itoring equipment in the spaceflight growth chamber used for this study showed active photosynthesis
(CO 2 uptake during light cycles) and respiration (CO 2 production during the dark cycles) by the potato
leaves in space [28].
Twelve sweetpotato stem cuttings were flown aboard the Space Shuttle in 1999 to study adventitious
root development in spaceflight. Results showed that all the cuttings rooted well, regardless of whether
the basal or apical end of the cutting was placed in agar (D. Mortley et al., unpublished), and suggest that
vegetative propagation of sweetpotato should be possible under weightless conditions.
V. CONSIDERATIONS FOR MARS “GREENHOUSES”
Perhaps some of the first opportunities for testing bioregenerative life support approaches will occur when
human missions reach Mars. With current propulsion technologies, minimal durations for Mars missions
would be in the range of 3 years. This would account for travel to Mars, time for orbital realignment, and
travel back to Earth. Such missions might be preceded by unmanned missions to stockpile consumables
and possibly establish in situ propellant production systems for returning to Earth [114]. Conceivably,
plant production systems or “greenhouses” might also be deployed prior to human arrival to provide some
food and O 2. As a human presence on Mars increases, larger and more sophisticated plant growing sys-
tems may be feasible.
A. What Would It Take to Set Up A Plant Growing System on Mars?
The Martian surface environment is cold in comparison with Earth, with an average temperature of 210
K (Earth average 275 K), although temperatures can rise above 273 K (0°C) at some locations during the
day [23]. Because of this, any plant growing enclosure would have to be well insulated, particularly at
night. Achieving this insulation may be difficult with transparent structures, but supplemental nighttime
covers or enclosures might be considered [115]. Temperature-sensitive hinges might be used to open in-
sulating covers in the morning and then close them at night. These covers could then also be designed to
reflect additional light at the transparent structure. Day lengths on Mars are 24.6 hr; hence the natural pho-
toperiod is close to the circadian cycle on Earth. But the incident solar radiation is only about 45% that of
Earth’s, and dust storms can reduce the amount reaching the surface even further [40].
The Martian atmosphere is tenuous (~0.6 kPa) compared with Earth’s (~100 kPa) and is composed
primarily of CO 2 [23,24] (Table 4). This Martian CO 2 could be used for sustaining photosynthesis, but
the ambient pressure is too low to support plants. Thus a Mars greenhouse would have to be pressur-
ized to some minimum level to sustain acceptable plant growth. The saturation pressure of water does
not change much with total pressure (across range of 5 to 100 kPa) [116], and this factor must be con-
sidered in assessing low-pressure thresholds for plants. If adequate water vapor pressure cannot be
maintained, this combined with increased gas diffusion coefficients at low pressures could increase
transpiration and result in water stress to the plants [117]. In addition to CO 2 and water vapor, some
minimal level of O 2 would be required to sustain respiration [118,119], particularly in root zones and
during dark cycles.
Determining the minimum pressures acceptable for plant enclosures is critical because lower pres-
sures could reduce structural mass and gas leakage, which would reduce system costs [115,117]. This in-
936 WHEELER ET AL.
TABLE 4 Gases and Elements Available on Mars
Atmospheric compositiona Regolith compositionb
CO 2 (95.3%) SiO 2 (40–60%)
N 2 (2.7%) FeO or Fe 2 O 3 (12–17%)
Ar (1.6%) Al 2 O 3 (7–11%)
O 2 (0.13%) SO 3 (5–8%)
CO (0.08%) MgO (2–7%)
H 2 O (~0.03%) CaO (6–7%)
K 2 O (0–1%)
aTotal pressure of ~0.6 kPa [23,24].
bData from Viking and Pathfinder landings [24,114].