Rocket engineer Jack James (right), with Mariner Project manager Dan Schneiderman (left).
The Florida sun glistened over the blue Atlantic waters, but Jack James was in a black mood. It was July 22, 1962, and it had been a very bad day. The Texas-born engineer was project manager for NASA’s Mariner program, which aimed to send America’s first emissary to another planet. James had gotten the job a little more than two years earlier. Like everything else in the space race of the early 1960s, James’s program had been rushed forward at breakneck speed, working nonstop. Now the fruit of all that effort lay in ruins at the bottom of the ocean.
James and his team had been given less than fourteen months to design, build, and launch a probe to Venus.1 Until then, the Moon had been the primary target of the space race, and America’s record for hurling oddly shaped boxes of electronics at Earth’s rocky satellite had been a mixed bag. The Russians were having better luck. They’d gotten three of their nine probes to the Moon. The US had only gotten one there.2 Now NASA was desperate for more than a win. It needed to upstage the Soviets in a big way. That’s why James was given the audacious job of thinking beyond the Moon and going interplanetary.
Mariner 1 was designed to perform a “fly-by” past Venus, a world that orbited 30 percent closer to the Sun, but with almost the same mass and size as Earth.3 The probe’s original design called for 1,250 pounds of scientific instruments, communications gear, solar panels, rocket motors, and fuel. But the new, more powerful generation of rocket boosters Mariner was supposed to ride into space kept blowing up, and NASA brass soon demanded a redesign. James’s engineers were forced to quickly shed more than two-thirds of Mariner’s weight.
James navigated his team through every design change and every challenge. That was how they came to this day. With the whole world watching, Mariner 1’s booster rocket lit up the Florida sky as it blasted off that July morning. For the first few moments, the launch looked clean. But then the Atlas booster began to fishtail. Every launch has a “range safety officer” whose job is to blow the rocket to bits if it looks as though the mission is failing and it’s going to crash back to Earth. Four minutes and fifty-three seconds into the flight—and just six seconds before Mariner 1 would have safely separated from the main launch vehicle—the safety officer hit the big red button.4
Boom!
For a full minute after the rocket exploded, telemetry continued to get signals from the probe as it tumbled from the sky to its ocean grave.5 At least Mariner 1 had been a tough bird.
They’d been so damn close. Just six seconds more and they’d have been on their way to Venus.
“Born to Lose,” by Ray Charles, played on the radio as James drove back to his rented apartment in Cocoa Beach. He was in despair. Years later, he’d recall the mantra of all space engineers: “To be a hero there are ten thousand parts that need to work properly on a spacecraft. To become a bum you just need one of them to fail.”6
But while James and his team were down, they weren’t out. The now-destroyed probe had a twin. Mariner 2 was waiting back at Cape Canaveral.7 There was still time to be a hero.
THE VENUS PROBLEM
The logic of the space race dictated that either Mars or Venus would be the next destination after getting probes to the Moon. Both were neighbor planets that could be reached in a matter of months, a step up from the three-day trip to the Moon. And each had its own long history of dreamers imagining a temperate world fit for extraterrestrial life.
Given its proximity to the Sun, Venus gets twice as much solar energy as Earth.8 That’s why many early astronomers imagined Venus as a jungle planet. In 1870, Claude Flammarion (the author of The Plurality of Worlds) thrilled his readers with images of a Venusian landscape made of broad, swampy plains ringed by mountains higher than the Himalayas.9
Flammarion assured his readers that Venus was a world rich with life: “Of what nature are the inhabitants of Venus . . . ? All we can say is that the organized life [there] must be little different from terrestrial life, and that this world is one of those that resembles our own most.”10
An imagined view of Venus from Flammarion’s 1884 book, Les Terres Ciel.
But with the increasing power of astrophysical observations, this pleasant dream of a jungle Venus would come under fire. First, astronomical observations in the late eighteenth century revealed Venus to be perpetually shrouded by clouds.11 Then, in the mid-twentieth century, the Venusian atmosphere was revealed to be heavy with carbon dioxide (CO2). The Earth’s atmosphere is 78 percent nitrogen, 21 percent oxygen, and one percent everything else. CO2 comes in at a mere 0.039 percent of the air you are breathing right now. That’s a pretty small fraction for a molecule that, as we will see, has a big role to play in our story. But for Venus, CO2 is pretty much all there is to the atmosphere, accounting for more than 95 percent of all its gases.12
The presence of so much CO2 was bound to make Venus a very different place from Earth, and by 1956, astronomers had gained their first evidence of just how different it might be. Using the same kind of radio astronomy technologies Frank Drake would soon employ in Green Bank, scientists from the Naval Research Laboratory found evidence that Venus’s surface temperature was well above 600 degrees Fahrenheit.13 That’s hundreds of degrees above the boiling point of water. If the NRL result was true, then Flammarion couldn’t have been more deluded. His Venusian swamps would have boiled away long ago. More importantly, 600 degrees was far too hot for any form of life to survive. It seemed the place Venus resembled most wasn’t Earth, but Hell.
