by Adam Frank
Sagan attended the University of Chicago, where he was trained to think as both a scientist and a humanist. It was a combination that would later prove so compelling to millions via his popular writings and television programs. After Chicago, he moved ninety miles northwest to the Yerkes Observatory in Wisconsin, where he started work on his PhD.18
Graduate work in astrophysical sciences takes years of dedicated effort. First, there are advanced classes on the basics of theory and observation. Only after this initial phase can students start independent study. Sagan arrived at Yerkes with an interest in life beyond Earth, so for his graduate thesis he chose three separate issues at the intersection of planetary science and what we now call astrobiology. The first of these would be the Venus problem.
Sagan’s question was straightforward: What process could turn the surface of Venus into a scalding hell? Combing through decades of scientific literature in search of an answer, he found one in what is now well known as the greenhouse effect.
A planet like the Earth would be a deep-freeze world without its atmosphere. That conclusion requires only a few lines of basic physics to demonstrate. Sunlight hitting a planet warms its surface. The warmed ground emits what is called heat radiation, which is just electromagnetic waves generated by the jiggling motions of heated atoms. Any object at any temperature above absolute zero spews heat radiation into its surroundings. That includes your own body as you read these words.
For our planet’s temperature to remain steady and unchanging, the energy flowing onto it must balance the energy flowing out. Heat is just another form of energy. That means incoming solar energy and outgoing heat radiation energy must balance if the Earth’s temperature is to stay constant. Scientists call this balance the planet’s equilibrium temperature.
Calculating a planet’s equilibrium temperature requires the kind of basic physics most students learn in their first-year astronomy classes. Once they work through the math, those students all come up with the same startling result: Earth without an atmosphere would have an equilibrium temperature around zero degrees Fahrenheit. That’s well below the freezing point of water.19
As we all know from daily experience, most of Earth’s surface is not frozen. In fact, the planet’s current average temperature is a balmy 61 degrees Fahrenheit.20 Somehow, our planet manages to stay warm enough for most of its water to be in liquid form, rather than as a solid (ice) or a gas (water vapor). It’s the atmosphere that raises the temperature. The blanket of gases surrounding the planet keeps Earth’s equilibrium temperature well above freezing. But how, exactly, does that happen?
The fact that you can see the Sun on a cloudless day gives testimony to the fact that Earth’s atmospheric gases are mostly transparent to our star’s incoming visible radiation. The Sun’s visible-range electromagnetic waves pass right through our atmosphere as unmolested as through a clean glass window. But the heat radiation emitted by the warmed planet’s surface isn’t in the visible part of the spectrum. Instead, the planet radiates at longer infrared wavelengths the eye can’t see. So, while incoming sunlight passes freely through the atmosphere, for the longer infrared wavelengths emitted by Earth’s warmed surface, it’s a different story entirely.21
Like a blanket you throw over yourself on a cold winter night, the blanket of gases surrounding our planet holds in energy that would otherwise get radiated away. It’s this trapped energy that raises Earth’s temperature above freezing. An actual greenhouse works along a similar principle, as the windows allow sunlight in but keep warmed air from rising away—hence the name, the greenhouse effect.
The greenhouse effect was old news for scientists studying the Earth. In 1896, Swedish Nobel Prize–winning chemist Svante Arrhenius had discovered the human impact on Earth’s greenhouse effect.22 Using a simple mathematical model, Arrhenius laid out the physics of Earth’s greenhouse warming, demonstrating how our planet was warmed by its atmosphere. Just as important, his calculation also revealed how our own activity was adding to that warming. Using records of coal consumption, Arrhenius saw we were already putting enough CO2 into the atmosphere to change the energy balance. Using the coal data, he predicted that human beings would eventually raise the planet’s temperature as we continued dumping CO2 into the air. His pencil-and-paper calculation predicted a global increase of about five degrees.23 This is remarkably close to modern estimates. In our current era of climate-change denial, it’s startling to recognize how far back the understanding of human-driven climate change begins.
