Is it possible? It is if you make it possible. You're the writer. Induce a willing suspension of disbelief in the reader. Set the stage so that nano-bots are plausible; we already have the rockets. Set the stage wherein Mars gravity would retain the oxygen after it is created. Now there's the trick!
So you see the problem, right? Diatomic oxygen normally reacts with iron in an exothermic reaction. If you want to reverse the process, you've got to put energy into the system. To do it on a planet-wide scale, you need to put a lot of energy in.
Catalytic agents normally help (or perhaps inhibit) a chemical reaction go in the direction of releasing energy, they don't make chemical reactions go in the opposite direction from normal.
That isn't to say it isn't possible. After all, plants do something similar with CO2 here on Earth. You might want to consider that Mar's already has an atmosphere predominated by CO2. It is also a very thin atmosphere, compared to that of Earth, under 1%. And you need to consider that in order to get the same "sea level" pressure, Mars would need a far larger atmosphere because it has a lower gravity.
So the simple answer is that it is probably possible, but utterly pointless. Without introducing vast quantities of gases from extramartian sources, you could never get even close to an Earth-like atmosphere (which I'm assuming is your goal). You would also need to supply the energy from somewhere, which would limit the utility of using self-replicating nano-bots. If they were solar powered, they could probably make atmosphere at about the rate the solar wind would blow it away
Even so, SF uses this idea all the time, so do what you like.
By the way, are they NINJA nanobots?
I would have a hard time believing nano-bots could alter the entire atmosphere of Mars for reasons that have already been stated.
However, I would very easily believe in nano-bots that cleaned up the atmosphere inside an enclosed structure in preparation for colonization by plants, animals, humans, and purple people eaters.
Of course, if these are ninja nano-bots, anything is possible.
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Is it possible to fire a missile at mars filled with self-replicating nano-bots whose primary function is to 'metabolise' the iron oxide in the geosphere in order to release oxygen into the air? Sort of catalytic agents.
The short answer is yes, it is possible. We could almost do it with current technology, though the self-replicating part still needs work. We might be able to do it with microorganisms sooner than nanobots.
The long answer is no, this would not give Mars a breathable atmosphere. I don't believe there is any way it ever could. To terraform Mars you would need to bring a lot of energy (and/or mass) to Mars. As it stands your nano-bot plan leaves out this vital step.
[This message has been edited by Doc Brown (edited December 14, 2004).]
Basically deals with the possibility of creating a breathable atmosphere on Mars using genetically enigineered bacteria (or something).
[This message has been edited by J (edited December 14, 2004).]
[This message has been edited by J (edited December 14, 2004).]
And even if you liberated the oxygen from all the CO2 and iron oxide on the planet, it wouldn't give you a breathable atmosphere.
If you're even going to ask our advice, the answer is no. But as I said (and movies like Total Recall and Red Planet illustrate), there is really no pressing need to ask our opinion before you do this.
I am trying to find ways that 'terraforming' can be accelerated.
Alternatively, if we are talking a sci-fi story set, lets say 1,000,000 years in the future, is it akin to writing fantasy anyway?
[This message has been edited by hoptoad (edited December 14, 2004).]
Every living thing (with one or two deep-sea exceptions) and every machine on earth is ultimately powered by the sun. Even oil-consuming machines, as petroleum is a (very old) plant byproduct.
Survivor's insistence that your machines need energy is true, but his insistence that the sun doesn't provide enough is incorrect.
Plants create enough energy to live, grow, and reproduce from carbon dioxide and sunlight. So do many bacteria. There is no reason why, with a little inventiveness, you can't co-opt the excess energy created in those processes to power both your nano-bots and the aeration of the martian atmosphere.
According to this site:
http://www.atmos.washington.edu/2002Q4/211/notes_greenhouse.html
the solar energy reaching Mars is 593 W/m^2. Some of this will be absorbed by the atmosphere, and no completely efficient energy conversion process exists, so I'll assume that as an upper limit we can use 500 W/m^2 of this, as it's a nice round number. 
