Life on Mars, from Viking to Curiosity
Life on Mars, from Viking to Curiosity
After midnight in a sweltering room in Pasadena in July 1976, Viking Mars team members sat hunched around a bulky monotone computer monitor, tensely awaiting the first data from the world’s first successful Mars probe lander, the only Mars lander ever specifically designed to detect life. Over the next weeks each of Viking’s first life-detection experiments came back with a striking signature. As the data trickled back into the Space Operations Facility, it became clear that carbon dioxide was released when organic compounds were added to Martian soil, though not when the mixture was superheated. This was a life signature, and exactly what had happened with the experiment on Earth. When water was added to the soil, oxygen was released, just as on Earth. The remote probe, panning for life, had found its signature in its first two experiments. The third experiment heated the soil, like warming food in the oven, and those results were mixed.
Arguments intensified, however, as the fourth experiment’s conflicting data came in. To claim life on Mars would be unprecedented. If they were wrong, no team member would live it down. Anything was better than striking out with a pompous grin on your face. Unbeknownst to most in the world watching, however, three of the four experiments on the primitive Viking lander could have been interpreted as testing positive for microbes, giving the same results as when they had been checked thousands of times on Earth. The researcher Patricia Straat told another mission staffer, Gil Levin, “That’s life!”
However, the fourth experiment, employing a gas chromatograph of the kind employed by James Lovelock, and a mass spectrometer, a delicate instrument for measuring the size of molecules, showed no life—and not only that, but absolutely no organics—on Mars. That was a stunning result: Organics exist all over space—on asteroids, comets, and meteors, and in interstellar dust. Not only that, but the experiment suggested that Mars’s surface was poisonous or self-sterilizing. Mission scientists had a heated argument, and NASA eventually decided to err on the side of caution. The surface must be self-sterilizing, they concluded, owing to the planet soil’s powerful oxidizing agents, which also helped to give it its red color. Viking had found a barren, windblown red planet, pockmarked by craters, cold and dead as the moon.
A few mission scientists disagreed, maintaining that the fourth experiment simply failed—as it had often in Earth tests. A group of activists, including Levin, wrote and spoke, goading NASA to release the full Viking data. On NASA’s 2016 celebration of the mission’s 40th anniversary, he repeated his call. He predicted the Mars Curiosity rover would find complex organics, which it did. When he saw the detection of methane bursts by Curiosity, he told me, he saw that the disappearance of the methane had happened too rapidly to have been caused by ultraviolet radiation: “This disappearance could have been caused by methanotrophs, which use methane and makes for a perfect little eco-cycle.”
Other Mars probes gave conflicting results. NASA’s Opportunity and Spirit rovers in the early 2000s, whose reports thrilled millions of fans around the world, including me, were designed and constructed by geologists and engineers, not by biologists. Evidence of water came from the 2008 Phoenix lander, whose camera pictured clear droplets on its cold steel legs. Simulations suggested that either water condensed around windblown grains of calcium perchlorate, the salt-type mineral whose properties enable it to scavenge water from the atmosphere, or that the landing had stirred up dirty ice beneath the surface and drops had formed and melted on the legs. The point was, said the University of Michigan’s mission scientist Nilton Renno, “On Earth, everywhere there’s liquid water, there is microbial life.” Such saline water on Earth housed microbes, in fact.
Suddenly everyone was interested in what could live in an ice-covered lake or plain, or in a remote cave or mine miles below the planet’s surface.
One of the best places to look for promising bits of Mars, paradoxically, was Earth. Walk along the frozen white expanse of the Antarctic and you will see bits of Mars, in the form of small stones. In fact, about 10 pounds’ worth of rock from Mars falls on Earth every year. If a large meteorite strikes Mars, it flings bits of rock into space beyond the small planet’s gravity. As that planet’s closest neighbor, our own, larger planet will find that its gravity traps some of the rocks, which fall to Earth’s surface and are most easily found in barren, ice-covered regions such as the Antarctic. Their authenticity is determined by chemical analysis of their shock glass, the melted glassy substance original to the rock. If a stone’s shock glass contains the exactly the same mix of gases as Mars’s atmosphere, identified in the many Martian probes, it is from Mars.
Tiny squiggles on a famous Martian meteorite found in Antarctica, ALH 84001, in 1996 led the NASA researcher Dave McKay and his team to claim that they had discovered fossils of microbes. Today most regard that as unlikely. But eons ago abundant water had certainly flowed in Mars’s oceans and rivers, the liquid’s mineralized remnants clearly visible across the planet—floodplains, alluvial basins, even oxbow curves in long-dried great rivers. The original name given in 1887 by the astronomer Luigi Schiaparelli to the great rift valleys seen through early telescopes was canali, Italian for “channels” (though English-speaking researchers translated it incorrectly as “canals”). At the turn of the century, in Arizona, Percival Lowell thought he glimpsed active Mars rivers featuring seasonal changes in vegetation. In fact, numerous probes have imaged morning haze in Martian canyons. Seizing on Lowell’s claim, Edgar Rice Burroughs, the author of the Tarzan books, wrote a wacky series of 1920s and ’30s Princess of Mars science-fiction novels that sent generations of young Americans to adventure. What Lowell saw was flaws in his mirrors. What Burroughs saw, as he divorced his wife for a Hollywood actress, was a gold mine of public gullibility.
Later NASA probes produced confounding results. The Mariner probes in the 1960s strongly suggested that Mars’s thin, cold atmosphere permitted no possibility of pure liquid water, though the final orbiter clearly pictured the beds of ancient streams and oceans.