Five Billion Years of Solitude

by Lee Billings

  “It’s simply very hard to abiotically build up large concentrations of both methane and oxygen in a planet’s atmosphere,” Kasting said to me later, as we began our drive to Black Moshannon. “Finding evidence of both in the atmosphere of a planet like ours—a rocky planet with surface water and enough mass to hold on to its internal heat and drive something like plate tectonics—would be, to me at least, an ironclad detection of life. People might say that’s like looking for your lost keys only under street lamps, where the light is brightest, but I don’t think that’s exactly true. The truth is that the public never wants to hear some scientist rule something out entirely, so it’s politically correct to say, ‘Oh, these alien biospheres could of course have very different chemical signatures than anything we know here on Earth!’ I think that’s totally wrong. Maybe someday I’ll eat my words, but I believe the only biosignatures we can reasonably look for are the ones that we can presently model and constrain—the ones we either see here on the present Earth or know the Earth had in its past. If life starts up on any planet that shares a few key characteristics with our own, I think you’ll get a biosphere we can spectroscopically recognize. A metabolism will work the same even if the cells are very different than ours and don’t depend on DNA and RNA molecules. Here or there, putting CO2 and hydrogen together to drive a metabolism will produce methane. Here or there, taking hydrogen from water and venting out the oxygen is a winning metabolic strategy if life can figure out how to do it. Chemistry and thermodynamics are the same everywhere.”

  As elegant as Lovelock’s criterion appears on paper, its great shortcoming is that the spectral signatures of oxygen and methane manifest at very different wavelengths. Oxygen absorbs starlight most efficiently in the near-infrared, creating prominent spectral “absorption bands” just outside the part of the spectrum that our eyes can see. Methane, being a very potent greenhouse gas, most efficiently absorbs at the longer wavelengths of the thermal-infrared. In astronomy, working at longer wavelengths translates to using bigger light-gathering areas—this is why radio telescopes are so much larger than optical ones. It’s also why detecting both oxygen and methane in an exoplanet’s atmosphere may well require the coordinated efforts of at least two space telescopes. One would be smaller and simpler, to observe oxygen in the visible and near-infrared, while the other would be larger and more complex, to observe methane in the thermal-infrared. Working together, the telescopes could also measure other gases in a planet’s atmosphere, notably water vapor and CO2, which would help constrain a world’s habitability and climate—extreme amounts of either gas would suggest a world too hot to support liquid water and life, while more moderate amounts would indicate the presence of surface water and more hospitable surface temperatures.

  “This won’t be an instantaneous thing, because planning and building multiple large space telescopes takes a lot of time,” Kasting explained to me. “A near-infrared telescope will probably go up first—something that can see water vapor, oxygen, and not much else. Maybe it will find that stuff in the atmosphere of a nearby planet in the habitable zone. From there you can go to thermal-infrared and look for methane, which you might not find unless it’s present in relatively high concentrations with the oxygen, as it might have been on Earth during much of the Proterozoic. Even if you do find it, there will still be ambiguities, and you probably won’t convince everyone at first. Another possibility would be finding planets like the Archean Earth: you wouldn’t see oxygen, but if you looked in the thermal-infrared, you’d probably see a lot of methane and maybe some organic hazes. The naysayers would kick and scream about that, because it seems to be much easier to abiotically build up significant concentrations of methane than oxygen—a dead planet with lots of volcanic activity and much more igneous, reduced ultramafic rock at its surface could do it. You’d have to start looking for other potential biosignatures that are much more difficult to detect, gases like nitrous oxide or dimethyl sulfide. For any interesting planets we’d find at first, there could be a whole series of follow-up missions done at greater and greater expense of time and money to nail down what exactly is being seen. It could go on for fifty years, a century, who knows.

  “So the real question is,” Kasting continued, “if the first mission does find oxygen in some planet’s atmosphere, can that alone be persuasive enough to drive investment in the rest of this process? It’s unquestionable that, on Earth, the rise of oxygen was the most fundamental change in our planet’s history, because it paved the way for the evolution of complex life, of us. But on other planets, we could be fooled if we aren’t careful.”

