The Copernican system showing the planets orbiting the Sun. (Credit: Ullstein Bild/Getty Images)

When Earth Lost its Place at the Center, Intriguing Possibilities Emerged

Scientific interest in life on Mars arguably started in earnest in 1543, when Renaissance astronomer Nicolaus Copernicus rocked everyone’s world by showing that the planets orbit the Sun. Suddenly, Earth was no longer the center of the universe, and life on our planet lost its privileged position. But losing our special status also opened a huge new possibility: that life could exist on any of the other non-special planets in our solar system. Life, perhaps, existed everywhere.

As telescopes became more powerful in the Victorian era, astronomers were finally able to reveal Mars in more detail, but without sophisticated instruments they relied heavily on terrestrial analogy. A whitish area on a pole was assumed to be a polar ice cap. A darker region was taken to be an ocean or lake. Changes seen from month to month were thought to be seasons. Mars was revealing itself to be sufficiently Earth-like—so much so that even astronomers who believed man was God’s sacred creation allowed for the possibility that there was another race of Martian men just next door.

Twinsies? Maybe Not. But 19th-Century Astronomers Kept Finding Similarities

Further observations revealed more commonalities between Mars and Earth. Astronomers found the planet has a roughly 24-hour day (it’s about 24 hours and 39 minutes), an atmosphere and seasonal changes, as well as observable surface features. From there, the natural conclusion was that life on Mars would be like life on Earth. Some hypothesized that Martians would be humanoid. Others went further, assuming that Mars’ varying climates would have produced different species in different places, just as they do on Earth.

It wasn’t until the 1860s that astronomers were able to bring science to the study of Mars with spectroscopy. Studying light passing through Mars’ atmosphere revealed its chemical composition. Most importantly, astronomers found water vapor on Mars. This strengthened the idea that the planet had Earth-like seasonal changes. Mars was looking more and more like another Earth.

You Say Canali, I Say Canals

Water became a focus for astronomers towards the end of the 19th century, particularly Italian astronomer Giovanni Schiaparelli. He focused on the polar ice caps and, drawing his observations by hand, saw what he took to be snow accumulating at the poles in the winter and melting into oceans in the summer. He drew maps identifying and naming surface features, and as the season wore on, he tracked the water as it traveled toward the equator in a system of canali. These naturally occurring channels, he thought, irrigated the planet.

Schiaparelli’s canali captured the imagination of the scientific community, particularly American businessman and scientist Percival Lowell—but not as Schiaparelli had intended. Lowell mistranslated canali as canals, assuming they were engineered marvels like the recently completed Suez Canal and not a naturally occurring surface feature. From there, Lowell made the leap that the existence of canals signified that intelligent Martians had created on their planet what humans had done on Earth. Lowell published the idea in his book Mars, complete with hand-drawn maps of the planet criss-crossed with Martian-built canals, a sure sign of intelligent life.

Lowell spent years studying the canals, which became fixtures of two more books about Mars. He assumed they were massive, running thousands of miles over distances equivalent to that between Boston and San Francisco. Such a massive undertaking just to keep the drying planet fertile for life had to indicate the presence of intelligent life, he believed. Lowell became so obsessed he built an observatory in Flagstaff, Arizona, just to study the canals. The site later gained fame as the observatory where Pluto was discovered.

The first images of Mars from NASA's Mariner 4. (Credit: NASA)

The first images of Mars from NASA’s Mariner 4. (Credit: NASA)

The Space Age Mandate Was Simple: Follow the Water

The dawn of the space age in the 1950s brought with it a renewed fascination with Mars, one fueled by competition. While the Soviet Union and the United States were locked in a highly visible race to put humans into space and then on the Moon, there was another, less publicized race between the two nations: to return an image of, and land on, another planet.

The Soviets took an early lead, launching history’s first interplanetary probe to Mars in 1960—but it failed to reach Earth orbit. Both countries experienced a handful of failures until July of 1965, when NASA’s Mariner 4 probe passed a little more than 6,000 miles from the red planet. The 22 close-up images it sent back showed craters that revealed Mars to be more Moon-like than Earth-like. Science instruments on board confirmed the CO2-heavy atmosphere was much less dense than Earth’s, found no magnetic field and measured daytime surface temperatures in the inhospitable vicinity of -100 degrees Centigrade. The idea of Mars being another Earth was starting to fall apart.

Subsequent missions brought Mars into focus. In 1969, Mariner 7 gave us the first global look at Mars, showing the polar ice caps that had so fascinated Victorian scientists to be very much present. But in 1972, Mariner 9’s more detailed images revealed there were neither channels nor canals crisscrossing the surface. That mission did, however, find surface features suggesting water once flowed on Mars’ surface. If Mars was this Earth-like and had the one ingredient we know life needs to thrive—water—it might still be possible for life to exist there. A new era of Mars exploration began, and this time the directive was simple: follow the water.

An image of Mars taken from Viking lander, 1976. (Credit: NASA)

An image of Mars taken from Viking lander, 1976. (Credit: NASA)

Human Emissaries Arrive in the Form of Robotic Rovers

In 1976, robots served as our proxy, taking the first steps on Mars. The twin Viking landers were designed to photograph the surface, collect science data, and use three biological experiments on board to search for life.

They found none—not even a microorganism living below the surface. Data revealed that the combination of solar ultraviolet radiation and the dry climate had made Mars barren. But rather than consider this finding a defeat, scientists explored an exciting new possibility: What if Mars was newly sterile? The missions photographed and measured so much about Mars—volcanoes, lava plains, canyons, craters, dust storms, wind-shaped areas, atmospheric pressure changes and gas movement between latitudes. It all pointed to Mars being more active and hospitable in its past. So, scientists changed their tack: They would look back in time, searching for fossils and chemical signatures of past life on Mars.

Views of Mars from the NASA's Curiosity Mars rover on April 4, 2016 and a selfie by the Curiosity rover taken January 2015. (Credit: NASA/JPL-Caltech/MSSS)

Views of Mars from the NASA’s Curiosity Mars rover on April 4, 2016 and a selfie by the Curiosity rover taken January 2015. (Credit: NASA/JPL-Caltech/MSSS)

This begat the latest wave of robotic exploration with rovers serving as field scientists. In 2004, the twin Mars Exploration Rovers Spirit and Opportunity reached the surface; Opportunity is still working hard 14 years later. These two robotic geologists were designed to take advantage of how easily accessible Mars’ rocks are—they’re littered all over the surface. Armed with a suite of instruments to break open and study sites up close as they roam, these rovers found evidence of rocks shaped by flowing water, minerals formed in the presence of salt water and silica deposited near hot springs. Though they found no fossils, they definitively confirmed that Mars used to be wet.

The Curiosity rover landed in 2012 with a chemistry set on board to build on this knowledge. Analyzing the chemical composition on the surface of Mars, this rover reaffirmed that the planet once had the right chemistry to support microbes and that there was once fresh, possibly drinkable water on the surface. It’s not a record of an ancient civilization—or little green men—but it’s still a compelling find.

Now, scientists are looking back in time on Mars to try to understand when life might have thrived there. The analogies between Earth and Mars have changed since the Victorian era, but the quest to learn from those parallels remains. We’re just looking back in time now. And now we want to know: If water existed on Mars when life arose on Earth, could life have existed on our neighbor? InSight is adding another key piece to that puzzle, helping us understand more about past Mars so we can get closer to answering the centuries-old mystery of why life exists on our own planetary rock.

Amy Shira Teitel is a spaceflight historian and author of Breaking the Chains of Gravity.

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