Many planetary bodies are thought to have produced hydrothermal activity — interactions between water and rock — as a result of hot-water circulation during the early stages of the Solar System, but Earth was the only one known to be sustaining such activity today. Then, in 2005, the Cassini spacecraft discovered eruptions of water vapour and ice emanating from long, warm fractures on the south pole of Saturn's moon Enceladus. The detection of salted, icy grains in Enceladus' erupting plume1 clearly pointed to an ocean environment below its icy crust and to the leaching of rocks by warm water, at least in the past. On page 207 of this issue, Hsu et al.2 report hints of presently active hydrothermal processes on Enceladus.

This story began about a decade ago during Cassini's approach to Saturn, when one of the spacecraft's instruments detected tiny dust particles, called stream particles, escaping into interplanetary space from the Saturn system3. Analysis of these particles revealed that they were mostly nanometre-sized and rich in silicon, in contrast to the ice-rich particles prevalent in the Saturnian environment. The origin of these particles has remained enigmatic for years.

Building on their earlier modelling work4, Hsu and colleagues simulated the dynamics of the particles' ejection, tracking them back to their most probable source region: Saturn's E ring, a tenuous ring mostly made of small ice grains, extending between the orbits of the moons Mimas and Titan. Because Enceladus is the source of particles in the E ring5, it must also be the ultimate source of the silicon-rich stream particles, which were presumably once incorporated in icy grains.

By analysing mass spectra of the stream particles, the authors concluded that the dominant constituent is silica (SiO2). This is much more probable than pure silicon or silicon carbide (SiC), two other potential candidates. Silica is extremely common on Earth, occurring mostly in the natural form of quartz. But finding silica nanoparticles in the Saturnian environment is unexpected. Hsu et al. ruled out fragmentation of larger grains as a possible process to explain the narrow size distribution of the stream particles. The composition and size distribution must therefore be inherited from the particles' formation process, which seems most likely to have been fast crystallization of silica nanoparticles from supersaturated aqueous solutions.

Using laboratory experiments, the authors finally showed that silica particles with the observed size distribution can be produced only under rather specific thermo-physical conditions, thus constraining the thermal state of Enceladus' interior. Specifically, a region of the rock core must have a temperature of at least 90 °C and be in contact with water of pH greater than 8.5 to dissolve silica in sufficient amounts; the oceanic salinity should be less than 4% and oceanic pH in the range of 8.5 to 10.5, to allow the formation of numerous nanometre-sized silica grains.

The inferred core temperature is unexpectedly high for a body the size of Enceladus (approximately 500 kilometres in diameter), especially given that deep water circulation should efficiently cool the core6. A strong heat source must exist to raise the core temperature above 90 °C — most probably tidal friction, and possibly also exothermic water–rock reactions known as serpentinization reactions7. But modelling is needed to determine whether tidal flow and serpentinization in the core could provide sufficient energy at present to allow hydrothermal activity, and, if so, for how long.

Intriguingly, the conditions inferred by Hsu and colleagues in Enceladus' water–rock system are similar to those found on Earth in an atypical hydrothermal field called Lost City (Fig. 1), which was discovered in the early 2000s in the mid-Atlantic Ocean8,9. This hydrothermal field consists of limestone chimneys 60 metres tall, which vent metal-poor, basic fluids (pH 10–11) at a temperature of 90 °C; the fluids are rich in hydrogen, abiotically produced methane and other organic compounds. For comparison, most other known fields are fuelled by acidic (pH 3–5), metal- and sulfide-rich fluids at temperatures greater than 300 °C (ref. 8).

Figure 1: The Lost City hydrothermal field under the mid-Atlantic Ocean.
figure 1

IFE, URI-IAO, UW, Lost City Science Party; NOAA/OAR/OER; Lost City 2005 Exp./CC BY 2.0

These limestone chimneys, which are up to 60 metres tall, vent fluids at a temperature of 90 °C. Hsu et al.2 report evidence of a similar aqueous environment in Saturn's icy moon Enceladus.

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Because it is relatively cold, Lost City has been posited9 as a potential analogue of hydrothermal systems in active icy moons. The current findings confirm this. What is more, alkaline hydrothermal vents might have been the birthplace of the first living organisms on the early Earth, and so the discovery of similar environments on Enceladus opens fresh perspectives on the search for life elsewhere in the Solar System.

Hsu et al. also conclude that the silica particles must be transported from the core hydrothermal source to the plume source near the surface in a fairly short time — from months to years at most — to limit the particles' growth. This implies that samples of materials erupted from Enceladus' warm fissures would provide a unique opportunity to directly probe aqueous, possibly prebiotic, processes occurring deep in Enceladus' rock core, in almost real time. Cassini's discoveries, together with Hsu and colleagues' findings, point to potentially complex chemical processes in Enceladus' watery interior. Cassini will fly through the moon's plume again later this year, but only future missions that can undertake improved in situ investigations10,11, and possibly even return samples to Earth11, will be able to confirm Enceladus' astrobiological potential and fully reveal the secrets of its hot springs.Footnote 1