Saturn’s moon Enceladus doesn’t get as much attention as Europa, but it too has an internal liquid water ocean. It may also be more active than Europa with frequent geysers erupting from the surface. Years back, when NASA’s Cassini-Huygens probe explored the Saturnian system, scientists were fascinated to see how much methane was present in Enceladus’ geysers. At the time, it seemed feasible the methane was naturally occurring, but that’s less likely now that new research has ruled out all the known geochemical processes. That leaves biological sources—life—as a leading possibility.
Saturn is more distant than Jupiter, and thus its moons are even more frigid. However, like Jupiter, Saturn has a lot of gravity. As its moons orbit, the tug of Saturn’s gravity is so intense that it stretches the moon’s crust. This flexing heats the interior, a process known as tidal heating. Add together geological activity and a liquid ocean, and you have a perfect recipe for hydrothermal vents. Cassini-Huygens detected several compounds in the geyser plumes (including methane) associated with hydrothermal vents on Earth.
While it’s possible to produce methane through non-biological processes, it’s also a common byproduct of biological metabolism. That’s why NASA and other space agencies are so keen to track down the source of methane wherever it pops up. For example, Curiosity and the Trace Gas Orbiter have been gathering data on Mars’ methane spikes for several years. While there are no missions currently exploring Enceladus, researchers from the University of Arizona were able to develop a mathematical model to try and determine what’s likely to be happening inside the moon.
First, the team had to confirm that the volume of dihydrogen in the plumes was sufficient to sustain a population of Earthlike hydrogenotrophic methanogens. That is: organisms that metabolize hydrogen and excrete methane. Next, they had to assess the potential impact of temperature and how that would affect the escape rate of methane and dihydrogen. In the end, the model says that no known geological process can explain the volume of methane coming out of Enceladus.
That doesn’t mean there’s definitely alien life clustered around the hydrothermal vents of Enceladus. It’s also possible there is an abiotic process that we don’t know about that can produce enough methane to match what we see in reality. For example, a large pocket of methane-bearing materials could have been trapped in the moon’s crust as it formed. All we can say for sure is the chances of life on Enceladus are somewhat higher than we might have estimated a few years ago.
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