SIMULATION OF PLANETARY (MARTIAN) ENVIRONMENTSAttempts 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 undercontrolled 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 intable 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 ofClostridium sporogenes.Increases in colony counts ofBacillus cereusappeared to be influenced by temperature cycling ([ref.72]).Table II.—Survival and Growth of Organisms in Simulated Planetary (Martian) EnvironmentsSpeciesSurvival, monthsMoistureTemperature, °CAtmospheric pressure, mm HgN2, percentCO2, percentSubstrateConditions on Mars:14µ±7µ-70 to +3085, 25±15, 113 to 30Anaerobic sporeformersClostridia,Bacillus planosarcina6Low, (CaSO4)-60 to +2076955Air-dried soilAnaerobic nonsporeformersPseudomonas,Rhodopseudomonas6Low, (CaSO4)-60 to +2076955Air-dried soilAnaerobesAerobacter aerogenes,Pseudomonas sp.GrowthVery wet-75 to +25760100(?)Difco infusion brothClostridium,Corynebacteria"Thin short rod"101 percent or less-25 to +2565100(?)SoilBacillus cereus20.5 percent soil-25 to +2565942.21Sandstone soilClostridium sporogenes1 (growth)8.4 percent-25 to +2565942Enriched soilClostridium botulinum10Lyophilized-25 to +2565950 to 0.5Lava soilKlebsiella pneumoniae6Lyophilized-25 to +2565950 to 0.5Lava soilBacillus subtilisvar.globigii42 percent-25 to +2585950.3MediaSarcina aurantiaca40.5 percent-25 to +2585950.3Desert soilClostridium tetani2 or less1 percent-60 to +2585950.3SoilAspergillus nigerOver 6 hrVery dry-60 to +257695.50.25Glass cloth on copper barAspergillus oryzaeOver 6 hrVery dry-60 to +257695.50.25Do.Mucor plumbeusOver 6 hrVery dry-60 to +257695.50.25Do.Rhodotorula rubraOver 6 hrVery dry-60 to +257695.50.25Do.Pea, bean, tomato, rye, sorghum, rice.0.3Moist+25751000Filter paperWinter rye0.6Moist-10 to +2376980.24SoilStudies 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 anaerobeClostridium botulinumsurvived 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 ofBacillus subtilisorPseudomonas aeruginosa.Bacillus cereusspores 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 109ergs/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, includingClostridium,Bacillus, andPlanosarcina, and nonsporeforming facultative anaerobes, includingPseudomonasandRhodopseudomonas, 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 primaryobjective 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 aerogenesandPseudomonas 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. subtilisvar.niger,B. megaterium,B. stearothermophilus,Clostridium sporogenes,andAspergillus 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, onlyB. subtilisandA. nigersurvived in appreciable numbers; at 107° C, onlyA. nigerspores survived; none were recoverable after exposure to 120° C.B. subtilissurvived 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, CO2detection 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 seemsextremely 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.
