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MARINER 9: PRELUDE TO FIRST FIELD TEST

OF A GENERAL THEORY OF BIOLOGY?

 

Patricia Ann Straat*, Gilbert V. Levin*, Paul D. Lowman, Jr.**

November 1974

 

*IRIS Team Associate; Biospherics Incorporated,

Rockville, Maryland; IRIS Co-Experimenter;

Biospherics Incorporated, Rockville, Maryland

**IRIS Co- Experimenter; Goddard Space Flight Center,

Greenbelt, Maryland

 

GODDARD SPACE FLIGHT CENTER

Greenbelt, Maryland

 

CONTENTS

 

INTRODUCTION

 

ABSENCE OF DIRECT OR INDIRECT EVIDENCE FOR MARTIAN LIFE

 

MARS AS A HABITAT: PRESENT

 

THE MARTIAN ENVIRONMENT: PAST

 

TERRESTRIAL ATMOSPHERE AND BIOLOGICAL EVOLUTION

 

PRIMITIVE EARTH, MARS TODA Y AND THE POSSIBLE ORIGIN OF LIFE ON MARS

 

Geology

Temperature, Pressure, Liquid Water

Atmospheric Composition

Ultraviolet Radiation

 

POSSIBLE LIFE FORMS ON MARS

 

THE VIKING MISSION

 

CONCLUSIONS

 

ACKNOWLEDGMENTS

 

REFERENCES

 

ILLUSTRATIONS

 

Figure 1 Coprates Canyon

 

Figure 2 Grand Canyon

 

MARINER 9: PRELUDE TO FIRST FIELD TEST

OF A GENERAL THEORY OF BIOLOGY?

 

INTRODUCTION

 

Beginning with Darwin, biologists have been constructing a general theory to explain the phenomenon of life. Building on the physicist’s explanation of how primordial matter evolved into the diverse physical universe, the theory seeks to establish a logical continuum of increasingly complex development embracing living forms. The order of development would be the elements, compounds, life precursors, single celled living organisms, differentiated organisms and might include Chardin-like (1) considerations of consciousness and intelligence. In essence, the theory poses the hypothesis that “life” is a property acquired by matter at a certain stage of its organization which is directed by physical laws in a conducive environment (2). Such a theory avoids the necessity for explaining the existence of life through some unique event which would constitute a sharp discontinuity in our developing understanding of the universe. The direct consequence of a “General Theory of Biology” is that life will evolve independently on countless planets which lie in “life zones” with respect to their stars and which experience the appropriate environmental history. Thus, the theory becomes subject to testing by planetary exploration. The National Aeronautics and Space Administration’s (NASA) 1976 Viking Mission to Mars provides the first opportunity for such a field test.

 

Whereas the existence of Martian life was a popular concept during the 1890’s (3, 4), by 1961 this probability had been discounted as “quite unlikely” (5) on the basis of data revealing a scarcity of water on Mars. Shortly thereafter, the estimate of Mars surface atmospheric pressure was lowered from 85 mb to about 10 mb (6), thereby further weakening the argument for Martian life by reducing the already slim prospects for liquid water. By 1965, Mariner 4 imaging (7) indicated a moon-like cratered surface for Mars which strongly implied that the planet was lifeless. Indeed, Mars was deemed unlikely to yield meaningful biological information.

 

Mariner 9, in 1972, produced a wealth of data which caused a major reevaluation of the red planet. This mission has rescued Mars from its biological nadir and has rekindled interest in the biologically oriented Viking Mission. As developed herein, the strong prospect now exists that the Viking complement of experiments (8) will have a significant impact in the further refinement or revision of the General Theory of Biology.

 

ABSENCE OF DIRECT OR INDIRECT EVIDENCE FOR MARTIAN LIFE

 

Mariner 9 did not have the capability to detect biological activity directly. However, the infrared interferometer spectrometer (IRIS) and the ultraviolet (UV) spectrometer could detect certain constituents of the atmosphere which might provide indirect evidence of metabolic activity on Mars. Several gases involved in terrestrial metabolism are present in the Earth’s atmosphere and reveal their biological origin by anomalies in atmospheric concentrations or in isotopic ratios (9, 10). Accordingly, it was thought that Martian life, if present, might be detected indirectly. The atmospheric constituents found in the IRIS and UV spectra are CO2, CO, O2, O3, water vapor, water ice crystals and silicate dust (11-13). However, the concentrations of these compounds can be accounted for by nonbiological processes such as volcanism (14) and atmospheric photodissociation (15, 16). Other constituents possibly indicative of metabolic activity were absent or present in amounts below the threshold of detection, leaving their possible existence and biological involvement undetermined. Also, within the detection limits of the IRIS spectra, carbon and oxygen isotopic ratios in the Martian atmosphere could not be determined with sufficient resolution for biological inferences.

