Cloud formation and composition on Titan with a Planetary Climate Model

1. Introduction

The thick atmosphere of Titan is home to complex, tightly coupled dynamics, photochemistry and microphysics. Methane is abundent, and the conditions of pressure and temperature allow for the development of a methane cycle similar to that of water on Earth, with convective methane clouds forming in the troposphere [1]. In addition to the methane cycle, stratiform hydrocarbon clouds at the poles, or HCN clouds at high altitude have also been observed [2]. Knowledge of Titan’s clouds mostly comes from the data collected during the Cassini-Huygens mission (2014-2017). In this context, climate models are invaluable tools for understanding the mechanisms at work and deepening our interpretation of these observations. The characteristics and conditions of formation of Titan’s clouds depends on many parameters and mechanisms, among which the atmospheric conditions, but also the properties of the photochemical aerosols that serve as privileged cloud condensation nuclei. The aim of this presentation is to assess the impact of these different factors on cloud formation and composition, and their repercussions on Titan's climate.

2. Methods

The Titan Planetary Climate Model (Titan PCM [3]), first developed at the Pierre-Simon Laplace Institute (IPSL), is a 3D model that can simulate Titan’s Climate at a global scale. By integrating couplings between dynamics, radiative transfer, chemistry and microphysics, it enables to study the interaction between these various processes and their influence on each other. In particular, it now includes a microphysical model in moments for haze and clouds [4]. Nevertheless, discrepancies with observations persist, and in its current state, the model only takes into account the condensation of a limited number of species (CH4, C2H2, C2H6, HCN) in the form of ice only, and independently of each other. Certain parameters, such as wettability or density of the aerosols, also remain poorly constrained. We want to further develop the microphysical model in order to improve the description of clouds on Titan and better understand the mechanisms of cloud formation and the methane cycle.

3. Results and objectives

Previous studies have hypothesized the existence of different cloud layers in the troposphere, with solid mehane cloud forming around 25 km of altitude, and liquid methane-nitrogen clouds closer to the surface, around 10 km [5,6]. While the model reproduces cloud formation at these two altitudes at a latitude and period coherent with observations, it does not consistently discriminate two distinct layers between 10 and 30 km (see Fig. 1). Therefore, forthcoming studies will focus on the implementation of the various phases of the droplets (solid and liquid), which would enable the impact of mixtures and interactions between species to be taken into account at a later stage. First, we will investigate the formation of liquid-phase clouds in the troposphere. In particular, the role of ethane and nitrogen in the condensation and stabilization of liquid methane will be studied. Model results will be compared with observations. These developments will first be tested in a 1D study before being incorporated into the 3D model.

Fig. 1 : Comparison between the modeled cloud extinction profile at 0.7 μm (in red) and the modeled temperature profile (in black dashed line) at low latitudes (30°N) during the middle of northern summer (Ls=135°). The extinction peak between 30-80 km corresponds to a mist layer of minor species condensates, while the extinction below is associated with methane clouds. The altitude scale on the right axis is approximate.

 

References

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[2] West, R. A., Del Genio, A. D., Barbara, J. M., Toledo, D., Lavvas, P., Rannou, P., . . . Perry, J. (2016). « Cassini Imaging Science Subsystem observations of Titan’s south polar cloud ». Icarus, 270 , 399-408. doi:10.1016/j.icarus.2014.11.038

[3] De Batz De Trenquelléon, Bruno, Lucie Rosset, Jan Vatant d’Ollone, Sébastien Lebonnois, Pascal Rannou, Jérémie Burgalat, et Sandrine Vinatier. « The New Titan Planetary Climate Model. I. Seasonal Variations of the Thermal Structure and Circulation in the Stratosphere ». The Planetary Science Journal 6, n^o 4 (1 avril 2025): 78. https://doi.org/10.3847/PSJ/adbbe7.

[4] De Batz De Trenquelléon, Bruno, Pascal Rannou, Jérémie Burgalat, Sébastien Lebonnois, et Jan Vatant d’Ollone. « The New Titan Planetary Climate Model. II. Titan’s Haze and Cloud Cycles ». The Planetary Science Journal 6, n^o 4 (1 avril 2025): 79. https://doi.org/10.3847/PSJ/adbb6c.

[5] Wang, Chia C., Sushil K. Atreya, et Ruth Signorell. « Evidence for Layered Methane Clouds in Titan’s Troposphere ». Icarus 206, n^o 2 (avril 2010): 787‑90. https://doi.org/10.1016/j.icarus.2009.11.022.

[6] Curtis, Daniel B., Courtney D. Hatch, Christa A. Hasenkopf, Owen B. Toon, Margaret A. Tolbert, Christopher P. McKay, et Bishun N. Khare. « Laboratory Studies of Methane and Ethane Adsorption and Nucleation onto Organic Particles: Application to Titan’s Clouds ». Icarus 195, n^o 2 (juin 2008): 792‑801. https://doi.org/10.1016/j.icarus.2008.02.003.