White with blue veins planet.

NASA is preparing to explore alien worlds—by investigating Earth’s dark corners

Three future missions are headed to faraway moons Europa, Enceladus, and Titan to search for life. To help, scientists are testing their techniques here in our backyard.

Beneath its frosty, fractured crust, Saturn’s moon Enceladus harbors a global ocean that could contain all the ingredients necessary for life as we know it to thrive.
False-color mosaic of 21 images by NASA/JPL/Space Science Institute

As I thumb the throttle on my snowmobile, it skitters and glides across a sea of snow and ice. In the dusky twilight, the landscape is painted in shades of ethereal blue. I’m heading back to town after spending the day crisscrossing a frozen fjord, one of many in Norway’s Svalbard archipelago, a cluster of mountainous islands in the high Arctic where auroras often dance overhead and narwhals, belugas, and walruses patrol the seas.

It’s March, and the sun finally returned to the sky here about a month ago. I’m with a half dozen scientists who are hunting peculiar landforms called pingos—or, more specifically, the microbes that live within them. Anchored in permafrost, these domes range in size from mounds to small hills and seasonally expand and contract as water seeping through them freezes and thaws. They’re like an icy eruption in slow motion. Temperatures hover around minus 15°F as the bundled-up, rifle-toting scientists make multiple trips each day to their study sites, where they collect ice cores and water samples while keeping an eye out for polar bears.

The microbes populating the pingos could offer a glimpse at how alien life might survive on other worlds in the solar system—icy moons with global seas tucked beneath frozen rinds. That’s because, in winter, life inside the pingo “doesn’t rely at all on solar energy—it’s only using chemical energy,” says microbiologist Dimitri Kalenitchenko of Norway’s University of Tromsø, who is leading the project.

The story of sunlight-starved life on Earth is relatively new. For a long time, “we still thought life on this planet was largely restricted to the surface … and entirely dependent on photosynthesis,” says Barbara Sherwood Lollar, a geologist at the University of Toronto who studies microbes living deep within the Earth. Then, in the late 1970s, the Alvin submersible explored a dark oceanic hydrothermal vent near the Galápagos Islands, discovering a flourishing ecosystem about a mile and a half beneath the ocean’s surface, and forever changed our conceptions of life’s limits. “It’s one of the things that forces us to be humble,” Sherwood Lollar says. “To think that even on our own planet we are still finding processes, still finding environments that we didn’t know existed.”

Similarly, scientists used to think a world’s habitability depended on its distance from the sun. But that picture is incomplete. Now three faraway moons, warmed by the gravitational tug and pull of the giant planets they orbit, tempt scientists with their promises of alien life in their oceans: Jupiter’s moon Europa, where a salty sea containing more water than Earth’s oceans sloshes beneath an ice shell; and Saturn’s moons Enceladus—a small, ice-encrusted world with a global ocean that erupts through fractures in its south pole—and Titan, with its unearthly terrain of liquid hydrocarbon lakes on its surface and an ocean nestled within its interior. Observations suggest that each moon has the chemistry, water, and energy needed to support life as we know it, and maybe don’t know it.

We might soon learn if these seas are inhabited. A European Space Agency spacecraft called JUICE is on its way to the Jovian system to surveil the giant planet and its icy moons, including Europa. Next year, NASA’s Europa Clipper spacecraft will also set sail for Europa, on a quest to solve the mysteries of its crust and salty sea. And later this decade, the U.S. space agency’s Dragonfly mission will send an octocopter to Titan, carrying a suite of instruments that could detect signs of life on the hazy moon’s surface. Missions to Enceladus are being planned too. “It’s a really exciting time to be a planetary scientist,” says Morgan Cable of NASA’s Jet Propulsion Laboratory (JPL). “We could, for the first time in human history, find life somewhere else.”

Decades in the making, these missions collectively cost billions of dollars. To prepare for astrobiological exploration so far afield, scientists are testing their instruments and techniques in the cold, dark corners of our own backyard. Even though Titan’s surface chemistry is hard to replicate in the field, the seas in the innards of these three moons may not be so unlike our watery environments. From Earth’s surface to its subterranean caverns, these investigations study some of the most alien creatures on our planet. Their discoveries could even write the story of life’s beginnings here, and maybe elsewhere.

