Greenhouse

The Eden Project] (near St Austell, Cornwall, UK) is a terrestrial example of the possible use of large biomes as greenhouses and life support for Mars colonies. Image credit: Jürgen Matern

Greenhouses and grow rooms are types of agricultural facilities. Growing plants in a Greenhouse delivers oxygen, biomass and food. It can play an important part in human recreation (Mars Garden) and may be the place for funerals. The sunlight is not bright enough on Mars to allow most terrestrial plants to thrive, but it can provide a valuable part of the light energy required for plants. Additional energy is necessary for lighting and heating to get higher yields. Food production facilities may include biological reactors for bulk protein and carbohydrates production, sidestepping plant production altogether. Grow rooms, with entirely artificial lighting conditions, are also an alternative.

The Tropical Islands (Germany) is a terrestrial example of a huge dome to create an inhouse habitat.

The greenhouse must be constructed from transparent material, allowing maximum sunlight to pass, generating an artificial "greenhouse effect". This effect may be enhanced by filling the greenhouse with potent greenhouse gasses such as sulfur hexafluoride. (very doubtful) The spectral properties of the transparent material should be optimized to match the absorption characteristics of chlorophyll, maximizing the energy gain, possibly using a layer of quantum dots (Speculative).

Plants need a mix of air pressure and temperature. The greenhouse must be strong enough to hold that air pressure, and it must be insulated to hold the interior temperature. Photosynthesis works best at fairly high temperatures, 20°C and more.

Greenhouse construction and conditions

The strain on the greenhouse structure from internal air pressure will make greenhouses on Mars very different from greenhouses on Earth. The intense temperature losses during the night will also impact greenhouse design. Greenhouses on Mars need to be pressure vessels, with pressures that might reach up to 10 tonnes per m2 (101 kPa, Earth standard air pressure) while greenhouses on Earth just need to survive wind loads and their own weight. Greenhouses operating at lower atmospheric pressure with higher oxygen proportions are possible, but the impact of the lower air density on heat transfer and the efficiency of evapotranspiration, and by analogy on the yields, has yet to be tested on a significant scale. Experiments have shown that higher plants need about 0.5 bar (7.2 psi or 1/2 of an atmosphere). Higher plants (non tropical species) can grow at 10C, though 15C is better. (reference needed)

It may be that the ultimate structural requirements are such that the gains from using natural lighting from the sun are outweighed by the costs of construction. If this is the case, agriculture on Mars will still be possible in underground grow rooms, using artificial lighting.

For underground grow rooms, walls need to be water resistant ans insulated to prevent condensation. A reflectance of 80% for the surface materials is recommended.

Greenhouse conditions

	Grow room	Tropical	Temperate	Minimum	Living area
Temperature (C)					22-25
Pressure (kPa)					103
Relative Humidity	50-80%				20-50%
CO2 (ppm)					300

Side-lit Greenhouse Concept

The Mars Foundation concept for a side-lit greenhouse.

The Mars Foundation concept for a greenhouse involves the maximum use of local materials to avoid waste, maximize energy input and optimize space. Spawned from the Hillside settlement design, the greenhouse would most likely be located inside/next to a hill side (possibly in the location of Candor Chasma). Therefore regolith or some other absorbent material could be suspended above the greenhouse to protect occupants and plants from harmful radiation. The source of light would therefore be directed from the side, via an array of adjustable mirrors. A system of vents and ducts would allow warm air to circulate, perhaps even used to heat the main habitat.

A significant part of sunlight on Mars is diffuse and cannot be reflected by a mirror, so the number of mirrors required must take this into account. The mirrors add to the cost of the installation, and this cost must be compared to using energy production and artificial lighting. In this concept, radiative heat losses may be significantly lower than for an 'open sky' greenhouse, as the side-lit greenhouse is not exposed to the sky at night, and the radiative environmental temperature is therefore higher.

