The possibility of life on Enceladus may depend on the size of its ocean, and its geysers have some clues.
Saturn's moon Enceladus is a tantalizing world of subsurface oceans and cryovolcanoes, features that have distinguished it as a top candidate in the search for extraterrestrial life.
Much of what we know about this tiny world's promising ocean is informed by an extraordinary collection of about 100 geysers situated at its southern pole, which forcefully shoot plumes of water and minerals into space at speeds of around 400 meters per second.
Now, researchers led by astronomer Marie Běhounková have taken a new step in explaining the existence of these geysers—and the oceanic forces that power them—by focusing on the strange timing of plume eruptions. Though the plumes are largely influenced by Saturn's significant tidal pull on Enceladus, contorting the ice shell and heating up its subsurface ocean, there is a delay of several hours between the most intense tidal forces and the eruption of the geysers.
In a paper published this morning in Nature Geoscience, Běhounková and her colleagues posit that "the delay in eruption activity may be a natural consequence of the viscosity structure in the south-polar region and the size of the putative subsurface ocean."
In other words, the observed lag is yet another card that Enceladus has tipped about its interior structure. The geysers don't immediately respond to extreme tidal heating due to the viscous nature of the southern ice shell and its subsurface ocean, which affects the timing of the plume eruptions.
To that end, Běhounková's team modeled two very different scenarios that account for this delayed reaction. The first is that the moon's southern pole is home to a regional ocean about 30 kilometers below the ice shell, while the second suggests that Enceladus has a much larger, global ocean under a convecting ice shell that extends about 60 to 70 kilometers between the surface of the moon and the subsurface water.
"Our two possible solutions have very different implications for Enceladus's thermal budget and possibly for stability of the ocean," Běhounková told me over email.
"For a regional sea, the heat loss is limited to the south polar terrain," she said. "For a global ocean, the heat loss outside the south polar terrain is predicted to be large, an even larger heat source than implied by thermal emission in the south polar terrain."
Essentially, determining whether the plumes are powered by a regional ocean or a global one has huge consequences when it comes to predicting the future of Enceladus and its oceans.
For example, many scientists have proposed that the unusual geological activity observed on the moon is a rare anomaly in its history, which we just happen to be around to witness. In some of these previous studies, researchers have suggested that tidal heating and liquid water might make Enceladus's oceans hospitable to life for temporary bursts, but if the world freezes over after its runaway heat loss, any potential organisms would likely be wiped out too.
For her part, Běhounková is planning to nab more data about the plumes in future Cassini flybys. She also intends to study the moon's libration—the way in which Enceladus slightly oscillates relative to Saturn—for clues to distinguish between regional and global oceans.
"Cassini's (sadly) last Enceladus flybys are planned later this year," she told me. "One of the goals of these flybys is to search for variability in the plumes. The predictions which our model makes can be tested with and hopefully improved by with these future observations."
"In my opinion, the libration of the moon is a mechanism that can provide complementary information which can distinguish between the regional and the global ocean," she continued. "The magnitude of libration can be determined by using repeat imaging of Enceladus's surface and it can hint at the presence of the global ocean."
On top of that, there is already a future mission to the moon, called the Enceladus Life Finder (ELF), in the concept phase, so we are poised to learn a lot more about this weird ocean world in the coming decades. Jupiter's moon Europa may be the oceanic ice moon that grabs the most headlines, but studies like Běhounková's demonstrate that Enceladus continues to be a very interesting up-and-comer.