Diurnal and Seasonal Variations of Titan’s Surface Temperature and Planetary Boundary Layer Structure Simulated with Dry and Moist GCMs

Understanding Titan’s Planetary Boundary Layer (PBL)—the lowest part of the atmosphere directly influenced by surface conditions—remains challenging due to Titan’s dense atmosphere and limited direct observations. Previous modeling efforts have produced a wide range of estimates for surface temperature and PBL behavior, particularly regarding diurnal variations, but have not clearly examined how subsurface parameters—specifically thermal conductivity, volumetric heat capacity, and their combined effect as thermal inertia—influence these outcomes. In this study, we revisit this issue using the Titan Atmospheric Model (TAM), a General Circulation Model (GCM) specifically developed for Titan (Lora et al. 2015; Lora 2024). Alongside, we present a theoretical framework that links the surface temperature variations to surface energy balance, providing a physically grounded interpretation of the simulation results.

First, we present simulation results under a dry setup to allow direct comparison with previous studies. Our theoretical framework explains the weak sensitivity of seasonal surface temperature structure to local thermal inertia, as reported by MacKenzie et al. (2019), who applied a global thermal inertia map to a GCM. This insensitivity arises from the minimal role of subsurface heat conduction in modulating surface temperature over Titan’s long annual cycle, where atmospheric damping is the dominant control. In contrast, at diurnal timescales, subsurface heat conduction plays a more significant role than atmospheric damping. As a result, diurnal surface temperature variations become sensitive to local thermal inertia, reaching magnitudes on the order of  O(10^-1) [K] near the equator during midsummer. Specifically, high thermal inertia surfaces (Hummocky terrain: 1,962 [TIU]; TIU is Thermal Inertia Unit [J•m^-2•s^-0.5•K^-1]) exhibit variations of approximately 0.1 [K], while low thermal inertia surfaces (Dunes: 246 [TIU]) show variations of up to 0.4 [K]. These thermal differences influence PBL structures as well: larger temperature amplitudes lead to higher daytime maximum surface temperatures, which in turn result in deeper adiabatic layers, increasing the PBL depth by several hundred meters to over 1,000 meters. These findings support the interpretation of the Huygens data as capturing the diurnal evolution of the PBL during the local morning (Charnay and Lebonnois, 2012).

Furthermore, we investigate the role of Titan’s methane cycle in shaping PBL structure. Our simulations show that including the methane cycle reduces the equator-to-pole temperature gradient, improving agreement with observational constraints and highlighting the importance of latent heat, consistent with previous studies (e.g., Mitchell et al., 2009; Lora and Adamkovics, 2017). The simulations also produce a nearly constant methane mole fraction up to 5 km near the equator throughout the year, corresponding to the model’s simulated lifting condensation level. This result aligns with the Huygens probe observations of a constant methane humidity layer up to 5 km (Niemann et al., 2005; 2010). These findings underscore the importance of incorporating Titan’s methane cycle for realistic simulations of surface temperature and PBL dynamics. 

References:

Charnay, B., & Lebonnois, S. (2012). Two boundary layers in Titan’s lower troposphere inferred from a climate model. Nature Geosci., 5 , 106–109.

Lora, J. M., Lunine, J. I., & Russell, J. L. (2015). GCM simulations of Titan’s middle and lower atmosphere and comparison to observations. Icarus, 250 , 516–528

Lora, J. M., & Adamkovics, M. (2017). The near-surface methane humidity on Titan. Icarus, 286 , 270–279.

Lora, J. M. (2024). Moisture transport and the methane cycle of Titan’s lower atmosphere. Icarus, 422 , 116241. 

MacKenzie, S. M., Lora, J. M., & Lorenz, R. D. (2019). A thermal inertia map of Titan. J. Geophys. Res. Planets, 124 , 1728–1742.

Mitchell, J. L., Pierrehumbert, R. T., Frierson, D. M. W. & Caballero, R. (2009). The impact of methane thermodynamics on seasonal convection and circulation in a model Titan atmosphere. Icarus, 203, 250-264.

Niemann, H. B. et al. (2005). The abundances of constituents of Titan's atmosphere from the GCMS instrument on the Huygens probe. Nature, 438, 779-784.

Niemann, H. B. et al. (2010). Composition of Titan's lower atmosphere and simple surface volatiles as measured by the Cassini-Huygens probe gas chromatograph mass spectrometer experiment. J. Geophys. Res., 115, E12006.