Year in review: Global ocean spans Enceladus

As it winds up its studies of Saturn’s moon Enceladus, NASA’s Cassini spacecraft is offering the best evidence yet that this moon’s buried ocean could be a great place to search for extraterrestrial life.

Cassini, which has orbited Saturn since 2004, has swooped past Enceladus more than 20 times. But only recently have measurements confirmed that, beneath the moon’s icy shell, an underground ocean spans the entire globe (SN: 10/17/15, p. 8). Scientists had suspected for a decade that the moon had a smaller sea, based on geysers spurting near its southern pole, but a widespread ocean means more room for otherworldly microbes to thrive.

Enceladus is freezing — around −200° Celsius at its surface — but heat generated by friction as Saturn’s gravitational pull tugs on the moon probably keeps its buried sea liquid. Where the ocean meets underlying rock, temperatures may even soar to a near-boiling 90° (SN: 4/18/15, p. 10). That’s how hot it has to be in some geothermal springs on Earth for silica in rocks to dissolve, producing a milky-looking fluid. Cassini spotted tiny silica particles in one of Saturn’s rings, probably delivered there from the bottom of Enceladus’ ocean by those gushing geysers, one research team proposes.

On Earth, hot springs are home to bacteria and other creatures; on Enceladus, the chemistry might also be right for life. Using measurements of carbon dioxide and salts from the geysers, scientists calculated that the ocean is probably quite alkaline, similar to household ammonia (SN: 3/21/15, p. 12). Alkaline lakes on Earth, like California’s Mono Lake, are among the most biologically productive aquatic environments on the planet.

Cassini took its deepest dive through the Enceladus geysers on October 28, a close call that will surely yield science for years to come. The swan song, Cassini’s final Enceladus visit, is scheduled for December 19.