Enceladus was expected to be a frozen planet, a frozen lump of solid ice orbiting Saturn indefinitely. However, between 2004 and 2017, the Cassini spacecraft visited the system and discovered an active moon that was virtually bursting at the seams with water, hydrogen, and methane: three chemicals that would go hand in hand with life in Earthly oceans.
Enceladus, on the other hand, isn’t Earth, and planetary scientists aren’t sure how to interpret the strange mixture of substances the Saturnian moon spews into space. Were they the consequence of extraterrestrial chemistry or biology? Researchers are still undecided. Alien “methanogens”—microbes that eat hydrogen and carbon dioxide and expel methane—may or might not live on Enceladus. However, a new study published yesterday in Nature Astronomy by a group of scientists indicates that producing that much methane from the most obvious chemical reaction is very impossible.
“Methanogens can explain how much methane there is,” says Antonin Affholder, a doctorate ecology student at ENS Paris and the study’s primary author.
Underneath the ice, there is a world.
Cassini discovered water geysers erupting into space in 2006, providing the first strong confirmation that Enceladus’ ice surface was concealing an ocean.
A decade later, the spacecraft plummeted right into one of the liquid plumes while circling Enceladus, skimming about 30 miles from the moon’s surface. The sensor analyzed the chemicals streaming out into space during its daring descent, basically sniffing the ocean spray.
It smelled like water, with hints of hydrogen and methane. Researchers discovered that hydrogen was a marker of potential life rather than life itself. It was most likely emitted by deep marine vents. On Earth, such vents are teeming with microscopic life that feeds on hydrogen. These locations are even being examined as possible contenders for the origins of terrestrial life.
Then there was the matter of methane. The ancient living forms that live in deep marine vents use hydrogen and carbon dioxide (CO2) and produce methane (CH4), giving them the moniker methanogens. Cassini demonstrated that Enceladus contained just about everything needed to make basic creatures comfortable—water, heat, and food—with the finding of just a few molecules.
The evidence, on the other hand, was circumstantial. Aside from microbial digestion, methane is produced via a variety of chemical processes on Earth. The quantity of hydrogen Cassini discovered also perplexed researchers: a livable Enceladus was teeming with potential food, yet nothing appeared to be eating it.
Methane is a mysterious gas.
Affholder and his biologically inclined coworkers decided to use their knowledge of populations and ecosystems to the situation and set out to reproduce all potential conditions where Enceladus’ ocean meets its rocky core.
They started with the methane’s most evident non-biological source. A process known as “serpentinization” results in hydrogen when hot water sloshes against particular minerals (such as in a deep-sea vent). Then, just like methanogens, additional chemical processes can combine hydrogen and carbon dioxide to produce methane. The scientists determined a range of feasible quantities of hydrogen and methane Enceladus may generate on its own using data from previous studies nailing down serpentinization rates.
The scientists next evaluated how the range of hydrogen and methane on Enceladus might alter if methanogens were present. To keep their assumptions about Enceladus’ life credible, the researchers included genuine creatures from Earth in their simulation. Affholder argues, “We can’t just imagine anything we want to imagine.” “We need to base our assumptions on what we already know.”
Finally, they compared the probability of the two sets of theoretical Enceladuses using a statistical framework known as Bayesian analysis. They discovered that the cascade of chemical events starting with serpentinization is insufficient to produce as much methane as Cassini collected during its geyser dive.
With a “score of zero,” Affholder argues the first hypothesis is “totally excluded.”
The abundant hydrogen is precisely exactly what an inhabited Enceladus would emit, according to the researchers. Near the vents, the molecules congregate in a zone much too hot for known methanogens to thrive. Any animals present would most likely nibble on molecules far enough away from the vents to have minimal effect on the total amount of hydrogen.
The findings of Affholder’s team do not suggest that Enceladus is teeming with methane-belching organisms. However, it does imply that the molecule is being produced by an unknown source.
Methane might be seeping up from the core, for example. If the moon was generated primarily by crashing comets, it should be crammed with rubbery, carbon-rich materials that, when heated, decompose into methane. Alternatively, a wholly unanticipated mechanism might be at work. Researchers are drowning in their lack of understanding of what lies underneath the ice shell of the moon.
“We have no idea where Enceladus came from. The age of Enceladus is unknown. The exact nature of the methane is unknown, according to Affholder.
With indirect conclusions, aliens are being hunted.
In the near future, no lander will be drilling through miles of alien ice, so scientists will have to rely on data from afar to figure out what’s going on on Enceladus.
Examining the type of methane ejected is one technique. If its carbon atoms came from comets and were buried in the moon’s core for billions of years, they could have different weights than the carbon atoms devoured and ejected by bacteria. A Cassini sequel with improved instrumentation may be able to tell the difference between the two.
“We could need a methane trip to learn more,” Affholder says.
However, with no additional missions to the Saturn system on the horizon, scientists are focusing their efforts on Jupiter’s Europa, another ice moon. The James Webb Space Telescope, which is scheduled to launch later this year, may be able to see the contents of geysers there. An orbiter capable of sample the plumes directly, as Cassini did, is also in the works. Europa may be livable, and if it isn’t, addressing a few fundamental questions about its past might help us learn more about its frigid neighbor.
Similar techniques are being developed by Affholder and his partners to determine the probability of alien life on exoplanets, where atmospheric mixes of oxygen, carbon dioxide, and other gases will be even more difficult to interpret than Enceladus’ molecules. Their study foreshadows an age of astrobiology in which extraterrestrial life will be discovered through the whimpers of consecutive statistical studies rather than the blast of a blazing gun.