Carl Sagan with a mock-up of a Viking Lander


A Short History of Life on Mars
February 15, 2010

The idea of “Men from Mars” has been with us for more than a century now, thanks to writers like H.G. Wells and Edgar Rice Burroughs. And movies like “Mars Attacks” and “War of the Worlds” are good fun. But what’s the real story of the search for life on Mars? Today we tell the tale of the search for life on the Red Planet…

In the 17th and 18th centuries, early telescopic astronomers glimpsed polar caps– much like Earth’s– that grew and shrank with the Martian seasons. The Martian day turned out to be about the same length as Earth’s. The axial tilt was similar to Earth, too, which meant Mars has seasons much as we do. And those strange dark surface markings… were they water? Or vegetation?

Then in the mid-1800’s, the Italian astronomer Giovanni Schiaparelli claimed to see long, thin lines on the surface of Mars. He called them canals, and he mapped them meticulously.

American astronomer Percival Lowell saw the canals too and loudly claimed they were irrigation structures built by an advanced Martian civilization. Inspired by Lowell’s claim, H.G. Wells wrote “War of the Worlds”, which has been re-purposed into radio events and movies over the decades. The possibility of “Men from Mars” stoked the imagination of science fiction writers and readers through the first half of the 20th century.

But as telescopes improved, few other astronomers could see the canals which were– correctly– dismissed as an optical illusion. Some denounced Lowell as a crank. And the existence of life on Mars remained tantalizing, but unproven.

Then, in 1965, space probes were dispatched to Mars to get a better view.

In 1965, the Mariner 4 space probe flew past Mars and snapped 22 black-and-white images of a tiny part of the Martian surface. The images showed craters– big ones– which suggested Mars was more like our moon than the Earth. So no Martian forests, or canals, or cities. The New York Times wrote a feature article declaring Mars “a dead world”. Later, Mariners 6 and 7 showed more craters, and many planetary scientists gave up hope of finding life on Mars.

But one scientist thought this conclusion was premature. Carl Sagan, along with a few colleagues, suggested the coverage and resolution of the early Mariner images were too poor to confirm the absence of life.

Then NASA sent Mariner 9.

In 1971, this probe became the first to orbit Mars. At first, the images showed only the white polar caps and a featureless surface. That’s because the probe arrived during a planet-wide dust storm which lasted weeks. As the dust cleared, the images revealed a startling display of surface features including immense volcanoes, canyons, and river beds that suggested the one-time presence of liquid water. The atmospheric pressure on Mars is too low to sustain liquid water now. But where did the water go? Underground? Frozen in the polar caps? If so, maybe there was still hope to find life elsewhere on the planet. The chase for life on Mars was on again.

Five years later, NASA landed the two Viking probes on the surface of Mars. They sent back thousands of pictures of a dry, rusty, rocky surface. And they grabbed samples of the Martian soil and conducted on-site chemistry experiments to look for the telltale signs of life.

The results?

At first, they looked promising. But after a little thought, most scientists concluded there was no definitive evidence for life on the surface of Mars.

Sadly, other surface probes since Viking, right up to the current Phoenix Lander, have found no evidence for life. No palm trees or hubcaps, no bacteria or organic molecules. More missions are planned in the coming years, including the European ExoMars mission which will dig two meters into the surface to look for signatures of life.

One more strange thing…

In 1984, a meteorite was found in Antarctica. Scientists were certain the meteorite came from Mars. It was likely knocked of by a volcanic eruption or asteroid impact, and its chemical composition was the same as the surface of Mars. In 1996, a group of scientists suggested they found fossilized evidence of bacteria in this Martian meteorite. But these results have been in dispute on and off ever since; no strong conclusions one way or the other have been declared. Though late last year, the same scientists concluded once again that this meteorite contains evidence of life on Mars.

So no one’s found clear-cut evidence of life on Mars, but we’ve only examined a tiny part of the surface. Upcoming missions may yet lead to the most startling scientific conclusion ever made… that life exists somewhere other than Earth.

Stay tuned…