Saturn's icy moon Enceladus appears to host seeps of hot, mineral-rich water rising from the moon's core into an ocean hidden under an icy crust – a process that raises intriguing prospects for life.
On Earth, hydrothermal vents at bottom of the ocean support deep-sea oases whose inhabitants range from humble bacteria to crabs, fish, and tube worms in a realm bereft of sunlight.
If the undersea hydrothermal vents on Earth are any indication, Enceladus's hydrothermal system has many of the ingredients scientists say are needed to support organic life. In Enceladus's case, those would include water, a source of energy, and potential nutrients.
The evidence for hydrothermal activity on Enceladus is indirect. It comes from vanishingly small grains of silica captured by NASA's Cassini spacecraft, which is orbiting Saturn. In its tour of the Saturn system, Cassini as it has repeatedly found silica in Saturn's second-farthest ring, which is fed by geysers of water ice erupting from Enceladus.
The only plausible explanation for silica's presence, researchers say, are hot, mineral-laden fluids reaching the interface between Enceladus’s core and ocean.
On Earth, silica is the ho-hum stuff of beach sand. At Enceladus, the results are exciting because they "point strongly to the presence of a hydrothermal system," one that is currently active, notes Jonathan Lunine, a planetary scientist at Cornell University in Ithaca, N.Y., who was not a member of the team reporting the results.
That could raise the stakes for sending another mission to Enceladus to sample the geyser plumes.
NASA is considering a proposal to send a relatively small spacecraft back to Enceladus to repeatedly sample the plume and hunt for evidence of life in the chemistry of the ices it captures.
"For any system where you don't have photosynthesis, which we don't expect to have here because this is under 40 kilometers [25 miles] of ice, the biologically active zone is going to be the interface between the rock and the water – the ocean floor," says Dr. Lunine, who proposed the Enceladus mission along with colleagues from the US and Germany.
The discovery has been a long time coming, says Frank Postberg, a planetary scientist at the University of Heidelberg in Germany and one of three scientists who led the effort.
Cassini's Cosmic Dust Analyzer first detected the silica grains nearly 10 years ago, he says, but the team didn't know quite what to make of the information.
Initially, the researchers thought the silica somehow had reached the E-ring from Saturn's main rings, which lie closer to the planet.
But by gathering and analyzing additional samples over several years, the researchers found that the silicate grains includes virtually no metals, not what one would expect if they originated from pulverized planetary rock from somewhere else.
The researchers noted that the formation of such tiny silica grains was unique to water carrying dissolved silica. So they conducted lab experiments, incorporating previous studies of the chemistry of Enceladus's under-ice sea. The experiments varied other conditions, including temperature and pH, until the right-size silica grains formed.
The results suggested that the grains formed in 90 degrees Celsius (162 degrees Fahrenheit) water. The grains also allowed the scientists to estimate the water's high alkalinity, as well as its salinity, which was comparable to levels other studies have estimated. The grains’ tiny sizes suggests they formed during Enceladus’s current eruptive phase – they didn’t have time to grow substantially larger.
On an icy moon, those temperatures would be generated by friction within Enceladus's core, which results from the moon’s interaction with Saturn's gravity as it orbits the giant planet.
The team suggests that the hydrothermal activity originates deep in a relatively porous crust. As water circulates through the crust and heats, it dissolves and carries minerals that precipitate out as grains when the water reaches the interface between ocean and core. As the water continues to rise through the ice crust toward eruption, the grains serve as seeds around which ice crystals can form. When these are ejected into Enceladus's plume, they become part of the E-ring. Over time the ice vanishes, leaving the grains behind.
Last month, researchers suggested in the journal Geophysical Research Letters that hydrothermal activity could be one of two potential sources for methane detected in Enceladus's plume. Methane can be a signature of biological activity, although it also has non-biological sources.
Conditions at Enceladus' hydrothermal sites appear to be similar to those at the Lost City, a hydrothermal field beneath the North Atlantic Ocean, notes Gabriel Tobie, a planetary scientist at the University of Nantes in France.
Temperatures, pH levels, and alkalinity are comparable between to the two, he writes in a commentary accompanying the team's study.
Given that life many have emerged on Earth first in such environments, "the discovery of similar environments opens fresh perspectives on the search for life elsewhere in the solar system, he writes.
For those interested in the question of life beyond Earth, at least for now, Enceladus appears to be the gift that keeps on giving.
Says Lunine: "It keeps producing stuff for Cassini to discover that strengthens the case for habitability."