 

Visual evidence of biological relevance had divergent implications. The biological explanation of the wave of darkening seems rejected (17) in favor of attributing the reoccurring changes to shifting aeolian deposits. However, the classic dark feature, Coprates, is not similarly explained. This feature was identified (18) as being coincident with the Coprates Canyon, i.e. , the floor of the Coprates Canyon is dark. A conservative, nonbiological, explanation is that the floor is covered with dark volcanic rocks. Terrestrial rift valleys, to which the Coprates Canyon appears analogous, as shown in Figures 1 and 2, are generally associated with such rocks. However, it has been pointed out (19) that lava flows would probably be covered by wind-deposited dust and sand rather rapidly. Terrestrial experience tends to confirm this reasoning in that orbital photography (20) shows low desert areas to be consistently light colored because of fine-grained aeolian and alluvial sediments. In view of the intensity of aeolian processes on Mars, it is hard to understand why the floor of Coprates is not rapidly covered. Thus, the biological explanation of a self-renewing dark layer of organisms cannot be ruled out in Coprates. This type of depression is distinct from that of the Hellas Basin which does appear to have been filled with light colored material. And, of course, it would be in the Coprates Canyon, with a relatively high atmospheric pressure and its strong implication (Figure 1) of past hydrologic activity that a water-limited life might survive rather than in the Hellas Basin.

 

MARS AS A HABITAT: PRESENT

 

Mariner 9 results (11, 21) have shown Martian surface temperatures ranging from 145ºK to about 280ºK with diurnal fluctuations of as much as 90ºK. Surface pressures in equatorial zones range from about 3 to 8 mb, averaging about 5 mb. The atmosphere consists almost entirely of CO2 with only traces of other constituents. Liquid water was not detected on Mars although it may exist transitorily whenever a source is available, the temperature is above 273ºK, and the surface atmospheric pressure exceeds 6.1 mb. The IRIS (11) and occultation (22) experiments have shown that the critical 6.1 mb level is exceeded in Argyre, Hellas, Isidis Regio, Western Margaritifer Sinus, and in large areas of the northern hemisphere near latitude 60°. From these data and from groundbased observations, it has recently been suggested (23) that liquid water may form diurnally in the regolith material surface between 30° and 40ºN latitude. There is evidence (24) for water ice in the polar caps, in permafrost, and in ice clouds over the Tharsis volcanoes and in the north polar hood (13). Recent reduction of IRIS data (13) indicates an average particle size of 2.0 μm for the ice clouds over Tharsis with an integrated cloud mass of 5 x 10-5 g cm-2. Water vapor in nearby Lower Arcadia is reported as 5 x 10-3 g cm-2 (13).

 

 

 

 

Intense ultraviolet radiation directly strikes the surface of the planet and is also considered limiting to life. On Earth, a protective upper atmospheric layer of ozone effectively screens out the UV. The ozone on Mars, however, is insufficient to afford a similar level of protection. Some protection may be afforded by atmospheric dust which is raised in frequent dust storms by the high velocity winds.

