This article was originally published at Eos. The publication contributed the article to Space.com’s Expert Voices: Op-Ed & Insights.
New laboratory research suggests that some organic molecules previously detected in plumes erupting from Saturn’s moon Enceladus may be products of natural radiation, rather than originating from the moon’s subsurface ocean. This discovery complicates the assessment of the astrobiological relevance of these compounds.
Enceladus hides a global ocean buried beneath its frozen crust. Material from this liquid reservoir is ejected into space from cracks in the ice near the south pole, forming plumes of dust-sized ice particles that extend for hundreds of kilometers. While most of this material falls back onto the surface, some remains in orbit, becoming part of Saturn’s E ring, the planet’s outermost and widest ring.
Between 2005 and 2015, NASA’s Cassini spacecraft flew repeatedly through these plumes and detected a variety of organic molecules. The detection was viewed as evidence of a chemically rich and potentially habitable environment under the ice, where molecules essential to life could be available. However, the new study offers an explanation in which radiation, not biology, is behind the presence of at least some of these organic molecules.
To test the role of space radiation, a team of researchers led by planetary scientist Grace Richards, a postdoc at the National Institute for Astrophysics in Rome, simulated conditions near Enceladus’s surface by creating a mixture of water, carbon dioxide, methane, and ammonia, the main expected components of surface ice on Enceladus. They cooled the concoction to −200°C inside a vacuum chamber and then bombarded it with water ions, which are an important component of the radiation environment that surrounds the moon.
The radiation induced a series of chemical reactions that produced a cocktail of molecules, including carbon monoxide, cyanate, ammonium, and various alcohols, as well as molecular precursors to amino acids such as formamide, acetylene, and acetaldehyde. The presence of these simple molecules indicates that radiation could induce similar reactions on Enceladus.
Richards presented these findings at the Europlanet Science Congress–Division for Planetary Sciences Joint Meeting (EPSC-DPS 2025) in Helsinki, Finland. She and her coauthors also published a detailed report in Planetary and Space Science.
Enceladus and beyond
The new research raises the question of whether the organic molecules detected in Enceladus’s plumes truly come from the moon’s buried ocean, whether they are formed in space, or whether they form close to the surface after the plumes leave the Enceladean interior.
While the finding doesn’t exclude the possibility of a habitable ocean on Enceladus, Richards urges caution in assuming a direct link between the presence of these molecules in the plumes, their origin, and their possible role as precursors to biochemistry.
“I don’t necessarily think that my experiments discredit anything to do with Enceladus’s habitability,” Richards said.
However, she added, “when you’re trying to infer this ocean composition from what you’re seeing in space, it’s important to understand all the processes that go into modifying this material.” Apart from radiation, these processes include phase changes, interactions with the moon’s ice walls, and interactions with the space environment.
“We need a lot of experiments of that type,” said planetary scientist Alexis Bouquet, a French National Centre for Scientific Research (CNRS) researcher at L’Université d’Aix-Marseille who wasn’t involved in the study. “They demonstrated that you can produce a certain variety of species in conditions that are relevant to the south pole of Enceladus.”
Bouquet highlighted the importance of simulating these environments in a lab for planning future missions to Enceladus and for interpreting the much-anticipated data from current missions to Jupiter’s icy moons. These missions are NASA’s Europa Clipper, which will explore Europa, and the European Space Agency‘s (ESA) JUICE (Jupiter Icy Moons Explorer), which will visit all three of the giant planet’s moons with subsurface oceans: Ganymede, Calisto, and also Europa.
The intense radiation around Jupiter makes these experiments especially relevant. “Radiation chemistry for Europa or the Jovian moons in general [is] a big deal, a bigger deal than in Enceladus,” Bouquet says.
Another Story Completely
As Richards’s work questions the origin of organic compounds around Enceladus, researchers keep adding more molecules to the puzzle.
After a new analysis of data gathered during one of Cassini‘s close approaches to Enceladus in 2008, researchers led by planetary scientist Nozair Khawaja at the Freie Universität Berlin and the University of Stuttgart reported the discovery of new types of organic molecules, seemingly emanating from the icy vents. They include ester and ether groups and chains and cyclic species containing double bonds of oxygen and nitrogen.
On Earth, these molecules are essential links in a series of chemical reactions that ultimately produce complex compounds needed for life. And while these molecules could have an inorganic origin, “they increase the habitability potential of Enceladus,”
Khawaja said. The findings appeared in Nature Astronomy.
Khawaja’s team’s analysis suggests that complex organic molecules are present in fresh ice grains just expelled from the vents. During its last flyby, Cassini got as close as 28 kilometers to the moon’s surface.
After modeling the plumes and the icy grains’ residence times in space, they think that the ice grains sampled by Cassini did not spend a lot of time in space, likely just “a few minutes,” Khawaja said. “It is fresh.”
This short duration in space questions whether space radiation had enough time to produce the organic molecules Khawaja detected. Just a few minutes would not be long enough for such complex chemistry to take place, even in a high-radiation environment.
“Big grains coming from the surface full of organics? That is much harder to explain through radiation chemistry,” Bouquet said.
While the types of experiments performed by Richards “are valuable and take the science to the next level,” Khawaja said, “our results tell the other story completely.”
Back to Enceladus
Both studies reinforce the complexity of Enceladus’s chemistry, upholding it as a prime target in the search for extraterrestrial life, or at least life’s building blocks. Enceladus has all three prerequisites for life: liquid water, an energy source, and a rich cocktail of chemical elements and molecules. Even if the subsurface ocean is out of reach—it lies at least a few kilometers beneath the ice close to the poles—the plumes offer the only known opportunity to sample an extraterrestrial liquid ocean.
Studies for a potential ESA mission dedicated to Enceladus are already underway, with plans that include high-speed flybys through the plumes and, potentially, a lander on the south pole. The insights from both recent studies will help researchers design the instrumentation and guide the interpretation of future results.
“There is no better place to look for [life] than Enceladus,” Khawaja said.