Underground Greenhouse Concept (Grow room)

Underground Greenhouse Concept

If geothermal energy or nuclear power is not available the heating will consume large amounts of electrical energy. In this case the sum of energy used for lighting and heating must be considered. An underground greenhouse is easier to insulate to hold warmth inside. On the other hand the effort of lighting is higher, since no direct sunlight is used. This concept has some additional advantages: It is meteorite-safe and radiation-safe.

Natural caves and artificial caves can be utilized to build such an underground greenhouse, which requires a preparation with high effort in either case. The maintenance may be less that for a surface greenhouse, for the ambient temperatures are steady and the radiation levels are low. A combination of greenhouse and living space for the settlers might be possible, but high productivity greenhouses will be warm, humid, rich in CO2 and with very high lighting levels. These are not necessarily the best conditions for humans, so grow rooms will probably be build as separate environments from the general habitat.

The Mars One concept for the initial settlement was an inflatable greenhouse with a thick cover of regolith.

Artificial lighting equivalent to the average illumination level on Earth is 600 W/m2 and more. With an insulated grow room, this energy must be removed from the grow room to avoid overheating. However, the lighting is required for proper growth. Therefore a grow room will need to be cooled using mechanical cooling, as is the case on Earth. Plant also evaporate a large quantity of water by evapotranspiration for their normal growth. The water must be removed by cooling or some other method from the air, as saturated air will no longer allow evapotranspiration and the plants will not survive. Compression work required to cool the air and remove the water is usually in the order or 50 to 80% of the light load to the area, as compressors operate in low efficiency ranges for grow rooms. So beyond the 600W/*m2 of lighting at an efficiency of about 80% for LEDs, up to another 4-500 W/m2 may be required to power the cooling of the grow rooms for a total peak load of 12-1500 W/m2. Alternatively, water cooled in the Martian regolith or through surface radiators might be used to cool the grow rooms without the added energy of compression.

Underground greenhouses will require a vast amount of power, see the calculations in the "Nutrition & Energy Calculations" section below.

Water-shield Greenhouse Concept

Water-shield Greenhouse Concept

Hydrogen does a good job absorbing cosmic radiation. Water contains highly concentrated hydrogen, and hence serves as a good radiation shield. On the other hand it is highly transparent for visible light and UV. The combination of both makes it an interesting material for greenhouse shielding. The spectrum of the light that gets through the water needs to be evaluated.

Under a strong pressure resistant housing the water is placed in a thick layer. It absorbs the dangerous parts of cosmic radiation and sunlight and passes most of the spectral parts needed by humans and plants. Additionally, it helps to buffer daily temperature variations because of its high specific heat capacity.

The layering could be as follows: The outer layer is a construction of steel and glass, providing enough strength for the difference in atmospheric pressure. It also serves as insulation for temperature differences. Additional sheets of glass or plastics improve the insulation effect. A self-healing puncture protection should be considered. The innermost layer is the water. It can be held by transparent canisters.

Multiplying Sunlight

Multiple Mirrors for Greenhouse

A solar concentrator‎ is a set of mirrors that can be used to bring more sunlight into the greenhouse than the base area of the greenhouse receives directly from the sun. Three times the amount of Martian sunlight should be enough to serve terrestrial plants. During good weather periods this allows growing vegetables without additional energy.

However, up to 40% or more of Martian light is diffuse light. This light cannot be focused by a mirror and therefore the surface required will be up to six times the surface area of the greenhouse, if this solution is adopted.

Flora and fauna

Plants can be grown either in liquid fertilizer (hydroponics) or in soil. Many plants live in symbiosis with microbes and insects. Bees and other insects can be used to pollinate the blossoms for fruit plants. The growth of flora and fauna under the low Martian gravity bears some uncertainties. the problem that Biosphere 2 had with ants are a good example of the problems that can happen in greenhouses and artificial habitats. (citation needed).

Maintenance

Greenhouses require significant amounts of work: planting, watering, in some cases replanting and moving seedlings into sufficiently wide areas. Contamination by other plants is also likely, reducing yields and adding plant control requirements. Soils require nutrients and aeration. compost requires both space and manutention of the dead plant matter and biological wastes added to it. The greenhouse structure itself will be attacked by humidify, fatigue form temperature cycles and UV damage from the sun. The typical polythene greenhouses sheets on Earth require replacement every decade or so.

