Light of the Stars

by Adam Frank

  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. “M
ars 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.