assessing the exchange of angular momentum between these geophysical fluids and surface/interior of Titan by analogy with terrestrial counterparts. The atmospheric and oceanic angular momenta were predicted by three-dimensional circulation models of the atmosphere and seas and analysed by standard statistical tools. These model data were used to calculate the rotation variations of Titan’s surface, i.e. libration and polar motion. The atmospheric angular momentum was verified by comparison with direct and indirect observational data including the atmospheric angular momentum vector and dune orientation. Several surprising results were obtained in the project. Titan’s atmospheric angular momentum vector is slightly tilted from Titan’s spin axis and precesses westward with a period of 1 Titan day as observed by Cassini. This rotation was shown to be a result of thermal tides in the stratosphere, which were previously thought to be absent. Virtually all dunes on Titan’s surface exhibit eastward streamline patterns, which were previously interpreted as evidence of global surface westerlies. This observation, however, was in contradiction to surface wind predictions and difficult to understand since such winds would continuously decelerate the atmospheric super-rotation and accelerate Titan’s rotation. This mystery could be solved by realizing that the general circulation model predicts weak easterlies in most seasons but westerly gusts during a brief equinoctial passage of the Inter-Tropical Convergence Zone. This wind field can explain the eastward streamline pattern on dunes without violating the angular momentum constraint. This topic attracted attention of science writers before and after publication of these results. Subsequent simulations showed that large-scale topography in combination with atmospheric tides gives rise to a mountain torque, which exchanges angular momentum with the surface on diurnal time scales. From a modelling point of view it was recognized that hydrostatic general circulation models have inherent difficulties in maintaining stratospheric super-rotation in balance with an equatorial bulge of the atmosphere. A quasi-hydrostatic general circulation model is able to circumvent this problem without applying an expensive non-hydrostatic general circulation model. The oceanic angular momentum of hydrocarbon seas was predicted by an ocean circulation model with realistic bathymetry maps of the seas. Saturn’s tides cause tides, which behave as quasistanding waves in the seas and have maximum amplitudes of several decametres. Strong hydraulic tidal currents are predicted in straits between interconnected basins, but otherwise tidal currents are weaker than wind-driven currents. The oceanic angular momentum primarily varies by meridional mass redistribution of liquids, whereas tidal currents are one order of magnitude less important for the angular momentum variation. Wind-driven circulation undergoes strong seasonal variations and becomes significant in summer, when the wind freshens. The wind set-up is generally smaller than the tidal range but the sea surface current caused by wind is stronger than that caused by tides. Wind causes irregular variations in the oceanic angular momentum whose amplitude is similar to that of tidally caused variations. However, given the small total mass of Titan’s seas compared to the atmospheric mass the oceanic torque are three orders of magnitude smaller than the atmospheric torque and can thus be neglected in the calculation of libration and polar motion to a first approximation. Titan’s polar motion consists of large seasonal wobbles and small diurnal wobbles. The amplitude and shape of the seasonal and diurnal wobbles are strong functions of the unknown thickness of the ice shell above the subsurface water. A strong resonance with the Chandler wobble occurs if the ice shell has a thickness of less than 50 km. In this case the wobble may have a radius of the order of a kilometre and a strongly elliptic shape because of the triaxial shape of Titan. Deviation of Titan’s ice shell from hydrostatic equilibrium amplifies the diurnal and semi-annual libration of Titan by one order of magnitude with respect to a hydrostatic ice shell. The semi-annual and diurnal axial atmospheric torque are predicted to cause a libration with ~1 km amplitude and a few hundred metres, respectively, whereas the impact of the polar seas on the libration is negligible.
Publications
- 2010. Relevance of fast westerlies at equinox for the eastward elongation of Titan’s dunes. Aeolian Res. 2, 113-127
Tokano, T.
- 2010. Westward rotation of the atmospheric angular momentum vector of Titan by thermal tides. Planet. Space Sci., 58, 814-829
Tokano, T.
- 2011. Polar motion of Titan induced by the atmosphere. J. Geophys. Res. 116, E05002
Tokano, T., Van Hoolst, T., Karatekin, Ö.
- 2011. Precipitation climatology on Titan. Science 331, 1393
Tokano, T.
- 2012. Mountain torque and its influence on the atmospheric angular momentum on Titan. Icarus 220, 863-876
Tokano, T.
(See online at https://doi.org/10.1016/j.icarus.2012.06.027) - 2013. Are tropical cyclones possible over Titan’s polar seas? Icarus 223, 766-774
Tokano, T.
(See online at https://doi.org/10.1016/j.icarus.2013.01.023) - 2013. Wind-induced equatorial bulge in Venus and Titan general circulation models: Implication for the simulation of superrotation. Geophys. Res. Lett. 40, 4538-4543
Tokano, T.
(See online at https://doi.org/10.1002/grl.50841) - 2014. Non-uniform global methane distribution in Titan’s troposphere evidenced by Cassini radio occultations. Icarus 231, 1-12
Tokano, T.
(See online at https://doi.org/10.1016/j.icarus.2013.11.030) - 2014. Numerical simulation of tides and oceanic angular momentum of Titan’s hydrocarbon seas. Icarus 242, 188-201
Tokano, T., Lorenz, R. D., Van Hoolst, T.
(See online at https://doi.org/10.1016/j.icarus.2014.08.021) - 2015. Wind-driven circulation in Titan’s seas. J. Geophys. Res. 120
Tokano, T., Lorenz, R. D.
(See online at https://doi.org/10.1002/2014JE004751)
