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Pockets of liquid water trapped in the thick ice shell of Jupiter鈥檚 moon Europa may be much shorter-lived than previously thought, but they may still be present today and potential habitats for life.

Europa, the fourth-largest moon of Jupiter, is believed to have a liquid water ocean tens of kilometres under its frozen surface. This water may be in contact with an ocean floor that provides the necessary mix of materials for life to arise.

Previous research suggested that parts of the icy shell might also be liquid in the form of pockets of water 10 kilometres or so wide that sit much closer to the surface, perhaps only a kilometre down. Now Chase Chivers at the Georgia Institute of Technology in the US and his colleagues have modelled these pockets in greater detail, finding that while they might be shorter-lived than thought, they are still promising locations for life.

鈥淲e find that they last for tens of thousands of years鈥� before they re-freeze, says Chivers. Previous research had suggested they would last perhaps hundreds of thousands of years.

Evidence for the pockets comes from images taken by NASA鈥檚 Galileo spacecraft in the 1990s and 2000s. It spotted pits and markings called lenticulae on the surface of Europa, some of which appear dark in colour 鈥� thought to be linked to salt in the subsurface pockets that keeps water liquid.

The appearance of these features suggests that water pockets are still present today, says Chivers, possibly hundreds of them. They may be a result of the ocean seeping into the icy crust or portions of the crust itself melting.

鈥淲e think there is still shallow water under some of these features,鈥� says Chivers. Some may even erupt onto the surface as plumes, which were previously thought to come directly from Europa鈥檚 subsurface ocean.

If these pockets do exist, they could be potential habitats for life, says Mark Fox-Powell at the Open University in the UK. 鈥淚f there is life in the subsurface ocean, and it gets incorporated into the ice shell and later re-melted, that could kick-start a community,鈥� he says. However, once the pockets re-freeze, that life would become trapped. 鈥淚t鈥檚 a doomed community.鈥�

The existence of the pockets so close to the surface would make them potentially detectable by upcoming missions such as NASA鈥檚 Europa Clipper spacecraft, scheduled to launch in 2024 and arrive in 2030. The craft will fly by the moon and use a radar to peer beneath the surface, and also a dust analyser that could detect material from one of these pockets 鈥� perhaps even microbial life itself 鈥� if it were to pass through a plume linked to one.

Steve Vance at NASA鈥檚 Jet Propulsion Laboratory in California, a member of the Europa Clipper science team, says having evidence of liquid water so close to the surface would be 鈥渞eally intriguing鈥�. And if these pockets do exist, they would be a much shallower target to perhaps directly sample with a future lander mission on Europa, says Vance.

鈥淒rilling through a kilometre of ice sounds a lot easier than drilling through five or more kilometres,鈥� he says.

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