On Earth, vast oceans are the source for the water cycle. On Titan, however, the known surface deposits of liquid methane are far too sparse to fully support the moon's methane cycle. That's because methane in the atmosphere readily breaks down into its components, carbon and hydrogen, when exposed to sunlight. Over the moon's 4.5 billion-year-old history, that process would have consumed all the known quantities of methane.
Something, it seemed, needed to be replenishing the moon's supply of surface methane. The lake in Titan's tropics, which appears to be a long-lived feature, may help identify one of the sources needed to sustain the cycle, researchers say.
The lake's location is key. The phrase "Titan's tropics" might seem like an oxymoron, considering that the moon's surface temperature averages minus 290 degrees Fahrenheit. Nevertheless, the moon's equatorial regions receive the most consistent sunlight throughout the course of its "year."
Previous climate studies "indicated convincingly that any liquid on Titan's tropical surface would quickly evaporate and be quickly transported to the pole," says Caitlin Griffith, a planetary scientist at the University of Arizona who led the team reporting the results.
Discovering the tropical lake "was completely unexpected," Dr. Griffith says.
Cassini's travel mate, the European Space Agency's Huygens lander, seemed to reinforce that when it parachuted to the surface in 2005. Upon touchdown at a landing site within the equatorial belt, its sensors detected a brief puff of methane rising from a surface covered with sand-like ice grains. This suggested that it landed on a surface slightly dampened with the liquefied gas – but certainly no hydrocarbon lake or ocean, as some had expected.
Indeed, as Cassini continued its flybys of Titan, its radar found no lakes in the tropics, though it did find deep, vast lakes in polar regions. Yet now at least one lake appears to be present. How could Cassini have missed something the size of the Great Salt Lake in Utah?