The kidney-shaped feature in this image of Titan’s south polar region is Ontario Lacus, which is thought to be filled with liquid hydrocarbons (Image: NASA/JPL/Space Science Institute)

The largest lake on Saturn’s moon Titan is as smooth as a mirror, varying in height by less than 3 millimetres, a new study shows. The find, based on new radar observations, adds to a deluge of evidence that the moon’s lakes are indeed filled with liquid, rather than dried mud.

“Unless you actually poured concrete and spread it really, really smoothly, you’d never see something like that on Earth,” says team member Howard Zebker of Stanford University.

Astronomers have waffled on whether Saturn’s largest moon is dry or wet, but the bulk of the evidence points to liquid lakes.


The radar on the Cassini spacecraft, which arrived at Saturn in 2004, turned up dark splotches at Titan’s poles. The darkness in radar indicates those regions are very smooth, like the signal expected from the surface of a liquid lake.

Converging evidence

Spectral data also showed that the apparent lakes seem to be filled with methane and ethane, which would be liquid on Titan’s frigid surface. And “geomorphologically, they just look like lakes“, Zebker says.

But previous radar observations viewed the apparent lakes at an angle, and therefore did not see bright radar glints reflected back from their surface, leaving open the possibility that the features were dry lake beds or patches of soot.

Now, researchers report seeing just that signal. In December, Cassini pointed its radar straight down over Titan’s largest lake, Ontario Lacus, which spans 235 kilometres at the moon’s south pole. The reflected signal was so strong, it maxed out the probe’s receiver.

“It’s like if you had a flashlight and you shined it straight down at a mirror, and it blinds you when you look down at it,” says lead author Lauren Wye of Stanford.

Weather cycle

The radar echoes revealed a surface covering thousands of square metres whose height varies by less than 3 millimetres – 10 times as flat as previous measurements were able to reveal. “It’s very hard to imagine a solid surface that is smooth on the order of millimetres,” Wye told New Scientist.

This provides strong evidence that the lake is currently liquid, not dried mud. “If you’ve ever walked outside and seen an area on the ground where there’s mud and the water dries up, even that is pretty flat – but you get cracks in the mud and pieces that curl up,” Zebker says. “You never see anything as smooth as what we’re inferring for Titan’s surface.”

Confirming the presence of liquid on Titan adds to the long list of similarities between Titan and Earth. Titan is the only other body in the solar system to show evidence of an active weather cycle, where liquid evaporates from lakes to form clouds, and rains back down to the surface, forming rivers and channels. This makes Titan one of the best candidates in the solar system for supporting extraterrestrial life.

Skipping rocks

But if it is liquid in the lakes, the winds must be unusually still. Earlier computer simulations predicted that waves on Titan would be seven times as high and long as those on Earth.

“This runs counter to that,” Zebker says. “In this one particular lake, the largest lake on Titan and therefore likely to be the roughest, we see no evidence whatsoever of big waves. You could skip rocks really well.”

It could be that the liquid is thick and viscous, like honey, though the material properties of methane and ethane at low temperatures are not well known.

Calm weather

It could also be that changing seasons have calmed the weather at Titan’s poles. The seasons change slowly – Titan’s year is 30 Earth-years long – and Titan’s south pole is currently shifting from summer to fall.

“This is the time of year that we would expect the polar regions to be quiescent,” says Jonathan Lunine of the University of Arizona.

Taken together, the evidence for liquid is convincing, Lunine adds. “I think Ontario is filled with liquid,” he told New Scientist. “Liquid is the simplest, most consistent explanation for all the data sets.”

But Zebker cautions that the only way to know for certain is to jump in. “You want to have your next probe plunk down in the middle of a lake,” he says. “Anything else is an indirect measurement and some kind of a model.”

Journal reference: Geophysical Research Letters (DOI:10.1029/2009GL039588)