Illustration of Europa’s icy surface with a water jet in the foreground and Jupiter and the sun behind (Image: K. Retherford, Southwest Research Institute)

An icy moon of Jupiter has been caught spitting into space. For the first time, a towering plume of water vapour has been seen coming from Europa. The discovery strengthens the case that the moon has a liquid ocean beneath its icy crust, and may even offer a way to taste its seas and search for signs of life.

Images from the Galileo spacecraft, which orbited Jupiter from 1995 to 2003, revealed a cracked and chaotic surface dominated by dark ridges and faults. This hinted that Europa has a relatively thin crust in which fissures sometimes open up and let water escape from a subsurface ocean. Similar rifts on Saturn’s icy moon Enceladus shoot spectacular water geysers. But the Galileo probe did not spot any plumes in action on Europa, and later efforts also came up empty.

Now images taken by the Hubble Space Telescope have revealed a large cloud of hydrogen and oxygen – most likely in the form of water vapour – extending from the moon’s south pole. A model suggests that it is a plume 200 kilometres high that is spouting 3000 kilograms of water per second.


“This is a big discovery,” says Cynthia Phillips at the SETI Institute in Mountain View, California, who was not involved in the find but had looked for plumes with Galileo. “If there are plumes erupting, there’s got to be liquid water, and it’s got to be pretty close to the surface.”

Shy geyser

Previous search efforts might have missed the plumes in part because they are intermittent, says Lorenz Roth of the Southwest Research Institute in San Antonio, Texas. The gravitational pull of mighty Jupiter squashes and stretches the small moon, and it is possible that the cracks only open when Europa is furthest from the planet. Even then, the sizes of any plumes may vary drastically.

“If there had been a plume of this magnitude erupting during Galileo, we would have seen it,” says Phillips. “This points to serious variability.”

Another issue may be that previous scans were done in visible light, says Roth. “If the plumes only contain vapour, but not ice or dust grains that scatter sunlight, you can hardly detect them in normal, visible-light images,” he says.

Scoop of life?

Roth and his colleagues instead went looking in Hubble’s ultraviolet images, which can capture the light emitted when hydrogen and oxygen molecules collide with stray electrons. The team used UV images of Europa taken in November and December 2012, as well as images from 1999. The more recent December image showed the plume, which remained visible for all 7 hours of Hubble’s observations.

It’s possible that Europa’s ocean sustains life, making it a tempting target for future space missions. Two spacecraft currently en route to the Jupiter system – the European Space Agency’s JUICE mission and NASA’s Juno mission – may be able to observe the plumes at close range. And if NASA’s Europa Clipper probe concept gets off the ground, that craft could one day get even closer.

If a future orbiter could collect some material from a plume, that would allow us to sample the seas without the difficult task of landing and drilling into the ice, says Phillips: “If there are life forms, they would be in the liquid layer. If the liquid is being thrown out into orbit, maybe you could catch something. How cool would that be?”

Journal reference: Science, DOI: 10.1126/science.1247051