Light of the Stars

by Adam Frank

  But the success in understanding Mars’s climate today highlights the single most important lesson Mars has taught us: climate changes, and with it, habitability changes, too.

  Habitability is a key concept for astrobiologists, who think of it in an intuitive way: the ability of a planet to be inhabited by life. In the Drake equation, the formal definition of habitability is the presence of liquid water on a planet’s surface.

  The robots we sent to Mars offer us pretty definitive evidence that Mars once had liquid water on its surface. Some of that evidence is geological and comes via mineralogy. The exposed Martian bedrock in front of Opportunity not only sent Steven Squyres into fits of joy, eventually it also revealed the presence of small, spherical pebbles termed “blueberries.” Instruments embedded in the tip of the rover’s multi-jointed arm allowed Squyres and his team to recognize these blueberries as hematite, a mineral that only forms in the presence of liquid water.62

  Some of the evidence for a wet version of Mars was more direct. Seven years after the discovery of the blueberries, the Curiosity rover found a set of carved rock features on Mars that could only have resulted from deep, fast-moving water flows. Curiosity scientists could even estimate the nature of the flow—about hip deep and rushing downstream at three feet a second.63

  So, Mars once had liquid water on its surface. But that means it must also have had a much thicker atmosphere keeping that water from flashing away into vapor. And if the water was rushing on the surface, that thick atmosphere must also have been warming the planet to temperatures well above freezing. Put it all together, and it seems the Red Planet was once blue, at least for a while.

  Scientists call this warmer, wetter period of Martian climate history the Noachian, for the story of Noah and the flood. Their best estimates place it between 4 billion and 3.5 billion years ago.64 There remain deep questions about what happened to the water on Mars. Getting answers to those questions may have to wait until we can send actual geologists to the Red Planet.

  But even as we wait for those answers, the recognition of Mars’s dramatic climatic change already offers us a critical astrobiological perspective on our own Anthropocene era. Mars shows us that habitability—that most critical of astrobiological concepts—is not forever. A planet can change its habitable state. Most importantly, it can lose it entirely.

  When we worry over our entry into the Anthropocene, we are inherently concerned with our project of civilization’s sustainability. But what is sustainability but a special example of habitability? What we are really concerned with when we talk about the Anthropocene is the habitability of the planet for a particular kind of energy-intensive, globally interdependent, technological civilization. The present climate epoch—the Holocene—has been particularly habitable for that kind of project.

  Mars shows us that habitability can be a moving target. The same is likely to be true of sustainability in the Anthropocene. Planets change, and that is a lesson Mars and its history help us come to terms with. It is not, however, the only lesson the other worlds in our solar system have to teach us.

  REMARKABLE JOURNEYS

  On June 12, 1982, Central Park hosted a sea of humanity. Spilling over the Great Lawn and onto Fifth Avenue, the park was thronged as never before in its 150-year history. The New York Times reported that there were “pacifists and anarchists, children and Buddhist monks, Roman Catholic bishops and Communist Party leaders, university students and union members.” Delegations had arrived from Vermont and Montana, Bangladesh and Zambia. “The smiling, hand-clapping line of marchers threading into the park stretched back three miles along Fifth Avenue.” According to the Times, it was “the largest political demonstration in US history.”65 All those delegations and all those people were in the park for one reason: to save the world.

  The shadow of nuclear warfare, which loomed so large as the final factor in Frank Drake’s equation, had grown longer and darker by the early 1980s. The election of Ronald Reagan as president, along with renewed aggressive actions by the Soviet Union, seemed, once again, to edge the world closer to an all-out nuclear exchange. By 1982, the two superpowers had increased their stockpile to more than fifty thousand nuclear weapons.66 The massive rally in New York was intended to build support for a “nuclear freeze”—an end to the weapons buildup and the beginning of a nuclear drawdown. But neither the US nor Russian government was listening.