SIMULATION OF PLANETARY (MARTIAN) ENVIRONMENTSAttempts 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 undercontrolled 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 intable 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 ofClostridium sporogenes.Increases in colony counts ofBacillus cereusappeared to be influenced by temperature cycling ([ref.72]).Table II.—Survival and Growth of Organisms in Simulated Planetary (Martian) EnvironmentsSpeciesSurvival, monthsMoistureTemperature, °CAtmospheric pressure, mm HgN2, percentCO2, percentSubstrateConditions on Mars:14µ±7µ-70 to +3085, 25±15, 113 to 30Anaerobic sporeformersClostridia,Bacillus planosarcina6Low, (CaSO4)-60 to +2076955Air-dried soilAnaerobic nonsporeformersPseudomonas,Rhodopseudomonas6Low, (CaSO4)-60 to +2076955Air-dried soilAnaerobesAerobacter aerogenes,Pseudomonas sp.GrowthVery wet-75 to +25760100(?)Difco infusion brothClostridium,Corynebacteria"Thin short rod"101 percent or less-25 to +2565100(?)SoilBacillus cereus20.5 percent soil-25 to +2565942.21Sandstone soilClostridium sporogenes1 (growth)8.4 percent-25 to +2565942Enriched soilClostridium botulinum10Lyophilized-25 to +2565950 to 0.5Lava soilKlebsiella pneumoniae6Lyophilized-25 to +2565950 to 0.5Lava soilBacillus subtilisvar.globigii42 percent-25 to +2585950.3MediaSarcina aurantiaca40.5 percent-25 to +2585950.3Desert soilClostridium tetani2 or less1 percent-60 to +2585950.3SoilAspergillus nigerOver 6 hrVery dry-60 to +257695.50.25Glass cloth on copper barAspergillus oryzaeOver 6 hrVery dry-60 to +257695.50.25Do.Mucor plumbeusOver 6 hrVery dry-60 to +257695.50.25Do.Rhodotorula rubraOver 6 hrVery dry-60 to +257695.50.25Do.Pea, bean, tomato, rye, sorghum, rice.0.3Moist+25751000Filter paperWinter rye0.6Moist-10 to +2376980.24SoilStudies 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 anaerobeClostridium botulinumsurvived 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 ofBacillus subtilisorPseudomonas aeruginosa.Bacillus cereusspores 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 109ergs/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, includingClostridium,Bacillus, andPlanosarcina, and nonsporeforming facultative anaerobes, includingPseudomonasandRhodopseudomonas, 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 primaryobjective 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 aerogenesandPseudomonas 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. subtilisvar.niger,B. megaterium,B. stearothermophilus,Clostridium sporogenes,andAspergillus 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, onlyB. subtilisandA. nigersurvived in appreciable numbers; at 107° C, onlyA. nigerspores survived; none were recoverable after exposure to 120° C.B. subtilissurvived 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, CO2detection 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 seemsextremely 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.
SIMULATION OF PLANETARY (MARTIAN) ENVIRONMENTSAttempts 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 undercontrolled 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 intable 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 ofClostridium sporogenes.Increases in colony counts ofBacillus cereusappeared to be influenced by temperature cycling ([ref.72]).Table II.—Survival and Growth of Organisms in Simulated Planetary (Martian) EnvironmentsSpeciesSurvival, monthsMoistureTemperature, °CAtmospheric pressure, mm HgN2, percentCO2, percentSubstrateConditions on Mars:14µ±7µ-70 to +3085, 25±15, 113 to 30Anaerobic sporeformersClostridia,Bacillus planosarcina6Low, (CaSO4)-60 to +2076955Air-dried soilAnaerobic nonsporeformersPseudomonas,Rhodopseudomonas6Low, (CaSO4)-60 to +2076955Air-dried soilAnaerobesAerobacter aerogenes,Pseudomonas sp.GrowthVery wet-75 to +25760100(?)Difco infusion brothClostridium,Corynebacteria"Thin short rod"101 percent or less-25 to +2565100(?)SoilBacillus cereus20.5 percent soil-25 to +2565942.21Sandstone soilClostridium sporogenes1 (growth)8.4 percent-25 to +2565942Enriched soilClostridium botulinum10Lyophilized-25 to +2565950 to 0.5Lava soilKlebsiella pneumoniae6Lyophilized-25 to +2565950 to 0.5Lava soilBacillus subtilisvar.globigii42 percent-25 to +2585950.3MediaSarcina aurantiaca40.5 percent-25 to +2585950.3Desert soilClostridium tetani2 or less1 percent-60 to +2585950.3SoilAspergillus nigerOver 6 hrVery dry-60 to +257695.50.25Glass cloth on copper barAspergillus oryzaeOver 6 hrVery dry-60 to +257695.50.25Do.Mucor plumbeusOver 6 hrVery dry-60 to +257695.50.25Do.Rhodotorula rubraOver 6 hrVery dry-60 to +257695.50.25Do.Pea, bean, tomato, rye, sorghum, rice.0.3Moist+25751000Filter paperWinter rye0.6Moist-10 to +2376980.24SoilStudies 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 anaerobeClostridium botulinumsurvived 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 ofBacillus subtilisorPseudomonas aeruginosa.Bacillus cereusspores 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 109ergs/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, includingClostridium,Bacillus, andPlanosarcina, and nonsporeforming facultative anaerobes, includingPseudomonasandRhodopseudomonas, 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 primaryobjective 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 aerogenesandPseudomonas 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. subtilisvar.niger,B. megaterium,B. stearothermophilus,Clostridium sporogenes,andAspergillus 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, onlyB. subtilisandA. nigersurvived in appreciable numbers; at 107° C, onlyA. nigerspores survived; none were recoverable after exposure to 120° C.B. subtilissurvived 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, CO2detection 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 seemsextremely 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.
