The Sirens of Mars

by Sarah Stewart Johnson

  Although I could sense the enormous power of these instruments, I had no inkling then of their potential as tools for space science. Years later in graduate school, when the opportunity to work on a mass spectrometer came along, I jumped at the chance. I began to plow through papers about how scientists were using them to uncover traces of life in ancient rocks. Under the right circumstances, certain molecules, like lipids in cell membranes, could stick around for billions of years. Even if a group of atoms here or there fell off, like fingers or toes, the backbone of the molecule could still tell stories about the cell it came from, the same way we can learn about dinosaurs from dinosaur bones. Patterns among very simple molecules could also serve as strong indicators of life. Even without patterns, the mere detection of organics was key to determining the conditions for life as we know it.

  When the Viking landers carried the first mass spectrometers to the surface of Mars in the 1970s, they found no definitive evidence of organics. But Curiosity had three things going for it: It had a far more sophisticated detector in SAM, with a resolution of better than one part per billion. Curiosity’s landing site was specially chosen for the fine-grained clays that could have once trapped and preserved organics. Also, the rover could sidle right up to the best sampling locations and use its drill to access the protected interior of rocks.

  When Curiosity began carving into the mudstones, it kicked out not hard orange oxidized rock but instead a soft gray powder. A pinch of pulverized clay, about half the size of a baby aspirin, wound its way down a sieve into the belly of the rover. There, diagnostic analyses revealed that the rocks were made of clay minerals that typically form under neutral pH conditions. Mars hadn’t been thoroughly soaked in acid after all. And unlike at Meridiani, there was hardly any salt in these mudstones. All six of the elements required for life as we know it were present in the sample: carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur. It was the context we needed. Not only was there water, it was the right kind of water, in the right kind of place. The Sheepbed mudstone was just where we should be looking for life. At the press conference that followed, John Grotzinger, the mission’s lead, announced that a habitable world had finally, definitely been discovered. “If this water was around and you had been on the planet,” he said, underscoring his point, “you would have been able to drink it.”

  What’s more, there was finally an indisputable detection of organic molecules. SAM’s tiny oven heated up the sample, turning the molecules in the mudstones to gas, then whiffed them into a long thin tube about the width of a dust mite. One by one, they popped out the other end, where the mass spectrometer identified them: simple compounds, chlorinated from interactions with perchlorate from the surface. And discovered later, even more-complex molecules, bound together by sulfur. The high levels of the detections, up to three hundred parts per billion, set to rest one of the longest-standing mysteries about Mars: The building blocks of life were indeed there.

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  BY THE TIME I joined the science team, Curiosity had holes in its wheels, and its poor drill was beginning to show signs of wear. It had been roving for four years. But the mission was about to enter into one of its most interesting phases: an ascent of the Vera Rubin Ridge, named after the extraordinary scientist who discovered the first evidence of dark matter, mysterious particles thought to comprise roughly 85 percent of the total matter in the universe. As a girl, Rubin had learned to tell time by how the stars outside her window wheeled across the sky. As a young woman, she was encouraged to pursue a career not in astronomy but in painting astronomical objects. Undeterred, she went on to earn a PhD at Georgetown and stayed on as an assistant professor at a time when many astronomy departments were not open to women. She was averse to sharp elbows, gravitating to a research question that “nobody would bother [her] about.” Working in this self-imposed obscurity, she transformed cosmology. She proved that the vast majority of the universe is invisible, a finding I have always found poignant.

  Near the end of her life, she wrote that she had succeeded in her two goals: “to have a family and to be an astronomer.” She had four children, three sons and a daughter. The eldest, born when she was just twenty-two, even became a planetary geologist. He now works on Curiosity’s science team, striving to understand the formation of the ridge named for his remarkable mother.

  Rubin’s old observatory is still on campus, on a hill just opposite my lab. I pass it sometimes in the mornings, as one of my children’s favorite things is to see the pollywogs in the adjacent pond before preschool. On the way back down the hill, they stop to pick up pine cones for their teachers and to stuff pebbles into their pockets. They are full of questions, shining their attention like lamps on everything before them. They spend all their time, every moment, stitching together clues about how things work. Watching them, I’m reminded how we’re born knowing so little and understanding even less about the context of our world. They look to me for explanations, for words they don’t know, for help connecting the data points. All of their questions seem to contain another “why.” Why is it this? Why is it this and not that? It reminds me of something Rubin once said: “I’m sorry I know so little. I’m sorry we all know so little. But that’s kind of the fun, isn’t it?”

  I want time to stop in these moments, with their tiny fingers laced into mine. Even though we walk slowly, pausing constantly to inspect things, we always arrive, and at the sight of their small friends, they run from my arms. I drop their lunches in their cubbies, then linger, watching them for a few more moments through the windows of the building, my fingertips against the panes of the glass.

