Architecture, Mars

Atmosphere design for the surface of Mars

The Hab

The advantages of a normal Earthian atmosphere apply equally well to the Hab as they do to the ISS and other spacecraft. However, there are several compelling reasons for a reduced atmospheric pressure for the Hab:

  • Lower Hab mass.
  • Less air to be manufactured, which therefore reduces the mass and energy requirements of the ISAP system.
  • Potential for a zero prebreathe protocol for EHA/EVA.

The higher the atmospheric pressure of the Hab, the stronger and therefore heavier it will need to be. This is contrary to our requirement of reducing the mass of the Hab as much as possible because of the significant challenge of landing such a heavy object on Mars.

Atmospheres in early spacecraft had low total pressure, low oxygen pressure, or both. However, not all of these atmospheres would be suitable for an 1.5 year stay on Mars. Research conducted in recent years has more clearly defined the limits for artificial atmospheres suitable for extended exposure.

The minimum partial pressure of oxygen required to support human physiology is considered to be 16kPa. However, for long-duration space missions, a minimum partial pressure of oxygen of 18kPa is recommended  (Duffield, 2003). This is based on a previous study about planetary surface habitats (Campbell, 1991), which reviewed 33 different considerations related to atmospheric pressure and composition.

From a physiological perspective, an O2 pressure of 18kPa is perfectly safe. This is equivalent to about 1370m altitude (approximately the altitude of Kathmandu, Nepal), which does not even qualify as “high altitude” in mountain medicine (1500 – 3500m). Acclimatisation to reduced O2 pressure at altitude is characterised by an increase in pulse and breathing rate. Most people can ascend to 2400m (where O2 pressure is about 16kPa) without difficulty, however, altitude sickness may occur above this level. Astronauts can be conditioned for an O2 pressure of 18kPa by training in a hypobaric chamber, or at a moderate altitude (e.g. Black Mesa, US). In a microgravity environment there would already be increased strain on the cardiovascular system , and it would be preferable not to cause any further strain; however, the habitat is in a gravity environment on the surface of Mars, and although this is still a reduced gravity environment compared with Earth, the increased load on the heart will be mitigated.

The next design question is how much buffer gas to include. A pure oxygen atmosphere introduces an unacceptably high risk of fire, such as the one that occurred in the Apollo 1 Command Module. The upper limit of oxygen concentration with regard to fire safety has not clearly defined, but 30% is considered a reasonable upper limit (Campbell, 1991). This gives us a total atmospheric pressure of 60kPa, about 60% of Earth.

Buffer gas refers to the component of the atmosphere comprised of metabolically inert gases, which usually means nitrogen (N2), plus the noble gases helium (He), neon (Ne), argon (Ar), krypton (Kr) and xenon (Xe). The buffer gas portion of the atmosphere of Earth is almost entirely N2 (99%), with about 1% Ar and trace amounts of He, Ne and Kr. As described in the section on In Situ Air Production, because we’re making buffer gas in an economical way by simply using the Martian atmosphere with dust, CO2 and contaminants removed, our buffer gas on Mars will be about half-half N2 and Ar, possibly with trace amounts of Ne, Kr and Xe.

Nominal atmospheric concentrations of CO2 and H2O must also be determined. According to JSC 20584 (Spacecraft Maximum Allowable Concentrations for Airborne Contaminants), the maximum CO2 concentration is 0.7%. A CO2 concentration of 1% can cause drowsiness, with more serious symptoms occurring at higher concentrations. A typical concentration in normal spacecraft operations is 0.5%, which is a reasonable design goal. This gives us a CO2 partial pressure of about 0.3kPa.

With regard to water vapour, NASA specifies a RH (Relative Humidity) of 30-70%, i.e. an average of about 50%. Our target temperature is 295K (about 72°F or 22°C, which is optimal for human comfort and productivity), and the saturated water vapour pressure at this temperature and pressure is about 2.6kPa. Our average water vapour partial pressure will be 50% of this, or about 1.3kPa.

Any other gases present in the atmosphere should be present in trace amounts only.

Proposed design for Mars habitat atmosphere.

Gas Partial pressure (kPa)
Oxygen (O2) 18.0
Carbon dioxide (CO2) 0.3
Water vapour (H2O) 1.3
Buffer gas (N2/Ar) 40.4
Total 60.0

This atmosphere will produce differences in sound quality that the crew will be required to adapt to. The higher density of Ar compared with N2 will have the effect of lowering audio frequencies, including astronaut voices. Sound will also not be as loud or travel as far due to the reduced atmospheric pressure.

