Imagine a world where rain falls, gathering in lakes and ponds, seeping into the surrounding rocks and evaporating until it falls again. There is only one catch: the world is the Saturn Moon, Titan where the rain is not water; it is liquid methane.
Two new articles examine how this incredibly familiar, waterless "water cycle" manifests on the surface of Titan. To this end, two separate research teams turned to data from the Cassini mission which ended their stay at the Saturn system in September 2017  . The spaceship flew past the massive moon more than 100 times, collecting crucial observations of this alien world.
Some of these observations showed the scientists something truly out of the ordinary: their first look at liquid just in the landscape and not just ghosts of such liquid features . "Titan is the only outside world where we see bodies of fluid 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 Titan the Earth of the outer Solar System."
Related: NASA may decide this year to land a drone on Saturn's Titan Titan
"Titan is the most interesting moon in the solar system, so I think that will bring me some enemies, but I think it's really true, "Shannon MacKenzie, principal author of one of the new studies and a planet scientist at the Applied Physics Laboratory at Johns Hopkins University, told Space.com. That does not mean that the moon is straightforward. "Titan throws many curvy balls," she said.
MacKenzie's study analyzes a potential curvy ball: three small features that were seemingly liquid-filled lakes when Cassini first discovered them, but apparently discovered them. When the spacecraft returned to the area, it was dried up , The observations indicate that the liquid has either evaporated or penetrated into the surrounding planetary surface.
These "phantom lakes" may indicate seasonal changes on the moon, MacKenzie and her co-authors believe. (Between the two observations of the spacecraft over the area where the northern hemisphere of the moon passed from winter into spring, seven earth years passed.)
However, the situation may not be quite as simple as the two series of observations were taken by different instruments. Cassini was designed to capture data with either its radar or its optical and infrared light cameras, but not simultaneously. And during the first sweep of the spacecraft, the region was too dark to use the cameras.
MacKenzie and her colleagues therefore had to consider changing the instruments as a possible variable. But she is still confident that something in the two passes is different and that it is quite plausible that liquid was there and then disappeared. Even though the different signals over the two fly-bys were caused by another phenomenon, MacKenzie said she was still fascinated by what could tell us about the strange moon, which is among the plausible candidates of scientists for those areas where life on Earth might be lurking] "If we look at some newly identified materials on the surface instead, that's also interesting because the sediments on titanium are really important to prebiotic chemistry," MacKenzie said.
But MacKenzie focused only on three small lakes that seem to have disappeared, leaving many lakes visible in Cassini's observations of the area. In the second study published today, scientists used radar data to study a handful of much larger lakes.
While Cassini's last pass over Titan in April 201
Mastrogiuseppe and his colleagues did this in their new work, recognizing the bottoms of lakes more than 100 meters deep and finding that their contents are dominated by liquid methane . "We have realized that the composition of the lakes is essentially similar to that of the mare and the sea," he said. "We believe that these bodies are fed by local rains, and then these basins drain liquid."
This suggests that the moon beneath the surface of Titan harbors another feature that is reminiscent of Earth: caves. On Earth, many caves are formed by water that dissolves surrounding rocks such as limestone, leaving behind a landscape type karst characterized by springs, aquifers, caves and sinkholes.
Researchers examine the lake region Titan think they see similar karst properties. Nor have they discovered any channels connecting all of these distinct different fluid traits which is why Mastrogiuseppe and others suspect that some of the fluid could penetrate into the surrounding terrain, much like karstic systems here on Earth.  "Titan is really this world that is geologically similar to Earth, and exploring the interactions between liquid bodies and geology is something we have not really been able to do before," said Lopes. The new studies are beginning to achieve this by seeing these interactions live on another planetary body.
Of course, it is much harder to study these interactions so far, in a world that was never the main focus of a world mission. "We've talked about possible missions with robotic researchers who might crawl into lava tunnels and caves on the moon and on Mars," Lopes said. "Could we send one of them in the future to crawl into this terrain and caves and find out what's in it?"
Such a mission is unlikely to happen so soon, but NASA is seriously considering a project called Dragonfly that would land a drone on the alien moon. If selected, the mission would start in 2025 and reach Titan nine years later. And if NASA does not opt for Dragonfly, chances are good that another mission concept comes along. "Titan is just cool not to go back," MacKenzie said.
Both the MacKenzie and Mastrogiuseppe were published today (April 15) in the journal Nature Astronomy.