New exploration of Titan's seas – Sky & Telescope

Since Cassini dove into Saturn's atmosphere in 2017, we haven't had any missions to observe the weather on this giant planet or its atmospheric moon, Titan. But researchers are still plumbing the depths of this mission's data to release new science. This week, the news is about Titan's lakes.

Saturn's moon Titan is ten times farther from the Sun than Earth, and a thick atmosphere of opaque orange smog hides its surface. But if you can look beneath the smog, you'll find a surface that looks strangely familiar.

Titan has wind and weather. It has deserts filled with sand dunes. It has white, fluffy clouds that overflow with rain. Rain falls on mountains, where it runs off into streams and collects in rivers. Rivers empty into seas, forming sedimentary deltas. Seasonal sea level rise clogs river mouths, creating estuaries.

But it's all strange. The mountains aren't made of rock, but of water ice, which is kept strong enough to form high peaks by hellish 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 the water cycle on Earth: liquid runs off, collects in lakes, evaporates, condenses into clouds, then rains back down to start the cycle all over again.

However, on Titan there is a key difference: when ultraviolet rays from the Sun hit methane (CH₄) in Titan's atmosphere, a set of reactions creates ethane (CH₆). Ethane is heavier than methane or nitrogen and dissolves easily in methane raindrops. Unlike methane molecules, once ethane falls on Titan's mountains and spreads across its seas, it does not evaporate again, but remains there. There is no ethane cycle.

Titan is mostly a desert; its only truly wet places are at its poles, where clouds gather and disperse during the long polar summers. As on Earth, it can rain in deserts, but it's not very common. So, given the fact that methane is recycled on Titan but ethane is not, theory predicts that wetter regions near the poles should have more methane relative to ethane in their lakes than lakes in drier regions.

This was a lot Theoretically predicted 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 compositional variation on Titan. But the Cassini team did perform several experiments that may offer clues about variations among different lakes.

In four bistatic Radar As part of its experiments, Cassini transmitted a radio signal to 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 and have wave heights measured in millimeters, are particularly good at reflecting radio waves.

In an article recently published in Nature CommunicationsValerio Poggiali (University of Bologna, Italy, but temporarily visiting Cornell University) and collaborators (including Lorenz) analyzed the challenging bistatic radar data and found systematic variations with latitude in how well Titan's lakes reflected radio waves from Cassini to a giant radio antenna in Canberra, Australia.

The most plausible explanation for these variations is that the more polar lakes are richer in methane, especially at their surfaces and near their shores, where rivers flow into the seas. Moreover, near Titan's coasts, winds throw larger waves (up to three millimetres instead of one!) through estuaries that are especially rich in methane.

Titan's waves are too small to surf unless you're an ant. Still, it's amazing to imagine the gentle sound of waves crashing on Titan's shores and how that sound might change after rains, when a methane spill spills into the heavier seas.