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Bright Patches on Saturn’s Largest Moon Are Dried-Up Lake Beds

New study tackles a 20-year-old mystery about Titan, the second-largest moon in the solar system

A near-infrared, color image taken by NASA's Cassini spacecraft shows the sun glinting off of north polar seas on Saturn's largest moon, Titan. Similar glints were spotted from Earth in 2000 at Titan's equator but Cassini found no evidence of liquid there. (NASA / JPL-Caltech / Univ. Arizona / Univ. Idaho)
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New research suggests mysterious bright spots seen at the equator of Saturn’s moon Titan may be dried up lake beds, reports Lisa Grossman for Science News. The new research, published this week in the journal Nature Communications, offers an explanation for a phenomenon first observed in 2000.

Titan is the ringed planet’s largest moon and is the second largest moon in the entire solar system. The mega-moon is also the only one known to have a substantial atmosphere.

Between 2000 and 2008, radio telescopes at the Arecibo Observatory in Puerto Rico and the Green Bank Observatory in West Virginia identified roughly a dozen spots at Titan’s equator that were bouncing anomalously bright radio signals back to Earth, reports Mike Wall for Space.com. Such signals, called specular reflections, happen when radio waves bounce off a surface at the same angle they went in at, like the sun glinting off a mirror, explains Grossman in Science News.

At the time, the prevailing wisdom was that Titan’s equatorial specular reflections were essentially sun glints on the surface of large bodies of liquid, which researchers had long suspected existed on the moon’s surface, Jason Hofgartner, a planetary scientist at NASA's Jet Propulsion Laboratory and the study’s lead author, tells Space.com.

When the Cassini spacecraft reached Saturn in 2004, more than 500 lakes and seas were spotted in the images Cassini collected. Those lakes and seas are filled with liquid methane and ethane. Titan is still the only place in the solar system besides Earth with stable liquid on its surface in the form of lakes and seas, Hofgartner tells Science News.

The catch of Cassini’s observations was that these lakes and seas were concentrated around Titan’s polar regions. None were seen at its equator, where the specular reflections were observed by the radio telescopes beginning in 2000, the researchers write.

To figure out why, Hofgartner and his colleagues went back to the data from Arecibo and Green Bank using the up-close images from Cassini to “ground truth” the telescopes’ observations, per Science News.

By using all these data streams in combination, the researchers were able to pick out a few specific places on the moon’s surface that had produced the specular reflections. The spots stood out from the surrounding landscape because they were smoother and looked to have a distinct composition, according to Space.com.

After considering a few options, the team concluded that the most likely explanation was that the equatorial bright spots were in fact dried up lake beds, similar to others seen in Titan’s wetter polar regions, per the study.

Other potential explanations that the researchers deemed less likely to explain the phenomenon include pooling rainfall—methane rain periodically falls from clouds in Titan’s atmosphere—and dunes. Science News reports that the researchers deemed Titan’s rainfall too infrequent to be the likely culprit and ruled out dunes because they’re located on the wrong parts of the moon.

Hofgartner tells Space.com that the dried up lake beds were probably depleted of their liquids by some mixture of solar radiation and natural shifting towards the poles as part of Titan’s methane cycle.

The study’s results may have implications for the search for far away planets with the potential to harbor life, which frequently involves looking for evidence of liquids such as water.

“The lesson is that we have to be very, very strict … when we’re trying to find, say, oceans on other planets,” Hofgartner tells Science News.

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