- Huge plumes of water vapor erupt from large cracks in the surface ice at Enceladus’ south pole. Scientists say the water vapor comes from the moon’s subsurface ocean.
- The Cassini spacecraft sampled the plumes, finding all the elements needed for life, including both simple and complex organic molecules. Do the organics actually originate in the ocean, as scientists have thought?
- At least some of the organics might be created by radiation hitting Enceladus’ icy surface instead, new lab simulations suggest.
Saturn’s moon Enceladus is an intriguing world that just might support life in its subsurface ocean. Indeed, NASA’s Cassini orbiter previously found a variety of both simple and complex organic molecules, both on Enceladus’ icy surface and in its water vapor plumes. Scientists said those plumes, at the south pole of the moon, most likely originate directly from the ocean below the ice crust. Do the organics also come from the ocean? Researchers in Europe said on September 9, 2025, that at least some of them might instead be formed by radiation hitting the frozen surface.
Grace Richards at the National Institute for Astrophysics (INAF) in Rome, Italy, led the research team. She presented their findings at Europlanet’s EPSC-DPS Joint Meeting 2025 in Helsinki, Finland, on September 9, 2025.
Organics in Enceladus’ plumes
The plumes on Enceladus break through the icy surface at the south pole of the moon. They erupt from long, deep cracks called Tiger Stripes. Scientists said the plumes originate from the salty ocean below the outer ice crust.
When Cassini flew through Enceladus’ plumes, it tasted them to see what they are composed of. Cassini found water vapor, salts, ammonia, nitrogen, oxygen, phosphorous, hydrogen cyanide and a variety of organic molecules. In addition, there is evidence for hydrothermal activity on the ocean floor. Altogether, these findings support the scenario that Enceladus’ ocean is potentially habitable, by earthly standards.
Are the organics from the ocean or radiation?
Richards and her team conducted experiments to see what could create the organics. Did they actually originate in the ocean, or could they be formed on the surface? Notably, the results suggest at least some of the organics could be formed from radiation hitting Enceladus’ icy surface. The radiation is trapped within Saturn’s magnetosphere, which Enceladus passes through in its orbit. Richards said:
While the identification of complex organic molecules in Enceladus’ environment remains an important clue in assessing the moon’s habitability, the results demonstrate that radiation-driven chemistry on the surface and in the plumes could also create these molecules.
Simulating the surface ice of Enceladus
To learn more, Richards and her colleagues simulated the ice of Enceladus in a lab. This was both ice on the surface and within the Tiger Stripes fractures. Specifically, to mimic conditions on Enceladus, the researchers added carbon dioxide, methane and ammonia and cooled the ice down to -320 degrees Fahrenheit (-200 C).
Then, the team simulated the radiation hitting Enceladus. They did this by bombarding the ice with ions, which are atoms and molecules stripped of an electron. Interestingly, as the ions reacted with the ice, they created organic molecules including carbon monoxide, cyanate, ammonium and the precursors to amino acids. Amino acids are the building blocks of proteins.
Previously, Cassini had detected some of those molecules on Enceladus’ surface and some in its plumes. This doesn’t prove definitively that the organics on Enceladus are all formed this way. However, it does show that it’s possible, at least for some of them. As Richards noted:
Molecules considered prebiotic could plausibly form in situ through radiation processing, rather than necessarily originating from the subsurface ocean. Although this doesn’t rule out the possibility that Enceladus’s ocean may be habitable, it does mean we need to be cautious in making that assumption just because of the composition of the plumes.
Other complex organics
Previous studies found a variety of complex organic molecules in the plumes, including esters, alkenes and ethers, as well as other complex molecules containing nitrogen and oxygen. They provide evidence for possible chemical reactions that could produce fats or nucleotide bases, the building blocks of DNA. Whether the radiation could account for these is currently unknown.
Bottom line: Scientists have thought the organics in Enceladus’ plumes originate in its ocean. A new study suggests at least some of them might be formed by radiation.
Source: Water-Group Ion Irradiation Studies of Enceladus Surface Analogues
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