Saturn’s magnetosphere contains trapped plasma and energetic charged particles which constantly irradiate the surface of Enceladus. The plasma consists of a variety of charged particles including water-group ions, formed primarily from high energy electrons interacting with plume material. Observations by instruments aboard NASA’s Cassini spacecraft show that on Saturn’s inner icy satellites, such as Mimas and Tethys, irradiation by the cold plasma darkens reflectance spectra and produces bullseye-shaped features on the moons’ surfaces. However, at Enceladus, the effects of plasma bombardment are unknown and difficult to determine.
Saturn’s moon Enceladus with a plume. Image credit: NASA / JPL-Caltech / SSI / Kevin M. Gill.
“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,’ said Dr. Grace Richards, a researcher at the Istituto Nazionale di Astrofisica e Planetologia Spaziale.
Enceladus’ plumes were discovered in 2005 by NASA’s Cassini spacecraft.
They emanate from long fractures called ‘tiger stripes’ that are located in the south polar region of Enceladus.
The water comes from a sub-surface ocean, and the energy to heat the ocean and produce the plumes is the result of gravitational tidal forces from massive Saturn flexing Enceladus’ interior.
Cassini flew through the plumes, ‘tasting’ some of the molecules within them and finding them to be rich in salts as well as containing a variety of organic compounds.
As organic compounds, dissolved in a subsurface ocean of water, could build into prebiotic molecules that are the precursors to life, these findings were of great interest to astrobiologists.
However, the new results show that the exposure to radiation trapped in Saturn’s powerful magnetosphere could trigger the formation of these organic compounds on Enceladus’ icy surface instead. This calls into question their astrobiological relevance.
In their research, Dr. Richards and colleagues simulated the composition of ice on the surface and in the walls of Enceladus’ tiger stripes.
This ice contained water, carbon dioxide, methane and ammonia and was cooled to minus 200 degrees Celsius.
The researchers then bombarded the ice with ions to replicate the radiation environment around Enceladus.
The ions reacted with the icy components, creating a whole swathe of molecular species, including carbon monoxide, cyanate and ammonium.
They also produced molecular precursors to amino acids, chains of which form proteins that drive metabolic reactions, repair cells and convey nutrients in lifeforms.
Some of these compounds have previously been detected on the surface of Enceladus, but others have also been identified in the plumes.
“Molecules considered prebiotic could plausibly form in situ through radiation processing, rather than necessarily originating from the subsurface ocean,” Dr. Richards said.
“Although this doesn’t rule out the possibility that Enceladus’ ocean may be habitable, it does mean we need to be cautious in making that assumption just because of the composition of the plumes.”
“Understanding how to differentiate between ocean-derived organics and molecules formed by radiation interacting with the surface and the tiger stripes will be highly challenging.”
“More data from future missions will be required, such as a proposed Enceladus mission that is currently under consideration as part of the Voyage 2050 recommendations for ESA’s science program up until the middle of the century.”
The team’s findings were presented earlier this month at the EPSC-DPS2025 Joint Meeting in Helsinki, Finland.
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Grace Richards et al. 2025. Water-Group Ion Irradiation Studies of Enceladus Surface Analogues. EPSC Abstracts 18: EPSC-DPS2025-264; doi: 10.5194/epsc-dps2025-264
