Thermal Inertia Controls on Titan's Surface Temperature and Planetary Boundary Layer Structure

Abstract

Understanding Titan's planetary boundary layer (PBL)-the lowest region of the atmosphere influenced by surface conditions-remains challenging due to Titan's thick atmosphere and limited observations. Previous modeling studies have produced inconsistent estimates of surface temperature variability, a critical determinant of PBL behavior, often without clear explanations grounded in surface energy balance. Here, we develop a theoretical framework and apply a three-dimensional dry general circulation model (GCM) to investigate how surface thermal inertia influences surface energy balance and temperature variability across diurnal and seasonal timescales. At diurnal timescales, lower thermal inertia surfaces experience larger temperature fluctuations and enhanced daytime sensible heat fluxes due to less efficient subsurface heat conduction. In contrast, at seasonal timescales, surface temperature variations show weak sensitivity to thermal inertia, as atmospheric damping tends to dominate over subsurface conduction. The PBL depth ranges from a few hundred meters to 1,000m on diurnal timescales, while seasonal maxima reach 2,000m, supporting the interpretation from a previous study that the Huygens probe captured the two PBL structures. Simulated seasonal winds at the Huygens landing site successfully reproduce key observed features, including near-surface retrograde winds and meridional wind reversals within the lowest few kilometers, consistent with Titan's cross-equatorial Hadley circulation. Simulations for the planned Dragonfly landing site predict shallower thermal PBLs with smaller fluctuation amplitudes, while maintaining similar wind patterns. This work establishes a physically grounded framework for understanding Titan's surface temperature and boundary layer variability, and offers a unified explanation of Titan's PBL behavior that provides improved guidance for future missions.