Life support

It's not simple to keep humans alive outside their home planet!

Live support systems are an essential and necessary part of a Martian settlement, since the natural Martian environment does not allow human beings to survive. More information on this topic is included in How living on Mars will be different than living on Earth.

Requirements

• Water is the most important requirement. No water no life.
• Oxygen must be combined with nitrogen to create breathing air inside the habitat.
• CO₂ must be removed, ideally by plants.
• Food must be provided.
• The temperature inside the habitat must be comfortable.
• Waste must be treated and recycled.

NASA table of inputs to support a person in space (1977)

Input	Per day	Per year	1000 people
Dry food	0,6 kg	219	219 tons
Sanitary water	2,3 kg	840	840 tons
Water	1,8 kg	657 kg	657 tons
Oxygen	0,9 kg	329 kg	329 tons
Total	5,6 kg	Example	Example

The numbers are slightly different from other references. So we should provide for a certain margin of safety!

Oxygen consumption, oxygen/CO2 cycle

Humans breath about 11 000 liters of air per day of which 21%, or 2310 l is oxygen. Since we breathe out air at about 15% oxygen, 25% of the oxygen is consumed, or about 550 liters of oxygen per day. The density of oxygen is 1,4 g/l, or 1,4 kg/m3. So each crew member 'uses' 0,7 kg of oxygen per day. The oxygen is mostly used to burn hydrocarbons in cells, producing both water and CO2. The combustion of carbon creates CO2 according to the following equation: C + O2 = CO2 + 395 KJ/mole, or 9000 kJ/kg. 1000 people will consume 700 kg of oxygen per day and produce about 1 ton per day of CO2. 1000 people will breathe 11 000 m3 per day.

Losses compensation

Any settlement will leak and have systemic losses. This needs to be taken into account in the life support system design

Artificial life support systems

The life support system on the ISS is optimized to low weight. It must be fuelled with large amounts of energy and it requires a constant replenishment with consumables.

Near-natural life support systems

On Mars we will not be able to maintain a life support system that needs consumables. The alternative is a near-natural carbon cycle with the growing of food and producing oxygen at the same time.

The challenge is the day and night rhythm as well as the lasting dust storms. Oxygen can only produced with light, either sunlight or artificial light. During the night or during a dust storm there may not be enough sunlight, either for direct lighting for the plants or for photovoltaic powered electric lights. The Biosphere 2 experiment has shown significant deviations from good oxygen conditions due to the day and night rhythm.

The first Martian settlement will probably be much smaller than Biosphere 2, making it even harder to keep a constant oxygen level. This article discusses solutions to mitigate this issue. The larger the settlement, the more stable it will be, as it will have more inertia.

Buffering the gases

Big tanks of water may be used to buffer some amount of oxygen as well as CO₂, since those gases are soluble in water according to Henry's law. A good side effect is the heat capacity of the water, helping to keep a comfortable temperature in the settlement. Oxygen produced for propellant purposes will need to be stored, and this storage can be used to buffer the oxygen.

Constant artificial lighting

Parts of the oxygen producing greenhouses may be lit at night, which requires stored electrical energy. The stored energy must be large enough to cover a few months of dust storms, unless nuclear reactors are used. wind turbines may help to produce part of the needed energy.

Water

Water can be extracted from the atmosphere of from frozen deposits on the surface. Once it enters the life support system, water become part of the life process and ends up either in food, at the water treatment plant mixed up with waste, or in the atmosphere as a result of respiration. Water is also the source of propellant, so large amounts of water are required for the settlement. Water has very high specific heat, therefore it can hold a lot of heat. Water storage of energy is also a good source of stability and inertia in a settlement.

Respiration from humans and evapotranspiration from plants will load the atmosphere of a settlement with humidity. This water needs to be removed by condensation, or the humidity will become too high and the plants will no longer function, and the humans will be very uncomfortable. Desiccant systems with regeneration or cooling systems with condensation are two methods for dehumidifying the air in the settlement. The cold outside the colony can be used at a handy heat sink for dehumidification as well.

Energy requirements

The Carbon Cycle (simplified)

Energy is required to run the life support system. The source of the energy is either the sun, though direct radiation of photoelectric systems, or nuclear fuel, using nuclear reactors. The energy required to run the life support system is considerably less than the energy required to produce propellant or to support industry.

The larger the life support system, the more stable it is.

Humans operate at about 100W of power. This comes from food, that eventually comes from photosynthesis. Photosynthesis has an efficiency of about 3-4%, but most of the production from photosynthesis is used by the plant itself. About 1% of the energy from photosynthesis turns into food, so the life support system of a settlement will require about 10 000W of solar power per inhabitant.

The other 99% is turned into plant and eventually biomass (about 1-2%), and into heat, in the form of evaporated water, known at latent heat, and a hot air, called sensible heat.