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Life on Mars? II. Physical restrictions
Ah. Space Res. Vol. 15, No. 3, pp. (3)171-(3)176, 1995 1994 COSPAB
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LIFE ON MARS? II. PHYSICAL
RESTRICTIONS
R. L. Mancinelli* and A. Bank*>** * NASA Ames Research Center, Moffett Field, CA, U.S.A. **Hebrew Universiv, P.O. Box 12, Rehovot, Israel
ABsTRAcr The primary physical factors important to life’s evolution on a planet include its temperature, pressure and radiation regimes. Temperature and pressure regulate the presence and duration of liquid water on the surface of Mars. The prolonged presence of liquid water is essential for the evolution and sustained presence of life on a planet. It has been postulated that Mars has always been a cold dry planet; it has also been postulated that early Mars possessed a dense atmosphere of CO2 (2 1 bar) and sufficient water to cut large channels across its surface. The degree to which either of these postulates is true correlates with the suitability of Mars for life’s evolution. Although radiation can destroy living systems, the high fluxes of UV radiation on the martian surface do not necessarily stop the origin and early evolution of life. The probability for life to have arisen and evolved to a significant degree on Mars, based on the postulated ranges of early mattian physical factors, is almost solely related to the probability of liquid water existing on the planet for at least hundreds of millions to billions of years. INTRODUCTION In this paper and its companion /I/. the chemical inventories and the martian environments’ physical conditions that determine its suitability for the origin and evolution of life are assessed. The primary physical factors important in the evolution of life on a planet include its temperature, pressure and radiation regimes. Because temperature and pressure regulate the state of water on the surface of a planet, the presence and duration of liquid water on the surface of Mars is governed by its temperature and pressure reghnes. The prolonged presence of liquid water is essential for the formation of pmbiotic macromolecules, their eventual assembly into organisms, and the sustained presence of life on a planet. The temperature and pressure regimes of Mars and its water inventory and history are not well understood. It has been postulated that Mars has always been a cold dry planet; it has also been postulated that 3.5 Gyr ago Mars possessed a dense Cq atmosphere @ 1 bar) and a hydrological cycle involving the movement of sufficient water to cut large valleys and channels across its surface. The degree to which either of these postulates is true correlates directly with the probability of Mars having been suitable for life to originate and evolve. The radiation regime of Mars may have played a role in where on Mars life may have arisen. TEMPERATURE AND PRESSURE The temperatures on the surface of Mars ate low and typically range from -100°C to -5°C. with a rare high to 17°C (R4/ see /5/ for a review). These temperatures overlap with those found on Earth. The total atmospheric pressure on Mars, mostly due to CO2, ranges from 7.4 to 10 mbar with an average of 8 mbar /6,7/. which is significantly leas than the 1013 mbar average of Earth. Carbon dioxide is postulated to have been the major constituent of the early atmospheres of Mars and Earth (e.g. /8,9/). Though not as abundant, water and N2 have also been hypothesized to be components of the early mattian atmosphere.. Furthermore, it has been hypothesized that if liquid water existed on early Mars then the atmospheric pressure was close to the one bar thought to exist on early Earth /8,9.10,11,12,13/.
