WHERE are the oceans? That鈥檚 the question everyone is asking after studying the latest images of Saturn鈥檚 largest moon, Titan.
The oceans and lakes scientists had been expecting to find would explain why 6 per cent of Titan鈥檚 atmosphere is methane. Methane could not have survived for much more than 10,000 years before being broken down by sunlight, so somewhere on Titan there must be a huge methane source that is constantly replenishing the atmosphere.
The most likely source seemed to be oceans of liquid ethane and other hydrocarbons from which the gas was produced, but so far images from the NASA spacecraft Cassini show no sign of the reflections that a liquid surface would be expected to produce. Radar images have also failed to show any signs of large craters that might be filled with liquid.
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To explain the presence of methane, new ideas are now emerging. One possibility is that the methane is a transient phenomenon produced by the recent impact of a large methane-rich comet, but as impacts of this type are thought to occur only once in a few million years this seems unlikely. Another idea is volcanic eruption, but this too is unlikely because the rate of volcanism would have to be perfectly tuned to maintaining the atmosphere鈥檚 composition without leaving large accumulations on the surface.
That leaves two other possibilities. The surface could be 鈥渁 thick regolith soaked with liquid鈥 big slurry鈥�, suggests Ralph Lorenz of Cassini鈥檚 radar imaging team. In other words, a global methane swamp rather than an ocean. Or there could be large bodies of liquid beneath the surface, like aquifers on Earth. These 鈥渕ethanofers鈥�, as they have been dubbed, would have to have outlets to the surface to provide the necessary exchange with the atmosphere.
鈥淭he surface of Titan could be a thick regolith soaked with liquid. In other words, a global methane swamp鈥�
Lorenz and his colleagues are now awaiting further clues from Cassini鈥檚 next fly-bys and from the touchdown in January of the European Space Agency鈥檚 Huygens lander.
