Well, I think we won't land humans on the surface for a long time to come, whether we want to colonize the planet or not - instead - we'll study it from orbit via telepresence, with avatars on the surface - or from Earth via our rovers as we are doing now.
That's because it is a major and totally irreversible change to the planet to introduce Earth life to it, and we are nowhere near the level of scientific understanding required to know what would happen next over the following decades and centuries if we do that.
If you want to terraform Mars, then the ideal order might well be to keep humans away from the planet to start with. Because - most plans start with microbes and primitive algae, to try to duplicate what happened on early Earth, somehow greatly accelerated.
But on Earth - of course it took hundreds of millions of years, but as well as that, there was no modern life there yet.
With modern life from human habitats eating the primitive algae and microbes we try to seed it with - would it work the same way? Is it not possible that our microbes would work to prevent terraformation on Mars?
Whatever clever solution we might come up, biological way to transform Mars - if we've landed humans there first - then we have the unpredictable factor of what would happen as a result of all the microbes from the human habitat - for that matter higher lifeforms also, as they spread over the planet.
Not at all likely that they just transform Mars to Earth automatically. Because even if we could somehow magically move Earth's oceans and atmosphere and lifeforms to Mars - it would not work, it would not be able to maintain itself but the whole thing would start to fall apart - right away (more on that later).
Also even an Earth like planet is hard to terraform quickly.
Short term - though everything is in balance, and we have quite large changes in oxygen levels between winter and summer, we also have the detritovores that return the CO2 to the atmosphere, consuming oxygen. We have enough oxygen in the atmosphere to last us for thousands of years even if all plant life was destroyed, for instance. It gets maintained, but builds up really slowly.
That same process on Mars, Chris McKay worked out, would take about 100,000 years to create an oxygen atmosphere similar to Earth. It takes about six times longer on Mars, because you have half the light levels because it is further from the sun, and a third of the gravity so need three times the mass of oxygen for the same partial pressure.
That's assuming you get to the point where Mars is already green like the Earth land masses, which with optimistic estimates takes a thousand years to achieve that first.
Ideally on Mars we'd want to have just the oxygen producers without detritovores.
If we do it that way, maybe we could build up an oxygen atmosphere in less than a hundred thousand years, some more rapid method?
But remember, on Earth it did take hundreds of millions of years to achieve this. So to do it in "only" a hundred thousand years is already a rapid acceleration over what happened on Earth.
And on Mars we need to "engineer" a different solution as the Earth ecosystems won't work on Mars.
Also - very long term, on Earth, we have volcanoes returning the CO2 to the atmosphere so not going to lose our atmosphere like Mars - that's impossible for Earth. Even if we got rid of all our current atmosphere somehow, it is just a tiny part of the total inventory of CO2 in carbonates etc. On geological timescales it would get recreated from the CO2 from volcanoes.
This doesn't happen on Mars which is probably part of the reason why it lost its atmosphere.
And it has no magnetic field. And has only a third of the gravity meaning you need three times as much mass of gas to get the same pressure.
So - transfer Earth to the Mars orbit, reduce its gravity, remove its magnetic field - and somehow stop continental drift - I mean that's a fantasy - but for comparision purposes, thought experiment. Or to put it another way - if you could move Earth's oceans and ecosystems to Mars and just dump them there unchanged
- well first it would freeze over to become an ice planet. Then over long time-scales it loses all its atmosphere and over longer timescales much of the ice probably gets lost to space also.
This is probably what happened to Mars. It had similar atmosphere to Earth in the early solar system, and a fair amount of water though probably not as much as Earth but enough for shallow seas covering most of the Northern hemisphere of Mars.
So - a terraformed Mars would also "unterraform" over millions of years leaving Mars similar to the way it is now, but probably harder to terraform, e.g. much of the dry ice now in the form of carbonates, from dissolving in the seas, so harder to liberate.
If rapid terraformation as quickly as a few millennia is possible it will surely also have a possibility of rapid unterraformation also.
Terraforming in just a few centuries seems likely to happen only in science fiction - or some future such as self replicating fusion powered factories or whatever.
Science fiction stories are sometimes full of interesting ideas - but of course are not predictions, they are the work of the imagination of a writer. Sometimes they are astonishingly right, sometimes they are just as amazingly wrong in their "predictions". E.g. the way that almost none of the stories before the Apollo landing "predicted" that the whole thing would be broadcast live on tv. Or the many science fiction stories from the C20 of a future with flying cars by the year 2000.
The Mars Society, who are amongst the most optimistic about terraforming, suggest 1000 years to get as far as trees and vegetation, with humans breathing using air breathers. That's with high technology orbiting mirrors, greenhouse gas factories on Mars etc.
In short the whole thing is just science fiction at present. There are interesting ideas for ways to terraform a planet. But we are nowhere near the level of understanding and technology to actually start on such a project.
