When designing their greenhouses for Mars, students must be introduced to the Martian conditions: the concept of gravity, the consistence of Mars’ atmosphere and surface, length of the year and the day, temperature, etc. and what, therefore, the main problems are, that scientists are confronted with in designing a suitable green house.
The following information is adapted from the article
“Greenhouse design for the Mars environment: Development of a prototype, deployable dome“
R.A. Bucklin, P.A. Fowler, V.Y. Rygalov, R.M. Wheeler, Y. Mu, I. Hublitz, E.G. Wilkerson
- Atmospheric pressure and gravity
Atmospheric pressure measured on Mars varies widely with location and season from 0.69 kPa to 0.9 kPa. Atmospheric pressure is always extremely low compared with that on Earth and from a structural analysis viewpoint effectively zero. Martian gravity is 0.38 of the Earth’s gravity (3.73 m/s2) so one’s weight will be less than on Earth. The Martian atmosphere has a density of about 0.01 of that of the Earth.
- Wind and dust
Because of low atmospheric density, the wind load will be low. However, the dust carried by wind is important, as dust suspended in the air changes the overall quantity of light and the distribution of direct and diffuse radiation. It is suggested that it would be necessary to develop a method of dust removal from the exterior of the greenhouse.
The average surface temperature on Mars is approximately –63°C with an average diurnal range of around –103°C to –5°C (Hiscox, 2000 as cited in Bucklin et al.,2004). The daily temperature range observed was –89° C to –32°C. Daytime temperatures in the summer at the equator may be suitable for plant growth, but nighttime temperatures are far below the temperature range where plants can survive.
- Light Levels
Estimates of light levels vary and it is difficult to determine whether values are for the Martian surface or for the Martian orbit. The distribution of direct and diffuse light is needed. Ambient light levels on Mars are high enough to sustain plant growth. However, because of extremely low temperatures and pressures, any plant production must be conducted inside an enclosure. Even the best transparent wall materials reduce light levels. An ideal wall material would allow transmittance of the wavelengths above 400 nm at angles of incidence from zero to 90° and zero transmittance out of the structure for all thermal wavelengths beyond 3000 nm (Aldrich and Bartok, 1994; as cited in Bucklin, et al., 2004).
The wall materials with the highest light transmisivity are thin films that have low thermal resistance and low mechanical strength. Thin films can be reinforced by straps or frames, but these reinforcing elements reduce the amount of light. It may be necessary to supplement ambient light with artificial lighting to achieve satisfactory plant growth. The power requirements of artificial lighting are very high; however, in contrast to most situations on Earth, the waste heat from artificial lights would be very useful on Mars.
- Structural needs
The main structural load on any configuration of Martian greenhouse will be imposed by internal pressure. Gravity loads and wind loads will be much smaller. The stresses in a curved shell are directly related to the internal pressure and the shell radius and are inversely related to the wall thickness. Stresses in flat sheets increase with pressure and sheet width. Bending stresses in flat sheets also increase as sheet thickness decreases. Walls must be as transparent as possible, which means walls should be as thin as possible. Most greenhouse films are less than 1 mm thick, so stresses can rapidly approach the film’s failure strength. Reinforcing material can be added to films and sheets, but reinforcing material blocks or reduces light levels. A spherical shape gives the best strength to weight ratio for carrying pressure loads and curved shapes such as hemispherical domes or half cylinders have better strength to weight ratios than shapes with flat sides. Curved shapes also have lower surface area to volume ratios, which is an advantage when considering heat loss through the wall surfaces. However, the lower surface area to volume ratio can be a disadvantage when light collection is considered. Many film materials exhibit large thermal expansion and contraction. Large stresses are produced if the film is restrained from changing length as the temperature decreases and wrinkles can appear when the temperature increases. Cycles of expansion and contraction can also produce stresses at joints. Many clear materials are sensitive to ultraviolet radiation.
