Remember that scene at the end of Total Recall when the alien machine melts the polar ice on Mars, and within a minute gives Mars a warm breathable atmosphere? Of course, there was a lot wrong with that scene – not the least of which is that the polar caps on Mars are not composed of anything close to a breathable atmosphere. The northern ice cap is mostly composed of water. There is a thin top layer of carbon dioxide (CO2) ice during the winter that sublimates (turns into gas) in the summer. The southern cap undergoes the same process, and is also made of water ice and CO2 ice, but apparently has more CO2 ice than the northern cap. You will notice there is no oxygen in there.

But could, theoretically, we melt the ice caps as part of a plan to terraform Mars? Elon Musk previously floated the idea of nuking Mars – exploding many nuclear bombs over the ice caps in order to quickly melt them, turning the water and CO2 into vapor and thereby warming the planet and thickening its atmosphere. The result would not be an instantly comfortable and breathable atmosphere, but could take us a long way to making Mars more habitable. Musk’s idea was soundly criticized, but recently he tweeted the idea again (although no one is sure if he is serious or not, or just trying to sell T-shirts), but it has resurrected the discussion about whether nuking Mars is a viable option.

A 2018 paper published in Nature crunched the numbers of what we know about non-atmospheric CO2 reserves on Mars. They counted not just the ice caps, but also the carbon bound in the soil. They concluded:

These results suggest that there is not enough CO2 remaining on Mars to provide significant greenhouse warming were the gas to be emplaced into the atmosphere; in addition, most of the CO2 gas in these reservoirs is not accessible and thus cannot be readily mobilized. As a result, we conclude that terraforming Mars is not possible using present-day technology.

That’s discouraging. There isn’t an atmosphere on Mars waiting to be melted or liberated. There is also the question of whether or not exploding nukes over the poles would even work to melt the ice. By some calculations we would need thousands of nukes per day over weeks to accomplish this. And then, of course, the gases would freeze again, because there is not enough CO2 to sustain warming. So – no “blue skies on Mars” anytime soon.

But what would it take to terraform Mars? We do need to define what this means. Optimally this would result in a stable breathable atmosphere with temperatures in the range of liquid water. The atmospheric pressure on Earth at sea level is by definition one atmosphere – or 101.3 kilopascals, or  14.7 psi. We probably don’t have to worry about the upper limit of pressure, and it seems humans can survive under several atmospheres of pressure at least – but what’s the lower limit? We actually have some idea about this, because of Mount Everest. The summet of this highest mountain is 1/3 of an atmosphere, 33.7 kilopascal. Most people cannot survive at this altitude, and most people who do reach the summit require extensive training and acclimation, and the use of supplemental oxygen. They also can survive only a short time, and many people die if they get stuck up there too long. So essentially, this seems to be at the lower limit of human tolerance.

But this also means that, with proper acclimation, an atmospheric pressure of 0.5-0.6 atmospheres might be survivable. We don’t necessarily have to get all the way up to one full atmosphere. For further context the atmospheric pressure in Denver is about 0.83 atmospheres.

The breathability of that atmosphere depends on the percentage of oxygen. If you combine that percentage with the atmospheric pressure you get the partial pressure of oxygen. So essentially, the thinner the atmosphere the higher the percentage of oxygen necessary, which is why Everest climbers need supplemental oxygen. When we breath we also need to blow off CO2, so there is also an upper limit of how much CO2 there can be in a breathable atmosphere. Earth’s atmosphere contains about 400 parts per million CO2, 0.04%. The toxic level that will kill all humans is about 6%. Lower amounts are still a problem, causing drowsiness, impaired function, and shortness of breath.

The CO2 limit is more of a problem than the O2 limit. If there were a warm atmosphere on Mars above half an atmosphere, but lacking oxygen, you could walk around with only a small tank of oxygen and a nasal cannula. If there were too much CO2 then you would need a full breathing mask to scrub out the extra CO2 before you breathed it in, or you would need to breath entirely tanked air, like a scuba diver.

Water vapor content also has a range of comfort. Too much humidity and it becomes difficult to breath, too little and we dry out too quickly. But this range is fairly forgiving, as would probably not be a problem as long as there was liquid water on the surface.

Finally, the rest of the atmosphere just needs to be devoid of poison. Nitrogen is probably best for this, and what we have on Earth. Hydrogen would not work, as it would combine with the oxygen and is also too light to be held by the Martian gravity. Helium is not reactive, but also is too light, and everyone would sound funny. Ideally we would have a massive source of nitrogen for the bulk of the atmosphere.

With all this in mind, it’s important to realize that any extent to which we increase the Martian atmosphere can be helpful. Right now the atmospheric pressure on Mars is about 0.01 atmospheres, or 1% that of Earth. As we increase the thickness of this atmosphere it becomes potentially easier to land on Mars, because parachutes and wings become viable. It becomes easier to pressurize living spaces. And, the atmosphere would provide more protection from radiation from space. This also would increase the negative effects of dust storms, but that’s probably a worthwhile tradeoff.

As pressure climbs higher, say to 50% that of Earth, we will no longer need to necessarily pressurize everything. Half an atmosphere would certainly require some acclimation, but it is starting to get theoretically livable. It would certainly be a lot easier to live on Mars with half an atmosphere than the currently wispy 1% that it has.

The thicker the atmosphere the warmer Mars will become, and this also reduces the strain on environmental systems. At some critical point, the surface temperature will become warm enough for there to be liquid water at the equator, and then spreading out toward the poles. This is also hugely beneficial. At this point we could theoretically grow crops, and plant grasses and even forests on Mars. This would release more oxygen. Each step of the way, Mars gets much easier to live on.

So how could we theoretically make this happen? Again, melting the poles won’t do it. Releasing the CO2 and oxygen from the surface soil won’t do it. It would take a massive terraforming program that would process much of the Martian surface to depth in order to liberate enough volatiles to make a usable atmosphere. We can’t do this today, but it is theoretically possible. This is a project that would take centuries, at least.

Another idea, however, is to redirect comets that are loaded with volatile compounds so they crash into Mars. We would obviously want to do this prior to having any colonies on Mars. Essentially we could deliver an atmosphere to Mars. This would also take centuries, however, to identify, get to, and redirect enough comets, and wait for them to crash into Mars. We could get the process started with a few well-placed comets, then continue with the gentler methods of terraforming that are not incompatible with existing colonies.

There is also the question of whether or not we should terraform Mars. So far we have no evidence of existing life on Mars, but it is reasonable to say that we should make sure there is no life on Mars before we terraform it. Also, I reject that argument that we need to terraform Mars because we are screwing up our own planet. This is nonsense – it is much easier to simply preserve our own environment rather than try to replicate it on Mars. This is not a reason for going to Mars.

But, if Mars is devoid of life, it is a nice little planet right next door and there is no reason not to spread out to its surface. It would be a good hedge against extinction from planetary impact. I think it would also be good for humanity. It’s not going to happen anytime soon, and really it’s a question for the next century or even further into the future.