Cassini data sheds light on Titan’s lakes

The liquid lakes near the north pole of Titan, Saturn’s largest moon, look smaller than they are, data recorded in 2017 by NASA’s now-destroyed spacecraft Cassini reveal.

To date, Titan is the only body in the solar system, other than Earth, known to have stable surface liquid. However, while Earth’s oceans and lakes are full of water, those on Titan comprise ethane and methane.

The atmosphere on the moon is so cold that it boasts a complete cycle of precipitation and evaporation involving the two hydrocarbons, which are gaseous at Earth temperatures.

On its final flyby of Titan, en route to a fiery end on Saturn’s surface, Cassini gathered radar information regarding the lakes.

In a paper in the journal Nature Astronomy, scientists led physicist Marco Mastrogiuseppe of the California Institute of Technology, US, report that although they are only a few tens of kilometres wide they are as much as 100 metres deep.

The finding that the lakes were methane-dominated was a surprise. An earlier examination of Ontario Lacus, the only major lake in Titan’s southern hemisphere, revealed an ethane composition. 

This suggests, Mastrogiuseppe and colleagues say, that the lakes in the north and south of Titan are formed by different processes.

“It is as if you looked down on the Earth’s North Pole and could see that North America had completely different geologic setting for bodies of liquid than Asia does,” says co-author Jonathan Lunine of Cornell University, US.

The northern lakes are situated at the top of large hills, a long way above sea level. This leads the researchers to suggest that they formed when the surrounding bedrock and ice dissolved and collapsed. Roughly analogous bodies of water, known as karstic lakes, are found on Earth in limestone deposits.

Mastrogiuseppe and colleagues think that the lakes – again, in a manner similar to karstic varieties on Earth – may drain from below as well as being topped up by methane rain falling from above.

“Subsurface reservoirs and flows therefore may be an important element of the Titan geochemical system,” they conclude.

The same idea is reflected in a second, related paper in the same journal, by physicists led by Shannon MacKenzie from Johns Hopkins University, US.

This team compared Cassini data gathered on three lakes close to Titan’s northern pole during flybys in 2010 and 2017.

The results show that in the seven years intervening, all three bodies of liquid largely disappeared, prompting the researchers to dub them “phantom lakes”.

MacKenzie and colleagues suggest that these particular lakes were very shallow and thus either drained or evaporated relatively quickly.

“Short-lived lakes might be nutrient-poor and thus have low astrobiological potential,” they conclude.