Enceladus is one of the most fascinating objects in the Solar System. Parked in orbit around Saturn, the ice-covered moon features a warm subterranean ocean and the basic chemical ingredients for life. But could alien microbes actually survive there? A new experiment suggests the answer is yes.
Methane-producing microbes can survive the conditions found on Enceladus, according to research published today in Nature Communications. An interdisciplinary research team led by Simon Rittmann from the University of Vienna came to this conclusion by taking microbes found around Earth's hydrothermal vents and exposing them to Enceladus-like conditions in the lab. Incredibly, these microbes didn't just survive - they actually thrived. Enceladus, perhaps more than any other place in our Solar System aside from Earth, has the potential to sustain life at this particular stage in our Solar System's history.
Every once in a while, Enceladus shoots a plume of water vapour and solid particles into space. Measurements made by NASA's late-great Cassini probe indicate the presence of life-sustaining chemicals within the plume, including organic and nitrogen-bearing molecules, salts, silicates, and molecular hydrogen. What's more, Enceladus has been able to sustain a warm ice-capped ocean for billions of years in yet another sign of potential habitability. Astrobiologists theorise that, like ancient Earth, life may have emerged around hydrothermal vents located around the moon's submerged rocky core.
Aside from Cassini's flybys, scientists haven't had the opportunity to visit the surface of Enceladus or explore its ocean, so we don't know if life actually exists there. But as Rittmann's new experiment shows, we can at least simulate the conditions of Enceladus right here on Earth and see how microbes fare within that setting.
Importantly, the scientists couldn't just pick a microbe at random - they had to find one that could conceivably emerge and survive on Enceladus. It just so happens that the University of Vienna researchers had an excellent candidate in the form of Methanothermococcus okinawensis, an deep-sea organism that likes to hang out near hydrothermal vents. This microbe, which comes from the Archaea family of microbes, uses carbon dioxide and molecular hydrogen as food, releasing methane as a by-product. These chemical conditions exist near hydrothermal vents on Earth, and very likely at similar vents produced by Enceladus' rocky core.
In experiments, the researchers took M. okinawensis, along with two other similar microbes, and exposed them to the gases and pressures thought to exist on Enceladus. Instead of shriveling away and dying a miserable death, the tiny organisms gobbled-up the carbon dioxide and hydrogen around them, helping them to grow and produce methane. To make life more complicated for the microbes, Rittmann's team also exposed them to some noxious compounds that might exist on Enceladus that are known to inhibit growth, such as formaldehyde, ammonia, and carbon monoxide. The microbes just shrugged these chemicals off and continued to thrive.
According to the new paper, low-temperature serpentinization - when rocks are geochemically altered by the conditions around them — also happens on Enceladus. In this case, low-temperature serpentinization could be generating enough hydrogen gas to support methane-producing microbes.
"This is the first interdisciplinary study about an Earth microorganism that could possibly produce methane under Enceladus-like conditions," Rittmann told Gizmodo. "This study included astrogeochemistry, planetology, microbial physiology, and high pressure biology. We were able to show that under [presumed] Enceladus-like conditions and the given environmental parameters, biological methane production did occur in the lab," and that a "microoorganism from a hydrothermal vent system on Earth could be grown in the presence of [presumed] inhibitors in combination with high pressure."
Jonathan I. Lunine, a professor of physical sciences as Cornell University who wasn't involved in the new study, says it's "great that they found an organism that could grow" in Enceladus-like conditions, but he's "actually not surprised organisms are viable in this type of soup."
One weakness of the new study, he says, is that the pH of the medium (i.e. the amount of acidity found in a liquid solution) used in the experiment was lower than the range found by Cassini. "Higher pH might inhibit growth," Lunine told Gizmodo. He also took exception to a claim made by the authors in a Nature Communications press release and in the paper itself that this study is the first to show that serpentinization could generate enough hydrogen to feed microorganisms. Lunine pointed out that he co-authored a paper in Science last year that had already demonstrated that finding.
Like Lunine, NASA astrophysicist Chris McKay isn't surprised by the results, saying he expected methane-producing microorganisms all along. "Methanogens are the pop-stars of astrobiology," he told Gizmodo. McKay's single complaint about the study is that the researchers didn't simultaneously include the pressure, temperature, pH, carbon dioxide, and ammonia found in the Enceladus vents, saying that it's "very hard to do."
The authors of the new study don't necessarily disagree, saying the only way to find out if life actually exists on Enceladus is to actually go there.
"Although we tried to be as broad as possible with the experiments regarding pressure, composition of medium and gases, temperature, microorganisms, inhibitors, [and so on], our study is no evidence for possible extraterrestrial life," said Rittmann. "Possibly a mission dedicated to...Enceladus...for searching for biosignatures of life, would be of interest to the scientific community."
So what are we waiting for?