Historv of water on Ma. The surface of Mars today exhibits carved features such as out-flow channels, valleys and lahars that date back to its early history /6.14-17/. These features have led some geologists to hypothesize that liquid water once flowed across the martian surface /6.14-19/. Further, the total amount of water thought to have existed on Mars early in its history has been calculated using several models, including those dealing with planetary formation, isotopic and chemical ratios of the martian atmosphere, and surface geomotphology. Estimates of the amount of liquid water that may have existed on the planet based on these models range from an amount that would cover the entire surface of the planet to a depth of -6 m to 1 km RO-25/. At least some of the carved features on the martian surface, however, may have originated by a cataclysmic outburst of water from subsurface sources, or have formed from ice sheet melting, or from sapping, rather than the sustained presence. of flowing water /26,27/. If these hypotheses am true then the amount of water that may have existed on the
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surface of Mars would be far less than required for the models cited previously. Indeed, their formation then does not require a dense atmosphere because the rush of water from the ground is rapid enough to came the surface in one short event 128. 29/. In addition, data from analysis of water from the Shergotty-Nakhla-Chassigny (SNC) meteorites, purportedly of martian origin, suggest that Mars had a hydrosphere that was decoupled from the lithosphere /30/. These data suggest that Mars did not have a hydrologic cycle involving the exchange of ground water with the atmosphere, decreasing the pmbability that large amounts of liquid water flowed across the martian surface for a billion years during its early history. Water vapor measurements made by the Viking orbiter indicated that the water content of the atmosphere fluctuated as a function of geographic location, topography, season, and time of day, with the maximum being 100 precipitable microns in the North Polar region during mid summer /31.32/. the relative humidity at the surface of the remainder of the planet is unknown. Regolith samples collected at the two Viking lander sites were found to contain approximately l-38 H20, by weight, when heated in a step-wise fashion to 5OtYC. Most of the water was released between 2tB300°C, indicating the presence of mineral hydrates of low to moderate thermal stability /33, 34/. These data support the notion that Hz0 in minerals is an important sink of Hz0 on Mars. The data gathered by Viking allow one to construct a tough descriptive model of the cut-tent martian environment at the two Viking lander sites. Although water ice does exist at the poles, and permafrost may also exist /31/, the environment comprising the vast majority of the the martian surface is desiccated. wand Water in the liquid state serves as the solvent for many biogenic compounds and mediates important geochemical (abiotic). prebiotic and biological reactions. Liquid water is important to the reactions necessary for the origin of life and its continued evolution because it is the primary intracellular solvent. As a consequence, the temperature regime of at least some portion of a planet must be above the triple point of water for life to have evolved. During the past 30 years numerous microbiological investigations have been conducted some in preparation for the Viking mission to Mars /35,36,37,38,39/. The data from these number of viable microorganisms in the soil of the Antarctic Dry Valleys declined as a negative water potential). The data suggest that water is the factor limiting microbial temperature f39l.
on Antarctic Dry Valley soils, investigations indicated that the function of soil moisture (more activity in the soil rather than
Investigations were undertaken during the 1960’s and 1970’s to determine the effect of freeze-thaw cycles at one atmosphere and at reduced atmospheric pressures. For example, it was demonstrated that a variety of soil microorganisms survived being subjected to freeze-thaw cycles using temperature ranges ftom +25 to do”C /40-45/, low atmospheric pressures down to 10 mbar /44-48/, and levels of moisture in soils of -0.5 96 /41.49/. The results of these studies suggest that the most important factor for the survival and growth of life is the availability of liquid water. It is known that changes in water content of soils have profound effects on microbial activity /50/. which result in changes in soil microbial populations /51/. Water potential is the free energy of water in a system, relative to the free energy of a reference pool of pure, ftee water. Pure free water has, by definition, zero water potential, and for example, the potential energy of water in unsaturated soils is negative. The tendency for water is to flow from high to low potential, making the availability of water for microbes in the soil less as the potential is lowered. The water potential of a microbial cell in soil is likely to be near equilibrium with its microenvironment within the soil (see e.g., /52/ for a review). The prospects for the origin of life on Mars would only have been favorable if the water potential was higher than it is today. Due to the necessity of liquid water, life undoubtedly arose in liquid water. Thus, life on Mats would be located in association with water-laden deposits, aquifers or soils that were once saturated with liquid water. Extremes for Life Although most organisms live at temperatures less than 40°C, the upper temperature limit for life is the upper limit for the stability of liquid water. For example, on the Earth’s surface the upper temperature limit for life is -100°C (the boiling point of water at 1 atm), but organisms isolated from near deep sea hydrothermal vents can grow at -110°C (the pressure at the deep sea hydrothermal vents is greater than 1 atm, increasing the boiling point of water) /53,54/. Life has also been found to live at low temperatures. In fact certain bacteria are metabolically active to temperatures well below 0°C so long as liquid water esists /55/. As stated in the previous section, studies of the Antarctic soil microbial ecology indicate that water potential rather than the low temperature was the factor limiting microbial activity. RADIATION
The distance from Mars to the sun is nearly half again more than the distance from the Earth to the sun As a consequence, the solar radiation incident on the mattian atmosphere is 43% of that incident on the Earth’s atmosphere /56/. On Earth approximately 55% of the total solar radiation reaching the atmosphere reaches the surface. On Mars most of the radiation teaches the surface. During summer the maximum daily solar radiation reaching the surface of the southern hemisphere of Mars has been calculated to be approximately 325 cal cmS /56/. During the winter the radiation reaching the surface of Mars is significantly less tban during the summer. The radiation reaching the surface at wavelengths greater than 280 nm is approximately 10% of the radiation incident on the Earth’s atmosphere. There is significant extinction of radiation near 250 mn in the mid- and high-latitudes due to adsorption of radiation by OZOIK..
