MODEL SYSTEMS FOR LIFE PROCESSES

ON MARS

M. A. MITZ National Aeronautics and Space Administration, Washington, D.C., U.S.A.

Abstract. In the evolution of life forms non-photosynthetic mechanisms have developed. The question remains whether a total life system could evolve which is not dependent upon photosynthesis. In trying to visualize life on other planets, the photosynthetic process has problems. On Mars, the high intensity of light at the surface is a concern and alternative mechanisms need to be defined and analyzed. In the UV search for alternate mechanisms, several different areas may be identified. These involve activated inorganic compounds in the atmosphere, such as the products of photodissociation of carbon dioxide and the organic material which may be created by natural phenomena. In addition, a life system based on the pressure of the atmospheric constituents, such as carbon dioxide, is a possibility. These considerations may be important for the understanding of evolutionary processes of life on another planet. Model systems which depend on these alternative mechanisms are defined and related to our presently planned and future planetary missions. Present knowledge of the environment and the chemistry of Mars (Mars Scientific Model, 1972) would lead one to suspect that life, if it exists on the planet, may have taken another path than the life processes on Earth. To identify these alternatives is a challenge but as any one who is trying to define a life detection experiment knows, it is necessary to study all possible alternatives in addressing this problem. The rationale for a search for terrestrial-like organisms was reviewed by Ponnamperuma and Klein (1970). This paper identified several possible alternatives to photosynthesis for a basis of life on Mars and discusses the impact on planning for future missions. The Martian atmosphere is composed mainly of carbon dioxide with small amounts of carbon monoxide and oxygen (Owen and Sagan, 1972). Low concentrations of water vapor are detectable in the atmosphere on a seasonable basis from time to time. In contrast to Earth, nitrogen in any form is not sufficiently high in the atmosphere to be measurable with the instrumentation used to date; although there remains the possibility that it is present at lower detection levels. Photodissociation of carbon dioxide by solar UV light reaching the planet is believed responsible for the carbon monoxide in the atmosphere. Based on calculations of UV intensity on the planet, a large portion of the carbon dioxide is subject to the dissociation annually but the amount of carbon monoxide present at any given time is relatively small (less than 0.1~) at the surface and the corresponding free oxygen is about the same order of magnitude. Complex reverse reactions to return that carbon monoxide to carbon dioxide are being considered because it is not obvious that the direct reversal can take place readily (McElroy and Donahue, 1972). In fact, biological oxidation of carbon monoxide is one of the mechanisms which has been proposed to account for the observed low concentration of CO and as a possible energy source for life (Wolfgang, 1970). An energetic intermediate of this type over comes the problem of organisms adapting with ability to receive light for photoOrigins of Life 5 (1974) 457462. All Rights Reserved Copyright 9 1974 by D. Reidel Publishing Company, Dordrecht-Holland

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synthesis in the presence of known destructive fluxes of UV light on the surface of Mars. There is also the problem of extended periods (months) of global dust storms which reduces the light level reaching the surface. Once liberated from direct contact with sunlight, Earth organisms do develop which live at great depths. On Mars this opens the possibility for life to exist under more favorable conditions of temperature, and water concentration than is possible on the surface. This paper considers possible alternative mechanisms for life which depend on carbon monoxide, carbon dioxide, and geothermal energy. These factors are considered individually as well as in combinations. Using these proposed mechanisms the question is discussed as to whether the experiment on the Viking mission to explore the surface of Mars in 1976 can detect these types of organisms if indeed they do exist. In addition, the model systems identified in this paper are examined for consideration in the design of future experiments.

1. Possible Mechanisms 1.1. CARBONMONOXIDE As a source of energy, carbon monoxide can yield considerable energy on conversion of carbon dioxide. Wolfgang pointed out the possibility of organisms making this conversion and thereby gaining the energy chemically rather than returning it to the atmosphere as heat (Equation (1)). Lederberg (1969) earlier speculated that carbon dioxide could react directly with water to synthesize carbohydrates (Equation (2)). C O .--[-1 0 2 --.-+C O 2 + 67 Kcal

(1)

2 C O + H 2 0 --> C H 2 0 + C O 2

(2)

In addition, there are other reactions of carbon monoxide and carbon dioxide which transfer energy and provide the cell with useful products. For example, nitrates and nitrites react with carbon monoxide under the proper conditions to produce ammonia (Equations (3) and (4)) which in turn reacts exothermically with carbon dioxide to produce ammonium carbamate (Equation (5)). HNO 3 + 4CO + HzO -~ N H 3 + 4CO z