  But eventually I walk away, and before I know it, I find myself on Mars, where Curiosity is still roving. From time to time, I get to help guide the rover. On those days, I either drive out to Goddard’s SPOCC—the Science and Planetary Operations Control Center, where SAM is operated, where life-sized cutouts of Spock from Star Trek decorate the hall—or I remotely access JPL’s server from my office at Georgetown. Downlinked images, caught by the giant dishes of the Deep Space Network, stream into the viewing software. They too transport me. They take my breath away. As the team decides what measurements to make and the engineers check the commands, I wonder how the rover is doing all those millions of kilometers away. What would it be like to be standing there, to hear the sounds of the instruments warbling in the darkness?

  When those days end, when eventually I realize I must go, again it feels almost impossible to leave, one love tearing me from another. Sometimes I’ll pan slowly through the last mosaic image of the Martian landscape, stopping at a certain angle where the land meets the sky. This is what I’ll see when I next flip open my laptop, my teleporter. What comes next? What’s the best way up the mountain? Is this new or something seen before? What does it mean? I carry the image in my mind, a wilderness stretching off into the horizon, vast and full of possibility.

  DEEP IN THE forests of western Bosnia is a village named Jezero. In many Slavic languages, “jezero” is the word for lake, and jezeros stretch all along the Adriatic. They fill the Julian Alps, dotting the spaces between low-lying meadows and frog-filled caves. There are sinkhole jezeros and karst jezeros, glacial jezeros and jezeros linked by a hundred waterfalls. But the jezero in the village of Jezero is green and quiet, almost mythic. It’s preternaturally still, rumored to be heavy with deuterium. The surface reflects the clouds like a piece of polished glass.

  Small craters on Mars are named after small towns, and on the western edge of Isidis Planitia is a small crater named for this small Bosnian town. Early in Mars’s history, Jezero Crater also held a lake with water that reflected the sky. Two rushing rivers emptied into the cavity—one from the west and one from the north. The lake was deep, its crater floor plunging hundreds of meters down from the rim, which on one unexpected day billions of years ago suddenly fissured, unleashing a catastrophic torrent of water over
the side.

  Among the bevy of spacecraft that will soon launch toward Mars is a NASA rover that will land on the spill of lava that covers the floor of Jezero Crater. The mission’s breathtaking goal is to collect samples from Mars to be brought back to Earth at some later date, samples that not only may harbor signs of ancient life but also could give us an unprecedented look into the history of our solar system.

  Here on Earth, our record of deep time has been forever lost. The seas have lifted into rain, and the rain has beaten the surface bare. Our planet has swallowed itself, plate by plate. Our original crust has almost completely disappeared; all but a few patches have been dragged back into the interior. The small blocks of rock that remain—in the cherts of Australia and the greenstone belts of Greenland—have been cooked, mostly beyond recognition. Our early days are irrecoverable.

  Mars, however, is all past. It is as if time stands still. There are no plate tectonics, no large-scale recycling of rocks. The rivers have stopped; the temperatures have plummeted. On the scale of humanity, Mars has been constant. To be sure, there is weather, like the spectacular dust storms that come and go. Barchans of sand shift across the surface. The polar caps wax and wane. The planet’s spin axis arcs into a deep bow every hundred thousand years or so. Yet the land beneath remains.

  The place we’re now visiting with spacecraft is almost the same world that it was three billion years ago. As a result, the right samples may even help us fill in the gaps in our own planet’s history. Right now it’s unclear what prebiotic chemistry dominated the early days of the rocky planets, or what dance of reactions made the first protocells. Perhaps life sprang from geothermal fields, with repeated cycles of wetting and drying helping to form complex mixtures of important molecules. Or perhaps not. The samples the rover collects might hold within them the echoes of the beginning of life, entombed deep in the planet’s ancient rocks.

  The rover’s chassis is the same as Curiosity’s, but it will carry a different scientific suite, even a small helicopter to test the viability of airborne craft. The rover’s two-meter-long arm, laden with new coring tools and instruments, looks like an outstretched lawnmower, and is just as heavy. Over at least two years of operations, the turret will drill several samples of rock and place them carefully in sample tubes, each about the size of a penlight. The rover will then deposit the tubes in a little pile on the surface. The cache will remain there for many years, glinting in the sun, until a fetch rover comes and launches them into orbit—to be caught by a passing spacecraft and brought home.

  Like the rocks we carried back from the moon, Mars rocks will be analyzed for decades to come. Once we have them in hand, we’ll have them forever. It’s been nearly fifty years since the last humans walked on the moon, yet the Apollo samples have been examined again and again, particularly as new tools and technologies have been developed. In that time, we’ve discovered astonishing, unexpected things, like the precise age of the moon and the fact that the rocks carry an indelible record of the history of solar activity.

  If we are going to archive samples of Mars, now is the time. One day, potentially one day soon, there will not only be rovers and robots, there will be people exploring the planet. SpaceX is already calling for a million passengers, sent on a thousand spaceships. But unlike rovers, which we can bake and clean, humans will shed life left and right, sloughing off cells, littering the planet with biological material. The next decades are thus critically important for the search for life because the window to explore an untrammeled planet—a pristine record of the past—is closing.

