Organic molecules detected in the watery plumes that spew out from cracks in the surface of Enceladus could be formed through exposure to radiation on Saturn's icy moon, rather than originating from deep within its sub-surface ocean. The findings, presented during the EPSC–DPS2025 Joint Meeting in Helsinki this week, have repercussions for assessing the habitability of Enceladus's ocean.
'While the identification of complex organic molecules in Enceladus's 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, of the Istituto Nazionale di Astrofisica e Planetologia Spaziale (INAF) in Rome, who is presenting the results at the meeting.
The plumes were discovered in 2005 by NASA's Cassini spacecraft. They emanate from long fractures called 'tiger stripes' that are located in Enceladus's south polar region. 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's 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, results of experiments by Richards and her colleagues show that the exposure to radiation trapped in Saturn's powerful magnetosphere could trigger the formation of these organic compounds on Enceladus's icy surface instead. This calls into question their astrobiological relevance.
Richards, with funding from Europlanet, visited facilities at the HUN-REN Institute for Nuclear Research in Hungary, where she and colleagues simulated the composition of ice on the surface and in the walls of Enceladus's tiger stripes. This ice contained water, carbon dioxide, methane and ammonia and was cooled to -200 degrees Celsius. Richards's team then bombarded the ice with ions – atoms and molecules stripped of an electron – 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,' said Richards. '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.'
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 the European Space Agency (ESA)'s science programme up until the middle of the century.
