Something is making methane on Saturn’s moon Enceladus. A new study has found that methane levels detected from the icy moon are far higher than can be explained by known geochemical processes – but they are consistent with microbes.
At a glance, the giant iceball of Enceladus may not seem like a particularly promising place to find life. But beneath that frigid shell is a slushy subsurface ocean, which makes itself known by spurting through the ice in giant water plumes, like an otherworldly Old Faithful. While exploring Saturn and its moons, the Cassini spacecraft dived through these plumes and detected unusually high amounts of molecules like methane, dihydrogen and carbon dioxide.
This suggested that at the seafloor, where the ocean met Enceladus’ rocky core, there were hydrothermal vents. And as we know from here on Earth, this dynamic environment is prime real estate for microbes that consume the dihydrogen and carbon dioxide produced there and in turn emit methane.
So does that mean there are alien microbes living around hydrothermal vents on the seafloor of Enceladus? Or could the detected molecules all be accounted for through those geochemical processes alone? To find out, the researchers ran models to see which scenarios best fit the observations.
The team modeled different conditions that could be possible in that environment on Enceladus, and introduced hypothetical methane-producing microbes, based on known strains from Earth. They examined whether the hydrothermal cycle could provide enough dihydrogen for the microbes to “eat,” whether the temperature was right for them, and importantly what effects the microbes would have on their surroundings – specifically the concentrations in the plumes. Then, the results of these models were compared to what Cassini actually detected in the plumes.
"In summary, not only could we evaluate whether Cassini's observations are compatible with an environment habitable for life, but we could also make quantitative predictions about observations to be expected, should methanogenesis (by microbes) actually occur at Enceladus' seafloor," says Régis Ferrière, co-lead author of the study.
Sure enough, conditions there seemed favorable to this kind of lifeform – but more intriguingly, no amount of known hydrothermal chemistry was able to explain the levels of methane detected in the plumes. Only when biological sources were added to the model did the levels match observations.
"Obviously, we are not concluding that life exists in Enceladus' ocean,” says Ferrière. "Rather, we wanted to understand how likely it would be that Enceladus' hydrothermal vents could be habitable to Earthlike microorganisms. Very likely, the Cassini data tell us, according to our models. And biological methanogenesis appears to be compatible with the data.”
Of course, it might not be life. But if not, it must be some other natural phenomenon that we don’t see here on Earth. One solution the team suggests is that primordial organic matter from Enceladus’ core could break down into methane, dihydrogen and carbon dioxide through hydrothermal activity. But the likelihood of that depends on how Enceladus formed.
We'll no doubt learn more as astronomers continue to investigate this fascinating world.
The research was published in the journal Nature Astronomy.
Source: University of Arizona
