Does life exist on Europa, the moon of Jupiter that has enticed astrobiologists for decades with signs of a vast and sunless ocean hidden beneath its crust? Data from NASA’s Jupiter-orbiting Juno spacecraft now suggest that if anything does lurk within this moon’s dark abyss, it may be starved of life-giving oxygen.
Despite being a prime target in the hunt for alien life, Europa is not necessarily a nice place to live. A spacesuit-clad astronaut spending 24 hours on its surface could be protected for the duration from the moon’s cold and essentially airless conditions but would still receive a lethal dose of radiation from an incessant rain of high-energy particles whirling through Jupiter’s powerful magnetic field. That constant bombardment has a potential upside, though: it also chips away ever so slightly at the moon’s icy surface, irradiating and splitting apart various molecules to create more complex chemistry. Because most of Europa’s crust is composed of water ice—that is, familiar H2O—this process’s primary byproducts are oxygen and hydrogen that either sticks to the surface or soars aloft to form a tenuous atmosphere around the moon. Some of that material may even seep down into the trapped ocean below, where it could conceivably serve as a source of energy and nutrients for living things.
That makes this frigid shell “like the lung for Europa,” says Jamey Szalay of Princeton University. “It’s constantly generating oxygen over the whole surface.” Just how much oxygen Europa makes, however, has been a mystery—until now. A new study led by Szalay offers best-yet estimates of the moon’s production of molecular oxygen—the form upon which we Earthlings rely—and has found that Europa manufactures it in woefully diminutive amounts. The result suggests that any oxygen making its way within the moon would be a trickle, not a torrent, making the substance’s paucity a potential top-down limit on life’s prospects there.
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The finding, published today in Nature Astronomy, comes from analyzing measurements beamed back by NASA’s Jupiter-orbiting Juno spacecraft, which had “sniffed” Europa’s frail atmosphere for the first time ever during a brief flyby in 2022. Szalay and his colleagues conclude that although the moon’s surface prolifically churns out hydrogen, it only manages to make at most 18 kilograms of oxygen per second.
As high as that number may sound, it’s far below the circa 1,000 kilogram-per-second estimates previously derived from overly simplistic computer models. Even so, the latest oxygen estimates “are still compatible with microbial habitability for life as we know it,” says Manasvi Lingam, an astrobiologist at the Florida Institute of Technology, who was not affiliated with the new work. Subsurface oceans such as those on Europa are generally expected to lack nutrients because they are so isolated; without sunlight to cook up prebiotic chemistry or to power biological photosynthesis, these buried bodies of water could be bereft of life. It is possible, however, that nutrients slowly accumulating on the oceans’ icy roofs across hundreds of millions of years could still reach the depths below, transported within by various geological processes, Lingam says. So a lack of sunlight is “not necessarily a death knell for life in subsurface oceans.”
“Sniffing” the Atmosphere
Europa’s atmosphere, vanishingly thin compared with Earth’s own thick blanket of breathable air, is known to be a delicate veneer of oxygen hugging the moon’s surface beneath a puffier extended layer of hydrogen. What little there is of the moon’s “air” is replenished by hydrogen and oxygen wisping out of the radiation-riddled shell of ice; these elements “are kind of bouncing around” on the surface, Szalay says, but “every once in a while, one of them pops off.” Such surface escapees inflate Europa’s atmosphere to be “higher than Everest,” says study co-author Fran Bagenal of the University of Colorado Boulder. “But as far as atmospheres go, this is pretty tenuous.”
As the Juno spacecraft glided in Europa’s wake for a few minutes in September 2022, its JADE instrument (short for Jovian Auroral Distributions Experiment) detected a plethora of these cast-off hydrogen atoms—but only in the immediate vicinity of the moon. That proximity, says study co-author and JADE instrument lead Frederic Allegrini of the Southwest Research Institute in Texas, means “there is no doubt that they come from Europa.” Based on that hydrogen’s abundance—JADE counted a whopping 100 million atoms during its brief encounter—the study team reverse-engineered how much in total must be wafting up from the moon’s surface and, from that, inferred the associated amount of oxygen that must be produced.