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On Mars there is no oxone, but the atmospheric CR adsorbs virtually all of the radiation at wavelengths < 190 mn. As a result none of it reaches the martian surface /56/.
The effects of the current Martian radiation regime on life have been reviewed recently and will not be discussed at length in this paper /S/. Suffice it to say that several studies have shown that exposure to direct solar radiation containing ultraviolet light similar to that falling on the surface of Mars is lethal to microorganisms, but if the organisms were shielded by dust pardcles a certain percentage survived /43, 57-59/. The high fluxes of ultraviolet light that radiated the surface of the early Earth were obviously not inhibitory to the origin and early evolution of life. Therefore, if the early atmosphere of Mars were similar to the atmosphere of early Earth it would not have been inhibitory to the origin and early evolution of life /60/. The highly absorptive 02 and @ ln the Earth’s atmosphere appeared -1.7 biion years ago /61/, after the origin and early evolution of life. As a consequence, life on earth evolved several repair mechanisms that protected them from ultra-violet radiation. The same selection pressure would have allowed such mechanisms to have evolved on any life-form that arose on early Mars /see 5 for a review/. In fact, Mars’ greater distance from the sun means that early Mars received nearly half as much radiation as early Earth making life less threatened by the sun’s radiation. If. however, the atmosphere of early Mars were as thin as it is today, then life could have only arose and evolved in protected niches (e.g., beneath the surface). CONCLUSIONS Water is one of the most important volatiles known to occur on Mars that is essential for the origin and evolution of life. Because liquid water is essential to the origin and evolution of life, Mars’ temperature and pressure regimes, which determine the state of water, are important physical features of its environment governing the planet’s suitability for the origin and evolution of life. Although missions to Mars gathered much information regarding this important biological compound, a number of unanswered questions remain regarding its state, distribution and history. For example, did significant quantities of water ever exist on the surface of Mars? And if they did was it long enough for life to originate? It is uncertain whether liquid water persisted on the surface of Mars long enough for the hundreds of millions to billions of years it took life to originate on Earth /62/. Indeed, the axial tilt of Mars may have changed significantly over the age of the solar system. implying that Mars’ paleoclimate may have been different than currently thought /63/. Clearly, the current martian environment that exists across the vast majority of the planet’s surface is inhospitable to life, primarily because of the lack of sufficient quantities of liquid water. Although certain wavelengths and intensities of radiation destroy living systems, the high fluxes of ultraviolet radiation on the surface of Mars would not necessarily stop the origin and early evolution of life, as evidenced by the origin of life on early Earth which had a high flux of ultra-violet radiation. The martian ultraviolet radiation regime, however, would affect the types of organisms that might evolve (i.e., possess UV absorbing pigments, e.g., carotenoids. purines, pyrimidines). and where they might evolve (i.e., areas shielded from UV. e.g., beneath a water column, buried in sediment, beneath the surface soil). From the above discussion it is clear that radiation would influence where and what types of organisms would evolve on Mars, but not inhibit the origin of life on that planet. Neither pressure nor the temperature regime taken individually would stop the origin and evolution of life on Mars. The pressure and temperature regimes of Mars are important In so far as they regulate the state of water on Mars. Did life arise on Mars during its early history? This is a primary question driving the exobiological investigation of Mars. To find the answer to this question other questions must first be answered. These questions include, 1) were the major and minor chemical building blocks of life (C, H. N, 0, P. S, Mg. Mn. Cu, MO. etc.) and compounds (e.g., NO3-, CO2, S042-, etc.) available in sufficient quantities on early Mars for life to arise? 