14 DAY TRIAL // The atmosphere of Saturn's largest moon contains mostly nitrogen, around 5% methane and other organic compounds in smaller amounts. But where did the methane come from? Scientists used to think that maybe Titan has methane oceans, but such ideas were put to rest by the Huygens probe, which landed on Saturn's moon on 14 January 2005.
The temperature on Titan doesn't allow liquid methane but only frozen methane. However, the idea of methane ice glaciers comes with a dilemma: how does methane evaporate from these glaciers into the atmosphere?
Gabriel Tobie, a planetary astrophysicist at Nantes University in France, has now created a geologic model capable of describing how Titan got its methane glaciers and its atmosphere. This model makes certain predictions that could be tested in the incoming months by NASA's Cassini probe, but a new Titan mission will probably be needed to settle the case.
The model states that the moon started out with roughly 0.1% methane. This may have been either one of its original ingredients or it may have been produced by various chemical reactions in its core. However, most of this methane reached the surface and had been transformed in ethane by the Sun's rays. The rest of it remained locked in a few kilometers thick layer of methane ice which floated on a deep ocean of ammonia-enriched water.
As the moon cooled, the subterranean water began to freeze causing an underground turmoil that pushed portions of mud through the surface crust. This mud was about 100 degrees Celsius warmer than the methane ice on the surface and caused some of it to melt. These raising trails of relatively hot mud are called cryovolcanoes - they are very cold compared to Earth standards, but hot relative to the surface. There exists some evidence from the Cassini-Huygens mission that such volcanoes actually exist on Titan. The driving forces behind these volcanoes are the tidal forces - caused by the gravitation attraction of Saturn.
In order to prove this theory the methane glaciers should be detected - which isn't exactly easy to do. "Cassini's instruments, in particular its Visible and Infrared Mapping Spectrometer (VIMS), should detect an increasing number of cryovolcanic feature and, if we are lucky, may eventually detect eruptions of methane," said Tobie.
The Cassini spacecraft will also make several measurements of Titan's gravity, which may confirm the presence of a liquid interior - the "fuel" of this supposed volcanic activity.
Credits: Titan's upper atmosphere (NASA/JPL/Space Science Institute); Possible cryovolcanoe (ESA/NASA/JPL/University of Arizona)