That evening in Longyearbyen, Svalbard’s largest town, wrapped in blankets and recovering from the bite of the Arctic, I looked out the window just in time to catch a man aiming his phone at the celestial light show. Outside, in the chill, serpentine ribbons wriggled overhead. Shimmering cosmic lights above an otherworldly landscape? It was almost too perfect.

I first pressed a boot print into the Svalbard snow four years ago, as I prepared to board the Norwegian icebreaker Kronprins Haakon—a shiny new vessel back then—along with three dozen scientists and engineers. We launched from Longyearbyen and sailed toward a fractured patch of seafloor north of Greenland. There, some two and a half miles down, the Earth heaves searing dark fluids into the sea, forming what is known as the Aurora hydrothermal vent field. Salt water mingling with hot rocks beneath the seafloor powers these eruptions, which produce the heat and chemistry needed for organisms to thrive. “They can host all kinds of weird and wonderful life-forms,” said Chris German, a marine geochemist from the Woods Hole Oceanographic Institution who has spent nearly four decades hunting hydrothermal vents, as we cruised north. Under permanent ice cover, the field could be a superlative earthly analogue for the seafloors of Europa and Enceladus.

To explore Aurora, German and his colleagues brought one of the most advanced uncrewed submersibles on Earth: an orange, minivan-size vehicle called NUI, short for Nereid Under Ice. Festooned with stickers, some reflecting a recent collaboration between Woods Hole and NASA, the three-million-dollar machine is designed to explore under-ice ecosystems. It can dive three miles down, swim more than 25 miles, and operate for half a day without recharging. NUI functions autonomously, but it can also be remotely piloted; scientists watching live feeds from its cameras can direct it to collect particular sediments or organisms.

“Our hope is that NUI is kind of like the Australopithecus or Homo habilis to the robotic spacecraft that would one day go to Europa,” Kevin Hand, a National Geographic Explorer and astrobiologist at JPL, told the team as the ship plowed through our planet’s frozen crust. For him, Europa is a tantalizing target in the quest to know whether we are alone in the universe.

For days, thick ice—too bulky for the ship to break—had slowed our journey to the vents. Although the going was tricky, the Arctic Ocean’s relatively thin icy rind is a friendlier version of the crusts encasing alien oceans. The shell at Enceladus may be less than a mile thick at the south pole, where the geysers erupt, but estimates for Europa’s rind suggest it’s considerably thicker. Characterizing Europa’s frozen shell and the compounds on its surface is one of the highest priorities for both the Europa Clipper and JUICE missions. Those spacecraft will make multiple flybys of Europa to measure the ice shell’s thickness and study its layers and ocean beneath, in hopes that with more information, future missions can get through the rind and access the water.

When NASA’s Cassini spacecraft zoomed around the Saturn system from 2004 to 2017, it sampled the Enceladian plume multiple times and found salts, silica, organic molecules, and molecular hydrogen, all telltale signs of seafloor activity. It also detected phosphorus, an element that’s crucial for life on Earth. NUI’s job on this cruise was to get the first good look at the Aurora vent field and help figure out whether it’s fueling chemical reactions that can support abyssal life-forms—perhaps the kind of life that might evolve in dark, permanently ice-covered alien seas.

Two days after arriving at the Aurora seamount, NUI dived to Aurora. As the orange sub sank in the long Arctic twilight, a gentle snow fell on fields of frost flowers carpeting the ice. It took hours for the submersible to descend to the seafloor. In the control room a pilot maneuvered the vehicle. At one point he handed me the joystick, and for a few moments we joked that I held the record for the deepest Arctic remotely operated vehicle dive piloted by a woman. (It was 9:19 a.m., and the sub was at 9,104 feet, but who’s keeping track?)

An hour later, the trouble started. As NUI neared its destination, its onboard systems blinked off one by one. Then the pilot reported that he’d lost control of the submersible. After a bit, the team commanded the vehicle to drop its dive weights and begin rising to the surface. Instead, NUI sank. A few minutes later, the sub’s depth reading marched across the screen in an ominous flat line—it was on the seafloor. Maybe the weights didn’t drop; or maybe a crucial housing had sprung a leak, the sub had flooded, and it was now too heavy to rise to the surface.