The number of workers required depends on automation levels and the production levels. The higher the production levels the more controls and intervention are required. Depending on the crop, personnel may vary from 1 (or less) per hectare for grains to 10 per hectare for tomatoes, or more for very intense grow rooms.

Although plants may grow with lower pressure and higher CO2 levels, workers will have difficulties in these conditions. Due to the many manipulations required for plant production, it may be difficult to use different pressure levels than for the rest of the habitats. CO2 and humidity may be significantly higher, however.

Minimum Light levels for house plants or during dust storms

Plants require a minimum of radiant power for survival and growth, and a temperature above freezing. Plants require at leart 20-25C for maintenance:

0,75 W/m2 and a preferred level of 3 W/m2 for 8-12 hours per day for survival.

3 W/m2 and a preferred level of 9 W/m2 for 8-12 hours per day for maintenance.

9 W/m2 and a preferred level of 24 W/m2 for 8-12 hours per day for propagation.

Nutrition and Energy Calculations

Based upon the figures in the food and sunlight articles the following calculations can be carried out for an artificially lit greenhouse:

The minimum size of cropland per person is about 365 m² for a plant based diet. The needed light energy can be assumed with 1000 kWh per m² and year. The result is an annual amount of 365 MWh per person. In other words: An average illumination power of 42 kW per person is required.

The usage of fluorescent lamps with an efficiency factor of 30% results in a requirement of about 140 kW per person in electrical energy. The overall efficiency of food production with artificially lit greenhouses is less then 1 permille, or in other words, to produce food with a content of 1 kWh the amount of more than 1 MWh in electricity must be spent. Progress in LED lighting will increase the overall efficiency, and some LED systems already reach 60% efficiency.

Assuming 60% efficient LED's and assuming solar cells with an efficiency of 15% (locally produced cells and ignoring dust covering them), and noting that solar power is weaker on Mars (further from the sun) we get:

--- Solar flux at Mars: 590 W/m^2. 15% solar cells = 88.5 W/m^2. [(42 kW per person (for a plant based diet) / 60% (efficient LED lights)] / 88.5 W/m^2 = 791 square meters per person. Along with various inefficiencies this will likely be closer to 800 m^2/person assuming the cells are dust free and that there is no dust storm. (This is a bit over 1/6 th of an American football field. So each football field of solar cells, including end zones, can feed 6 people with a bit of margin.)

So each person will require about 800 square meters of solar cells to provide food for them. Cleaning these cells of dust may require going outside in space suits (which is laborious and dangerous) or automation, (which is costly). During thick dust storms, this area would increase tenfold. (During the large dust storm of 2018, Opportunity rover's solar cells went from over 300 Wh to 29 Wh, so a 90% reduction in power is a historical reality.) Of course, we could avoid food production altogether, as during winter periods in northern climates on Earth. In that case, we need MORE than 800 m^2 per person, (say 850 meters squared per person), to build up a surplus for the cold dark weeks of dust storms, and provide a buffer.

The above calculation assumes average light all year long. If we consider summer and winter months, we will want to have more solar cells for the summer to build up a surplus for the winter. If we assume people go out into a vacuum to reposition solar cells, (say a monthly adjustment so that they are always at the right angle to directly face the sun), then perhaps a 33% increase would be sufficient. This would bring us to 1.05K to 1.2K m^2 of solar cells per person.

In order to reduce this vast power requirement needed by greenhouses, part of the required light could be provided by direct or indirect sunlight. This would be a huge help, but we would wish to have emergency lights for when there are thick dust storms, or to help in the winter months.

If underground, heat loss is likely to be slow (in fact cooling would be likely needed) requiring more energy to move heat around.

If on the surface, and if the greenhouses are spread out or poorly insulated against the cold ground, heating the greenhouse may require additional energy. Energy is also required to offset night losses and maintain a minimum temperature. If stacked vertically for minimum volume usage, the energy added for lighting may cause the space to require active cooling. Although tall greenhouses may have a larger surface area which would speed cooling. Careful design may cause these two problems to cancel out.

