Science & Technology

Astrobiology takes the spotlight: Saturn’s moon Enceladus may offer a ‘free lunch’

The sea that lies beneath the icy surface of Enceladus, one of Saturn’s moons, could provide even more fuel for extraterrestrial organisms than previously thought.

That’s the upshot of a study to be presented at AbSciCon 2019, an astrobiology conference taking place next week in Bellevue, Wash. Hundreds of researchers will be sharing their findings about the prospects for life elsewhere in the solar system and the universe.

Among them will be Lucas Fifer, a doctoral student in Earth and space sciences at the University of Washington. He’s the lead researcher for the Enceladus study.

It’s been more than a decade since NASA’s Cassini mission revealed that plumes of water are rising up from fissures in Enceladus’ thick surface layer of ice. Scientists have gone on to find increasing evidence that Saturnian moon’s subsurface sea could be hospitable to marine microbes or perhaps even higher forms of life. Analyses of the plumes indicate that they contain molecules of hydrogen as well as minerals and organic molecules that could serve as fuel.

But do the plumes accurately reflect the composition of the seas beneath the ice? Fifer and his UW colleagues, Jonathan Toner and David Catling, fed the chemical analysis of the plumes observed by the Cassini probe into a computer model, and found that the seas are likely to provide even more of the things that life could take advantage of.

The reason is that the proportions of gases dissolved in the water should change as the plumes erupt through the ice fissures, due to a phenomenon known as fractionalization. Some of the gases would get preferentially left behind.

In Enceladus’ case, those gases include hydrogen, methane and carbon dioxide — which serve as energy sources for microbes on Earth. Having more of those gases should improve the chances for any life existing in the hidden ocean.

“It’s better to find high gas concentrations than none at all,” Fifer said in a news release. “It seems unlikely that life would evolve to consume this chemical free lunch if the gases were not abundant in the ocean.”

Higher levels of carbon dioxide also would imply that the ocean is less alkaline than previously thought, and closer to the acid-alkaline pH balance found in earthly oceans. That’s another advantage for Earthlike life.

“Although there are exceptions, most life on Earth functions best living in or consuming water with near-neutral pH, so similar conditions on Enceladus could be encouraging,” Fifer said. “And they make it much easier to compare this strange ocean world to an environment that is more familiar.”

What’s more, the analysis suggests the ocean could hold high concentrations of ammonium, which is another potential fuel for life.

Fifer said the findings could be interpreted in two ways. One interpretation would be that the Enceladusians have a big menu to choose from. The other interpretation is less reassuring: Having so much fuel lying around in the ocean could mean “that there is hardly anyone around to eat it,” he said.

Enceladus has traditionally shared the spotlight with Europa, an icy moon of Jupiter, when it comes to the search for life in subsurface oceans. NASA has plans on the books for a mission to Europa in the 2020s, but a mission to Enceladus is likely to be farther off.

One proposed project, known as the Enceladus Life Finder, would send a probe on repeated passes through the moon’s plumes. And Russian-Israeli billionaire Yuri Milner has voiced interest in helping an Enceladus mission get off the ground sooner rather than later.

Fifer said the analysis that he and his colleagues conducted could help scientists on such missions calibrate their own findings. “Future spacecraft missions will sample the plumes looking for signs of life, many of which will be affected just by the eruption process,” he said. “So, understanding the difference between the ocean and the plume now will be a huge help down the road.”

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