Obviously, we can only capture this energy during daylight hours, so overall we get 250 W/m^2.
I'm finding it hard to get a precise figure for the minimum amount of energy required to reduce iron oxide. I have this from here: http://www.saskschools.ca/curr_content/chem30/modules/module3/activities.html
"a) 2Fe(s) + 1½O2(g) Fe2O3(s)
H= -824 kJ/mol Fe2O3"
This suggests to me that to reverse the process, we're going to require at least 550 kJ per mole of oxygen released (although my chemistry is extremely rusty ).
So, over one earth year, we're going to have 250 W * 365 * 24 * 60 * 60 = 8 GJ/m^2 of energy available, allowing us to release approximately 14 kmoles of O2 per square metre of surface.
Earth's atmosphere apparently has 2.77 x 10^-4 moles/litre of O2. ( http://www.physics.umanitoba.ca/~davison/henry.html )
Given the large size of our upper limit on production compared with this figure, I'd say there's probably enough energy to do this.
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If your story is set 1,000,000 years into the future, then it will likely be a science fantasy. Nothing wrong with that, that's why the HUBs come with handwavium pellets!
Once the bacteria have oxygenated the atmosphere to a certain point, they'll start dying--literally poisoned in their own excrement. Without the energy produced by the symbiotic bacteria, the nanobots stop working.
So you can set the nanobots loose like a swarm of locusts without worrying about them getting out of control or consuming everything consumable.
Using the astroid bombardment idea, could that be used to build up the mass?
As earths atmosphere consist of only 20% oxygen anyway will an approximate 13% be much of a problem? Can we boost the atmosphere past the 20% of Earth barrier and have a higher oxygen ratio?
Or am I being silly? I am an illustrator not a scientist.
[This message has been edited by hoptoad (edited December 15, 2004).]
Jules' solar energy conversion assumptions are absurdly optimistic for any near future science (meaning in the next thousand years), but probably easy for a technology advanced a million years beyond our own. Still, at that level creating a new planet out of what we would imagine to be essentially nothing would probably be pretty easy.
So I'm with RH on this. A million years in the future means that you can write your own ticket where the science is concerned.
So if you were to bombard Mars with enough hydrogen sulfide, you could turn all of the iron oxide into iron sulfide (which is black and would turn Mars into the Black Planet) and water and oxygen.
Whether you want a black planet or not is another question, but the black might absorb more energy from the sun, at least.
Someone else already commented that colliding comets into the planet would be the most effective way to introduce water and free oxygen into the atmosphere. Additionally the clouding that would take place as a result would effectively thicken the atmosphere and heat the planet.
As to the terraforming grunt work. Using a genetically modified animal would be preferable to a mechanical device. It would also add a nice wrinkle to the story as you are faced with what to do with this lifeform after the process is complete. In any event with current tech standards you are looking at a 1000 year plus process.
Use a genetically engineered "water bear", it can survive the rigors involved with deep space travel and the harsh environments present on Mars, have it process Co2 and excrete oxygen, as mentioned by another writer, further by "mixing" in some plant DNA you could make it chlorophyl based and it would metabolize the sunlight for it's primary energy source.
It's all far fetched, but that is the point of Sci-Fi to address the implausible or impossible.
Good luck.
It almost looks that way!
They look like fascinating little bugs. Very gummy bearish. They usually live in moss and one length I saw was 300 microns. It can go into a dry state where it "resists storage in liquid nitrogen, contact with mineral acids, organic solvents, radioactive radiation and boiling water" (See this site). After putting the poor creature through all these tortures, they add water and poof! It goes back to living its ordinary little life.
Fascinating.
[This message has been edited by Keeley (edited December 17, 2004).]
Hey, maybe there are already some on Mars, existing as tuns. All we need to do is apply water.