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 undercontrolled 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 intable 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 ofClostridium sporogenes.Increases in colony counts ofBacillus cereusappeared to be influenced by temperature cycling ([ref.72]).
Species
Survival, months
Moisture
Temperature, °C
Atmospheric pressure, mm Hg
N2, percent
CO2, percent
Substrate
Conditions on Mars:
14µ±7µ
-70 to +30
85, 25±15, 11
3 to 30
Clostridia,Bacillus planosarcina
6
Low, (CaSO4)
-60 to +20
76
95
5
Air-dried soil
Pseudomonas,Rhodopseudomonas
6
Low, (CaSO4)
-60 to +20
76
95
5
Air-dried soil
Aerobacter aerogenes,Pseudomonas sp.
Growth
Very wet
-75 to +25
760
100
(?)
Difco infusion broth
Corynebacteria"Thin short rod"
10
1 percent or less
-25 to +25
65
100
(?)
Soil
Bacillus cereus
2
0.5 percent soil
-25 to +25
65
94
2.21
Sandstone soil
Clostridium sporogenes
1 (growth)
8.4 percent
-25 to +25
65
94
2
Enriched soil
Clostridium botulinum
10
Lyophilized
-25 to +25
65
95
0 to 0.5
Lava soil
Klebsiella pneumoniae
6
Lyophilized
-25 to +25
65
95
0 to 0.5
Lava soil
Bacillus subtilisvar.globigii
4
2 percent
-25 to +25
85
95
0.3
Media
Sarcina aurantiaca
4
0.5 percent
-25 to +25
85
95
0.3
Desert soil
Clostridium tetani
2 or less
1 percent
-60 to +25
85
95
0.3
Soil
Aspergillus niger
Over 6 hr
Very dry
-60 to +25
76
95.5
0.25
Glass cloth on copper bar
Aspergillus oryzae
Over 6 hr
Very dry
-60 to +25
76
95.5
0.25
Do.
Mucor plumbeus
Over 6 hr
Very dry
-60 to +25
76
95.5
0.25
Do.
Rhodotorula rubra
Over 6 hr
Very dry
-60 to +25
76
95.5
0.25
Do.
Pea, bean, tomato, rye, sorghum, rice.
0.3
Moist
+25
75
100
0
Filter paper
Winter rye
0.6
Moist
-10 to +23
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 anaerobeClostridium botulinumsurvived 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 ofBacillus subtilisorPseudomonas aeruginosa.Bacillus cereusspores 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 109ergs/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, includingClostridium,Bacillus, andPlanosarcina, and nonsporeforming facultative anaerobes, includingPseudomonasandRhodopseudomonas, 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 primaryobjective 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 aerogenesandPseudomonas 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. subtilisvar.niger,B. megaterium,B. stearothermophilus,Clostridium sporogenes,andAspergillus 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, onlyB. subtilisandA. nigersurvived in appreciable numbers; at 107° C, onlyA. nigerspores survived; none were recoverable after exposure to 120° C.B. subtilissurvived 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, CO2detection 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.
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.
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 seemsextremely 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.