SAN FRANCISCO, CALIFORNIA—It’s springtime on Titan, Saturn’s giant and frigid moon, and the action on its hydrocarbon seas seems to be heating up. Near the moon’s north pole, there is growing evidence for waves on three different seas, scientists reported here today at a meeting of the American Geophysical Union. Researchers are also coming up with the first estimates for the volume and composition of the seas. The bodies appear to be made mostly of methane, and not mostly ethane as previously thought. And they are deep: Ligeia Mare, the second biggest sea with an area larger than Lake Superior, could contain 55 times Earth’s oil reserves.

The evidence is coming from NASA’s Cassini spacecraft, which has being exploring the Saturn system since 2004. In 2009, the northern hemisphere of Titan passed its spring equinox, when it begins tilting toward the sun, and climate models predicted that the increased light would kick up winds as the moon approaches summer in 2017.

That appears to be happening. In a handful of flybys of Titan in the past 6 months, Cassini scientists have seen signs of waves on three different seas: Kraken Mare, Ligeia Mare, and Punga Mare. Some of the evidence is based on radar reflections, which detect roughness at the sea surface. Particularly intriguing has been a feature on Ligeia Mare dubbed the Magic Island because it appeared, disappeared, and reappeared over the past 2 years. Jason Hofgartner, a planetary science graduate student at Cornell University, says that a likely explanation is transient episodes of waves. “It is neither magical nor an island. But the name has stuck,” he says.

Scientists involved in the discoveries have been cautious, saying that the features could also be floating debris or bubbles. At Kraken Mare, however, Cassini researchers detected a wavelike feature with both the spacecraft’s radar and a mapping spectrometer. That double detection gives Alexander Hayes, a planetary scientist at Cornell, extra confidence. “It’s most likely waves,” Hayes says. He calculates that the waves are moving at about 0.7 meters per second and at heights of about 1.5 centimeters. “They’re not huge,” he says. Right now, Hayes says, the waves seem to be appearing only in scattered patches where islands or canyons could be funneling winds—a phenomenon that sailors call cat’s paws. In January, Cassini will make another flyby of Titan that will allow the spectrometer a chance to confirm a radar feature detected in Punga Mare.

NASA Chief Scientist Ellen Stofan, who has spent much of her career studying Titan, calls the results a “vindication” for those who predicted seasonal change. “To me, it’s exciting,” she says. “It says that Titan is a dynamic place.” She says that Cassini scientists can now look for evidence that the waves, now or in the past, have eroded into the jagged, frozen shorelines and created long, straight beaches—features that have been mostly lacking in Cassini data.

Other scientists at the meeting reported on using Cassini’s radar to assess the size and contents of the seas. The maximum depth of Kraken Mare appears to be 160 meters, and Ligeia Mare could be as much as 200 meters deep, reported Marco Mastrogiuseppe of Sapienza University of Rome. The fact that the radar signals could bounce off the sea bottom suggests that the seas were more transparent than expected and thus must contain mostly methane, not ethane. Hayes says his best estimate is about 90% methane. Essam Marouf, a planetary scientist at San José State University in California, reported on the first results from a separate radar experiment that sent radar reflections to Earth instead of back to the spacecraft. Those tests provide independent evidence that the seas are dominated by methane, Marouf says, and it implies that the lakes are kept filled by precipitating methane.

Decades ago, planetary scientists such as David Stevenson of the California Institute of Technology in Pasadena had predicted that the seas might be mostly ethane. “It certainly wasn’t obvious that they would be methane-dominated,” Stevenson says. Part of the reason for that presupposition is that light coverts methane in the atmosphere to ethane. Over billions of years, this process would deplete Titan’s surface stores of methane unless it was kept resupplied by a reservoir. Some scientists have proposed that erupting cryovolcanoes or deep underground aquifers of liquid methane occasionally recharge Titan with methane. “There is an unsolved question underlying this,” Stevenson says. “Where does all the methane come from?”

*Correction, 17 December, 11 a.m.: This item originally used the phrase “bodies of water” to describe methane seas. We have struck the words “of water.”