  Kasting had come up with two plausible ways a lifeless planet might masquerade as a living, oxygenated world for future space telescopes. The first probably played out in the early history of our own solar system, when Venus lost its water to a runaway greenhouse: as the hydrogen from water escaped into space, it should have left behind an ocean’s worth of free oxygen that would have gradually reacted with carbon to form CO2, leaving scorched Venus with an oxygen-rich atmosphere for perhaps hundreds of millions of years. This “false positive” didn’t worry Kasting very much—the truth could be revealed by such a planet’s location near the habitable zone’s inner edge, and by the lack of any water vapor accompanying the oxygen in its near-infrared atmospheric spectrum. Kasting’s second scenario was more troublesome, and involved a small, frozen planet at the outer edge of the habitable zone: if the planet was between perhaps two and three times the mass of Mars, it would likely be too small to long retain the internal heat that drives volcanoes and anything like plate tectonics, but large enough to prevent a thick atmosphere from being stripped away by stellar winds. Ultraviolet photolysis of even small amounts of water vapor would produce infinitesimal quantities of free oxygen in the upper atmosphere of such a “super-Mars,” Kasting said; but without any volcanic gases to react with, and with surface water locked in ice and unable to expose oxygen-absorbing minerals in rocks, that slow trickle of oxygen could build up over billions of years to fill the atmosphere and give the illusion of life. Interpreting possible biosignatures grew even more fraught and uncertain when considering planets around stars significantly different from our own Sun—some red dwarfs, for instance, though smaller and cooler than our star, emit significantly more ultraviolet radiation, enough to radically alter the atmospheric photochemistry of the planets in their habitable zones.

  As we drove out of State College along U.S. Route 322, bound for Black Moshannon, we passed across Bald Eagle Ridge, an Appalachian spur of quartzite, sandstone, and shale. The road forked, cut through the ridge, and split off into an extension of Interstate 99, a new stretch of highway that had been built in the first decade of the 2000s. As we continued on Route 322, running alongside I-99, I noticed that the long, sloping hillsides surrounding the roadcut looked strangely slick and denuded. I realized that I wasn’t looking at rock and soil at all, but at sheets of thick gray and black plastic held in place by wire mesh. I pointed and asked Kasting if he could tell me what I was witnessing.

  “There was bad acid runoff here a while back. When they made the roadcut, the Pennsylvania Department of Transportation dug right through the sandstone and used all the leftover pulverized rock as base and fill. The sandstone was laced with veins of pyrite, fool’s gold, that the surveyors somehow missed in their rush to get this thing built.” Kasting shook his head. “You could call pyrite a ‘reduced’ rock. It’s made of iron and sulfide, so when you expose it to oxygen it breaks down into iron oxides and sulfates. Mix that with rainwater, which naturally contains carbonic acid from dissolved atmospheric CO2, and the carbonic acid reacts with the sulfates to make concentrated sulfuric acid, which corrodes rock and leaches heavy metals from the ground. They started getting nasty runoff almost immediately from this whole stretch, and it was flowing into the groundwater and some good trout streams, so the highway got delayed by four years and many tens of millions of dollars. They had to go back and dig out a million cubic
meters of this stuff and plant it in a landfill; then they covered up the rest. They could’ve saved themselves a lot of trouble by paying just a bit more attention, don’t you think?”

  If there is a theme underlying Kasting’s work, it must be that having the patience to deeply ponder seemingly simple interactions of air, rock, water, and sunlight can yield surprising and sometimes profound insights. In fact, Kasting’s deep patience was exactly what led to the greatest breakthrough of his career—an insight that revolutionized all subsequent studies of planetary habitability. It occurred to him one day in late 1979, while defending his PhD thesis at the University of Michigan, a few years before his father would urge him to “get a real job.”

