creases the probability of finding acceptable, transparent materials that could be used for low-mass, in-
flatable greenhouses [115].
If an initial charge of water and O 2 were added to the greenhouse (i.e., brought from Earth to start
the system), could local CO 2 be used as a pressurizing gas? Water in the greenhouse could be recycled,
although there would be some leakage and incorporation into biomass, and O 2 produced from photosyn-
thesis would have to be removed and stored. If, for example, a minimum pressure of 10 kPa (0.1 atm) is
required for good plant growth, and 2 kPa of water vapor and 5 kPa of O 2 were needed [115–117], then
the balance (3 kPa) might be CO 2. Studies with CO 2 levels0.5 kPa indicate that these high levels can
cause unexpected results in some plants (Figure 3) [54,65,68]. Obviously, much more research is needed
to further define the potential for growing plants at low pressures and at vastly different partial pressures
than have been studied before.
Some essential elements for plant growth are available on the Martian surface [114] (Table 4) but
whether these would be useful for early efforts to establish plant systems is unknown. A more likely sce-
nario would be to supply the necessary nutrients initially and then incorporate recycling approaches with
the inedible biomass from previous plantings and even human wastes following human arrival
[91,97,101,103]. Eventually, residual biomass might be composted and incorporated with local regolith
to generate soils for supporting plant growth as systems expand [104].
VI. CONCLUDING REMARKS
As humankind advances and technologies improve, the exploration and colonization of other planets in
our solar system seem inevitable. This exploration will require safe and reliable life support technologies
that minimize stowage and resupply costs. Green plants (crops) could serve a vital role for these regener-
ative life support systems, where photosynthesis is used to provide oxygen and food while removing
waste carbon dioxide. In addition, plant transpiration in combination with root-zone microbes could be
used to process and purify wastewater. For bioregenerative systems to succeed, the growing environment
and horticultural approaches must be carefully managed to optimize crop outputs. In some settings, e.g.,
Mars “greenhouse,” this might involve the use of low atmospheric pressures and/or gas partial pressures
vastly different from the terrestrial environment. A key factor for implementing crop production systems
will be the development of energy-efficient lighting approaches. For early missions, stowage and physic-
ochemical technologies will provide most of the consumables, with plants possibly grown to provide a
modest supply of fresh foods. As mission distances and durations increase, the role for plants could ex-
pand, where crops are then used for most of the atmospheric regeneration and provide major portions of
carbohydrate, protein, and oil for the crew. The self-regenerating nature of biological systems could also
provide a degree of autonomy for surviving potential system failures or mission delays.
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