麻豆传媒

The mysteriously replenishing source of methane in Titan鈥檚 atmosphere may be outgassings from chemical structures within the moon鈥檚 icy crust that have caged the gas for billions of years.

Methane currently makes up less than 5% of the hazy, nitrogen-dominated atmosphere around Saturn鈥檚 giant moon Titan. No one can say exactly how the methane got there in the first place or what is sustaining it, though there must be a source or process topping up the gas, since radiation from the Sun breaks methane down over time.

Prior to the Cassini-Huygens mission that arrived at Saturn in 2004, scientists had long hypothesised the atmospheric methane was evaporating from large lakes or seas of hydrocarbons on Titan鈥檚 surface. But the mission has not seen evidence for such lakes.

The new model developed by researchers lead by Gabriel Tobie at the University of Nantes, France, which includes a subsurface ocean of water and ammonia, suggests heat rising through the moon鈥檚 interior layers has released the trapped methane during three major episodes over the past 4.6 billion years, when the solar system first formed.

Thermal plumes

The most recent episode began between 1 billion and 500 million years ago, says Christophe Sotin, a member of a team at Nantes.

The team鈥檚 new model takes Titan鈥檚 unusual orbital features into account, and incorporates the characteristics of the moon鈥檚 interior.

The researchers say hot thermal plumes driving up through an underlying layer of ice trigger the release of methane into a chamber whose contents may later be squirted into the atmosphere by eruptions of ice volcanoes.

Observations by Cassini over the next few years should allow the researchers to refine, and possibly verify, their model. Earlier this week, Cassini gathered measurements of Titan鈥檚 gravity field during the first of four flybys that will help scientists determine whether Titan boasts a subsurface ocean.

Liquid layer

Scientists will be watching to see how much the moon deforms over time. 鈥淚f you have [an underground] liquid layer, the moon is going to deform much more easily than if you don鈥檛 have a liquid layer,鈥� Sotin told 麻豆传媒.

If it were not replenished by some source, the methane on Titan would have been removed billions of years ago by chemistry driven by ultraviolet radiation. Sotin says methane could only last about 10 or 20 million years in Titan鈥檚 atmosphere with no top-ups.

Cassini has spotted no large hydrocarbon lakes, as once hypothesised, during its 11 close flybys of Titan. And while the Huygens probe snapped shots of channels or canyons 鈥� probably carved by liquid methane 鈥� and detected a whiff of methane shortly after landing on the planet-like moon, there was no surface liquid anywhere in sight.

The new model suggests the most recent methane-releasing episode could have injected enough atmospheric methane to produce heavy methane rains sufficient to soak the planet鈥檚 surface, possibly accounting for Huygens鈥� observations.

But co-author Jonathan Lunine of the University of Arizona, US, says: 鈥淭itan is now in an era where there鈥檚 enough outgassing to add methane to the atmosphere, but not enough for widespread seas of methane.鈥�

Streaky clouds

Lunine suggests that Titan will stop releasing methane into the atmosphere within the next few hundred million years, at which point Titan will completely dry out.

Henry Roe of the California Institute of Technology, Pasadena, US, says he is intrigued by the new model鈥檚 prediction that this 鈥渇inal鈥� methane-emitting period should produce more localised and perhaps, intermittent, bursts of methane.

Roe鈥檚 group reported what they described as long streaky methane clouds in Titan鈥檚 southern mid-latitudes in October 2005. Using the Gemini Observatory in Hawaii, his team observed many clouds between December 2003 and February 2005. But the clouds have been absent since then.

鈥淚t seems like whatever was happening at that 40掳 South latitude聟 was active for a year or two and then turned off,鈥� he told 麻豆传媒. That 鈥渟eems to fit鈥� what the new model is predicting, he says.

Journal Reference: Nature (vol 440, p 61)