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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?

Our focus is use of Mars resources (air and soil) to grow food crops. This project will not develop a Closed Ecological Life Support System, the goal is not to recycle all air, water, and sewage; the Green CELSS Task Force is working on that. The assumption for this group is that a manned mission to Mars will include a chemical/mechanical air and water recycling system like that used on the International Space Station, as well as packaged food for the trip. The Mars Greenhouse we develop will provide fresh food to supplement stored provisions, and prove food crops can grow on Mars.

First, what do we have to work with?
Regolith analysis by Sojourner as presented at Lunar and Planetary Science XXXII (2001)
Results are presented as percentage by weight.
Sojourner samples
Carbon content is debated. It is either less than 0.8% or less than 0.3%
Nitrogen content is below the minimum measurable level. (<0.1%)
Oxidation states are projected assuming stoichiometric oxygen for all elements detected.

A CIPW analysis of these results produces the following minerals:
SojournerOpportunity / Meridiani Planum
A-2A-4A-5A-9A-10A-15TarmacHema2trench1tranchwall2Les HaunchesJack RussellBeagle BurrowNougatAverage

The atmosphere is very thin and dry. Its content is, by percent:
N2  2.7
Ar  1.6
O2  0.13
CO  0.07
H2O  0.03
Ne  0.00025
Kr  0.00003
Xe  0.000008
O3  0.000003
Atmospheric pressure is an average of just under 7 millibars, but it can vary from 9 millibars in the deepest basin to 1 millibar at the top of Olympus Mons.

The regolith will have to be treated to create a suitable soil for plants. The first step is to create a plastic bag for the greenhouse structure, pressurise it with Martian air, and shovel in Regolith. Plants require some atmospheric oxygen for respiration. Adding water to the regolith will cause several chemical reactions, and release oxygen. If we assume Mars soil is oxides and not minerals, the chemistry is simpler but it would appear many more reactions would occur. Sodium oxide [Na2O] will combine with chlorine [Cl] to form salt [NaCl] and release oxygen. Dissolved carbon dioxide will combine with water to form carbonic acid [H2CO3], which is an ionic compound consisting of 2 H+ ions and 1 CO3-2. Sodium oxide [Na2O] will also combine with carbonic acid to form soda. Dissolved soda will be indistinguishable between sodium carbonate [Na2CO3] and sodium bicarbonate (baking soda) [NaHCO3] due to the presence of H+ ions in water. Potassium is as ionic as sodium, so potassium oxide [K2O] will form potassium salt and potassium soda. Calcium oxide [CaO] will combine with sulphur trioxide [SO3] and water to form gypsum [CaSO4•2H2O]. Calcium oxide is also known as quicklime, which will react with water to form slaked lime and give off heat: CaO + H2O → Ca(OH)2. Phosphoric oxide will react with water to become phosphoric acid: P2O5 + 3 H2O → 2 H3PO4. Sulphur trioxide [SO3] will also react with water to form H2SO4, when dissolved in water that is sulphuric acid. Since sulphur exists as a trioxide instead of dioxide, all sulphur reactions will be quicker. How much oxygen will be released? Will it be enough? Should we bubble Martian air through the water/soil mixture to accelerate the creation of soda? Will the salt level be toxic? Soda is an alkali, will it make the soil too alkaline? With an alkali and acid together they will react. What will the final pH be?

My rough calculations, assuming enough CO2 is bubbled to consume all sodium and potassium, and based on sample A-2, each kilogram of regolith will produce:
salt11.5 grams
baking soda113.1 grams
gypsum12.7 grams
left over CaO1.8 grams
phosphoric acid11.0 grams
sulphuric acidconsumed
oxygen1.6 grams
Is that right? That result would be very alkaline.

This depends on the composition of regolith. For example, if CaO is combined with silica and alumina to form tricalcium silicate [3CaO•SiO2], tricalcium aluminate [3CaO•Al2O3] and dicalcium silicate [2CaO•SiO2], then that together with magnesium oxide, iron oxide and gypsum is portland cement. When water is added to portland cement it will form concrete. When concrete sets the silicate forms jellylike hydrated silica [SiO2•NH2O]. This could consume nitrogen from greenhouse air and turn the soil to concrete.

Nitrogen content in regolith is negligible. Nitrogen fixing plants can take atmospheric nitrogen to create nitrates for other crops. Is the atmospheric nitrogen content enough? The Insitu Propellant Production facility will extract carbon dioxide for its use from Martian atmosphere using a sorption bed. We could use the carbon dioxide depleted exhaust to inflate the green house. Exactly how much CO2 will be depleted? Derrick Davis suggests the sorption bed will extract 95% of CO2. Assuming the sorption bed does not remove carbon monoxide, that should leave:
CO2  2.80
O2  2.7
CO  1.45
H2O  0.62
Plus the trace gasses.

Assuming a greenhouse with 280m3 volume and 20 tonnes of regolith, soil reactions should release 32kg of oxygen. If the temperature was 20°C and pressure 257.86 mbar (3.74 psi) before adding the oxygen, the result should be 344.96 mbar (5.00 psi) and the gas mix would be:
CO2  2.24
CO  1.16
Plus trace gasses. Water will evaporate from the soil.
Is that right? Is this a suitable atmosphere?

More important than these simple treatments are the soil nutrients that are more difficult to alter. What are the nutrient requirements for desired food crops? What levels are toxic? For example, iron [Fe] is extremely high. What food crops can survive this? What can we do to reduce it? Could the carbonyl process extract iron without destroying the rest of the soil? The carbonyl process has been proposed, but it requires metallic iron to start with; iron on Mars exists as rust. Could you dissolve iron with acid? Phosphoric acid will be created by adding water to regolith, and it is a phosphorus fertiliser for plants. What is the process? Is there enough potassium [K]? Adding compost from toilets would add nutrients to the soil. This would add nitrates as well as potassium. Is that enough potassium? Relying on compost means you ship potassium from Earth in the form of food.

To approach the problem from the other end, what food will the astronauts require for human nutrition? What foods should we examine? Raising livestock for meat will not be possible for some time. What meat substitutes would be appropriate? What nutrition is provided by a harvest or veggie burger, and what crops go into them? What processing is required?

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