Lunar Settlement Artificial Atmosphere

Introduction

A lunar colony that is economically viable, with agricultural production to feed the colonists, air recycling, industrial production to support housing construction, transportation and exports, will need both industrial process atmospheres and a human life support atmosphere. Since there will need to be artificial environments for agriculture and air recycling, these processes will be less differentiated from other industries than they are on Earth. Air recycling on Earth is nearly free. Only within the last century have industrial concerns about preserving a healthy atmosphere on Earth been widely recognized as important. On Luna the atmospheres for the various industrial processes will be a bigger concern than the atmosphere that supports the life of the colonists, even if life support is more urgently important. Vast amounts of industry per person will support the colonists. Some processes run best in reducing atmospheres. Others require oxidizing atmospheres, inert atmospheres, or vacuum.

A lunar habitat will require a pressurized mixture of gasses to sustain the life of the inhabitants. There is some experimental data with artificial atmospheres from space stations, the ISS and 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 dependence on the artificial life support. ESA is doing controlled ecological life support system research in Barcelona, Spain.. On the Moon, the atmosphere will be made in accordance with the architecture followed. Air Locks, will prevent loss of atmosphere to the outside and prevent the mixing of different atmospheres in different compartments. Ambient vacuum as part of sintered brick walls or regolith fines packed in walls will provide adequate insulation. Air conditioning will include cooling with radiators.

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. 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 , 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.

See Also

• Atmosphere
• Lunar Atmosphere
• Lunar Life Support Parameters
• Lunar Settlement