While oxygen recovery and reuse is well developed, the ISS does receive regular shipments of make-up oxygen. See for example this and this and this answer.

For an early, manned presence on Mars, a good source of make-up oxygen available early-on would be desirable. Losses from incomplete recovery, leakage, EVA suit venting(?), accidents, catastrophes, and others that I can't think of may not always be covered by oxygen supplies brought from Earth.

The three local sources of oxygen on Mars that I can think of are

  • atmospheric CO2;
  • sub-surface H2O (eventually a source for fuel for return to Earth);
  • ClO4- (perchlorate) in Martian regolith, order of magnitude of 1 part-per-thousand (see here and here).

Of these, which one is most likely to be used first as a source of breathable oxygen for make-up and emergency needs?

Answer should evaluate required electrical or thermal power, mass and reliability of equipment hauled from Earth, and challenges collecting the raw materials where these turn out to be limiting parameters.

  • $\begingroup$ Possible duplicate space.stackexchange.com/questions/5094/… ? $\endgroup$
    – mike
    Nov 14, 2017 at 7:31
  • $\begingroup$ @mike That question is specifically about an experiment for a rover to produce a small amount of oxygen as a demonstration. This one differs; I've asked for the answer to address "which is likely to be used first?" for the first astronauts living on the surface of Mars. Also, there's nothing there about the more recently proposed use of perchlorate. $\endgroup$
    – uhoh
    Nov 14, 2017 at 7:42
  • $\begingroup$ @uhoh I calculated very roughly that more than 100 kilos of Martian soil is needed each day for each person to get the necessary oxygen, so i think at least for the early presence on Mars perchlorate is not an suitable option. Although in the future sub-surface water could be cleaned with oxygen producing, perchlorate reducing microorganisms ! $\endgroup$
    – Cornelis
    Jun 9, 2018 at 15:54
  • 1
    $\begingroup$ @Conelisinspace I think it's fantastic the way you've dug so deep into this issue! Your link doesn't work for me, but this does: commons.wikimedia.org/wiki/… That's a beautiful plot and I like that most of the lower latitudes are single digit percents (water content of top 1 meter) because that sounds more reasonable. I think the J. P. Morgan of Mars will get rich shipping water from the ice near the poles to the people near the equator where it's sunny $\endgroup$
    – uhoh
    Jun 11, 2018 at 11:46
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    $\begingroup$ Read a bit about perchlorate candles, that's an interesting compound. When you burn it-- it releases more oxygen than is consumed? Seems crazy. $\endgroup$ Aug 23, 2018 at 14:24

3 Answers 3


The atmospheric CO2 is likely to be used first.

This is simply for reasons of availability.

Accessing underground water sources or harvesting perchlorate from the regolith would both require large amounts of infrastructure. Mining, drilling, and other material moving equipment, which amounts to a lot of launched mass just to get the raw materials before you even start the chemical processing.

They are also both likely to require careful site selection and surveying which might not be possible until you already have some presence on the ground. Some of this can be prospected from orbit, but if you land in the wrong spot (an area with no underground water or hard rock instead of regolith) and you won't have the resources you need to survive.

The atmospheric CO2 on the other hand is readily accessible anywhere on the planet and requires very little infrastructure to access.

The Mars 2020 rover plans to carry an experiment named MOXIE, that does this, extracting oxygen from the atmosphere.

  • $\begingroup$ Interesting bit about MOXIE. I wonder if they'll bring a balloon or something to "stockpile" some of it. Doubt there would be a point to it though. $\endgroup$ Aug 23, 2018 at 21:46
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    $\begingroup$ @MagicOctopusUrn, their website specs mention 10 grams per hour and a run time of 2 hours, so not a lot to store. Although doing the math that is more than 10 liters of gas at STP, which is more at Mars low pressure. But, I would guess they would just run it past an O2 sensor and vent it. $\endgroup$
    – Josh King
    Aug 23, 2018 at 21:59
  • $\begingroup$ Awwh... not even a latex balloon for the 2020's first birthday? Could invite Opportunity over to sing for it. Jokes aside, O2 sensor sounds like the way to go. $\endgroup$ Aug 23, 2018 at 22:09
  • $\begingroup$ This article was discussed a bit in chat recently. It's about a complete system to produce rocket fuel as well materials for human life support, but you might find it interesting. thespacereview.com/article/3484/1 I like your answer, it makes sense that since the atmosphere only requires a pump to harvest (rather than a mining infrastructure) it would certainly be the method of choice at least in the beginning. $\endgroup$
    – uhoh
    Aug 23, 2018 at 23:44

Looking at perchlorate and a few of those compounds have gotten me to this article on chemical oxygen generators.

