Research on greenhouses, the assumptions included, and assumptions not included

We do not want a greenhouse on Mars to get down to 2 degrees Celsius, at which temperature our desired plants would not be efficiently producing high energy chemical reactions by photosynthesis. Also we do not want to maintain a constant 20 degree Celsius which would put too much of an economic burden upon the greenhouse growth system. A middle ground should be found by testing various models and maximizing for food production per dollar. A solar heated greenhouse may be more economic than and artificially lighted and heated greenhouse.

Some researchers investigating solar heated greenhouses on Mars have assumed a dome shaped greenhouse with 1/2 the solar light intensity available on Earth.^[1] This is a valuable set of assumptions to research but sunlight concentrated by mirrors and entering a greenhouse only through the roof while all walls are regolith insulated is another set of assumptions that could be modeled. With the flat roofed sunken greenhouse it would be possible to have a thermally insulating cover on the roof during the night and only open the roof to sunlight in that portion of the day during which light is strong enough to maintain operating temperature. The light in turn would directed horizontally by flat panel mirrors then directed vertically downward by a cylindrically curved concentrating mirror. This might provide more than adequate heat for temperature maintenance . The flat transparent roof would require strong squares of glass in a steel framework to form a triple or quadruple paned insulating window. In the quadruple pane case, the pressure in the three insulating spaces would be: outer layer, 2.5 psi; second layer, 5.0 psi; third layer, 7.5 psi; interior ten psi. An 8 foot by 8 foot section of such a greenhouse roof would carry 42 tons of upward force from the contained atmosphere. This is well within the range of what can be carried by structural steel tensile members sunk into the Mars soil along the sides of such a section of roof. Each 8 inch by 8 inch square of glass held in the steel framework would hold 160 pounds of force, also well within what can be achieved with current materials. To limit the damage from possible accidents 8 foot by 8 foot sections of greenhouse could be separated from each other and from living spaces by pressure bulkheads with access through a pressure lock set of double doors.

Other researchers claim that there is no material which can safely contain greenhouse atmosphere pressure while providing proper thermal insulation, puncture resistance and admitting sufficient light for crop growth.^[2] It would be interesting to know if this research was based upon assuming a dome shaped green house with 1/2 Earth standard solar radiance as the competition for their artificially lighted growth rooms.

All available options for greenhouse architecture should be considered before settling on one or two architectures to be fully developed.

It may be best to set up a colony initially using nuclear power for artificially lighting growth rooms, because this would be the quickest and easiest method to set up. Later solar heated greenhouses could be developed and used, allowing the diversion of electrical power to competing uses as they are developed.