Saturn’s largest moon, Titan, is a true space oddity. It’s the only natural satellite in the solar system that has a thick, dense atmosphere, and it’s the only extraterrestrial object we know of to feature persistent lakes and seas—though ones filled with liquid methane and ethane rather than water.
Now, using data from the late, great Cassini 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 Geoscience, some 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 may have been triggered by the explosive release of nitrogen gas trapped beneath the moon’s icy surface.
The team’s model is far from a slam dunk, in part because it only looks at certain lakes, and also because it’s unclear what might have heated up the nitrogen to spark such savage outbursts.
But solving the riddle could reveal plenty more about this odd moon’s geologic—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 cracking the case of the befuddling lakes may help scientists better understand how the moon crafted such intriguing ingredients.
From Earth to maars
Although it’s tempting to blame any round depressions on asteroid or comet collisions, many of Titan’s lakes are irregularly shaped and don’t look like typical impact craters. Some scientists therefore suspected that ponderous chemical erosion chewed out basins. On Earth, vulnerable rocks are dissolved by acidic waters, creating lakes, so it’s possible that on Titan, liquid methane might dissolve a “bedrock” made of organic compounds and water-ice.
But for several years, other scientists have been comparing 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 conclude that Titanic lakes with raised rims and jagged, rampart-like borders really do look like maars that have since filled up 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 may instead come from the moon’s ancient cycles of heating and cooling. Today, Titan is a frigid -290 degrees Fahrenheit at the surface, but several climate models suggest it went through even colder epochs in the past, with a methane-depleted, nitrogen-rich atmosphere providing a pretty pathetic greenhouse effect.
During these extra-chilly times, the moon may have been cold enough to host rain made of liquid nitrogen, 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’s frenzied expansion could then have led to crust-excavating paroxysms.
When Titan’s climate warmed and methane and ethane began to dominate the hydrological cycle, these liquid hydrocarbons would have flowed or rained into the resulting crater bowls, forming the lakes we see today.
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 be able to test the theory by watching for subsurface explosions on today’s methane-rich Titan.
The team’s model also can’t yet pinpoint a source of the heat that might have once cooked those ancient nitrogen caches. There are a few options, Lunine says, including warmth emerging from a convecting core powered by radioactive decay.
The best way to investigate the mystery further would be to send a spacecraft to Titan to explore the lakes up close, says Michael Malaska, a scientist at NASA’s Jet Propulsion Laboratory who wasn’t involved with the work. Dragonfly, a nuclear-powered dual quadcopter arriving on Titan in 2034, won’t go to the poles, Lunine says, but it may still find evidence of internal activity elsewhere that could hint at the origins of any nitrogen-vaporizing heat.
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’t currently account for the tall crater rims, but it does a good job at explaining how you’d make the pits in the first place. Right now, no one can say for sure which model, if either, may emerge victorious.
“Each idea has good points, and a few holes,” Malaska says. “Pun intended.”