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Mars Greenhouse
inflatable structure for Mars and Earth
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A greenhouse is a simple technology that holds the promise to provide a continuous supply of all the food Mars astronauts or colonists will need. The question is what atmosphere constituents, and soil nutrients are required? What soil content levels are toxic to crops? What crop sequence can process native Martian regolith into soil that can sustain food-bearing crops? What crops will provide human nutrition requirements? Is there a plant selection and regolith conditioning process that can be done simply enough to test on the first manned mission?

The current focus is design of the enclosure. How to design an inflatable structure to grow plants in the cold of Mars. Work on soil preparation has been moved to the Soil page. This inflatable greenhouse can also be used on Earth to grow plants in winter in Canada. This project could permit grade schools to get involved with Mars research by gowing plants. The Mars Society can design and sell an inflatable greenhouse to schools, a design that can be deployed on grass. A design that can grow plants in Winter may have commercial possibilities as well.

The first concern is heat. The temperature on Mars is cold. Pathfinder found the temperature measured by its topmast sensor (1m above ground) to vary between -8C at 2:30pm to -77C at 5:30am, and pressure varied from 6.77mbar to 7.08mbar. Viking 2 recorded temperature over more than a Martian year, the low was -111C. Mars Global Surveyor measured temperatures from -140C at the winter pole to +27C on the day side during summer. We wouldn't land a manned mission at the pole, but tropical latitudes will experience -111C...+27C. A plastic bag could contain greenhouse atmosphere. An embedded layer of a silver based compound would transmit visible light and short wave infrared (radiant heat from sunlight) while reflecting long wave infrared (radiant heat from warm objects). This would trap heat in. Commercially this is known as Heat Mirror. If that is not enough, the pressure envelope could have two layers, with the greenhouse interior pressurised more than the inter-layer gap. That would hold the layers in place. The gap could be filled with argon gas. Argon is used in low emissivity (Low-E) windows, and is a constituent of Martian atmosphere. To prevent convection, the gap should be between 1/4" and 3/4" wide. Heat loss to the ground is more important. Would spray foam insulation be enough? Would we have to use a rigid floor to prevent crushing the insulation with astronaut's feet? Would bubble-wrap filled with argon be better? Can bubble-wrap be made strong enough to walk on without popping the bubbles?

The ideal material for the enclosure on Mars is PolyChloroTriFluoroEthylene (PCTFE). This is sold by the brand names Kel-F, Neoflon PCTFE, or Clarus. The premier qualtiy greenhouse film on Earth is Tefzel, but UV stability is reported not quite good enough for space.
1-mil filmFEPTefzelPCTFE
Tensile strength3000 psi6000 psi5300 psi
Bursting strength11 psi33 psi19.4 psi
Density54.6 g/m244.38 g/m254.1 g/m2
Service temperature-240...+204C-185...+150C-240...+132C

Gas permeability of 1-mil film at 25C in cm3/m2dayatm
FEPTefzelPCTFE
CO2259003875?
N2500046514.5
O216001550107   
water permeability in g/m2day
H2O7.0<0.1

Click to enlarge Transmission of visible light is good; Tefzel 1.0 mil film is 94-95%, but PCTFE transmission is higher, especially in UV light.

Click to enlarge Spectrally Selective low-e glazing can the most dramatic protection for UV and IR. By trapping InfraRed light in, radiative heat is kept in. IR is radiant heat. According to the graph to the right, spectrally selective transmits 82% of violet light, 85% of blue and green, but 45% of orange and 30% of red. IR transmission tapers off from 30% to 10% at just over 1 micrometer wavelength; it rises back to 45% at 2.5 micrometer wavelength. This does have the slight problem that it reflects more radiant heat from sunlight than from warm objects. Heat Mirror is a product from one manufacturer. It is based on the same principle. UltraViolet reflectance for these products is 98-99.5% depending on brand. Since the ozone layer on Mars is almost non-existant, UV protection is important. The surface of Mars not only gets UV-A and UV-B, but UV-C as well.

Radiation on Mars is reduced by its atmosphere. The MARIE instrument on Mars Odyssey produced a map of estimated radiation dosage for the surface. At the near-surface permafrost deposits at the equator, radiation is 14 rems/year. Astronauts on the ISS typically receive 20-40 rems/year. You can view NASA's web pages for Estimated radiation dosage and epithermal neutrons (water).

The inflatable design has several advantages. First, it has the lowest mass for transport to Mars. On Earth that translates to lower freight cost. Since there are no structural members holding the roof up, this greenhouse can be set-up on grass or dirt; as long as there are no sharp rocks to puncture the enclosure. As an educational tool for schools, the airlock is very important. It demostrates the need for the airlock: if a student opens both doors at once the air will be released causing the "roof" to collapse on the plants. Although the roof is only a plastic sheet, this makes the airlock real so student's cannot ignore it.

Thermodynamics are quite a challenge. One member has produced a spreadsheet to estimate it; it can be viewed here.

To join this project and help design the greenhouse, click the Yahoo Groups button above.

   
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