by Deborah Blum
The migration of Jupiter reshaped the solar system in many other ways. It cleared out the original asteroid belt on the way in and filled it with new objects on the way out. It reorganized the distribution of comets. It stunted the growth of Mars, making it into the cold and nearly airless world it is today—bad luck for prospective Martians. At the same time, Jupiter deposited enough material closer to Earth that our planet ended up colliding with one of the leftover planetary cores. The moon is thought to have formed from the wreckage of that cataclysm.
After Jupiter finished its wanderings, the solar system looked stable, but the appearance was superficial. Instead, it had set the stage for a second great upheaval, one that has been baffling scientists for half a century.
NASA’s Apollo missions achieved many notable things, but for the purposes of this story their greatest legacy was bringing back 842 pounds of moon rocks. On Earth, almost all evidence of the solar system’s unruly early history has been worn away by erosion, biological activity, and the slow dancing of the continents. On the moon, there is no erasure. The moon never forgets.
During the 1970s, even as NASA was retreating from the majestic Apollo missions to a rickety Skylab, several groups of researchers set out to decode the lunar samples brought back by the astronauts. The moon’s surface contains an intact chemical record of all the asteroids that have pummeled it over the eons. Planetary scientists expected to find a steady progression from disorder to order: lots of impacts right after the formation of the solar system, then a rapid tapering off as the moon (like Earth and the other planets) mopped up the last bits of debris. That is not at all the story written in the rocks.
When three geochemists—Fouad Tera, Dimitri Papanastassiou, and Gerald Wasserburg—sifted thoroughly through the lunar material, they saw instead that most of the material created by impacts had an age of about 3.9 billion years. The moon apparently experienced another intense barrage of asteroids at that time, a full 700 million years after the formation of the solar system. The researchers called it the “terminal lunar cataclysm”; now it’s known as the Late Heavy Bombardment. Either way, it stayed on the books for decades as one of the solar system’s biggest mysteries.
Around 2005, Morbidelli decided to take a crack at solving it. In conjunction with three other researchers, including Harold Levison, Walsh’s neighbor and collaborator at the Southwest Research Institute, he wrote up a series of papers in which he connected the Late Heavy Bombardment to a previously unknown, belated instability in the formation of the solar system. The resulting “Nice model” (that’s Nice as in the French town where Morbidelli works) is now the most widely accepted explanation for the solar system’s second wave of devastating impacts.
According to this model, the solar system never quite found its steady groove after Jupiter migrated back out and the sun blew out its birth nebula. An enormous cloud of comets orbiting the sun beyond Neptune—the region that now marks the Kuiper Belt—was slowly but inexorably working gravitational mischief.
At first Neptune’s orbit was synchronized with Jupiter’s, a pattern called a resonance. Jupiter circled the sun three times, perhaps, for each one orbit of Neptune. Resonances tend to keep things stable. Over hundreds of millions of years, however, that cometary cloud dragged Neptune into a new orbit. “When Neptune gets off resonance with Jupiter, the system goes ‘boom,’ completely unstable. Then the violent evolution starts,” Morbidelli says. Neptune migrated out, flinging comets inward; those comets reached Jupiter, which batted them even farther out; Jupiter, in response, migrated inward.
In the end, Saturn and Uranus, as well as Neptune, moved into more distant orbits. Jupiter settled into its present, closer one. In one version of the theory, developed by Morbidelli’s colleague David Nesvorny at the Southwest Research Institute, our solar system originally had a fifth giant planet that got ejected entirely during this commotion; if so, it is currently wandering alone among the stars. Most comets got exiled to the Oort Cloud, far beyond the planets. Many other comets and asteroids went careening closer to the sun, where great numbers of them smacked into the moon, Earth, and the other inner planets.
Although almost all traces of this hellish era have vanished from Earth, some fleeting bits remain. Bruce Simonson of Oberlin College is tracking down extremely ancient impacts from the most distinctive evidence they leave behind: glass spherules the size of BBs (created from rock melted by the asteroid or comet) and elevated concentrations of the element iridium (much more common in meteorites than in Earth’s surface). Earth’s two oldest rock beds, one in western Australia and the other in South Africa, preserve records going back at least 3.4 billion years. For the past two decades, Simonson has been prospecting there for signs of what the Late Heavy Bombardment did to our planet.
One of his most intriguing results is that the rain of asteroids may have continued for a staggeringly long time, until 2.5 billion years ago or even more recently. “There’s evidence it was a more gradual ramp-down, and we think it’s convincing evidence, but of course it’s ours,” Simonson says. If he’s right, regular asteroid blows were occurring well into the era of life on Earth, which began around 3.5 billion years ago. He also sees signs of what he technically calls “big-ass impacts,” many times bigger than the impact that helped kill off the dinosaurs.
Surprisingly, Simonson thinks life soldiered on just fine amid all the falling rocks. “I’m not a big fan of impacts and extinction,” he confides. “The only one where we have clear coincidence between impact and a change is the end-Cretaceous [when the dinosaurs went extinct].” He thinks that, overall, extinctions are more likely to have been caused by enormous volcanic eruptions and by the changing configuration of the continents and oceans. That’s a little comfort, at least, considering that everywhere on Earth where he looks, he finds hints of ancient asteroid blasts.