The dominant environmental parameter in a Mars deployable greenhouse will be temperature. A heating system will be a necessity at night. Solar collectors can be used to increase the amount of energy, but collectors will not be effective during times when light is diffuse because of dusty conditions or clouds. Even on the best days, supplemental heating will be required. If a transparent film is used for wall material, the heating system will consume major quantities of energy, so utilising as much solar energy as possible will be critical. Significant quantities of solar energy are available on the Martian surface, but as on Earth, solar energy on Mars is not always available when required and is never available at night. If supplemental lighting is used, cooling may be necessary because electrical lights produce very large quantities of waste heat. Because of the cold surroundings, cooling should consume much less energy than heating. The quantity of solar energy available to heat a greenhouse can be increased by the use ofsolar collectors and concentrators when direct sunlight is available. Thermal storage is necessary when using solar systems in order to provide a steady supply of energy throughout the day and night. Glass is transparent to visible wavelengths of light and opaque to infrared wavelength and is an ideal wall material for the greenhouse effect. Unfortunately, many plastic films are relatively transparent to infrared radiation. The radiation characteristics of wall materials must be carefully selected to optimise transmission of photo synthetically Active Radiation (PAR) and block as much radiation in the infrared range as possible. Gas leakage will occur from the greenhouse. All practical closed systems holding gas under pressure leak because of the pressure differential across wall surfaces and the difficulties of maintaining tight seals of flexible materials. Heating of replacement gases will add to the energy load of the greenhouse. Carbon dioxide can be replaced from the Martian atmosphere, but water vapor and oxygen will be difficult to make up. The greenhouse will require a ventilation system. Plants will require some minimum air velocity over leaves for gas exchange. Plants transpire and release oxygen as a byproduct of photosynthesis. Even if the overall system is closed, the plant growth volume must be maintained within a certain range of relative humidities and at some point, surplus oxygen must to be removed from the system and carbon dioxide will need to be added.
Temperature and relative humidity must be constantly controlled to maintain a satisfactory environment for plant growth. An overall environmental control system will be required to manage the interactions between lighting, temperature, relative humidity, oxygen level, carbon dioxide level, pressure, the hydroponics system and plant growth.
The plant consideration that has the largest impact on structural design is the internal pressure of the greenhouse. The absolute minimum internal pressure is the sum of the partial pressures of carbon dioxide, water vapor and oxygen inside the greenhouse. The partial pressure of carbon dioxide in Earth’s atmosphere is 0.035 kPa. The partial pressure of carbon dioxide in the Martian atmosphere is about 0.57 kPa. The partial pressure of water vapor in Earth’s atmosphere, referred to as the vapor pressure, varies with temperature and relative humidity. At comfortable room conditions of 25°C and 50% relative humidity, the vapor pressure for Earth’s atmosphere is 1.6 kPa. The variation of humidity can be neglected in open systems operating at Earth atmospheric pressure, but the variation is important in closed systems operating at reduced pressures. Tests in the vacuum test chamber at Kennedy Space Center (KSC) (Fowler et al, 2000 as cited in Bucklin, et al., 2004) indicate that plants tolerate pressures down to 20 kPa without problem, but begin to wilt below this value. In other tests at KSC, plants survived below 10 kPa for short periods of time.
Plants have a region of temperatures in which they function best and also upper and lower limits beyond which they display heat or cold damage. Temperature also has a major influence on transpiration rate and on dissolved oxygen levels in root moisture.
The internal gas mix must contain minimum levels of carbon dioxide, water vapor and oxygen. The maximum desirable levels of these components will not total 10 kPa so some inert gas will be required to supply the remainder of the desired pressure. The pressure and the gas mix in the greenhouse will require monitoring and control. In a closed system, the carbon dioxide partial pressure will drop as carbon dioxide is consumed by photosynthesis. At the same time, oxygen is released by the plants and the oxygen partial pressure will increase. Water vapor partial pressure will increase as the plants transpire and add water to the gas mix. Excess water vapor will have to be removed from the gas mix and recycled into the soil media or the hydroponics system’s water. Water vapor pressure will fluctuate by several kilopascals as relative humidity varies. Carbon dioxide will have to be replaced as it is consumed by photosynthesis and oxygen will have to be harvested and stored or discharged before it reaches undesirable levels. Just as with temperature, plants require a certain range of relative humidity to function. Relative humidity is the ratio of the ambient water vapor pressure to the water vapor pressure at saturation for the same temperature. For a given temperature, relative humidity is a function of ambient water vapor pressure. Vapor pressure increases as moisture is added to the air through transpiration or evaporation from leaks in the hydroponics system. Under Earth atmospheric pressure in an open system, the change in vapor pressure is not important, but in a totally closed system at low pressure, fluctuations in vapor pressure will significantly influence total pressure. Hydroponics systems will need to be as tight as possible to reduce the quantity of water that evaporates from leaks. At high relative humidities, condensation on interior wall surfaces will occur. Condensation by itself will reduce light levels and over time will promote dirt and mineral collection on wall surfaces that will further reduce light levels.