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  JEZERO CRATER IS home to a relict river delta, and that is the reason it was chosen as the landing site. There are two deltas, in fact—but the larger and more magnificent, which fans gorgeously out to the east, accumulated rocks and debris along the crater’s western rim.

  In many ways, deltas are the perfect place to investigate life. As rivers move into steady water, they slow down and spread out. Frictional drag is what suspends small grains of sediment and keeps them leaping along. But when the water slows, the somersaulting particles fall. The grains sort into sizes—coarse sand settles first, then silt, then clay. The finest-grained material is the last to settle and the most likely to trap things. These gooey clays bind and bury organics. They harden into impervious mudstones, and the molecules within them are protected from oxidation and other forms of chemical attack.

  This is what we hope we’ll find at Jezero. The large delta there was fed by headwaters that stretched for scores of kilometers, all the way to the horizon. The clay-bearing distal edge of the delta is one of the rover’s main targets, its rich bottomset beds offering a chance to find the archived traces of ancient life.

  Herodotus named deltas after noticing that the triangle shape at the mouth of the Nile resembled the Greek letter delta, Δ. He was a great historian, determined to prevent the traces of human “events being erased from time,” but he was also an explorer. Reaching for the edge of his ancient world, he first sailed to Egypt—the “gift of the river”—in around 450 B.C. He documented the trip in an “autopsy,” or personal reflection. One of the first things he noticed was a great “silting forward of the land” as far distant as a day’s sail from the shore. He described how the river spilled out plaits of fine-grained clay when it emptied into the Mediterranean, how you could let down a sounding line and bring up nothing but mud.

  The same sludge carpeted the delta, transforming the desolate Sahara into a place with flamingos. The fine-grained material was filled with nutrients, perfect for growing crops: durum wheat, emmer, flax, barley, rape, black mustard. From the ground rose chicory and parsnip and the spices of caraway, anise, and hop. From the end of the Paleolithic, the ancients plowed and seeded during the long winter growing season. In the spring was the rich harvest, when flint-bladed tools would reap the land.

  Then in summer, the fields would flood. The Egyptians had a word for it, known today as akhet. (Like an eroded relic, the word’s vowels are lost to history, with only consonants remaining in the hieroglyph.) Akhet was the inundation. Under the dog star, Sirius, with the Nile swollen, people mended their tools and tended their livestock. They lifted the mud from beneath the water to make pots. Among the sycamores and reeds, they formed the wet clay with a kind of potter’s wheel, hand turned. They smoothed the surface and fired the receptacles in makeshift kilns. They learned how smoke could darken the surface and how oxides of copper could brighten it. They decorated the jars and jugs with pictures, thoughts, and poems, then filled them with water and wine, oil and grain. They carried those vessels with them, often into the grave, where the patterns would remain thousands of years later, their colors still resplendent.

  It was the Pelusiac branch of the Nile, off toward distant Sinai, that Herodotus took as he voyaged up the delta. He sailed alongside primeval thickets of papyrus, spangled with feathery umbels. Stalks sprang from the shallow water, some reaching almost as high as five meters. The fens had a special place in Egyptian cosmology. The world was created when the first god stood on the first piece of land: a rise that appeared, like the end of akhet, from the boundless dark water.

  The fens were a dark and mysterious place, and from them sprang the germs of creation. Like the black mud, they were a “gift of the river,” particularly their papyrus stalks. He noted how the Egyptians ate the young shoots, roasted in red-hot ovens. They made garlands from the flowering heads and mashed the reeds into the seams of boats. They pressed out sails from the spongy white pith, affixed to masts of acacia. And, importantly, they made paper, the perfect surface for recording language.

  Herodotus’s writings found their home on papyrus scrolls in the Great Library of Alexandria, on the other end of the delta, where winter waves and longshore drift flushed silt eastward. The library served for generations as a hub of unbroken scholarship. A great lighthouse signaled visitors in the
night, reflecting fire with polished-metal mirrors. When ships arrived into the port, manuscripts were sent to be copied by scribes. In time, the library’s collection grew to tens then hundreds of thousands of scrolls.

  Never before had such a powerful repository of knowledge been amassed. A gathering of ideas, like the gathering of sediment. A place of sifting, sorting, synthesis. Traditions came in from the Persians, the Babylonians, the Assyrians, and the Phoenicians. And in that fertile space, new masterpieces emerged.

  Among them was one of my favorite books, Euclid’s Elements, spare and uncluttered. Euclid didn’t invent the mathematics underlying the Elements, at least not all of it, but he synthesized the work of his predecessors in a novel way. There at the edge of a delta, he laid out thirteen sections of geometry and arithmetic: definitions, postulates, theorems, proofs. He charted a course through plane geometry and incommensurability, from the infinitude of primes to the cubature of pyramids, cones, cylinders, and spheres. It was an internally coherent system of mathematics, built from first principles. It was an unprecedented accounting of the physical universe.