Enceladus makes the short list for most interesting places in the Solar System. After the Cassini mission discovered plumes of water ice erupting from Enceladus’ surface, other work confirmed that a salty sea probably hides below its surface. This raised some obvious questions. What’s it like in that ocean? Could there be life?

Even on its approach to Saturn, however, Cassini was inadvertently gathering evidence. It ran into tiny particles fleeing the gas giant’s gravity at high speed. This wasn't completely unexpected, but what the heck were these particles? Some modeling led researchers to conclude that they likely came from Saturn’s faint “E ring”—the portion of its ring system that we now think is fed by Enceladus’ icy geysers. That means the particles Cassini encountered on its approach would have come from those geysers.

Most of the particles in the “E ring” are ice droplets, but these particles are not. They’re miniscule, at less than 20 nanometers across. While they might once have been held inside ice droplets, they've been freed by the slow erosion of the ice by collisions with charged particles.

Apart from determining their size, Cassini was also able to make some measurements of the elements present in the nanoparticles. Silicon is clearly a major constituent, with a possible oxygen signal alongside it. Sodium and/or magnesium were the only other elements potentially registering, but the signal only barely poked its head up above the noise. Given the possibilities, it’s believed the nanoparticles were composed of silica—SiO 2 .

Silica nanoparticles with a narrow range of sizes—4 to 16 nanometers across—are unusual critters. There aren’t a whole lot of ways to produce them. In fact, a team of researchers led by University of Colorado Boulder’s Sean Hsu, University of Heidelberg’s Frank Postberg, and University of Tokyo’s Yasuhito Sekine think there is only one plausible explanation: such particles tend to form at certain seafloor hydrothermal vents. And that leads them to infer that there is hydrothermal activity at the bottom of Enceladus’ ocean, too.

To nail down the conditions required, the researchers ran some lab experiments simulating hydrothermal vent reactions. High-temperature water containing ammonia and sodium bicarbonate was added to powdered olivine and pyroxene minerals (which we would expect to find in Enceladus’ rocky core) and kept at high pressure for several months. Then, the samples were cooled, as would occur when hydrothermal fluid exits a seafloor vent. When water is cooled, it can hold less dissolved silica, and so some begins to precipitate as tiny particles.

Through the experiments, the researchers were able to calculate that hydrothermal fluid temperatures of at least 90 degrees Celsius are necessary, with pH between 8.5 and 10.5. So long as that’s true, the fluid becomes over saturated with silica and quickly precipitates nanoparticles upon cooling—nanoparticles in the same size range as the ones Cassini ran into beyond Saturn’s rings.

The picture they paint looks like this: beneath Enceladus’ seafloor, high-temperature, alkaline fluid is loading up with dissolved silica from the rocky core and rising up into the ocean. The nanoparticles that form rise to the base of the moon’s icy shell within a few years—much longer and they would be able to grow to larger sizes. From there, they would get a lift into Saturnian orbit from Enceladus’ jets.

The ice droplets launched by those jets tell us something about the ocean just beneath the ice. Tiny silica particles within those droplets may tell us about deeper places.

In an article accompanying the new paper in Nature, Gabriel Tobie of the University of Nantes in France notes that the temperatures implied for fluids in Enceladus’ rocky interior are surprisingly high and that modeling work would be necessary to assess their likelihood.

Still, those temperatures are lower than most of Earth’s hydrothermal vents, although the ones on Earth are acidic. However, the “Lost City” field of vents in the mid-Atlantic matches the description: lower temperature, higher pH. “Because it is relatively cold, Lost City has been posited as a potential analogue of hydrothermal systems in active icy moons,” Tobie writes. “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.”

That’s a lot to say about some very little particles.

Nature, 2015. DOI: 10.1038/nature14262, (About DOIs).