(3)

NO~- + 3CO + 2H20 ~ NHa + 3 CO z + O H -

(4)

2NH3 + CO2 ~ NH2COONH4 + 44Kcal

(5)

Under suitable conductions ammonium carbamate is converted to urea (Equation (6)) then to ammonium cyanate (Equation (7)). The latter can also be converted to cyanic acid. NH2COONH4 ~ NH2CONH2 + H 2 0

(6)

N H 2 C O N H 2 --* NH4CNO

(7)

Most of these reactions will take place slowly at room temperature and more rapidly at elevated temperatures. For example, ammonium carbamate is converted to urea in 53~ yield at temperature of 150~ in 30 min. Once the ammonium cyanate is

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present, it may be involved in several interesting reactions. Of particular interest is the reaction with phosphate (Equation (8)) to produce an active ester. KCNO + KH2PO 4 -0 NHaCOOPO4H 2 + 2KOH

(8)

According to Jones and Lipmann (1960) this reaction takes place at room temperature at pH 5 to 6 with 40~ conversion. They indicate that equilibrium is reached 'fairly rapidly'. The carbamyl phosphate produced in Equation (8) is a phosphorylating agent which reacts with adenosine diphosphate (ADP) to form adenosine triphosphate (ATP). NH2COO-P + ADP ~ NH3 + CO2 + ATP (9) The ammonia and carbon dioxide generated can be recycled. However, the carbon dioxide would be diluted by the pool of carbon dioxide in the process. Except for reaction 2 and 6, the original carbon monoxide may transfer its energy to several new compounds which do not p e r s e involve incorporation of the carbon or oxygen into the material produced. 1.2. CARBONDIOXIDE Another potential source of enelgy is the atmosphere of carbon dioxide. Assuming that any moisture present will be saturated with CO2, then the first reaction to consider is the solubilization and subsequent reaction of CO2 with water (or ice) (Equation (10)). This assumes that there is liquid water somewhere on the planet. C O 2 g a s Ai- H20 ~ C O 2 liq. -~- H20 ~- H 2 C O 3 ~-

HCO 3 + H +

(10)

The heat of solution of CO2 in water alone is + 4.7 Kcal mole- 2. The dissolved carbon dioxide then reacts slowly with water to form bicarbonate which may react with other ions in contact with the solution. If insoluble salts are present, they may be solubilized. For example, in the presence of carbon dioxide saturated water insoluble calcium carbonate is converted to slight more soluble calcium bicarbamate (Equation (11)). CaCO3 + HCO3 + H + ~ + Ca(HCO3)z (11) In this way the water saturated with carbon dioxide slowly leaches out some of the soluble minerals needed by the organism. If the organism possesses even a simple primitive ionic membrane, the extraction process might be accelerated and extended. In this case the carbon dioxide may be used for generating stronger acid to extract the needed nitrogen and phosphorous containing minerals without depending on large volumes of water for leaching at great distances from the organisms. In the past, the extraction has been postulated based on excreted organic acids or biologically derived chelating agents which are generally also organic in nature. It is theoretically possible (Mitz, 1971) to produce significantly high hydrogen ion concentrates using the pressure of carbon dioxide and the appropriately charged membrane. The overall reaction can be written as salt and carbon dioxide going to sodium bicarbonate and a strong acid, Equations (12) and (13).

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M.A.MITZ NaC1 + CO 2 + HzO ~- NaHCO3 + HCI

(12)

MNO3 + CO2 + H : O ~ MHCOa + HNO3

(13)

