In short, it is just fantasy and science fiction, and not realistic. It’s not at all as easy as the science fiction suggests. Kim Stanley Robinson’s book suggests just a few generations but he does it with a lot of artistic license fudging of the numbers for the sake of a good story - as is of course normal in science fiction writing.
RELEASING CO2 WITH NUCLEAR WEAPONS
Elon Musk himself suggested dropping nuclear weapons on the Mars poles some time back. But many people came in with criticism of that - which was just an off hand remark. The numbers don’t work out, even the largest nuclear weapon ever built is not nearly powerful enough. You'd need 25,000 gigaton bombs each one 20 times more powerful than the largest nuclear weapon ever built, to equal the effect of a single 10 km diameter comet hitting the poles of Mars. And that's a size of comet that hits Mars from time to time so it’s obviously not enough to warm it up.
Coming at it another way if you use a Tsar bomba, 50 megatons, largest nuclear weapon ever detonated, and all the energy went into sublimating dry ice, then each Tsar Bomba would liberate 50,000*4184000/184, or 1.137 billion tons of CO2 which sounds a lot until you realize the current Mars atmosphere is 25 trillion tons. To double the atmospheric pressure, enough to have water liquid on the surface, needs 25 trillion / 1.137 billion or 22,000 Tsar Bombas.
But that wouldn’t be a runaway greenhouse - the CO2 would just all condense back to the poles after a while. To have a runaway greenhouse you need 10% of Earth’s atmospheric pressure or nine times that, or around 200,000 Tsar Bombas - or around 10,000 one gigaton bombs.
The Mars atmosphere is currently at A in this graph- an equilibrium of temperature and CO2 pressure. For a runaway greenhouse it has to get to B which requires 10% of the Earth’s atmosphere as CO2 released, or in other words increasing the atmospheric pressure on Mars ten times.
To liberate this much CO2 just the process of turning dry ice into gas, without any temperature rises, involves 10,000 one gigaton bombs. with all the energy from all those bombs only used to turn dry ice into gas. Obviously in practice it would require a lot more than that as only a tiny fraction of the yield would liberate the CO2
And - that’s with all the energy from the explosion used only to turn dry ice into gas, none needed to warm it up and none of it escaping into space or used to damage the surface etc. That’s obviously unrealistic.
As well as that then there’s the problem that we don’t know if there is enough CO2 on Mars. There is enough dry ice to nearly double its atmosphere, but there probably isn’t enough to make it ten times in mass.
Thickness Map of Buried Carbon-Dioxide Deposit - Mars Reconnaissance Orbiter - discovery of a deposit up to 600 meters thick nearthe south pole of Mars in 2011.
Red here indicates a thickness of 600 meters of dry ice, yellow is about 400; dark blue is less than 100, tapering to zero
It's about 9,500 to 12,500 cubic kilometers - so about 15.2 - 20 teratons at the density of dry ice of 1.6 tons per cubic meter - not quite enough to double the Mars atmosphere of 25 teratons, but close to it.
So no, nuclear weapons are no good here, despite the media hype about them. For more on that, including detailed working out for these calculations, see my Why Nukes Can't Terraform Mars - Pack Less Punch Than A Comet Collision
RELEASING WATER WITH NUCLEAR WEAPONS
It is similar for the idea of turning ice into water. If we used 100,000 gigaton bombs (5 million Tsar Bombas), this would melt the estimated 11 meters of water equivalent over the surface of Mars thought to be available on Mars according to one estimate. Again all of that has to go into melting the ice, no allowance for warming it up, and no allowance for energy lost from the explosion through its other effects.
Also though it sounds impressive, 11 meters of water, remember that the Mars equatorial region is dry to depths of hundreds of meters. It’s like pouring the water into the Sahara sands. Just about all of Mars except the higher latitudes, is as dry as the Sahara to considerable depths.
And remember that unless Mars is far warmer than Earth, it’s ice cap would be similar to Earth’s in size on a terraformed world with water, rain and snow. As it is now, it is much smaller than Earth’s. So it’s not clear you’d have much free water at all if you melted the ice somehow and then somehow kept it as warm as Earth from then on. It would still have to have ice caps at both poles, and most of the water left over, if any, would just disappear into the desert sands.
It did have oceans in the past but Mars seems to have lost all that water, possibly underground but most of it into space, dissociated into hydrogen and oxygen, lost the hydrogen, oxidized the iron on the surface of Mars removed the oxygen.
Elon Musk later said he meant that we could warm up Mars using a mini sun created by continuously exploding nuclear weapons above the poles of Mars. That is a science fiction future scenario that we are nowhere near capable of doing, and obviously requiring a far larger output of energy than the numbers above.
