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By Leah Crane
If you visited Saturn’s biggest moon a few hundred million years ago, you may have witnessed explosions that left behind huge craters. Those craters would have turned into some of Titan’s strange lakes of liquid methane, the only lakes we’ve ever seen on a world other than Earth.
The simplest way lake basins form is by liquid dissolving the ground into a divot, so that is how we have long assumed that Titan’s lakes were formed. But NASA’s Cassini spacecraft found some lakes with rims around them that could not have been formed that way.
These lakes also tend to be irregularly shaped rather than round, which led Jonathan Lunine at Cornell University in New York and his colleagues to discount the possibility that they were formed from impacts. Instead, the team say these craters may be remnants of explosions just beneath Titan’s surface hundreds of millions of years ago.
“They’re not very circular which implies that there are multiple popping points, which we also see in explosion craters on Earth,” says Lunine. On Earth, this kind of popping happens when hot magma seeps up from underground, hits cool water and explodes.
On Titan, the most likely way to make such an explosion is with underground liquids that get warmed up, perhaps by an asteroid strike or a change in the convection of the icy moon’s core, and turn into pressurised vapour. “There isn’t enough oomph in the methane to really do this,” says Lunine. Even though methane is the main liquid on Titan now, it would not explode to create these craters.
But nitrogen would, and there is reason to suspect that earlier in Titan’s history it may have had a colder, more nitrogen-rich atmosphere and liquid nitrogen on its surface. The researchers calculated that exploding nitrogen could have led to the weird lake beds that we see today.
“Multiple processes form lakes on the Earth, so why not on Titan?” says Lunine. “Titan is very much like the Earth in having multiple geological processes that produce features rather than being driven by just one or two processes like some moons in our solar system.” These explosions are yet another way that Titan is more similar to Earth than to any other moon we know of.
Journal reference: Nature Geoscience, DOI: 10.1038/s41561-019-0429-0
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