The Titan Middle Atmosphere Intercomparison Project

The atmosphere of Titan, the largest moon of Saturn, is unique in the Solar System.  Like Earth, its atmosphere is mostly composed of molecular nitrogen, though unlike Earth, there is no molecular oxygen and instead the second most abundant molecule is methane.  The interactions between the products of the photodissociation of these two dominant molecules give rise to a complex suite of hydrocarbons (molecules of the form C_xH_y) and nitriles (C_xH_yN_z) [1].  Continued reactions (through their collision and agglomeration) between these species ultimately lead to Titan’s characteristic orange haze, which shields Titan’s surface from most shortwave sunlight.  Akin to the absorption of ultraviolet light by Earth’s stratospheric ozone, shortwave radiative heating by Titan’s haze and methane leads to the formation of Titan’s stratopause, the local thermal maximum that separates the stratosphere (about 40 to 250 km above the surface) and mesosphere (about 250 to 600 km above the surface).

Across all latitudes, the zonal winds in Titan’s middle atmosphere are westerly, exclusively blowing from the west towards the east, and have been inferred to reach speeds up to ~280 m/s [2,3,4].  This is in stark contrast to the zonal winds of Earth’s stratosphere, which include both westerly and easterly blowing winds.  The maintenance of Titan’s stratospheric superrotation is thought to be by the Gierasch-Rossow-Williams mechanisms [5,6]: Zonal angular momentum is delivered to the stratosphere from ascending motion from the surface and then transported to the high latitude by the meridional circulation; this transport is then balanced by the transport of zonal momentum equatorward by atmospheric eddies, likely made up of Rossby-Kelvin waves [7,8,9].

Trace molecules (e.g., C₂H₆, C₂H₄, C₂H₂, HCN) in Titan’s stratosphere are enriched above the winter pole, in some cases by several orders of magnitude [10,11].  The enrichment is generally thought to be driven by the descending branch of Titan’s meridional overturning circulation delivering molecules from their high-altitude source region into the lower stratosphere.  Once delivered to the high-latitude stratosphere, the molecules are thought to be trapped by the strong stratospheric jet.  Some molecules (e.g., C₂H₆) exhibit ‘tongues’ extending away from the high latitude, suggestive of mixing processes transporting high latitude air into the mid latitudes [12].  This, however, has yet to be confirmed.

In this presentation, we directly compare three Titan general circulation models (GCM) to determine the characteristics of Titan’s middle atmosphere that are robustly present across different model assumptions and parameterizations.  Included in this intercomparison are the Titan Atmospheric Model (TAM, [13]), Titan Planetary Climate Model (Titan PCM, [14]), and TitanWRF [9].  This intercomparison of fully three-dimensional GCMs aims to provide the first multi-model resource to explain the observed seasonal-scale changes in Titan’s middle atmospheric thermal, dynamical, and compositional structure.

References

[1] Vuitton et al., Icarus, 2019

[2] Sharkey et al., Icarus, 2021

[3] Achterberg, PSJ, 2023

[4] Vinatier et al., A&A, 2020

[5] Gierash, JAS, 1975

[6] Rossow & Williams, JAS, 1979

[7] Lombardo & Lora, JGR: Planets, 2023

[8] Lewis et al., PSJ, 2023

[9] Lian et al., Icarus, 2025

[10] Teanby et al., GRL, 2019

[11] Mathe et al., Icarus, 2020

[12] Shultis et al., PSJ, 2022

[13] Lombardo & Lora, Icarus, 2023

[14] de Batz de Trenquelléon, et al., PSJ, 2025