OTHER WAYS TO WARM UP MARS
There are other ideas for warming up Mars, but they make optimistic asssumptions about the amount of CO2 there or use comets to bring them in. And they are all mega technology.
One way is to use a thin film mirror to reflect light to Mars. You need to double the amount of sunlight to get to Earth like conditions, and thin film mirrors might way one gram per square meter, so that’s one ton per square kilometer, or 36 million tons for half the cross sectional area of Mars. If you have very thin films maybe you can get that down to a million tons??
Another approach is to use a smaller mirror positioned to focus light on the poles only, this can be 110 km in diameter and with a mass of of the order of 30,000 tons (or using 6 grams per square meter material, 200,000 tons), see Technological Requirements for Terraforming Mars
Or you can use greenhouse gases. That needs 25 billion tons of greenhouse gases they work out, with fluorine as a key component . So that then involves mining 13 cubic kilometers of fluorite ore a century on Mars - of course you have to find the ore also to mine it. Then making greenhouse gases needs a lot of power too. To do the greenhouse gas manufacture requires an estimated 500 half gigawatt nuclear power plants running continuously for that same century, all just used for greenhouse gas production.
But you still have the problem that it probably doesn’t have enough CO2.
OTHER RESERVOIRS OF CO2 ON MARS
Zubrin's ideas, in Case for Mars. is based on modeling exchange of CO2 with the regolith The Case For Mars
There's a 1974 paper saying much the same thing here
However this paper suggests that H2O adsorption would displace the dry ice, reducing the estimate 6 or 7 times, this is from 1994
And this is a more modern estimate, it goes through the various possibilities which also include clathrates in the cryosphere. Tracing the fate of carbon and the atmospheric evolution of Mars
A lot of the early CO2 turned to carbonates. They go through various other options such as clathrates and adsorption on regolith, and conclude:
"The amount of CO2 contained in the identified atmospheric, polar and subsurface reservoirs under its molecular form is probably not in excess of a few tens or one hundred or so millibars."
So if you were very optimistic there you might think 10% was achievable.
So, I think he is right that there could be CO2 in the regolith, perhaps enough to raise atmospheric pressure to 10% though over a long period of time surely, as it would only warm up slowly from the top with those greenhouse gases or mirrors or whatever. If it did get as far as an optimistic 10% then it would be stable at that point and could stay at that level without more greenhouse gases - though Mars would be very cold still, without greenhouse gases, with 10% CO2 atmosphere - if you look at the graph above, the average polar temperature is around 162 °K or about -111 °C.
To get it to 100% of Earth’s probably you have to import it using comets, or you could perhaps release it from the carbonates using a cyanobacteria, an idea suggested here: Terraforming Mars: dissolution of carbonate rocks by cyanobacteria.
Could that be used to set up a carbon cycle on Mars to return the CO2 to the atmosphere? Based on a different method from the volcanoes and continental drift of Earth?
SUPPOSE YOU ARE OPTIMISTIC ABOUT RESERVOIRS OF CO2 IN THE REGOLITH OR USE CYANOBACTERIA OR LOTS OF COMETS - THOUSAND YEAR PROJECT
Anyway supposing that you optimistically say it does, that there are huge reservoirs in the regolith, supplemented maybe using many comets, to reach 1 bar, and you somehow find the water needed, on Mars or through cometary impacts.
In that case, then the estimate of the time taken to reach an atmosphere similar to Earth but made of CO2, with trees growing on Mars in the tropical areas, would be around 1,000 years according to the optimisitic Mars society projections.
After a thousand years, this is not an atmosphere that animals, birds or humans can breathe even with oxygen breathers. CO2 is poisonous to us above 1%. You would need a closed system more like an aqualung than the oxygen masks used by mountain climbers and high altitude pilots. Also even with a pure Earth atmospheric pressure CO2 atmosphere it would probably be too cold for trees to grow even in equatorial regions so you’d need to continue with the global mirrors or the greenhouse gases into the indefinite future to keep it warm enough for trees.
TO MAKE IT BREATHABLE - ANOTHER 100,000 YEARS
Having got that far, then to make it breathable, somehow you have to sequestrate all that carbon out of the atmosphere to turn it to oxygen ,and at the same time introduce some neutral gas like nitrogen (or the atmosphere would be so flammable it would be dangerous).
To get the carbon out of the atmosphere as algae or peat or trees would take around 100,000 years - faster if you can increase the amount of sunlight with those planet sized thin film mirrors. This is quite optimistic as on Earth of course it took many millions of years.
