What counts as a successful terraforming?
If you mean it in its usual sense - a planet like Earth able to continue to support an Earth like biosphere - without mega-engineering - without giant mirrors or greenhouse gas factories - then - neither of those fit the bill.
To achieve that - we would need to have the technology to move planets - and in case of Mars - increase the mass of the planet, restart continental drift and reinstate its magnetic field. And you'd also need to increase the spin rate of Venus - and for Mars - you would need to add a large Moon sized cmpanion moon to stop its axial tilt from varying (it sometimes tilts so far it has equatorial ice belts instead of polar ice caps).
Ideally also - since we are re-arranging the solar system anyway - might as well try to find a way to stabilize its orbit so it doesn't keep changing in eccentricity - whether life can survive in such an eccentric orbit - with asymmetry between summers in the two hemispheres - probably it can - but not very Earth-like.
Venus is too close to the sun and so needs shades to cool it down.
Mars is too far away and also hasn't got any continental drift or magnetic field. Venus is too volcanic and its entire surface is resurfaced every few hundred million years - even if you somehow get rid of all its CO2, you would have to keep doing so as an on -going process.
The ideas for "terraforming Mars" - are - first - not based on experimental data , but instead - mainly on "guestimates" - educated guesses about what might happen - since we simply don't have the data that would be needed to do a proper projection. So - lots to go wrong and nobody can give you a well grounded projection.
But in any case - it's really more like temporary partial terraforming. If it did work, then the planet would probably unterraform over a period of millions of years - and quite probably a shorter timescale than that. If you can indeed terraform it in a few thousand years - well - does that not mean it could also unterraform as quickly? Given that it is far too far from the sun, too cold, not enough mass, and insufficient magnetic field - and also - that we can't predict long term evolution of a biosphere full of microbes and higher life forms all continually evolving in unpredictable ways.
And - we don't know what level of gravity humans need.
On the face of it, Venus seems the greater challenge because it is really hard to get rid of its CO2 atmosphere - even if you expel it into space - unless at way beyond escape velocities - the planet will gather most of it up again and within a short time geologically you are back where you started. And the problem of its slow rotation rate seems hard to surmount - unless you use the mega-engineering solution of a simulated 24/ hour day caused by mirrors nad shades in space continually adjusting under influence of some program, say, to give it earth like conditions.
But - Mars in some ways hardest to terraform permanently. And both really require either continuous on -going mega-engineering to keep the planet at close to Earth like conditions - or else - mega engineering to re-arrange the solar system to put them into new orbits more conducive for Earth like life.
If we assume the level of technology needed to move planets around in our solar system - then spinning up a planet, and removing most of its atmosphere - might not be so hard to do - might even seem easy. If so, maybe Venus would be the more attractive of the two, at close to Earth mass and in many ways -apart of course from its lon day and thick atmosphere a near Earth twin.
Especially if we can get it back to more normal volcanism. It might be that we just need more water on the planet to get it to return to a more Earth like pattern of continental plates instead of its current occasional global upheaval of the entire surface.
Anyway - if that is ever done - it's a decision for future generations.
Easiest by far is to build habitats in space itself where you can engineer everything to your specifications in a Stanford Torus type habitat - if it can be made sufficiently low maintenance.
Far easier than that also - probably - to build floating colonies in the Venus atmosphere -at any rate weight per person is far less. And many other advantages, see
Robert Walker's answer to Why are we thinking about a Mars colony when a Venus colony would be more technically feasible? It seems that radiation shielded floating colonies could be assembled on Venus, with plastic film and aluminum wire bags, filled with breathable air.
But far easier than either of those is to look after our own Earth - and help it to recover from whatever things might happen to it. Because there is nothing in the next few hundred million years which is at all likely to make it anything like as uninhabitable and hard to terraform in the first place - or to keep "terraformed" - as either Venus or Mars.
See my science20 article: Why We Can't "Backup Earth" On Mars, The Moon, Or Anywhere Else In Our Solar System
If you mean it in its usual sense - a planet like Earth able to continue to support an Earth like biosphere - without mega-engineering - without giant mirrors or greenhouse gas factories - then - neither of those fit the bill.
To achieve that - we would need to have the technology to move planets - and in case of Mars - increase the mass of the planet, restart continental drift and reinstate its magnetic field. And you'd also need to increase the spin rate of Venus - and for Mars - you would need to add a large Moon sized cmpanion moon to stop its axial tilt from varying (it sometimes tilts so far it has equatorial ice belts instead of polar ice caps).
Ideally also - since we are re-arranging the solar system anyway - might as well try to find a way to stabilize its orbit so it doesn't keep changing in eccentricity - whether life can survive in such an eccentric orbit - with asymmetry between summers in the two hemispheres - probably it can - but not very Earth-like.
Venus is too close to the sun and so needs shades to cool it down.
Mars is too far away and also hasn't got any continental drift or magnetic field. Venus is too volcanic and its entire surface is resurfaced every few hundred million years - even if you somehow get rid of all its CO2, you would have to keep doing so as an on -going process.
The ideas for "terraforming Mars" - are - first - not based on experimental data , but instead - mainly on "guestimates" - educated guesses about what might happen - since we simply don't have the data that would be needed to do a proper projection. So - lots to go wrong and nobody can give you a well grounded projection.
But in any case - it's really more like temporary partial terraforming. If it did work, then the planet would probably unterraform over a period of millions of years - and quite probably a shorter timescale than that. If you can indeed terraform it in a few thousand years - well - does that not mean it could also unterraform as quickly? Given that it is far too far from the sun, too cold, not enough mass, and insufficient magnetic field - and also - that we can't predict long term evolution of a biosphere full of microbes and higher life forms all continually evolving in unpredictable ways.
And - we don't know what level of gravity humans need.
On the face of it, Venus seems the greater challenge because it is really hard to get rid of its CO2 atmosphere - even if you expel it into space - unless at way beyond escape velocities - the planet will gather most of it up again and within a short time geologically you are back where you started. And the problem of its slow rotation rate seems hard to surmount - unless you use the mega-engineering solution of a simulated 24/ hour day caused by mirrors nad shades in space continually adjusting under influence of some program, say, to give it earth like conditions.
But - Mars in some ways hardest to terraform permanently. And both really require either continuous on -going mega-engineering to keep the planet at close to Earth like conditions - or else - mega engineering to re-arrange the solar system to put them into new orbits more conducive for Earth like life.
If we assume the level of technology needed to move planets around in our solar system - then spinning up a planet, and removing most of its atmosphere - might not be so hard to do - might even seem easy. If so, maybe Venus would be the more attractive of the two, at close to Earth mass and in many ways -apart of course from its lon day and thick atmosphere a near Earth twin.
Especially if we can get it back to more normal volcanism. It might be that we just need more water on the planet to get it to return to a more Earth like pattern of continental plates instead of its current occasional global upheaval of the entire surface.
Anyway - if that is ever done - it's a decision for future generations.
Easiest by far is to build habitats in space itself where you can engineer everything to your specifications in a Stanford Torus type habitat - if it can be made sufficiently low maintenance.
Far easier than that also - probably - to build floating colonies in the Venus atmosphere -at any rate weight per person is far less. And many other advantages, see
But far easier than either of those is to look after our own Earth - and help it to recover from whatever things might happen to it. Because there is nothing in the next few hundred million years which is at all likely to make it anything like as uninhabitable and hard to terraform in the first place - or to keep "terraformed" - as either Venus or Mars.
See my science20 article: Why We Can't "Backup Earth" On Mars, The Moon, Or Anywhere Else In Our Solar System
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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