 

Some terrestrial forms might survive, although perhaps not reproduce, under known Martian conditions. Terrestrial life forms have adapted to a wide range of environmental conditions including extremes of temperature, atmospheric and osmotic pressure, pH and gases (for a comprehensive review, see reference 25). Relevant to Martian conditions, many terrestrial obligate anaerobes have an absolute requirement for a carbon dioxide atmosphere, psychrophilic Antarctic microorganisms grow at –5ºC (26, 27) and terrestrial bacteria are relatively insensitive to pressure changes (26). Also many bacteria withstand extreme desiccation under vacuum. Indeed, the survival of Antarctic organisms under high vacuum improves as the temperature is lowered from +20ºC to –30ºC (28). Thus, the cold Martian surface temperatures may support life at low pressures. Several organisms, including Bacillus subtilis, have survived conditions which, in 1963-1965, were believed to simulate Mars: 93 percent N2, 85 mm Hg, temperature fluctuations from –60ºC to +30ºC, 0.5 percent moisture and visible radiation (29, 30). Other experiments indicate that near surface survival is decreased but not eliminated if simulations include UV light (31). Recently (32), Micrococcus luteus and two unidentified Antarctic microorganisms added to soil survived and multiplied under a CO2 atmosphere at 10 mm Hg and with temperature variations between –25ºC and +25ºC. Although ultraviolet radiation was omitted from this study, taken together, experiments such as these suggest that Martian conditions are not necessarily lethal to all terrestrial forms of life.

 

If survival is possible for any terrestrial organisms on Mars, indigenous Martian life is clearly not precluded. Such Martian life must have adapted to extreme aridity or have become restricted to limited areas of low altitude where liquid water may be transitorily available. Possible adaptive mechanisms may include the ability to increase the internal water concentration over that of the environment, analogous to the “bioaccumulation” phenomenon whereby terrestrial organisms concentrate metal ions (33, 34). Adaptations to extreme aridity occur on Earth in the cold, dry, saline deserts of Antarctica where soil moisture content is approximately 1.4 percent by weight or less. Here, terrestrial microbes are abundant and rapidly multiply upon the addition of water (35). However, the growth of microorganisms in certain areas within the Antarctic dry valleys has been questioned (36). Certain bacteria and invertebrates survive extreme drying for as long as 40 years in a state of “cryptobiosis” (37). These organisms are revived upon the addition of water. Martian organisms may “hibernate” during dry seasons and flourish during periodic “wet” seasons or eras, as previously postulated (38, 39).

 

Comparable adaptation to UV radiation, estimated to reach the Martian surface with a flux of about 2000 erg sec-l cm-2 (19) , may be postulated. Possible adaptations include protective coatings, restricted habitats and enzymatic repair mechanisms such as those described (40) for terrestrial nucleic acid systems. Recent studies (41) indicate that some terrestrial organisms can survive the UV flux, that Escherichia coli resistance to UV radiation increased when growth is anaerobic, and that some microorganisms develop resistance to UV exposure. That protective mechanisms exist as a function of habitat has been demonstrated using the “labeled release” technique (42). Desert soil microbes were found more resistant to UV exposure than soil microbes growing in climes where humidity increases UV screening, a finding with obvious implications for Mars.

 

Finally, Martian life would have to adapt to the extensive erosional, transportational and depositional effects of the high surface winds. Mariner 9 results (43) have shown that the surface of Mars is pervasively blanketed by sand and dust. Strong winds (44) continually distribute this material and the fine dust particles can be raised to form thick clouds with extensive planetary coverage. These dynamic processes would also have a positive influence, insuring the widespread distribution of microbial heterotrophs and nonphotosynthetic autotrophs. Microbial phototrophs, however, may be absent or restricted to protected environments where the sun’s energy is not excluded by the shifting dust.

 

THE MARTIAN ENVIRONMENT:  PAST

 

We have seen that the present Martian environment, though relatively harsh by Earth standards, may not preclude the support of life. A more fundamental question is whether life could have originated independently on Mars. We shall approach this question by comparing the environment of primordial Earth, in which life presumably did originate, with the environment of Mars as revealed by Mariner 9.

 

Mariner 9 results (43, 45) indicate that Mars is geologically in a stage of planetary evolution between the states represented by the moon and the Earth. The moon, Mars, and Earth, in that order, show increasing mass, density, internal energy and degree of geologic evolution (46): About half the surface of Mars consists of heavily cratered primitive terrain analogous to the lunar highlands. Much of the remainder, the smooth plains of the northern hemisphere, appears to consist of lava flows analogous to lunar maria such as Oceanus Procellarum. But Mars also shows extensive tensional fracturing , in particular the Coprates Canyon, that has no lunar counterpart. It may, instead, be the equivalent of the African rift valleys or the central valleys of the terrestrial mid-ocean ridges, and has been interpreted (47) as incipient fragmentation of the Martian crust. Other Martian features without lunar analogs are the immense shield volcanoes, Nix Olympica and North, Middle and South Spots, as well as isolated volcanoes in other places (48). An important feature of these volcanoes is their youth, indicated by the fresh topography and low crater populations. It appears possible that Nix Olympica, for example, is currently active. In any event, the recent vulcanism shows that Mars has progressed farther in planetary evolution than has the moon, and is now actively evolving from internal causes, in contrast to the moon (49, 50). From a general viewpoint, then, Mars is geologically broadly similar to Earth as it was perhaps four billion years ago.

