Effect of subsurface on the methane cycle in Titan’s atmosphere with a Planetary Climate Model

1 Introduction
In Titan’s atmosphere and on its surface, methane plays a crucial role in the global climate. Since the methane cycle resembles Earth’s water cycle, and one important aspect of Earth’s water cycle is infiltration into the ground and storage in underground aquifers, it is reasonable to draw parallels. We know from the Cassini mission (2004 -2017) that methane precipitation occurs on Titan’s surface [1], and from the Huygens probe that the surface is moist [2]. It is therefore easy to imagine that, just like on Earth, methane infiltrates Titan’s soil, creating a subsurface cycle that alters the soil’s properties and its temperature. Certain aspects of the methane cycle are not well understood, and models struggle to reproduce some observations. The lack of data on many parameters of Titan’s soil and atmosphere makes accurate modeling challenging. Understanding the distribution of methane in the subsurface and its overall effect on surface temperature could improve the interpretation of observations of lakes and surface moisture, as well as predict regions of precipitation. Like the asymmetry of the seasons which creates an uneven distribution of methane across Titan’s surface and atmosphere. The Titan Planetary climate model of Titan, first developed at the Institut Pierre-Simon Laplace [3], is a useful tool for exploring the interaction between the surface and the atmosphere. Adding a subsurface model to this existing framework which already includes surface precipitation and evaporation would make it possible to visualize the distribution of methane within Titan’s soil and its interaction with atmospheric dynamics. The microphysical processes associated with the clouds of various species present in Titan PCM allow for a better representation of cloud formation through nucleation and condensation.

2 Method and objective
For the inclusion of a liquid infiltration and diffusion model in Titan’s PCM, inspired by [4], the subsurface is  given the ability to retain liquid methane within its layers. A liquid flux through a porous medium is added to the thermal transfer processes, based on mass conservation and Darcy’s law. The transfer of liquid mass within the subsurface impacts the soil’s energy balance. Transfers are calculated in three dimensions, with diffusion and gravity driving the movement of the liquid. Vertically, the gravitational potential dominates, but horizontally, the slope between a flat surface and that inferred from the topography is weaker. The subsurface reservoir creates methane storage zones that can become lakes, depending on the height of the saturated layer and Titan’s topography. A sufficiently detailed topography could reproduce methane basins where underground methane forms shorelines at the surface. This would affect the availability of surface methane. Many parameters, particularly the characteristics of the soil, remain unknown. By comparing model results to observations, refining the free parameters can provide estimates of Titan’s subsurface conditions such as soil porosity, tortuosity, and the degree of saturation with liquid methane. In this presentation we will present the result of simulations perfomed with the PCM that accounts for surface methane without and with interaction with the subsurface. We will focus on the difference in the cloud maps, precipitations and methane planetery flux in the atmoshere and in the subsurface. Several scenario will be considered and our model will be combared to available data.

References
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