2) were the building blocks of life and compounds in an appropriate state (liquid, solid, gas) for life to arise? and 3) was the physical state of Mars suitable for life to arise? The data obtained thus far suggest that early Mars did possess sufficient quantities of the building blocks of life for life to arise /l/. The data also suggest that a number of the chemical compounds necessary for life to arise were available /l/. It is not known, however, if water existed in the appropriate state (i.e., liquid) ln sufficient quantities long enough for life to arise. It has been postulated that the physical environment of early Mars may have been too harsh for life to arise /41-50/, but from the discussion presented in this paper it is clear that the physical environment was suitable for life to arise. REFERENCES 1. A. Banin and R.L. Mancinelli, Life on Mars? I. 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Primed in Great Britain. 0273-l 177195$7.00 + 0.00
LIFE ON MARS? II. PHYSICAL
RESTRICTIONS
R. L. Mancinelli* and A. Bank*>** * NASA Ames Research Center, Moffett Field, CA, U.S.A. **Hebrew Universiv, P.O. Box 12, Rehovot, Israel
ABsTRAcr The primary physical factors important to life’s evolution on a planet include its temperature, pressure and radiation regimes. Temperature and pressure regulate the presence and duration of liquid water on the surface of Mars. The prolonged presence of liquid water is essential for the evolution and sustained presence of life on a planet. It has been postulated that Mars has always been a cold dry planet; it has also been postulated that early Mars possessed a dense atmosphere of CO2 (2 1 bar) and sufficient water to cut large channels across its surface. The degree to which either of these postulates is true correlates with the suitability of Mars for life’s evolution. Although radiation can destroy living systems, the high fluxes of UV radiation on the martian surface do not necessarily stop the origin and early evolution of life. The probability for life to have arisen and evolved to a significant degree on Mars, based on the postulated ranges of early mattian physical factors, is almost solely related to the probability of liquid water existing on the planet for at least hundreds of millions to billions of years. INTRODUCTION In this paper and its companion /I/. the chemical inventories and the martian environments’ physical conditions that determine its suitability for the origin and evolution of life are assessed. The primary physical factors important in the evolution of life on a planet include its temperature, pressure and radiation regimes. Because temperature and pressure regulate the state of water on the surface of a planet, the presence and duration of liquid water on the surface of Mars is governed by its temperature and pressure reghnes. The prolonged presence of liquid water is essential for the formation of pmbiotic macromolecules, their eventual assembly into organisms, and the sustained presence of life on a planet. The temperature and pressure regimes of Mars and its water inventory and history are not well understood. It has been postulated that Mars has always been a cold dry planet; it has also been postulated that 3.5 Gyr ago Mars possessed a dense Cq atmosphere @ 1 bar) and a hydrological cycle involving the movement of sufficient water to cut large valleys and channels across its surface. The degree to which either of these postulates is true correlates directly with the probability of Mars having been suitable for life to originate and evolve. The radiation regime of Mars may have played a role in where on Mars life may have arisen. TEMPERATURE AND PRESSURE The temperatures on the surface of Mars ate low and typically range from -100°C to -5°C. with a rare high to 17°C (R4/ see /5/ for a review). These temperatures overlap with those found on Earth. The total atmospheric pressure on Mars, mostly due to CO2, ranges from 7.4 to 10 mbar with an average of 8 mbar /6,7/. which is significantly leas than the 1013 mbar average of Earth. Carbon dioxide is postulated to have been the major constituent of the early atmospheres of Mars and Earth (e.g. /8,9/). Though not as abundant, water and N2 have also been hypothesized to be components of the early mattian atmosphere.. Furthermore, it has been hypothesized that if liquid water existed on early Mars then the atmospheric pressure was close to the one bar thought to exist on early Earth /8,9.10,11,12,13/.
