Once upon a time, water flowed across the surface of Mars. Waves lapped against shorelines, strong winds gusted and howled, and driving rain fell from thick, cloudy skies. It wasn’t really so different from our own planet 4 billion years ago, except for one crucial detail—its size. Mars is about half the diameter of Earth, and that’s where things went wrong.
The Martian core cooled quickly, soon leaving the planet without a magnetic field. This, in turn, left it vulnerable to the solar wind, which swept away much of its atmosphere. Without a critical shield from the sun’s ultraviolet rays, Mars could not retain its heat. Some of the oceans evaporated, and the subsurface absorbed the rest, with only a bit of water left behind and frozen at its poles. Unrelenting radiation, along with electrostatic discharge from planet-wide dust storms, drove chemical reactions in the arid Martian dirt, ultimately leaving it rich in pesky toxic salts called perchlorates. If ever a blade of grass grew on Mars, those days are over.
But could they begin again? What would it take to grow plants to feed future astronauts on Mars? In science fiction, it isn’t much of a problem. Matt Damon’s character in the 2015 movie The Martian simply had to build a greenhouse, spread out human excrement, add water, and wait. The film got a lot of things right—bacteria in the human biome will be useful—but it didn’t account for the perchlorates. The potato plants that sustained him would never have grown, but even if they had, two years of eating contaminated, carcinogenic potatoes would have nuked his thyroid, boxed his kidneys, and damaged his cells—though he might not have realized it, because perchlorates are also neurotoxic. It would have been Matt Damon’s finest death scene.
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