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Mary Beth Wilhelm’s right arm shot out of the passenger window, and the convoy of vehicles behind her crunched to a stop. Spread among two trucks and an SUV, her four colleagues and I squinted and craned our necks, wondering what had caught her eye in the colorless wasteland of the northern Atacama Desert. She opened the door of our SUV and hoisted herself out. The temperature was somewhere around 90 degrees Fahrenheit, and the mid-day gales were already picking up. Nothing beckoned. Nothing but tan spread out before us in every direction. I steeled myself against the wind, got out, and looked around. Behind our convoy and down a hillside was the great salt flat called Salar Grande, a scorching, parched expanse about the size of San Francisco Bay. Ahead of my silver SUV, I saw a rolling Martian world of sand-colored rock spread beneath a blazing blue sky. No mosquitoes buzzed near our ears; no birds flew overhead. Wilhelm walked a few dozen feet away from the convoy, stopped, and stooped. Then everyone saw it. Related Stories Life Can Survive in the Most Mars-Like Place on Earth

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Is Colonizing Mars the Most Important Project in Human History? A pebble field roughly the area of a two-car garage was dappled with chartreuse flakes: lichen. The first life we’d seen in days. Wilhelm crouched in the heat and squinted, flashing her hot-pink eyeshadow. She scooped some rocks into a sterile canister. Later, Wilhelm would ship the rocks to her lab at NASA’s Ames Research Center, in Silicon Valley, where she works as an astrobiologist. She would scrape bits of lichen from the rocks, liquefy them, and sequence their DNA. She would do the same for microbes she collected from ravines, and preserved cells she scraped from salt rocks. In many places in the Atacama, such hardy creatures are the only life forms, and Wilhelm and other scientists think that they might be similar to the last surviving life on Mars—if Martian life ever existed.

The odds seem pretty good that we will find extraterrestrial life, someday. Everything Earth’s history has taught us suggests that once life takes hold, it is really, really hard to snuff out. Almost everywhere that astrobiologists like Wilhelm look—from sunless caves in the Italian Apennines to hydrothermal vents on the seafloor—they find something respiring and reproducing. Life is everywhere on this planet. Life can handle a lot. And yet we remain, so far, alone. No one has visited. Nobody has called. Nobody has turned up in our telescopes, our robotic instruments, or our collected space rocks. In the Atacama, whose waterless, wind-worn landscapes mimic the surface of Mars, Wilhelm and her sunburned crew are mapping the edges of life’s domain, learning what life needs to survive, form communities, and produce future generations. And they are approaching the question they most long to answer: Are we really alone in the universe? On every leg of my journey to the Atacama, the density of life dropped as the heat intensified. I traveled from wintry St. Louis; to Dallas; to late-summer Santiago, Chile; and then to Antofagasta, a small coastal city creaking under growth fueled by lithium and copper mining. From there, loaded with groceries, outback supplies, and Chilean wine, Wilhelm’s team and I drove north up the coast toward Salar Grande. The Atacama Desert stretches 600 miles south from the Peruvian border, nestled between the Pacific Cordillera and the Andes, “a cross extended over Chile,” in the words of the Chilean poet Raúl Zurita. Some parts of it are so devoid of life that their microbe-per-inch count can compete with near-sterile hospital surgical suites. Some areas of the Atacama, Earth’s driest nonpolar desert and the oldest desert anywhere, have been rainless for at least 23 million years, and maybe as long as 40 million years. Carbon cycling happens on timescales of thousands of years, comparable to Antarctic permafrost and places deep within Earth’s crust; the Atacama contains some of the most lifeless soils on the planet. The Atacama is one reason that Chile has become a haven for astrobiologists and astronomers: Its pristine dark skies offer an unparalleled view of the stars, and its depleted desert offers a peerless lab for studying the dry limits of life, including how life might survive among those stars. And honestly, it just looks a lot like Mars. It is the closest that these astrobiologists will ever get to the planet that occupies their grant proposals and their imaginations.

