Cassini reveals surprises with the Titan lakes

On its last flyby of Saturn's largest moon in 2017, NASA's Cassini spacecraft collected radar data revealing that the small, liquid lakes of Titan's northern hemisphere are astonishingly deep, perched on hilltops and filled with methane.

The new discoveries, published on April 15 in Nature Astronomy, are the first confirmation of the depth of some Titan lakes (more than 100 meters) and their composition. They provide new information on how liquid methane evaporates and infiltrates into Titan – the only planetary body in our solar system other than the Earth that is known to have a stable liquid on its surface.

Scientists know that Titan's hydrological cycle works in the same way as Earth's, with one major difference. Instead of the water evaporating from the seas, forming clouds and rain, Titan does everything with methane and ethane. We tend to consider these hydrocarbons as a gas on Earth, unless they are pressurized in a tank. But Titan is so cold that they behave like liquids, like gasoline at room temperature on our planet.

Scientists knew that the much larger northern seas were filled with methane, but it was surprising to find that the small northern lakes, mostly filled with methane Previously, Cassini data measured Ontario Lacus, the only major lake in Canada. Southern hemisphere of Titan. There, they found an almost equal mixture of methane and ethane. Ethane is slightly heavier than methane, with more carbon atoms and hydrogen in its composition.

"Every time we make Titan discoveries, Titan becomes more and more mysterious," said lead author Marco Mastrogiuseppe, a Cassini radar researcher at Caltech in Pasadena, California. "But these new metrics help answer a few key questions.We can now better understand Titan's hydrology."

The fact that the hydrology of one side of the northern hemisphere is completely different from that on the other side, adds Jonathan Lunine, scientist and co-author of Cassini, adds to the curiosities of Titan, with its Earth-like features carved by exotic materials. Cornell University in Ithaca, New York.

"It's like you're looking at the North Pole of the Earth and you could see that the geological environments of North America were completely different from those in Asia," Lunine said.

On the east side of Titan, there are large low-lying seas, canyons and islands. In the west: small lakes. And the new measurements show the lakes perched on top of big hills and plateaus. The new radar measurements confirm previous findings that lakes are well above sea level, but they evoke a new image of landforms – such as mesas or mounds – sticking to hundreds of feet above sea level from the surrounding landscape, with deep and liquid lakes.

The fact that these western lakes are small (a few tens of kilometers) but very deep also reveals to scientists something new about their geology: it's the best evidence to date that they've probably formed when the surrounding bedrock of ice and solid organics dissolved and chemically collapsed. On Earth, similar water lakes are called karst lakes. Occurring in areas such as Germany, Croatia and the United States, they form when the water dissolves the limestone bedrock.

In parallel with research on deep lakes, a second article in Nature Astronomy helps to better understand the mystery of Titan's hydrological cycle. The researchers used Cassini's data to reveal what they call transient lakes. Different sets of observations – from radar and infrared data – seem to show that fluid levels have changed significantly.

The best explanation is that there have been some seasonal changes in surface liquids, said lead author Shannon MacKenzie, a global scientist at the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland. "One possibility is that these transient characteristics could have been less deep bodies of liquid that, during the course of the season, evaporated and infiltrated into the subsoil," she said.

These results, along with those of Nature Astronomy's paper on Titan's deep lakes, support the idea that hydrocarbon rains feed the lakes, which can then evaporate back into the atmosphere or drip into the atmosphere. basement, leaving liquid tanks underneath.

Cassini, who arrived in the Saturn system in 2004 and completed her mission in 2017 by deliberately diving into the atmosphere of Saturn, has mapped more than 1.6 million square kilometers of lakes and liquid seas on the surface of Titan. He performed the work with the radar instrument, which emitted radio waves and collected a feedback signal (or echo) providing information on the terrain, depth and composition of the liquid bodies, as well as two systems of Imaging capable of penetrating the thick atmospheric layer of the moon. mist.

The crucial data for the new research was gathered during Titini's last flyby by Cassini on April 22, 2017. It was the last look of the mission on the smaller lakes of the moon, and the team took the most advantage. The collection of echoes on the surface of small lakes while Cassini compressed by Titan was a unique challenge.

"It was the last Cassini race at Titan, and it was really a feat," Lunine said.