Mars clearly had a warm and wet past, a time when streams, lakes, and even an ocean were present on its surface. Currently, however, most water on the planet appears to be locked in its icy poles, and the atmosphere is so thin that water would quickly evaporate even if temperatures were held at Earth-like levels. But could we go back to the future? Is there enough material on Mars to form a dense atmosphere filled with enough greenhouse gasses to keep things warm enough for liquid water?

SpaceX CEO Elon Musk attracted a bit of attention when he suggested that we could get there simply by nuking Mars' poles, liberating the ice (both water and carbon dioxide ices) into the atmosphere. When asked about the prospects for the plan, a scientist said, "Whether it would really work, I don't think anyone has worked up the physics in enough detail to say it would." Now, a couple of planetary scientists have accepted the challenge of working up the physics, and they have bad news for Musk.

Greenhouse and pressure

The researchers, Bruce Jakosky and Christopher Edwards, focus on two significant questions. The first is whether we can put enough gasses back into the atmosphere to create an Earth-like air pressure so that people who need to do something on the surface don't need to wear bulky suits to isolate themselves from the environment. The second is whether we can warm the surface enough so that liquid water could persist on it.

Those situations require a lot more gas in the atmosphere. In addition to increasing the pressure, gases can mediate the greenhouse effect, capturing outgoing radiation before it reaches space and converting some of it to kinetic energy. As is happening on Earth, this process has the net effect of raising temperatures. If you liberate enough of the right gas, you can handle both of these issues.

The trick is finding the right gas. Water vapor is a potent greenhouse gas, but it's difficult to get much of it into a cold atmosphere before it starts precipitating back out; it enhances warming driven by other factors but can't drive it on its own. People have also suggested pumping in CFCs, which are also strong greenhouse gasses, but these aren't chemically stable, so they couldn't drive a long-term warming. All of which brings us back to carbon dioxide, a greenhouse gas that is already 90 percent of the Martian atmosphere.

Unfortunately, that atmosphere is only thick enough to generate about 0.6 percent of the pressure that Earth's atmosphere does. And it's clearly not enough to keep the planet very warm. In fact, it's so cold that a part of the atmosphere freezes out at one of the poles during Mars' seasonal cycle.

After considering the options, the researchers' challenge became simplified: can you identify enough sources of CO 2 on Mars to refill its atmosphere?

Taking inventory

Turning to that seasonal ice, the researchers estimate that it's about a third of what's already in the atmosphere. Radar imaging of the polar ice caps indicates that the ice also contains pure carbon dioxide ice layers. If that carbon dioxide could be put back into the atmosphere, you'd double the current atmospheric concentrations. Combined, this brings us up to roughly 1.5 percent of Earth's atmospheric pressure.

From there, however, things get very difficult and speculative. For example, water ice can form in such a way that it encapsulates individual gas molecules in its crystal structure (on Earth, this is typically in the form of methane/ice mixtures). Though these "clathrates" could contain carbon dioxide on Mars, it's impossible to tell how much ice (if any) is in this form. But even if every single bit of ice on the planet is a carbon dioxide clathrate, it's only enough to get the atmosphere up to 15 percent of Earth's pressure—assuming we were willing to nuke the poles and every glacier to melt the ice.

Other bits of Mars' carbon dioxide are associated with its rocks. Some of it is just loosely attached to rock surfaces; this is especially true of sandy, loose material, which has more surface area to adsorb the gas. This material can be heated back off, but the process faces a number of problems. To begin with, even if we took a high estimate for the CO 2 locked up this way, it only pushes the atmospheric levels up to about half of what we'd want. And doing so would require heating pretty much everywhere on the planet where there's loose rock—think of a Mars-scale strip-mine. Finally, the rock-held carbon dioxide is at equilibrium with that in the atmosphere. It would start being adsorbed right back the moment we stopped heating the rock.

The last possibility that Jakosky and Edwards look at is carbonate minerals, which can undergo chemical transformations that release carbon dioxide, leaving a different mineral behind. But we've imaged much of the Martian surface, and there's simply not much carbonate there. There is one region, called Nili Fossae, where the minerals are visible. If the researchers assume that what we see at the surface is a sign of larger deposits beneath them, there is a substantial amount of carbon locked up there. If we strip-mined the region and released it into the atmosphere, it would probably triple the current levels. At a wildly optimistic maximum, it could give us 15 percent of Earth's atmospheric pressure.

If you combine the mostly excessively optimistic estimates, you'd get somewhere near 80 percent of the Earth's atmospheric pressure. But this would require us to melt both poles, strip-mine the entire planet's surface, and heat every bit of loose rock—and keep it heated indefinitely. More realistically, Jakosky and Edwards think we could probably triple the Martian atmosphere's current levels of carbon dioxide, which would only get us up to 1.5 percent of the pressures we see on Earth. If we generously put the figure at two percent, then we're looking at 10K of additional greenhouse warming, which is nowhere close to allowing liquid water on the surface.

Lost to space

So how did Mars ever end up with streams and lakes? Conveniently, we've been studying that issue, using instruments on orbiting craft like MAVEN and Mars Express. These craft have been tracking the ongoing loss of gasses from Mars' atmosphere to space as the solar wind and other radiation slam into the planet. Using numbers on oxygen loss from these observations and tracking the amount of radiation produced over the history of Sun-like stars, it's possible to estimate how much oxygen has been lost over Mars' history.

The numbers suggest that there's been enough lost to either raise Mars' atmospheric pressure to Earth-like levels or to cover its surface in a layer of water tens of meters deep. Estimates based on carbon isotope ratios (lighter isotopes are lost to space more readily) indicate that Mars has lost half its original carbon as well. "Loss to space was the dominant process for removing the ancient CO 2 greenhouse atmosphere," Jakosky and Edwards conclude.

Unfortunately, once something is lost to space, it's not coming back. To terraform Mars, we'd be looking at getting this material from a non-local source. Of course, smashing comets and asteroids into the poles would also melt the ice caps, so it's a bit of a two-for one. But it's well beyond anything that even Elon Musk is considering in his more optimistic moods.

Nature Astronomy, 2018. DOI: 10.1038/s41550-018-0529-6 (About DOIs).