Carbon dioxide

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Carbon dioxyde molecule

Carbon dioxide[1] (chemical formula: CO2) is a chemical substance that occupies about 96 % of the Martian atmosphere.

Molar Mass of 12(C)+32(O2)=44

Biological significance

The metabolism of human beings, animals and various microbes depends on the oxidation of carbohydrates, resulting in carbon dioxide and water exhalation. Plants use the carbon from carbon dioxide to produce carbohydrates and release the oxygen back to the atmosphere, completing the cycle.

The reaction is: CO2(carbon dioxide) + 2H2O (water) + photons (light energy) → C(n)H2O(m) (carbohydrate) + O2(oxygen)+ H2O (water)[2]

Humans require that carbon dioxide levels be low in the air, so that CO2 in the blood can diffuse outwards into the air in the lungs. There can be plenty of oxygen in the air, but if CO2 goes much over 0.5% of the air pressure, then flow of CO2 out of the body is slowed. OSHA (Occupational Safety & Health Administration) has established a permissible Exposure Limit of 5,000 parts per million (ppm) or 0.5% of CO2 in the air, averaged over an 8 hour work day. At 10,000 ppm (1.0%) CO2 may cause drowsiness. At 4% to 5% it is immediately dangerous to life or health (gasping for breath, dizziness, confusion, headaches, shortness of breath). At 80,000 ppm (8% of the air pressure) it will cause unconsciousness and then death. These responses vary by individual (depending on how healthy they are), and on the length of the exposure. The treatment for too high CO2 concentration is to immediately move the subject to an area with low CO2, to allow the dangerous CO2 level in the blood to drop.

Settlement atmosphere

Colony CO2 treatment.png

Carbon dioxide is required in the settlement atmosphere for plant metabolism. Standard concentration on Earth is increasing, so the value is a moving target. However, a concentration between 300 ppm (0,03%) and 1,000 ppm (0,1%) is considered acceptable[3]. Nuclear submarines have varying carbon dioxide levels that can reach 9000 ppm in normal operations, but average between 3-4000ppm[4]. A CO2 enriched environment may be beneficial for the growth of plants in greenhouses or photobioreactors.

The Sabatier process can be used in place of photosynthesis to complete the atmospheric part of the carbon cycle. Methanotrophic synthesis of carbohydrates from methane would be required to complete the carbon metabolic cycle without the use of plants. Or food can be supplied from Earth or Mars for a partial cycle, where Methane from the Sabatier process can be stored for use as a propellant.

In situ Production

CO2 will be extracted in-situ by atmospheric processing, or from carbonate rocks to provide larger industrial quantities to feed industry[5] Carbonate rocks are common on Earth, but seem to be rarer on Mars. It is possible that Mars has more carbonate rock formations which are buried.

It is very likely that large amounts of CO2 will be found in local clays which will out-gas if the clay is warmed. (Some clays can hold 9% of their mass in CO2 when cold.)

Notes on Atmosphere

CO2 forms clouds in the Martian atmosphere, which is rare. (Most planetary atmospheres do not form clouds of its primary constituent.) Mars is so cold that CO2 freezes out in the winter, causing the planet's air pressure to fall by as much as 30%. See Atmosphere for more details.

CO2 is a Greenhouse gas and if more carbon dioxide could be outgassed from the crust somehow, the planet would warm. See Terraforming for more details.


Concentration of CO2 on Earth is was 275 parts per million (ppm) in pre-industrial times. Currently it has risen above 400 ppm. Increasing concentration improves plant production rates, however the effect is non linear and reaches a peak of 20% improvement in yields at about 1,200 ppm.[6] This may also be affected by pressure, if low pressure greenhouse are developed for Mars.

Effect of CO2 concentration on plant production