Two moons in the outer solar system—Jupiter’s Europa and Saturn’s Enceladus—are becoming the undisputed top targets in the search for life elsewhere in the solar system, scientists and NASA officials said at a press conference Thursday. Beneath their icy crusts both moons have deep, global oceans of liquid water, kept warm by tidal tugs from the gas-giant planets they orbit. Whether anything swims in those oceans remains unknown—but perhaps not for long.

At Enceladus, NASA’s Cassini spacecraft has discovered molecular hydrogen—a potential foodstuff for bacteria and a sign of hydrothermal activity—within plumes of water vapor venting from the ocean into space through cracks in the moon’s surface ice. At Europa, earthbound telescopes have spied signs of similar plumes, tracing one that seems to repeat—an “Old Faithful” of the outer solar system—to a mysterious thermal anomaly on the moon’s surface. Both findings boost the case for sending future life-finding missions to each moon as part of NASA’s burgeoning “Ocean Worlds” program of interplanetary exploration.

“These ‘ocean worlds’ have just been discovered,” said Jim Green, director of NASA's planetary science division. “They are in our solar system. We need to probe them because they are among the best locations—we believe—that may harbor life today.”

Free Energy at Enceladus

Cassini has been periodically diving through plumes of Enceladus ever since it first discovered them, shortly after its arrival at Saturn in 2004. The plumes were a total surprise—no one had expected such robust activity upon the diminutive moon, which is roughly the size of England. Sniffing the plumes with an onboard mass spectrometer, the spacecraft has found they contain not only water vapor, but also trace gases such as ammonia, carbon dioxide and methane. It looked for hydrogen, too, because microbes in Earth’s deep oceans use hydrogen's reaction with carbon dioxide to obtain “free energy” to drive their metabolisms. The reaction produces methane as a byproduct. Talk of “methanogenic” microbes on Enceladus briefly spiked in 2009, when reports emerged of Cassini’s spectrometer finding signs of hydrogen in a plume. But team members traced that signal to water vapor dissociating as it slammed into the instrument’s titanium walls at more than 17 kilometers per second. For this latest work, published in the journal Science, the team managed to feed plume material to Cassini’s spectrometer at slower speeds without it touching the walls, ruling out alternate production sources and allowing any hydrogen emitted from the moon’s ocean to be seen. What they found shocked them: hydrogen constituted 1 to 2 percent of the plumes—more than enough to serve as a source of free energy for microbial life.

“One or 2 percent doesn’t sound like a lot to most people, but it really is—it’s an amazing amount,” says Frank Postberg, a planetary scientist at the University of Heidelberg who studies Enceladus’s plumes but was not involved in the new work. “Molecular hydrogen is hardly found on Earth at all because it’s so light and volatile—it is eaten by bugs or reacts with other substances or just floats away to space.” Such a large amount, researchers believe, must be steadily replenished somewhere within the moon. On Earth, molecular hydrogen chiefly comes from lifeless processes, such as when hot water circulates through rocks rich with iron or organic molecules. This liberates the gas, which can flow out at seafloor hydrothermal vents to nourish methanogenic bacteria that form the base of the food chain for light-starved ecosystems. On Enceladus, researchers believe, similar hydrothermal activity must be taking place.

“This free energy is really a game-changer for Enceladus,” says lead author Hunter Waite, a researcher at the Southwest Research Institute (SWRI) in San Antonio, Tx. “The presence of molecular hydrogen shows there is the chemical potential there to support metabolic systems like methanogenic microbes. This suggests we’ve found a potential food source that would support the habitability of Enceladus’s interior ocean.”

Whether or not any life within Enceladus would be able to actually use molecular hydrogen as a food source remains to be seen. “You have to know how much free energy there actually is—what’s the calorie count of this food source?” says study co-author Christopher Glein, a geochemist at SWRI. How much energy any microbes could gain from the hydrogen likely depends on many things still unknown about this hidden ocean—its temperature, salinity and alkalinity, as well as the availability of other nutrients such as phosphorus and sulfur. “Enceladus is showing us, ‘Here’s the supply.’ But the key question is, ‘What’s the demand?’"

