by Lee Billings
Kasting began working hard in math and science at school, and read all the science fiction he could get his hands on. One of his favorites was Isaac Asimov’s Foundation series, books about the rise and fall of a galactic empire. Much of the story revolved around the empire’s capitol planet of Trantor, a stand-in for what, at the time, passed as a plausible guess at Earth’s not-too-distant future: a planet where land and sea, where nature itself, had been wholly smothered and subdued beneath the footprints of forty billion people and a glittering techno-utopia of skyscrapers, superhighways, and domed farms and habitats. “I liked books with big ideas, ones that dealt with the future of humanity, or how to run a society,” Kasting told me. “Foundation had a cool one: ‘psychohistory,’ the idea that if you have enough people they will behave just like atoms or molecules, individually unpredictable but foreseeable in aggregate, so a civilization’s behavior becomes like that of an ideal gas, something controlled through statistical mechanics. I don’t know if that’s true—people are pretty complicated—but it made me think more about what can be predicted.”
Late one evening when the boys were in middle school, Kasting’s father arrived home from work with a tripod-mounted 2.5-inch refractor telescope, suitable for viewing all that the new rockets were bringing into reach. On dark, clear nights, they could see Saturn’s rings, the ruddy disk of Mars, and the plains and craters of the Moon where men soon would walk. Through the viewfinder, the jagged, magnified lunar surface looked close enough to touch, like some monochrome impasto landscape hung on a museum wall. Jim’s interests exceeded the limits of the solar system a few years later when he upgraded to a more powerful 4.25-inch reflector, and he began searching the sky for nearby planetary nebulae and neighboring galaxies. Sometimes he would wonder how the Earth or another inhabited world would appear, viewed from so very far away, if only there was a telescope big enough to look.
After high school, Jim plotted a trajectory he hoped would intersect with NASA’s orbit: undergrad at Harvard, then a PhD in atmospheric science at the University of Michigan, and finally a series of postdoctoral positions. In 1981, he achieved his dream, securing a research fellowship at NASA’s Ames Research Center in Mountain View, California.
Not long after Jim’s NASA debut, his father paid him a visit out in California. By then, Jim had met and married his wife, Sharon, and their first child, a son, Jeff, had just been born. Kasting’s father listened attentively, smiling and nodding as Jim showed off his burgeoning efforts to model the early atmospheric evolution of Venus, Earth, and Mars—working on the problems full-time with NASA’s winds at his back, he was making progress fast, and getting further than anyone had before. Perhaps skeptical that Jim could raise a family by predicting a planet’s far-distant past and future, or maybe just habituated to always push for greatness, when Jim had finished explaining Kasting the elder promptly asked his son when he planned to get a real job. In fact, Kasting’s work had already begun to revolutionize planetary science and had placed him on the NASA fast track. In 1983, when his fellowship expired, he was immediately hired as a research scientist at Ames, where he would remain until his 1988 emigration to Penn State. With NASA money as their nest egg, Jim and Sharon would have two more sons, Patrick and Mark.
Kasting’s Penn State office was adorned only with a blue-and-white Oriental rug and a few yellowing astronomy-themed posters that broke the spartan regiments of books, papers, and reports. One side of the room was occupied by three large filing cabinets, collectively filled with a good half ton’s worth of astrobiology’s primary literature. The other side was taken up by bookshelves mounted on the cinderblock walls. The shelves brimmed with well-thumbed, dog-eared volumes with such titles as Biogeochemistry of Global Change, The Chemical Evolution of the Atmosphere and Oceans, and Fundamentals of Atmospheric Radiation. An adjacent whiteboard was filled top to bottom with scribbled shorthand references to stellar flux, atmospheric partial pressures, and surface temperature, as well as three frenzied, overlapping strata of differential equations, each distinguished by its own shade of erasable marker.