  Kasting’s breakthrough idea concerned how exactly the Earth had maintained a relatively narrow range of clement surface temperatures for billions of years despite the Sun’s slow, steady brightening over that time. When our planet was freshly formed, the Sun should have been some 30 percent less luminous than it is today—a diminution more than sufficient to completely freeze the Earth’s surface for the entire first half of its history. And yet scientists have found abundant evidence for liquid water on Earth throughout that time. Though researchers understood the stellar astrophysics underlying this “faint young Sun problem” in the 1950s, it was not until 1972 that planetary scientists became widely aware of it, via a paper by Carl Sagan and his colleague George Mullen. After that paper, prior estimates of habitable zones were left in disarray.

  Reconstructive efforts began in the late 1970s, when an astrophysicist named Michael Hart at NASA’s Goddard Space Flight Center simulated the faint young Sun’s effects on the evolution of Earth’s atmosphere and climate. Hart found his virtual Earth would only survive and evolve to resemble our current world if the inventory of greenhouse gases in its early atmosphere was significantly boosted. This came as no surprise—most researchers believed (and still do) that this was essentially how the early Earth avoided freezing. But Hart’s other findings were more disturbing: If he moved the Earth 5 percent closer to the dimmer Sun, the enhanced greenhouse effect rapidly boiled the planet’s oceans. Worse, if he moved the Earth only 1 percent farther away from the faint young Sun, once the planet acquired oxygen two billion years into its life, the subsequent decrease in greenhouse gases such as methane created bright glaciers that spread all the way down to the equator, reflecting ever more sunlight in an “ice albedo” feedback loop that ended with the entire ocean frozen solid. No matter how long Hart’s model ran, the frozen planet remained forever locked in ice. No evidence had yet been found for our planet’s own Snowball Earth episodes and its eventual recovery, so Hart believed the runaway glaciation to be an inescapably fatal problem. Hart’s solar habitable zone was minuscule, and only by the rarest happy chance could Earth have formed in its midst. He chillingly concluded that there were far fewer habitable planets in our galaxy than had previously been thought. By Hart’s estimation, Earth could well have been the only one.

  James Lovelock had a wildly different idea. He believed that Earth had indeed endured the faint young Sun through some potent atmospheric mixture of greenhouse gases, probably mostly CO2. But he posited that the reason our planet had avoided a runaway greenhouse early in its life was that photosynthetic organisms had pulled the excess CO2 out of the air and locked it away in buried organic carbon at precisely the right rate to stabilize Earth’s temperature. In his view, it was life itself that actively, unconsciously maintained the Earth’s habitability by closely coupling and coevolving with the world’s geophysical systems. The coupling was so close, he argued, that at the largest scales differences between living things and their inanimate environs became indistinct, and the world could rightly be viewed as a complex system analogous to a planetary-scale organism. He called this union of the biosphere and the rest of the Earth “Gaia” after the goddess of Mother Earth in Greek mythology. With a collaborator, the American biologist Lynn Margulis, Lovelock went on to author a large body of literature further developing the theory.

  Kasting’s contribution to this debate came from his study of carbon cycles for his PhD thesis, which was on the rise of oxygen on the prebiotic Earth. Specifically, Kasting was examining whether photolysis of CO2 could have pumped significant amounts of oxygen into the atmosphere long before the advent of cyanobacteria and oxygenic photosynthesis. To tackle the problem, he first needed to estimate how much CO2 had existed upon the primordial Earth, then feed that information into one of his custom-made numerical models. Much of the CO2 present in Earth’s atmosphere today is regulated by the biosphere, in an “organic” carbon cycle in which living things sequester carbon as they grow, only to release it back into the environment when they die and decay. But there is an older, inorganic carbon cycle as well, the carbonate-silicate cycle, one that operated before life had taken hold on Earth and still operates to this day on roughly million-year timescales. Kasting and I had glimpsed a small, isolated component of this inorganic cycle earlier, as we passed the roadcut with its acid runoff.

