Concepts for landing missions on Mars often include greenhouse structures for plant production. The types of structures proposed vary from small automatically deployed structures for research purposes to larger structures that would be used for food production. Present plans are that greenhouses on Mars will be operated at internal pressures as low as 0.1 to 0.2 Earth atmospheres. Low internal pressures permit the use of structures with lower mass, but complicate the heat and mass transfer processes involved in maintaining a suitable environment for plant growth and raise questions about the requirements of plants for growth at low pressures. Results are given from preliminary tests of lettuce growth at 25 kPa.

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Reduced atmospheric pressures may be used to minimize mass and engineering requirements for plant growth habitats used in some extraterrestrial applications. A chamber with high vacuum capability and thermal control at Kennedy Space Center was used to measure water loss of lettuce plants at reduced atmospheric pressures. A test stand with three, high-pressure sodium vapor lamps was used to determine short-term plant responses to reduced pressure. Initial experiments with lettuce showed that a pressure of 10 kPa (≈0.1 atm) resulted in a 6.1-fold increase in the rate of water loss compared to water loss at ambient pressure. However, due to low relative humidity, plants wilted after 30 minutes exposure to 10 kPa. A follow-up experiment in which relative humidity was controlled between 70% and 85%, demonstrated that water loss was directly proportional to the vapor pressure gradient, regardless of atmospheric pressure in the pressure range of 10 to 101 kPa. However, the response was curvilinear, suggesting effects on the pathway resistance. Results indicate that plant growth at atmospheric pressures of 5 to 10 kPa should be achievable. Further work will necessitate better relative humidity control and carbon dioxide control in order to separate vapor pressure deficit effects from diffusion effects.