Why mapping Mars completely changed how we see it

Mars has drawn our attention for millennia as the planet next door and a scientific object, but it was when we started mapping the planet that we began to see it as a place. Early mapmaking led to scientific discoveries—and a few far-fetched theories—which changed how we see the planet today.

Today, humans have a clear-eyed view of Mars and its bleak, ruddy surface. Since the 1960s, nearly two dozen spacecraft have visited Mars and beamed back images of its landscapes. Just this week, two more robotic explorers joined the fleet—one from China and another from the United Arab Emirates—and next week, a new NASA rover will attempt a risky landing during its quest to search for signs of life.

Among their many accomplishments, these Mars missions have helped us create detailed maps of a planet where humans have yet to set down their boots. But people were trying to map Mars long before the advent of spacecraft, when Mars was only fleetingly in focus.

By the mid-19th century, the quest to map the Martian surface had reached a fever pitch. Through a telescope, its milky, shadowy face swam in and out of resolution. So night after night, astronomers would stare at the hazy sphere and draw its features by hand. In the absence of photography, these maps were considered authoritative representations of truth, and they captivated audiences who were eager to understand the mysterious, reddish world next door.

But rather than presenting an objective reality, those maps were rife with bias, often reflecting the dreams and prejudices of their makers. Perhaps the most dramatic episode in Martian mapmaking occurred in the late 1800s, when astronomers presented two very different views of the same planet—a challenge for science, because both maps could not be right.

The winner of this mapping duel fueled decades of frenzied observations and inspired narratives about Mars that still ripple through modern cultures. And arguably, the Mars mania spawned by those early maps fed our current fascination with the red planet, leading to a new generation of cartographers who are helping off-world exploration to be more daring and more rewarding.

Dueling visions of Mars

In 1877, Mars made a very close approach to Earth, a predictable planetary alignment that offered astronomers their best views yet of the Martian surface. It was a perfect chance for them to map the features on the planet’s face, an endeavor that might sound simple but in practice is quite complicated.

“This is being done by astronomers—not by geographers or surveyors or cartographers in the field—but they’re pulling all those concepts from the geographic disciplines and applying them to Mars,” says Maria Lane, a historical geographer at the University of New Mexico.

One of the mapmakers, British amateur astronomer Nathaniel Green, was a professional artist and a long-time Mars observer. During the 1877 close approach, he left his normal observing spot in his garden near London and traveled to Madeira, Portugal, where the overhead air is stiller and affords sharper views of the faraway planet. Over two months, Green made 41 new sketches of Mars.

After comparing his observations with those of his colleagues, he compiled all of his sketches and ultimately produced the most detailed map of the Martian surface that anyone had yet made. Sculpted by Green’s hands, Mars was a delicately shaded world with landforms that gradually rose from vast plains and features that blended into one another. He labeled the planet’s locations traditionally, using a system established by previous observers of Mars.

Meanwhile, Italian astronomer Giovanni Schiaparelli was also sketching Mars. Unlike Green, Schiaparelli was a first-time Mars observer. Over eight months beginning in 1877, Schiaparelli stared at the planet through a telescope on a Milan rooftop, quickly jotting down its surface features and refining his drawings later.

Based only on his personal observations, Schiaparelli’s map revealed a vision of Mars with harshly delineated terrains—discrete islands amid dozens of strangely linear canals, which he colored blue. Schiaparelli labeled the exotic features on his map after places in Mediterranean mythologies.

“That was a really massively bold statement to make,” Lane says in the March issue of National Geographic. “It’s basically him saying, ‘I saw so much stuff that was so different from what anyone else had seen, I can’t even use the same names.’ ”

Green disagreed with Schiaparelli’s interpretations, and the two fought to prove the legitimacy of their maps, each describing their work as unbiased and objective. However, Green’s map ultimately faded from public view, while Schiaparelli’s work fueled a growing belief that intelligent aliens resided on the next planet out.

Schiaparelli’s map was more appealing to various audiences because it was crammed with information and features that Green’s did not depict. Plus, the topographic certainty of the Italian map lent it greater authority than the somewhat hazy view from Green. And Schiaparelli was, after all, a professional astronomer. It was hard to argue that Green’s map, which seemingly contained less information than Schiaparelli’s, was the more legitimate representation of Mars.

