The New Titan Planetary Climate Model. II. Titan’s Haze and Cloud Cycles

Observations of Titan have long revealed the presence of seasonal cycles, including haze, clouds, and gases. The lack of information on the different processes that govern these cycles prevents us from understanding all the phenomena taking place in Titan’s atmosphere. In this work, we survey Titan’s cycles exhaustively to understand their mechanisms, in particular the haze and cloud cycles that govern a large part of Titan’s climate and its surface. The Titan Planetary Climate Model (Titan PCM)—an upgraded version of the IPSL Titan Global Climate Model—incorporates a microphysical model for haze and clouds, along with a methane cycle (evaporation, transportation, condensation, precipitation). The model also features improved radiative transfer and updated spectroscopic data on gases. The Titan PCM includes phenomena hitherto omitted from Titan 3D simulations, such as cloud nucleation and condensation. Furthermore, we have coupled microphysics with radiative transfer in order to take account of feedback from haze and clouds on the thermal structure. We find that the haze follows a seasonal cycle constrained by the pole-to-pole circulation in the stratosphere. Below, the cold regions between 30 and 90 km trigger the condensation of minor species, forming a condensate mist layer throughout the year. Methane clouds form near the surface (5-25 km), in the ascending branches of the circulation, while polar clouds form higher up (60-300 km) at the winter pole after enrichment in trace compounds. The Titan PCM now enables detailed exploration of the long-term variations in Titan’s cycles, providing insights for future observations.