Modelling River - Dunes Interactions on Titan
Authors: Bohaceck Eleni Vassilia , Bahia Rickbir Singh , Braat Lisanne , Boazman Sarah , Sefton-Nash Elliot , Orgel Csilla , Wilson Colin , Riu Lucie 
Published: February 2023

The surface of Titan displays evidence of fluvial and aeolian activity. Rainfall on Titan results in fluvial landforms (FLs), lakes, and seas. Unlike Earth, this rainfall is predominantly liquid methane. Titan&#8217;s surface conditions allow for liquid methane and ethane to be stable. Although the rainfall is primarily methane, this methane (liquid density ~424 kg/m3) can be photolyzed to form ethane (liquid density ~544 kg/ m3), resulting in lakes and rivers of ethane. Liquid ethane is more likely to be fed back into rivers and lakes by springs and play a formative role in the lower reaches of rivers. Changes in fluid density from the source (methane) to the terminus (ethane) of Titan&#8217;s rivers may affect the flow dynamics of the river. Methane fed rivers are likely episodically active since rainfall, which is concentrated in the poles, lasts 10-100 hours each Titan year (30 Earth years). Although precipitation is limited in the mid-latitudes, FLs have been observed in these regions.
Titan is also covered by vast regions of active dune fields, primarily within the equatorial latitudes. They are composed of hydrocarbon and nitrile sand-sized particles forming from photochemical reactions in Titan&#8217;s atmosphere. Although observations of Titan are limited, interactions between rivers and dunes have been observed. Limited data availability means modelling fluvial and aeolian processes is one of the best methods to understand active and previously active processes on Titan.
Here we report the initial study by the Working group on Aeolian-Fluvial Terrain Interactions (WAFTI), based at the European Space Agency, which examines the effects of these processes in synergy under Titan conditions, using a combination of modelling and geomorphological analysis. We hypothesise that these interactions could have implications for the distribution and planforms of Titan FLs.
To simulate the interactions between fluvial and aeolian processes on Titan, we developed the Titan Aeolian-Fluvial Interactions model. This is a landscape evolution model based on a coupled implementation of the Caesar-Lisflood fluvial model, and Discrete ECogeomorphic Aeolian Landscape model (DECAL) dunes model. The Caesar-Lisflood fluvial model routes water over a digital elevation model and calculates erosion and deposition from fluvial and slope processes and changes elevations accordingly. The DECAL model is based on the Werner slab model of dunes, which simulates dune field development through self-organization.
Several scenarios shall be modelled: (1) a continuous methane river, flowing in a straight channel with linear dunes migrating towards the channel parallel to its length; (2) a continuous methane river flowing towards a dune field with crest lines perpendicular to the direction of flow; (3) simulation scenario (1) but altered slope to represent the three different reaches (source, mid-reaches, and termination) of the channel and simulate for both methane and ethane flows by altering fluid density; (4) simulation scenario (1) with an episodically active river and continually active dunes.
The findings of these simulations may help understand the drainage patterns and distribution of FLs and methane/ethane across Titan.