While geology and its study of the Earth had been around a long time, planetary science—which takes all planets as its subject—was a young field. The NRL results set off a firestorm among the small group of researchers who considered themselves planetary scientists. Part of the conflict came because, just a few years earlier, another team had predicted Venus to be covered by a vast, planet-girdling ocean.14 But neither oceans nor lakes nor even cups of tea could be squared with the new NRL data, which suggested temperatures on Venus comparable to the inside of a pizza oven.
In response, some scientists claimed the NRL’s data had been misinterpreted. Its source, they claimed, wasn’t Venus’s surface but violent, atomic-scale processes occurring at the boundary of its atmosphere and the harsh conditions of interplanetary space.15
Resolving the dilemma required more power than earthbound instruments could provide. The best telescopes of the day could not see the disk of Venus in enough detail to distinguish between a hot surface or processes occurring high in the atmosphere. Getting up close with a space probe was one means of getting that level of detail.
But a space mission wasn’t the only key that astronomers needed to unlock the mystery of Venusian climate. The NRL result was shocking to scientists because no one could understand how it might be true. Venus is closer to the Sun, but that proximity should only raise its surface temperature a few tens of degrees, not hundreds.16 If the surface temperatures really were 600 degrees, how could a planet so like the Earth in so many ways have ended up so different from our world? What was needed was a theory explaining how Venus might end up with such insanely high temperatures.
That task would be taken on by the young, untested, and not-yet-minted PhD student Carl Sagan. Though no one at the time could have guessed it, not only would Sagan’s work solve the Venus problem, it would also set the stage for a deeper understanding of our own world’s entry into the Anthropocene.
THE GREENHOUSE EFFECT
Though he died in 1996, Carl Sagan remains one of the most recognizable scientific faces in the popular imagination. Born sixty-two years earlier to working-class Jewish parents in Brooklyn, Sagan’s love affair with science began as a young boy during a trip to the 1939 World’s Fair. The passionate interest in life on other planets that defined his life came a bit later, as a teenager, with a steady diet of Astounding Science Fiction magazine and writers like H.G. Wells.17
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 would 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.
THE RED PLANET SHUFFLE
In a world of instant electronic access to all human knowledge and of routine jet travel five miles above the Earth, it’s easy to miss the audacity of the Mars rovers. Getting Spirit and Opportunity (and, later, Curiosity) safely on Martian ground was crazy enough. But the genius embodied in the rovers is reason to be proud of humankind. These robot scientists have trundled across miles of Martian landscape, drilling into rocks, sniffing for critical chemical compounds, and imaging the Red Planet at high resolution. The missions represent the best of our collective vision and capacity for solving the most challenging problems.
But the exploration of Mars by these rovers and other international probes represents something else that transcends engineering. Each was a step on the ladder of our coming of age as a planetary species. By literally giving us visions of another world through their high-resolution cameras, a new understanding of other worlds—and, perhaps, other worlds with civilizations—could be born. But climbing to that understanding was fraught with difficulties, as reality shattered our expectations and then shattered them again.
Like Venus, Mars was an early target of our interplanetary explorations. Just two years after Jack James’s JPL team flew Mariner 2 inward toward the Sun, their Mariner 4 probe made the journey outward to Mars, a planet with an even longer and more storied place in our extraterrestrial imaginings.
For the Mariner 4, mission Carl Sagan was again on the design team, and this time he won the debate about cameras. Mariner 4 carried a primitive (by today’s standards) analog TV camera. The pictures it sent back instantly changed our dreams of what Mars might be and what it might mean to us.