Read: Meet the endoterrestrials I’m neither an astrobiologist nor a professional astronomer, but I spend a lot of time thinking about Mars. I keep tabs on the robots spread across its surface and in its orbit, and sometimes I check their nightly photo downloads. The Atacama is not a giant leap from the Mars of my mind. As I drove up the coast, I found the view so much more like Mars than Earth. There are no palm trees or tourists or bleating gulls. There is nothing but brown, tumbling tanly down the hills, darkening to chocolate inside shadowy ravines and runnels, bleaching to an impoverished shade of cardboard, and crumbling into fine white beach before being swallowed by the cobalt hues of sea and sky. With no trees or succulents or even a blade of grass—not a smidge of green—the only disruption in the brown is a strip of asphalt, Ruta 1. With my cruise control set and David Bowie blaring, I pictured myself driving through Meridiani Planum, a vast equatorial Martian plain, en route to visit the Opportunity rover. The only reminders of other humans were the grim commemorations of car-wreck victims: Almost every mile of Ruta 1 is marked with roadside shrines to the dead. Four hours north of Antofagasta, our convoy peeled off Ruta 1 and turned east onto a dirt road. Alfonso Davila, a soft-spoken NASA astrobiologist who also works at Ames, was in the lead vehicle. He was a calming presence, quintessentially Spanish in his affect and manners. He gently translated the language and the landscape for me after I arrived, and I found him easy to trust. As he approached a featureless hill, I looked in vain for a tunnel. I followed his cherry Toyota Hilux up the hill, then watched him wrench the truck 180 degrees around a bend and up an even steeper incline. The switchback was dense with sand and the loosest gravel I have ever tried to clear—and I grew up in Colorado. I was certain I would get stuck, and I hung back. Davila saw me hesitate and reversed, compressing the dirt for my SUV. He hopped out and ran over to my window. He was wearing a black San Francisco Giants hat and, improbably, a tan corduroy shirt. I thought, He must be really hot, and also, I am definitely going to die. “Just, um, gun it,” he told me, in his Catalan accent. I waited for him to clear the path. I gunned it.

The hill was so steep that no road was visible past my dashboard; my SUV might as well have been a rocket ship launching me off the Earth. I felt my back left wheel slide and start inching toward the edge of the roadway, such as it was. Wilhelm, cool as a cucumber in my passenger seat, commented that the view was incredible. My palms were soaking wet. I wiggled the steering wheel and gunned it again, and the SUV lurched forward, finding purchase. Clear! I still couldn’t see the road beneath my wheels, but I could see the hillside to my right, and I hugged it until we cleared the summit. I let out a half-sigh, half-cheer “Whoo!” as we turned east, over the cordillera, and down into the shimmering Salar Grande. From a distance, Salar Grande looks like a sea of pebbles, glistening underneath the bluest possible sky. Up close, the pebbles resolve into a field of fissures and slabs called polygons, which form only in the Atacama’s ultra-dryness. They are edged with countless creamy, windswept nodules, as smooth and round as carved marble. They are made of pure salt, but are deceptively slippery, so the six of us moved slowly after leaving our trucks at camp. We could not see it, but we were walking on a forest. The nodules, called halites, harbor a community of microbes. They include a member of Archaea, the third domain of life, named Halobacterium salinarum. The salt also contains a few species of cyanobacteria adapted to ultra-salty conditions. The bacteria eke out a living by drawing energy from sunlight, which they absorb through the translucent salt knobs. The archaea and other bacteria feed on those cells.

They might be the only permanent residents of Salar Grande. “These are the last outposts of life in the Atacama,” Davila said. “These are the last outposts of life on Earth, in terms of dryness.” And they might be similar to the last permanent residents of Mars. Today, the fourth planet has environments like the Atacama, with similar polygons and salt flats and humidity levels. From 1977: Life on Mars Salar Grande was once a coastal inlet, much like today’s San Francisco Bay. It dried up between 1.8 and 5.3 million years ago, leaving behind a salt flat between 225 and 300 feet thick. The salar is therefore an analogue for the last time Mars was habitable, after Mars’ oceans, if there were any, dried up, when Martian ecosystems became concentrated in smaller places. And, like Mars itself, the Atacama is a glimpse into Earth’s own future. One day, billions of years from now, all of Earth may resemble this parched land of fissures and knobs, after our own oceans boil away, after the last trees fall, after the algae are all that is left of us. “In the beginning,” Davila said, “there was bacteria. And at the end, there will be bacteria.” We stopped after walking the length of a football field or two into the salar. Wilhelm, wearing a Sia concert T-shirt and yoga pants, began laying red rope in a straight line. Davila helped, wearing his long-sleeved corduroy shirt. He didn’t seem to mind the heat.

