One of the biggest discoveries made by the Cassini spacecraft is that Titan—the mammoth moon of Saturn—has lakes of liquid methane and ethane on its surface. Radar maps of the surface of Titan confirmed that the north pole is dotted with them, and combined cover far more of the surface of that moon than the Great Lakes do the Earth.

Smooth lakes of liquid natural gas don’t reflect radar waves well, so the maps made of Titan show the lakes as dark. Cassini’s instruments are sensitive enough that they have even ruled out constant waves on the lakes; they would show up as bright streaks in the images. The lakes are extremely smooth.

So what’s going on with the images above? In 2007, radar maps showed Ligeia Mare very near the moon’s north pole, looking pretty much as usual. The lake looks dark, and solid material (land) shows up as white. But in 2012 a new feature appeared, just off shore! It disappeared, but then turned up again in radar maps taken in 2014 … but shaped differently.

What the what? What are we seeing here? Fun answer: No one knows.

Scientists have apparently ruled out errors in the imaging techniques or artifacts in the detector, meaning whatever this thing is, it’s real.

It could be any number of things. Clearly it’s some transient feature, something that can come and go. That indicates it’s probably not solid land. It could be waves, or some sort of solid material just beneath the surface (methane ice)?

My first thought was bubbles, and I was pleased to see this on the list of candidates. Titan has seasons, and summer is coming for the moon’s northern hemisphere. The warming temperatures could be releasing bubbles buried in sediment under the lakes, for example, or the liquid could be warming up enough to release dissolved gases.

Right now no one knows, which is wonderful. A mystery! And it’s a good one.* Saturn and its system are full of ‘em. But what this does show is that even in the outer solar system, where temperatures reach a balmy -180° C, worlds can still be dynamic, interesting places. So much so that even after 10 years in orbit there, Cassini still has the ability to amaze and delight us.

*I’ll admit I’m still hoping for sea monsters. The biggest lake on Titan is named Kraken Mare!