Reason I'm asking, it's seemed to me that the best way to create an atmosphere on Mars is by moving an icy planetoid/comet/Kuiper Belt object and basically smashing it into mars, though I think a low orbit, perhaps near the Roche Limit where it breaks apart pretty easily would be more practical than a crash landing.
Roughly speaking, comets are mostly too small to be that interesting. To create a 1 ATM atmosphere you'd need something in the range of 120 KM in radius - figuring some of it would be water, some of it would be dust and rock, some of it would be CO2 that you'd want to turn into plant life.
In terms of moving the object, you could do a gravity assist around Neptune and using Neptune, you could adjust the trajectory either towards mars or towards another planet for another gravity assist. The object could be moved by large mirrors, melting some of the ices on the surface creating a propulsion or by explosions. I think that would be the easiest way.
And once in orbit, the process is rather simple, the sun would melt some of the lower freezing point ices, and in a near roche limit orbit, it could be broken apart fairly easily given the tidal forces already in play.
Figuring a Kuiper Belt object is made up of a combination of dust, frozen H2O, CO2, CH4, NH3 some frozen N2, a bit of CO, maybe some NO, some O2, a sprinkling of more complex molecules, a little Argon, Hydrogen and Helium, it's not something you'd want to breath, but it would create atmospheric pressure, perhaps similar to earth. (120 KM radius, figure 1.5 density, mass would be about 10 billion billion KG - about twice the mass of the earths' atmosphere, but if half of it is rock, dust and water-ice - then you've got about what you need for 1 ATM on mars).
So, while this would by no means be easy or quick, it certainly seems possible - given a thousand years or so, and we'd want to be careful not to miscalculate and send a 100 KM radius object into a path where it might hit earth - as that would be a bad day, but, with care, this should be doable - so you have an object in orbit around mars that's gradually breaking up and it gives the planet an atmosphere and water.
Is there a way to turn this quantity of NH3 and CH4 as well as CO and NO and other things we maybe wouldn't want to breath, into N2 and CO2 and O2? again in a relatively short (1,000 years or so) span of time, or would that be even more difficult and take a lot longer than getting the object where we wanted in the first place?
In short, the question is, can icy space objects be used to create planetary atmosphere for terraforming? It's always seemed so, to me, but I'm curious if my assumption is incorrect.