People have long thought about converting Mars from the cold, dry planet it is today to one that would be inhabitable by humans. This notion of “terraforming” Mars has been popular for decades in science fiction and popular media—including, for example, several of the Star Trek movies and television episodes. In some sense, the idea of a terraformed Mars harkens back to the time when Percival Lowell postulated that Mars was inhabited by Martians, using giant canals to transport water from the Martian polar caps to the equator to save a dying civilization.

Is terraforming Mars feasible today? Is there enough CO2 locked up in the planet that, if it could be mobilized back into the atmosphere, would create a thicker atmosphere and a warmer environment? We have used spacecraft measurements from the last 20 years to estimate how much CO2 remains on the planet and recently published a paper in Nature Astronomy that assessed how much of the remaining CO2 can be released with present-day technology. We didn’t address what technology might be used, but Elon Musk has suggested, for example, that we could terraform Mars simply by exploding nuclear bombs over the polar caps. The heat from those would release the CO2 locked up in the polar caps back into the atmosphere, and the thicker atmosphere would produce greenhouse warming to heat the planet.

Before we reveal the answer, a bit of background:

Today, we know that Mars is not inhabited by Martians, although it’s possible that there could be microbes living underground. More than 50 years of robotic missions, mostly by NASA and the European Space Agency, have revealed an understanding of the planet’s once-strong-but-now-miniscule magnetic field; evidence of huge river channels and precipitation-created valley networks; standing lakes; and a complicated evolution of the polar caps. These and other discoveries have changed our understanding of the Martian environment and caused a dramatic reconsideration by the planetary science community of Mars as a dynamic planet with a complex history.

In our study, we found that much of the planet’s CO2 has been lost to space, and that very little of what remains can be put back into the atmosphere. Mars’ current atmospheric pressure is approximately six millibars (mbar), or just over one half of one percent of the pressure at the surface of the Earth. CO2 is present as ice in the polar caps; as gas “adsorbed” (or physically bonded) to regolith grains; as carbon-bearing minerals (carbonates) either in the shallow regolith or deep within the crust; and possibly as “clathrates,” which consist of water molecules forming a solid with a cage-like, open structure in which a “guest” molecule such as CO2 can reside. The total amount of gas present in these reservoirs is likely to be less than the equivalent of some 100 mbar (as an equivalent pressure if it were all released into the atmosphere) and maybe only about 20 mbar of it could be readily put into the atmosphere. This falls well short of the one bar atmosphere of CO2 that would be needed to warm the planet enough to allow liquid water to be stable.

The scale of the task of terraforming Mars becomes truly daunting if we consider the massive amounts of CO2 that humans have poured into our own atmosphere through the burning of fossil fuels. Orders of magnitude more CO2 are needed than that released by the entirety of humanity throughout history. It would take the equivalent of a million CO2 icebergs a kilometer across to terraform Mars.

There may be ways to do it without a lot of CO2, such as by manufacturing and releasing greenhouse gases with a much higher heat-trapping efficiency. That activity has to be far in the future, however, as we haven’t even sent a first human expedition to Mars. For the foreseeable future, at least, any humans that do go to Mars will be using spacesuits and enclosed habitats to explore the red planet, much as we did for the human exploration of the moon in the late 1960s and early 1970s.

Perhaps humanity’s greatest resource is the imagination of its people. This imagination can be articulated by visionaries such as scientists, entrepreneurs, inventors or world leaders—individuals who can see beyond our current limitations to a new and different future. For outer space, their science fiction serves as a guide to a future where humans are not limited to our own planet, space travel is routine and distant, and fantastic adventures await. As technology continues to revolutionize our everyday lives, science fiction may become reality, and our current problems with terraforming Mars may be looked upon like a telegraph wire in the time of the smartphone.