Enceladus’s oceans may be the right saltiness to sustain life

The geometry of the icy shell around Saturn’s moon Enceladus suggests that the ocean beneath is a little less salty than Earth’s oceans and could potentially sustain life

By Karmela Padavic-Callaghan

The way ice covers the surface of Saturn’s moon Enceladus suggests that the oceans trapped beneath it may be only a little less salty than Earth’s oceans. The finding adds to the possibility that this moon might be able to sustain life.

The surface of Enceladus is encased in clean, bright ice. Wanying Kang at Massachusetts Institute of Technology and her colleagues wanted to determine what the characteristics of this ice shell indicate about the ocean beneath it.

Samples taken by the Cassini spacecraft of geyser-like jets of water from Enceladus’s surface previously showed that there is some organic matter that could sustain potential life on the icy moon. Considering the waters under Enceladus’s ice was the logical next step for inferring its habitability, says Kang.

The team devised a theoretical model detailing how ocean salinity, ocean currents and ice geometry affect each other on a planet or a moon, then tweaked it to best reproduce the properties of Enceladus’s ice.

The researchers found that saltier subsurface oceans correspond to thicker ice on a planet’s poles than over its equator and vice versa for less salty water. On Enceladus, the ice over the poles is thinner than the ice over the equator. The specific variation in thickness suggests that the ocean’s salinity could be as high as 30 grams of salt in a kilogram of water. For comparison, Earth’s oceans have a salinity of 35 grams of salt per kilogram of water.

The researchers also determined details of water circulation under the moon’s ice. These currents are related to temperature differences in the water so understanding them is also important for determining habitability, says Kang.

The team found that some heat emanates from the bottom of  Enceladus’s ocean, possibly indicating the existence of heat vents in the ocean floor. Kang says that some astrobiologists have previously suggested that, like on Earth, such hydrothermal vents could be where life is found in the future.

David Stevens at the University of East Anglia, UK, says that the behaviour of ice and water on other planets is directly related to their habitability. At the same time, salinity is only one factor, he says.

Kang and her team are currently working on applying the new model to Jupiter’s moon Europa, whose oceans are thought to have a higher salinity than those on Earth and Enceladus. Ultimately, they want to pin down the details of the oceans of all icy moons and planets observed by space missions as a step towards better determining how habitable they all are.

Journal reference: Science Advances, DOI: 10.1126/sciadv.abm4665