A perplexing Saturn moon mystery appears to be solved at long last.
Strange bright patches observed in the southern tropical regions of Saturn's biggest moon, Titan, more than a decade ago are likely the beds of dried-up hydrocarbon lakes and seas, a new study reports.
The results could shed light on Titan's climate history and also inform the hunt for potentially habitable environments on alien planets, study team members said.
Between the years 2000 and 2008, the big radio telescopes at Arecibo Observatory in Puerto Rico and the Green Bank Observatory in West Virginia spotted about a dozen anomalously bright regions on the 3,200-mile-wide (5,150 kilometers) Titan, the second-largest moon in the solar system.
At the time, the patches were viewed as likely evidence of lakes or seas on Titan, which was widely expected to harbor such bodies, said study lead author Jason Hofgartner, of NASA's Jet Propulsion Laboratory in Pasadena, California.
That early expectation was borne out after NASA's Cassini spacecraft arrived in orbit around Saturn in 2004. Cassini observed many lakes and seas on Titan and showed that the moon has an active weather system based on liquid hydrocarbons. Methane and ethane fall from the sky as rain, course down river systems and pool in lakes and seas, some of which are bigger than North America's Great Lakes.
Titan remains the only cosmic body beyond Earth known to harbor bodies of stable liquid on its surface.
But the lakes spotted by Cassini sit primarily near Titan's poles, especially the moon's far northern reaches. The probe didn't see bodies of liquid where the patches detected by the Arecibo and Green Bank dishes lie, in the southern tropics.
So Hofgartner and his colleagues decided to delve into the mystery. They pored over all the available data sets, using Cassini's observations to "ground-truth" the information gathered by Arecibo and Green Bank.
The researchers tied the reflective patches identified by the radio telescopes to a single "terrain unit," which has smoother surfaces and a different composition than the surrounding landscape. Such features are characteristic of dry lake or sea beds, Hofgartner said.
Here on Earth, for example, evaporated seas leave behind relatively flat surfaces that are saltier than their surroundings. Vanished Titan seas would not have been salty, but they may well have contained dissolved organic molecules that could make a similar mark on the landscape.
"The preponderance of evidence all seems to be consistent with that" explanation, Hofgartner told Space.com. The evidence includes dry lake beds Cassini observed in the polar regions, which look a lot like what the radio dishes saw closer to the equator.
The team cannot rule out the possibility that the mysterious bright regions are actually shallow pools of recently fallen hydrocarbon rain. But that seems quite unlikely, given how infrequently cloudbursts occur on Titan, Hofgartner said. During its 13-plus years in the Saturn system, Cassini observed just two rainfall events — one in 2004 and another one in 2010.
The new study opens a window on Titan's climate evolution, showing that conditions on the big moon have changed over time. But the nature and extent of that change aren't clear at the moment. For example, have liquid hydrocarbons just shifted position on Titan's surface, moving from the tropics to the poles? Or are the dry lake beds evidence that Titanic methane and ethane are being depleted, and will eventually disappear completely? Solar radiation, after all, is known to destroy methane in the big moon's atmosphere.
"My personal suspicion is, there's going to be a little bit of both at play here," Hofgartner said.
The new results could also be useful to astrobiologists, planetary scientists and anyone else interested in characterizing potentially habitable environments on alien worlds, he added. The Cassini, Arecibo and Green Bank data show that anomalous bright patches by themselves are not sufficient evidence to establish the presence of a current-day lake or sea, be it filled with water or methane; such features could just show where such a body used to be.
Indeed, active lakes and seas on Titan have a different signature — a brightness that's much more coherent, as a result of these bodies' incredible smoothness. (Winds on Titan are virtually nonexistent, so the moon's lakes and seas are almost mirror-flat.)
"It's telling us that we need to make sure we're really careful," Hofgartner said of the new study.
And there's another message as well, he stressed. Though the Cassini spacecraft is dead — its handlers steered the low-on-fuel probe intentionally into Saturn's atmosphere in September 2017 — the mission will keep on giving to the scientific community for many years to come.
"I think this just speaks to how wonderful a mission Cassini was," Hofgartner said. "There's still lots of work to be done analyzing Cassini data, and I think there are amazing discoveries just waiting to be found in that data set."
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Mike Wall is the author of "Out There" (Grand Central Publishing, 2018; illustrated by Karl Tate), a book about the search for alien life. Follow him on Twitter @michaeldwall. Follow us on Twitter @Spacedotcom or Facebook.