On Earth, water spends eons eroding rock to carve magnificent canyons. It turns out that the same process occurs on Titan, Saturn's largest moon. But instead of liquid water, the frigid moon's grand canyons are full of liquid methane.
The canyons, which appear to contain the same liquid methane seen in Titan's oceans, represent the first direct evidence of liquid-filled channels on the moon.
Titan is widely considered to be the most Earthlike world ever studied, thanks to its dense atmosphere and stable lakes, rivers and oceans. But while that thick atmosphere produces some strangely Earthlike processes — like a rain cycle — temperatures that hover around -290 degrees Fahrenheit turn familiar phenomena topsy-turvy. At those temperatures, any H2O on the surface would be hard as rock (not unlike the mountains of water ice on Pluto). But methane — which melts into liquid under Titanesque conditions but would boil away into gas given just a few tens of degrees more warmth — flows through the moon's uncanny valleys with ease.
How do scientists know these distant canyons are full of liquid? There are a few clues. For starters, there are images that show the channels as dark compared with the higher surfaces around them. Scientists suspected that these regions were full of liquid methane, but they couldn't be certain the coloring wasn't due to shadows or solid frozen sediment.
By bouncing radio signals from Cassini off of Titan's surface, the researchers were able to map out the canyons themselves and found that the bottoms of the features are remarkably flat and generally sit at the same level as the moon's seas. The smoothness of the features suggests flowing water created them (it's hard to imagine another geological process creating something so flawless).
Hayes and his colleagues aren't sure exactly what happened that sent Titan's methane into these gorgeous gorges. But in puzzling out the origin story of this Earthlike-but-not moon, they can improve our understanding of basic geological processes on our planet.
“On Earth we can’t vary the conditions like surface temperature and atmospheric density to see how geologic processes would behave,” Rosaly Lopes, a planetary geologist at NASA’s Jet Propulsion Laboratory who wasn't involved in the study, said in a statement.
We know how liquid water behaves when it flows down a hill — and how, over time, the hill beneath it is affected. But we can't ask what parts of the phenomenon are directly related to water's molecular makeup and which are inherent in the act of flowing. If liquid methane flows down a rocky hill, how does the process compare? Those are variables we can't really play with on Earth.
But on places like Titan, conditions are perfect for taking water out of the equation. In fact, some scientists have started playing with computer models to see how Titan's strange conditions might enable obscure molecules to form the building blocks of alien life.
“Although the term is overused, Titan is really a ‘natural laboratory’ for understanding geological processes,” Lopes said.
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