Also any project like that - it's bound to take thousands of years. So we need to have a stable technological civilization before we start on projects like that. It's the sort of thing you wouldn't want to abandon half way - as it could easily end up with Mars far worse than when you started (e.g. lose all its surface ice as water that sinks deep into the dry equatorial desert sands - or as water vapour dissociated into hydrogen and oxygen with the hydrogen lost to space) - so - you should only start it if you think you can complete it and are reasonably confident you can deal with the many issues likely to arise along the way.
(BTW though they did detect "water" in the equatorial sands, surprising discovery - it is only surprising for Mars. It is bound water, and all the sands on Earth have this, the amount of water found there is similar to the amount you get in the Sahara desert sands in the driest parts of the Sahara. It is still dry - and dry for kilometers depth into the surface probably in equatorial regions - only ice at the poles. Surprising amount of ice at the poles, but not at all clear that if you melt the polar ice and let it spread over the planet, that you'd have surface water, at any rate not found anything like enough ice to recreate the ancient oceans - nobody knows for sure what happened to them, lost to space, or sunk deep into the ground or a bit of both?).
So - need better understanding than we have now, and a stable technological civilization able to confidentally undertake a project that may take many millennia, possibly millions of years, to complete.
It's far easier to build Stanford Torus type habitats in space. Those can be completed in a few decades. For that matter, also easy to customize to any climate, any gravity level also.
If really interested in Mars for its own sake, then build settlements in orbit around Mars, keep humans well away from the surface, no possibility of contaminating it - and then explore it using telerobotic avatars on the surface. Safer for humans - and also more immersive experience than actually being there (enhanced binocular vision colour adjusted, blue skies even if you prefer, haptic feedback, walking and running using omnidirectional treadmills), more freedom of movement also especially as the telerobots get more capable.
See also
Trouble With Terraforming Mars
Imagined Colours Of Future Mars - What Happens If We Treat A Planet As A Giant Petri Dish?
Would Microbes From This Astronaut Make It Impossible For Anyone To Terraform Mars - Ever?
Those who plan to colonize Mars haven't given these matters much thought as far as I can tell. But at some point down the way then it will need to be examined.
But - before that - there's the whole issue, that we are going to Mars to find signs of ancient life there and present day life as well - or to find out why it doesn't have life if it doesn't have any.
So how can we make sure that we don't just find the life that we bring to the planet ourselves?
Even if life on Mars is similar to Earth life, that would be very interesting also, last thing you should do is to introduce Earth life to the planet to confuse study of how similar or different it is and what is the cause of the similarities or differences.
That's because it is a major and totally irreversible change to the planet to introduce Earth life to it, and we are nowhere near the level of scientific understanding required to know what would happen next over the following decades and centuries if we do that.
If you want to terraform Mars, then the ideal order might well be to keep humans away from the planet to start with. Because - most plans start with microbes and primitive algae, to try to duplicate what happened on early Earth, somehow greatly accelerated.
But on Earth - of course it took hundreds of millions of years, but as well as that, there was no modern life there yet.
With modern life from human habitats eating the primitive algae and microbes we try to seed it with - would it work the same way? Is it not possible that our microbes would work to prevent terraformation on Mars?
Whatever clever solution we might come up, biological way to transform Mars - if we've landed humans there first - then we have the unpredictable factor of what would happen as a result of all the microbes from the human habitat - for that matter higher lifeforms also, as they spread over the planet.
Not at all likely that they just transform Mars to Earth automatically. Because even if we could somehow magically move Earth's oceans and atmosphere and lifeforms to Mars - it would not work, it would not be able to maintain itself but the whole thing would start to fall apart - right away (more on that later).
Also even an Earth like planet is hard to terraform quickly.
Short term - though everything is in balance, and we have quite large changes in oxygen levels between winter and summer, we also have the detritovores that return the CO2 to the atmosphere, consuming oxygen. We have enough oxygen in the atmosphere to last us for thousands of years even if all plant life was destroyed, for instance. It gets maintained, but builds up really slowly.
That same process on Mars, Chris McKay worked out, would take about 100,000 years to create an oxygen atmosphere similar to Earth. It takes about six times longer on Mars, because you have half the light levels because it is further from the sun, and a third of the gravity so need three times the mass of oxygen for the same partial pressure.
That's assuming you get to the point where Mars is already green like the Earth land masses, which with optimistic estimates takes a thousand years to achieve that first.
Ideally on Mars we'd want to have just the oxygen producers without detritovores.
If we do it that way, maybe we could build up an oxygen atmosphere in less than a hundred thousand years, some more rapid method?
But remember, on Earth it did take hundreds of millions of years to achieve this. So to do it in "only" a hundred thousand years is already a rapid acceleration over what happened on Earth.
And on Mars we need to "engineer" a different solution as the Earth ecosystems won't work on Mars.
