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Welcome to Titan鈥檚 lake district. Situated near the moon鈥檚 north pole, this region of聽the alien world might seem quite familiar if you were to visit. Some of the lakes on Titan are at least 100 metres deep, while their neighbours are phantom lakes that drain and refill over time, as shallow lakes do in Earth鈥檚 deserts.

鈥淭he lakes and seas have very similar composition,鈥� says Marco Mastrogiuseppe at California Institute of Technology. 鈥淭his is different than Earth, where we have salty water for the sea and fresh water in the lakes. But what we observe on Titan is also similar to Earth because we have rain that feeds these lakes and subsurface drainage of the liquid.鈥�

Rather than a water cycle, Titan has a cycle of hydrocarbons 鈥� liquid methane and ethane. Using radar data from the final flyby of the Cassini spacecraft 2017, Mastrogiuseppe and his colleagues mapped the shape and depth of seven of Titan鈥檚 deep lakes. One is S-shaped, another looks a bit like a seahorse, and one was so rich in methane that it was possible to detect the floor of the lake, which sloped gently down to about 100 metres below the surface.

鈥淲e figured out we could take these measurements after Cassini left Earth, when it was on the way there,鈥� he says. 鈥淲e didn鈥檛 know the radar would be able to measure the depth because we didn鈥檛 know the composition of the liquid 鈥� but they are dominated by methane which is transparent to microwaves.鈥�

They also found that the surfaces of these deep lakes聽sit hundreds of metres above sea level, which indicates that they are fed by rainfall.聽They聽may form by a process we see on Earth called karstic formation, says Mastrogiuseppe. Soluble rocks are dissolved and washed away underground, leaving a basin that gets filled by rain or groundwater.

The team didn鈥檛 see evidence of streams or rivers, which indicates any drainage聽likely happens below the surface, but ground level runoff can鈥檛 be fully discounted because rivers could be too small for the resolution of Cassini鈥檚 instruments to pick up.

Shannon MacKenzie at Johns Hopkins University in Maryland also used Cassini data to explore the lake district. She and her team used both radar and optical imaging to measure the shoreline change in shallow lakes.

They found three phantom lakes that disappeared during Cassini鈥檚 mission. One of the lakes emptied in the six-month period between two Cassini flybys in July and October 2006, and two others appeared shallow in 2006 but dried up in 2013.

This gives us confirmation that evaporation and drainage are happening on seasonal timescales, MacKenzie says. It鈥檚 reasonable to assume it鈥檚 happening in deeper lakes, she adds, though we haven鈥檛 detected it. That鈥檚 because they have tall ramparts along their rims. It鈥檚 a bit like how it would be more difficult to tell that water had evaporated from a bucket when viewed from above than to see a shallow puddle dry up entirely.

鈥淭hey saw this change over 6 months and 7 years. That鈥檚 quick,鈥� says Rajani Dhingra at the University of Idaho, who wasn鈥檛 involved in the study. 鈥淣ow we know that the surface of Titan is porous enough that in 7 years, a lake of some depth emptied itself.鈥� She points out that limited data from Cassini means that the team saw the lake filled in using radar, and measured it empty using an infrared image.

鈥淚t鈥檚 more dependable if you see it in the same instrument, but we don鈥檛 have enough data and they did the best they could with what we have,鈥� says Dhingra.

鈥淥ne of the caveats here is that we have a compelling argument that there were liquids in the radar data, but we can鈥檛 rule out that this could instead be the first identification of a different kind of sediment whose properties may be unexpected,鈥� MacKenzie says. To confirm, they鈥檇 need reflection data 鈥� which would require another mission to Titan.

Journal reference:听Nature Astronomy, ;听