At the time of the Late Heavy Bombardment, asteroids were hitting Earth at least a thousand times as often as they do now. Could something like that happen again? No, both Morbidelli and Walsh answer without hesitation. The first two planetary rearrangements cleared out 99.9 percent of the asteroid belt and Kuiper Belt; there is just not enough left to re-create the kind of chaos that ruled 3.9 billion years ago.
So are we home free? No again. “The terrestrial planets, they are not totally stable,” Morbidelli says. That instantly captures my attention: Earth is one of the four terrestrial planets. “Mercury is on the edge of the instability, and it could go nuts, start to encounter Venus, then the orbits of Venus and the Earth could become unstable themselves.” From there, Venus could collide with Earth, or Earth could go careening off on a totally new orbit, sterilizing the planet. The odds are not great, but they’re not all that small either—about 1 percent over the next few billion years.
I question Morbidelli to make sure I’m understating him correctly. A 1 percent chance of disaster is surprisingly high odds in the cosmic-doomsday business. He sets down the phone for a moment and I hear him in the distance, double-checking with someone else in his office (“Do you know the probability that Mercury gets crazy?”). Then he’s back on the line: “Yes, 1 percent.” And he warns that the subtle divergences that would set the whole cataclysm in motion are like the weather, chaotic and impossible to forecast far in advance. They could be building up right now.
We are back to a probabilistic view of the solar system, in which nature builds some inherent uncertainty into the system. “It may be that instabilities are just a natural part of life for planetary systems as complicated as ours, and chaos keeps us from really understanding it,” Walsh says.
There are definitely other, less catastrophic, and more comprehensible forms of instability at work right now. Comets still leak out of the region around the Kuiper Belt, a lingering hangover from the Late Heavy Bombardment. Comets in the Oort Cloud get sent flying by passing stars. In addition to gravitational mischief by the planets, a slight pressure created by sunlight, known as the Yarkovsky Effect, keeps shifti
ng the paths of the asteroids, guaranteeing that the risk of impact will never go away. A year ago, the standard line was that events like Chelyabinsk happen once every 200 or 300 years. The updated estimate is a couple times a century, maybe more often.
But Morbidelli is not at all gloomy or apocalyptic about his work. The more I speak to him, the more I absorb his perspective. Instability is a mechanism that transforms things from generic and boring into particular and interesting—whether those things are people or planetary systems. “If you want to describe the global evolution of a person, well, he’s born and then he dies. That’s it,” Morbidelli says. “If you want to describe a specific person in detail, I cannot do it in a general scheme. There is a general scheme, but there are plenty of specific ramifications that drive you to be the person that you are. For a planetary system it is the same thing. That’s chaos: extreme sensitivity to tiny changes.”
To Morbidelli, we’re not at war with a hostile universe, we’re part of it. The astronomer has clearly spent a lot of time contemplating the personal implications of his work. “Planetary systems evolve by instable steps until they find their final peace; it is almost a Buddhist view of the universe,” he concludes, as lyrical as ever. “Everything evolves to wisdom and peace—and stability—through big revolutionary events.”
ROY SCRANTON
Learning How to Die in the Anthropocene
FROM The New York Times
I.
Driving into Iraq just after the 2003 invasion felt like driving into the future. We convoyed all day, all night, past army checkpoints and burned-out tanks, till in the blue dawn Baghdad rose from the desert like a vision of hell: flames licked the bruised sky from the tops of refinery towers, cyclopean monuments bulged and leaned against the horizon, broken overpasses swooped and fell over ruined suburbs, bombed factories, and narrow ancient streets.
With “shock and awe,” our military had unleashed the end of the world on a city of 6 million—a city about the same size as Houston or Washington. The infrastructure was totaled: water, power, traffic, markets, and security fell to anarchy and local rule. The city’s secular middle class was disappearing, squeezed out between gangsters, profiteers, fundamentalists, and soldiers. The government was going down, walls were going up, tribal lines were being drawn, and brutal hierarchies savagely established.
I was a private in the United States Army. This strange, precarious world was my new home. If I survived.
Two and a half years later, safe and lazy back in Fort Sill, Oklahoma, I thought I had made it out. Then I watched on television as Hurricane Katrina hit New Orleans. This time it was the weather that brought shock and awe, but I saw the same chaos and urban collapse I’d seen in Baghdad, the same failure of planning and the same tide of anarchy. The 82nd Airborne hit the ground, took over strategic points, and patrolled streets now under de facto martial law. My unit was put on alert to prepare for riot control operations. The grim future I’d seen in Baghdad was coming home: not terrorism, not even WMD’s, but a civilization in collapse, with a crippled infrastructure, unable to recuperate from shocks to its system.
And today, with recovery still going on more than a year after Sandy and many critics arguing that the Eastern Seaboard is no more prepared for a huge weather event than we were last November, it’s clear that that future’s not going away.
