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Lakes on Saturn’s Moon are Really Sinkholes Filled With Liquid Methane and Ethane

Strange and changeable lakes might form just as certain water-filled lakes do on Earth

Colorized radar images from the Cassini spacecraft show some of the many lakes on Titan (NASA/JPL-Caltech/ASI/USGS)
smithsonian.com

For a long time, nobody knew what the surface of Titan looked like. One of Saturn's moons, a thick atmosphere of methane and other gases kept the surface obscured. It wasn't until the Huygens probe landed on the Titan's surface, and the Cassini orbiter used its infrared and radar sensors, that scientists were able to peer beyond the haze.

With their newfound imagery of Titan, researchers learned that the moon is spotted and marked with liquid — not water, but hydrocarbons like ethane and methane. Now, a team of scientists has figure out how Titan’s lakes form, reports Jessica Mendoza for The Christian Science Monitor.

Titan is home to three large seas called mares, the largest of which (the Kraken Mare) stretches about 680 miles long. Rivers of hydrocarbons flow from them. The many shallower lakes however, are generally in flat areas and didn’t have rivers feeding them. Those depressions were a mystery for researchers who wondered how they formed, especially since they can change depth and shape. Geology on Earth gave them clues. Mendoza writes:

Though the moon’s icy surface temperatures – roughly minus 292 degrees Fahrenheit — means that liquid methane and ethane, not water, dominate its surface, Cornet and his team found that Titan’s lakes resemble Earth’s caves, sinkholes, and sinking streams.

These Earthly features, known as karstic landforms, result from erosion of dissolvable rocks, such as limestone and gypsum, in groundwater and rainfall. How fast the rocks erode depends on factors such as humidity, rainfall, and surface temperature. The scientists, assuming that Titan’s surface is covered in solid organic material and that the main dissolving agent is liquid hydrocarbons, calculated how long it would take for parts of Titan’s surface to create these features.

The team reports in the Journal of Geophysical Research, Planets that in the rainy polar regions, a 300-foot depression could form in about 50 million years. That rate is about 30 times slower than such lakes form on Earth’s surface. Closer to the equator, a drier region, the same depression might take 375 million years. 

"Of course, there are a few uncertainties: The composition of Titan's surface is not that well constrained, and neither are the long-term precipitation patterns, but our calculations are still consistent with the features we see today on Titan's relatively youthful billion-year-old surface," says Thomas Cornet of the European Space Agency in a press statement by Emily Baldwin from NASA’s Jet Propulsion Laboratory.

For his blog "Life Unbounded" at Scientific American, Caleb Scharf adds:

Once again, Titan - for all of its utterly un-earthly characteristics - is seemingly sculpted by a set of universal planetary processes. It's an excellent example of how our quest to discover and explore new worlds is ultimately deeply connected to understanding the Earth itself.

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