The mechanism is much like that found in the human stomach which produces one normal hydrochloric acid from salt. With a negatively charged membrane, the negative ions outside the cell are exchanged for the bicarbonate ions generated from carbon dioxide reacting with water from within the cells. The driving force",for this reaction is the pressure of carbon dioxide working with the enzyme, carbonic anhydrase inside the cell. The enzyme accelerates the normally slow reaction Of CO2 and H 2 0 to form bicarbonate ions. The increased ion concentration also increases the osmotic pressure which attracts water into the cell. The net effect is to actively 'pump' needed nitrate, nitrite, etc., and water into the cell. The dilute nitric acid is then available to react with carbon monoxide in the cell as indicated above to produce ammonia (Equation (3)) shifting the pH toward neutrality and providing a reactive form of nitrogen (Equation (5)). By removing the nitrate as it enters the cell, the maximum gradient between inside and outside the cell is maintained. Phosphate needed for activated phosphate may also be mobilized by this mechanism. This type &reaction, if it exists at all, can be visualized as acting on the mineral in close contact with the organism. Water can be externally attracted or recycled from within the cell. Small amounts of moisture supplied by the organism on its exterior with intermittent periods of reabsorption favor the concentration of the acid produced by the cell. In either case the amounts of water required in the biosphere is minimal in this model. 1.3. GEOTHERMALENERGY If the interior of Mars has any geothermal activity it may be warmer deep within the crust than at the top. The surface may be a dehydrated porous layer which overlays a permafrost layer. Trapped below the permafrost may be an area saturated in water vapor and, if the pressure is high enough, even liquid water. Because of the porosity and the lack of liquid water on the surface, the atmospheric gases may penetrate deep into the permafrost region. The organisms not dependent on photosynthesis should be able to take advantage of this situation. If the temperature is hot enough, urea and other organic compounds could be generated by many of the same reactions indicated above. By a process of 'gardening of the soil' the organics may move up towards the surface. In which case, conditions may be suitable for organisms to develop which live on these compounds as a primary source of energy. It is also possible to think about a life process which combines geothermal abiogenic synthesis as a source of a few compounds like urea with carbon monoxide as a major energy source, augmented by the carbon dioxide-acid extraction of needed minerals. 2. Discussion It would be fortuitous if one or a combination of the mechanisms indicated were the basis for life on Mars because the possible reactions described above were selected to

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fit a working model. On the other hand, there may be many other reactions using many of the same starting materials and/or principles which are equally reasonable and lead to the same result under the same or similar conditions. The specific reaction is not as important at this point as the possibility that there are at least two energy sources for biota outside of photosynthesis. What does this model tell us about the presently planned Viking and future missions? There are a number of things. First, we have already seen that if carbon monoxide or carbon dioxide is an energy source, one does not necessarily expect the carbon of that molecule to be incorporated into the organic backbone of the compounds synthesized. The energy may be derived without direct incorporation. However, if one monitois both the atmospheric carbon dioxide and carbon monoxide as one does in two of the three reactions in the Viking biology experiment package (Horowitz, 1972, Oyama, 1972) one should detect incorporation or changes if the atmosphere is a direct source of carbon. However, if the carbon is first converted into organic compounds abiogenically deep within the crust and then utilized by organisms, these same two experiments might be negative but the third biology Viking experiment (Levin, 1972) which depends on releasing a label gas from supplied organics should show that organisms have developed which depend on such compounds. In addition, any organic material present will be detected and may be identified by the Viking gas chromotograph-mass spectrometer (Biemann, 1971). Furthermore, the model tells us something about the water concentration, temperature, and pressure limitations. Specifically, it allows for a wide range of water, pressure, and temperature conditions. Certainly it is not limited to the condition of the surface or in the atmosphere of the planet. 3. Conclusion A model has been developed for an alternative non-photosynthetic basis for life which leads to several conclusions about the search for life on Mars. (1) The Viking investigations are a good first step towards a search for Martian biota. (2) If we do not find life on the surface, we need to look for it deep within the crust, near or below the permafrost layer with the same or similar instruments. (3) A search for organic materials at various depths in the crust should also prove extremely rewarding. (4) The models developed in this paper indicate that subsurface profile measurements of temperature, pressure, and water vapor, as well as the presence of nitrates, nitrites, and phosphates are important measurements that should be considered in the design of future missions. References Biemann, K.: 1971, in R. Buvet and C. Ponnamperuma (eds.), Chemical Evolution and the Origin of Life, North Holland Publ. Co., pp. 541-547. Horowitz, N. H., Hubbard, J. S., and Hobby, G. L.: 1972, Icarus 16, 147. Jones, M. E. and Lipmann, F.: 1960, Proc. Nat. Acad. Sci. 46, 1194. Lederberg, J.: 1969, Appl. Opt., 8, 1269. Levin, G. V. : 1972, Icarus 16, 153.

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Mars Scientific Model: 1972, JPL Document, No. 606-1. McElroy, M. arid T. Donahue, 1972, Science 177, 986. Mitz, M. A. : 1971, in R. Buvet and C. Ponnamperuma (eds.), Chemical Evolution and the Origin of Life, North Holland Publ. Co., p. 355-362. Owen, T., and Sagan, C. : 1972, Icarus 16, 557. Oyama, V. : 1972, Icarus 16, 167. Ponnamperuma, C. and Klein, H. P.: 1970, Quart. Rev. Biol. 45, 235. Wolfgang, R. : 1969, Nature 225, 876.