This March, Admiral Samuel J. Locklear III, the commander of the United States Pacific Command, told security and foreign policy specialists in Cambridge, Massachusetts, that global climate change was the greatest threat the United States faced—more dangerous than terrorism, Chinese hackers, and North Korean nuclear missiles. Upheaval from increased temperatures, rising seas, and radical destabilization “is probably the most likely thing that is going to happen . . .,” he said, “that will cripple the security environment, probably more likely than the other scenarios we all often talk about.”
Locklear’s not alone. Tom Donilon, the national security adviser, said much the same thing in April, speaking to an audience at Columbia’s new Center on Global Energy Policy. James Clapper, the director of national intelligence, told the Senate in March that “extreme weather events (floods, droughts, heat waves) will increasingly disrupt food and energy markets, exacerbating state weakness, forcing human migrations, and triggering riots, civil disobedience, and vandalism.”
On the civilian side, the World Bank’s recent report, Turn Down the Heat: Climate Extremes, Regional Impacts, and the Case for Resilience, offers a dire prognosis for the effects of global warming, which climatologists now predict will raise global temperatures by 3.6 degrees Fahrenheit within a generation and 7.2 degrees Fahrenheit within ninety years. Projections from researchers at the University of Hawaii find us dealing with “historically unprecedented” climates as soon as 2047. The climate scientist James Hansen, formerly with NASA, has argued that we face an “apocalyptic” future. This grim view is seconded by researchers worldwide, including Anders Levermann, Paul and Anne Ehrlich, Lonnie Thompson, and many, many, many others.
This chorus of Jeremiahs predicts a radically transformed global climate forcing widespread upheaval—not possibly, not potentially, but inevitably. We have passed the point of no return. From the point of view of policy experts, climate scientists, and national security officials, the question is no longer whether global warming exists or how we might stop it, but how we are going to deal with it.
II.
There’s a word for this new era we live in: the Anthropocene. This term, taken up by geologists, pondered by intellectuals, and discussed in the pages of publications such as The Economist and the New York Times, represents the idea that we have entered a new epoch in the earth’s geological history, one characterized by the arrival of the human species as a geological force. The biologist Eugene F. Stoermer and the Nobel-Prize–winning chemist Paul Crutzen advanced the term in 2000, and it has steadily gained acceptance as evidence has increasingly mounted that the changes wrought by global warming will affect not just the world’s climate and biological diversity, but its very geology—and not just for a few centuries but for millennia. The geophysicist David Archer’s 2009 book, The Long Thaw: How Humans Are Changing the Next 100,000 Years of Earth’s Climate, lays out a clear and concise argument for how huge concentrations of carbon dioxide in the atmosphere and melting ice will radically transform the planet, beyond freak storms and warmer summers, beyond any foreseeable future.
The Stratigraphy Commission of the Geological Society of London—the scientists responsible for pinning the “golden spikes” that demarcate geological epochs such as the Pliocene, Pleistocene, and Holocene—has adopted Anthropocene as a term deserving further consideration, “significant on the scale of Earth history.” Working groups are discussing what level of geological timescale it might be (an “epoch” like the Holocene, or merely an “age” like the Calabrian), and at what date we might say it began. The beginning of the Great Acceleration, in the middle of the twentieth century? The beginning of the Industrial Revolution, around 1800? The advent of agriculture?
The challenge the Anthropocene poses is a challenge not just to national security, to food and energy markets, or to our “way of life”—though these challenges are all real, profound, and inescapable. The greatest challenge the Anthropocene poses may be to our sense of what it means to be human. Within one hundred years—within three to five generations—we will face average temperatures 7 degrees Fahrenheit higher than today, rising seas at least 3 to 10 feet higher, and worldwide shifts in crop belts, growing seasons, and population centers. Within a thousand years, unless we stop emitting greenhouse gases wholesale right now, humans will be living in a climate the earth hasn’t seen since the Pliocene, 3 million years ago, when oceans were 75 feet higher than they are today. We face the imminent collapse of the agricultural, shipping, and energy networks upon which the global economy depends, a large-scale die-off in the biosphere that’s already well on its way, and our own possible extinction.
If Homo sapiens (or some genetically modified variant) survives the next millennium, it will be survival in a world unrecognizably different from the one we have inhabited.
Geological time scales, civilizational collapse, and species extinction give rise to profound problems that humanities scholars and academic philosophers, with their taste for fine-grained analysis, esoteric debates, and archival marginalia, might seem remarkably ill suited to address. After all, how will thinking about Kant help us trap carbon dioxide? Can arguments between object-oriented ontology and historical materialism protect honeybees from colony collapse disorder? Are ancient Greek philosophers, medieval theologians, and contemporary metaphysicians going to keep Bangladesh from being inundated by rising oceans?
Of course not. But the biggest problems the Anthropocene poses are precisely those that have always been at the root of humanistic and philosophical questioning: “What does it mean to be human?” and “What does it mean to live?” In the epoch of the Anthropocene, the question of individual mortality—“What does my life mean in the face of death?”—is universalized and framed in scales that boggle the imagination. What does human existence mean against 100,000 years of climate change? What does one life mean in the face of species death or the collapse of global civilization? How do we make meaningful choices in the shadow of our inevitable end?
