Thera Macula may lie above a pocket of liquid water (Image: JPL/NASA)

Jupiter’s icy moon Europa is pockmarked by curious domes and depressions. How they formed has been a mystery, but now it seems they are areas where liquid water once appeared close to the surface.

Europa is thought to harbour a saltwater ocean, sandwiched between a 20-kilometre-thick layer of surface ice and a rocky core below. For clues as to what might be happening there, Britney Schmidt of the University of Texas, Austin, and colleagues looked at studies of subglacial volcanoes and ice shelves on Earth. They concluded that ice rising from the bottom of Europa’s surface layer created its 300-metre-high “chaos terrains”.

As Europa orbits Jupiter, it flexes as a result of slight variations in the gravitational tug of the giant planet. The energy that goes into this bending is converted into heat that warms the bottom of the surface ice, pushing plumes of it upwards. This changes the pressure in the ice above, creating pockets of liquid water. The water breaks up the overlying ice and refreezes over tens of thousands of years, creating jumbled domes.


A large dark spot on Europa called Thera Macula (shown right) could result from warm ice rising beneath it, says Schmidt. “We are probably witnessing active chaos formation.”

Could this liquid water close to the surface support life? Not unless it was already in the icy crust, says team member Paul Schenk of the Lunar and Planetary Institute in Houston, Texas. “Because these water pockets are short-lived – [it takes] 10,000 to 100,000 years before they refreeze – it seems doubtful anything could grow unless it were already embedded within the ice,” he says.

But if there is any life in Europa’s oceans, the process might bring it up into the icy crust, making the domes intriguing targets for future landers, he says.

Journal reference: Nature, DOI: 10.1038/nature10608