Ocean scientists and space enthusiasts have been dreaming for years about what might exist in alien oceans, like the one thought to slosh beneath the ice of Jupiter’s moon Europa. While actually drilling through the frozen crust presents several challenges, what if we could send a probe to simply taste extraterrestrial seas that are leaking into space?
New observations from the Hubble Space Telescope suggest we may be able to do just that: The data offer a second round of tantalizing hints that plumes of water vapor are erupting from Europa’s mysterious interior. (Explore Europa's under-ice ocean with this interactive graphic.)
Astronomers are careful to note that while the observations are promising and cleverly designed, Hubble’s latest look still doesn’t show for sure that plumes exist.
“We found a couple of candidates, evidence of plumes, but don’t consider the features as proof of plume existence,” says William Sparks of the Space Telescope Science Institute. “These are challenging observations pushing the limits of Hubble.”
Yet, given NASA’s planned mission to the icy world, observations that even hint at the presence of Europan plumes are exciting.
“Are they definitive? No. Are they compelling? Quite. Do we need missions to Europa to find out more about this beautiful little ocean world? Absolutely,” says NASA’s Kevin Hand, another member of the team reporting the finding in The Astrophysical Journal.
Tasting Moon Spit
Europa is a small but intriguing world, with a global ocean tucked beneath a thick, icy rind. It’s considered one of the most promising places to search for signs of extraterrestrial life—but, buried beneath a solid shell that’s many miles thick, that ocean and any potential Europan cephalopods are tough to get to.
So, scientists are eager to find out whether Europa’s sea is bursting into space.
If it is, then a visiting spacecraft could fly through the resulting plumes, collect some of the moony spittle, and study it for signs that the ocean could support life—which is exactly what NASA’s Europa Clipper mission, set to launch in the 2020s, is designed to do. After all, there’s a decent chance Europa’s putative plumes are fueled by that alien ocean, although they could also be tapping into pockets of melted water trapped in Europa’s icy rind.
The first hints that Europa may be launching its ocean into the ether were reported in late 2013, when a team aimed Hubble at the icy sphere and spotted what looked like plumes erupting from the moon’s southern hemisphere. At roughly 125 miles high, these plumes were “extremely exciting” and harbingers of “a new chapter” in the search for otherworldly life, scientists said at the time.
But follow-up observations, as well as searches through archival data collected with NASA’s Cassini and Galileo spacecraft, failed to see any sign of the plumes. In 2014, plume tensions flared at a meeting of the American Geophysical Union, where scientists argued about whether one of those subsequent observing campaigns could even detect the plumes it claimed to be able to see.
Other questions focused on how dense the plumes were, and if they were continually jetting into space or were transient phenomena, visible only to the sharpest eyes looking at the right times.
Since then, the question of the plumes’ existence has remained unresolved.
Most recently, Sparks and his colleagues asked Hubble to watch as Europa passed between Jupiter and Earth. With the moon in silhouette, any eruptions near its edges would filter the far ultraviolet light coming from Jupiter in a way that could reveal the presence of plumes. Ten times, Hubble watched as Europa crossed Jupiter’s face—and on three of those occasions, it spied features that could potentially be explained by plumes.
“Our team deliberated for many months, actually a couple of years at this point, and scrubbed the data as much as possible, trying to be our own best devil's advocates,” Hand says. “In the end, these three cases persisted and so they could be good candidates for plume activity.”
Where Plumes Abound
If those plumes are there, they must be an intermittent phenomenon, says Sparks. That would make them an intriguing contrast with the most famous icy geysers in the solar system: jets seen streaming from Saturn’s moon Enceladus.
In 2005, when the Cassini spacecraft flew by that small moon, it saw enormous jets exploding out of cracks in the south pole. So powerful they create a ring around Saturn, those geysers are now known to be venting water from a global ocean sloshing beneath the Enceladian crust.
Over the last decade, Cassini has flown through those plumes several times, gathering data about the sea that lies beneath. Now, we know that its ocean is salty and contains organic materials and other elements considered vital for life as we know it. Still unknown—though currently being studied—is whether hydrothermal vents may be churning away on that alien seafloor and generating an environment analogous to those that have proven so rich for life on Earth.
“We’ve amazed ourselves at Enceladus by how much we’ve been able to learn by flying through the plumes themselves,” says the Southwest Research Institute’s Hunter Waite, who’s in charge of the instrument on board the Clipper spacecraft that would sniff Europa’s plumes, if they’re there.
“I’m sure somebody will eventually want to land, but it’s amazing how far you can go with just flying above the plumes.”
At Europa, Waite says, the team will be looking for many of the same things as on Enceladus: organic compounds, salts, gases, and the ratios of various molecules. The difficulty will be in capturing enough of the plume material to analyze, and then carrying out those studies in an environment where oodles of charged particles can break molecules apart and obliterate their original state.
Still, plume samples will be vastly more informative than any information gathered remotely. In other words, Waite says, “sniffing it is better than looking for it.”