Note that on Earth, thin plastic walls of greenhouses stop 15% of the sunlight. On Mars the much thicker pressure vessels will likely stop more. However, we get so much energy from the sun, that this loss will likely be worth it.

Note that Smart Windows may be a huge help for Greenhouses on the surface.

All this suggests that if underground greenhouses are planned, then nuclear power might be required. Alternately or some form of energy storage which can hold a surplus for the long winter months might be possible. However, if we have extra solar cells, and more growing space, this extra energy can be stored in the form of food crops in root cellars. (So you have enough surplus energy to grow more than enough food in summer, and then you can store the energy rich food for the ~11 Earth month long winter.)

Wind and air movement

Most plants need air circulation over the leaves to improve their evapotranspiration. Plants, in particular genetically modified plants with large seeds or fruit, also need to be stirred using air circulation to increase the strength of their stalks. Mars with its lower gravity might allow for weaker stalks. Heat transfer in a greenhouse depends on air movement as well, as much of the heat is removed through convection, and the humid air must be moved over a cooling system to remove the humidity. Practically all greenhouses on Earth have fans for extra air circulation and these will also be required on Mars. All grow rooms on Earth are air conditioned. Excessive air speed, however, can be detrimental to growth. The recommended average velocity is 0,5 m/s.

Dust storms

Greenhouses will be affected by dust storms. For greenhouses with reflectors and no artificial lighting a large dust storm will be catastrophic, possibly reducing lighting levels bellow the survival level of the plants. Dust storms which block 90% of the sunlight occur irregularly, and Mars gets less sunlight than Earth. If the light on Mars is halved, then low light crops can barely survive. More than that and artificial lights are needed. During dense dust storms (which can last up to 1 to 2 months) power may be diverted from industry to lighting & heating the greenhouses. Alternatively, the greenhouses may be shut down to reduce energy demand if sufficient food is stored to last this period. As Mars has a significant axial tilt, greenhouses at higher latitudes will probably shut down for winter anyway. (Providing the summer months can create enough surplus to last the 11 month winters.)

Greenhouse development on Mars

For early missions, enough food will be sent to avoid needing greenhouses. However, for future growth of the Martian settlement local food production is an eventual requirement. Therefore, some early experimental greenhouses will be built. They may be used to provide a more varied diet.

Radiation damage

Most crops are fairly resistant to radiation damage. In addition, many crops are by their very nature regularly harvested and radiation damage does not significantly accumulate. Damage to fruit trees may be more important.

List of Shadow Tolerant Vegetables

• Arugula (likes cool temperatures)
• Asparagus
• Beets (taste better in cool climates)
• Bok Choi
• Broccoli
• Brussel Sprouts
• Cabbage
• Carrots
• Cauliflower
• Celery (likes cooler climates)
• Chard
• Chinese Cabbage (will not tolerate hot temperatures)
• Collards
• Culinary herbs
• Fiddlehead of the Ostrich Fern
• Garlic (in low light conditions cloves are smaller, but just as flavourful)
• Horseradish
• Kale
• Kohlrabi (likes cooler climates)
• Leeks
• Lettuce
• Meslun
• Mizuna (likes cooler climates)
• Mustard Greens (low light makes it taste better)
• Parsnip
• Peas (especially snow or snap peas)
• Potatoes (prefer cooler weather, less sun will result in smaller tubers)
• Radish
• Rhubarb
• Rutabaga (also known as Canadian turnips)
• Scallions
• Spinach
• Swiss chard
• Stinging Nettles (harvest with care, but edible and highly nutritious).
• Tatsoi (likes cooler climates)
• Turnip (tastes best at cooler temperatures)

(Note, I'm no expert on plants, feel free to revise this list if something seems off to you, or if you know of an ideal plant for Mars.)

See Also

• Research on greenhouses & assumptions
• Experimental setup

External links

• The Mars Foundation Side-lit Greenhouse design.

References