For the first time, scientists using data from NASA’s Cassini spacecraft have identified liquid methane flowing through canyons carved into Saturn’s moon, Titan.
The findings, described in Geophysical Research Letters, fill in a picture of Titan as an increasingly dynamic, geologically fascinating world – one that, perhaps, might be friendly to microbial life.
Titan is king of ringed Saturn’s moons, more massive than dwarf planet Pluto and larger in diameter than the planet Mercury. It is also boasts the second-thickest atmosphere of the solar system’s rocky worlds, ranking below broiling Venus and above Earth.
“If it wasn’t orbiting around Saturn we would say it was a planet in its own right,” said Rosaly Lopes, a planetary geologist at Jet Propulsion Laboratory who studies Titan but was not involved in this study.
Scientists have found evidence of lakes and seas pooled near Titan’s poles, particularly in the north — making it one of the only rocky worlds in the solar system to have bodies of liquid on the surface. (Jupiter’s moon Europa and Saturn’s moon Enceladus are thought to have underground seas or oceans.) But there’s a key difference: Titan’s lakes are full of methane, not water. Methane can only exist as a gas in Earth’s environment, but it’s so cold on Titan (about minus 290 degrees Fahrenheit) that methane can remain a stable liquid on the moon’s surface. Water, meanwhile, is frozen so solid that it’s hard as rock.
Aside from that key detail, parts of Titan look remarkably familiar, Lopes said.
“The interesting thing is Titan looks very much like the Earth ... you have a lot of fundamental differences and yet the geology looks very much the same,” Lopes said. “We say that Titan is the Earth of the outer solar system.”
When the European Huygens probe detached from Cassini in late 2004 and landed on Titan in early 2005, it found icy pebbles that resembled the water-polished stones from rivers found on our own planet.
In 2012, scientists announced that Cassini had spotted Vid Flumina, a 249-mile-long river of methane that drained into Ligeia Mare, Titan’s second-largest sea. Channels branching off from the river seemed to resemble canyons but researchers couldn’t say for sure that anything was flowing through them.
It could have been a shadowing effect; there could have been some strongly light-absorbing material filling the channels, instead of liquid methane. And after all, just because something looks like a feature that’s found on Earth, doesn’t mean that it’s being caused by the same terrestrial process. (Consider the gullies on some Martian slopes that look like they’re sculpted by liquid water but in fact are probably carved by sliding slabs of dry ice – which would never happen on Earth, because our planet isn’t cold enough to keep carbon dioxide frozen solid.)
To solve this mystery, an international team of researchers used Cassini’s instruments to examine the hit the surface with radar, allowing them to determine which surfaces were solid and rocky and which were smooth liquid.
The scientists found that the canyon walls must rise at least as sharply as 40 degrees, and may plunge to a depth of 1,870 feet. The eight gorges were likely carved by flowing methane that drained into Vid Flumina on its way to the northern sea.
“Our study reports the first direct detection and characterization of liquid-filled canyons on Titan,” the study authors wrote. “Understanding the processes that led to the formation of such hydrological features will be crucial in understanding the evolution and the present state of Titan’s geomorphology. Regardless, any model of polar landscape evolution on Titan needs to explain the generation of such greatly incised, hundred meter deep canyons that drain into the Mare.”
Titan’s chemistry might be very different from Earth’s, but it does have plenty of organic molecules, including hydrocarbons like methane and ethane. Unappetizing as that may sound, it does mean that there’s a chance that microbial life could have originated here, though it would probably look wildly different from anything found on Earth.
“I’m not saying it would be life as we know it or even that it’s the most likely place in the solar system to have life but it’s certainly one of the candidate locations,” Lopes said. “It’s an interesting place to assess for habitability but we’ve still got to do that assessment.”
Scientists will continue to pore over the data Cassini has already sent back. Still, deeper study of the planet’s geology and its potential habitability may have to await a future mission to land on and explore the surface of the frigid moon.
The next moon to get its own mission will be Jupiter’s icy satellite Europa, considered by some to be the best chance for alien microbial life in the solar system.
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