Sagan wanted to go farther than Arrhenius—literally. He saw that what was true for the Earth must also be true for distant planets. The greenhouse effect had to be universal. So Sagan set himself the task of calculating the extent of the greenhouse effect on Venus to see if it could explain that planet’s extreme temperatures. Across many cold Wisconsin winter days, Sagan pored over old papers in the Yerkes library, teaching himself the basic physics of infrared atmospheric absorption and its subsequent planetary warming. After months of exhausting work, he had his answer. With its CO2-rich atmosphere, Venus was trapping enough energy to raise the surface temperature near to the staggering 600-degree level implied by NRL data.24 The planet was a cauldron because of the greenhouse effect.
Today, scientists recognize that planets anywhere in the universe must be subject to the same set of forces and processes. While each world has its own unique story, those stories are all enacted by the same list of players: the flow of winds, the pull of gravity, the dance of chemistry. Earth is no different, and this, as we will see, is the principal lesson of the Anthropocene. But when Carl Sagan was working alone in the Yerkes library, the application of this universal vision of the universe’s planets was still young. Other than a few nearly forgotten studies, Sagan was alone in bringing the earthbound process of greenhouse warming to another world. “Almost nobody on the planet as far as I could find, was interested in the Venus greenhouse effect . . . ,” he would later recall. “I sort of stumbled on it myself.”25
A LIVING HELL
Rocket engineer and project manager Jack James only had a day to mourn the loss of Mariner 1. The launch window in which Earth and Venus were positioned just right for the calculated flight path would close in a month. James’s team needed to get Mariner 2 ready for launch immediately. Twenty-eight days later, at 2:53 a.m. on August 27, 1962, another Atlas-Agena rocket lifted from the ground atop another pillar of fire.
This time, the launch was successful, but just barely. A few seconds before the Atlas booster was to separate from Mariner, one of the rocket’s control engines shut down, driving it into an uncontrolled spin. As fears rose for another failure, the first of the mission’s “seven miracles” occurred. Control was regained at just the right moment to undo any damage the spin had imparted to the probe’s calculated flight path. The rocket’s second stage fired and Mariner 2 was on its way to Venus.
It would take three months for the probe to cross more than twenty-five million miles of interplanetary space. Six more times, critical elements in Mariner’s systems would fail: a solar panel stopped working; temperatures on the space probe climbed to dangerous levels; the onboard computer failed to switch instruments to “encounter mode” as Venus approached. But each time, disaster was averted as the problem either fixed itself or James’s team from NASA’s Jet Propulsion Laboratory (JPL) found a workaround.26
“I’d get called at all times of the night,” James recalled later. “My nerves had become so taut by this time that I instructed everyone that called me to start out with one of two sentences: ‘There is no problem,’ or, ‘There is a problem.’ ” More than a few calls began with “There’s a serious problem.”27
In spite of all the difficulties, on December 14, 1962, Mariner 2 flew within twenty-two thousand miles of Venus, a distance about six times the diameter the planet. As data from Mariner 2 trickled into JPL, it became clear that the NRL study and Carl Sagan’s greenhouse effect theory had been right. The scalding temperatures
were not high in the atmosphere, but down on the planet’s surface. Venus was indeed a living hell.28
The evidence for Sagan’s greenhouse model for Venus got stronger as the space age matured. Over the next forty years, more than twenty other probes would visit our sister planet. Some mapped its surface at high resolution via cloud-penetrating radar. Others made detailed explorations of atmospheric conditions, including winds whipping around the planet at hundreds of miles per hour. The Russians even managed to get probes down to the surface. The probes worked for just a few hours before succumbing to the planet’s intense heat and nuclear submarine–crushing pressures.29
What emerged from these studies was a picture of a world where the CO2 greenhouse effect had run amok. The catastrophe was called a runaway greenhouse effect, and its discovery proved to be essential for understanding the climate cycles that run our own world.