“He’s Mr. Atacama,” Wilhelm reassured me, and laughed. He travels here twice a year, at least. “I’m trying to be,” he replied. As they worked, Davila compared notes with Jocelyne DiRuggiero, a French biologist at Johns Hopkins. I chatted with Kathryn Bywaters, a biologist at the Ames-affiliated SETI Institute. I kept my gaze high to keep sweat from pouring into my eyes. I started to worry I would contaminate the site. In this part of the Atacama, fog is the only water source keeping the microbes alive. It rolls over the coastal range at night and temporarily moistens the salt, which awakens as a shape-shifting blob in a phenomenon called deliquescence. On a previous expedition, Davila and others had wrapped a rock in gauze, and returned to it a year later to see what had happened. The gauze had been swallowed, as the salt rock continually melted and resolidified around it. “The salt is very dynamic,” Davila said, cracking one rock open with a hammer. Inside were bright layered stripes, in every shade of pond scum: pine, jade, pea-soup. DiRuggiero hypothesizes that the pigments are chromatic adaptations, essentially a form of sunscreen that protects the microbes from DNA-stripping solar radiation. She thinks that the halobacteria are changing their color based on the wavelengths of light they receive within the rock. Closer to the surface, they are lighter; down below, they are darker. Or it could be that the layers are made of different organisms entirely. Only DNA will tell, and to get that, the scientists have to bring samples home. DiRuggiero also planned to collect viruses, and later, to drill into a rock and load it with sensors to track what happens inside it over one year.

Zoë van Dijk

The researchers wanted to figure out what lives in the Atacama kiln, how the communities work together, how they signal to one another using molecules such as RNA, and how they are adapting to the Atacama’s changing climate. Their story may echo on Mars: Here lie the last leftovers, and this is what happened to them.

DiRuggiero passed me her rock hammer, and I used it to smack my own Seussical tuft of white. The meringue broke in two, and I saw small open pockets within the salt crystal, which were colonized in green. The other half of the rock stayed attached to the ground, its insides now exposed. I felt guilty for cleaving this teeming microbial city. “You are a destroyer of worlds,” Bywaters joked. As our first night approached, we set up camp and cooked dinner: Beef tacos with the best avocados I have ever had. While we ate, the golden hour finally brushed the brown away, and suddenly the Atacama was all color. Hilltops glowed pink and mauve. The far horizon darkened from cobalt to purple. The salt rocks of the salar became amber jewels. Stars started emerging well before they had any right to, well before the sun had vanished into the Pacific. To the west, to my relief, I found the waxing crescent moon, but it looked wrong: I had forgotten that it hangs backward in the Southern Hemisphere. After sunset, the wind shut off as abruptly as if someone had closed a door. By the time everyone turned in for the night, the air was utterly still. The emptiness seemed to press down on me. I thought of home, where even on the darkest, quietest night, sensory reminders of life are ever present. I see airplane lights blinking 30,000 feet above my yard. Leaves rustle in the wind or under animal feet. In my house, I hear my family; I see dog fur piling up in corners. But I had said goodbye to all that to sleep on Mars, where there is nothing.

I lay awake waiting for the moon’s scythe to sink below the western horizon. The Milky Way crossed my open-ceilinged tent, and I found that I could see by its light. To my right was the Southern Cross. The Chilean poet Gabriela Mistral instructs: “Lift up your face, child, and receive the stars. When you first look / they all pierce and freeze you, and then the sky begins to sway / like a cradle they’re rocking / and you give yourself up wholly / to be carried away, away.” If there were ever life on Mars, and if it could see, it would have witnessed a similar sky. I looked for the Centaurus constellation, but it had not risen yet. The centaur’s raised front hoof contains the nearest star to our own: Proxima Centauri. Astronomers think that star has a planet. Each of the other numberless, anonymous glitterings might have one, too. Some of those planets might be warm, nestled close to cool stars or safely distant from fiery ones, and some of those warm planets might have water. Add an atmosphere, and one of those warm, wet planets might be hospitable to life. I am comforted by this possibility, but I know that for others, the certainty of life elsewhere would be world-rending, faith-shattering. While some people of faith question the possibility of extraterrestrial life, others may question their faith in the face of its discovery. Some may associate its discovery with doom. Growing up in the Catholic Church, I was never taught that aliens were doctrinally impossible, though theologians have long debated how a discovery would affect church teaching. Catholicism’s relatively neutral stance may well be informed by its institutional embrace of astronomy; in 1891, Pope Leo XIII founded the Vatican Observatory so that “everyone might see clearly that the Church and her Pastors are not opposed to true and solid science, whether human or divine.” The Vatican’s current chief astronomer, Guy Consolmagno—an MIT graduate, a Jesuit, and a practicing planetary scientist—assured me that Scripture accommodates aliens. He cautioned against imposing our own understanding of religion onto the science that might find those aliens.