“Schiaparelli’s map ended up being definitive—the names, the way he represented it with dark lines, the definitive edges between features,” Lane says. “It was really visually authoritative.”

Schiaparelli’s map went on to inspire the imaginings of Percival Lowell, an influential man with a major-league Mars obsession who believed that the canals were artificial, planet-spanning fingerprints of alien technology.

“The original canal observer Schiaparelli did not assert that the lines on Mars were of synthetic origin,” Lane writes in an email. “But neither did he refute them.”

In 1894, Lowell raced to build himself a hilltop observatory outside Flagstaff, Arizona, so he could sketch Mars during its next close approach; and later that year he dutifully studied the red planet, scribbling its surface features onto his own maps. During those nights, Lowell spotted an additional 116 canals, and he believed them to be part of a massive irrigation network that shunted water from the Martian poles to the arid equatorial region.

Together, Lowell and others constructed a story that seduced the popular imagination with the promise of extraterrestrial life, and it was a tough tale to refute until the early 1900s, when photography made it possible for people to see Mars with their own eyes.

“That story was so intense that it’s hard to remember now that, about a hundred years ago, your ordinary educated citizen would, in all good conscience, believe that the astronomers might have been telling them there was a civilization on Mars,” says author Kim Stanley Robinson, who set an epic, three-part novel on near-future Mars.

Coming into focus

As the 20th century rumbled on, our view of Mars changed. Early on, photography dispelled the imaginings of Lowell and revealed a planet with a shaded, blurry surface that looked a lot more like Green’s carefully drawn map. And in the mid-20th century, the space age brought Mars into sharper focus when a fleet of spacecraft snapped images of its surface from up close.

Ultimately, the mysterious canvas we’d once populated with dreams of highly intelligent aliens turned into a sharply detailed, arid, sterile-looking landscape.

“The accuracy with which we’ve been able to map planetary bodies has just evolved tremendously over time,” says Robin Fergason of the U.S. Geological Survey, who makes current maps of Mars. But even our first close-up views of Mars, captured by spacecraft such as NASA’s Mariner 9 in 1971, and the Viking missions later in the 1970s, transformed our conceptions of what the red planet really was like.

“Starting with Mariner 9, and through the Viking era, those were the images where all of the ideas that had been formed through the telescope were turned on their side, and we got a completely new view of what Mars looked like,” Fergason says.

When NASA’s Perseverance rover attempts to land on the Martian surface on February 18, it will do so with the help of two detailed onboard maps. Made by Fergason and her team at USGS, the maps are a crucial part of the rover’s entry, descent, and landing sequence—a dramatic, complicated set of maneuvers that will deposit the one-ton rover in Jezero Crater.

Fergason and her team constructed the maps using a mosaic of high-resolution images from the Mars Reconnaissance Orbiter. Each one is of the rover’s four-mile-wide landing ellipse in the crater, where some areas are safe to touch down in, and others are not. Hazards such as large boulders, cliff edges, and potentially problematic sandy areas are marked.

As Perseverance parachutes through the thin Martian atmosphere, it will snap images from five different altitudes and compare what it sees with the onboard maps. Based on what it learns, Perseverance will automatically adjust its trajectory to avoid the marked hazards and land in a safe spot.

That technology has “revolutionized how we approach landing sites,” Fergason says. “For the first time we can have far more hazards in our landing ellipse than we’ve ever had before.” (See what Perseverance is slated to do on Mars in this immersive interactive.)

But contemporary Mars maps aren’t simply a bonus for science. In many ways, they’re just as compelling as the hand-drawn works of a century ago. Now, instead of inspiring the War of the Worlds, these maps are helping us imagine what a human future might be like on Mars.

When Andy Weir set out to tell the story of astronaut Mark Watney in The Martian, for example, it wasn’t the 700,000 images from NASA’s many rovers that excited him.

“The images from the surface are awesome, and they gave me a feeling for what it would be like to be Mark, looking out on a vast wasteland,” Weir says. “But what I really loved were the satellite maps. I loved mapping out his trip."

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