A minimum internal air velocity is needed for the gas exchanges required for photosynthesis to occur. Velocities in excess of this minimum should be produced by the ventilation system operating to remove moisture from the system, so maintaining the minimum required velocity is not expected to be a problem. The plant will also require some minimum volume for its canopy. The biggest challenge for the design of a deployable Martian greenhouse is to achieve maximum light transmittance while keeping heat loss to acceptable levels. Radiation heat transfer will dominate for Martian conditions. The low density atmosphere will reduce conductive and convective heat transfer through the atmosphere outside the greenhouse. Operating greenhouses at internal pressures as low as 0.1 Earth atmosphere has been discussed, but it appears that plants may not be productive at pressures below 0.2 or 0.3 Earth atmospheric.Conduction and natural convection inside the structure will be greatly reduced at 0.1 Earth atmosphere and hence a higher pressure may be required to maintain a good thermal and mass transfer balance. During the day, the greenhouse will receive direct radiation from the sun and some diffuse radiation. The greenhouse will lose radiant energy to all of the very cold surrounding objects and depending on sky conditions; it will lose radiant energy to cold portions of the sky away from the sun. At night, the surrounding objects will be even colder and the greenhouse will lose radiant heat to the cold sky. The presence of plants complicates the heat transfer analysis by adding latent heat transfer (evaporation and condensation) to sensible heat transfer (conduction, convection and radiation). The changing mix of carbon dioxide, oxygen and water vapor in the greenhouse must be accounted for in heat transfer analysis. Other factors of importance include leakage from the hydroponics plumbing and condensation on the inside of the greenhouse wall.
Several research groups are developing facilities to study the behaviour of plants at low pressure (Brown and Lacey, 2002; Chamberlian et al, 2003; Ferlet al, 2002; Goto et al, 2002; all cited in Bucklin et al., 2004). The tests described here are from preliminary studies with a 1 meter diameter dome shaped low pressure growth chamber developed as a prototype of a Mars Greenhouse by the Advanced Life Support group at KSC and the University of Florida. Tests conducted to clarify air and moisture relationships at low pressure have been conducted in a large vacuum chamber used to test space suits and several small chambers (Fowler et al, 2000; Fowler et al, 2002; Rygalov et al, 2002 as cited in Bucklin et al., 2004). An automated closed environmental growth chamber (see Figure 8) was developed at KSC that operates at pressures down to 25 kPa.
The base of the dome is stainless steel and the dome is made of clear Lexan. Internally, a monitoring and control system regulates the atmosphere to predetermined set points. The system is controlled electronically by a microcontroller which interacts with sensors and appropriate relays and solenoids to enact systems for each parameter. Algorithms were developed to control each parameter. The main component of the system is a central tower, the Automated Tower Management System (ATMS.
The ATMS consists of a tube with a fan and heater at the top to create airflow in the system. Directly below the fan a cooling coil is used both for air temperature and humidity control. Underneath the cooling coil is a water collection pot that receives condensation from the cooling coil. This pot of water is then pumped back through a selective manifold that distributes the water back to the plants, thus completing the water cycle of the system. An outside PC is used to log data from the experiment. A regulated vacuum pump is used to maintain the desired pressure. The ability of the system to grow a crop was tested by growing nine plants of Waldmann's Green Lettuce at 25 kPa pressure, 0.2 kPa carbon dioxide and 5 kPa oxygen. Plants grown in arcillite medium with Osmocote time release fertiliser. An automated watering system was used. The watering system was based on scales continuously weighing each plant. Water and CO2 were the only two outside inputs to the system. The experiment lasted 45 days with the first week (germination) done at regular pressure and then the plants were trans-planted and put into a 25 kPa environment. At the end of the experiment, the plants were harvested and analysed. One plant died and was removed from statistics. Six plants displayed tip burn, four plants had rusty spots on older leaves and there was mold on the soil surface under dead leaves of one plant. The system successfully grew a crop of lettuce past the typical harvest date.