Although Titan's atmosphere is mostly composed of nitrogen, a small amount of methane (about 1.6%) is also present. But because this methane should have been destroyed long ago through exposure to the Sun, astrophysicists think that Titan must somehow be replenishing its stocks through another process. Given the lack of evidence for methane-rich deposits on the surface, the belief to date has been that the methane must come solely from underground.

Now, however, Ellen Stofan at University College London in the UK together with colleagues from the USA and Europe think otherwise. By performing a careful analysis of radar data taken from NASA's Cassini-Huygens mission they have confirmed that numerous dark patches known to exist at high latitudes in Titan's northern hemisphere are in fact lakes of methane. According to the researchers, some of these lakes could be fed by both rivers and rainfall, while others could be fed by a methane "groundwater table". In other words, while underground deposits are still possible, there is definitely methane on the surface – and it is likely to be part of a continuous "methane cycle" that maintains the levels of atmospheric methane.

"We're excited to demonstrate that Titan is the first solar system body besides Earth that has an ongoing, active exchange of fluids," said Stofan. "Studying what is undoubtedly a complex system is going to help our understanding of climate dynamics."

This is not the end of the debate about Titan, however, and it looks as though it will continue to surprise physicists as the Cassini probe continues on its mission. Christophe Sotin at the Université de Nantes in France, another expert on Titan, thinks there is much left to learn. "A big [question] is the nature of the component that seems to coat the dark [patches]," he told Physics Web.