The overall reaction of photosynthesis is:
carbon dioxide + water + solar energy -> glucose + oxygen
6 CO2 + 6 H2O + solar energy -> C6H12O6 + 6 O2
See: Carbon cycle
So whenever you create oxygen using life processes, you have to take some carbon out of the atmosphere.
That's actually likely to be the limiting thing for Mars if we ever used photosynthesis to create oxygen in the Mars atmosphere.
First of all you need enough CO2 for the process as well as water.
But as well as that - you need to store and capture all those organics. If aerobes eat them and they decay - then the CO2 gets returned to the atmosphere and you lose the oxygen.
In the case of the Earth, carbon capture and oxygen release happened over millions of years during the early oxygen event.
There's a very rapid seasonal cycle and optimists sometimes look at that and think it means you can create oxygen quickly by looking at changes in oxygen between winter and summer. But that doesn't take account of the need to keep the organics captured to make permanent changes in the oxygen levels.
You also have to take account of the low Mars gravity - which means you need more oxygen in the atmosphere - more mass per square meter - for the same partial pressure of oxygen. And also take account of the lower light levels at the distance of Mars.
Chris McKay estimated that to get Earth like levels of oxygen, at the distance of Mars, with the light levels and gravity levels there, then you would need to have photosynthetic life on Mars covering the planet, similar to the plant life on Earth - for at least 100,000 years.
That's quite a speed up over the original first creation of oxygen on Earth.
The process would capture several meters thickness of peat or wood over the surface of Mars which would need to be permanently captured.
Maybe that can be speeded up - but surely well into several thousands of years.
And - there is much that can go wrong. Mars is very different from Earth. First of all - if you moved Earth out to the Mars orbit then it would go into a snowball phase. Then Mars has no continental drift which means that long term, all the CO2 will get captured into the oceans as limestone again. You can try to engineer cycles that return the CO2 to the atmosphere - but if that does work - it's a totally new form of planetary cycle, not present on Earth.
Many other things different. And things to go wrong. You might well end up making Mars worse for humans than it is now. And if Mars can be terraformed in as short a timescale as a few thousand years (which I'm doubtful about) - then it can surely unterraform just as quickly when you stop doing whatever mega-engineering thing it is you did to terraform it.
At any rate not likely to remain terraformed for as long as a million years. And the time when we are most likely to need to escape from Earth - or rather a future species not likely to be us quite by then - is about 500 million to a billion years from now when the oceans are expected to boil dry.
For more on this see my
Imagined Colours Of Future Mars - What Happens If We Treat A Planet As A Giant Petri Dish?
and
Trouble With Terraforming Mars
carbon dioxide + water + solar energy -> glucose + oxygen
6 CO2 + 6 H2O + solar energy -> C6H12O6 + 6 O2
See: Carbon cycle
So whenever you create oxygen using life processes, you have to take some carbon out of the atmosphere.
That's actually likely to be the limiting thing for Mars if we ever used photosynthesis to create oxygen in the Mars atmosphere.
First of all you need enough CO2 for the process as well as water.
But as well as that - you need to store and capture all those organics. If aerobes eat them and they decay - then the CO2 gets returned to the atmosphere and you lose the oxygen.
In the case of the Earth, carbon capture and oxygen release happened over millions of years during the early oxygen event.
There's a very rapid seasonal cycle and optimists sometimes look at that and think it means you can create oxygen quickly by looking at changes in oxygen between winter and summer. But that doesn't take account of the need to keep the organics captured to make permanent changes in the oxygen levels.
You also have to take account of the low Mars gravity - which means you need more oxygen in the atmosphere - more mass per square meter - for the same partial pressure of oxygen. And also take account of the lower light levels at the distance of Mars.
Chris McKay estimated that to get Earth like levels of oxygen, at the distance of Mars, with the light levels and gravity levels there, then you would need to have photosynthetic life on Mars covering the planet, similar to the plant life on Earth - for at least 100,000 years.
That's quite a speed up over the original first creation of oxygen on Earth.
The process would capture several meters thickness of peat or wood over the surface of Mars which would need to be permanently captured.
Maybe that can be speeded up - but surely well into several thousands of years.
And - there is much that can go wrong. Mars is very different from Earth. First of all - if you moved Earth out to the Mars orbit then it would go into a snowball phase. Then Mars has no continental drift which means that long term, all the CO2 will get captured into the oceans as limestone again. You can try to engineer cycles that return the CO2 to the atmosphere - but if that does work - it's a totally new form of planetary cycle, not present on Earth.
Many other things different. And things to go wrong. You might well end up making Mars worse for humans than it is now. And if Mars can be terraformed in as short a timescale as a few thousand years (which I'm doubtful about) - then it can surely unterraform just as quickly when you stop doing whatever mega-engineering thing it is you did to terraform it.
At any rate not likely to remain terraformed for as long as a million years. And the time when we are most likely to need to escape from Earth - or rather a future species not likely to be us quite by then - is about 500 million to a billion years from now when the oceans are expected to boil dry.
For more on this see my
Imagined Colours Of Future Mars - What Happens If We Treat A Planet As A Giant Petri Dish?
and
Trouble With Terraforming Mars
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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