A chlorate candle, or an oxygen candle, is a cylindrical chemical oxygen generator that contains a mix of sodium chlorate and iron powder, which when ignited smolders at about 600 °C (1,112 °F), producing sodium chloride, iron oxide, and at a fixed rate about 6.5 man-hours of oxygen per kilogram of the mixture.

They're actually in use in mines around the world for emergency oxygen in case of extreme emergencies that don't produce ignitable gasses (you don't want to use these in a flammable atmosphere). They're also the drop-down source of oxygen in airplanes!

Commercial aircraft provide emergency oxygen to passengers to protect them from drops in cabin pressure. Chemical oxygen generators are not used for the cockpit crew, who are typically supplied using compressed oxygen canisters also known as oxygen bottles.

How these canisters are produced, and the feasibility of mining/producing them on Mars-- I don't have any idea about, however-- but it sounds promising for stockpiling:

The mixture has an indefinite shelf life if stored properly: candles have been stored for 20 years without decreased oxygen output. Thermal decomposition releases the oxygen. The burning iron supplies the heat. The candle must be wrapped in thermal insulation to maintain the reaction temperature and to protect surrounding equipment.


  • 3
    $\begingroup$ There are safety issues - one of these candles started a fire on Mir that nearly killed the crew. io9.gizmodo.com/5978305/… $\endgroup$ Aug 23, 2018 at 16:51
  • 2
    $\begingroup$ @OrganicMarble I could see that, however I'm guessing a fire planet-side may be a little bit preferred over a space station fire. Though the end result would likely be the same, at least on a surface base they probably have more resources that could extinguish it, or at least be able to only use these generators in an isolated chamber apart from the rest of the base, only venting the gasses. $\endgroup$ Aug 23, 2018 at 17:04
  • $\begingroup$ @OrganicMarble speaking of-- isn't Mars' atmosphere mostly CO2? Wouldn't Mars' atmosphere automatically extinguish the fire as soon as the depressurization of the isolated chamber occurred? Once again my chemistry isn't very good, but I know at minimum CO2 can be used to extinguish/starve a flame when oxidizer runs out. $\endgroup$ Aug 23, 2018 at 17:16
  • 1
    $\begingroup$ I am not sure if these candles need anything from the ambient to burn or not. For the Mir fire, depressurizing the compartment with the crew in it to put out the fire would not have been a good option - as we used to say, "committing suicide to keep from getting killed". $\endgroup$ Aug 23, 2018 at 17:30
  • $\begingroup$ @OrganicMarble man... no wonder they require low ambient heart rates... I would've probably panicked and done just that! Calm, cool and collected seems to be a requirement for space travel. $\endgroup$ Aug 23, 2018 at 17:38

I am currently researching a possible application of algae to do this on mars, along with making food and bio-plastic from it. This is actually fairly easy to do given that there's water available on mars. it would be possible to have a tank half filled with dilute urine and algae. Algae are known to efficiently process light and CO2 into food for themselves and oxygen. By bubbling the CO2 through the water, the algae would be able to absorb it and produce oxygen. The amount of power required is close to none, the reliability of algae is extremely high, as they are capable of withstanding gamma rays, which are abundant on Mars's surface. The tank that holds the water and algae could just be a clear, inflatable, rubber balloon that can withstand the pressure difference.

Links: Algae absorbing gamma rays, Feasibility of growing algae in space

  • $\begingroup$ Seems like a really cool study! Do you have sources on claims :)? You can use in-line links like this [text for link](actual link). Things I would source: viability of culminating algae in a controlled environment on Mars, the process of bubbling the CO2 through the water, and having the algae be able to absorb it/produce oxygen and a few other things. Sourced answers are more readily upvoted, and it gives us additional reading! $\endgroup$ Aug 23, 2018 at 14:21
  • $\begingroup$ @MagicOctopusUrn will do $\endgroup$ Aug 23, 2018 at 14:26
  • $\begingroup$ Not sure I'm reading your first link right, but it looks like it's talking about algae surviving gamma radiation (albeit with reduced growth rates), not algae photosynthesizing with it. $\endgroup$ Aug 23, 2018 at 18:21
  • $\begingroup$ @NathanTuggy Sorry, that must be the wrong link, i'll find the right one soon $\endgroup$ Aug 23, 2018 at 18:34
  • $\begingroup$ @NathanTuggy I can't find the link that I used to have, oh well $\endgroup$ Aug 23, 2018 at 18:41

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