Undersea jacuzzi could give life to Saturn's moon

Space 11 March 2015

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Hot at its heart


COLD shell, warm heart. Tiny sand grains gathered from Saturn’s rings reveal that Enceladus, already famous for its impressive water jets, is hiding a warm ocean deep beneath its surface. The finding suggests this moon is the only known place besides Earth with ongoing hydrothermal activity, and boosts the chances of finding life below its cold exterior.

NASA’s Cassini probe had already shown that Enceladus sprays plumes of dust and ice kilometres into the air. Combined with gravity data, this points to a subsurface ocean around 40 kilometres beneath the moon’s south pole. Such an ocean could be a boon to the search for alien life, but anything living there would need a source of energy, because the sun is too far away to provide much warmth.

Now painstaking detective work by Hsiang-Wen Hsu of the University of Colorado, Boulder, and his colleagues has made a connection between sand grains picked up by Cassini over a decade ago and the moon’s sea floor.


Cassini isn’t able to scoop dust from Saturn’s rings directly, but instead samples material accelerated away from the planet by its magnetic field. Hsu traced some of these grains to Saturn’s E ring, which is thought to be created by Enceladus’s plumes.

Cassini’s Cosmic Dust Analyzer instrument revealed that the dust consists of silica particles between 4 and 16 nanometres across – like sand, but much smaller. That means the grains couldn’t have formed from larger particles colliding within the rings, because then you would expect a greater range of sizes.

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The team wondered if chemical reactions between rock and water on Enceladus’s ocean floor could explain the finding. To investigate, they mixed water and silicon-rich rocks commonly found on asteroids and comets, then heated them under pressure for months to replicate conditions within Enceladus.

Out popped grains much like the ones Cassini had caught. What’s more, they provided a measure of the temperature inside Enceladus: the right kind of particles only formed above 90 °C, suggesting the icy moon’s interior must be at least that warm.

“All this independent evidence points in one direction, that means these particles most likely form from hydrothermal interactions within Enceladus,” says Hsu (Nature, DOI: 10.1038/nature14262).

Hsu thinks the moon’s entire core is probably porous and filled with warm water that diffuses up to the subsurface ocean. But the source of all this heat is still a mystery. Any heat left over from Enceladus’s formation should have radiated away long ago. Tidal heating, generated by the squeeze of Saturn’s gravity, wouldn’t provide enough energy and chemical reactions or radioactive decay that could provide warmth should also have died out by now.

Enceladus may once have received a tidal heating boost from Saturn’s other moons that we are now seeing effects of, but the details aren’t clear, says Hsu.

There may be other reasons for the diminutive size of the grains seen by Cassini, says William McKinnon of Washington University in St Louis, but it’s a promising find. “If there is a source of energy, that’s one of the prerequisites for life,” he says. “If the story that they’ve put together is true, then the implications for what’s going on inside Enceladus are profound.”

“It is a promising find. If there is a source of energy, that’s one of the prerequisites for life”

Other icy moons are thought to harbour subsurface oceans, but the hydrothermal regions on Enceladus are particularly exciting because they closely match those found in the Atlantic, says Carolyn Porco, head of the Cassini imaging team. Bacteria thrive at such sites. “When we can draw a connection between an environment on Earth and an environment on Enceladus, that just ups the ante for the possibility we might find life,” she says.

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