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Tropical lake on Titan? Surprising find could solve moon's methane mystery.

Scientists have wondered whether some unseen process replenishes the lakes of liquid methane on Titan, Saturn's biggest moon. A newly found lake suggests intriguing possibilities. 

By Staff writer / June 13, 2012

A mosaic of the Huygens probe landing site, as seen by the descent imager/spectral radiometer (DIRS) on the Huygens probe. The mosaic is overlaid on a Cassini orbiter radar image, taken during a 2005, flyby. The landing site, marked by the red 'X,' is located in Titan's southern hemisphere.

ESA/NASA/JPL/University of Arizona/USGS

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Scientists have spotted a Great Salt Lake-scale patch of liquid methane on the surface of Saturn's moon Titan, along with smaller swamp-like features, in an unexpected and intriguing place. 

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Data from the Cassini spacecraft currently touring the Saturn system have already revealed lakes at Titan's poles, fed by summertime methane rain. But Cassini's radar found nothing similar at lower latitudes, and climate models have suggested that long-lived lakes might be impossible there.

Now, the discovery of the large, shallow lake in the moon's tropics may offer scientists clues about the processes driving a moon that has fascinating similarities to Earth before life emerged.

If the finding is confirmed by additional observations, it could imply the existence of significant subsurface methane deposits feeding the tropical lake. Or perhaps the moon recycles methane, with the liquid in polar lakes migrating underground back to the tropics, where it wells up again in lakes.

Whatever the cause, the discovery could have important implications for understanding how Titan maintains a methane cycle that is strikingly similar to the water cycle on Earth – with methane evaporating as gas, falling as rain, and gathering in lakes.

The report in Thursday's issue of the journal Nature represents a "significant discovery," says Tapio Schneider, a scientist at the California Institute of Technology who models planetary climates, including the moon Titan's.

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.

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