On a scale of zero to 10, I asked German, how worried are you? “Ten,” he said. “Ten. There’s a legitimate risk of a very bad thing.” Returning home without NUI—the team’s prized instrument, the potential forebear of tomorrow’s spacefaring submersibles—would be “pretty atrocious,” he said. It’s so close to the vents that if we could turn on NUI’s camera, he continued, we’d probably be staring Aurora in the face.

Three days went by. The NUI team monitored the sub’s location for any changes while Hand and several other people MacGyvered an orange, shoebox-size underwater vehicle from spare parts. They were planning a desperate rescue mission that, as far as I could figure, involved fishing for NUI with the makeshift device.

But that morning there was good news: NUI was on its way back up. The final fail-safe—a corrodible wire attaching the dive weights to the submersible—had worked. It had just taken longer than the anticipated 24 to 48 hours for salty seawater to gnaw through the wire, probably because chemistry happens at a slower pace in subfreezing Arctic seawater. By that afternoon, NUI was back on board, having helpfully surfaced under a rare patch of thin ice rather than the thick floes covering the area. (“I think it’s sentient,” German said.)

NUI wouldn’t explore Aurora during this cruise, but it turned out that the vents were still within reach. That same evening, the ice drift cooperated, and the ship sailed right over the vent field, towing a high-tech camera just above the seafloor. Aurora, the imagery revealed, was home to a massive black smoker, a rupture more than five feet across that hurled hot, sulfidic minerals into the sea. But as far as hydrothermal vents go, Aurora seemed remarkably unpopulated, said Eva Ramirez-Llodra, the cruise’s co-chief scientist. We only spotted a handful of snails and crustaceans—none of the dramatic tube worms or clams that cluster around other deep-sea vents. Amid the desolation, though, a garden of glass sponges bloomed. These filigreed creatures, seemingly the only organisms in abundance down there, are sometimes said to be barely alive. Their skeletons are made mostly of silica rather than calcium carbonate. And that makes sense. In the deep sea, silica is common. Life—these otherworldly sponges—found a way to use it.

Despite all the trouble on this cruise, the group managed to get its first good look at Aurora. And the vents were active and more mysterious than expected. The largest vent in the field, German said, was among the biggest black smokers he’d seen in his career.

At times it’s colder in the Arctic than in those alien seas. This is not fun for someone as sensitive to the cold as I am. However, the damp, lingering chill of an Italian cave turned out to be even worse. It burrowed straight into my bones and refused to leave. Wouldn’t it be nice if the solar system’s alien incubators were more tropical?

This past February I visited central Italy’s Frasassi cave system with scientists searching for some of Earth’s least known creatures: microbes that live deep within the cave’s underwater passages. Growing in toxic, oxygen-starved waters, these improbable communities are powered by the slow burn of water interacting with the rock itself, producing metabolic fuels such as hydrogen sulfide and methane—just like the organisms that depend on oceanic hydrothermal vents.

Scientists think these chemical reactions could exist on icy moons. They also suspect that Frasassi’s chemistry is similar to Earth’s ancient oceans, where the seeds of terrestrial biology might have sprouted. “Earth was a very different planet when it was born,” said Jennifer Macalady, a geomicrobiologist at Penn State University who’s been visiting the cave system for more than 20 years. “If we think this aquifer was like Earth’s early oceans, and we think that Earth’s early oceans might have some similarities with oceans on other planets, this is a great place to hone our skills for life detection.”

The system’s largest chamber was discovered in the 1970s and is 65 stories tall. It’s so big that the cathedral of Milan, an important unit of measurement for Italians, would fit inside. But the only gargoyles here have been sculpted, one drop at a time, by the slow seepage of mineral-rich water. Walking through this chilled, humid chamber is a bit like scoring a golden ticket to Willy Wonka’s Chocolate Factory—except the confections are crafted by limestone, water, and acid. Glistening, slimy, and sparkling with crystals, some of the stalagmites are 65 feet tall and as big around as old-growth redwood trees.

No one knows the extent of Frasassi’s network of lakes, which are connected by a vast subterranean aquifer. “There hasn’t been much diving in the Frasassi aquifer because, one, it’s toxic. Two, it’s hard to obtain access. You have to be very good on a rope, and you have to be a pretty sturdy individual. And then you have to be a diver,” Macalady told me. When she first visited Frasassi, she had zero caving experience; now she’s a skilled caver and speaks fluent Italian—her dog is named Lavastoviglie, which is Italian for “dishwasher” (as many dogs are).

