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I recently read about a machine called MOXIE that synthesised oxygen from carbon dioxide on Mars. I haven't been able to get the exact idea of how the thing works(though I've noticed analogies to trees). So my question is, why can't we use this machine on Earth? We know the crisis global warming is causing, and such a machine might be just what we need to tackle climate change. But I strangely see that NASA has plans to use it for astronauts only. Why?

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  1. By thermodynamics, to produce O2 and C from CO2 it will require more energy than what was delivered from producing the same quantity of CO2 by burning O2 and C1.

  2. It produces oxygen at a rate that is insignificant at planetary scales.

The idea will be to provide oxygen for a small habitat holding a few astronauts, not to change the composition of Mars atmosphera, using energy obtained from sources other than burning fuel (solar, RTGs....).

The utility for the NASA would be if the mass of the machine plus its energy sources is less than that of the oxygen that they would have to bring from Earth if they did not have this machine.

On Earth the production would be irrelevant in the scope of global climate change.

And if you powered it with a coal/gas/oil generator you would end even worse that if you had not done anything (and that is without considering the ecological footprint -CO2 and other- of building it).

If you powered it with solar/wind/sea energy... well, you would end better by using that solar/wind/sea energy directly into the grid to reduce the amount of gas/oil/coal burnt to produce energy globally.

From the energy POV this could make sense after all of the energy on Earth is produced from carbon-free sources, and even in that case the problems of scale continue.

Right now, the options we have are to rationalize the use of energy and to opt for carbon-free sources.

1There are some theoretical nitpicks about the law applying to reversed chemical reactions (if the MOXIE produces the C in a form other than the original the law would not apply). But in practice the rule holds.

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    $\begingroup$ And, of course, if you want to produce O$_{2}$ from CO$_{2}$ on Earth using solar energy, you can already do it with plants... $\endgroup$ May 21, 2021 at 11:54
  • $\begingroup$ @PeterErwin But note that plants are not very efficient, capturing at best about 1% of solar energy. In addition, land plants require hundreds of kilograms of water per kilogram of carbon dioxide reduced. $\endgroup$
    – John Doty
    May 21, 2021 at 21:45
  • $\begingroup$ @PeterErwin You can do that on Mars too if you've got potatoes. :) $\endgroup$
    – Graham
    May 21, 2021 at 21:57

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