“The only theological point is to remember it’s always a mistake to think there’s something God can’t do,” Consolmagno told me, “including making parallel universes, much less other worlds with other inhabitants.” I woke up chilled and thirsty, and eager for the sun—“the bitter sun, Lord Burn-All-Things,” as Mistral wrote. DiRuggiero’s graduate student, German Uritskiy, volunteered to hike another couple of miles into the wastes to retrieve a GPS device that had recorded the salar’s temperature, humidity, and other vitals for the past year. He volunteered in part because he is color-blind, meaning he was hard-pressed to find our quarry in the salt forest: He is an algae-hunting biologist who cannot see green. His absence meant DiRuggiero needed my help. I slathered sunscreen over every inch of my skin, swaddled myself in a white linen scarf, and stepped into the salar. DiRuggiero gave me a quick lesson in how to scrape bits of compressed salt into a sterile sample bag, and handed me her hammer. I pounded on a nodule until it cracked apart, then donned purple sterile gloves, spritzed my knife with alcohol solution, and picked up a chunk. I looked for the stripe of color that would indicate life within. I saw nothing. I turned the snowy sample toward the sun, then cupped it in my palm to shade it. In different light, with my sunglasses on and off, I still saw no evidence of life. I wanted so badly to see a line, even a faint one, but this rock, it seemed, was barren. I put it down.

Read: South America’s otherworldly seabird DiRuggiero and the others had found life in Salar Grande before. Other researchers had even grown Atacama microbes in the lab. They expected to find life again, but also knew that difficulty was part of the point. They were trying to find the limit beyond which life becomes hard or impossible to sustain. It might be a temperature, or a moisture level, or a rate of change. If they could pinpoint some threshold, that might be a clue to life’s minimum requirements. This liminal state, between finding and not finding, is characteristic of astrobiology in general. We don’t know whether we’re alone, but we don’t know whether we aren’t. We do know that there is one planet with life, and on this planet, life is everywhere; because of us, we can be sure life in the universe is possible. If we don’t find life on Mars pretty soon, or on Enceladus or Titan or Europa or Trappist-1b, that doesn’t mean we should stop trying. But it’s also possible that life has happened only once. We might be it. I took DiRuggiero’s hammer, stood up, walked a few feet, and thwacked another nodule. The hammer twanged in my hand and a foamy chunk of salt fell apart beneath. Dark green! I scrambled back to the sterile bags and started scraping. Uritskiy had told me that as long as I got a tiny bit of green, he could get to work; in the lab later, he would pour the salt into a vial, liquefy it, and purify the sample before running the contents through a DNA sequencer, to find out what was living in the rock and what it was doing. We collected samples from eight rocks, scraping both light and dark stripes, so DiRuggiero could test her chromatic adaptation hypothesis.

DiRuggiero packed my samples into a cooler for their journey home before we left for Salar Grande’s northern reach. After the lichen pit stop, we arrived at the second field site, which welcomed us with a dust devil. Everyone took iPhone videos as I imagined my rental car tossed like a tumbleweed. The scientists all seemed at home in the desert, despite its hostility. A few weeks before the trip to Chile, Wilhelm and Davila had returned from Antarctica, home to the world’s driest desert, the McMurdo Dry Valleys. But Davila, who spends four weeks a year in the Atacama, finds the frozen continent a breeze by comparison. “You set up a much more comfortable camp. There are lab tents, heated tents,” he said. “Outside, it’s extreme, but inside it’s fine. Here, there is no inside.” As Lord Burn-All-Things beat down on us, DiRuggiero set up a tabletop light experiment. She took a rock from the salar, placed it under a sawed-in-half milk crate, and used a modular spectrometer to measure the colors of the sun. Then she got out the drill. Brushing her straw-colored bangs from her face, DiRuggiero set her jaw and squeezed the trigger. The rock fought back; like my halite earlier, this one was very dry, and firmer than usual. She pressed on, and drilled a hole halfway through the rock. “We want to know what the organisms are seeing, the wavelengths of light and the intensity,” she explained. A spectrometer probe nestled deep in the rock would tell her.+

Read: An epochal discovery: A habitable planet orbits our neighboring star Later, she told me that the light inside the rock was red-shifted, meaning toward the infrared range of the electromagnetic spectrum. In her lab this spring, she and Uritskiy isolated cyanobacteria and sequenced their DNA and RNA. In April, they identified genetic markers for an adaptation called FaRLiP, for far-red light photoacclimation. This summer, she grew the cells in white light and near-infrared, and in still-unpublished research she found that they are using a newly discovered pigment called chlorophyll f. This pigment, only isolated in 2010, allows some species of microbes to harvest energy from far-red light. On Earth, this light penetrates salt rocks in the Atacama. On a distant planet orbiting a faraway sun, this wavelength of light may dominate. So chlorophyll f, just like the salt-loving microbes of the salar, offers tantalizing hints of life’s possibilities beyond this planet. By the end of our stay in Salar Grande, my face looked like it was red-shifted, too, crisped by sun and wind. It was time to go. The wind buffeted my SUV, keeping me company as I drove seven hours solo down the coast of Chile. “The hurting, healing, flying wind,” Mistral wrote. The next morning, as I drove to the beach in Antofagasta, I passed a park and thought my eyes were tricking me. The green looked somehow off. I wondered, Is that an accurate grass rendering, or is the simulation failing? I had forgotten what plants should look like.

