Constraints on the presence or absence of river deltas on Titan

Titan is the only known body to possess an active hydrological cycle besides the Earth, involving instead of liquid water hydrocarbons, mostly methane and ethane, and some dissolved nitrogen from the atmosphere [1]. Observations by the Cassini-Huygens mission revealed the presence of hydrocarbons polar lakes, widespread river channels, clouds and rains on Titan [1]. On Earth, rivers transport sediments that can be deposited at the mouth of the rivers, into the lakes or seas, in the form of a river delta [2]. However, despite numerous rivers entering lakes in the polar regions, sedimentary deposits at the mouth of the rivers are almost absent on Titan. Only one delta was identified at Ontario Lacus in the south pole [3], and Birch et al. [4] showed their global scarcity is not due to the Cassini RADAR resolution. If the sediment transport on Titan was similar to that of the Earth, more deltas should be seen. Still, the lack of deltas on Titan is not yet understood. Our work aims to improve the understanding of Titan’s hydrological cycle by investigating the specificities of Titan’s sediment transport and thus the processes that could explain the apparent lack of deltas on Titan.

The presence or absence of deltas on Earth is linked to several processes such as the presence and quantity of river bedload, or effects of waves and tides [5], but also to sea-level variation [6] or density contrasts between the rivers and the lakes or seas [7]. Dissolution of surface material was proposed on Titan’s polar terrains [8], and could also be of effect to dissolve sediments or erode deltas. The identification of one delta and many alluvial fans [3, 9], plus numerical work [10], suggest that transport and deposition of river bedload is possible on Titan. The importance of all processes on the bedload sediment deposition and reworking will need to be assessed. We start by investigating the effects of density contrasts at the river mouth on delta formation on Titan. The river flow is called hyperpycnal when its density is higher than the lake fluid, and hypopycnal when it is the inverse [7]. We begin this study by the case of hyperpycnal flows, for which sediments (both bedload and suspended load) could be transported by the river to the bottom of the lakes, making the sediment deposits invisible to the Cassini RADAR and explaining the apparent lack of deltas on Titan.

To investigate in which conditions hyperpycnal flows may develop on Titan, we first review the mixtures of liquids that could exist at the surface of Titan. A particularity of Titan is that rivers and lakes may not be formed by the same mixture of phases, creating density contrasts at the river mouth, and favoring or not hyperpycnal flows. Lakes [11, 12] could contain less or more ethane than the rain feeding the rivers [13, 14], affecting in return the quantity of dissolved nitrogen [15], and therefore their densities. Considering different end-member composition scenarios, we provide constraints on the density of the river flow and the lake, and discuss the concentration and characteristics (composition, grain size) of the suspended load, that permit the onset of hyperpycnal flows. We also discuss the liquid depth required for the hyperpycnal flows to plunge in the lakes [16, 17], in regard to the bathymetry of Titan’s polar lakes. Together, these results provide new constraints on the conditions required for creating river deltas within lakes on Titan.

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