Except for Earth, the moons of the giant planets, especially the icy moons such as Enceladus, Europa, Ganymede and Callisto, are the only other places in the Solar System with considerable evidence of the existence of liquid water in abundance. The oceans under the icy crusts on these moons may be able to host living organisms1,2,3,4,5,6,7,8 and thus be able to host habitable environments. Two critical factors that can make these moons compelling environments for living systems, besides the presence of water, are the availability of energy and a chemical disequilibrium based on reductant-oxidant pairs f or biological processes3. The surface of the Jovian moon Europa is young, with a resurfacing age determined to be approximately 30–70 Myr9, and it represents one of the most important targets for astrobiology research in the Solar System10. Studies also indicate active geological processes on a global scale on this icy moon such as the movement of lithospheric blocks and the upwelling of material that fills the rocky core3,11,12. These aspects could help to maintain the chemical disequilibrium in the oceans under its icy layer, which is powered by tidal and radiolysis phenomena (the latter is emphasized in this paper).

Radiolysis has already been proposed for the Europan framework. However, it was mainly focused on radiolysis caused by the bombardment of the superficial ice with energetic charged particles, such as electrons or ions, that are accelerated by the magnetic field of a nearby planet (in this case, Jupiter)3,13. Another possibility for the radiolysis on Europa comes from the bottom of the ocean, where there is a water-rock interface. Radioactive decay also occurs from fissionable materials that exist in every rocky celestial body in the Solar System. These materials, primordial radioactive elements, emit ionizing radiation that can interact with the ocean water that breaks its molecules, causes excitation and ionization, and consequently and locally forms very reactive ionized or radical species4,14,15,16,17. This defines the water radiolysis, the focus of this work. Research on the radiation effects on water and mainly radical formation induced by radiolysis increased largely in the mid-20th century14; however, the association of radiation from nuclear decay as a possible source of energy for a living system was proposed near the end of the century18. Recent studies of the so-called fossil natural reactors on Earth18 have provided a basis for the debate on the importance of ionizing energy from radioactive decay as a localized source of energy for biological processes. Notwithstanding, the recent discovery of peculiar ecosystems in deep subsurface environments, which are maintained by nutrients produced via radionuclide radiolysis19,20,21, has garnered attention for its feasibility.

On Earth, water radiolysis is significant in the deep environments where water and fissionable materials exist17,19,22,23 and consequently form several chemical species that contribute to microbial activity20,21. Chivian et al.20 and Lin et al.21 reported an important occurrence in nature of metabolism dependent on this type of radioactivity interaction. In the depths of the Mponeng gold mine in South Africa20,21 and located at the region of the Witwatersrand basin, it was found that a single-species ecosystem based on the bacterium Candidatus Desulforudis audaxviator, which uses this source of energy, was independent of sunlight. This discovery opened new venues to the study of other non-illuminated environments of the Solar System and the Universe, including Europa and other icy moons.

Recently, the debate on radiolysis under the surface of Europa has gained new perspectives. Atri24 discusses the importance of galactic cosmic rays (GCR), which are primary charged particles, mostly protons, that originated beyond the Solar System. If a celestial body has a reasonably thick atmosphere, primary GCR particles strike the atmospheric molecules, producing secondary particles such as kaons, pions and muons that can propagate deep underground and are highly unstable, quickly decaying to produce particles such as β and γ particles and possibly triggering radiolysis. The radiolysis discussed in that work24 is galactic cosmic ray-induced and may be important when considering small rocky bodies such as planets not tied to any planetary system or comet, but it depends on the presence of an atmosphere. However, radiolysis from radioactive isotope decay has shown potential importance in powering life on the deep subsurface of icy moons where solar energy cannot reach and galactic cosmic rays cannot provide enough energy. Considering charged particles of reasonable primary energy, only muons could reach 3 km below the surface level of an icy moon, and the energy deposition rate would still become nearly zero below this depth25,26.

However, there is still a modest number of references in the literature related to the effect of water radiolysis as a consequence of radioactive minerals in the deep subsurface icy moons and its implications for habitability. Recently, the radiolytic production of H 2 in the subsurface of several of the Solar System’s icy moons was proposed13,27, although there is a necessity for complementary models to associate radiolytic energy production with biological metabolism to assess the actual habitability of extensive extraterrestrial water bodies.

Models related to the survival of bacterial cells based on radiolysis-produced chemical species, such as H 2 , have been proposed28. This model focuses on terrestrial context and on the primary radiolysis product. In contrast, in this study, we present the model based on the production of a secondary chemical species, sulfate, and apply it to the extraterrestrial context. For this model, we compared the radiolysis-produced sulfate rate to in situ sulfate demand for a deep subsurface environment where Ca. D. audaxviator was found.