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Titan's surface sports many small lakes, which scientists studied in two new papers.



(Image: © NASA/JPL-Caltech/ASI/USGS)

Picture a world in which rain drops, gathers in ponds and lakes, seeps into the surrounding rock, and evaporates away, just to fall again. There is only one catch: The planet is Saturn’s moon, Titan, where the rain isn’t water; it is liquid methane.

Two new papers explore how this exceptionally familiar, waterless”water cycle” manifests on Titan’s surface. To accomplish this, two separate research teams turned to data from the Cassini mission, that finished its stay in the Saturn system in September 2017. The spacecraft flew past the huge moon more than 100 times, gathering key observations of the strange world because it did so.

Some of these observations showed scientists something truly exceptional: their very first glimpse of liquid currently on the landscape, instead of mere ghosts of these liquid attributes . “Titan is the only world away from the Earth where people see bodies of fluid on the surface,” Rosaly Lopes, a planetary scientist at NASA’s Jet Propulsion Laboratory who worked about the Cassini mission but was not involved in either of the new papers. “Some of us like to call Titan the Earth of the outside solar system.”

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“Titan is the most fascinating moon in the solar system. I think that gets me some enemies, but I think that it’s actually true,” Shannon MacKenzie, lead author on one of the new studies and also a planetary scientist at Johns Hopkins University’s Applied Physics Laboratory, told Space.com. But that doesn’t mean that the moon is simple. “Titan yells us a lot of curveballs,” she explained.

MacKenzie’s study examines one potential curveball: three little features that appeared to be liquid-filled lakes when Cassini first spotted thembut appear to have dried up from the time that the spacecraft returned to the region. The observations imply that the liquid \evaporated or seeped into the surrounding surface.

These”ghost lakes” may be evidence of seasonal changes around the skies, MacKenzie and her coauthors think. (Seven Earth years passed between the spacecraft’s two observations of the region, where the northern hemisphere of the moon transitioned from winter to spring.)

But the situation may not be quite that simple, because the two sets of observations have been taken by different tools. Cassini was built to gather data with \its radar instrument or its visual and infrared light cameras, but not \simultaneously. And through the first pass of the spacecraft, the area was too dim to use the cameras.

Thus MacKenzie and her coworkers had to factor in the change in devices as a possible factor. But she is still convinced that something differs in the two moves, and that it’s fairly plausible that liquid was still there, then vanished. When some other happening brought on the different signals over the two flybys, MacKenzie said she intrigued by the things that could tell us about the moon, which is one of scientists’ candidates for where existence may be lurking beyond Earth.

“If we’re looking at some recently identified materials on the outside, then that is interesting, also, because the sediments around Titan are really important for chemistry,” MacKenzie explained.

But although MacKenzie focused on just three small lakes that appear to have vanished, lots of lakes remained observable throughout Cassini’s observations of the area. In the next paper scientists employed radar data to examine a couple of lakes.

During Cassini’s very last pass over Titan in April 2017, the spacecraft was designed to gather a very specific type of data, called altimetry, over the lake region to gauge the height of unique materials. Marco Mastrogiuseppe, a planetary scientist at Caltech, had used similar statistics to quantify the depths of some of Titan’s seas, much bigger bodies of liquids, and also the Cassini team hoped he would be in a position to do the same with lakes.

Mastrogiuseppe and his colleagues did so in their fresh paper, differentiating the bottoms of lakes more than 328 feet (100 meters) deep and establishing that their contents were dominated by fluid methane. “We realized that essentially the makeup of the lakes is very, very similar to this one of their mare, of the sea,” he said. “We think that these bodies have been fed by local drains and then these basinsthey drain liquid”

Which implies that under Titan’s face, the moon could host still another characteristic reminiscent of Earth: Earth. On Earth, several caves have been formed by water stripping off surrounding stone types such as limestone, leaving behind a type of landscape known as karst, characterized by springs, aquifers, caves and sinkholes.

Researchers studying Titan’s lake region believe they see comparable karst-type characteristics. Additionally they haven’t seen channels linking all these different liquid attributes , which is why Mastrogiuseppe and others suspect that a number of the liquid might be seeping into the surrounding terrain, so much like karst systems here on Earth.

“Titan is really this world that geologically is comparable to the Earth, also analyzing the interactions between the liquid bodies and also the geology is something that we haven’t really been able to do before,” Lopes said. The studies begin to make that occur by seeing those interactions live on the other planetary body.

Obviously, it is far more challenging to study these interactions far off, on a planet that has never become the main focus of a mission. “We have been talking about potential missions with robotic explorers that might crawl down into lava tubes and caves on the moon and Mars,” Lopes said. “Can we at the future ship one of these to sort of creep into this terrain and to caves and discover what’s under there?”

Such a mission likely won’t happen anytime soon, but NASA is seriously contemplating a job called Dragonfly which would land a drone to the strange moon. If selected, the assignment would start in 2025 and hit Titan nine decades later. And if NASA doesn’t pick Dragonfly, odds are good that another mission concept will come together. “Titan’s simply to cool to not return to,” MacKenzie said.

The two MacKenzie‘s and Mastrogiuseppe‘s papers were printed today (April 15) in the journal Nature Astronomy. 

Mail Meghan Bartels in mbartels@space.com or trace @meghanbartels. Follow us on Twitter @Spacedotcom and forth Facebook.  

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