NASA/JPL/University of Arizona

It's official. NASA scientists have found evidence of present-day liquid water on Mars. But before you start thinking about a second home there, know this: that water isn't drinkable. It's chockfull of salts called perchlorates that can be toxic to humans.

Several states, including California and Massachusetts, have issued limits on the amount of perchlorates that can be in drinking water, because it can cause thyroid problems and harm the gastrointestinal tract, skin, breast tissue and the placenta. That's bad news on Earth, but on Mars, where medical care may be hard to come by, it would be an even bigger problem.

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Just to put it in perspective, it's saltier than the saltiest water on Earth: Antarctica's Don Juan Pond. "Nothing can live in the brine of Don Juan Pond," NASA planetologist Chris McKay, who wasn't involved in the Mars water discovery, told me in an email.

So how are we going to quench our thirst if we do start building condos on Mars? Importing water from Earth is expensive and unsustainable long-term. And trying to imitate the survival steps taken by the stranded astronaut from The Martian—burning oxygen and hydrogen to produce water—would be extremely dangerous. Some have suggested setting up a huge humidifier to extract water from Mars' thin atmosphere. Others think that loading up soil, which contains trace ice crystals, into a heater is the way to go. You could trap the water vapor, then condense it back to a liquid.

But the most effective way to keep us hydrated on Mars will be to tap its fresh water. There's tons of it at the polar caps, according to scientists. The northern ice cap alone is 621 miles across. There's also dust-covered glaciers that wrap around the planet. It's a huge amount of ice—the equivalent of all of Mars being covered in one meter of ice, according to a recent study. That's a nice, stable (though probably non-renewable) source of H20.

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By comparison, the new salty water found on Mars is transient—it only appears during the warm season. Although there's a lot of it, it's spread out over very large areas, so the volume at any one place is insignificant. So it wouldn't make sense to go after this water, said John Logsdon, professor emeritus at George Washington University's Space Policy Institute.

To get at Mars' frozen water, then, we'd have to drill.

Using radar, scientists discovered ice under these thick layers of Martian dust.
Credit: ESA/DLR/FU Berlin

But that water, too, is unlikely to be potable from the get-go. Perchlorates are everywhere on Mars. Some estimates suggest soils contain 0.5 to 1% of the stuff, which is definitely toxic to humans. So if we melted this, we'd still end up with salts and other gunk we don't want in our drinking water.

Here on Earth, water with perchlorate goes through desalination to remove it. It's the way we turn seawater into fresh water. According to the U.S. Environmental Protection Agency, there are three available methods: ion exchange, reverse osmosis, or biological treatment.

Luckily for future Martians, "these methods for desalinating water on earth should work on Mars," said McKay. But because the salinity is so high, it would require more energy.

Ion exchange, which is the most common method to de-perchlorate water in the U.S., involves swapping out perchlorate ions for other molecules with the same charge. Reverse osmosis uses pressure to push a liquid through a membrane with tiny holes that only let water through. Everything else is trapped behind it. Think of it like a club bouncer, who only lets desirable customers through. Water's in. Perchlorates are out.

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"The big challenge is getting the equipment necessary to Mars and ensuring the technology is reliable for a mission to Mars because if something breaks it would take at least another six months to get a replacement there," said NASA spokesperson Stephanie Schierholz.

Another option is biological treatment, which means using bacteria to eat up the perchlorate. Using microbes has a clear advantage over the other two methods: there's no need to deal with concentrated perchlorate waste. That would help us keep Mars clean. But if we don't find Martian life forms that feed off perchlorates, we might have to import our own to do the job. NASA scientists want to keep the Red Planet Earth-microbe free as long as possible so they can study the local fauna without worrying about contamination. So we'd have to take special precautions to contain them.

Because we don't want to deplete Mars of its natural resources, we'd also want to set up a water production factory there. That's where 20 years of NASA research on life support systems in space will come in handy. On the International Space Station, there's a system that "recycles urine to purify it to water, providing 80% of the water the astronauts use aboard the station," Schierholz said.

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Yes, if you go to Mars, you'll eventually be drinking your own purified pee. It's the ultimate green alternative.

Daniela Hernandez is a senior writer at Fusion. She likes science, robots, pugs, and coffee.