Because of Venus’s eternal cloud cover, it never appeared as anything more than a white disk. But for Mars, the story was very different. By the mid-1800s, astronomers knew Mars had surface features that changed over time. This led many nineteenth-century scientists to a dramatic conclusion: Mars had a climate like our own.37
Most importantly, astronomers saw that Mars had that most essential of climatic features: seasons. White polar caps on the Red Planet had been seen as far back as the seventeenth century. The polar caps grew and retreated as Mars progressed through its 687-day orbit. It was with good reason that in 1870, Claude Flammarion envisioned Mars as a world rife with life.38
By the turn of the twentieth century, the Mars story gained a new level of drama via Percival Lowell’s obsession with the Red Planet. Lowell’s fascination had begun with earlier studies by the Italian astronomer Giovanni Schiaparelli, which appeared to show long, straight features on the surface. Lowell claimed these were canals, representing the work of an intelligent civilization.39 In popular books, Lowell argued forcefully that Mars was inhabited and its society was, in effect, a victim of climate change. The planet was drying up and the canals were a desperate attempt to bring water from the polar ice caps. While most astronomers dismissed Lowell’s observations as wishful thinking, in the popular imagination the die had been cast. Through books like H.G. Wells’s War of the Worlds, Mars became the alien world most people imagined to host an alien civilization.
By the mid-twentieth century, astronomers had already accumulated enough telescopic evidence to be confident that Mars was not home to an advanced civilization. The atmosphere appeared to be thin and the planet cold. Still, the possibility that life existed in some form on that world remained very real. Periodically, the planet experienced significant changes in color that some argued had a biological origin.40 As Mariner 4 was launched, Carl Sagan remained hopeful that Mars might be home to at least some kinds of vegetation or, at the least, microbes.
But when Mariner 4 sailed past the Red Planet on July 14, 1965, the twenty-two images it sent back killed the dream of life on Mars in both the public and scientific imaginations.
It was the craters that did it.
Mariner 4 saw a lot of craters on Mars, and some of them were vast. On Earth, craters don’t last long. Whether they form from volcanoes or from meteor impacts, most craters on Earth get erased after many millions of years. It’s the familiar processes of weathering by wind and water that wipe the craters away. Seeing large craters on Mars meant its surface hadn’t changed in billions of years. Mariner 4 showed us a Mars that looked a whole lot like the empty, desiccated Moon.41
In the wake of the new pictures, a New York Times editorial announced to its readers, “The astronomers of past decades who thought they detected canals on the Martian surface and speculated that it might have bustling cities and beings engaged in lively commerce were victims of their own fantasies.” Concluding, “The red planet is not only a planet without life now but probably always has been.” 42
First Venus and then Mars. The main accomplishment of humanity’s first interplanetary emissaries seemed to be the death of our interplanetary dreams of life on other worlds.
Luckily, Mars didn’t stay dead for long. In 1971, Mariner 9 became the first spacecraft to park at a planet’s doorstep. Rather than just zipping by at ten thousand miles per hour, Mariner 9 went into orbit around the Red Planet. By taking up residency this way, the probe found Mars’s story to be far more complicated and far more interesting.43
Mariner 9 was built to map a good deal of the planet’s surface. When it arrived, however, it found Mars covered in a globe-swaddling dust storm. The surface was totally obscured. Because the space probe had been built with some inherent software flexibility, NASA engineers were able to delay the mapping till the storm abated (two Russian probes that arrived at the same time as Mariner 9 had no such flexibility and returned little useful data). While Mariner’s work was delayed, the planet-encircling storm highlighted the critical role airborne particles (that is, dust) could play in shaping climate.44 In the years to come, that link would become a political football for earthbound policy makers.
Eventually, the storm cleared and Mariner 9 returned more than seven thousand images. In those pictures was our first hint that, while today’s Mars may be bone-dry and frozen, Mars of the past might have been a very different kind of world. The pivot depended entirely on water.
Mariner 9 revealed landscapes that looked a whole lot like they’d been carved by flowing water. There were dry riverbeds and broad deltas. There were floodplains and rainfall basins. Confirmation that these features really were shaped by torrents of liquid water would have to wait for future missions. But what Mariner 9 immediately told us was simple and profound: the planet had changed in a big way.45
Mariner also revealed that our smaller neighbor was a planet as unique as our own. Mars was home to Olympus Mons, a towering volcano that rises almost fourteen miles from the planet’s surface. It also hosts Valles Marineris, a four-mile-deep canyon the size of North America that put the puny crack in Arizona we call “Grand” into a new, cosmic perspective.46 Mars, it turned out, had volcanoes and valleys, craggy highlands and smooth, broad lowlands. It was a place all its own, with tourism-worthy sites unlike anywhere on Earth. And all this topography would matter as the first attempts to understand the Martian climate got underway.
The view of the Nirgal Vallis channels on Mars taken by Mariner 9 in 1971.Images like these were the first indication that Mars once had water flowing on its surface.