Historv of water on Ma. The surface of Mars today exhibits carved features such as out-flow channels, valleys and lahars that date back to its early history /6.14-17/. These features have led some geologists to hypothesize that liquid water once flowed across the martian surface /6.14-19/. Further, the total amount of water thought to have existed on Mars early in its history has been calculated using several models, including those dealing with planetary formation, isotopic and chemical ratios of the martian atmosphere, and surface geomotphology. Estimates of the amount of liquid water that may have existed on the planet based on these models range from an amount that would cover the entire surface of the planet to a depth of -6 m to 1 km RO-25/. At least some of the carved features on the martian surface, however, may have originated by a cataclysmic outburst of water from subsurface sources, or have formed from ice sheet melting, or from sapping, rather than the sustained presence. of flowing water /26,27/. If these hypotheses am true then the amount of water that may have existed on the
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surface of Mars would be far less than required for the models cited previously. Indeed, their formation then does not require a dense atmosphere because the rush of water from the ground is rapid enough to came the surface in one short event 128. 29/. In addition, data from analysis of water from the Shergotty-Nakhla-Chassigny (SNC) meteorites, purportedly of martian origin, suggest that Mars had a hydrosphere that was decoupled from the lithosphere /30/. These data suggest that Mars did not have a hydrologic cycle involving the exchange of ground water with the atmosphere, decreasing the pmbability that large amounts of liquid water flowed across the martian surface for a billion years during its early history. Water vapor measurements made by the Viking orbiter indicated that the water content of the atmosphere fluctuated as a function of geographic location, topography, season, and time of day, with the maximum being 100 precipitable microns in the North Polar region during mid summer /31.32/. the relative humidity at the surface of the remainder of the planet is unknown. Regolith samples collected at the two Viking lander sites were found to contain approximately l-38 H20, by weight, when heated in a step-wise fashion to 5OtYC. Most of the water was released between 2tB300°C, indicating the presence of mineral hydrates of low to moderate thermal stability /33, 34/. These data support the notion that Hz0 in minerals is an important sink of Hz0 on Mars. The data gathered by Viking allow one to construct a tough descriptive model of the cut-tent martian environment at the two Viking lander sites. Although water ice does exist at the poles, and permafrost may also exist /31/, the environment comprising the vast majority of the the martian surface is desiccated. wand Water in the liquid state serves as the solvent for many biogenic compounds and mediates important geochemical (abiotic). prebiotic and biological reactions. Liquid water is important to the reactions necessary for the origin of life and its continued evolution because it is the primary intracellular solvent. As a consequence, the temperature regime of at least some portion of a planet must be above the triple point of water for life to have evolved. During the past 30 years numerous microbiological investigations have been conducted some in preparation for the Viking mission to Mars /35,36,37,38,39/. The data from these number of viable microorganisms in the soil of the Antarctic Dry Valleys declined as a negative water potential). The data suggest that water is the factor limiting microbial temperature f39l.
on Antarctic Dry Valley soils, investigations indicated that the function of soil moisture (more activity in the soil rather than
Investigations were undertaken during the 1960’s and 1970’s to determine the effect of freeze-thaw cycles at one atmosphere and at reduced atmospheric pressures. For example, it was demonstrated that a variety of soil microorganisms survived being subjected to freeze-thaw cycles using temperature ranges ftom +25 to do”C /40-45/, low atmospheric pressures down to 10 mbar /44-48/, and levels of moisture in soils of -0.5 96 /41.49/. The results of these studies suggest that the most important factor for the survival and growth of life is the availability of liquid water. It is known that changes in water content of soils have profound effects on microbial activity /50/. which result in changes in soil microbial populations /51/. Water potential is the free energy of water in a system, relative to the free energy of a reference pool of pure, ftee water. Pure free water has, by definition, zero water potential, and for example, the potential energy of water in unsaturated soils is negative. The tendency for water is to flow from high to low potential, making the availability of water for microbes in the soil less as the potential is lowered. The water potential of a microbial cell in soil is likely to be near equilibrium with its microenvironment within the soil (see e.g., /52/ for a review). The prospects for the origin of life on Mars would only have been favorable if the water potential was higher than it is today. Due to the necessity of liquid water, life undoubtedly arose in liquid water. Thus, life on Mats would be located in association with water-laden deposits, aquifers or soils