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Imagine a world where the rain falls, gathers in lakes and ponds, infiltrates the surrounding rock and evaporates to no longer fall. There is just one catch: the world is the moon of Saturn, Titan, where the rain is not water; it's liquid methane.
Two new articles explore how this oddly familiar, waterless "water cycle" is manifesting itself on the surface of Titan. To do this, two separate research teams turned to data from the Cassini Mission, who ended his stay at Saturn in September 2017. The spacecraft flew over 100 times over the massive moon, collecting crucial observations about this strange world.
Some of these observations have shown scientists something truly extraordinary: their first glimpse of the liquid currently in the landscape, rather than mere ghosts of these liquid features. "Titan is the only world outside the Earth where we see liquid bodies on the surface," said Rosaly Lopes, scientist in planetary science at NASA's Jet Propulsion Laboratory, who had worked on the Cassini mission but was not involved in none of the new documents. "Some of us like to call Titan the Earth's outer solar system."
Related: NASA could decide this year to put a drone on Titan, the lunar of Saturn
"Titan is the most interesting moon in the solar system, I think it gives me enemies, but I think that's the truth." Shannon MacKenzie, senior author of one of the new studies and global scientist at Johns Hopkins University's Applied Physics Lab, told Space.com. But that does not mean that the moon is simple. "Titan throws us a lot of bullets," she said.
MacKenzie's study analyzes a potential curve: three small features that seemed to be lakes filled with liquid when Cassini spotted them for the first time, but seems to have been drained by the time the spacecraft returned to the area. The observations suggest that the liquid has evaporated or infiltrated into the surrounding planetary surface.
These "ghost lakes" can be the proof of seasonal changes on the moon, MacKenzie and his coauthors believe it. (Seven terrestrial years elapsed between the two spacecraft observations on the region, during which the northern hemisphere of the moon moved from winter to spring.)
But the situation may not be so simple, the two sets of observations being taken by different instruments. Cassini was designed to collect data with its radar instrument or infrared and visual cameras, but not both simultaneously. And during the first pass of the probe, the region was too dark to use the cameras.
MacKenzie and his colleagues therefore had to consider changing instruments as a potential variable. But she is still convinced that something different in both passages and that it is quite plausible that liquid was there, then disappeared. Even though the different signals on both overflights were caused by another phenomenon, MacKenzie said that she was still intrigued by what that might tell us about the strange moon, which is one of the plausible candidates scientists for a life beyond the Earth.
"If we look at more newly identified materials on the surface, it's also interesting because the sediments on Titan are really important for prebiotic chemistry," MacKenzie said.
But while MacKenzie focused on three small lakes that seem to have disappeared, many lakes remained visible during Cassini's observations of the area. In the second article published today, scientists used radar data to study a handful of much larger lakes.
During Cassini's last run over Titan in April 2017, the aircraft had been programmed to collect a very specific type of data, called altimetry, on the lake region in order to measure the height of different substances. Marco Mastrogiuseppe, scientist in planetary sciences at Caltech, had already used similar data to measure the depths of some of Titan's seasbodies of liquids much larger and the team Cassini hoped to do the same with the lakes.
Mastrogiuseppe and his colleagues did in their new document, identifying the bottoms of lakes more than 100 meters deep and establishing that their content was dominated by liquids. methane. "We realized that the composition of the lakes is very very similar to that of the sea, the sea," he said. "We think that these bodies are fed by the local rains and that these basins drain the liquids."
This suggests that under the surface of Titan, the Moon could still host another feature reminiscent of that of the Earth: the caves. On Earth, many caves are formed by water that dissolves the surrounding rock types like limestone, leaving behind a type of landscape called karst, characterized by springs, aquifers, cellars and sinkholes.
Researchers studying the Titan Lake District believe they have characteristics similar to those of karst. They also did not find any chains linking all these different liquid characteristicsThis is why Mastrogiuseppe and others suspect that liquid seeps into the surrounding terrain, much like the karst systems of the Earth.
"Titan is really this world geologically similar to Earth, and studying the interactions between liquid bodies and geology is something we have not really done before," said Lopes. New studies are beginning to make this possible by seeing these interactions unfold live on another planetary body.
Of course, it is much more difficult to study these interactions so far, in a world that has never been central to the mission. "We talked about possible missions with robotic explorers that could crawl into lava tubes and caves on the Moon and Mars, "said Lopes. Could we in the future send one of them to somehow crawl on this ground and into caves and find out what lies underneath? "
Such a mission will probably not happen soon, but NASA is seriously considering a project called Dragonfly it would land a drone on the strange moon. If selected, the mission will be launched in 2025 and will reach Titan nine years later. And if NASA does not choose Dragonfly, there is a good chance that another mission concept is coming up. "Titan is just too cool not to go back," MacKenzie said.
Both MacKenzie& # 39; sand MastrogiuseppeThe articles were published today (April 15) in the journal Nature Astronomy.
Email Meghan Bartels to [email protected] or follow her @meghanbartels. follow us on Twitter @Spacedotcom and on Facebook.
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