The books and equations revealed Kasting’s true interests, which go far beyond our own small planet and its history, harking back to his musings at a backyard telescope. He is widely considered the world’s foremost authority on planetary habitability—how a life-friendly planet can emerge and evolve over geologic time. Like the Earth itself, he has spent most of his time within the Precambrian’s murky frontiers. Among other things, he has had a hand in calculating how much longer photosynthesis can support complex life on Earth (about a billion years), the minimum size an impacting asteroid must be to vaporize Earth’s oceans (one 270 miles wide would do the trick), and whether by burning all available fossil fuels humans could force the Earth into a Venus-style runaway greenhouse (the jury is technically still out, but Kasting believes the answer is, thankfully, “no”).
Over dinner the night before, I had suggested that we hike through some of the surrounding Pennsylvania wilderness, so that Kasting could use examples from the landscape to illustrate his Big Picture view of Earth as a system, of habitability as a process unfolding over geologic time. “If you take me out in the field, I’m pretty useless,” he initially demurred. “I’ve actually had no formal education in geology. I probably couldn’t tell you if a rock was a carbonate or a silicate. I’d be lucky to know a glacial till from a landfill.” After finishing a margarita, he had changed his mind, and offered to take me to Black Moshannon State Park, five square miles of forest and wetland located a twenty-minute drive northwest from Penn State’s campus. “I still won’t be very useful,” Kasting said, “but it will be a nice walk.”
• • •
Behind each modern announcement that scientists have found yet another possibly habitable world is a well-worn process that, simplified, unfolds as follows: Astronomers first measure the newly discovered planet’s mass, and, if possible, its radius, generating an estimate of the planet’s density and its likelihood of being rocky like the Earth. They also determine the rocky planet’s orbital distance from its star, as well as the intensity and color of the star’s light. Armed with this scant data, the entirety of which you could jot in ballpoint pen on the palm of one hand, they then interpret it through numerical modeling. In particular, they consult one of Kasting’s most-cited papers, “Habitable Zones around Main Sequence Stars,” published in the journal Icarus in 1993. In that paper, Kasting and two colleagues, Dan Whitmire and Ray Reynolds, used a climate model developed by Kasting to determine which orbits around stars are most likely to allow rocky planets to harbor liquid water upon their surfaces. Inward of the habitable zone, a planet’s surface would be so scorched that any water would flash to steam, suffusing the atmosphere and gradually escaping into space, similar to what occurred on Venus; outward of the zone, a planet’s surface water would freeze, similar to what we see on Mars. If a newfound rocky planet proves to be within Kasting’s habitable zone, shortly thereafter its discoverers contact their funding institution’s press office, and soon their names appear on the nightly news and in the New York Times. Kasting coauthored a paper in January 2013 gently revising his twenty-year-old calculations, but the tweaks did not greatly alter his earlier work’s core conclusions.
Using a literal handful of data points to estimate the habitability of a faraway planet is a practice fraught with uncertainty, where major assumptions and leaps of faith become inevitably routine. That it is possible at all is only because, as far as we can tell, the laws of nature are everywhere the same throughout our observable universe, whether in the solar system or around some far-distant alien star. Anywhere in the universe where starlight falls upon a planet, it pumps radiant energy into the system of that world. How much energy filters in depends on the planet’s atmosphere, and upon the starlight’s wavelength, or color. For those canonical 1993 calculations, Kasting and his colleagues gave their virtual planets atmospheric compositions thought to be the most typical outcome of ter
restrial planet formation: lots of inert nitrogen, accompanied by substantial fractions of CO2 and water vapor. Evidence suggests this was the bulk atmosphere of the early Hadean Earth, but for distant rocky exoplanets with atmospheres that have yet to be measured, any particular mix can presently be seen as only a hopeful guess.
