Lakes of methane have been spotted on Saturn's largest moon, Titan, boosting the theory that this strange, distant world bears beguiling similarities to Earth, according to a new study.
Titan has long intrigued space scientists, as it is the only moon in the solar system to have a dense atmosphere — and its atmosphere, like Earth's, mainly comprises nitrogen.
Titan's atmosphere is also rich in methane, although the source for this vast store of hydrocarbons is unclear. Methane, on the geological scale, has a relatively limited life. A molecule of the compound lasts several tens of millions of years before it is broken up by sunlight.
Given that Titan is billions of years old, the question is how this atmospheric methane gets to be renewed. Without replenishment, it should have disappeared long ago. A popular hypothesis is that it comes from a vast ocean of hydrocarbons.
But when the US spacecraft Cassini sent down a European lander, Huygens, to Titan in 2005, the images sent back were of a rugged landscape veiled in an orange haze. There were signs of methane flows but nothing that pointed to any sea of the stuff.
But a flyby by Cassini last year has revealed 75 large, smooth, dark patches between three and 70 kilometres across that appear to be lakes of liquid methane, scientists reported on Thursday. They believe that the lakes prove that Titan has a 'methane cycle' -- a system that is like the water cycle on Earth.