Lakes on Saturn’s giant moon Titan can stratify and overturn, scientists say, much like lakes on Earth. They may also be able to produce giant gas eruptions akin to popping the cork on an enormous bottle of Champagne.
Stratification occurs when lakes form layers of different density. On Earth, this most commonly occurs when summer sun heats up the top layer of the lake, causing it to expand and literally float on top of the cooler water below — a process that is most common in temperate zones such at the US, Europe, and parts of Australia.
The same process can also occur when saltwater intrudes beneath fresh water, as sometimes occurs in caves in Mexico.
“It’s a very sharp boundary,” says Jordan Steckloff, a planetary scientist with the Planetary Science Institute who works out of Whitmore Lake, Michigan.
“If you’ve ever dived into a lake past maybe 10 to 20 feet down, it gets very cold,” he says. “That’s because you’ve passed through this layer of stratification from the warm water above to the cold water below.”
Overturning occurs when something, such as autumn cooling, reduces the density of the top layer until eventually it no longer floats, and the top and bottom layers mix. On Earth this helps cycle nutrients between the surface and the depths and is a crucial aspect of temperate-zone lake ecosystems.
Titan is both vastly different and amazingly similar.
It is different because its surface is a frigid minus 183 degrees Celsius and its lakes are made of methane and ethane like giant ponds of liquified natural gas, some of which – especially the methane – evaporates into the atmosphere to form clouds and fall back as rain.
But there is also dissolved nitrogen from Titan’s thick, nitrogen-rich atmosphere. And that, Steckloff says, turns out to be the interesting part.
Because liquid methane is less dense than liquid ethane, he says, it has long been assumed that methane-rich liquid would rise to the top of the lakes. But because it evaporates more readily than ethane, it wouldn’t accumulate there. Instead, the lake would be constantly mixing as the denser, methane-depleted liquid from the surface sinks back into the depths.
But, he says, that ignored the effect of the dissolved nitrogen.
Nitrogen, he says, dissolves much more readily in methane than in ethane. And it is heavy—heavy enough that if there is enough dissolved nitrogen, such methane-nitrogen mixtures can actually be denser than ethane (with its lesser component of dissolved nitrogen).
This, he and colleagues calculated in a paper published in the Planetary Science Journal, means that Titan’s lakes can indeed stratify as the evaporation of methane from the surface also causes the surface liquid to lose nitrogen.
The result is that there can be a buoyant, nitrogen-depleted layer dominated by ethane floating on top of a nitrogen-rich layer higher in methane.
Eventually, however, so much methane evaporates that the ethane-rich upper layer can sink, and the lake turns over, starting the process afresh.
The process, he adds, won’t work at Titan’s average temperature. That’s because the solubility of gases in liquids is highly dependent on temperature, and at of minus 183 degrees Celsius there isn’t enough dissolved nitrogen to make it happen.
But if they are just slightly cooler, it does work. And it’s easy to envision a process that could chill them, ranging from evaporative cooling to seasonal variations.
A sedate overturn, however, isn’t the only thing that can happen. There could actually be quite a bit of nitrogen gas dissolved in the lower layer, and when the overturn occurs, it could fizz out – fast. “This is where we get to the idea of an exploding lake,” Steckloff says.
“This seems strange,” he adds, but a very similar process has been seen in Lake Nyos in Cameroon, where carbon dioxide from underwater volcanism can build up in the lake’s depths.
At some point something causes the lake to overturn, and all that carbon dioxide explodes out, like opening the top of a soda can after you’ve shaken it. (Not a good thing for people who happened to be nearby at the time.)
Nobody knows how strongly this might occur in Titan’s lakes. But, Steckloff says, in lab experiments designed to mimic Titan’s lakes it can sometimes be very abrupt – abrupt enough that one of the experiments literally blew its top and threw a temperature sensor into the air.
If such things occur in Titan’s larger lakes, they would indeed be spectacular to watch. But only if your billion-dollar rover is at a very safe distance.