Research at our Institute: Surface and Interior
1. Surface Properties
The properties of Titan’s surface were unknown until the arrival of the Cassini/Huygens mission. However, the thermal and mechanical characteristics of the surface are crucial for the dynamics and energy budget of the atmosphere. In the general circulation model (GCM) of the atmosphere several surface models (porous icy regolith, compacted ice, lakes) have been assumed and the predicted surface temperature have been compared with the available few observational data. The simulation indicated that a porous icy surface is most likely.
Furthermore, the model for the first time showed that, contrary to previous expectations, the polar surface temperature can seasonally vary by a few degrees, which would have a large impact on the pattern of the atmospheric circulation and cloud formation.
Publication:
Tokano, T. Meteorological assessment of the surface temperatures on Titan: constraints on the surface type. Icarus, 171, 222-242, 2005.
2. Variation of Titan’s Rotation Induced by the Atmosphere
The above seasonal variation in the surface temperature also causes seasonal reversal of the near-surface zonal wind direction by thermal effects. Accordingly angular momentum is exchanged seasonally between the surface and atmosphere. Considering the large moment of inertia of Titan’s atmosphere and the relative small moment of inertia of Titan’s interior, this would have a large influence on the rotation rate or length-of-day of Titan. Should the upper crust of Titan is mechanically decoupled from the core by a liquid subsurface ocean, the rotation variation would further amplify by an order of magnitude.
Furthermore, exchange of atmospheirc angular momentum with the surface can cause a polar motion of Titan, i.e. the rotation axis of Titan precesses around the geographical pole.
The model prediction has been the basis for Cassini’s geodetic measurements of possible crust movement, but also raised some controversial discussion.
Publications:
Tokano, T., F. M. Neubauer. Wind-induced seasonal angular momentum exchange at Titan’s surface and its influence on Titan’s length-of-day. Geophys. Res. Lett., 32, L24203, doi:10.1029/2005GL024456, 2005.
Karatekin, Ö., T. Van Hoolst, T. Tokano. The effect of internal gravitational coupling on Titan’s non-synchronous rotation. Geophys. Res. Lett., 35, L16202, doi:10.1029/2008GL034744, 2008.
Tokano, T., T. Van Hoolst, Ö. Karatekin. Polar motion of Titan forced by the atmosphere. J. Geophys. Res., 116, E05002, doi:10.1029/2010JE003758, 2011.
3. Formation of Dunes
A large portion of Titan’s low latitudes is covered by numerous linear dunes that are aligned parallel to the equator. The streamline pattern is everywhere eastward, so this was interpreted as evidence of westerly surface winds. However, there is controversy as to which type of winds may have caused the dunes.
Simulation with the GCM indicates that dunes on Titan may be reversing dunes caused by seasonally reversing north-south winds. The eastward streamline pattern can be produced by fast, turbulent westerlies that transitionally occur during the passage of the intertropical convergence zone (ITCZ) around equinoxes. In other seasons easterlies prevail whose peaks are weaker than those of westerlies. This solution would disagree the previous assumption that equatorial surface winds would be mostly westerlies, which are hard to understand in the context of meteorology and geophysics.
Publications:
Tokano, T. Dune-forming winds on Titan and the influence of topography. Icarus, 194, 243-262, 2008.
Tokano, T. Relevance of fast equatorial westerlies at equinox for the eastward elongation of Titan’s dunes. Aeolian Res., 2, 113-127, 2010.
Press release of NASA (29 July 2010): Blowing in the Wind: Cassini Helps with Dune Whodunit
4. Dynamics, Thermodynamics and Astrobiology of Seas/Lakes
Previous studies about the then hypothetical lakes on Titan were largely concerned with the chemical composition without regard to seasons. Considering the large temperature dependence of the density of liquid hydrocarbons, even a tiny temperature increase of the lake surface in summer can stabilize the thermal stratification of the lakes. In winter, cooling of the lake surface initiates convection. However, if evaporation occurs in summer, the lake surface would cool down even in summer. Model simulations show that the fate of lakes would strongly depend on the chemical composition and season.
Such differences may be important for the putative astrobiological potential since convection, stable stratification or shrinkage would affect the distribution of gases and matter in the lake.
In short timescales the dynamics of the lakes is governed by Saturn’s tide. For the first time the tides in particular and fluid dynamics in general of two lakes on Titan (Kraken Mare and Ontario Lacus) were simulated by a 3-dimensional ocean circulation model taking into account the geography and geometry of the lakes. The model predicts tides that are substantially larger than in terrestrial seas of similar size (e.g. Baltic Sea, Black Sea).
The popular article about in New Scientist, 1 December 2009 "Long-lived Titan lakes are boon to life" in New Scientist discusses these topics.
Veröffentlichungen:
Tokano, T. Thermal structure of putative hydrocarbon lakes on Titan. Adv. Space Res., 36, 286-294, 2005.
Tokano, T. Limnological structure of Titan’s hydrocarbon lakes and its astrobiological implication. Astrobiology, 9, 147-164, 2009.
Tokano, T. Simulation of tides in hydrocarbon lakes on Saturn’s moon Titan. Ocean Dyn., 60, 803-817, 2010.