Answer is, unknown, except for Earth. Mars is the one usually suggested. But there is quite a lot different between Mars and Earth.
And - it's a long process also. On Earth it took millions of years. The Mars Society hope that with mega technology - giant space mirrors to reflect more light onto Mars to warm it up, and greenhouse gas factories and such like - that it could take as short a time as 1000 years - plus several more millennia to get the oxygen levels up.
But - is that what our descendants, 1000 years from now, will want us to do to Mars?
Is clear we aren't doing it for ourselves. For quite a few centuries it would be hardly changed. And all the way to the end of that 1000 years - you'd still need oxygen - and most of the time, need complete spacesuits to get around.
Then - once terraformed - it's going to gradually unterraform - the only question is how long it takes - at least - unless you keep supplying comets to replenish the atmosphere - if so - sustained long term megatechnology.
What seems pretty clear is - that nature won't do it for us automatically. What we see on Mars is the end result of Mars which might well have had life in the past.
And Earth's biological cycles won't work on Mars because of the cold, lower gravity, and long term - lack of magnetic field and continental drift.
I don't think that necessarily means it is impossible. But means that any solution to terraform Mars would need to take account of all these differences - and also need to look into the future - and decide - are we doing this for our descendants 30 generations from now? Or perhaps 30,000 generations from now if you allow an extra 100,000 years to build up an oxygen atmosphere on Mars?
If so - why? And will it matter for them that Mars would unterraform? What would the implications be for our descendants even further into the future, say a few 100,000 generations from now, as Mars loses it's atmosphere again and its water and returns to its current lifeless state, or some other state?
That needs to be thought through first!
And - lots of things to go wrong.
See my Trouble With Terraforming Mars
And
Imagined Colours Of Future Mars - What Happens If We Treat A Planet As A Giant Petri Dish?
As for Venus, then it's just got too much CO2, and is so close to the sun. There are ideas for terraforming it - but - you have to somehow get rid of all that atmosphere - or else - in some way combine it, maybe by delivering huge amounts of water.
In some ways - cooling Venus down - then delivering gigantic amounts of water to it - and using that to react the water with the CO2 to it to create organics - and try to restore it to an Earth like state by adding those huge amounts of water - maybe from giant comets - in some way might be the best way to do it. Could make it into a near Earth twin. If you do that you also have to find a way to spin it up to a higher rotation rate - so you are talking about pretty huge mega technology there. Either that or a system of space mirrors and space "sun shades" that simulates a daily cycle of the sun moving around the planet by tilting different mirrors appropriately for different times of the day.
But is a huge project that, again we are nowhere near the level of techology - and more important - understanding - needed for it.
But who knows - in the future maybe we might do projects like this. Not impossible, in a long stable civilization - and maybe with lifetimes extended, people living for tens of thousands of years etc etc, then you might get the understanding and knowledge needed to do stuff like this - and also to understand its consequences.
I think is great to think over ideas -but to actually attempt it now - especially - in some simple minded way just throwing microbes onto Mars and hoping they turn it into a copy of Earth - that's not going to work. And adding mirrors and greenhouse gases - again - just too crude, when we don't even understand how Earth works too well, and Mars is so different - and so much to go wrong.
- Further from the sun - move the Earth out to Mars orbit and its oceans would freeze over turning it into "Snowball Earth"
- No magnetic field. Mars has only a few magnetic field hot spots but no protection from the solar wind. Mars has lost nearly all its water - and if we add water to it again - will lose it eventually - not known how exactly but part of the reason may be dissociation of hydrogen from H20, which then escapes to space - in upper atmosphere - because of its lack of a magnetic field to protect it.
- No continental drift - long term this returns CO2 to our atmosphere and is what got Earth out of its occasional snowball phase in the past. So if Mars turns into a snowball (which it is basically already but without the ice) - it can't get out of it again
- 40% of Earth's gravity - this means - that for the same atmospheric pressure you need nearly three times as much mass of the gas per square meter
- Orbit elongated a lot more than the Earth's
- No Moon to stabilize the axial tilt - sometimes tilts so far that it has equatorial ice sheets.
And - it's a long process also. On Earth it took millions of years. The Mars Society hope that with mega technology - giant space mirrors to reflect more light onto Mars to warm it up, and greenhouse gas factories and such like - that it could take as short a time as 1000 years - plus several more millennia to get the oxygen levels up.
But - is that what our descendants, 1000 years from now, will want us to do to Mars?
Is clear we aren't doing it for ourselves. For quite a few centuries it would be hardly changed. And all the way to the end of that 1000 years - you'd still need oxygen - and most of the time, need complete spacesuits to get around.
Then - once terraformed - it's going to gradually unterraform - the only question is how long it takes - at least - unless you keep supplying comets to replenish the atmosphere - if so - sustained long term megatechnology.
What seems pretty clear is - that nature won't do it for us automatically. What we see on Mars is the end result of Mars which might well have had life in the past.
And Earth's biological cycles won't work on Mars because of the cold, lower gravity, and long term - lack of magnetic field and continental drift.
I don't think that necessarily means it is impossible. But means that any solution to terraform Mars would need to take account of all these differences - and also need to look into the future - and decide - are we doing this for our descendants 30 generations from now? Or perhaps 30,000 generations from now if you allow an extra 100,000 years to build up an oxygen atmosphere on Mars?
If so - why? And will it matter for them that Mars would unterraform? What would the implications be for our descendants even further into the future, say a few 100,000 generations from now, as Mars loses it's atmosphere again and its water and returns to its current lifeless state, or some other state?
That needs to be thought through first!
And - lots of things to go wrong.
See my Trouble With Terraforming Mars
And
Imagined Colours Of Future Mars - What Happens If We Treat A Planet As A Giant Petri Dish?
As for Venus, then it's just got too much CO2, and is so close to the sun. There are ideas for terraforming it - but - you have to somehow get rid of all that atmosphere - or else - in some way combine it, maybe by delivering huge amounts of water.
In some ways - cooling Venus down - then delivering gigantic amounts of water to it - and using that to react the water with the CO2 to it to create organics - and try to restore it to an Earth like state by adding those huge amounts of water - maybe from giant comets - in some way might be the best way to do it. Could make it into a near Earth twin. If you do that you also have to find a way to spin it up to a higher rotation rate - so you are talking about pretty huge mega technology there. Either that or a system of space mirrors and space "sun shades" that simulates a daily cycle of the sun moving around the planet by tilting different mirrors appropriately for different times of the day.
But is a huge project that, again we are nowhere near the level of techology - and more important - understanding - needed for it.
But who knows - in the future maybe we might do projects like this. Not impossible, in a long stable civilization - and maybe with lifetimes extended, people living for tens of thousands of years etc etc, then you might get the understanding and knowledge needed to do stuff like this - and also to understand its consequences.
I think is great to think over ideas -but to actually attempt it now - especially - in some simple minded way just throwing microbes onto Mars and hoping they turn it into a copy of Earth - that's not going to work. And adding mirrors and greenhouse gases - again - just too crude, when we don't even understand how Earth works too well, and Mars is so different - and so much to go wrong.
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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