In the aquifer’s depths, a layer of clear fresh water sits atop salt water that’s saturated with toxic hydrogen sulfide. That lower layer bubbles up from deep within the Earth, and in addition to being smelly, it’s anoxic—there’s no dissolved oxygen to power microbial metabolisms. “In that hydrogen sulfide layer, we expected to find almost nothing,” Macalady said. “Instead, we found a forest of microbes unexpectedly making a living.”

In 2004, Italian divers discovered that the toxic layer in one of the Frasassi lakes, Lago Infinito, was surprisingly inhabited. Black, stringy biofilms—cooperative communities of microbes—hung from the underwater rocky ceiling like tattered Gothic curtains, some of them three feet long. In the lake’s seemingly sterile depths, the divers were spooked and fled. “If you see alien cave goo, your first instinct is to turn around,” Macalady said, deadpan.

The Frasassi microbes Macalady is curious about are autotrophs, or organisms that make their own food from gases and minerals. These microbes have turned up in several underground lakes, sometimes looking dark and tentacular, other times gray and feathery. Preliminary work points to thousands of species collaborating to craft these eerie appendages. Some have recognizable genetic sequences, but none are fully known to science. And a good fraction are what Macalady calls “genetic dark matter”—the single-celled unknown. Finding those unknowns is not unprecedented, but as scientists sequence more microbes, it’s “more rare to find a community that has so much of this genetic dark matter,” she said.

On this trip, Dani Buchheister, one of Macalady’s graduate students, was hoping the divers could collect biofilms so she can try to coax the mysterious autotrophs to grow in a lab. If she can do that, perhaps she can solve the mysteries of the microbes’ identities and figure out how they survive in these pungent, suffocating waters—with an eye to what that means for life farther afield. “[I’ve become] more and more convinced that if we find life in our solar system, it’s probably in subsurface environments,” said Buchheister, who grew up in Florida and could sometimes see the fiery streaks of rockets launching from nearby Cape Canaveral. “And my draw to microbiology was the idea that microbes are our best examples of the extremes of life poking at the edges of what’s possible with the life we know.”

And that’s how we found ourselves on the muddy edge of Lago dell’Orsa, an 80-foot rappel from the tourist walkway above. Two divers had sunk beneath the cold, aquamarine waters to collect biofilm samples, but 10 minutes later, only one diver, Kenny Broad, had returned. He went back to search for his partner, Nadir Quarta, among the poorly mapped passages. “Why they’re not both up here, I don’t understand,” Macalady said, aiming her headlight into the darkness as she balanced on a slippery rock. We all went quiet, and the plink, plink, plink of water hitting the cave floor echoed through the chamber.

After an eternity (or perhaps several minutes), we heard the gurgling of two cave divers returning to the surface. “Well, that was slightly terrifying,” Broad said to the crew as he floated. The 2011 Rolex National Geographic Explorer of the Year and an environmental anthropologist at the University of Miami, Broad was one of a handful of divers qualified for this expedition, which required proficiency with a rebreather as well as experience with underwater photography and scientific sample collection. Quarta had driven down from Switzerland. As they dived, the pair got split up, with Quarta heading down a passage he called “a mess.” Underwater, amid poor visibility, Broad couldn’t find him—a worrying situation during any dive.

Once it was clear they were OK, Macalady asked, “Did you see some fuzzy gray stuff?”

During Broad’s second dive in frigid Lago dell’Orsa—a 65-foot plunge that he described as “pretty shallow”—he filled five large syringes with diaphanous biofilms, material so fragile it fell apart at the slightest jiggle. Back at the field station, he thawed and stretched. “It wasn’t visually stunning,” Broad reflected. “But there’s this otherworldly element to caves that’s undeniable. They’re a portal to some other place.”

When we met up with the others in the field station’s lab, Macalady had some of the biofilms under a microscope. Through the eyepiece, I finally came face-to-face with these perplexing intraterrestrials. Their cottonlike, wispy gray tendrils were punctuated by dark clumps of pyrite—perhaps the telltale sign of a primordial metabolic pathway that converts smelly hydrogen sulfide into pyrite and hydrogen gas. Scientists suspect such an energetically cheap pathway powered the first life-forms on Earth, but no one has found any evidence for it. Maybe until now. “Those are pyrite framboids, the waste product of this metabolism we’re looking for,” Macalady said, indicating the dark sphere. “It’s a clue.”