The next great step in reviving the possibility of life on Mars came with the two Viking landers that touched down via parachutes and retro-rockets in the summer of 1976. Once again, Carl Sagan played an integral role, designing lander experiments that looked for microbial life in the Martian soil. The biology experiments returned ambiguous results, but the Viking landers’ meteorological stations allowed us to see, for the first time, what the weather was like on another planet.47 Each Martian day (called a sol), the Viking landers sent back measurements of temperature, pressure, and wind. The data flowed for six years, until one lander failed and the other was turned off by mistake.48 Through Viking we were on our way toward seeing weather and climate on other worlds as a cousin to our own.
With the advent of the Martian rovers in the 2000s, the mantra of NASA’s Mars program became “follow the water.” If life had once existed in Mars, we’d first have to prove the planet was once wet enough and warm enough to support life.49 But the presence of surface water can never be separated from the question of climate. So by following the water, NASA also committed itself to unpacking the story of Martian climate and Martian climate change. Like Venus, the Red Planet was acting as a guide for understanding our own world.
THE GREAT MARTIAN CLIMATE MACHINE
Robert Haberle wasn’t planning on becoming a world expert on the Martian climate. After serving in Vietnam, Haberle returned to civilian life in 1968, kicking around Europe for a while, “being young and anxious to explore the world.” Finally, starting in college at San Jose State, he needed to declare a major. “I was looking through the catalogue and saw meteorology,” he recalled to me in an interview. “I thought that meant the study of meteors. My wife had to explain to me it was about the weather.”50 It was an unlikely beginning for a man who would eventually help develop NASA’s premier Mars Global Climate Model, one of the world’s most powerful tools for studying the Red Planet’s history.
The model’s own history dates back to the late 1960s, when pioneers Conway Leovy and Jim Pollack took a climate model developed for Earth and began adapting it for Mars.51 Pollack was one of Carl Sagan’s first graduate students, and they collaborated together for years. Leovy was an atmospheric pluralist. He wanted to build a version of climate studies that reached beyond Earth to embrace every planet with an atmosphere.
For scientists, the word climate refers to long-term patterns of weather. While the weather changes from day to day (sunny on Tuesday but raining on Wednesday), climate represents the long-term patterns of winds, precipitation, ice cover, and ocean flow. To make a climate model, scientists must solve the physics equations governing these processes. That means a climate “model” is really a mathematical physics model. It’s a description of the world that uses the highly specific and very exacting language of mathematical physics.
Just as architects make models of a skyscraper out of paper, balsa wood, and plastic, scientists use the laws of physics, expressed in the language of mathematics, to construct models of a physical system. If it’s a gas engine they’re modeling, then the mathematics lets them understand and predict something like the engine’s fuel consumption. If it’s a bridge they’re modeling, then the mathematics lets them understand and predict how many cars can safely travel from one side of the bridge to the other. And if it’s a planet’s climate they’re modeling, then the mathematics lets them understand and predict the long-term patterns of temperature, cloud cover, and so on.
To be effective, however, a climate model needs a lot of “moving parts.” It needs to describe a lot of different kinds of physics, chemistry, and, perhaps, other processes as well. It must account for the flow of atmosphere on a spinning planet. It has to describe how radiation from the Sun warms the air near the surface, causing gases to rise. It must deal with how some of those gases, like water vapor or carbon dioxide, will condense into liquids or ice when they get cold (that’s how the models track cloud formation, rain, and snowfall). Building a climate model that gets the answers right (meaning it matches observations) requires years of insanely hard work.
It also requires a lot of equations to describe the combined action of atmospheric flow, condensation, and the movement of radiation. Each one is pretty complicated on its own, taking a lot of human ingenuity to master. But solving all the complicated equations together at the same time is simply beyond the intellectual power of any one person. So to make progress, scientists must turn to digital computers that solve the equations in tiny steps, over and over again, billions of times each second. In this way, the computers animate the equations. They bring details hidden in the mathematical complexity to life. And the models Haberle and others built did just that. They brought Martian climate to life for scientists. Through the models, researchers could see the full complexity of Mars’s climate. Most important, they could see both the similarities and the differences in how it worked relative to that of our own world.
JUST LIKE EARTH, ONLY IT’S NOT
“All planets are subject to the same basic forces,” says Robert Haberle. “It’s just that the strength of those forces will be different on different planets.”52 While Mars today may be a frozen, arid world utterly unlike Earth, the mechanics of its climate bear essential similarities to ours. Let’s start with its differences from Earth. While Venus has a lot more atmosphere than our planet, Mars has a lot less. The surface pressure read off by the Viking landers and the other Martian weather stations is less than one percent of what we get on Earth. That means the total weight of Mars’s blanket of gases is 99 percent less than Earth’s. Like Venus, most of Mars’s atmosphere is made up of CO2. But with so little atmosphere to go around, Mars doesn’t get a whole lot of greenhouse warming. Typical nighttime lows go down to –128 degrees Fahrenheit, while daytime highs only get as high as –24 degrees Fahrenheit.53 Mars is definitely a place to chill.
