Smooth as Glass

Titan's Ontario Lacus
Titan's Ontario Lacus

The dark spot in the middle of this image is Ontario Lacus, what is thought to be a lake filled with liquid methane or ethane near the south pole of Titan, Saturn’s largest moon.


Scientists are not in full agreement whether or not these features on Titan are filled with liquid or dry material, but the radar data transmitted back from the Cassini orbiter during its recent flybys seem to indicate liquid-filled features on the surface of the frigid, overcast moon.

Radar images do not work like visible light photos, instead they indicate rugged, irregular terrain as light areas and smoother, highly-reflective areas as dark. The “lakes” show up as dark areas, having smooth surfaces highly reflective to radar. The visual translations of the radar maps seem to also indicate familiar lake-shaped structure to the dark areas, nearly identical to lakes on Earth.

The difference here though, at least in Ontario Lacus’ instance, is that there seems to be no surface movement at all in the lake. The radar data shows less than a 3mm variance in the surface….it’s literally as smooth as glass.

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

Although the weather this time of year on Titan’s south pole is expected to be calm, a body of liquid as large as Lake Victoria would logically have some wave motion in it. In fact, due to the gravity and the estimated viscosity of liquid hydrocarbons, waves on Titan have been computer-modeled to be seven times as high and long as they would be on Earth under similar conditions. Yet the Ontario Lacus data shows almost no height variance at all, leaving researchers puzzled. Could the liquid ethane be just that thick, and smoothed over like tar? Since the exact properties of large bodies of cold liquid ethane aren’t yet known, it’s hard to say.

Stanford University researcher Howard Zebker says the only way to know for certain is to “have your next probe plunk down in the middle of a lake. Anything else is an indirect measurement and some kind of a model.”

Titan is the only other world in our solar system found to have an active weather cycle, where liquids from the surface evaporate into the atmosphere, form clouds, and return to the surface as precipitation. But instead of being water-based, like here on Earth, the process operates in extremely cold conditions and ethane and methane serve as the liquid.


Image: NASA/JPL/Space Science Institute