Earth Microbes Survive Simulated Enceladus Ocean, Study Finds

The organisms are methanogens, microbes that thrive near hydrothermal vents on the seafloor by converting hydrogen and carbon dioxide into methane. In experiments led by William Orsi, a professor of geomicrobiology at Ludwig-Maximilian University in Munich, scientists combined water with salts and carbonates and added powdered rock to recreate the extremely alkaline environment of Enceladus’s ocean as well as reactions between water and rocky seafloor. The reactions produced hydrogen, a process thought to occur in the moon’s ocean. The researchers then tested whether earthly methanogens, originally found near deep-sea hydrothermal vents in the Okinawa trough between Japan and Taiwan, could survive in the solution. Orsi called it "a long shot."

The microbes grew despite the simulated ocean reaching pH 11, far more alkaline than their known limit. They also adapted their metabolism to cope with low concentrations of carbon dioxide in the water. Orsi said: "If we use the cellular definition of life that we understand on Earth today, the experiments tell us that if this organism, or something similar to it, was on Enceladus, there’s a good chance it could survive."

Enceladus, a 300-mile-wide ice ball, has become a focus in the search for life beyond Earth since robotic spacecraft began observing it. Since the 2000s, NASA’s Cassini probe and, more recently, the James Webb space telescope have revealed gigantic plumes of water vapour and ice grains erupting from the moon and evidence of a huge saltwater ocean beneath its frozen crust. The seafloor is thought to be dotted with hydrothermal vents that warm surrounding water. Last year, scientists found organic substances in samples collected by Cassini as it flew through one of Enceladus’s plumes.

David Rothery, professor of planetary geosciences at the Open University, said the work removes another barrier to the viability of microbial life there by showing that methanogenesis was possible in the simulated Enceladus ocean.

Orsi stressed that any life that evolved on Enceladus could be radically different from life on Earth. Even if similar microbes arose, it is unclear whether methane-making microbes would thrive in the moon’s ocean. His experiments lasted only days. "Could they survive for a year? Could they survive for a million years? We don’t know," he said.

In a second paper in the journal, some of the same researchers describe how ice grains in Enceladus’s plumes freeze and fragment in a way that separates different constituents of the ocean. If ocean droplets contained bits of alien microbes, these might appear in a small fraction of ice particles but be highly concentrated within them. Frank Postberg, professor of planetary sciences at the Free University of Berlin, said that would be "great news" for the search for life and that it would be relatively easy to identify signs of life with today’s technology.

Interest in returning to Enceladus has grown. The European Space Agency’s L4 mission aims to combine a Saturn orbiter with an Enceladus lander that would touch down at the moon’s south pole and hunt for biosignatures in the plumes erupting from its ocean. With a launch proposed for about 2042, it would not reach Saturn until the 2050s. Rachael Hamp, also at the Open University, who has worked on Enceladus’s plumes, said: "It’s fantastic to see new research pushing our understanding further and providing additional evidence for the potential habitability of this little icy world. I think this makes the European Space Agency’s proposed mission to Enceladus even more exciting."

Editor's Summary

The study found that earthly methanogens can survive in a simulated Enceladus ocean, adding evidence that the moon may be habitable for some microbial life. A second paper suggests plume ice grains could concentrate potential biosignatures, while ESA’s proposed mission would not arrive until the 2050s.