For some reason most people asked this will say it's because of the lack of a magnetic field. And yes it's true, that is one of many reasons why Mars is no longer Earth like. But it's not the main reason it can't be terraformed now.
It took Mars hundreds of millions of years into the billions of years to lose most of its atmosphere. If you could somehow return an Earth like atmosphere to Mars it could take as long to lose it again.
If that was the only issue you could terraform it and hope it to stay terraformed for at least a few million years. And if it had a thick Earth density atmosphere, that would protect against most of the effects of solar storms.
Without the Earth's magnetic field, we'd still be protected from cosmic radiation and from solar storms, except for the magnetic effects of solar storms which would be devastating for long range power lines and so forth, and even that could be protected against by various methods.
With present day Mars with its near vacuum for an atmosphere, of course solar storms are a big issue. If we did send humans there, they would be limited in the amount of time they could spend per year on the surface in unprotected EVAs before the risk of cancer became too high to be acceptable.
But if you could terraform Mars so that it has an atmosphere as thick as Earth has, then it would protect against solar storms and radiation. Our atmosphere is equivalent to about ten meters of water in its capability to shield against solar storms and radiation.
So you don't really need a magnetic field for that. In the Solar storm of 1859 the main effects were the magnetic effects on long range telegraph lines. For humans, it wasn't an issue at all, no significant radiation levels.
Whether it is ethically responsible to terraform a planet for only a few million years is an issue of course, see my Our Ethical Responsibilities To Baby Terraformed Worlds - Like Parents.
But there's a lot more to it than that.
The fundamental issue isn't so much this lack of a magnetic field, as that it is further away, on the outside edge of the "goldilocks zone" so too cold for a planet like Earth.
It just doesn't get enough sunlight for an ecosystem like ours. To keep warm enough it would need a much warmer "blanket" with significant amounts of some stronger greenhouse gas than CO2, like methane. Which may be how it managed to have liquid water in the early solar system. It may be possible to develop a methane rich atmosphere for Mars if you knew a lot about exoplanets and lifeforms, and how to design ecosystems for them than we do now. An atmosphere like that could be warm enough for liquid water, but it would not be an Earth like atmosphere.
Yes, because of it's lack of magnetic field, it has lost nearly all its atmosphere and all of its oceans. It seems now that it has lost most of it to space, not just lost it as frozen materials in the crust.
A few years back it was reasonable to suppose that the water is still there in the form of ice and the atmosphere in the form of dry ice underground. But that's becoming increasingly unlikely. We know of only enough CO2 to raise it's atmospheric pressure to 2% of Earth's. And though there probably is more, still, it's rather unlikely that there is enough for Earth pressure CO2.
It also has very little by way of ice, and not enough probably for oceans. You sometimes read about how the ice at its poles would be enough for many meters of water over the entire surface. That's true, in a way. But remember that most of the surface is dry to considerable depth. And it may have no water to depths of kilometers below the surface in the equatorial regions. So what would happen to all that water if you warmed up the planet? I don't think there would be much left on the surface.
Then another issue is that Earth's atmosphere wouldn't work at the distance of Mars with half the amount of sunlight. It would become a cold frozen planet, like the past episodes of snowball or slushball Earth.
Even with a 100% Earth pressure CO2 atmosphere, if that was achievable, you might get some liquid water, but it would still be too cold for trees, probably, even at the equator. CO2 is not a strong enough greenhouse gas to make up for that factor of two reduction in the amount of sunlight reaching Mars.
So, ideas for terraforming Mars involve huge megaprojects to double up the amount of sunlight reaching Mars, or to retain most of the heat that gets there with its existing low levels of sunlight.
One idea is a space mirror of size at least 144 million square kilometers, to double up the amount of sunlight received.
Another idea, which is thought to be easier - is to build 200 power plants each with 500 megawatt capacity and leave them running 24/7 for a century to make greenhouse gases from fluorine. You also need to mine 11 cubic kilometers of fluorite ore.
The aim is to release enough CO2 to take it up to the magic figure of 10% of Earth's atmospheric pressure (if it exists) which would then trigger a runaway greenhouse effect, and release any remaining CO2 (if you don't get to that point, then since 1% is a point of stability, any CO2 you release would soon condense back to the poles).
But that's harder to suppose working now that it seems quite probable it has nothing like the right amount of CO2.
But even with a CO2 atmosphere, you'd still need to create greenhouse gases into the indefinite future or use those space mirrors to keep it warm.
And that's just the start of the problems with terraforming it.
You also need somehow to get rid of all the carbon in the CO2, because CO2 is poisonous for humans and animals at percentages above 1%. And you have to add a buffer gas such as nitrogen because a thin pure oxygen atmosphere is a fire risk.
And the result then may not be stable long term. You need to introduce many cycles to get it to work.
Even if there was a second planet same mass as Earth, same distance from the sun etc with no life, even if Earth was one member of a double planet - the Moon was as large as Earth but uninhabited - it still would be a huge challenge to terraform the Earth sized Moon. Because on Earth it took hundreds of millions, and billions of years for the various stages. So you are talking about a massive speedup, and where life is involved, then the results could be unpredictable. Who is to say an oxygen rich atmosphere like ours is the only end state it could end up in? It could easily end up in some different end state that is inhospitable to our form of life.
And - if it was possible to terraform it as quickly as that, within a few thousand years as the Mars Society optimistically estimates for Mars - what's to stop it unterraforming just as quickly? I mean - not necessarily lose its atmosphere again- if it was a runaway greenhouse effect, it might not be possible to lose its atmosphere - but to go to some other end state other than what we find hospitable for humans, or animals? An atmosphere without oxygen for instance?
I think the academic papers are interesting. Intellectually it's a challenge, which can help us understand about how Earth works, about how exoplanets might work, and so on. It might help us understand Mars' past. And who knows, maybe a few centuries from now it may be a possibility for Mars. But it might just as well be that we decide never to terraform Mars, either because we don't know how to do it, or because we think that Mars ismore valuable in itsunterraformed state.
If you terraform Mars, or attempt to, you are doing something that you hope will be valuable a thousand years from now or more, and something that's a mega project with our technology that would cost probably hundreds of billions or trillions of dollars - building 10,000 megawatts of powerplant capacity and mining operations to mine cubic kilometers of fluoride ore on another planet can't come cheap. We are bound to have other priorities, and it's hard to see such a project lasting for a year, or perhaps for a presidential term of eight years if some president was really keen on it. Never mind ten thousand or a hundred thousand years.
But more than that - how do you know that we will want this probably badly terraformed world a thousand or ten thousand years from now? Is it not far more likely that a thousand years from now, they will look back at what we have done to Mars and think "If only they had just left well alone!".
That's especially so given the potential to learn so much from Mars about the origins of life, evolution possibly with a different biochemistry, or if there is no life there, about how a planet works without life on it. It is actually a varied interesting planet, far more than we thought a couple of decades ago, and lots to find out there as it is.
Would you support a project to melt the Antarctic ice cap so that humans can live in Antarctica a little more comfortably? That's a very minor project compared with terraforming Mars.
See also my Trouble With Terraforming Mars and several other articles on Science20 such as Our Ethical Responsibilities To Baby Terraformed Worlds - Like Parents
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