  In response, a new kind of peace movement grew. It was larger and broader than anything the cold warriors of the 1960s had been forced to contend with. While the Central Park rally marked the nuclear freeze movement’s rise to political relevance, its framing of humanity’s basic nuclear dilemma differed significantly from that of the Cold War era twenty years before, when Frank Drake formulated his final factor. This shift became apparent a year after the massive rally, when a group of scientists published a study that changed the language of nuclear war.

  The paper was titled “Nuclear Winter: Global Consequences of Multiple Nuclear Explosions.” Carl Sagan and James Pollack were both on the team of authors who were collectively referred to as TTAPS (Richard P. Turco, Owen Toon, Thomas P. Ackerman, Pollack, Sagan). The TTAPS argument was straightforward: even a medium-scale nuclear exchange would lead to so many fires that soot lofted into the atmosphere would significantly cool the planet. Agricultural production would seize up and the world would be plunged into hunger and chaos. The lesson from their study was straightforward too: almost any nuclear exchange could transform the planet in dangerous ways. The weapons could never be used.67

  By this time, Carl Sagan had become a celebrity via his best-selling books and his TV appearances. He highlighted the TTAPS study with an extended essay in Parade magazine.68

  While the Reagan administration publicly dismissed the science of nuclear winter, the majority of the scientific community took it seriously. From that point, there was no going back. “Nuclear winter” entered the world’s vocabulary and its imagination. Years later, both Soviet and American officials would openly discuss how nuclear winter’s doomsday scenario helped draw the two nations to the negotiating table.69

  The entry of nuclear winter into the political landscape was notable for two reasons. First, it was a result based on a climate model. Pollack, Sagan, and their collaborators had used the mathematical physics governing global atmospheric flows to track the behavior of particles blown into the air by global fires. For the first time in human history, a model of a planet’s climate would frame global political debate. But it’s a second feature of the debate that matters most for our moment. A key argument for nuclear winter came from Mars.

  The globe-engulfing Martian dust storms, first observed in detail by Mariner 9, provided critical data for the nuclear winter researchers. The behavior of tiny particles carried high into the atmosphere would have been mere theory without the flotilla of probes we’d sent to Mars. With the data they supplied, the Martian climate models were expanded to include newly realized physical principles of how solar and infrared radiation interacted with dust. Thus, the space probes and the climate models revealed the powerful effect of dust on the Martian atmosphere. That understanding was then transferred to the distinctly terrestrial problem of fires ranging across the planet after a nuclear war. The TTAPS paper was explicit in calling Mars out as a test bed for nuclear winter physics.

  The history of TTAPS and nuclear winter shows us that knowledge gained from an alien world has already influenced earthbound debates about our future. Now, thirty years later and in the midst of our modern climate debates, we must recognize how deeply our understanding of climate is rooted in what we’ve learned from “wheels-on-the-ground” studies of other planets. The desperate attempt by climate-change deniers to sow doubt on climate science (and its modeling efforts) willfully ignores what five decades of space travel have taught us: we have more than one world, and one story, to school us in the ways a planet can change.

  We humans sent exemplars of our ingenuity to Venus an
d Mars. Later, humanity’s robot emissaries would reach the outer worlds of Jupiter and Saturn (and their remarkable ocean-bearing moons). By 2016, every planet and every class of solar system object had been visited at least once by our probes. Asteroids, comets, and dwarf planets—we had “touched” them all and we had learned from them all.

  In making those remarkable journeys, we did more than simply satisfy our curiosity or beat other nations for bragging rights. While we might not have known it at the time, these missions to other planets were also giving us the conceptual tools we now need to make fateful decisions about our own still-unknown fate.

  We could not have understood the greenhouse effect as we do now without what was learned from the robot probes to Venus. We could not have understood the process of climate modeling as we do now without the rovers trundling across Mars. And the atmospheres of Jupiter, Saturn, and other solar-system worlds have each provided their own lessons. We traveled billions of miles only to see our planet and our own predicament come into high resolution.

  CHAPTER 3

  THE MASKS OF EARTH

  AIR LESSONS

  Imagine you are a time traveler who just landed on Earth 2.7 billion years ago. As you step outside on this younger version of our planet, what’s your first experience? The answer is pretty simple.

