Green Mars, pp.135, 177, 251-253

One problem with schemes for the terraforming of Mars is the need for a source of an inert gas to give an atmosphere of similar composition to Earth. A higher partial pressure of oxygen than we have here would ‘vigorously accelerate combustion’, tend to intoxicate us, and make technological civilisation impossible, while a much lower total atmospheric pressure would not be able to support Earth-like weather. The partial pressure of oxygen required to support combustion declines with declining total pressure, so even this lower total atmospheric pressure option might be dangerously combustible if it had enough oxygen present to support life. So, where is this inert gas to come from?

Getting nitrogen from Titan won’t stand up to any sort of cost-benefit analysis.

There are unlikely to be enough noble gases trapped deep underground from radioactive decay to amount to a hill of beans.

Thus, it has been suggested that molecular nitrogen could be obtained by ‘burning nitrates’, which seem to be present (or may be present) in considerable amounts in the Martian crust.

However, simply heating nitrates will not be very effective as a way to ‘dilute’ oxygen. For instance:

2Na(NO₃)₂ + heat → 2NaNO₂ + O₂

2NaNO₂ + more heat → Na₂O + NO + NO₂

2Ca(NO₃)₂ + heat → 2CaO + O₂ + 4NO₂

Thus ‘burning’ nitrates generates lots of toxic gas, and some extra oxygen.

Of course, with more energy input:

2NO₂ + much more heat → N₂ + 2O₂

So we have got some nitrogen eventually, but at a cost of 2.5 oxygen molecules per nitrogen molecule.

If we don’t want to add lots of extra oxygen to the atmosphere, we will have to add a reducing agent instead, and do something more like ‘burning’ . On Earth, if we had lots of extra nitrogen dioxide we wanted to get rid of, we would do something like:

NO₂ + 2H₂ → N₂ + 2H₂O

Or

NO₂ + C → N₂ + CO₂

Or more realistically, something like

4NO₂ + C₃H₈ → 3CO₂ + 4H₂O + 2N₂

The problem is that there is not a lot of carbon or hydrogen on Mars that is not already incorporated in carbon dioxide or water. I haven’t googled to find out how much hydrogen has been located/postulated on Mars, but a crude atom balance suggests that if we want to burn nitrates with enough hydrogen to generate one nitrogen atmosphere, we need to burn at least two whole hydrogen atmospheres. I don’t think this is available, it would surely have outgassed long ago. I have found references to methane clathrates on Mars, which may be there in similar amounts to the nitrates (perhaps) and would allow the reaction

2NO₂ + CH₄ → CO₂ + 2H₂O + N₂

The problem here is that it would be a very significant bit of geo-engineering to mine the methane and get it to the nitrates, or vice versa, and we are adding to the carbon dioxide load that we need to get rid of later.

So what other reducing agents are available? I suggest that much more cost-effective than bringing nitrogen from Titan would be to bring down some Iron-Nickel asteroids and rust them in nitrogen dioxide. The mass that would be transported would be much larger, but the distance would be much shorter, and there would be no need to do any complicated collection and packaging and transport, just provide the right nudge of energy to send the asteroid on a collision course with Mars.

The following reaction is certainly thermodynamically favourable, though I don’t have an idea of what its activation energy might be:

6NO₂ + 8Fe → 3N₂ + 4Fe₂O₃

While this uses considerably more mass of reductant to produce the same amount of nitrogen than methane would, instead of having to be painstakingly mined and collected like the methane, the asteroids could be crashed down into the nitrate deposits in one foul swoop. The reaction does not produce any extra carbon dioxide that will need to be scrubbed out later.