Enceladus’s wealth of hydrogen, Postberg says, could be seen as a sign that the moon is lifeless. “It’s probably a little bit too much, if you ask me. If little organisms are all over the place in the ocean and they like to eat molecular hydrogen, they probably wouldn’t let so much reach the surface and go out into space.” Even so, hydrogen’s abundance can’t rule out biology, he says: “There are many different kinds of metabolisms beyond methanogenic ones—and it could be that methanogenesis is only taking place in localized regions of the ocean.”

There is so much hydrogen, Waite says, that it may simply overwhelm the appetites of anything living within Enceladus. “Think of it this way—you’re sitting at home and they’re delivering a pizza every hour. At some point you’re gonna give up eating pizza, and it will be everywhere. There could just be too much food to eat.”

Old Faithful at Europa

Cassini’s discovery and ongoing scrutiny of Enceladus’s plumes was a key motivator for similar speculations about Europa, a far heftier moon that is larger than Pluto and thus more likely to harbor enough interior heat to drive hydrothermal processes and plumes. Its ocean, it is thought, possesses more water than all of the seas on Earth, beneath an ice shell perhaps 15 or 25 kilometers thick. Yet studies of Europa have been limited to observations with telescopes on Earth or in space, paired with archival data from NASA’s Galileo orbiter, which repeatedly flew by Europa while orbiting Jupiter between 1995 and 2003. (The only mission presently at Jupiter, NASA’s Juno spacecraft, is in a far-out orbit poorly suited for detailed observations of this moon.)

The first telltale signs of Europa’s plumes emerged in 2012, with Hubble Space Telescope observations led by SWRI astronomer Lorenz Roth that spied a haze of water vapor around Europa’s south pole. These early observations were bolstered in 2016 when a team led by William Sparks of the Space Telescope Science Institute in Baltimore, Md. spied more potential plumes in a larger set of Hubble data. Sparks’s studies took a different approach, using ultraviolet light to look for dark shadows of plumes erupting from Europa’s surface when the moon was silhouetted against Jupiter’s bright face. The study revealed several plume candidates seeming to spring almost at random from Europa’s surface. Now, however, one of them may have been observed erupting a second time. The findings are published in Astrophysical Journal Letters.

“When we saw the recurrence, we got very excited,” Sparks says. “Because if you’ve got a difficult-to-observe and intermittent phenomenon, if it actually repeats then all of a sudden things change. It changes the balance of likelihood that this is real.”

The candidate plume first appeared in a Hubble image taken in March of 2014; a separate image from February of last year shows another shadowy plume-like feature erupting from the same small region near the moon’s equator, northwest of a mid-sized young crater called Pwyll. Based on Cassini’s studies of Enceladus—which showed that moon’s plumes come from vents that are “hot” relative to the icy surface, Sparks’s team examined the region in thermal maps of Europa’s nighttime side derived from Galileo data.

These images of Jupiter's moon Europa, taken by NASA's Galileo spacecraft, highlight a thermal anomaly on the moon's icy surface. The green oval in the image at left outlines the likely source location of potential plumes of erupting water vapor detected by NASA's Hubble Space Telescope in 2014 and again in 2016. At right, a thermal map shows that the plume's likely source overlaps with an abnormally warm region of Europa's surface. Credit: NASA, ESA, W. Sparks (STScI), and the USGS Astrogeology Science Center

“To our astonishment, we found a thermal anomaly right smack bullseye on the location we had just identified” for the potential recurring plume, Sparks says. “I think we’ve now stepped beyond having a couple of intriguing features seen at the edge of Hubble’s capabilities to being in a place where the preponderance of evidence really now has to favor [these plumes] being real.”

