Dr Grace Richards of the Istituto Nazionale di Astrofisica e Planetologia Spaziale (INAF) explained that while Cassini's detection of complex organics in Enceladus's plumes remains compelling for habitability studies, laboratory simulations suggest that Saturn's radiation environment could generate similar molecules directly on the moon's icy surface.
Discovered in 2005, the plumes rise from south polar fractures known as tiger stripes, powered by tidal heating from Saturn's gravitational pull. Cassini revealed salts and varied organic molecules in the ejected material, raising hopes that the ocean beneath Enceladus could host prebiotic chemistry and potentially life.
Richards's team, supported by Europlanet, replicated Enceladus-like ices at -200C containing water, carbon dioxide, methane and ammonia at Hungary's HUN-REN Institute for Nuclear Research. Bombarding the ice with energetic ions produced compounds including carbon monoxide, cyanate, ammonium and amino acid precursors. Some of these chemicals match those already observed in both Enceladus's surface and its plumes.
"Molecules considered prebiotic could plausibly form in situ through radiation processing, rather than necessarily originating from the subsurface ocean," Richards said. "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."
Differentiating between ocean-derived and radiation-formed molecules will require data from future missions. One concept is a dedicated Enceladus mission under consideration in ESA's Voyage 2050 program, which would extend exploration planning into the middle of the century.
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