It’s also a desert. There’s very little water in Mars’s atmosphere—just 0.01 percent of what’s found in Earth’s.54 Since the atmospheric pressure is so low, exposed liquid water boils away in seconds. This is the same effect you get when you try to boil water high in the mountains—the water doesn’t need to get very hot before it turns to vapor. That’s why the water that does exist on Mars is either gaseous (water vapor) or locked up in ice at the poles. There may, however, be a lot of water underground, as ice or even in liquid form.
So, depending on which part of your spacesuit failed, conditions on Mars today would quickly kill you, either from asphyxiation or hypothermia. And yet, for all Mars’s differences from Earth, the Martian climate machine still operates in ways very familiar to earthlings.
Imagine for a moment you are a Portuguese sailor in the 1400s. You’re trying to get from West Africa, where you’ve been trading, back to Portugal. If you try sailing directly northward, you’ll find storms and variable winds that move you along at a sluggish pace. But if you try something crazy and sail west—out deeper into the Atlantic and away from Portugal—you get a pleasant surprise. Sail far enough west and you hit beautiful, steady winds that will carry you back east and north. You’re home in Portugal in no time. What you’ve discovered are the trade winds.55
A couple of hundred years after European sailors stumbled on the trade winds, English lawyer and naturalist George Hadley found their explanation. The trade winds are giant rivers of air, driven by solar heating and the Earth’s rotation. Hadley recognized that hot air in the tropics always rises upward, while cold air at the poles always sinks. The air in between has to fill in the gaps, leading to a giant equator-to-pole pattern of circulation.56
If the planet weren’t spinning, that would be the end of the story: up/down and north/south motions. It’s Earth’s rotation that bends the equator-to-pole atmospheric conveyor belt through what’s called the Coriolis force, which twists the flow, adding an east/west component to the circulation. The big circular flow in the North Atlantic is one of these giant rivers of air. In the southern hemisphere, there’s a mirror-image trade wind pattern (the east/west direction is flipped because the direction of the Coriolis force changes across the equator). In total, Earth has six of these vast, circulating atmospheric flows, and the strongest of these, flowing just above and below the equator, are called Hadley cells.
Mars, like Earth, is spinning. At 24.7 hours, the length of its day is remarkably close to Earth’s.57 Since the laws of physics don’t care where you live, Mars’s earthlike spin should mean Hadley cells appear on the Red Planet, just as they do on our world. “It’s one of the first things that comes out of a good Mars climate model,” says Haberle. “You see big circulation patterns from the Martian equator to pole and back again.”58
The Hadley cell is not the only familiar climate pattern on Mars. “Mars has jet streams,” says Haberle, referring to the rivers of fast-moving air that exist high in Earth’s atmosphere. “Every rotating planet with an atmosphere has them.” And, just as on Earth, sometimes those jet streams will buckle and wander. Atmospheric scientists call these flow patterns “Rossby waves,” and they were the cause of the dreaded “polar vortex” that brought record-cold air to inhabitants on the East Coast in the winter of 2014.59
While their technical details can be daunting, Hadley cells, jet streams, and Rossby waves all show us something profoundly simple and important: the physics of climate is universal. All worlds obey the same rules: Earth, Mars, Venus, even an exoplanet a hundred light-years away. Most importantly, they are rules that we now understand because we’ve seen them working on more than one planet.
HABITABLE WORLDS, SUSTAINABLE WORLDS
If you want to know the weather on Mars right now, there is an app for that.60 The Curiosity rover, which landed in 2012, includes a meteorological station that beams conditions back to Earth for any and all to see. Follow the app for a whole day, and you’ll see the temperature rise and fall between very un-earthlike extremes. You’ll also see the atmospheric pressure change in ways that are definitely not witnessed on our world.
On any given day, the amount of atmosphere pressing down on the Martian surface can change by as much as 10 percent. That’s almost like being in Los Angeles in the morning and then climbing the mile up to Denver’s thinner air a few hours later, only to return again to sea level by nightfall. For our story, what’s important about these changes is that the dramatic pressure swings are completely captured in the Martian climate models. There is so little air on Mars that, once the Sun begins warming the surface and driving hotter air upward, the entire planet’s atmosphere readjusts, sending pressure waves from one side of the globe to the other. All the models track these readjustments and nail the daily air pressure variations. In other words, the climate models get these answers right.61 That alone is an important point for our earthbound climate debate. We understand climate well enough to predict it on other planets.
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.
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 and 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.