Solving that mystery and identifying the microbes will depend on whether Buchheister can persuade them to grow in a lab. That might be tricky for multiple reasons, one being that the team’s preliminary estimates suggest these microbes divide on very long timescales, perhaps once every century or millennium. There’s so little energy available in the depths of the Frasassi aquifer that, like the glass sponges populating the Arctic abyss, these microbes have figured out how to survive on what they can get.

“That’s kind of my sweet spot,” Buchheister said. “Learning about the limits of life on Earth, the weird places they live, and how that information might influence where and how we search beyond Earth.”

When I return to Longyearbyen in March, Kalenitchenko—who’d been aboard the Kronprins Haakon—is collaborating with Hand, who is here again too. (Tom Cruise is also in town to shoot a sequel to Mission: Impossible.) A lot has changed since we last met in the frozen north. And Ramirez-Llodra, co-chief scientist from my cruise, and her crew had revisited the Aurora vent field in 2021 and collected the deep-sea lifeblood erupting from two black smokers. Those vents now have names: Enceladus and Ganymede, the latter a reference to Jupiter’s giant icy moon, the largest in the solar system.

But our target on this trip is terrestrial ice—in the pingos—not sea ice. Over several field seasons, this team will collect ice cores and characterize the microbial communities populating three sites near town—both in the pingos’ reservoirs and locked in their ice. The work already suggests these microbes share characteristics with species that normally live in deep-sea biodiversity hot spots. That’s because what occurs in the pingos is “what we see in the deep sea,” Kalenitchenko says.

As in the Frasassi aquifer, hydrogen sulfide leaves its smelly calling card in some pingo reservoirs. The ice is full of methane, a compound Hand refers to as “life’s first lunch.” These two molecules can be both microbial food and microbial waste. Appropriately, the metabolic pathways that consume and excrete hydrogen sulfide and methane dominated the chemistry of the earthly environments I visited for this story. These pathways are arguably among the simplest and least energetically demanding in the single-celled repertoire, meaning they are, perhaps, a good model for what might arise elsewhere.

By melting ice cores and studying that methane, Hand can determine whether the gas is biologically produced or is the work of geology and chemistry—the exact kind of science we might someday do with a slice of alien ice. But identifying life’s work on another world isn’t as simple as studying the carbon atoms in methane. Remotely finding the fingerprints of extraterrestrial biology, perhaps locked in ice or floating in a teaspoon of seawater, will mean looking for multiple biosignatures.

On moons like Europa and Enceladus with chemistries like Earth’s, scientists can make educated guesses about what they’re looking for. But on Titan’s surface, which Dragonfly will explore, the terrain might look deceptively Earthlike, but the chemistry is anything but familiar. There it’s so cold that liquid methane and ethane fill the lakes and seas, while the landscape is rock-hard water ice. Identifying life’s fingerprints on Titan will be a different challenge. “One just has to be very patient in the outer solar system,” says planetary scientist Elizabeth “Zibi” Turtle of Johns Hopkins University Applied Physics Laboratory, who’s spearheading the Dragonfly mission to Titan. “It’s the nature of the part of the solar system we’re exploring.”

Learning whether these moons—or any world in the solar system—are inhabited is worth the challenge. “We need to find out,” says JPL’s Robert Pappalardo, science lead for the Europa Clipper mission. “It really could bring a new Copernican revolution in our understanding of our place in the universe.”

As a longtime contributor to National Geographic, Nadia Drake has written about people’s obsession with Mars, the mysteries of Saturn’s rings, and why women make the best astronauts. She loves reporting from the field and one day would like to ride a rocket into Earth orbit or dance on the moon.

An Explorer since 1998, the German photographer, filmmaker, and adventurer Carsten Peter specializes in extreme environments. For the November 2022 issue of National Geographic, Peter captured the violent volcanic eruption in Spain’s Canary Islands. This issue features his images of otherworldly settings on Earth. 

Based in Washington, D.C., Chris Gunn focuses on science and technology. As a contract photographer for NASA, he took the lead on documenting the building of the James Webb Space Telescope. His images have appeared in publications such as the New York Times and Popular Science.

A version of this story appears in the October 2023 issue of National Geographic magazine.

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