The Cassini space probe's observations of Saturn's largest moon show that its lakes seem unusually long-lived for harbouring methane, which evaporates rapidly.

Titan, Saturn's largest moon, is the only other place in the Solar System apart from Earth to harbour stable liquid on its surface. Like water plays a dominant role on Earth by evaporating from lakes, raining from clouds, and carving channels, methane plays a similar role on Titan.

However, scientists at NASA's Jet Propulsion Laboratory (JPL) have found that without any known mechanism to replenish the lakes with methane, which evaporates quickly, and only occasional hydrocarbon cloudbursts, the bodies have been remarkably long-lived. The lakes were observed by the Cassini space probe, which has been studying the Saturn system since it arrived there in 2004.

Christophe Sotin, who led a team at JPL on the study, offered an alternate explanation: That the lakes are not filled with methane but its more slowly evaporating sibling ethane. Feeding his team's results into a model that calculates when Titan's supply of such hydrocarbons could be exhausted, Sotin suggests that the lakes will go dry in tens of millions of years. On geological timescales, that's not much time at all.

Sotin and his team's conclusions were announced on April 15. They suggest that as opposed to having been part of the moon's make up when it formed 4.6 billion years ago, as was believed, methane could have been released in one gigantic outburst at some time in Titan's past, possibly from a meteorite impact.

The rest of the volatile hydrocarbon's journey through the moon is well-understood. It is exchanged between sub-surface and surface lakes. When it rises up, it evaporates into the moon's nitrogen-dominated atmosphere. As it floats to the upper strata, the hydrocarbon is broken apart by sunlight and loses its hydrogen atoms to space.

The carbon atoms then go on to form more carbon-rich compounds such as ethane and aerosols, whose presence in the moon's atmosphere and surface has been detected by the Cassini and ESA's Huygens probes. Ultimately, these compounds will form a fossil record of the moon that Titan once was.

Scientists believe that Titan's early methane-rich atmosphere could provide clues about Earth's at around the same time. This is because the Titanic methane-driven photochemistry could have produced prebiotic molecules necessary for the formation of life just like on Earth. Now, with Sotin's results, methane's cycle could be shorter than previously believed, pointing to similar changes in Earth's early days before life evolved.