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ISRU, Mars

In Situ Food Production (ISFP)

Growing food on Mars is an exercise in efficiency. The facilities will not (at least initially) be available to grow every kind of fruit, vegetable, grain, nut and herb that we are used to. We may only be able to grow small amounts of a small number of crops. For a while there may be restrictions on the number of available ingredients and therefore the meal choices. The challenge is to find which crops deliver the most nutrition for the amount of volume, mass and energy required to produce them.

Top of this list would surely be spinach. Tomatoes, mushrooms, cabbages, garlic, kale, carrots would perhaps also make the list. What is the best way to grow each of these?
There are 3 primary mechanisms proposed for producing food on Mars:

  • In soil
  • Hydroponics
  • Aeroponics

Experiments are already underway to grow food in Martian soil simulant. Because all the chemical elements necessary for life are available in Martian soil, it should be possible to grow plants in it; however, it will be necessary to analyse the crops thus produced, in order to determine if they have healthful levels of vitamins and minerals. They will not necessarily have the exact same nutrient profile as their counterparts on Earth, because of the availability of those elements in the soil.

Some chemical processing of the soil may be necessary to prepare it for plant growth; for example, it may be too acidic or salty. Therefore the addition of a specially prepared fertiliser may be necessary. It may be beneficial to introduce worms to the soil and feed them with food scraps, so they can process the dirt grains and organic material together to make fertile soil. Of course, for this to happen we would initially need food scraps, which would have to come from somewhere, so this would not be an option for the first crops.

In any case, the first crops are more likely to be grown using hydroponics or aeroponics. These are similar setups in that the plants are grown in a dirt-free environment, fed with nutrient-rich water. In the case of hydroponics, the water flows through pipes in which sit the roots of the plants, so they can access the nutrients in the water. In the case of aeroponics, the plants are suspended, with their roots exposed to the air; nutrient-rich water is provided to the roots as a mist.

The primary advantage of aeroponics over hydroponics is that the water requirement is minimal, which will be important for a Mars base where water may be scarce in the early years. The disadvantage, however, is that the mist greatly increases the relative humidity of the greenhouse atmosphere. Relative humidity of controlled environments like spacecraft atmospheres should not exceed 70% (according to NASA guidelines), as this can interfere with electronics or cause build-up of mould. This could be addressed by separating the greenhouse environment from the main habitat environment by a gate.

A double-gate, whereby a person transitioning from the habitat to the greenhouse would open one gate, step through, close that gate behind them, open a second gate, step through and close that one behind them, may be an effective method of containing humidity in the greenhouse. However, this level of control is probably unnecessary, and a single gate will work fine if people don’t leave it open. The small amount of water vapour that would travel across from the greenhouse to the habitat would be easily soaked up by the habitat’s ECLSS.

One way to mitigate the migration of water vapour from the greenhouse into the habitat would be to place intake fans near the gate, which draw air form the region around the gate into the THC (Thermal and Humidity Control) subsystem of the habitat’s ECLSS, which will remove any surplus water vapour from the atmosphere.
Aeroponics may therefore be the preferred choice. The questions remain:

  • What crops would be good to commence experimentation with? (e.g. spinach and tomatoes)
  • What nutrients are added to the water provided to the system, if any?
  • What mass and volume of equipment is required to produce what mass, volume and nutrient value of food?

Another downside of aeroponics is that in some cases the crops will not grow as large as they would in a hydroponic system or if they were grown in soil. This is usually due to lack of available nutrients in the provided water. When using water enriched with sufficient nutrients, and with the proper equipment, aeroponic crops can grow to full size.

Therefore, if we are to experiment with an aeroponic system on Mars, it will be important to take with us a supply of nutrients optimised for aeroponic crop production.

Aquaponics combines hydroponics with fish-farming, and is another approach to food production that has been suggested for Mars. The fish can be fed food scraps, possibly supplemented with special food; the water in which the fish swim becomes nutrient-rich due to the metabolic outputs of the fish, and is then provided to the plants via the hydroponic system.

The main problem with aquaponics is the amount of water required, which is naturally much higher than for the other proposed methods. It is far more likely that Martians will be vegetarians, which is perfectly safe and healthy; many millions of people on Earth live healthfully on plants only. Producing food from plants is considerably more efficient in terms of energy and water, which will be crucial on Mars. In fact, it’s also crucial on Earth, but humanity is still learning this. Some scientists predict that by 2050 everyone (or perhaps the majority of people) on Earth will be vegetarian due to expansion of the population. In any case, if aquaponics is implemented on Mars it will probably not be for a few years, when water and energy production are much higher.