Attempts have been made to simulate to some degree the various
parameters of the Martian environment, such as atmospheric composition,
pressure, radiation flux, temperatures, and the day-night as well as
seasonal cycles. Certain factors for Mars cannot yet be simulated, such
as soil composition, gravitational field, magnetic field, and electrical
field.
Caution is required in interpreting all simulation experiments. How
Earth organisms respond to simulated Martian environments probably has
nothing to do with life on Mars, but these experiments may show whether
or not anything in the environment of Mars makes life as we know it
impossible. We must expect that on Mars, life will have evolved and have
adapted over long periods of time under conditions which are quite
different from conditions on Earth. The simulation experiments also
provide some information about the possibility of contaminating the
planet Mars, or any planet, with organisms from Earth. In addition, they
give us some clues about the possibilities of adaptation and evolution
of life under these conditions.
From an evolutionary point of view, if life has developed on Mars, we
expect it to have evolved at least to a microbial stage. On Earth,
micro-organisms are the most ubiquitous and numerous forms of life. This
fact should be considered in studying extraterrestrial bodies.
Micro-organisms have been selected as the best test organisms, and
bacteria and fungi have been used because they are durable and easy to
grow. Also, because of their rapid growth, many generations can be
studied in a relatively short period of time. The organisms include
chemoautotrophic bacteria, which are able to synthesize their cell
constituents from carbon dioxide by energy derived from inorganic
reactions; anaerobic bacteria, which grow only in the absence of
molecular oxygen; photoautotrophic plants such as algae, lichens, and
more complex seed plants; and small terrestrial animals.
Organisms have been collected from tundra, desert, hot springs, alpine,
and saline habitats to obtain species with specialized capabilities to
conserve water, balance osmotic discrepancies, store gases, accommodate
to temperature extremes, and otherwise meet stresses. An attempt is made
in these simulation experiments to extend these processes across the
possible overlapping microenvironments which Earth and Mars may share.
Scientists have developed various special environmental simulators,
including "Mars jars" and "Marsariums." These have made possible
controlled temperatures, atmospheres, pressures, water activities, and
soil conditions for duplicating assumed Martian surface. A complex
simulator, developed by Young et al. ([ref.52]), reproduces the
formation of a permafrost layer with some water tied up in the form of
ice beneath the soil surface. This simulator serves as a model to study
the wave of darkening, thus supporting the hypothesis that the
pole-to-equator wave of darkening is correlated with the availability of
subsurface water. The simulator is a heavily insulated 2-cu-ft capacity
chamber with an internal pressure of 0.1 atm. The chamber contains a
soil mixture of limonite and sand and an atmosphere of carbon dioxide
and nitrogen. With the use of a liquid nitrogen heat exchanger at one
end and an external battery of infrared lamps at the other end, the
temperature simulates that of Mars from pole to equator. Thermocouples
throughout the soil monitor the temperatures in the chamber.
Zhukova and Kondratyev ([ref.69]) designed a structure measuring
100×150×180 cm. Micro-organisms were placed at the surface of a copper
bar made in a special groove separated by glass cloth. Copper was
selected as one of the best heat-conduction materials permitting a rapid
change of temperature. The lower end of the bar was immersed into a
mixture of dry ice and ethyl alcohol, which made it possible to create a
temperature of -60° C. Heating was performed by an incandescent spiral.
As the knowledge concerning the Martian environment becomes more
refined, scientists can more accurately simulate this environment under
controlled conditions in the laboratory. Determination of the effects of
the Martian environment on Earth organisms will permit better
theorization on the forms of life we might find on Mars and will permit
us to estimate the potential survival of Earth contaminants on Mars.
However, until the environmental conditions of Mars are defined more
accurately, the experiments must be changed continually to fit newly
determined conditions. Therefore, existing simulation data are made less
valid for comparison. The data resulting from the simulation experiments
for Mars have been compiled in table II, and the experiments are
summarized below.
The earliest simulation studies were carried out by the Air Force, and
the studies during the past 6 years have been supported by NASA.
Recently, these studies have received less support or have been
terminated in favor of critical studies on the effects of biologically
important environmental extreme factors on Earth organisms. These
critical studies permit establishing the extreme environmental factor
parameters in which Earth life can grow or survive. These data will have
valuable application to the consideration of life on any planet, to the
design of life-detection instruments, to the sterilization of space
vehicles, and to the problem of contamination of planets.
Some exploratory experimental studies are in progress to study the
capabilities of organisms to grow under the assumed conditions on
Jupiter. These include studies at high pressure with liquid ammonia,
methane, and other reducing compounds.
