Since Cassini plunged into Saturn’s atmosphere in 2017, we’ve had no mission to watch the weather on this giant planet or its moon with atmosphere, Titan. But researchers are still plumbing the depths of this mission’s data to publish new science. This week, the news is about Titan’s lakes.

Saturn’s moon Titan is 10 times farther away from the Sun than Earth, and a thick atmosphere of opaque orange smog hides its surface. But if you can peer beneath the smog, you find a surface that is uncannily familiar.

Titan has wind and weather. It has deserts filled with sand dunes. It has white puffy clouds that spill over with rain. The rain falls on mountains, where it runs off into streams and collects in rivers. The rivers debouch into seas, building sedimentary deltas. Seasonal sea level rise chokes river mouths, creating estuaries.

But it’s all alien. The mountains aren’t made of rock, they’re made of water ice, kept strong enough to build tall peaks because of unearthly low temperatures. Water is as unlikely to melt on Titan as rock is on Earth; the moon’s rain is made of methane and nitrogen. Titan has a complete methane cycle like Earth’s water cycle: The liquid runs off, collects in lakes, evaporates, condenses in clouds, then rains again to begin the cycle anew.

There’s a key difference at Titan, however. When ultraviolet rays from the Sun strike methane (CH₄) molecules in Titan’s atmosphere, a set of reactions creates ethane (CH₆). Ethane is heavier than methane or nitrogen, and it readily dissolves into methane raindrops. Unlike methane molecules, once ethane falls onto Titan’s mountains and runs into its seas, it doesn’t evaporate again; it stays. There is no ethane cycle.

Titan is mostly desert; its only really wet locations are at its poles, where clouds concentrate and spill during the long polar summers. Just like on Earth, rain can happen in deserts, it’s just not very common. So, given the fact that methane cycles on Titan but ethane does not, theory predicts that wetter regions nearer the poles should have more methane relative to ethane in their lakes than would lakes in drier regions.

This much was predicted theoretically in 2014 by Cassini radar scientist and Titan expert Ralph Lorenz (Johns Hopkins University Applied Physics Laboratory). Cassini didn’t have an instrument that could actually sample the compositional variation across Titan. But the Cassini team did conduct several experiments that can offer hints about variations among different lakes.

In four bistatic radar experiments, Cassini broadcast a radio signal at Titan that bounced off the surface and was then received on Earth by a Deep Space Network antenna. The glassy surfaces of Titan’s lakes – which are almost perfectly smooth, with wave heights measured in millimeters – are especially good at reflecting radio waves.

In a paper recently published in Nature Communications, Valerio Poggiali (University of Bologna, Italy, but temporarily visiting Cornell University) and coworkers (including Lorenz) crunched the challenging bistatic radar data and found systematic variations with latitude in how well Titan’s lakes reflected Cassini’s radio waves to a giant radio dish in Canberra, Australia.

The most plausible explanation for those variations: The more polar lakes are richer in methane, especially at their surfaces and near their shores, where rivers enter seas. Moreover, near Titan’s shores, winds toss larger waves (as many as three millimeters instead of just one!) across estuaries that are especially methane-rich.

Titan’s waves are far too small to surf, unless you’re an ant. Still, it’s wild to imagine the soft sound of waves lapping at Titan’s shores, and how that sound might change after rainfall, when an outpouring of methane spills in to the heavier seas.