The principal way that CO2 gets added naturally to a planet’s atmosphere is through volcanic eruptions. Molten rock explodes through the surface, venting huge amounts of CO2. Radar imaging of Venus shows ample evidence for volcanism in the recent past (meaning the last hundreds of millions of years). But what volcanoes give, water can take away. “Weathering” by water, in the form of rain and rivers, breaks rocks down to their chemical components. Later, these molecular components can bind with CO2 and get packed back into solid forms—that is, as new rocks. This is the basic process that creates what are called “carbonate” minerals like the limestone under Miami.
So, CO2 belched into a planet’s atmosphere via volcanoes can go back into the ground in rocks. Eventually, the rocks are subducted (dragged down) into lower regions of the Earth, where they melt, allowing the CO2 to find its way back into the atmosphere through future volcanoes. It’s a cycle that regulates the carbon dioxide in the atmosphere, and therefore the planet’s greenhouse effect. It’s also a cycle that appears to have been broken on Venus.30
At some point, Venus likely had more water. It may even have had oceans and been hospitable to life. But when some of that water evaporated, it made its way high into the atmosphere, where a deadly process began. Close to the edge of space, ultraviolet radiation from the Sun (the same kind that causes skin cancer) zapped the water molecules and broke them apart into hydrogen and oxygen. Hydrogen, being the lightest of all elements, easily escaped into interplanetary space as soon as the water molecules were broken apart. With the hydrogen gone, there was no chance for the broken water molecules to reform. Over time, and high in its atmosphere, Venus was bleeding its precious water into space.31
The planet’s water loss resulted in what scientists call a positive feedback loop on climate. More water loss meant less rock erosion and less CO2 bound up in rocks. More CO2 in the atmosphere meant a more pronounced greenhouse effect and higher temperatures. But higher temperatures meant more water loss, which . . . well, you get the picture.
On Earth, there is no danger of losing our water in the way that Venus did. Our planet’s atmosphere has a particularly cold layer, about twelve miles above the ground, that causes water to condense out and fall as rain or snow, keeping it from ever making it to very high altitudes. This “cold trap,” as scientists call it, may have existed at one time on Venus. But at some point, its atmospheric layers changed, allowing water molecules to begin diffusing up to the heights where they could be split apart and lost forever.32
With its water safely trapped closer to the surface, Earth’s carbon cycle acts as a negative feedback on climate. Negative feedback cycles keep small changes in temperature from growing out of control. Imagine if Earth’s temperature were to jump by a few degrees. The negative feedback begins when this higher temperature leads to more evaporation. Then more evaporation leads to more rain; more rain leads to more weathering; and more weathering leads to more atmospheric CO2 being drawn into rocks. Now there’s less CO2 in the air, meaning the greenhouse effect is reduced and the planet’s temperature comes back down.
By giving us an explicit example of the greenhouse effect gone wrong, Venus helped teach us about the effects of negative and positive feedback loops on planetary climate. It made us think more deeply about the cycles of matter and energy that give a planet its character—or cause that character to change. From Mariner 2 onward, the probes we sent to Venus let us see exactly how a planet that might have been a kindred twin had, instead, become a monster. Using the early understanding developed purely from studying Earth, the Venus missions allowed us to flex the muscles of a young climate science and broaden the reach of its knowledge. Like a doctor studying pathological cases of a disease to understand the basic workings of a healthy physiology, Venus’s runaway greenhouse became a laboratory for understanding the complex interplay of atmospheres and geology that shape a world like ours.
By taking our first steps toward the planets, we were also taking the first steps toward understanding the laws of planets. We were beginning the process of using the worlds of our solar system to unpack the general and generic laws all planets must obey. Our early missions to the planets, led by pioneers like Jack James and early theoretical studies by Carl Sagan, were also our first steps in growing up as a planetary species. We were seeing, for the first time, the depth of our commonality with the rest of creation.