  The inorganic carbon cycle begins when volcanoes belch CO2 into the air, some of which then mixes with rainwater to fall as carbonic acid. On land, the carbonic acid weathers and erodes silicate rocks, releasing carbon-rich minerals that accumulate in groundwater, streams, and rivers. We had witnessed this first step as we crossed Bald Eagle Ridge; most of the cycle’s subsequent steps took place beyond the spatial and temporal boundaries of human life. The carbon washes into the ocean and eventually falls to the seafloor, forming layers of carbonate rock such as limestone. When the actions of plate tectonics push the carbonate-loaded seafloor down into the Earth’s mantle, the carbon cooks out of the rocks, generating CO2 that streams back into the atmosphere through erupting volcanoes, closing the cyclic loop. While working on his PhD thesis, Kasting gathered the best estimates he could find to constrain the early Earth’s abiotic carbonate-silicate cycle, and ran all the data through his model. At the end of the exercise, he concluded that photolyzed CO2 could have perhaps formed a tenuous layer of stratospheric ozone, but little else, and certainly not enough oxygen to enrich the atmosphere.

  Kasting’s thesis leaned heavily on the ideas of James Walker, an eminent atmospheric scientist at the University of Michigan who had taken Kasting under his wing. Consequently, Walker sat on the panel that reviewed Kasting’s thesis. Kasting successfully defended his thesis before the review panel, and afterward the newly minted Dr. Kasting sat down for lunch with his former interrogators. Kasting, Walker, and another panelist, the atmospheric scientist Paul Hays, began discussing Hart’s troubling results and potential solutions to runaway glaciation on the early Earth. Lovelock’s theory seemed plausible but remained frustratingly vague and tautological: the Gaia hypothesis suggested that for a planet to be habitable, it must first be inhabited. Perhaps, Walker offered, something independent of life had acted to circumvent or counteract the runaway glaciation—a dearth of clouds on the early Earth, for instance, could have allowed more ice-melting sunlight to reach the planet’s surface, or volcanic eruptions could have over time covered a glaciated planet in a dark blanket of ash that absorbed more sunlight and melted the ice. But these explanations felt unsatisfactory—whether or not they would occur seemed more a matter of chance than necessity.

  With all the details of the inorganic carbonate-silicate cycle fresh in his mind from his thesis defense, Kasting paused for a moment to think, then said he had a simpler idea. “If the Earth were completely covered in ice, its interior would remain hot and volcanic activity would continue pumping CO2 into the atmosphere,” he haltingly began. “But there would be less exposed silicate rock, and the low temperatures would have frozen the water vapor out of the air. . . . So where could the CO2 go? Why wouldn’t it just keep building up in the atmosphere until the greenhouse effect melted the ice? Shouldn’t weathering rates depend on temperature? Maybe that’s the way out.” Though they tried for the remainder of lunch, neither Walker nor Hays could find any o
bjections to Kasting’s observation. The next day Kasting left Michigan to begin a stint of postdoctoral research at the National Center for Atmospheric Research in Boulder, Colorado.

  After taking us across Bald Eagle Ridge, Route 322 had dumped us in the adjacent valley, where we took another highway northeast toward the town of Philipsburg. Five miles of tree-covered ridgelines, grassy fields, and railway tracks passed by out the window, until Kasting motioned for me to turn left off the highway. As we began a looping ascent up a rough-paved road into rolling, oak-forested hills, he chuckled next to me in the passenger seat and with whimsy in his voice said, “That was probably the single best idea I’ve ever had, but I didn’t even realize it at the time—I was still more interested in the rise of oxygen.”

  Ten months after his PhD defense, Kasting was working in Boulder when he received a large package in the mail. Inside was a thick manuscript titled “A Negative Feedback Mechanism for the Long-Term Stabilization of Earth’s Temperature.” Kasting was listed as the paper’s third author, after Walker and Hays.

  “Walker had gone off and worked out this whole thing,” Kasting recalled. “Hays, I think, had helped him with some of the math. [Walker] had gathered all the information available about silicate weathering rates, looking mostly at lab data, and he pretty convincingly demonstrated that, yes, they do depend on temperature and rainfall. From all that data he derived an expression for the weathering rate as a function of CO2 partial pressure and the temperature of the planet.”