Early experiments simulating Martian conditions using soil bacteria were
carried out by Davis and Fulton ([ref.70]) at the Air Force School of
Aviation Medicine, San Antonio, Tex. Mixed populations of soil bacteria
were put in "Mars jars" with the following conditions: 65-mm Hg
pressure, 1 percent water or less, nitrogen atmosphere, sandstone-lava
soil, and a temperature day-night cycle of +25° to -25° C. The moisture
was controlled by desiccating the soil and adding a given amount of
water. Experiments, conducted up to 10 months, demonstrated that
obligate aerobes died quickly. The anaerobes and sporeformers survived.
Although a small increase in the total number of organisms indicated
growth, the increases in the number of bacteria may have been due to
breaking up clumps of dirt.
Roberts and Irvine ([ref.71]) reported that, in a simulated Martian
environment, colony counts of a sporeforming bacterium, _Bacillus
cereus_, increased when 8 percent moisture was added. Moisture was
considered more important than temperature or atmospheric gases inasmuch
as a simulated Martian microenvironment containing 8 percent moisture
permitted germination and growth of endospores of _Clostridium
sporogenes_. Increases in colony counts of _Bacillus cereus_ appeared to
be influenced by temperature cycling ([ref.72]).
Table II.—_Survival and Growth of Organisms in Simulated Planetary
(Martian) Environments_
------------------------------------------------------------------
Species Survival, Moisture Temperature,
months °C
------------------------------------------------------------------
Conditions on Mars: 14µ±7µ -70 to +30
------------------------------------------------------------------
Anaerobic 6 Low, -60 to +20
sporeformers (CaSO₄)
_Clostridia_,
_Bacillus
planosarcina_
------------------------------------------------------------------
Anaerobic 6 Low, -60 to +20
nonsporeformers (CaSO₄)
_Pseudomonas_,
_Rhodopseudomonas_
------------------------------------------------------------------
Anaerobes Growth Very wet -75 to +25
_Aerobacter
aerogenes_,
_Pseudomonas sp._
------------------------------------------------------------------
_Clostridium_, 10 1 -25 to +25
_Corynebacteria_ percent
"Thin short rod" or less
------------------------------------------------------------------
_Bacillus cereus_ 2 0.5 -25 to +25
percent
soil
------------------------------------------------------------------
_Clostridium sporogenes_ 1 8.4 -25 to +25
(growth) percent
------------------------------------------------------------------
_Clostridium botulinum_ 10 Lyophilized -25 to +25
------------------------------------------------------------------
_Klebsiella pneumoniae_ 6 Lyophilized -25 to +25
------------------------------------------------------------------
_Bacillus subtilis_ var. 4 2 percent -25 to +25
_globigii_
------------------------------------------------------------------
_Sarcina aurantiaca_ 4 0.5 percent -25 to +25
------------------------------------------------------------------
_Clostridium tetani_ 2 or less 1 percent -60 to +25
------------------------------------------------------------------
_Aspergillus niger_ Over 6 hr Very dry -60 to +25
------------------------------------------------------------------
_Aspergillus oryzae_ Over 6 hr Very dry -60 to +25
------------------------------------------------------------------
_Mucor plumbeus_ Over 6 hr Very dry -60 to +25
------------------------------------------------------------------
_Rhodotorula rubra_ Over 6 hr Very dry -60 to +25
------------------------------------------------------------------
Pea, bean, tomato, rye, 0.3 Moist +25
sorghum, rice.
------------------------------------------------------------------
Winter rye 0.6 Moist -10 to +23
------------------------------------------------------------------
Table II.—_Survival and Growth of Organisms in Simulated Planetary
(Martian) Environments_
----------------------------------------------------------------------
Species Atmospheric N₂, CO₂, Substrate
pressure, percent percent
mm Hg
----------------------------------------------------------------------
Conditions on Mars: 85, 3 to 30
25±15, 11
----------------------------------------------------------------------
Anaerobic 76 95 5 Air-dried
sporeformers soil
_Clostridia_,
_Bacillus
planosarcina_
----------------------------------------------------------------------
Anaerobic 76 95 5 Air-dried
nonsporeformers soil
_Pseudomonas_,
_Rhodopseudomonas_
----------------------------------------------------------------------
Anaerobes 760 100 (?) Difco
_Aerobacter infusion
aerogenes_, broth
_Pseudomonas sp._
----------------------------------------------------------------------
_Clostridium_, 65 100 (?) Soil
_Corynebacteria_
"Thin short rod"
----------------------------------------------------------------------
_Bacillus cereus_ 65 94 2.21 Sandstone
soil
----------------------------------------------------------------------
_Clostridium sporogenes_ 65 94 2 Enriched
soil
----------------------------------------------------------------------
_Clostridium botulinum_ 65 95 0 to Lava soil
0.5
----------------------------------------------------------------------
_Klebsiella pneumoniae_ 65 95 0 to Lava soil
0.5
----------------------------------------------------------------------
_Bacillus subtilis_ var. 85 95 0.3 Media
_globigii_
----------------------------------------------------------------------
_Sarcina aurantiaca_ 85 95 0.3 Desert
soil
----------------------------------------------------------------------
_Clostridium tetani_ 85 95 0.3 Soil
----------------------------------------------------------------------
_Aspergillus niger_ 76 95.5 0.25 Glass
cloth on
copper
bar
----------------------------------------------------------------------
_Aspergillus oryzae_ 76 95.5 0.25 Do.