It’s worth noting that, while Carl Sagan got the credit he deserved for predicting Venus’s hyperactive greenhouse effect, his name was not on the paper reporting Mariner 2’s results.33 Early in the project, Sagan had been put on Mariner’s design team, where he had, among other things, argued for a camera to be included on board (his proposal was rejected). But as Jack James’s group pushed hard to make its deadlines, some felt Sagan was not pulling his weight. Their misgivings proved to be correct. A crisis had erupted in Sagan’s personal life that kept him from making the expected contributions to the mission.
In 1957, when he was still working on his PhD, Carl Sagan married Lynn Margulis, a brilliant but as-yet-undirected student (at the time, her last name was Alexander). When they met, Margulis had not yet settled on science as a career. Sagan helped introduce her to questions concerning life and planets. A fire was lit in the young woman’s imagination, and even as their children were born, she took on the task of graduate work in biology. But Sagan’s relentless work schedule left the full burden of raising their children and managing the household to Margulis. After five years of trying to hold the demands of family and graduate work together, Margulis had had enough. She packed up the children and left Sagan to his overcommitted work schedule.34 But in one of the great turns of scientific history, Lynn Margulis would return to play an equally important role in understanding the coupled histories of life and planets. Before that story could play out, however, Sagan and the rest of the world would have to come to terms with Mars.
BEDROCK MARS
Steven Squyres, the chief scientist for the multibillion-dollar Mars Exploration Rover program, was not nervous. Sure, the plan was insane, but that didn’t mean he had to be nervous. It was January 25, 2004, landing night for the robot rover Opportunity. Squyres was waiting in the flight control room at NASA’s Jet Propulsion Laboratory while, more than three hundred million miles away, the Opportunity rover was bundled in its descent capsule, hurtling at twelve thousand miles per hour toward Mars. Since blasting off from Earth six months earlier, Opportunity had been on a direct path toward the Red Planet. But it wasn’t going to slow down and ease into orbit, as in some previous missions. Instead, the $400 million probe was on a straight shot toward its landing zone in the Meridiani Planum, a broad plain just south of Mars’s equator.
The entry, descent, and landing (EDL) phase called for Opportunity to dive straight in from space, shedding speed via friction with Mars’s thin atmosphere. A supersonic parachute would then blow open, slowing the capsule further. After that, if all went according to plan, the lander would spool down, away from the rest of the spacecraft, via a sixty-five-foot-long tether. As the descent continued, a cocoon of giant airbags wou
ld explosively inflate around the lander. Approximately one hundred feet from the ground, retro-rockets would fire, bringing the whole spacecraft to a halt. The lander, surrounded by its airbags, would hang forty feet from the ground. Then the tether would be cut away, dropping the airbag-enshrouded lander to the surface, where it would bounce like a beach ball on steroids. Eventually, after a mile or so of bouncing, the lander was supposed to come to a safe resting place on the Martian surface.35
Yeah, the idea was insane.
But it was an insane idea that had already worked. Just three weeks earlier, Opportunity’s twin, the Spirit rover, had bounced to safety on the other side of the planet. That six-wheeled mobile geology laboratory was already wandering the Martian surface, taking data. So Squyres was not nervous. Well, not too nervous.
There was a long wait as the JPL flight team searched for signals that Opportunity had survived its ordeal. Then the EDL manager yelled out to the room, “We’re down, baby!” The room exploded in cheers. Opportunity was safe on the surface.
Within the hour, Squyres switched to the rover operations room as Opportunity’s cameras came on and his team tried to see exactly where their creation had come to rest. “The picture comes [up on the screen] and it’s dark,” Squyres recalled later. “There’s something there but it’s underexposed.” Slowly, the image gets calibrated, or “stretched.” “The stretch hits and instantly I realize what I’m seeing,” Squyres writes. “It’s impossible, it’s too good to be true, it’s too good to believe.”36
Right in front of the rover was an exposed layer of bedrock— the kind of thing you see on Earth when you’re driving on a road cut through hills. And, just as on Earth, the layer of exposed rock Squyres was staring at represented a record. It was a sandwich of compressed Martian history going back millions or billions of years. They were staring at Mars’s planetary evolution written in rock: the scientific equivalent of pure gold.