Also - very long term, on Earth, we have volcanoes returning the CO2 to the atmosphere so not going to lose our atmosphere like Mars - that's impossible for Earth. Even if we got rid of all our current atmosphere somehow, it is just a tiny part of the total inventory of CO2 in carbonates etc. On geological timescales it would get recreated from the CO2 from volcanoes.
This doesn't happen on Mars which is probably part of the reason why it lost its atmosphere.
And it has no magnetic field. And has only a third of the gravity meaning you need three times as much mass of gas to get the same pressure.
So - transfer Earth to the Mars orbit, reduce its gravity, remove its magnetic field - and somehow stop continental drift - I mean that's a fantasy - but for comparision purposes, thought experiment. Or to put it another way - if you could move Earth's oceans and ecosystems to Mars and just dump them there unchanged
- well first it would freeze over to become an ice planet. Then over long time-scales it loses all its atmosphere and over longer timescales much of the ice probably gets lost to space also.
This is probably what happened to Mars. It had similar atmosphere to Earth in the early solar system, and a fair amount of water though probably not as much as Earth but enough for shallow seas covering most of the Northern hemisphere of Mars.
So - a terraformed Mars would also "unterraform" over millions of years leaving Mars similar to the way it is now, but probably harder to terraform, e.g. much of the dry ice now in the form of carbonates, from dissolving in the seas, so harder to liberate.
If rapid terraformation as quickly as a few millennia is possible it will surely also have a possibility of rapid unterraformation also.
Terraforming in just a few centuries seems likely to happen only in science fiction - or some future such as self replicating fusion powered factories or whatever.
Science fiction stories are sometimes full of interesting ideas - but of course are not predictions, they are the work of the imagination of a writer. Sometimes they are astonishingly right, sometimes they are just as amazingly wrong in their "predictions". E.g. the way that almost none of the stories before the Apollo landing "predicted" that the whole thing would be broadcast live on tv. Or the many science fiction stories from the C20 of a future with flying cars by the year 2000.
The Mars Society, who are amongst the most optimistic about terraforming, suggest 1000 years to get as far as trees and vegetation, with humans breathing using air breathers. That's with high technology orbiting mirrors, greenhouse gas factories on Mars etc.
In short the whole thing is just science fiction at present. There are interesting ideas for ways to terraform a planet. But we are nowhere near the level of understanding and technology to actually start on such a project.
Also any project like that - it's bound to take thousands of years. So we need to have a stable technological civilization before we start on projects like that. It's the sort of thing you wouldn't want to abandon half way - as it could easily end up with Mars far worse than when you started (e.g. lose all its surface ice as water that sinks deep into the dry equatorial desert sands - or as water vapour dissociated into hydrogen and oxygen with the hydrogen lost to space) - so - you should only start it if you think you can complete it and are reasonably confident you can deal with the many issues likely to arise along the way.
(BTW though they did detect "water" in the equatorial sands, surprising discovery - it is only surprising for Mars. It is bound water, and all the sands on Earth have this, the amount of water found there is similar to the amount you get in the Sahara desert sands in the driest parts of the Sahara. It is still dry - and dry for kilometers depth into the surface probably in equatorial regions - only ice at the poles. Surprising amount of ice at the poles, but not at all clear that if you melt the polar ice and let it spread over the planet, that you'd have surface water, at any rate not found anything like enough ice to recreate the ancient oceans - nobody knows for sure what happened to them, lost to space, or sunk deep into the ground or a bit of both?).
So - need better understanding than we have now, and a stable technological civilization able to confidentally undertake a project that may take many millennia, possibly millions of years, to complete.
It's far easier to build Stanford Torus type habitats in space. Those can be completed in a few decades. For that matter, also easy to customize to any climate, any gravity level also.
If really interested in Mars for its own sake, then build settlements in orbit around Mars, keep humans well away from the surface, no possibility of contaminating it - and then explore it using telerobotic avatars on the surface. Safer for humans - and also more immersive experience than actually being there (enhanced binocular vision colour adjusted, blue skies even if you prefer, haptic feedback, walking and running using omnidirectional treadmills), more freedom of movement also especially as the telerobots get more capable.
See also
Trouble With Terraforming Mars
Imagined Colours Of Future Mars - What Happens If We Treat A Planet As A Giant Petri Dish?
Would Microbes From This Astronaut Make It Impossible For Anyone To Terraform Mars - Ever?
Those who plan to colonize Mars haven't given these matters much thought as far as I can tell. But at some point down the way then it will need to be examined.
But - before that - there's the whole issue, that we are going to Mars to find signs of ancient life there and present day life as well - or to find out why it doesn't have life if it doesn't have any.
So how can we make sure that we don't just find the life that we bring to the planet ourselves?
Even if life on Mars is similar to Earth life, that would be very interesting also, last thing you should do is to introduce Earth life to the planet to confuse study of how similar or different it is and what is the cause of the similarities or differences.
About the Author
Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
Lives in Isle of Mull
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