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Saturn's largest moon, Titan, is a true space oddity. It is the only one in the world that has a thick, dense atmosphere, and it is the only extraterrestrial object we know of to be persistent lakes and seas-though ones filled with liquid methane and ethane rather than water.
Now, with the help of the spacecraft, a team of scientists has suggested that some of Titan's liquid-filled basins are even more bizarre than imagined: Based on their size, shape, and freakish features, these lakes may have been formed by underground explosions.
As the researchers report this week in Nature GeoscienceSome of the moon's small lakes have unusually high rims, which make them appear similar to volcanic craters on earth that were created through underground blasts. In Titan's case, the violent excavation of these craters has been triggered by the explosive release of nitrogen gas trapped beneath the moon's surface.
The team's model is far from a slam dunk, because it's unclear what it's like to be able to go out of the water and smell it outbursts.
But solving the riddle could reveal plenty more about this odd moon's biologic-and maybe even biologic-history.Carbon-based molecules found inTitan's seas and skies suggest it has the building blocks for some kind of life, and it can help to make it easier.
From Earth to maars
Although it's a long time ago, we've seen many collisions, many of the Titan's lakes are irregularly shaped and do not look like typical impact craters. Some scientists do not know that ponderous chemical erosion chewed out basins. It is possible that Titan, liquid methane might dissolves a "bedrock" made of organic compounds and water-ice.
Titan's lake basins to volcanic features on our world named maars. These craters can be circular or more erratically shaped, looking somewhat like the chasms left behind by underground nuclear weapons tests. Maars appear when magma mixes explosively with groundwater, triggering eruptions of fresh volcanic material, or when hot rock superheats this water, creating bursts of steam that fling rock into the sky. When the blasts stop, maars often then fill with water.
The new study painstakingly compared Titan's lake basins with terrestrial maars, and the researchers that titanic lakes with raised rims and jagged, rampart-like borders really do look like they have filled with liquid methane.
But there's a snag: "We actually have no unimpeachable evidence for volcanic features on Titan," says study co-leader Jonathan Lunine, a planetary scientist at Cornell University.
The answer can be taken from the moon's ancient cycles of heating and cooling. Today, Titan is a frigid -290 degrees Fahrenheit at the surface, but with many methane-depleted, nitrogen-rich conditions providing a pretty pathetic greenhouse effect.
During these extra-chilly times, the moon has been created, which in turn created underground reservoirs of the frigid stuff. It would have been coldest at the poles, so that's where liquid nitrogen caches would have concentrated.
The liquid nitrogen would have been relatively unstable, though, and just a little bit of subsurface heating would have turned it into a gas. Effortless vaporization would have created pockets of high-pressure nitrogen gas, and the gas expansion could then be crust-excavated paroxysms.
When Titan 's climate warmed and methane and ethane began to dominate the hydrological cycle, these liquid hydrocarbons would have resulted in the creation of new watersheds.
The missing cook
The potential for explosivity is controlled by how much methane is in the liquid nitrogen mix, says study leader Giuseppe Mitri, a geoscientist at Italy's Università d'Annunzio. That means we are unlikely to test the theory by subsurface explosions on today's methane-rich Titan.
The team's model can not yet pinpoint a source of heat that might have been used to cure those ancient nitrogen caches. There are a few options, Lunine says, including warmth emerging from a convecting core powered by radioactive decay.
Each idea has good points, and a few holes. Pun intended.
NASAs Jet Propulsion Laboratory
Michael Malaska, a scientist at NASA 's Jet Propulsion Laboratory, who was not involved with the work. Dragonfly, a nuclear-powered dual quadcopter arriving on Titan in 2034, said Lunine said, but it may still find evidence of internal activity that could not be ignored.
If these really are Titan's maars, Malaska adds, then it would explain features like those jagged ramparts, but the lack of a clear explosion trigger is problematic. By contrast, the erosion model can not be done for the first time, but it is a good job at explaining how to make the pits in the first place. Right now, no one can say for sure which model, if one, may emerge victorious.
"Every idea has good points, and a few holes," Malaska says. "Pun intended."
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