Earth isn’t the only world with lakes. Saturn’s moon, Titan, is another place where liquid rains, evaporates and seeps into the surface.

During its final flyby of Titan in 2017, NASA’s Cassini spacecraft was gathering radar data. Therefore, revealing that the small liquid lakes in Titan’s northern hemisphere are surprisingly deep, perched atop hills.

New data confirms just how deep some of Titan’s lakes are (more than 300 feet, or 100 meters). Scientists believe they formed when ice and solid organics dissolved and collapsed around them.

“Titan is the only world outside the Earth where we see bodies of liquid on the surface,” Rosaly Lopes, a planetary scientist at NASA’s Jet Propulsion Laboratory who worked on the Cassini mission but wasn’t involved in either of the new papers. “Some of us like to call Titan the Earth of the outer solar system.”

However, instead of water, hydrocarbons like liquid methane and ethane fill Titan’s lakes. But these hydrocarbons are not gas. Instead, Titan is so cold that they behave as liquids, like gasoline at room temperature on our planet.

Researchers previously had found evidence that Titan’s lakes shrink during the moon’s summer. But recent data analyses from the defunct Cassini spacecraft, reveal lakes completely disappearing off the surface of the moon. Researchers have published their new findings online April 15 in Nature Astronomy.

Planetary scientist Shannon MacKenzie and colleagues uncovered the disappearing lakes by comparing Cassini observations from two different seasons of Titan’s year, which lasts 29.5 Earth years. In the midst of Titan’s winter in 2006, Cassini’s radar observations indicated that liquid fills all three lakes. But when scientists trained Cassini’s infrared cameras on the lakes in 2013, during the moon’s spring, all three had dried up.

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“The fact that they just do not look like liquids at all to the [infrared] instruments is so weird,” says MacKenzie, of Johns Hopkins University Applied Physics Laboratory in Laurel, Md.

The observations suggest that the liquid either evaporated or seeped into the surrounding planetary surface. These “phantom lakes” may be evidence of seasonal changes on the moon, researchers suggest. They may have been extremely shallow. Perhaps just centimeters deep, and evaporated or seeped into the ground as winter turned to spring.

That suggests that below Titan’s surface, the moon may host yet another feature reminiscent of Earth: caves. On Earth, many caves are formed by water dissolving away surrounding rock types like limestone. Thus, leaving behind a type of landscape called karst, characterized by springs, aquifers, caves, and sinkholes.

They also haven’t spotted channels connecting all these different liquid features, which is why Mastrogiuseppe and others suspect that some of the liquid may be seeping into the surrounding terrain, much like karst systems here on Earth.

“Every time we make discoveries on Titan, Titan becomes more and more mysterious,” said lead author Marco Mastrogiuseppe, Cassini radar scientist at Caltech in Pasadena, California. “But these new measurements help give an answer to a few key questions. We can actually now better understand the hydrology of Titan.”

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Thumbnail image: The largest moon of Saturn and the second in our solar system. Credit: NASA/JPL-Caltech/Univ. Arizona/Univ. Idaho