Food for microbes found on Enceladus

Saturn’s icy moon Enceladus packs snacks suitable for microbial life.

Data from the Cassini spacecraft show that the vaporous plume shooting out of the moon’s southern pole contains molecular hydrogen. It is probably generated when water in the moon’s subterranean ocean reacts with rock in its core, researchers report in the April 14 Science. Such reactions at hydrothermal vents and in other extreme environments on Earth produce high abundances of hydrogen, which some microbes use for food. There’s enough hydrogen on Enceladus to sustain microbial life, the team suggests.

“We are not saying Enceladus has life, but the discovery does move the moon higher on the list of potentially habitable places in the solar system,” says study coauthor J. Hunter Waite of the Southwest Research Institute in San Antonio.

Enceladus became a good target for finding life beyond Earth when researchers found a global ocean under the moon’s icy exterior and hints of hydrothermal activity (SN: 10/17/15, p. 8; SN: 4/18/15, p. 10). The big question was whether the ocean harbored molecular hydrogen, an energy source that could help to fuel microbes in the absence of sunlight, says Chris McKay, an astrogeophysicist at NASA Ames Research in Moffett Field, Calif., who was not involved in the study.

Researchers had tried to measure molecular hydrogen in the plume during previous flybys with Cassini. But, Waite says, the spacecraft was moving too quickly, about 64,800 kilometers per hour. On October 28, 2015, Cassini took a deep dive into the plume at a slower speed, about 30,600 kilometers per hour, giving the team enough time to make a precise measurement: Molecular hydrogenmakes up 0.4 to 1.4 percent of the gas in the plume. The majority of the ejected material is water, with traces of carbon dioxide, methane and ammonia, the team reports.

“The level of H₂ is way above the limit for life,” McKay says. Laboratory studies show that microbes fueled by hydrogen need as little as 10 parts per million of itin their environment to survive: 0.4 percent, or 4,000 parts per million, would provide an abundance of food, he says.

Such an abundance of hydrogen was surprising, Waite says. He and colleagues considered whether the gas might come from hydrothermal activity, other processes or possibly material left over from the moon’s formation. Calculations showed that only ongoing hydrothermal activity could produce enough hydrogen.

The result, McKay says, “confirms that the ocean of Enceladus would be a nice place for life.” It’s so ripe for life that NASA wants to ensure Cassini does not contaminate the moon, or its neighbor Titan, with stray Earthlings. The spacecraft is running out of fuel and could accidentally crash into one of these icy worlds. To prevent that, Cassini will start a series of orbits around Saturn on April 23 that will end with a deadly plunge into the planet’s atmosphere in September.