  You die.

  To be specific, you asphyxiate. For about the first two billion years of Earth’s history, its atmosphere contained only minute traces of oxygen, even though it had long been a home to life. For almost half the planet’s history, its “air” was composed almost entirely of nitrogen and CO2.1 Today, however, the Earth’s atmosphere is almost all nitrogen and oxygen, with only a tiny fraction of CO2. What happened to make so great a change?

  This one all-important detail about Earth’s history—the rise of its oxygen—is a lesson for us today. It was life, acting on a global scale billions of years ago, that altered the planet’s atmosphere. In doing so, it also changed the future history of the Earth, leading to humans and our project of civilization. Now life, in the form of our civilization, is once again poised to alter the planet’s atmosphere and its complex machinery of evolution. The comparison of that time, billions of years past, with our own moment of climate change offers a doorway into the remarkable story of the “masks of Earth.” Its narrative bears a truth few of us recognize.

  Our world has been many planets in the past.

  These other versions of Earth were profoundly different from the cloud-mottled, blue-green world we know today. Each was a consequence of planetary forces shaping and then reshaping our world. Together, they reveal how deeply humans and our project are part of a much longer story. When it comes to life changing the planet, we are neither unique nor unusual. That’s why the story of our planet’s past, a story that is fundamentally astrobiological, is so critical to us. Knowing the Earths that were will give us the vocabulary to craft a new story, one that keeps us part of the Earth soon to be.

  NO EASY DAY

  The Polecat arctic transports were beasts. Designed for duty in the harshest conditions, each was the size of a minibus. The caterpillar-treaded special-purpose vehicles were built wide to keep them steady on uneven terrain, with powerful diesel engines for hauling cargo and personnel across ice, snow, or even up the steep side of a glacier.2

  On October 16, 1960, the side of a glacier was all young Soren Gregersen saw as he looked out the window of his assigned Polecat. Just a few hours earlier, Gregersen, a seventeen-year-old Danish Boy Scout, had been stuffed into the Polecat’s cab by a smiling GI. Two days before that, he’d landed at the US Air Force’s Thule Air Base on the western shore of Greenland and been outfitted with regulation cold-weather military gear. Gregersen watched in wonder as the Polecat began its long trek up the “ramp,” a sloping road carved into the glacial ice. He was beginning a 150-mile trek out onto one of Earth’s most inhospitable locales.3

  Bouncing around the Polecat’s cab, Gregersen was caught somewhere between excitement and terror. After all his hopes, preparations, and travel, it was really happening. He was on his way to Camp Century, a city the Americans built under the ice.

  At the same time that Jack James was blasting Mariner probes to the planets and Frank Drake was tuning his radio telescope in search of alien civilizations, the US military was pushing audaciously across a different kind of boundary. This one, where Boy Scout Soren Gregersen was bound, lay at the top of the world.

  Greenland is a giant ice slab where seven hundred thousand square miles of glacier rise a mile and a half above sea level.4 Temperatures at the center of its vast ice plateau typically drop to a Marslike –70 degrees Fahrenheit. Winds routinely sweep across its barren plains of snow at 125 miles per hour.5 And yet, in 1959, the US government chose to build a military base and a scientific laboratory right in the middle of Greenland’s frozen emptiness.

  The logic of the Cold War led the US to plan the impossible in the form of Camp Century. The base consisted of twenty-one trenches dug into the ice, each up to three football fields long. Each trench was twenty-six feet wide and twenty-six feet deep, with snow packed across steel arches to create a ceiling. Prefab buildings, hauled across the glaciers, had been laid into each trench to serve as barracks for the camp’s two hundred servicemen and scientists. Powering the base required a $5 million portable nuclear reactor that the military dragged out across the ice sheet.6 Taken together, building Camp Century required a monumental effort, but one that would achieve a monumental breakthrough.

  The barracks built inside one of the ice tunnels at Camp Century.

