In 2005, a NASA spacecraft flew past Enceladus, an icy moon of Saturn, snapping pictures and recording observations as it went. When scientists processed the data, they saw plumes of mist erupting from the cracked surface of the moon’s south pole and into the emptiness of space. The plumes, the spacecraft’s instruments had found, were made of water vapor.
Scientists were stunned. Enceladus is small, just 300 miles wide, and its surface reflects sunlight rather than absorbs it. For these reasons, they had expected the moon to be frozen solid, yet here was some evidence to the contrary. Later observations found more proof that Enceladus was alive, geologically speaking, and hiding a liquid ocean between an icy crust and a rocky core.
Close flybys of Enceladus revealed that the moon’s plumes contained a smorgasbord of chemicals, including methane, carbon dioxide, ammonia, nitrogen, hydrogen, ammonia, and formaldehyde. Enceladus quickly became an extremely attractive candidate in the search for other life in the solar system. Life, at least life as we know it on Earth, arises from some molecules and a little water, and this Saturnian moon had both.
But the NASA spacecraft, Cassini, couldn’t tell us much more than that. It was equipped to sample the contents of the moon’s geysers, not determine whether they showed signs of microbial life. (Also, Cassini plummeted into Saturn’s atmosphere and disintegrated last September #RIP.) To investigate that tantalizing possibility, scientists had to bring Enceladus closer to Earth.
Researchers at the University of Vienna have spent the last five years recreating the predicted conditions of Enceladus’s subsurface ocean in the lab. They designed experiments with temperatures, pressures, and pH levels similar to that of the Enceladian ocean. Into these environments they placed three strains of single-celled microorganisms called methanogenic archaea—bacteria that hate oxygen, love carbon dioxide and hydrogen, and produce methane as waste. Then the researchers watched the strains grow.
One grew successfully, even when the scientists changed the environmental conditions, dialing them up or down, and introduced some compounds, like ammonia and formaldehyde, that were supposed to stunt growth. The researchers concluded that methane-producing microorganisms could thrive on Enceladus. Perhaps, they say, some of the methane Cassini detected may have come from such tiny aliens.
On Enceladus, “under these conditions, life could exist,” says Simon Rittmann, a researcher at the University of Vienna’s division of archaea biology and ecogenomics who led the research. The findings were published Tuesday in Nature Communications.
Methanogenic microorganisms are known to be tough creatures, capable of surviving many extremes, says Christa Schleper, the head of the division at the university and one of the study’s authors. The new research shows that methanogens are “much more resilient and robust than maybe we anticipated,” she says. Scientists suspect methanogens may have even been the earliest forms of microbes on Earth, chomping away on carbon dioxide and hydrogen before enough oxygen rolled around to give rise to the first photosynthetic organisms, Schleper says.
The terrestrial microbe used in this study is called Methanothermococcus okinawensis—say that three times fast!—and came from a hydrothermal vent that lies more than 3,000 feet below the surface in a basin in the East China Sea, near Japan. Hydrothermal vents are fissures in the seafloor that spew hot water chock-full of minerals into the ocean. They give rise to wonderfully diverse organisms that feed on the expelled nutrients, rather than on sunlight, which can’t reach them all the way down there.
Scientists say Enceladus has these hydrothermal vents, too. Chemical reactions between water and rock at the seafloor that produce hydrogen could also be occurring, generating enough of the gas to fuel the growth of methane-producing microorganisms, Rittmann says.
If we could send this bacteria to Enceladus and drop it into one of the long cracks in the moon’s surface and into the ocean, it might feel quite comfortable there, says Hunter Waite, a planetary scientist at the Southwest Research Institute and a Cassini principal investigator who was not involved in the study. Waite’s own research has shown that Enceladus is rich in microbe-fueling gases, like hydrogen.
“This is just a further indication that it’s not a stretch of the imagination at all to think that there might be microbes living in some form in these ocean worlds, even in our own solar system,” says Waite, who’s working on a mission to another ocean world, Europa, a moon of Jupiter. NASA is planning to launch an orbiter in the 2020s to the moon, which, like Enceladus, is icy on the outside and swirling with liquid below.
The researchers at the University of Vienna are careful to point out that their research doesn’t mean that Enceladus is inhabited. Their work only suggests that microbes like one found on Earth could thrive in Enceladus’s ocean under the conditions scientists predict exist there. “We can only make assumptions about Enceladus,” Rittmann says. “It can always be that maybe life is not out there. But this is for others to prove.”
The methane Cassini detected may have come from entirely different sources. Methane can be produced by nonbiological processes on Earth, like the chemical reactions in seafloor sediment observed in the Arctic Ocean, but these are still poorly understood.
To know for sure, humans would need to send another robot to the Saturn system with different instruments designed to detect, among other things, the distinct chemical signatures of the production of methanogenic bacteria. But they couldn’t touch down on the surface, which would risk contamination with terrestrial bacteria.
“We need to go back with better instruments, learn more about the ocean chemistry, look for signs of life,” Waite says. “Flyby missions would do that just fine.”
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