Cassini might have gone to the great beyond, but radar data from its last flybys of Titan has recently revealed something really bizarre about Saturn’s moon.
While there is an entire subsurface ocean rumored to lurk beneath the icy surface of Jupiter’s moon Europa, Titan is the only planetary body in our solar system, other than our own planet, that is known to have stable liquid on its surface. It just rains methane and ethane instead of water over there (no big deal), and these gases turn to liquid in the freezing climate. We thought we know how those lakes formed — but new research published in Nature Geoscience suggests their origins were more explosive.
Previously, a phenomenon on Earth was thought to explain how the mysterious lakes on Titan formed. Karstic lakes occur when water dissolves surrounding limestone, and scientists have been convinced that the liquid methane on Titan has done kind of the same thing. Now it is thought that these lakes actually happened when craters exploded into being because of subsurface bubbles of liquid nitrogen that finally burst as the crust warmed.
No wonder radar images some of the smaller lakes on Titan showed them to have rims so steep that they tower above its chemical oceans. The formation process of karstic lakes can’t really explain that. These sharp, sheer rims are definitely leftovers from craters blowing up.
"The rim goes up, and the karst process works in the opposite way," said team lead Giuseppe Mitri of Italy's G. d'Annunzio University. "We were not finding any explanation that fit with a karstic lake basin. In reality, the morphology was more consistent with an explosion crater, where the rim is formed by the ejected material from the crater interior. It's totally a different process."
Methane on Titan has behaved like a greenhouse gas for at least the last half-billion, and probably the last billion, years, keeping the moon warm, or at least as warm you can get on Titan. The warming of the surface that led to craters exploding also suggests that the moon, however frigid it is now, could have gone through several ice ages and warmer eras, since methane will get depleted by sun-powered chemistry and then resupplied.
"These lakes with steep edges, ramparts and raised rims would be a signpost of periods in Titan's history when there was liquid nitrogen on the surface and in the crust," Mitri said. "Even localized warming would have been enough to turn the liquid nitrogen into vapor, cause it to expand quickly and blow out a crater."
Cassini left behind even more data, so who knows what else is buried in the data it collected before plunging to its demise in Saturn’s atmosphere.