But just because these new results — also relying on Hubble data and set to be published in the Astrophysical Journal — don't have us gasping in shock doesn't mean they aren't significant and cool. This is the first time plumes have been spotted again since that one-off in 2012.
Plumes would be exciting because they'd probably be signs of geological activity under the ocean surface, which would mean a source of energy for potential life-forms under the alien sea. Scientists are fairly certain that Europa is covered in a global, salty ocean full of genuine liquid H2O kept warm by the powerful tidal forces of Jupiter. There's a rocky core deep beneath that precious liquid, and if hydrothermal vents or volcanoes are heating up the ocean floor and spurring on chemical interactions between rock and water, microbes could make a cozy home there. At least in theory, anyway. On Earth, life is found pretty much wherever there is water and energy. Until we probe potentially habitable places like Europa, we can't be sure how often that basic equation works on other worlds.
Those tell-tale plumes would also make the hunt for microbial life easier by giving us a way to sample the ocean without drilling into miles of ice. The Cassini spacecraft recently made a pit stop near Enceladus, a moon of Saturn known to shoot geysers out of its subsurface ocean with far more regularity, for the sole purpose of diving through its plumes.
“Europa’s ocean is considered one of the most promising places that could potentially harbor life in the solar system,” Geoff Yoder, acting associate administrator for NASA’s Science Mission Directorate, said in a statement. “These plumes, if they do indeed exist, may provide another way to sample Europa’s subsurface.”
Scientists detected the plumes by borrowing a trick from exoplanet hunters: When an exoplanet passes in front of its host star, the light that passes through its atmosphere changes based on the molecules it encounters. So by looking at a star with a planet crossing in front of it, scientists can read the chemical makeup of its atmosphere — a crucial step in determining a world's habitability.
Led by William Sparks of the Space Telescope Science Institute in Baltimore, researchers retooled this method to look at an alien world much closer to home.
"The atmosphere of an extrasolar planet blocks some of the starlight that is behind it," Sparks said in a statement. "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."
In searching for signs of Europa's atmosphere, they saw water vapor venting from its surface. The team watched Europa transit in front of Jupiter 10 times over the course of 15 months and saw what might have been plumes on three occasions.
At Monday's news conference, the researchers stressed that these results push the limits of the Hubble's capabilities — they can't be too certain of what they're seeing.
“We do not claim to have proven the existence of plumes but rather to have contributed evidence that such activity may be present," Sparks said.
But no other known natural phenomenon could explain the water vapor they've seen, he added.
Experts not involved with the study agreed that the results, while far from definitive, could prove to be significant if true.
"When you push the envelope with any astronomical facility and you’re really at the limit, there’s always the possibility that it might be, something else, a background atmosphere for example," Cornell University's Jonathan Lunine, who wasn't involved in the new study, told The Washington Post. "But it looks pretty confined, not spatially uniform, so that argues for it being a plume."
If Europa really is displaying its plumage, Lunine explained, the big question remaining is whether the jets of water come from the ocean itself or the icy crust that contains it.
"You can have a plume coming from warm ice," he said. A plume coming from the icy crust might carry the same molecules as the ocean far below, but it might not. So scientists hope to sample water propelled up from the deepest depths, where life might flourish in warm volcanic vents. In that case, the plumes would carry the molecular signatures of life.
"If it’s coming from the ocean, it means that we’re sampling material that might be part of a habitable environment," Lunine said.
NASA already has a mission to Europa in the works, scheduled to launch in 2022 and arrive a few years after that. That probe won't land or even orbit the icy moon — Jupiter's radiation makes the area too dangerous for a robot — but it will orbit Jupiter and make a few close passes of the watery world. If Europa does indeed shoot geysers into space, the spacecraft could use those plumes to sniff out the chemical composition of an ocean without needing to get too close.
But unless follow-up observations from home find more definitive geyser evidence, a probe hoping to dive through the spray might not know where to go looking for watery eruptions. Unlike Enceladus – where certain geological features make it obvious where the frequent plumes will occur – Europa has a complex surface with many potential hot spots.
“Whereas on Enceladus the geology makes it a little bit obvious where the activity has been, on Europa there’s activity everywhere that could lead to plumes or could muddy the interpretation,” Georgia Tech’s Britney Schmidt said at Monday's news conference.
If any areas of the moon show particular promise in ground-based observations or during the upcoming flyby, a future mission could land on the surface and drill into the ice. But don't hold your breath for that subsurface dive. Scientists think Europa has about 60 miles of ice in its crust, compared to the half mile or so that we have to drill through to study subglacial lakes on Earth. It's going to take a lot of work to create a spacecraft that can survive Europa's radiation levels, land on an icy surface, operate at temperatures of minus-180 degrees Fahrenheit and tunnel down through the massive layer of solid ice before it even starts collecting data.
In the meantime, scientists are hopeful that the powerful James Webb Telescope, launching in 2018, will paint a better picture of Europa's possible plumes, pushing past the limits of Hubble's line of sight.
This post was originally published on September 26. It has been updated.