With its subsurface ocean and geysers spewing water and complex organic molecules, scientists say Saturn's moon Enceladus is one of the most promising places to look for extraterrestrial life in the solar system.
But what exactly would life on Enceladus look like and how would it function?
After all, the theoretical organisms growing and multiplying deep below the ice moon's surface would have no access to the sunlight that fuels the vast majority of life forms on Earth.
There is no available oxygen to work with, and a tremendous amount of pressure to contend with if an organism hopes to derive energy from the chemical reaction between Enceladus' subsurface ocean and its rocky core.
Despite these hurdles, a team of researchers found that there is at least one life form on Earth that likely would do just fine living under the presumed conditions on Enceladus. It's a single-celled organism known as Methanothermococcus okinawensis that lives in hydrothermal vents more than 3,000 feet below sea level off the coast of Japan.
In a paper published Tuesday in Nature Communication, researchers show that M. okinawensis could theoretically thrive in what scientists believe to be the conditions on Enceladus by turning molecular hydrogen and carbon dioxide — both believed to be present on the ice moon — into methane.
No spacecraft has penetrated Enceladus' frozen crust, but NASA's Cassini spacecraft did fly through great plumes of material that were spewing from the moon's south pole. These passes revealed that Enceladus' internal ocean is made primarily of water, but the spacecraft's instruments also detected methane, carbon dioxide, ammonia, molecular nitrogen and molecular hydrogen, among other compounds.
To see if any life forms on Earth could survive on Enceladus, the authors turned to three types of microbes known as methanogenic archaea. These are single-celled organisms that grow in the absence of both sunlight and oxygen, and produce methane as a metabolic product. Here on Earth you can find methanogenic archaea in marine sediments, salt marshes and in the human gut, where they are partly responsible for flatulence and belching.
After attempting to grow the archaea in a variety of different conditions, the authors decided to work exclusively with M. okinawensis because it grew predictably in several scenarios.
Over the next series of experiments, they found that M. okinawensis was able to reproduce and make methane under the chemical and pressure conditions presumed to be found on Enceladus. In addition, the authors showed that the presence of molecular growth inhibitors, which also are known to be part of Enceladus' chemical profile, did not hinder M. okinawensis' growth.
"M. okinawensis grew under all tested putative [presumed] Enceladus-like conditions," said Simon Rittmann, an archaea biologist at Wien University in Vienna, who led the work. "Even formaldehyde and ammonium at the highest-tested concentrations and pressure up to 90 bars resulted in methane production."
The new study shows how life as we know it could survive on Enceladus, but Rittmann cautions that it is still too soon to say whether these microbes would survive.
"Our study is a physiological, microbiological, geological study, and the results from lab experiments must be very carefully interpreted," he said.
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