Titan's subsurface reservoirs modify methane rainfall

Titan's subsurface reservoirs. Credit: ESA/ATG medialab

The international Cassini mission has revealed hundreds of lakes and seas spread across the icy surface of Saturn's moon Titan, mostly in its polar regions.

These lakes are filled not with water but with hydrocarbons, a form of organic compound that is also found naturally on Earth and includes methane.

While most of the liquid in the lakes is thought to be replenished by rainfall from clouds in the moon's atmosphere, the cycling of liquid throughout Titan's crust and atmosphere is still not well understood.

A recent study led by Olivier Mousis at the Université de Franche-Comté, France, and involving colleagues at Cornell University and NASA's Jet Propulsion Laboratory in the USA, probed the hydrological cycle of Titan by examining how Titan's methane rainfall would interact with icy materials within underground reservoirs.

They found that the formation of materials called clathrates changes the chemical composition of the rainfall runoff that fills these hydrocarbon reservoirs, leading 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 be connected with hidden bodies of liquid beneath Titan's crust, but we just didn't know how they would interact", says Mousis.

"Now, we've modelled the moon's interior in great detail, and have a better idea of what these hidden lakes or oceans could be like."

Mousis and colleagues modelled how a subsurface reservoir of liquid hydrocarbons would diffuse throughout Titan's porous icy crust.

They found that this diffusion could cause a new reservoir – formed from clathrates - to form where the bottom of the original reservoir meets layers of non-porous ice.

Clathrates are compounds in which water forms a crystal structure with small cages that trap other substances like methane and ethane. On Earth, clathrates that contain methane are found in some polar and ocean sediments.

On Titan, the surface pressure and temperature allow clathrates to form when liquid hydrocarbons come into contact with water ice, a main component of the moon's crust. These clathrates could remain stable as far down as several kilometres below the surface of Titan.

Lakes on Titan. Credit: NASA/JPL/USGS

"One of the interesting properties of clathrates is that they cause fractionation, in this case, they trap and split molecules into a mix of liquid and solid phases," adds Mousis.

Because of this, astronomers have suggested that clathrates may be responsible for many unusual phenomena on Titan, including the depletion of the heavy noble gases in the moon's atmosphere, and variations in the moon's polar radius.