Presuming that the alignment is not coincidence, and that the features Hubble has observed are in fact plumes, Sparks and his team see two possible explanations. “One possibility is that there is liquid water below the ice, and its heat flows up to cause the thermal anomaly and cracks are opening in the surface to cause the plume activity,” Sparks says. That “internal heat” explanation would suggest the water is only two kilometers below the surface. Alternatively, a plume erupting from below could create the anomaly by changing the region’s “thermal inertia.” That is, the plume could alter the texture and compactness of the surface ice, forming chunks and boulders of dense, snow-covered ice that cool more slowly than their surroundings after being warmed by light from the distant sun.

In either case, the plume’s source might be a “chaos region” of cracked, jumbled ice too small to have shown up in Galileo’s images. Study co-author Britney Schmidt, a planetary scientist at Georgia Tech University, has developed a model for the formation of such regions that suggests they are linked to shallow reservoirs of liquid water thought to develop in the crust midway between the surface and the interior ocean. “You could get water pockets that form over time and slowly begin to crack up the surface,” Schmidt says. “This could be one of them. You’d expect the area to be warmer, but until the surface cracks open you wouldn’t have much surface expression of anything going on.”

Sparks and his colleagues are already devising other ways to peer deeper at Europa’s potential plumes—not only with Hubble but with other telescopes operating at different wavelengths. NASA’s infrared James Webb Space Telescope, slated to launch in 2018, could potentially settle the debate about Europa’s plumes, and Sparks has already booked time on the agency’s currently operational SOFIA platform, an infrared telescope mounted to a modified Boeing 747 aircraft.

In the meantime, other astronomers are already beginning to examine their own data for corroborating evidence of an Old Faithful plume at Europa. Told of the discovery, Michael Brown, an astronomer at the California Institute of Technology, rushed with graduate student Samantha Trumbo to examine thermal maps of Europa’s dayside they had recently produced using the European Southern Observatory’s Atacama Large Millimeter/sub-millimeter Array (ALMA) in Chile. The hot spot appears in their data, too.

“[The Sparks team’s] hot spot is indeed one of the weirder spots on Europa,” Brown says. “It's actually a little cooler than we would predict on the dayside, and the temperature doesn't change much on the Galileo nightside measurement. So: internal heat? Maybe. Huge thermal inertia? Also maybe.” Further analysis of ALMA’s thermal map of Europa’s dayside in coming months, Brown says, could clarify the hot spot’s source by comparing it to Galileo’s old nightside observations.

Things to Come

Ultimately, however, the mysteries of Europa and of Enceladus may well require sending more spacecraft there.

NASA is already developing Europa Clipper, a spacecraft intended to launch in the 2020s to perform 45 flybys of Jupiter’s moon, festooned with state-of-the-art instruments that could confirm its putative plumes and link them to habitable conditions—and perhaps the presence of life in the ocean hidden below. Spurred by powerful advocates such as Representative John Culberson (R-Texas), the agency is also considering a life-finding Europa surface lander, although the Trump administration’s budget proposal provides no funds for this latter project. Additionally, the European Space Agency’s Jupiter Icy Moons Explorer spacecraft—also known as JUICE—will study Europa when it enters orbit around the gas giant planet in 2024.

Plans for Saturn are less certain: after more than a decade of maneuvering around the Saturnian system, the Cassini mission is now perilously low on fuel. Even if its fuel tanks were full, its instruments are now antiques that were never designed for close-up detailed studies of a phenomenon like plumes. Mission planners will send the craft plunging into Saturn’s atmosphere this September, precisely to avoid the risk of contaminating Enceladus and other promising Saturnian moons with any trace of terrestrial biology. This fall, shortly after Cassini’s fiery demise, NASA will select a new interplanetary mission for its “New Frontiers” program. But the multiple proposals to send a spacecraft to Enceladus will have to contend with many others targeting different destinations.

“The real story here is that we have the chance to do for Europa what we’ve already done for Enceladus,” Schmidt says. “If we get Clipper to Europa, that would be really cool. And what if NASA’s New Frontiers selection is for an Enceladus mission? Then we could be doing the same science in two places. At Europa, Clipper will be able to do what, honestly, we wish we could be doing right now at Enceladus.”