----------------------------------------------------------------------
_Mucor plumbeus_ 76 95.5 0.25 Do.
----------------------------------------------------------------------
_Rhodotorula rubra_ 76 95.5 0.25 Do.
----------------------------------------------------------------------
Pea, bean, tomato, rye, 75 100 0 Filter
sorghum, rice. paper
----------------------------------------------------------------------
Winter rye 76 98 0.24 Soil
----------------------------------------------------------------------
Studies of the effects of simulated Martian environments on sporeforming
anaerobic bacteria were carried out by Hawrylewicz et al. ([ref.49]).
They showed that the encapsulated facultative anaerobe, _Klebsiella
pneumoniae_, survived under simulated Martian atmosphere for 6 to 8
months, but were less virulent than the freshly isolated organisms.
Spores of the anaerobe _Clostridium botulinum_ survived 10 months in the
simulator. Hagen et al. ([ref.53]) found that the addition of moisture
to dry-simulated Martian soil did not improve the survival of _Bacillus
subtilis_ or _Pseudomonas aeruginosa_. _Bacillus cereus_ spores
survived, with added organic medium plus moisture, but no germination of
the spores resulted.
Hawrylewicz et al. ([ref.49]) put rocks from Antarctica bearing various
lichens in simulated Martian conditions in a large desiccator. They
found that the algal portion of a lichen, _Trebouxia erici_, showed only
slight resistance to the Martian environment. They also pointed out the
effect moisture had on the physical condition of lichens. The
undersurface of a lichen has great water-absorbing capability, and the
slightest amount of moisture on a rock surface is absorbed by the lichen
which can turn green in 15 minutes.
Scher et al. ([ref.51]) exposed desert soils to simulated environmental
conditions and diurnal cycles of Mars. The atmosphere consisted of 95
percent nitrogen and 5 percent carbon dioxide (no oxygen) and was dried,
using calcium sulfate as a desiccant. The total atmospheric pressure was
0.1 atm. The temperature ranged from -60° to +20° C in 24-hour cycles.
One hour was spent at the maximum and at the minimum temperatures. The
chambers were irradiated with ultraviolet, 2537 Å, with a dose of 10⁹
ergs/cm², which is comparable to a daily dose found on Mars, and easily
exceeds the mean lethal dose for unprotected bacteria. Soil aliquots
were removed weekly and incubated at 30° C. The scoring was done both
aerobically and anaerobically. Sporeforming obligate and facultative
anaerobes, including _Clostridium_, _Bacillus_, and _Planosarcina_, and
nonsporeforming facultative anaerobes, including _Pseudomonas_ and
_Rhodopseudomonas_, were found. The experimental chambers were frozen
and thawed cyclically up to 6 months. Organisms that were able to
survive the first freeze-thaw cycle were able to survive the entire
experiment. The ultraviolet irradiation did not kill subsurface
organisms, and a thin layer of soil served as an ultraviolet shield. All
of the samples showed survivors.
Young et al. ([ref.52]) assumed that water is present on Mars, at least
in microenvironments, and that nutrients would be available. The primary
objective of their experiments was to determine the likelihood of
contaminating Mars with Earth organisms should a space probe from Earth
encounter an optimum microenvironment in terms of water and nutrients.
The experiments used bacteria in liquid nutrient media. The environment
consisted of a carbon dioxide-nitrogen atmosphere, and the temperature
cycling was -70° to +25° C, with a maximum time above freezing of 4½
hours. _Aerobacter aerogenes_ and _Pseudomonas sp._ grew in nutrient
medium under Martian freezing and thawing cycles. Atmospheric pressure
was not a significant factor in the growth of bacteria under these
conditions.
Silverman et al. ([ref.47]) studied bacteria and a fungus under
extreme—but not "Martian"—conditions. Spores of five test organisms (_B.
subtilis_ var. _niger_, _B. megaterium_, _B. stearothermophilus_,
_Clostridium sporogenes,_ and _Aspergillus niger_) and soils were
exposed while under ultrahigh vacuum to temperatures of from -190° to
+170° C for 4 to 5 days. Up to 25° C there was no loss in viability; at
higher temperatures, differences in resistivity were observed. At 88° C,
only _B. subtilis_ and _A. niger_ survived in appreciable numbers; at
107° C, only _A. niger_ spores survived; none were recoverable after
exposure to 120° C. _B. subtilis_ survived at atmospheric pressure and
90° C for 5 days, but none of the other spores were viable alter 2 days.
