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Science fiction has long dreamed of turning Mars into a second Earth, a place where humans could live without having to put on a space suit. The easiest way to do that would be to use carbon dioxide already on Mars to create a new atmosphere, but now researchers say that is impossible.

Terraforming Mars to make its surface habitable for Earth life would involve raising both its temperature and pressure by adding an atmosphere made of heat-trapping greenhouse gases. The only ones present on Mars in any significant amounts are carbon dioxide and water vapour, both of which are currently frozen.

“If there is enough carbon dioxide, we could warm up Mars in 100 years once we start,” says Chris McKay at NASA’s Ames Research Center in California. “We know how to warm up a planet – we’re doing it on Earth. The fundamental question is, is there enough stuff?”

No, it turns out. Bruce Jakosky at the University of Colorado, Boulder, and Christopher Edwards at Northern Arizona University used results from several spacecraft to build an inventory of all the carbon dioxide on Mars to figure out whether, if we moved all of it from the ground into the atmosphere, we could create high enough temperatures and pressures for life.

Under pressure

Right now Mars has an atmospheric pressure of about six millibars – tiny compared to the one bar at sea level on Earth. “We would need something like a million ice cubes of carbon dioxide ice that are a kilometre across in order to do get to one bar,” says Jakosky.

At one bar, the temperature would be just above 0°C, allowing liquid water, and thus life, on the surface. The atmosphere wouldn’t be breathable, but humans could get by with breathing masks, not full space suits, and plants could grow freely, slowly building up oxygen over the course of the next few centuries.

But Jakosky and Edwards found that there’s probably only enough carbon dioxide in the Martian polar ice caps, dust and rocks to raise the pressure to 20 millibars at most. So we can’t terraform Mars with existing technology, because there simply isn’t enough carbon dioxide. “It’s not that terraforming itself isn’t possible, it’s just that it’s not as easy as some people are currently saying,” says Jakosky. “We can’t just explode a few nukes over the ice caps.”

It ain’t easy

There may be hidden reservoirs of carbon deep under the surface that could make the job easier, says Robin Wordsworth at Harvard University. “If you could develop the technology to look for those and extract it, that might get you close to the bar,” he says. “But it’d be kind of a fishing expedition – there’s no guarantee that these things exist.”

Without enough carbon, we would have to warm up Mars some other way, perhaps by making chlorofluorocarbons (CFCs) or bombarding the planet with comets or asteroids. That’s going to be difficult, and it will still not be enough to truly make Mars a home. For that, we need nitrogen – and we’re still not sure how much of that Mars has.

“If there’s not enough carbon dioxide, terraforming would take thousands of years or more but it’s still possible,” says McKay.  “If there’s not enough nitrogen, you need Star Trek. You need warp drive and tractor beams, you need to pull nitrogen from the atmosphere of Jupiter. It becomes science fiction.”