After a particular atmospheric cocktail is chosen, the core of Kasting’s numerical approach kicks in, most of which he developed during his seven years at NASA. During all that time, he devoted himself to perfecting his models, hand-coding each important way that starlight interacts with an atmosphere. In the real world, and in Kasting’s models, a photon of a certain wavelength might simply bounce off the top of the atmosphere, while a photon of another wavelength might instead pass without incident all the way down to the planetary surface. Inside the atmosphere, real or virtual, a photon might be reflected by a cloud, or by bright ice on the ground. It might be absorbed by a greenhouse gas, or by the dark water of a sea. When a photon is particularly energetic—ultraviolet or higher on the electromagnetic spectrum—it might even create entirely new substances in the air and on the ground by knocking into molecules and splitting them apart—a process called “photolysis.” The photolytic products could then have their own secondary effects on the absorption and reflection of starlight, all of which must be taken into account. Over the years, Kasting accumulated all the necessary data he could find, building up a vast library of radiation-absorption tables, photochemical reaction rates, atmospheric lifetimes of different gases, and the global pace at which certain gases are emitted from volcanoes or absorbed by rocks. Collectively, all these various interactions and inputs have an enormous effect upon a planet’s atmospheric composition and average surface temperature—its climate.
If you naively calculated the average temperature of the modern Earth’s surface based only on the amount of sunlight it receives and its average reflectivity, or albedo, you’d obtain a value of -18 degrees Celsius, well below the freezing point of water. If you calculated it using one of Kasting’s climate models, you’d get a result of 15 degrees Celsius, which is, of course, what the Earth’s average surface temperature actually is. The discrepancy is mostly due to warming from several different greenhouse gases, each of which Kasting must account for in painstaking detail.
Water vapor, for instance, must be treated very carefully, as it is actually a much more potent greenhouse gas than CO2, efficiently absorbing a much broader swath of the thermal-infrared portion of the spectrum. Further, its effect on climate is qualitatively different: unlike CO2, which stays gaseous at typical Earth temperatures, water vapor is intimately affected by Earth’s temperature changes. Low temperatures will cause it to condense into clouds and fall out of the sky as rain, snow, and hail, which removes its greenhouse effect and drives temperatures even lower. Conversely, high temperatures increase the evaporation rate of surface water, pumping more water vapor into the air to raise temperatures even further. Water vapor thus acts in a positive feedback loop to amplify other climate changes, such as the steady heating forced by rising levels of atmospheric CO2. If CO2 is the fulcrum about which Earth’s climate change pivots, water vapor is the lever.
The key output of one of Kasting’s climate models is something called a temperature-pressure profile—scientific jargon for how starlight shining on any given atmosphere will influence not only its warmth, but also its vertical structure. Earth’s atmosphere, for instance, reflects a quarter of the incoming sunlight and absorbs another quarter through greenhouse gases, allowing approximately half of the sunlight that strikes it to filter down to the surface. This means that, on average, Earth’s atmosphere is colder than its surface, and is warmed from the bottom up by convection, like a pot of water being heated on a stovetop. Most of the surface heating and convection occurs around the equator, where, as a cursory examination of any globe will show, there is more surface area to absorb the sunlight that beats down from almost directly overhead. Convective cells of moist air undulate from the warm surface, cooling as they rise and expand, eventually growing cold enough to dump their moisture as condensed water vapor—that is, as clouds and rain. Atmospheric convection helps to explain why the tropics are hotter than the poles, why air around high mountaintops, though fractionally closer to the Sun’s radiance, tends to be thinner, colder, and drier than the air at sea-level plains, and why thunderstorms typically occur on torrid afternoons and early evenings, hours after the Sun’s zenith.
Earth’s temperature-pressure profile creates a feature in the atmosphere called the tropopause, a dividing line which runs above the warm, weather-filled troposphere and below the colder, thinner stratosphere. Since water vapor condenses when exposed to cold temperatures, it is effectively trapped beneath the tropopause by the colder overlying atmospheric layers. Just how important this “cold trap” effect is for Earth’s prolonged possession of water became apparent in the 1980s through a series of studies by Kasting, his colleague James Pollack, and a handful of their peers at NASA Ames. They were interested in understanding why Venus, our planet’s near twin, had developed such a dramatically different climate than Earth, despite evidence that in its very early history our sister planet had been hospitably tepid and wet, rather like our own world now.
