Oxygen

O	8
Oxygen

Abundance: 0.15% atmosphere 46,6% crust

Oxygen (periodic table symbol: O⁸) is a chemical element that can be found in the atmosphere and in most minerals on Mars. Almost half of the mass of the Martian crust is Oxygen, bound up in various minerals. Oxygen is created in stars from the fusion of Carbon and Helium. Oxygen has 8 protons and the most common isotope is oxygen 16. O₁₇ and O₁₈ exist and are stable. Oxygen needs two electrons to fill its outer electron shell, which makes it a powerful oxidizer. (Only Fluorine is a more powerful oxidizer, but Fluorine is a fairly rare element.)

Alga producing oxygen

Relevance for life

The metabolism of human beings, animals and aerobic microbes depends on oxygen. The atmosphere of Mars contains only 0,15 % oxygen, which is not enough to support animal or human life. The minimum concentration required is about 15% oxygen, or 100 times more (depending on the atmospheric pressure)(reference needed).

• Oxygen is used by life within the Carbon cycle, where CO2 is consumed by plants through photosynthesis, liberating oxygen than can then be used by plants for other processes or by animals to support the combustion of carbohydrates, that liberates CO2, completing the cycle.
• Biological combustion also produces water, and therefore consumes oxygen that is removed from the biological cycle. But in tiny amounts compared to the other requirements that life has for water as a solvent and media for chemical reactions. However, Oxygen is eventually returned to the atmosphere through photodissociation of water with UV light in the upper atmosphere.

• If Oxygen becomes common in a future Mars' terraformed atmosphere, an Ozone layer will also build up 'naturally' which will protect the surface from Ultraviolet light.

Production

Oxygen can be produced in situ:

• in greenhouses by plants.
• in photobioreactors by algae.
• by reduction of carbon dioxide from the martian atmosphere, or carbonate minerals via either molten salt reduction or via MOXIE style solid oxide electrolysis.
• by reduction of oxide minerals, either at low temperature in aqueous solution or at high temperature in molten salts. This is required to free up metals from many ores.
• by electrolysis of water
• by the decomposition of perchlorates in the soil.
• by thermal decomposition of water through the Sulfur/Iodine or Zinc/Sulfur/Iodine cycles.

Electrolysis

Submarines are a fairly good analog for a Mars habitat, a sealed system that has to manufacture oxygen and remove carbon dioxide. How Submarines produce oxygen using electrolysis.

Two electrodes, positive and negative are charged using an electric current. Pure water is required because salt water will create chlorine gas which can be deadly if not adequately separated. On a submarine, reverse osmosis is used to filter out the salt, the same might be done on Mars. The pure water has potassium hydroxide (KOH) added as a catalyst to make the electrolysis more efficient. The hydrogen created in a submarine is exhausted from the boat, but would represent a vary valuable co-product on Mars.

Biosynthesis

Oxygen is produced by plants, algae and monocellular organisms as a byproduct of photosynthesis. Since food is required for the settlement inhabitants and oxygen is a natural by-product of food production, oxygen is inherent in the operation of a life support system that uses any form of photosynthesis in its operation, such as Melissa(reference) or greenhouses.

Metal production

Most metals are present on Mars as oxides. Processes that remove the oxygen from the metal are required to create metals for industry. On Earth, the most common methods use carbon and produce a metal + CO2. On Mars, since there are no known carbon reservoirs, the required carbon would need to be produced through processing of the CO2 atmosphere, that also liberates oxygen. Alternatively, hydrogen could be used with catalysts to create a metal + water +O2, in varying quantities depending on the process. Again, producing hydrogen from water frees up oxygen. So the metal processing industry on Mars is likely to generate excess oxygen, than can be stored or liberated into the Martian atmosphere.

Oxygen removal

Excess oxygen in the atmosphere is both a health and a fire risk. Excess oxygen can cause Hyperoxia.

Oxygen can be removed from the atmosphere by refrigeration, adsorption or combustion.

Refrigeration

Refrigeration is the classic way of producing oxygen on Earth, and can be used to reduce oxygen concentration in a Martian settlement. A stream of atmosphere is passed through a chiller, where is is cooled to its condensation temperature. the liquid oxygen is removed through a valve, and can be used in various processes for the settlement. This process also removes water, that can easily be added back.

Adsorption

Adsorption can selectively remove oxygen from the atmosphere using an appropriate media bed through which the oxygen rich atmosphere is passed. The media eventually saturates with oxygen and can be regenerated, generally using heat to free up the oxygen, and be reused indefinitely.

Combustion

Combustion is used on Earth in industrial processes to produce a non corrosive atmosphere of CO2 and nitrogen. The oxygen is burned with natural gas, the water is removed by condensation and the resulting CO2+nitrogen mix is relatively inert and can be used, for example, as an atmosphere for processes that involve explosive iron dust.

Gas monitoring

Monitoring the atmosphere in every compartment of a Martian habitat is required in order to ensure the proper distribution of oxygen and to measure the concentration of contaminants, as these can cause health or fire risks. CO2 is usually monitored as a proxy for oxygen, as the toxic concentration of CO2 is reached much faster than the lowest threshold for respiration.

The main gasses that should be monitored are: O₂, CO₂, H₂, N₂ and H₂O. Other gasses monitored should be: CO, refrigerants, if present and Volatile Organic carbons (VOCs). Regular calibration of gas monitoring systems need to be carried regularly, as all these sensors are susceptible to signal drift and contamination of the sensor elements. Redundancy of the sensors should be implemented when possible.

Handheld gas probes are often used in an industrial context on Earth, or in submarines. These need to be calibrated, and can be used in situations where permanent sensors are not available.

Gas concentrations in areas where there are plants and greenhouses are very dynamic. CO₂ concentrations can vary dramatically, especially in small habitats. Plants tend to overproduce oxygen over time, and therefore a method for removing excess oxygen from the atmosphere of a Martian settlement is required.

Emergency supplies

Emergency supplies or backup supplies in small vehicles is provided by burning a Chlorate candle (aka an Oxygen candle). Iron (Fe) and Sodium Chlorate (NaClO₃).
Fe + O₂ -> Fe₂O₂ + heat
heat + NaClO₃ -> NaCl + O₂

• Oxygen candles are used on the space station.
• Fires are classified in various ways. A Class Delta fire (self oxidizing fire), like an oxygen candle, cannot be extinguished, it has to burn out.

Uses for oxygen

• Propellant. Oxygen is often chosen as an oxidizer for chemical propulsion.
• Atmospheric component. The standard Earth atmosphere contains 21% Oxygen.
• Chemical reagent as an oxidizer.

References

Oxygen^lunarp on Lunarpedia.