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, T