In Ridley Scott’s new movie, “The Martian,” (in theaters today) Mark Watney, played by Matt Damon, gets caught in a sandstorm on Mars. He’s left for dead by his crew, who manage to escape the planet. To save himself, Watney needs to get in touch with NASA and arrange a rescue mission. In the meantime, he has to survive for a few hundred days—on a planet where nothing grows.
Unfortunately, Watney is apparently nowhere near where NASA just found water on Mars. But, fortunately, he is a botanist. Almost immediately, he heads back to NASA’s Martian base, MacGuyvers together a device that distills water from the air, and figures out how to grow potatoes, living off them and leftover food from NASA. But does the science stand up to scrutiny?
We spoke to Andy Weir, who wrote the novel on which the movie “The Martian” is based. He did copious research to make sure the novel is as scientifically accurate as possible, and was in frequent contact with Drew Goddard, who adapted the book into a screenplay, to transfer that accuracy on screen. Weir also said he was in frequent contact with the crew during the shoot to consult on technical questions.
And, yes, it is possible to grow plants on Mars—kind of. Alone, Martian soil doesn’t have the necessary elements for plant life. “The main thing that’s not in Martian soil is a bunch of nutrients and biological materials that plants rely on to grow,” Weir says. “It’s not there because, obviously, there’s no life on Mars.”
So to get biological material into Martian soil, Watney uses the only spare biological material he has: astronaut poop. He mixes it in with the Martian soil, plants some potatoes that NASA had sent up with his crew, and, voila, you have plant life on Mars.
Yes, it is possible to grow plants on Mars—kind of.
There’s just one problem that Weir didn’t address, because he didn’t know about when he wrote the novel: Martian soil has perchlorates, a type of salt that’s hazardous to the human body. The perchlorates would either make it more difficult for plants to grow, or would make the plants toxic. The solution is actually very simple, but it wasn’t included in the book or movie. “You can literally just rinse them out of the soil,” Weir says. “Wash the soil, soak it in water, and the water would wash the perchlorates away.”
While using manure to fertilize soil is common on Earth, there are some obstacles—especially when it comes to using human feces. Human waste has human pathogens in it. Crops grown from soil fertilized that way would have those pathogens on or inside of them. If humans eats those crops, they will contract those pathogens.
Eating food grown from someone else’s poop, in other words, can get you sick. In Watney’s’s case, he uses his own poop, so he would contract only the pathogens he already has. “You can get away with it in a desperate survival situation, where you are a single person using your own manure to grow crops that only you eat,” Weir says.
But Watney also uses the crew’s leftover waste from the station’s toilet, which could mean that he could have contracted his crew members’ pathogens as well. (The movie isn’t as gross as it sounds, we promise.) This is addressed with a bit of explanation in the book, but isn’t said explicitly in the film.
“The crew’s waste was all completely desiccated, freeze-dried, and then dumped out on the surface of Mars and bagged,” Weir says. “Any pathogens in there would have been dead.”
Watney rehydrated his crew’s feces and mixed it with his own. That way, he could take advantage of the nitrates and other elements from the crew’s feces, and still have the bacteria from his own feces into it. “So what he ended up with was a big tub of shit that only has his pathogens,” Weir says. Good advice if you’re ever stuck on Mars.
The next question is whether a person can actually live off practically only potatoes for hundreds of days, but that’s fodder for another article.