The new results are “very intriguing,” says Kevin Hand, a scientist at NASA’s Jet Propulsion Laboratory (JPL), who was not involved with the new study, “but oxygen is only part of the story.” In previous research, Hand and colleagues found that tangled skeins of yellowish-brown gunk that crisscross much of the moon’s surface were composed of sodium chloride, or table salt. Along with other findings, such as the recent discovery of carbon dioxide ice on Europa’s surface, this is what you’d expect if the moon’s ocean, like Earth’s, was in direct contact with rock at the seafloor and enhanced with life-friendly nutrients. The salt spotted on its surface could even hint at hydrothermal vents on the moon’s ocean floor, a tantalizing but as yet unconfirmed prospect that would serve as an energy hub for microbial life just like it does on Earth. And speaking of salts, Hand’s study also showed parts of Europa’s frozen crust are abundant in magnesium sulfate—Epsom salt—which either bubbled up from the underlying ocean or baked on the surface after blowing in from its sulfur-spewing neighboring volcanic moon Io. Microbial life on Earth “loves to chew away on sulfate,” Hand says, so even if a small fraction of the sulfate spotted on the moon’s surface oozes into the ocean sloshing below, “that’s got tremendous implications for the habitability of Europa’s ocean.”
Life without Oxygen
The earliest life on Earth may have emerged roughly four billion years ago, when no continents had yet arisen to punctuate our planet’s global primordial ocean. A leading theory postulates that life’s cradle could have been hydrothermal vents on the seafloor—akin to those that may exist within Europa and other oceanic icy moons—which supplied the required energy to form complex organic molecules. Earth’s early life didn’t need oxygen, at least to start with, notes Catherine Neish, a planetary scientist at the University of Western Ontario. Our planet was in fact oxygen-deprived for the first half of its life. Geologic and genomic clues indicate most life back then had anaerobic metabolisms and relied on sulfur, iron and other oceanic minerals as biochemical stand-ins for oxygen to get the energy required for growth, movement and reproduction. The rise of Earth’s atmospheric oxygen is linked to an evolutionary innovation that seems to have emerged some two billion years ago, when microbes called cyanobacteria developed oxygenic photosynthesis, releasing the gas as a byproduct of their sun-powered metabolism. Yet even from there it would still be another 1.5 billion years or so before atmospheric oxygen approached modern levels, aiding the subsequent emergence of complex multicellular life.
For most of Earth’s history, its seas have boasted unrestricted interaction with its atmosphere—a privilege unavailable to oceans such as Europa’s, surmounted by thick shells of ice. For instance, Saturn’s largest moon, Titan, also an ice-sheathed orb, has in its atmosphere abundant glycine, one of the simplest organic compounds used by life. Recent research led by Neish suggests that comet strikes—a key delivery mechanism that makes pools of melted ice on Titan’s surface—dump only a pittance of glycine (and thus, presumably, of other organic compounds) into the ocean below. How much, exactly? In terms of glycine mass, only “about an elephant’s worth,” says study co-author Michael Malaska of JPL. “That’s not a lot, but it’s nonzero.” Whether that’s good, bad or neutral news for life’s prospects is unclear because any organisms arising on such moons could have wildly unearthly biochemistries—and also because there may be myriad mysterious ways by which different nutrients in varying amounts make their submarine migrations through icy crusts. “I don’t think any number is going to be an absolute deal-breaker,” Malaska says. Rather, nutrient abundances are useful indicators of “how sensitive we need to make our instruments to look for ‘life’ signals.”
A “Snapshot in Time”
Whether gauging life’s prospects on Europa, Titan or elsewhere, such studies tend to only consider a “snapshot in time” and don’t necessarily capture a world’s changing state across a multibillion-year history, notes Emily Martin, a geologist at the National Air and Space Museum in Washington, D.C., who wasn’t involved with either of the recent papers. Scientists have very little hard data about what Europa or any other icy moon was like billions of years ago. Whether they had habitable oceans back then—or, for that matter, whether they had oceans at all—is unknown. Extrapolating from Earth—a world we know endured numerous profound global changes across its history—gives “no reason to think that any other planetary body would have experienced an unchanging atmosphere for 4.5 billion years,” Martin says. Pegging studies about such dynamic worlds in a temporal context “is really hard to do.”
With the Juno spacecraft now settled in an orbit around Jupiter that will never take it past Europa again, many astrobiologists are already planning for 2030, when NASA’s Europa Clipper mission is slated to arrive at Jupiter after a five-and-a-half-year journey. Much like Juno, Clipper will not orbit Europa or any other Jovian moon. Instead it will loop around the gas giant itself to avoid lingering overlong within Jupiter’s hardware-frying radiation belts. That approach, however, will still allow the orbiter to swing past the moon about 50 times over the course of four years, during which it will map the moon’s interior and assess the depth of its subsurface ocean. In light of the new study’s findings, Clipper could even directly measure the abundance of oxygen when it swoops down just 25 kilometers above Europa’s frozen surface. These insights would reveal more potentially crucial clues about the complex chemistry abiding in the icy orb that is so alien from Earth.
“We don’t know what we don’t know,” Martin says. “Every time we learn a new thing, we find out how much weirder these places are.”