Four groups of soil organisms (mesophilic, aerobic, and anaerobic
bacteria, molds, and actinomycetes) were similarly tested in the vacuum
chamber. From one sample only actinomycetes survived 120° C, while one
other soil sample yielded viable bacteria after exposure to 170° C.
Several organisms resisted 120° C in ultrahigh vacuum for 4 to 5 days.
When irradiated with gamma rays from a cobalt 60 source, differences
were observed between vacuum-dried spores irradiated while under vacuum
and those exposed to air immediately before irradiation. A reduction of
from one-third to one-ninth of the viability of spores irradiated in
vacuum occurred with vacuum-treated spores irradiated in air.
Siegel et al. ([ref.73]), in approximate simulations of Martian
environments, studied tolerances of certain seed plants, such as
cucumbers, corn, and winter rye, to low temperatures and lowered oxygen
tensions. Lowered oxygen tensions enhanced the resistance of seedlings,
particularly cucumber and rye to freezing, and lowered the minimum
temperature required for germination. Germination of seeds in the
absence of liquid water has also been studied. In this case, seeds of
xerophytes have been suspended in air at 75-mm Hg pressure above water.
The air was thus saturated. Germination was slow but did occur.
Siegel et al. (refs. [ref.73] and [ref.74]) found that the growth rate
of several higher plants was enhanced by certain gases usually thought
to be toxic, such as N₂O. This finding is significant inasmuch as the
presence of nitrogen oxides in the Martian atmosphere has been cited as
evidence for the nonexistence of plants on that planet by Kiess et al.
([ref.75]). Exploratory survival tests showed that various mature
plants, as well as the larvae, pupae, and adult specimens of a
coleopteran insect, were undamaged when exposed to at least 40 hours of
an atmosphere containing 96.5 percent N₂O, 0.7 percent O₂, and 2.8
percent N₂.
Lichens are of interest because of their ability to survive and thrive
under extreme environmental conditions on Earth. Biological activity of
slow-growing lichens was detected by metabolic gas exchange, CO₂
detection being especially convenient. Siegel points out that this
method is sensitive and nondestructive, to be preferred to staining
techniques, which at present are limited because they are only
semiquantitative, subjective, and destructive of the lichen.
A Russian study of simulated planetary environments has been performed
with good simulation but for periods of only 2 to 6 hours. Comments on
simulation experiments made by Zhukova and Kondratyev ([ref.69]) are
presented as follows:
On the basis of modern conceptions on Martian conditions it is
difficult to imagine that higher forms of animals or plants
exist on the planet. A Martian change of seasons similar to that
of our planet empowers us to think that there is a circulation
of an organic substance on Mars, which cannot exist without
participation of microbic forms of life. Microorganisms are the
most probable inhabitants of Mars although the possibility is
not excluded that their physiological features will be very
specific. That is why the solution of the problem concerning the
character of life on Mars is of exceptional interest. But still
the answer to this question can be verified only by simulating
Martian conditions, taking into account the information obtained
from astrophysicists.
Experiments aimed at creating artificial Martian climatic
conditions have been started quite recently; their number is not
large since they cannot be combined with the results of numerous
experiments investigating the effect of extreme factors on
microorganisms. The result of the effect of such physicochemical
parameters of the medium as pressure, sharp temperature changes,
the absence of oxygen and insolation, depends on their
combination and simultaneity. These examples convincingly show
that while simulating Martian conditions one should strive to
the most comprehensive complex of simultaneously acting factors.
The creation of individual climatic parameters acting
successively leads to absolutely different, often opposite
results. It should be mentioned also that refusal to imitate
insolation and the performance of experiments with specimens of
soil which itself has protective effect on cells of
microorganisms, but not with pure culture of bacteria, are usual
shortcomings in the bulk of studies on this problem.
It appears that organisms from Earth might survive in large numbers when
introduced to Martian environment. Whether these organisms will be
capable of growth and explosive contamination of the planet in a
biological sense or not is highly questionable. The likelihood of an
organism from Earth finding ideal conditions for growth on Mars seems
extremely low. However, the likelihood of an organism from Earth serving
as a contaminant for any life-detection device flown to Mars for the
purpose of searching out carbon-based life is considerably higher. The
chance that life has originated and evolved on Mars is a completely
separate question and much more difficult to answer.
It would be interesting to attempt to determine possible evolutionary
trends which might occur on a planet by means of selection of organisms
in a simulated planetary environment. Rapid genetic selection combined
with radiation and chemicals to speed up mutation rate under these
conditions should reveal possible evolutionary trends under the
planetary environmental conditions. This could be attempted after the
planetary environments are more accurately defined.