Astronomically forced climate changes on Saturn moon Titan

• Applicant: Dr. Tetsuya Tokano
• Subject Area: Astrophysics and Astronomy ; Atmospheric Science
• Term: from 2014 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 262022543

• Final Report Year: 2021

Final Report Abstract

The influence of astronomically caused variations in insolation on the climate and landscape of Saturn’s moon Titan was investigated. Simulations with a global climate model were carried out under Saturn’s orbital parameters of various epochs within the past 45 kyr. Subsequently, a lake mass balance model in combinations with topography/bathymetry maps of individual basins was run to simulate temporal variations in the size and distribution of hydrocarbon lakes. Large eccentricities globally give rise to large seasonal variations in temperatures and condensation of condensable species in the troposphere. They cause condensation of nitrogen, Titan’s major atmospheric constituent, in the polar region in the season following aphelion. However, the seasonal nitrogen condensation is too little to cause Mars-like seasonal atmospheric pressure variations. The combination of variations in eccentricity and longitude of perihelion can change the dune orientation by up to 70 degrees since surface winds sensitively respond to orbital parameter changes. Saturn’s precession causes a steady change in the seasonal asymmetry of climate mainly due to its impact on methane condensation at high latitudes. However, simulations with and without topography yield qualitatively different methane precipitation patterns. The global topography distorts the meridional circulation pattern to such an extent that the annual precipitation peaks at the north pole under all orbital configurations, while in the absence of topography the polar precipitation would alternately peak at the north pole and south pole due to Saturn’s precession. The lake mass balance model for Ontario Lacus, the largest lake near the south pole, predicts variations in the lake size by a factor of four. However, this lake is unlikely to have entirely desiccated or become as large as the observed palaeosea. Consequently, the previous perception that the hemispheric asymmetry in the polar lake precipitation periodically reverses due to orbital forcing appears unrealistic. The modelling also showed that small isolated lakes can stably exist in enclosed tropical basins despite the arid climate, but may have occasionally desiccated in the past. Obliquity variations have a generally minor impact on Titan’s climate. As a whole, the impact of orbital forcing on Titan’s climate is more subtle than that on Earth’s or Mars’ climate because of the lack of glaciation and deglaciation under the present atmospheric mass and composition. This importantly helps maintain the present characteristics of Titan’s climate beyond the timescale of Croll-Milankovitch cycles.

Publications

• 2016. Eclipse-induced changes of Titan’s meteorology at equinox. Planet. Space Sci. 121, 94-102
  Tokano, T.
  
• 2016. Variations in Titan’s dune orientations as a result of orbital forcing. Icarus 270, 197-210
  McDonald, G., Hayes, A. G., Ewing, R. C., Lora, J. M., Newman, C. E., Tokano, T., Soto, A., Chen, G., Lucas, A.
  
• 2017. Nitrogen condensation in Titan’s atmosphere under contemporary atmospheric composition. Icarus 289, 120-133
  Tokano, T.
  
• 2019. A model intercomparison of Titan’s low-latitude climate. Icarus 333, 113-126
  Lora, J. M., Tokano, T., Vatant d’Ollone, J., Lebonnois, S., Lorenz, R. D.
  
• 2019. Modeling of seasonal lake level fluctuations of Titan’s seas and lakes. J. Geophys. Res. Planets 124, 617-635
  Tokano, T., Lorenz, R. D.
  
• 2019. Orbitally and geographically caused seasonal asymmetry in Titan’s tropospheric climate and its implication for the lake distribution. Icarus 317, 337-353
  Tokano, T.
  
• 2019. Titan surface temperatures during the Cassini mission. Astrophys. J. Lett. 877, L8
  Jennings, D. E., Tokano, T., Cottini, V., Nixon, C. A., Achterberg, R. K., Flasar, F. M., Kunde, V. G., Romani, P. N., Samuelson, R. E., Gorius, N. J. P., Guandique, E., Kaelberer, M. S., Coustenis, A.
  
• 2020. Stable existence of tropical endorheic lakes on Titan. Geophys. Res. Lett. 41, 32019GL086166
  Tokano, T.,
  
• 2021. Orbitally forced variation in the size of Ontario Lacus simulated by a lake balance model. Icarus 354, 114090
  Tokano, T.