Listen, I get it. You want to go to Mars. I want to go to Mars. (Sort of.) And the plan—it’s good. A rocket with people. A base on the moon. Then more rockets and more people. Start making fuel on the surface, maybe depot it along the way. An outpost becomes a base becomes a domed city. And then: terraforming.

Bring dead Mars back to life, build it a new atmosphere with whatever’s left in its soil—frozen carbon dioxide, most likely—to up the air pressure, rely on greenhouse warming (you know, like climate change?) to make the place warm enough so frozen water, locked away underground, melts and comes roaring back. Oceans! Air! Maybe breathable, but at least enough so you don’t have to walk around in a spacesuit. Boom (where the value of “boom” = 10,000 years, plus or minus). Up the gravity well we go, and we can get moving on the Earther-Martian Colony Revolution all the hard sci-fi keeps promising.

It ain’t crazypants. The astronomer Carl Sagan, an upright symbol of scientific rectitude, pitched “planetary engineering” in 1971, melting water vapor from Mars’ polar ice to create “much more clement conditions.” Twenty years later, the astrobiologist Christopher McKay rounded the idea out, suggesting that terraforming of Mars was possible as long as the planet still had enough carbon dioxide, water, and nitrogen squirreled away to volatilize and pump into the atmosphere.

But a couple of scientists who study Mars are trying to burst that hermetically-sealed, oxygen-recirculating, radiation-shielded bubble. If a new analysis is correct, conditions on Mars make it impossible for existing technology to turn it into a garden of Earth-like delights.

“We were able to put together for the first time a reasonably clean inventory of the CO 2 on Mars,” says Bruce Jakosky, a planetary scientist at the University of Colorado and co-author, with Northern Arizona University's Christopher Edwards, of the new paper. “The bulk has been lost to space, a small amount to polar ice and shallow carbon-bearing minerals, and an unknown amount to deep carbonates.” Even adding in bits of CO 2 stuck onto rocks—“adsorbed” onto their surfaces—and a little more locked into water-molecule cages called clathrates doesn’t help. “Even if you put it all back into the atmosphere, it doesn’t add up to enough to warm the planet,” Jakosky says.

Atmospheric pressure on the surface of Earth is about 1 bar; you need about that much CO 2 on Mars to bring the surface temperature up to freezing; even just 250 millibars would change the climate there significantly. And some time in the past, Mars had that and more—geology and surface morphology strongly hint at the existence of liquid water on the planet’s surface in its distant past, which means it had to be warm enough and pressurized enough to retain that liquid water. If the planet had CO 2 in the same proportions of Earth and Venus, Jakosky says, you’d expect the equivalent of 20 bars of the stuff somewhere—mineralized as carbonate, frozen in polar ice, something. “For the past 40 years, the mantra of Mars science has been looking for carbonate deposits that had to exist, because the CO 2 had to have gone somewhere,” he says. “Down into the crust, it would be accessible, perhaps. If it went up to the top and got lost out of the atmosphere, it’s gone.”