Picture a world where rain falls, collects lakes and ponds, sucks into the surrounding rock and evaporates, just to fall again. There is only one catch: The world is Saturn moon, Titan where the rain is not water; It is liquid methane.
Two new papers explore how this known, waterless "water bike" manifests itself on Titan's surface. To do this, two separate research groups turned to data from the Cassini mission which ended their stay on the Saturn system in September 1917 . The spacecraft flew past the massive moon more than 100 times and gathered important observations of this strange world as it did.
Some of these observations showed that scientists were truly extraordinary: their first glimpse of fluid currently on the landscape, rather than just the ghosts of such floating properties . "Titan is the only outside world where we see liquid bodies on the surface," Rosaly Lopes, a planetary scientist at NASA's Jet Propulsion Laboratory, who worked on the Cassini mission but was not involved in any of the new papers. "Some of us like to call the Titan Earth in the outer solar system."
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"Titan is the most interesting moon in the solar system. I think it gets me some enemies, but I think it's really true, "said Shannon MacKenzie, lead author of one of the new studies and a planetary scientist at Johns Hopkins University's Applied Physics Laboratory, Space.com told. But that doesn't mean the moon is uncomplicated. "Titan throws us many basket balls," she said.
MacKenzie's study analyzes a potential curve ball: three small features that appeared to be liquid-filled lakes when Cassini first discovered them but appears to have dried up at the time the spaceship returned to the area. The observations indicate that the liquid either evaporates or seeps into the surrounding planet surface.
These "phantom lakes" may be evidence of the seasonal changes on the moon, believe MacKenzie and her co-authors. (Seven years of Earth passed between the spacecraft's two observations of the area, during which the moon's northern hemisphere passed from winter to spring.)
But the situation cannot be so simple, since the two sets of observations were taken with different instruments. Cassini was built to collect data with either its radar instrument or its visual and infrared light cameras, but not both at the same time. And during the spacecraft's first pass, the region was too dark to use the cameras.
So MacKenzie and her colleagues had to factor in the change of instruments as a potential variable. But she is still convinced that something is different in the two passages, and that it is quite likely that liquid was there, then disappeared. Although the various signals over the two airbanks were caused by some other phenomenon, MacKenzie said she is still fascinated by what can tell about the strange moon, which is among scholars credible candidates for where life can lurk beyond the earth. [1
But although MacKenzie focused on only three small lakes that seem to have disappeared, much of the lakes were visible through Cassini's observations of the region. In the second paper that was published today, researchers used radar data to study a handful of much larger lakes.
During Cassini's very last pass over Titan in April 2017, the spacecraft was programmed to collect a very specific type of data, called altimetry, across the lake region to measure the height of various subjects. Marco Mastrogiuseppe, a planetary scientist at Caltech, had already used similar data to measure the depth of some of Titan's sea much larger bodies of liquid and the Cassini team hoped he could do the same with lakes.
Mastrogiuseppe and his colleagues did so in their new paper and identified the bottoms of lakes more than 328 feet deep and found that their contents were dominated by liquid . "We realized that the essentials of the lakes are very, very similar to that of the sea lakes," he said. "We believe these bodies are fed by local rain and then these waterways drain."
It suggests that under Titan's surface, the moon may host another property reminiscent of the earth: caves. On the earth, many caves are formed of water that dissolves surrounding rocks such as limestone, leaving a type of landscape known as karst characterized by feathers, water feet, caves and sinkholes.
Researchers who study Titan's lake region believe that they see similar karst type properties. They have also not discovered channels connecting all of these various floating properties why Mastrogiuseppe and others suspect that part of the liquid can seep into the surrounding terrain, just like karst system here on earth.  "Titan is really this world that geologically resembles the earth, and studying the interaction between the floating bodies and the geology is something we have not really been able to do before," Lopes said. The new studies begin to do so by seeing the interactions that play live on another planet.
Of course, it is much more difficult to study these interactions so far away, in a world that has never been the primary focus of a mission. "We've talked about possible missions with robot explorers who can crawl down in lava tubes and caves on the moon and Mars," Lopes said. "Would we be able to send one of these in the future to crawl down this terrain and into caves and find out what lies below?"
Such a mission is unlikely to happen sometime soon, but NASA is seriously looking at projects called Dragonfly that would land a drone on the strange moon. If selected, the mission would start in 2025 and reach Titan nine years later. And if NASA does not choose Dragonfly, chances are good that another mission concept will come along. "Titan is just to cool down not to go back to," MacKenzie said.
MacKenzie s and Mastrogiuseppe s paper was published today (April 15) in the journal Nature Astronomy.