that were once saturated with liquid water. Extremes for Life Although most organisms live at temperatures less than 40°C, the upper temperature limit for life is the upper limit for the stability of liquid water. For example, on the Earth’s surface the upper temperature limit for life is -100°C (the boiling point of water at 1 atm), but organisms isolated from near deep sea hydrothermal vents can grow at -110°C (the pressure at the deep sea hydrothermal vents is greater than 1 atm, increasing the boiling point of water) /53,54/. Life has also been found to live at low temperatures. In fact certain bacteria are metabolically active to temperatures well below 0°C so long as liquid water esists /55/. As stated in the previous section, studies of the Antarctic soil microbial ecology indicate that water potential rather than the low temperature was the factor limiting microbial activity. RADIATION
The distance from Mars to the sun is nearly half again more than the distance from the Earth to the sun As a consequence, the solar radiation incident on the mattian atmosphere is 43% of that incident on the Earth’s atmosphere /56/. On Earth approximately 55% of the total solar radiation reaching the atmosphere reaches the surface. On Mars most of the radiation teaches the surface. During summer the maximum daily solar radiation reaching the surface of the southern hemisphere of Mars has been calculated to be approximately 325 cal cmS /56/. During the winter the radiation reaching the surface of Mars is significantly less tban during the summer. The radiation reaching the surface at wavelengths greater than 280 nm is approximately 10% of the radiation incident on the Earth’s atmosphere. There is significant extinction of radiation near 250 mn in the mid- and high-latitudes due to adsorption of radiation by OZOIK..
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On Mars there is no oxone, but the atmospheric CR adsorbs virtually all of the radiation at wavelengths < 190 mn. As a result none of it reaches the martian surface /56/.
The effects of the current Martian radiation regime on life have been reviewed recently and will not be discussed at length in this paper /S/. Suffice it to say that several studies have shown that exposure to direct solar radiation containing ultraviolet light similar to that falling on the surface of Mars is lethal to microorganisms, but if the organisms were shielded by dust pardcles a certain percentage survived /43, 57-59/. The high fluxes of ultraviolet light that radiated the surface of the early Earth were obviously not inhibitory to the origin and early evolution of life. Therefore, if the early atmosphere of Mars were similar to the atmosphere of early Earth it would not have been inhibitory to the origin and early evolution of life /60/. The highly absorptive 02 and @ ln the Earth’s atmosphere appeared -1.7 biion years ago /61/, after the origin and early evolution of life. As a consequence, life on earth evolved several repair mechanisms that protected them from ultra-violet radiation. The same selection pressure would have allowed such mechanisms to have evolved on any life-form that arose on early Mars /see 5 for a review/. In fact, Mars’ greater distance from the sun means that early Mars received nearly half as much radiation as early Earth making life less threatened by the sun’s radiation. If. however, the atmosphere of early Mars were as thin as it is today, then life could have only arose and evolved in protected niches (e.g., beneath the surface). CONCLUSIONS Water is one of the most important volatiles known to occur on Mars that is essential for the origin and evolution of life. Because liquid water is essential to the origin and evolution of life, Mars’ temperature and pressure regimes, which determine the state of water, are important physical features of its environment governing the planet’s suitability for the origin and evolution of life. Although missions to Mars gathered much information regarding this important biological compound, a number of unanswered questions remain regarding its state, distribution and history. For example, did significant quantities of water ever exist on the surface of Mars? And if they did was it long enough for life to originate? It is uncertain whether liquid water persisted on the surface of Mars long enough for the hundreds of millions to billions of years it took life to originate on Earth /62/. Indeed, the axial tilt of Mars may have changed significantly over the age of the solar system. implying that Mars’ paleoclimate may have been different than currently thought /63/. Clearly, the current martian environment that exists across the vast majority of the planet’s surface is inhospitable to life, primarily because of the lack of sufficient quantities of liquid water. Although certain wavelengths and intensities of radiation destroy living systems, the high fluxes of ultraviolet radiation on the surface of Mars would not necessarily stop the origin and early evolution of life, as evidenced by the origin of life on early Earth which had a high flux of ultra-violet radiation. The martian ultraviolet radiation regime, however, would affect the types of organisms that might evolve (i.e., possess UV absorbing pigments, e.g., carotenoids. purines, pyrimidines). and where they might evolve (i.e., areas shielded from UV. e.g., beneath a water column, buried in sediment, beneath the surface soil). 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