While DiRuggiero, Davila, Wilhelm, and the others collected fresh samples, their NASA colleagues were busy at another Mars stand-in a day’s drive south, in a ghost town called Yungay. For a few weeks every year—it would be longer if the team had more funding—NASA engineers and scientists descend on a ramshackle five-room building near the ghost town’s cemetery, on land owned by a copper mine. Their five-year project, the Atacama Rover Astrobiology Drilling Studies, is the space-robot version of the work in Salar Grande. The decibel level was shocking after my desert baptism. It was like entering a sports bar during the Super Bowl after a week-long silent meditation. In Salar Grande, I knelt like a penitent and quietly scraped green crud into sample bags. In Yungay, a laser-equipped metal box on wheels trundled around, noisily spinning its three-foot drill. In Salar Grande, a handful of us shared quiet dinners around a campfire; in Yungay, two dozen engineers—mostly, but not all, young men—passed tortillas down a loud, long table as they tried to make one another laugh. In Salar Grande, I basked in a silent shower of stars and saw my shadow under their light. In Yungay, an engineer stood in front of the rover and dabbed, making sure his friends could see his jagged LIDAR shadow on the laptop screen. Yungay was hotter than Salar Grande, and less beautiful, though no less like Mars. The grit still covered every inch of me. The wind consumed me, and the sunlight was starting to make me feel manic. I felt as if I were hallucinating. But there were some upsides. A watercooler meant we could actually enjoy staying hydrated, and the University of Antofagasta Desert Research Station even had a shower. At the close of the NASA mission, Yungay would fall silent, and the building would soon be picked clean by local mine workers scavenging for home-improvement materials.

If the goal in Salar Grande is to uncover life’s last holdouts, the goal in Yungay is to teach a robotic emissary to do it for us. Wilhelm’s keen eye spotted the yellowish-green speckles of lichen from a moving truck. But no human scientists are going to Mars anytime soon. A robot geologist would be able to identify the algae-fungus composite lichen flakes only after drilling into the planet, bringing up a deep sample of dirt, liquefying and purifying it, and running some kind of peptide or DNA-sequencing test. This is tricky even for a human; from 100 grams of soil, Wilhelm extracts just 40 microliters, four-tenths of one gram, of organic material. From that minuscule sample, she can detect life and its makeup, but a robot will not be able to pipette. “You have to turn your dirt into liquid, and liquid into data. The liquid is the technology gap we’re trying to bridge,” Davila told me. The rover, nicknamed K-Rex, was designed to do all this. In Yungay, its mission was to drive where it was told, drill, scoop, run an analysis, and beep “Eureka!” Like K-Rex, Mars rovers planned for 2020 and beyond will look for chemistry that cannot be explained without the busyness of life. But this is much more complicated than it sounds. Amino acids are abundant in space, so that would not be enough. Same with hydrocarbons. It’s the type that matters: Some organic molecules come in mirror varieties, like our hands; on Earth, life uses all left-handed amino acids and right-handed sugars to build proteins and DNA. If we sift the sands of an alien world and find an abundance of certain hydrocarbons, amino acids, and lefty sugars, then things could get interesting. And yet—will that be enough? Chiral molecules are quite a long way from little green microbes, let alone little green humanoids. Getting to “Eureka! We are not alone!” is a lot to ask of a miniature car driving alone on a world 300 million miles from home. The thing is, everything we know about Earth suggests that we should keep trying. When you take away almost all the water, add copious heat, eliminate all vegetation, and turn up the bitter sun, it is still possible to find something alive, even a whole community of living things. Yes, we’re on Earth, a rock that has spent 4 billion years crawling with creatures desperate to survive and make copies of themselves. Nowhere else that we know of has such a history. If life took hold on Mars in its watery past, or if microscopic life persists on the moons of Jupiter or Saturn, it is not going to be easy to find its exhalations, or its remains. It will be even harder for the robots we send in our stead. But we have reason to hope. We know life arose in the universe once. We just don’t know whether it can happen again. The only way to be certain, to end this state of unknowing, is to find life somewhere else, and to answer yes. Until then, the search will continue.