Europa, one of the four largest moons of Jupiter, has surprised astronomers with the discovery that plumes of water vapour appear to be erupting from beneath its surface. 

They could allow space scientists to discover whether liquid water beneath the moon’s surface could be home to alien aquatic life, a possibility that has long intrigued astrobiologists.

The plumes, rising 200km (125 miles) high, have been revealed in photographic images taken by NASA’s Hubble Space Telescope as the Galilean satellite passed in front of its parent planet.

Bright features rising from the edge of Europa were observed by the space telescope three times out of ten separate occasions when planetary scientists observed the planet.

The discovery has excited the scientists because Europa was already believed to have a salty subterranean ocean beneath the whole of its surface crust. This underground sea, discovered by NASA’s Galileo probe in the 1990s, may be more than 100km deep, and hold twice as much water as all the Earth’s seas put together.

If the sea really is bursting free from the crust as vapour, it will make it a whole lot easier for NASA to take samples of Europa’s water with future space probes. It had been imagined that they would have to drill through several kilometres of ice to reach it. The European Space Agency’s Juice mission will also study Europa and other moons of Jupiter for hints of life.

NASA announced the discovery at a special press conference today. Geoff Yoder, acting associate administrator for NASA’s Science Mission Directorate in Washington, said in a statement: “Europa’s ocean is considered to be one of the most promising places that could potentially harbor life in the solar system.

“These plumes, if they do indeed exist, may provide another way to sample Europa’s subsurface.”

The finger-like plumes were found by a team led by William Sparks of the Space Telescope Science Institute (STScI) in Baltimore. Their original aim was to detect whether Europa had a thin atmosphere by using a transit technique similar to that used by observers of exoplanets.

Whereas the exoplanet expert watched to see if starlight revealed the existence of an atmosphere around alien worlds, Sparks’ team wanted to see if Jupiter’s reflected light indicated something similar around Europa. But they also realised that if water vapour was venting from Europa’s surface, that would be revealed too.

Sparks said: “The atmosphere of an extrasolar planet blocks some of the starlight that is behind it.

“If there is a thin atmosphere around Europa, it has the potential to block some of the light of Jupiter, and we could see it as a silhouette. And so we were looking for absorption features around the limb of Europa as it transited the smooth face of Jupiter.”

More energetic jets of water vapour have previously been observed spouting from the polar region of Saturn’s icy moon Enceladus. NASA’s Cassini probe has even flown directly through the plumes a number of times to “taste” them.

Europa is a rocky world with a surface that was first revealed to be smooth and icy and covered with a surprising pattern of cracks by the Voyager probes in the early 1980s. The pattern resembles ridge ice found in the Arctic Ocean, and is thought to be caused by the tidal forces exerted by Jupiter and also other moons.

Water in the underground ocean is kept liquid by the powerful pull of Jupiter, raising the intriguing prospect that simple life might have flourished there. Europa is the smallest of the four main moons of Jupiter discovered by the Italian astronomer Galileo Galilei in the 17th century.

It has a diameter of 3,140 km and orbits at a distance of 671,100 km, once every 3.55 days. It is tidally locked, or in other words, always keeps the same face towards Jupiter.