Fantasy or reality? (Image: NASA)

Now you see it, now you don’t. Europa’s 200-kilometre-high water jets may have been downgraded from major discovery to major mystery. Follow-up searches have yet to see the geysers again, while older observations don’t seem to support their existence. Some people are now wondering if the jets are far rarer than expected – or if they were ever there to begin with.

“It’s a real puzzle now,” said Donald Shemansky of the University of Southern California in Los Angeles, who presented the analysis of spacecraft data at the American Geophysical Union conference in San Francisco on 18 December that contradicted the idea of regularly erupting plumes.

We already suspected that Jupiter’s icy moon Europa had a vast ocean of water beneath its frozen crust. But excitement surged last year when a team led by Lorenz Roth of the Southwest Research Institute in San Antonio, Texas, announced that the Hubble Space Telescope had spotted a small bump of water coming from Europa’s south pole, meaning the moon was shooting its insides out into space.


This made the moon an ideal target for orbital probes to attempt to fly through the jets and detect the presence of life.

Shy geysers

But the geysers have yet to reappear. That doesn’t necessarily mean they don’t exist, says Roth – but they “are more transient than we would have hoped”.

Shemansky’s results further muddle the spouting waters. His team looked at data from NASA’s Cassini spacecraft, which flew by Jupiter in 2001 on its way to Saturn.

Another moon of Jupiter, Io, is extremely active, with constant volcanic eruptions on its surface that shoot charged particles of sulphur and oxygen out into space. These ions get swept up in Jupiter’s strong magnetic field, forming a ring of plasma around the gas giant.

If Europa’s plumes had been active in 2001, some of their water molecules should have been split by Jupiter’s radiation and dumped hydrogen atoms into space. For a short time, these hydrogen atoms would have joined Jupiter’s plasma torus, cooling the other charged particles.

Thin air

But when Shemansky and his team looked at the Cassini data, they saw nothing like that. Furthermore, they argue that Europa’s atmosphere is about two orders of magnitude thinner than previously believed, which seems impossible if it is regularly being replenished with water from the inside.

Not so fast, says Kurt Retherford, also of the Southwest Research Institute and another of the plume’s original discoverers. Cassini zipped by Jupiter at a significant distance, making hydrogen atoms difficult to detect even if they were present.

“We would say using their technique, they couldn’t possibly find water,” he said.

Retherford, Roth and their colleagues are now preparing a paper to rebut Shemansky’s analysis. Their main worry is about how Shemansky was modelling the plasma around Jupiter.

“It contradicts everything that’s been done before in Europa’s environment,” said Roth.

At the limits

The original detection of plumes was at the limit of Hubble’s capabilities, and finding them again may be difficult for many reasons. Until a definitive repeat observation is made, many in the field are hedging their bets.

“The Hubble observation was so borderline that maybe they were fooled, or they got lucky and caught an event that’s not so common,” said Robert Pappalardo of NASA’s Jet Propulsion Laboratory in Pasadena, California.

NASA is expected to decide next year whether to send a robotic mission to the frozen moon, so knowing if the jets are real will be crucial in guiding how researchers design instruments to either try to confirm their existence or sample their contents.

Whether the geysers exist or not, Pappalardo still sees Europa as a great scientific destination. “Either way, the plumes certainly kicked Europa up in the public consciousness.”

Journal reference: Astrophysical Journal, DOI: 10.1088/0004-637X/797/2/84