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Hundreds of lakes and seas are known of on Titan, Saturn's largest moon, filled not with water but with hydrocarbons such as methane.

Most of these bodies of liquid are thought to be replenished by rainfall from clouds in the moon's atmosphere, but now research says underground reservoirs may also be feeding them.

In a study scientists found that subsurface 'springs' of propane and ethane may be contributing to some bodies of liquid - and the effects may be noticeable on the surface.

A researcher in France says bodies of liquid on Titan may be fed by underground springs. This artist's rendering shows a cross-section of the surface and subsurface of Titan Saturn's moon Titan. An 'alkanofer' is a resevoir of liquid hydrocarbons, while clathrates are compounds that trap other substances

The study was carried out by Dr Olivier Mousis, a research associate for the Cassini spacecraft, currently in orbit around Saturn, at the University of Franche-Comté, France.

He examined how Titan's methane rainfall would interact with icy materials in underground reservoirs.

And with colleagues he found that runoff from this rainfall was likely leading to the formation of springs of propane and ethane, which in turn were feeding the lakes and seas.

The results of this could be noticed on Titan's surface, meaning scientists may be able work out what is happening underground simply by looking at the moon from space.

The scientists 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.

Importantly, this process would continue up to Titan's surface. 

Lakes fed by these propane or ethane subsurface reservoirs would show the same kind of composition, whereas those fed by rainfall would be different and contain methane, nitrogen, and trace amounts of argon and carbon monoxide. 

In other words, the composition of the lake could indicate what is happening deep underground.

'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.' 

The finding was made when studying how methane rain interacts with lakes on Titan (pictured). It was found that rainfall runoff was likely feeding underground resevoirs. These in turn would change in chemical composition before adding to existing lakes and seas

The process of underground reservoirs feeding lakes and seas (illustrated) could be spotted on the surface, meaning scientists may be able to work out what is happening underground on Titan just by observing the moon from space

Mousis and his colleagues came to the conclusion when they found that the formation of materials called clathrates changes the chemical composition of the rainfall runoff that feeds hydrocarbon 'aquifers' - reservoirs of liquid underground.

One of the peculiar properties of clathrates is that they trap and split molecules into a mix of liquid and solid phases, in a process called fractionation. 

Titan's underground clathrate reservoirs would interact with and 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 Dr Mathieu Choukroun of Nasa's Jet Propulsion Laboratory (JPL), one of three co-authors of the study with Dr Mousis.

Importantly, 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.

Pictured is Titan (foreground) in orbit around Saturn. With Titan's low gravity and dense atmosphere, methane raindrops could grow twice as large as Earth's raindrops. As well as this, they would fall more slowly, drifting down like snowflakes. But scientists think it rains perhaps only every few decades