Reducing the atmospheric pressures for a closed plant growth system (greenhouse) will allow significant reductions in structural mass of the greenhouse and gas leakage and increase the potential for finding a thin, transparent material that can be used for a greenhouse, thus allowing direct capture of ambient light for photosynthesis. However, the reduced pressures pose considerable challenges; for example, thermal and humidity control capabilities are altered, as well as the operation of pressure-sensitive water delivery systems and maintenance of acceptable dissolved oxygen levels. A thorough understanding of plant growing system performance under reduced pressures is needed to assess these issues, along with monitoring and control capabilities for temperature, carbon dioxide, humidity, and dissolved oxygen levels over a range of operating pressures.

The focus of the project includes the testing and management of the internal environment of an approximately 1-square-meter greenhouse at relatively low pressure (about 10 kilopascals or 0.1 atmosphere). Obviously a 1-square-meter greenhouse could not provide much total oxygen and food for human life support, but it represents a first step toward testing the environmental management system, structural and materials systems, and plant growth in an integrated testbed. Results from this project can then be used to design and deploy a working module for a future Mars mission, as well as provide information for designing and building larger greenhouses to support more autonomous human colonies in space.

Critical to the concept of using greenhouse structures designed for low pressures is the assumption that plants will grow and develop acceptably in these environments. Short-duration tests with lettuce plants showed that transpiration rates increased as pressure decreased. This result was consistent with other observations from low-pressure tests but was complicated by the difficulties in maintaining constant humidities at the different pressures. Subsequent tests to track weight loss from pans of water (direct evaporation) at different pressures but with better humidity and vapor pressure deficit control showed that evaporation rates increased as pressures decreased. This response is likely related to increased gas diffusion rates at lower pressures and in part can explain the increased transpiration observed with plants at reduced pressures.

In a related series of tests, water vapor saturation pressures at different pressures were studied. To measure this, a water source was enclosed in a small, dark vacuum chamber, and the atmosphere was pumped down. The change in pressure was then observed as the chamber humidity was allowed to rise to approximately 100 percent. As expected, saturated humidity pressures were a strong function of temperature but were not affected by the total pressure. In several tests, rapid drops in pressure resulted in boiling of liquid water in the test chamber, which in turn caused a pressure increase.

Future activities include performing tests to determine whether low-pressure environments with only oxygen, carbon dioxide, and water in the atmosphere are acceptable for plant growth. A Lexan structure (1-meter-diameter dome) for growing plants was developed (see figure 1), which will be placed in a vacuum chamber and pressurized to approximately 10 kilopascals against an external pressure of approximately 1 kilopascal of carbon dioxide. The dome includes a cooling and humidity control system using externally supplied chilled water, condensed water recirculation system to the plants, air circulation, oxygen removal membrane, and the ability to add carbon dioxide as a pressurizing gas (see figure 2). Lighting would be provided externally to simulate Mars irradiance levels. The goal is to grow candidate crops for Advanced Life Support Systems through a typical production cycle.