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New study explains movement of liquids on Titan

Jenny Winder, News Writer
Sep 8, 2014, 14:18 UTC

Sen—A new study seeks to explain how liquids move and cycle through the crust and atmosphere of Saturn's largest moon, Titan to form reservoirs of propane and ethane that may feed into some rivers and lakes.

The NASA and European Space Agency Cassini mission has revealed hundreds of lakes and seas across the north polar region of Titan, filled with hydrocarbons, a form of organic compound found naturally on Earth which includes methane. Titan's lakes are thought to be replenished by rainfall from clouds in the moon's atmosphere. How liquids move and cycle through Titan's crust and atmosphere is still not well understood.

A recent study led by Olivier Mousis, a Cassini research associate at the University of Franche-Comté in France, found that the formation of materials called clathrates changes the chemical composition of the methane rainfall runoff that charges these hydrocarbon "aquifers". This process leads to the formation of reservoirs of propane and ethane that may feed into some rivers and lakes.

"We knew that a significant fraction of the lakes on Titan's surface might possibly be connected with hidden bodies of liquid beneath Titan's crust, but we just didn't know how they would interact," said Mousis. "Now, we have a better idea of what these hidden lakes or oceans could be like."

Mousis and his colleagues modeled how a subsurface reservoir of liquid hydrocarbons, also called an "alkanofer," once filled with methane rainfall runoff, would diffuse through Titan's porous, icy crust. They found that this diffusion could cause a new reservoir to form where the bottom of the original underground reservoir meets layers of non-porous ice. This secondary reservoir would be composed of clathrates.

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This artist's rendering shows a cross-section of the surface and subsurface of Saturn's moon Titan, with a possible model for the structure of underground liquid reservoirs there. Image credit: ESA/ATG medialab

Clathrates are compounds in which water forms a crystal structure that traps other substances like methane and ethane. Clathrates containing methane are found in some polar and ocean sediments on Earth. On Titan, the surface pressure and temperature should allow clathrates to form when liquid hydrocarbons come into contact with water ice, a major component of the moon's crust. These clathrate layers could remain stable as far down as several miles below Titan's surface.

Clathrates  trap and split molecules into a mix of liquid and solid phases, in a process called fractionation. Titan's clathrate reservoirs would fractionate the liquid methane from the original underground hydrocarbon lake, slowly changing its composition. Eventually the original methane aquifer would be turned into a propane or ethane aquifer.

"Our study shows that the composition of Titan's underground liquid reservoirs can change significantly through their interaction with the icy subsurface, provided the reservoirs are cut off from the atmosphere for some period of time," said Mathieu Choukroun of JPL, one of three co-authors of the study published in the printed issue of the journal Icarus.

The chemical transformations taking place underground would affect Titan's surface. Lakes and rivers fed by springs from propane or ethane subsurface reservoirs would show the same kind of composition, whereas those fed by rainfall would be different and contain a significant fraction of methane. This means researchers could examine the composition of Titan's surface lakes to learn something about what is happening deep underground, said Mousis.