“To someone like me, the single most interesting thing about Venus is what it says about the inner boundary of the habitable zone,” Kasting explained as we chatted in his office. “It sets a reasonable empirical limit on what you can expect for other planets outside the solar system—you don’t need to do much modeling to guess that something getting Venus’s amount of starlight probably won’t be habitable. So if you want to know what happens when an otherwise Earth-like planet forms too close to its star, or what can happen to a habitable planet as its star gets brighter over time, Venus can tell you a lot.”
Building on previous work performed by several other planetary scientists, most notably Caltech’s Andrew Ingersoll, Kasting modeled how the Earth’s atmospheric structure—Earth’s temperature-pressure profile—would react to increased intensity of sunlight, as would occur if the Earth’s orbit were moved in toward the Sun to a more Venusian orbit, or when the Sun slowly increases its luminosity over geological time. He found that with a relatively modest 10 percent increase in the starlight’s intensity, equivalent to moving the orbit of our planet to 0.95 AU, 5 percent closer to the Sun, the additional warming would saturate the troposphere with water vapor, pushing the tropopause up to altitudes of 90 miles or more.
As Kasting watched the tropopause soar in his numerical model, he knew he was witnessing what would lead to the end of that virtual world, and someday, our own: much of the water vapor that lofted to such heights would rise above the protective ozone layer, where it would be photolyzed by ultraviolet light from the Sun. A small percentage of the liberated atomic hydrogen would escape entirely into outer space, taking with it any potential to ever bond again with Earthbound oxygen to create water. Within a few hundred million years, enough hydrogen would be lost to space in this manner that Earth’s oceans would essentially boil away, leaving the planet lifeless and dry as a bone, with not a drop of water left upon its surface or in the air. In a billion years, long before it swells into a red giant and threatens to physically engulf our world, the Sun will have brightened by that crucial 10 percent, and the Earth will begin to rapidly lose its water and its life. This “moist stratosphere” mechanism is now thought to be how Venus began losing its oceans early in our solar system’s history, and its threshold of 0.95 AU for our own planet conservatively approximates the inner edge of Kasting’s habitable zone from his canonical 1993 paper.
As Venus lost its oceans, the rising temperatures baked CO2 out of the planet’s crust, and the gas began to fill the atmosphere. As a result, Venus’s atmosphere is now some 90 times denser than Earth’s and almost pure CO2, creating a greenhouse effect so potent that the planet’s surface temperature is hot enough to melt lead. In a second series of studies, Kasting and h
is coworkers modulated the CO2 content of Earth’s atmosphere to examine whether increased CO2, rather than increased sunlight, could on a much faster timescale independently lead to the loss of oceans through a moistened stratosphere.
To his surprise, Kasting found that even as rising CO2 levels sent temperatures skyrocketing, the vast amounts of water vapor released acted like the lid on a pressure cooker, pressurizing the lower atmosphere to such an extent that the oceans never boiled, keeping Earth’s stratosphere relatively dry. For the stratosphere to become saturated with moisture, for the oceans to vaporize and escape into space, the numerical models indicated that Earth’s atmospheric CO2 would have to reach more than twenty-five times its present concentration—more than could be released by burning the entirety of our planet’s known “conventional” fossil fuel reserves of oil and coal, but just maybe within reach if all the planet’s unconventional sources, like the Marcellus’s shale gas, were burned as well. While humanity could readily give the planet a fever that could shrivel societies and severely diminish existing biodiversity, Kasting’s calculations suggested it would be very much harder—though not definitively impossible—for humans to create a moist stratosphere. By his reckoning, forcing the planet to give up its ocean to space by burning fossil fuels appears to be just beyond the reach of present-day civilization.
