Saturn's moon Titan shares many of Earth's features, including clouds, rain and lakes. And now scientists know the two are similar in another way: they both have waves. Cameras on NASA's spacecraft Cassini recently saw what appear to be waves on one of Titan's largest methane lakes—a signal scientists have long searched for but never found.

“I was starting to despair that we were going to see them at all,” says Jason Barnes, a physicist at the University of Idaho who presented the evidence in March at the 45th Lunar and Planetary Science Conference in The Woodlands, Tex. If confirmed, the discovery would mark the first time waves have been seen outside Earth.

Barnes and his team found patterns in the sunlight reflecting off a northern lake called Punga Mare that they interpret as two-centimeter-high waves. There is a different explanation, others caution: Punga Mare may be a mudflat instead of a deep lake, and a shallow film of liquid on top may be the cause of the unique light signature. “It's compelling, but it's not definitive,” says Jonathan Lunine, a planetary scientist at Cornell University who was not involved in the study.

Waves on Titan would be noteworthy for several reasons. Such a finding would confirm that the lakes actually are deep reservoirs of methane and ethane, the dominant forms of liquid on that moon. If life on Titan exists, it would probably be primitive, so the best place to look for self-assembling structures, scientists say, is in large bodies of liquid—the kind that form waves.

True liquid bodies would also make a robotic spacecraft mission to explore Titan's habitability more feasible. After all, landing is easier in liquid than in a thicker substance or on solid ground.

By 2017 scientists should know for certain whether what they are seeing is indeed caused by waves. So far Cassini has been observing the moon during its northern winter, when weak winds are at work. As spring settles in over the next few years, bringing stronger winds to kick up seas, the probe should capture more definitive evidence of waves if they exist. Those waves will probably be larger than two centimeters.