 

A highly important Mariner 9 discovery was that Mars has geochemically differentiated, resulting in a global SiO2-rich crust. This has since been disrupted by the basaltic lava flows of the northern hemisphere, which themselves represent further differentiation (11). Planetary differentiation is deemed necessary to the origin of life in that it makes the lighter. bioessential elements more readily available at the surface. The discovery of early global differentiation strengthens the resemblance between Mars and primitive Earth since it now appears that the Earth’s continental crust was largely formed by differentiation 2.5 to four billion years ago (51).

 

Mariner 9 has produced evidence (52) for liquid water on Mars, at some time in the past, which is probably the most important biological inference produced by the mission. Many photographs of landforms show terrain almost certainly formed by running water, including dendritic canyons with structural control tributary to Coprates Canyon, braided canyons and canyons probably related to melting of subsurface ice (52). The similarity of the dendritic tributaries of Coprates Canyon to those of the Grand Canyon, suggesting water erosion on Mars, is shown in Figures 1 and 2. The laminated structures of the polar regions are also indicative of the existence of water ice (53, 54). Together, these geomorphic findings provide strong evidence for the intermittent presence of liquid water. The origins of this water were presumably volcanic outgassing and melting of glacial ice (55).

 

The existence of liquid water in the geological past also implies past temperature and pressure conditions considerably different from the present. It has been postulated (39) that periodic warmings could completely vaporize the polar caps, raising the surface pressure to as high as one atmosphere. A more conservative calculation (53) indicates that, even if the entire caps were volatilized, the atmospheric pressure would increase only about five-fold to an average pressure of 15-30 mb. However, this increased pressure would also favor liquid water if a water source were available and the temperature between about 293° and 300ºK.

 

An important aspect of the water-carved landforms is their relative youth; they appear to be among the youngest Martian landforms except for possible aeolian deposits and the Nix Olympica lava flows. This means that liquid water has been present in relatively recent times. As such, it has presumably been biologically available at a late date in the planet’s history.

 

TERRESTRIAL ATMOSPHERIC AND BIOLOGICAL EVOLUTION

 

In speculating on the possibility of life on Mars, it is instructive to examine some of the special conditions under which life is believed to have developed on Earth. The origin and early evolution of life on Earth are intimately related to the development of the Earth’s atmosphere, which is now universally believed to be of secondary, internal origin rather than being residual from an initial atmosphere. For convenience, we consider Earth to have undergone four major atmospheric stages, each accompanied by a corresponding stage of biological or prebiological evolution.

 

Phase I, the earliest, may be termed solar in that the gases of the accreting or primordial Earth were largely of solar nebula composition; hydrogen, helium and small proportions of other gases. This atmosphere was lost by gravitational escape (14) or was swept away by the T-Tauri solar wind (56) within a few million years after the Earth’s formation. There is no geological or biological evidence of this phase.

 

The Phase II atmosphere, termed reducing, was of secondary origin, having been formed by volcanic outgassing and to a lesser degree by rock weathering (57, 58). There is definite, though sparse, geologic evidence of this phase in the form of detrital pyrite (FeS2) and uraninite (UO2) from rocks uncovered which are approximately two billion years old (59). Such reduced compounds could not have survived in an oxidizing atmosphere. The original composition of this reducing atmosphere has been the subject of considerable debate. Methane and ammonia have been considered strong possibilities (60) because of the cosmic abundances of their constituent elements and by analogy with the giant planets. Others (58, 61, 62) have accepted the geologic evidence pointing to an early atmospheric composition consisting of H2, H2O, CO, CO2, N2 and SO2. This evidence is based on the composition of magmatic gases as indicated by analyses of volcanic steam, hot springs and igneous rocks. Such gases include an unknown proportion of recycled atmospheric constituents. However, analyses of gases of meteorites revealed a composition similar to those of volcanic rocks (58). It has also been recently suggested (62) that HCl may have been present in the primordial atmosphere.

