No ocean here.
The dry landing of the Huygens probe--shown in this artist's impression--may imply that methane is locked inside ice on the moon's surface.

Titan's Methane Kept on Ice

The origin of the atmospheres found in our solar system is often shrouded in mystery, and Titan is no exception. As much as 5% of the air enveloping Saturn's largest moon contains methane, but astronomers aren't sure where the gas comes from. Now researchers may have found their culprit: volcanoes that spew icy slush warm enough to melt Titan's glacial crust, freeing methane trapped inside.

There was very little known about the surface of Titan until 14 January 2005, when the European probe Huygens landed on there with a thud, not a splash (ScienceNOW, 21 January 2005). Theories of methane oceans quickly dried up, leaving scientists to wonder where the methane in the moon's atmosphere comes from. One idea is that the gas is trapped in a kind of water ice, called methane clathrate, which is believed to exist in the outer crust of Titan. But there's debate over how methane would escape this frozen cell.

To crack the case, planetary astrophysicist Gabriel Tobie of the Université de Nantes in France and his colleagues modelled Titan's geologic history. In their picture, the moon started out with roughly 0.1% methane--either as one of its original ingredients or as the result of chemical reactions in its core. Most of this disappeared, however, when the methane seeped to the surface and was transformed into ethane by the sun's rays. What didn't reach the atmosphere became locked in a layer of methane clathrate. This sheet, a few kilometers thick, floated on a deep ocean of ammonia-enriched water.

As temperatures inside the moon dropped, the water in this subterranean ocean began to freeze, causing a churning that forced plumes of icy slush to burst through the hard crust. Although the slush was nearly frozen, it might as well have been magma, since it was as much as 100 degrees Celsius warmer than the methane ice on the surface. The resulting cryovolcanoes--of which there is some evidence on Titan--heated the clathrate enough to release the gas to the atmosphere, the authors propose 1 March in Nature.

Detecting the layer of methane clathrate will be hard to do, admits Tobie, because it's very similar to water ice. But in the coming months, NASA's Cassini spacecraft will make several measurements of Titan's gravity, which may confirm that liquid--the "engine" of this volcanic activity--is present in the moon's interior. Still, the main uncertainty is whether the ice plumes would puncture the clathrate and provide enough heat, says planetary scientist David Stevenson of the California Institute of Technology in Pasadena. He is not entirely convinced that cryovolcanism has been detected on the moon's surface. "It may be necessary to have a dedicated Titan mission," he says.

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