  In our era, when people who know nothing of climate science make sweeping claims of sweeping ignorance, it’s important to remember the risks required to make that science happen. The soldiers and scientists at Camp Century lived at the edge of the world, and their work carried considerable dangers. Transport flights and crews had to contend with extremes of weather unlike almost anywhere else on the planet. In the summer of 1961, a helicopter crash outside the base took the lives of all six aboard.7 But those GIs, their officers, the scientists, and even Boy Scout Soren Gregersen had all come to Greenland’s glacial wasteland on a mission. “It was the most exciting thing that ever happened to me,” recalled Gregersen, now a retired professor of geophysics, when I spoke with him. “That experience is what got me started in science.”

  Camp Century was a joint US–Danish effort (Greenland is a Danish territory). To create publicity for the polar mission, the Boy Scouts in both countries held competitions seeking “junior scientific aides.” In late 1959, Gregersen and American Boy Scout Kent Goering each won their chance to spend five months on, and in, the ice.

  “We lived right alongside the GIs,” says Gregersen. “Every day, we got some task required to maintain the base. Sometimes it was chopping away the ice that constantly grew inside the tunnels. Sometimes it was working on the pumps that fed an enormous freshwater reservoir deep in the ice. I loved it all, and all of it was thrilling.”

  But for young Gregersen, it was the science that made the strongest impression. There were many reasons why the US military built Camp Century. Plans had been discussed to house nuclear missiles in the ice (the ever-shifting glaciers killed that idea).8 There was also the need to keep watch on the Soviets. Gregersen remembers the vast radar arrays, pointing north, at Thule Air Base. But the military was especially interested in climate. The history of warfare was, after all, full of military campaigns done in by weather. Just as the funding for the exploration of space was opened by the Cold War, the Earth’s climate and its history had also become a military concern. That translated into funding for climate science. The money took scientists to the most remote corners of the planet. It was also how young Soren Gregersen first saw the drills at Camp Century.

  In rooms carved from centuries of fallen snow, Camp Century scientists set up drilling derricks, like the kind you’d see in oil country. Their goal was to dive downward through almost a mile of ice and tho
usands of years of planetary history.9 “I saw the effort being made in those ice drilling labs,” says Gregersen. “And it made a huge impression on me. What they were trying to do—it just seemed amazing—recovering the history of the planet using ancient snow.”

  Transformative visions of the world usually come when we find new ways to see it. In science, the ability to get to new kinds of data—literally new ways of seeing—allows us to revise and refresh our understanding. Jack James’s Mariner mission to Venus, Carl Sagan’s Martian dust data, and the radio telescopes at Frank Drake’s Green Bank observatory all rewrote our understanding of astronomy and planetary science. In the decades after World War II, our understanding of the Earth was also being reimagined by new data that had been beyond the reach of earlier generations of researchers. Camp Century was one critical chapter in the story of that change.

  Ice ages were still a mystery in 1960. The most certain thing scientists could say about them was that they’d happened. Over the last few million years, mile-thick slabs of ice covered much of the Northern Hemisphere. At least four different times, they ground their way south and then retreated back.10 Each glacial epoch left the planet cold and dry. Ocean levels dropped almost four hundred feet—the height of a forty-story building—as so much of the Earth’s water became locked in ice. In between the ice ages, the planet got reprieves in the form of warmer, wetter interglacial states.11

  The Earth endured the last ice age for almost a hundred thousand years. Only after the final laggard glaciers retreated did the project of human civilization begin. Our history of farming and cities, writing and machine building fits entirely within the Holocene: the current ten-thousand-year-old interglacial period.12 And even though scientists knew the basic sequence of events leading to the Holocene, the details of how the climate slipped from one state to another eluded them. They simply didn’t have the data to see the details of the change. What they needed was a way to follow the planet’s temperature, year by year, all the way back to when glaciers were last king. Under the auspices of the US Army’s Cold Regions Research and Engineering Laboratory, Camp Century’s drilling operation gave scientists that record.