Study of Titan's thermal structure and seasonal variations with the Titan PCM

Titan, the largest of Saturn’s moons, possesses a complex atmosphere controlled by deeply interconnected processes. Space exploration missions, like the Cassini-Huygens mission, highlighted the presence of strong seasonal effects, resulting in various phenomena whose underlying mechanisms remains poorly understood. Due to the strong coupling of the various processes, the development of a complete climate model is therefore essential to better understand these phenomena.

The Titan Planetary Climate Model (Titan PCM), formerly known as the IPSL Titan GCM, has thus been improved, and now includes an updated radiative transfer and a new microphysical model for haze and clouds.

The radiative transfer scheme now relies on a flexible k-correlated method. The photochemical solver was extended and the composition is now computed above the top of the Titan PCM (roughly 500km) up to 1300km. Updated spectroscopic data on gases were also included. Finally, a new microphysical model in moments for the haze and clouds was developed. The nucleation, condensation and precipitation of the clouds are now modeled. This microphysical model and the radiative transfer are now coupled, resulting in a full radiative coupling of both microphysics and chemistry within the Titan PCM.

Taking into account the formation of tropospheric and polar clouds allows to represent the corresponding depletion of the troposphere in aerosols. In the stratosphere, the haze distribution is lead solely by the dynamic. This results in an improved thermal structure in the troposphere and low stratosphere, despite biases above 10Pa due to the vertical limitations of the model.

The simulations obtained from this new version allow to discuss the impact of coupling on the thermal structure. Particular interest is paid to the equinoctial reversal of the circulation at high latitudes and the radiative destabilisation in the low stratosphere. Observed at the end of the winter through Cassini radio-occultations, the model allows to trace its evolution through fall, and results indicates a dynamic interplay between haze and gases that influences the thermal structure.