Something’s cooking on Enceladus

The core of Saturn’s moon Enceladus may be cooking the water in its subsurface sea.

Silicon-rich particles embedded in one of Saturn’s rings originated in water on Enceladus that had been heated to at least 90° Celsius, researchers report in the March 12 Nature. The debris probably was dredged up from the bottom of the moon’s ocean by water percolating through the rocky core, and then blasted into space through cracks in the moon’s icy shell (SN: 5/3/14, p. 11). Warm water-rock interactions are found in hydrothermal vents on Earth as well. If they’re occurring on Enceladus, it’s yet another sign that the moon has conditions favorable for life.  

Hsiang-Wen Hsu, a planetary scientist at the University of Colorado Boulder, and colleagues used data collected by the Cassini spacecraft, orbiting Saturn since 2004, to measure the size and composition of the particles. They found that the debris was made mostly of silicon dioxide grains, or silica, each just a few nanometers across. Silica is a common by-product of rock in contact with water. The Blue Lagoon in Iceland, for instance, gets its milky appearance from fine silica particles in the water. Hot water from a nearby geothermal power plant dissolves silica in the rocks, forming particles similar to those in Saturn’s E ring, Hsu says.

To replicate the process, Hsu and colleagues let a powdery mix of minerals commonly found in asteroids and comets sit in pressurized cocktails of water, ammonia and sodium bicarbonate ranging from 120° to 300° C for several months. The resulting silica concentrations allowed the researchers to deduce the chemical reactions at play and calculate a minimum temperature at which silica particles can form: 90° C. The results suggest that there may be a tremendous source of heat at the bottom of Enceladus’ ocean.

“This is an extraordinary claim,” says Christopher Glein, a geochemist at the University of Toronto. While he agrees that the debris is created by interactions between rock and water on Enceladus, he’s cautious about the idea of ongoing hydrothermal activity. It’s very difficult, he says, to generate such high temperatures on Enceladus. The most likely source of heat is friction generated by the gravity of Saturn alternately squishing and stretching the moon. But there doesn’t seem to be enough energy to crank the thermostat up to 90° C.

Nevertheless, he says, “Enceladus seems to be a master at defying expectations.” One way to test Hsu’s claim is to look for molecular hydrogen blasting out of Enceladus. Seawater interacting with rock at some hydrothermal vents on Earth produces high concentrations of hydrogen. If there is hydrothermal activity on Enceladus, says Glein, Cassini could detect molecular hydrogen in the water plumes.

A lack of hot water on Enceladus isn’t necessarily fatal to life. Even in the absence of high temperatures, the interaction of the different chemical environments at a water-rock boundary creates a source of energy that organisms can tap into, says Glein. Hot water is just an added bonus. The energy released where hot and cold water mix creates more opportunities for life to flourish. “We see those types of things on Earth,” he says, such as around hydrothermal vents on the bottom of the ocean. “Organisms like to take advantage of those situations.”