Carbon dioxide

Carbon dioxide (CO2) is a material which can be obtained via in situ resource utilization on Mars. It is the primary component of the Martian atmosphere. It is also a waste product of the respiration processes of living organisms, and a soluble gas.

Plants require carbon dioxide for life support, and their environment can be improved by slightly enriching their environment atmosphere in carbon dioxide relative to that of the crew quarters. Carbon dioxide is a human waste product, however, and any manned mars expedition must make provision to scrub the cabin atmosphere to remove excess carbon dioxide.

Carbon dioxide can be used to quench less energetic reactions, or as an oxidizer for energetic reactions such as the Sabatier process. It can be electrolyzed to produce oxygen and graphite. When dissolved in sufficient concentration in water, it forms carbonic acid, which can be reacted with hydroxides to form carbonates useful for soil reclamation. Liquid carbon dioxide is a suitable solvent for many purposes and can be used in place of water for clothes washing and other processes. Dry ice is a convenient refrigerant.

Storage of Carbon Dioxide

Small amounts of carbon dioxide can be stored as pressurized gas. Larger amounts can be stored at atmospheric pressure as a cryogenic solid, dry ice. Liquid carbon dioxide requires cryogenic or high pressure storage, but is easily produced on demand from dry ice. Although carbon dioxide can be produced from the action of aqueous acids on carbonate minerals to yield carbon dioxide, the ability to obtain carbon dioxide directly from the Martian atmosphere would make manufacture using potentially valuable carbonates economically undesirable.

Simple compression of carbon dioxide to 5.2 ATM pressure is sufficient to solidify it at room temperature. If allowed to sublime, it will quickly cool to below its freezing point due to evaporative cooling. It may then be stored at atmospheric pressure or even in vacuum.

Physical Properties

 Molecular Weight 44 g/mol Freezing Point -78.5 oC Latent Heat of Vaporization 571.08 J/g Latent Heat of Fusion 196.104 J/g Liquid Density 1032 g/L Solid Density 1562 g/L Gas Density 1.87 g/L @ 15 oC, 1 ATM 2.81 g/L @ Sublimation Point

The thermal conductivity of carbon dioxide at a given temperature can be estimated using the formula:

$J = -2.2403 \cdot 10^{-8} T^2 + 1.0208 \cdot 10^{-4} T - 0.01200$

where T is the temperature magnitude on the kelvin scale and the result units are W/Km.

The specific heat of carbon dioxide at a given temperature can be estimated using the formula:

$c_{p} = 215.80 + {156580 \over T} + {6.086 \cdot 10^{7} \over T^{2}}$

where the result units are J/kgK.

A similar equation exists for compressibility.

Crew Tolerance

While carbon dioxide is not inherently toxic, the human body can only withstand exposure to small concentrations and still maintain proper respiration. Excessive levels are uncomfortable, and levels in excess of 10% the ambient oxygen level can be lethal.

Tolerance Limit = 5 mb

Discomfort Level = 5 to 30 mb

Asphyxiation Danger = 30 to 70 mb

Lethal = 90 mb

CO2 Enrichment for Plant Growth

Many plants grow best at 10 to 50 mb CO2 partial pressure. This concentration can be maintained in a separate greenhouse, which can be accessed by the crew using oxygen masks to prevent discomfort. Plants are generally tolerant of a much broader range of CO2 partial pressures than people, however, and can also be sustained in the crew quarters environment without carbon dioxide enrichment.

If properly scrubbed and filtered first to remove carbon monoxide and dust, the Martian atmosphere is sufficiently rich in carbon dioxide that it can be used as a source of carbon dioxide for plant growth without further treatment.