Lunar Settlement Artificial Atmosphere
Introduction
A lunar habitat will require a pressurized mixture of gasses to sustain the life of the inhabitants. There is some experimental data with some artificial atmospheres, mostly from space stations like ISS or the MIR. These stations have been manned by human crew for extended periods of time. However, projected lunar habitats will manned for longer time frames, and more experimentation will be required on the effects of long term exposure. On the Moon, the atmosphere will be made in accordance with the architecture followed. Some variables include the use of Air Locks, the thickness and materials of the walls, and other variables.
Pressure/Gas Combination
Exact choice of a habitats internal pressure and gas combination will have various effects on the inhabitants and the structure itself. Pressure is expected to be used to support the lunar structure under normal operation, though the level required for this is low enough that virtually any livable level will be more than enough (see Roof Support). As such, the higher the habitat pressure, the stronger a habitat must be constructed to withstand it. In addition, a high pressure interior will loose atmosphere faster than a low pressure one if a leak is developed. The choice of gas combination will greatly effect the achievable pressures.
Since Oxygen is expected to be a major by-product of manufacturing activities on the moon, a pure oxygen atmosphere is attractive as it is likely to be the easiest (and hence, cheapest) gas to procure on the moon. To avoid fires, the partial pressure of oxygen in the habitat must be kept to earth like levels (no more than 21 kPa / 3 psi). This low pressure is advantageous, and the lack of any filler gasses would greatly simplify atmospheric processing equipment. For these reasons, a pure oxygen atmosphere was utilized in the Gemini project, early designs of the Apollo spacecraft, and is currently used in spacesuits. However, some studies suggest that a pure oxygen atmosphere becomes poisonous to the inhabitants on long term exposure, making it unsuitable for a lunar habitat.[1] However, as mentioned previously, a pure oxygen environment is useful for space suits, as pressure must be kept low in order for the suit to stay pliable. Using a pure oxygen environment for space suits and a high pressure mixed environment for the habitat would require a period of breathing pure oxygen before donning the suit in order to remove all nitrogen from the blood and avoid decompression sickness (also known the bends). This is currently practiced on the ISS.
A combination of one or more inert gases with Oxygen would allow proper oxygenation over longer time-frames. Examples include nitrogen, helium, and argon, all of which are present in the lunar regolith and can be extracted by heating (see volatiles. Nitrogen is attractive as it would allow for an earthlike mix, though the low availability of nitrogen in lunar soil (compared to other volatiles) could raise difficulties in this regard. Argon, even less abundant than nitrogen, would also have this problem. Helium could also be added to the oxygen mix, as it is more abundant in lunar soil, and its low density means significantly less mass is required for a given volume. However, the addition of any appreciable quantities of helium to the atmosphere would result in a higher vocal pitch for those persons breathing it, similar to (though less intense than) the effects of inhaling pure helium. This effect is currently seen on earth in very deep diving operations, where helium-oxygen mixes are utilized, sometimes facilitating the need for a digital voice alteration device.
Another factor to consider with any combination is the direct effects of pressure. The most notable effect is in the boiling point of water, which would decrease with decreasing pressure.
| Total Pressure | Boiling Point Of Water | Comments | ||
|---|---|---|---|---|
| kPa | psi | °C | °F | |
| 101.3 | 14.2 | 100 | 212 | Sea Level, ISS |
| 84 | 12.17 | 95 | 203 | Denver, a high altitude city |
| 81.4 | 11.74 | 94 | 201 | Mexico City, a high altitude city |
| 74.0 | 10.2 | 92 | 197 | Open airplane, ISS ports |
| 59.1 | 8.3 | 86 | 187 | ISS spacesuit |
| 33.5 | 4.7 | 72 | 162 | Apollo spacesuit |
| 30.6 | 4.3 | 70 | 158 | Shuttle spacesuit |
| 26.0 | 3.65 | 66 | 152 | Top of Mount Everest |
The main problem this would pose would be cooking of food, as lower boiling points would make many foods impractical to cook. However, these foods could still be prepared using a pressure cooker. Many hikers utilize lightweight pressure cookers for this purpose when climbing to high altitudes, and adapting such devices to lunar use should not be difficult.
Agriculture
Plants [2], like humans, are capable of tolerating lower atmospheric pressure so long as all required gasses are available. Some studies indicate that plants can survive at pressures lower than any human can. Whatever the pressure, an environment optimized for plant growth would benefit from having an atmosphere enriched with carbon dioxide, as they would grow faster. This enrichment is used in some terrestrial greenhouses for both growth boosting and pest control, as plants will tolerate co2 levels that will kill insects.