As for obtaining all that nitrogen, maybe from comets? Sending ammonia rich comets to Mars?
And then once you have oxygen instead of CO2, of course, that means that it gets much colder. So you need more mega technology at that point, to warm it up more than you are doing already, into the foreseeable future to keep it warm, keep producing those greenhouse gases, century after century, or maintain those planet sized mirrors.
It is nothing like the easy process and timescale of the Mars trilogy science fiction you might have expected if you are keen on terraforming.
But for an optimistic take on it, see Zubrin and McKay’s Technological Requirements for Terraforming Mars
IS THIS WHAT OUR DESCENDANTS 100,000 YEARS FROM NOW WILL WANT? AND WHAT ABOUT POLITICAL WILL TO COMPLETE THE PROJECT?
- How do we know this is what our descendants will want 100,000 years from now?
- When have we managed to sustain a space project for more than a few decades, never mind 1000 years or 100,000 years?
- Would politicians on Earth approve the countless trillions of dollars to do all this mega technology on Mars for 100,000 years? Or would they stop the project after a decade or two?
There is no way that they would do it themselves, they’d be struggling to survive at all.
And wouldn’t it all be better spent protecting and preserving Earth? Or setting up habitats that can be completed in a few decades, such as lunar cave colonies or Stanford Torus colonies or colonies floating in the Venus clouds, or domed cities?
OTHER ISSUES
There are many other issues, biological, planetary protection of Mars and Earth, setting up all the necessary ecosystem cycles - with many gaps needing to be filled in (One example: how is CO2 returned to the atmosphere when it turns into carbonate rocks? Here on Earth then it’s done by continental drift and volcanoes returning CO2 as a constant input into the atmosphere but Mars has no continental drift). And is an oxygen rich atmosphere a likely endpoint anyway? What if nature doesn’t co-operate and it turns into a methane rich atmosphere instead, on Mars ,say. What if the lifeforms that flourish there are harmful to humans?
But here I’m just looking at the engineering challenges. For the other challenges see list of articles at the end.
ROLE FOR HUMANS IN SPACE
I think humans do have a role in space. But I think this idea of trying to create a second Earth in order to go “multiplanetary” is rather quixotic. We are nowhere near the stage as a civilization where we can embark on a career as planetary ecoengineers and hundred thousand year long projects. Nor do we know enough yet about planets or about ecosystems and life support.
While, if it is just domed cities and cave habitats, then let’s drop the “multiplanetary” as it is of little or no significance whether they are on Mars or on the Moon or in the Venus clouds or in the asteroid belt. And in that case is it not best to start in the safest and easiest places first? I.e. our Moon.
What we can do is to create habitats in space that have small closed system ecosystems in them. Not try to do a whole planet. Do a habitat first a few meters then tens of meters across, then kilometers across. Those are projects we can complete on realistic human timescales of a decade or so. They would be hugely challenging too but within our realistic possibility of doing something about.
And amongst the best places to do this would be in the lunar caves which are quite possibly 100 kilometers long and kilometers in diameter in the lesser gravity there. Another good place to try early life support like this might be the lunar poles. But we need to explore using robots first I think, controlled from Earth. We will start that in a small way next year with the Google X prize winners.
But we need some other reason to be there, there’s no way space is going to be as easy a place to live as Earth in the near future for some time to come. I think the Moon is most likely to have other reasons to be there sufficient to perhaps eventually have large numbers of people there, maybe into the tens of thousands, who knows, maybe eventually millions. Can’t see that happening further afield in the near future.
I also don’t think our present priority in space is to get as many humans as possible living in space as fast as possible. That’s also rather quixotic, a bit like the adventurers who discovered Antarctica deciding that they had to find a way to get as many people as possible living in Antarctica as quickly as possible. Space is far more hostile than Antarctica, even the most habitable places.
Let’s take it a bit more slowly and find out what is there and make later decisions once we know what the situation is that we face better.
See also
- Wait, Let's Not Rush To Be Multiplanetary Or Interstellar - A Comment On Elon Musk's Vision
- Trouble With Terraforming Mars
- Imagined Colours Of Future Mars - What Happens If We Treat A Planet As A Giant Petri Dish?
- To Terraform Mars with Present Technology - Far into Realms of Magical Thinking - Opinion Piece
- Why Nukes Can't Terraform Mars - Pack Less Punch Than A Comet Collision
- Our Ethical Responsibilities To Baby Terraformed Worlds - Like Parents
- How Valuable is Pristine Mars for Humanity - Opinion Piece?
- Case For Moon First
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