Space / Solar System

Titan’s central mystery has long been its methane-rich atmosphere. Incoming solar radiation should have shattered these buoyant methane molecules long ago. The fact that thick haze still blankets Titan suggests that the methane is replenished constantly by evaporation from some liquid reservoir. Indeed, the controllers of the Huygens probe were prepared for a splash when it parachuted down near Titan’s equator. Instead it landed with a thump. Huygens did spot what looked like a lake bed near its landing site, with apparent feeder channels—riverbeds, in essence—snaking into it. But they were bone-dry. Maybe, scientists postulated, methane doesn’t stay on the surface most of the time, but only seeps out periodically from a subsurface source.

Then Stofan found those northern lakes, which suggest that Titan is the only other place in the known universe with Earthlike bodies of liquid on its surface. “Titan turns out to be a very complicated place,” says Cassini team member Jonathan Lunine, of the University of Arizona’s Lunar and Planetary Laboratory. “Trying to understand the whole surface by going to just one place is as misleading as trying to do the same thing on Earth.”

Close-up exploration of other locations on Titan’s surface isn’t an option. The now-frozen Huygens probe can go no further; as expected, its power supply ran out less than two hours after it landed. But Cassini continues to aim its instruments at Titan as it periodically glides past it. Because the probe sweeps very close to the moon, its radar can illuminate only one narrow strip of Titan at a time. “It’s a bit like uncovering a mummy,” Lunine says. Not until last July did Cassini fly over and scan Titan’s arctic region—a place especially intriguing to scientists because it is colder than the equator and therefore is a region where methane is more likely to condense and rain down to the ground.

Sure enough, Cassini’s radar showed 75 or more dark spots of various sizes dotting the northern Titanic landscape. The spots appear black because the radar beams that strike there are not scattered back toward the spacecraft; rather, they bounce cleanly off into space. That suggests that these areas are extraordinarily smooth—a powerful indication that they are liquid. (A liquid naturally assumes a flat surface; a smooth solid area is very rare in nature.) The perimeters of the dark areas are irregular, as would be expected if fluid had flowed into a natural basin, and the transition from the dark flat areas to the much brighter surrounding regions is abrupt, as though a smooth liquid were lapping up against rough, dry land. Finally, many of the lakes are fed by what look like river channels.