From the moment scientists spotted plumes of water erupting from Enceladus, one of Saturn’s moons, it became the most tantalising place in the solar system to search for life beyond Earth. Now the case is even more compelling after researchers found that organisms that live in deep-sea waters off Japan can survive the harsh conditions expected in the briny ocean that lurks beneath Enceladus’s icy crust. The microbes, which thrive near hydrothermal vents on the seafloor by converting hydrogen and carbon dioxide into methane, survived in a brine that mimicked the Saturnian moon’s ocean.
The work suggests the distant world may be habitable, if not exactly hospitable, to at least some forms of life. Saturn has more moons than any other planet in the solar system, with nearly 300 known to orbit the gas giant. For more than two centuries, Enceladus, a 300-mile-wide ice ball, did not seem particularly remarkable.
But that changed with the arrival of robotic spacecraft. Since the 2000s, observations from Nasa’s Cassini probe and, more recently, the James Webb space telescope have revealed gigantic plumes of water vapour and ice grains spewing from Enceladus and evidence for a huge saltwater ocean under its frozen crust. The seafloor is thought to be dotted with hydrothermal vents that warm the water around them.
Last year, scientists found organic substances in samples collected by Cassini as it swooped through one of Enceladus’s spectacular plumes. Orsi and his colleagues wanted to see if they could concoct a miniature version of Enceladus’s ocean to study in the laboratory. By combining water with salts and carbonates and adding powdered rock, they recreated not only the extremely alkaline environment of the ocean but also crucial reactions that take place between the water and the rocky seafloor.
The rock-water reactions were active enough to start churning out hydrogen, a process that seems to happen in the ocean on Enceladus. It led the researchers to wonder whether Earthly microbes called methanogens, which survive by converting hydrogen and carbon dioxide into methane, could survive in the solution. To find out, the scientists turned to microbes originally found near deep-sea hydrothermal vents in the Okinawa trough between Japan and Taiwan.
Hydrothermal vents are hot springs on the seafloor that release hot, mineral-rich water. Surprisingly, the microbes grew in the simulated ocean despite its pH reaching 11, which is far more alkaline than the organisms’ known limit. The microbes even adapted their metabolism to cope with low concentrations of carbon dioxide in the water.
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