Beyond Mars and the asteroid belt, half a billion miles from the sun, the solar system might seem frigid, bleak, and lifeless. But scientists believe there’s a chance tiny alien creatures could reside on a distant moon, and you might find them if you look in the right place. For many researchers, that place is Europa, below its thick, icy crust.
Planetary scientists are discovering more about Jupiter’s fourth-largest moon, one of Earth’s nearest ocean worlds—places like Saturn’s moons Titan and Enceladus that have bodies of salty water and other liquids that could be amenable to the emergence of life. They’re presenting new findings this week about Europa’s cracked surface, hidden ocean, and geological activity at the biggest annual planetary conference in the United States, organized by the American Astronomical Society, held virtually for the second year in a row. The research serves as a prelude to tantalizing opportunities for new observations by upcoming NASA and European Space Agency missions.
“Europa is fantastic. Of anywhere in the solar system, outside the Earth, it has the greatest potential, I think, for maintaining a habitable environment that could support microbial life,” says Michael Bland, a US Geological Survey space scientist in Flagstaff, Arizona. After modeling the moon’s dynamic, rocky interior, Bland believes the conditions on its deep seafloor could be amenable to life, according to new work that he and NASA Jet Propulsion Laboratory scientist Catherine Elder presented at the conference on Monday.
Europa’s ocean is buried under about 10 miles of ice, but that doesn’t necessarily mean it’s too cold for life. As the moon orbits Jupiter, tidal forces generate heat that melts around 5 percent of the moon’s mantle, far below ground. Some of that magma could migrate up 100 miles through small cracks in the cold, rocky material above it, erupting onto the seafloor, Bland argues. If this process indeed happens, and frequently enough, it would function like hydrothermal vents do on Earth: These volcanic seafloor fissures provide the energy and chemical ingredients for life, far below the reach of sunlight and photosynthesis. Hardy organisms thrive in such dark, high-pressure environments on our world, and perhaps they do on others as well.
But for the process to work, the magma has to reach the underground sea quickly, before it freezes and hardens. Its upward velocity might be just barely fast enough for it to work out that way, Bland’s models show, meaning there’s a chance for life on Europa’s seafloor. “It’s plausible, but specific conditions have to be met, and it’s not guaranteed,” he says.
Europa is considered one of Jupiter’s four Galilean moons, first spotted by Galileo Galilei with his pre-NASA telescope four centuries ago. Its compatriots include Io, a volcanic, sulfuric, radiation-bombarded wasteland close to Jupiter, and, orbiting further out beyond Europa, massive Ganymede and cratered Callisto. The latter two may harbor underground oceans, as well, but if so, the water would lie much deeper beneath even thicker crusts.
But Europa is unique. Not only is its crust relatively thin, but its surface is covered with thousands of narrow, criss-crossing ridges and crevices, some extending for hundreds of miles. By mapping currently available images, Michelle Babcock, a planetary scientist at Georgia Tech in Atlanta, identified around 70 “tortuous ridges” among them: wiggly, irregular structures unlike the straight and arced ridges scientists can already explain.
While they're not yet sure what causes the ridges’ convoluted paths, all the laceration marks on the moon’s exterior could originate in some way from its slightly elliptical orbit, which repeatedly carries it closer to—and then farther from—Jupiter. “As it’s orbiting Jupiter, the shell is being stretched and pulled, and that tidal stress is causing fractures and cracks, contributing to a lot of surface features,” Babcock says. They presented their findings with colleagues Britney Schmidt and Chase Chivers on Monday.
Such geological activity might play a role in explaining another mystery: Europa’s seemingly young surface. Unlike its neighbors, which have pockmarked appearances thanks to being pummeled by comets and asteroids for eons, Europa has few craters to show for its age. Somehow the moon is continually giving itself a facelift, and its many ridges could be involved in covering or erasing such craters over time, Babcock and their colleagues argue.
Other Europa-focused researchers at the conference are exploring which telltale signs of alien bacteria might be there, how such pivotal evidence could bubble up onto the surface or be spewed into space in plumes, as well as the effects of Jupiter’s radiation zapping the surface, which could destroy these signs of life before they’re found.
To definitively answer any of these questions, however, requires new space missions. Europa was last visited by a dedicated spacecraft—named in honor of its discoverer, Galileo—two decades ago. It made close passes by the moon and its neighbors and took images with its cameras in the process. Before that, Voyager 2 snapped photos as it flew by in 1979. NASA’s Juno spacecraft, which has been orbiting Jupiter since its arrival in 2016, will make some observations during flybys over the next couple of years. The European Space Agency’s JUICE probe, scheduled to launch next year, will do so as well, though it will be spending more time around Ganymede.
After that, Bland, Babcock, and their colleagues look forward to NASA’s Europa Clipper, a mission years in the making that’s planned for launch in 2024. “The Europa Clipper will assess Europa’s habitability and how we might be able to use these investigations for other ocean worlds, thinking about the potential for life there as well,” says Kathleen Craft, a planetary scientist at Johns Hopkins University Applied Physics Laboratory in Baltimore, who will be presenting at the conference on Thursday.
The car-sized orbiter, with 100-foot solar panels unfurled on each side, will use radar, radio signals, and gravity science to study the structure of the moon, including measuring the thickness of the ice shell and the depth of the underground ocean. It will also try to snag samples from its plumes, which could include droplets from the ocean itself that might reveal information about how conducive to life it really is, Craft says. A baguette-sized instrument will ingest gas and vapor, analyze and classify the contents, and then beam the crucial data back to scientists at home.
Its mission also includes conducting aerial surveillance for a potential lander mission to Europa, which could scoop up material on the surface, or drill down for it, looking for that coveted evidence of extraterrestrial lifeforms.
To prepare for a future lander, researchers are scoping out somewhat similar places on Earth, such as in Greenland. Satellite imagery of proxy places like these will give them a realistic idea of what Europa’s terrain will be like, and that’s crucial, because right now their photos don’t have high enough resolution. And that could be dangerous: An icy region that might suggest a smooth landing place could hide a more complexly structured glacier, with smaller, unseen hazards. “But it’s a completely alien environment,” says Curt Niebur, NASA’s lead scientist for Planetary Science Flight Programs and Europa Clipper program scientist.
They will have to weigh the scientific value of a landing site—one with good odds of showcasing signs of life—versus the risk involved in touching down there. “Invariably, scientists say, ‘The most interesting spot to land is over there,’ and the engineers say, ‘We will blow up if we try to land there,’” he says.
Before Niebur and his colleagues can nail down the details of where the lander will go and how it will touch down, they need high-resolution images, better than anyone has yet, because landing in the wrong spot could scuttle the long-term investment in research and development leading up to such a flagship mission. Imagine, he says, if aliens sent a poorly placed probe to Earth, which sampled the wrong part of the ocean and only got a teaspoon’s worth of lifeless water. That’s why, he says, “everything hinges on Europa Clipper.”