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

Two new papers explore how this eerily familiar, waterless "water cycle" manifests on Titan's surface. To do so, two separate research teams turned to data from the Cassini mission, which ended its stay at the Saturn system in September 2017. The spacecraft flew past the massive moon more than 100 times, gathering crucial observations of this strange world as it did so.

Some of those observations showed scientists something truly extraordinary: their first glimpse of liquid currently on the landscape, rather than mere ghosts of such liquid features. "Titan is the only world outside the Earth where we see bodies of liquid on the surface," said Rosaly Lopes, a planetary scientist at NASA's Jet Propulsion Laboratory who worked on the Cassini mission but wasn't involved in either of the new papers. "Some of us like to call Titan the Earth of the outer solar system."

An image showing the surface of Saturn's moon Titan. (Image: © NASA/JPL-Caltech/ASI/USGS)

Some of those observations showed scientists something truly extraordinary: their first glimpse of liquid currently on the landscape, rather than mere ghosts of such liquid features. "Titan is the only world outside the Earth where we see bodies of liquid on the surface," Rosaly Lopes, a planetary scientist at NASA's Jet Propulsion Laboratory who worked on the Cassini mission but wasn't involved in either of the new papers. "Some of us like to call Titan the Earth of the outer solar system."

"Titan is the most interesting moon in the solar system. I think that gets me some enemies, but I think it's actually true," Shannon MacKenzie, lead author on one of the new studies and a planetary scientist at Johns Hopkins University's Applied Physics Laboratory, told Space.com. But that doesn't mean the moon is straightforward. "Titan throws us a lot of curveballs," she said.

MacKenzie's study analyzes one potential curveball: three small features that appeared to be liquid-filled lakes when Cassini first spotted them, but seem to have dried up by the time the spacecraft returned to the area. The observations suggest that the liquid either evaporated or seeped into the surrounding planetary surface.

These "phantom lakes" may be evidence of seasonal changes on the moon, MacKenzie and her coauthors believe. (Seven Earth years passed between the spacecraft's two observations of the area, during which the northern hemisphere of the moon transitioned from winter to spring.)

But the situation may not be quite that simple, since the two sets of observations were taken by different instruments. Cassini was built to gather data with either its radar instrument or its visual and infrared light cameras, but not both simultaneously. And during the spacecraft's first pass, the region was too dark to use the cameras.

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