 

The Phase II atmosphere must gradually have changed to a nonreducing (or less reducing) atmosphere, which is here designated as Phase III. During the transition, hydrogen gradually escaped the Earth IS gravitational field. Volcanic outgassing added water, accumulating to form the primitive oceans. Perhaps as early as 2.5 billion years ago, the chemical composition approached that of the present oceans (59). The relative proportions of CO2 and the reduced carbon in CO and CH4 (if present) gradually changed to favor increased CO2. Among those processes favoring this conversion are rock weathering, which releases the CO2 trapped in inclusions (63) , and photodissociation of water vapor, which dissociates to form oxygen which, in turn, readily combines with CO to form CO2 (16). Atmospheric CO2 would be regulated by absorption and precipitation in carbonates as the oceans became widespread.

 

It is generally believed that life originated on Earth during Phase II or Phase III. Objects found within the 3.1 billion year old Figtree chert of South Africa have been claimed (64) to be fossils extant at that time. Extensive prebiological chemical evolution is believed (65, 66) to have occurred prior to that time. Chemical synthesis of biologically relevant organic molecules has been achieved (67) and verified in many laboratories by subjecting appropriate gaseous mixture to electric discharges, ultraviolet radiation and even shock waves (for a comprehensive review, see reference 68). Reacting gases have included mixtures of methane and ammonia as well as H2, H2O, CO and CO2. These studies do not indicate a unique atmospheric composition required for chemical organic synthesis. They were limited, however, to reducing atmospheres as in Phase II. It is also possible that the ab initio synthesis of organic precursors during Phase II was supplemented by the infall of carbonaceous chondrites. The significance of carbonaceous chondrites with respect to the General Theory of Biology has been recognized for many years inasmuch as these meteorites are evidence that chemical evolution of organic compounds has actually occurred elsewhere in the solar system. The probable mechanism is by Fischer-Tropsch type reactions in the solar nebula (69). Such reactions tend to support an early atmosphere consisting minimally of H2, CO and CO2. Recent work, however, has shown (70) that organic matter is also formed under a nonreducing gaseous mixture simulating that of the present Mars atmosphere.

 

The transition from simple organic compounds to primitive life forms presumably proceeded by polymerization of the biological precursors and formation of coacervates. The last step required the presence of liquid water (71). Both polymerization and coacervation may have been facilitated by accumulation of insoluble organic compounds along shores of waterbodies (72).

 

Intermediary stages in this chemical evolution have also been simulated in various laboratories. Clays, such as montmorillonite, have been observed to promote amino acid polymerization (73). High molecular weight “proteinoids” have been produced by thermal amino acid polymerization (74). The resemblance of these proteinoids to certain Precambrian microfossils has been noted (72).

 

Terrestrial conditions at a very early stage were clearly conducive to the origin of life. It has been suggested (75) that the origin of life was contemporaneous with the first formation of sedimentary rocks and that the genetic replicating system itself arose in the first billion years of the Earth’s history. An estimate (76) of 3.4 billion years for the age of the genetic code, based on mutation frequencies and protein evolution studies of cytochrome c, has been obtained. This estimate is coincident with the estimate of 3. 5 billion years for the oldest sediments.

 

The present atmosphere, Phase IV, is oxidizing. The beginning of Phase IV , marked by the appearance of significant quantities of O2 in the atmosphere, can be dated by geologic evidence at about two billion years ago (62). This is approximately one and a half billion years after the first appearance of life forms (64). At this time, deposition of banded iron formations ended and the first red sediments were formed as the atmosphere became oxidizing (62). The probable source of this oxygen was photosynthesis by oxygen-producing life forms, such as blue-green algae, which appear to be associated with the last banded iron formations (62). As oxygen accumulated, formation of ozone in the stratosphere increasingly protected the Earth’s surface from UV radiation. This allowed development f the more advanced eucaryotic, oxygen-utilizing organisms characteristic of our Phase IV atmosphere. Eventually, perhaps 700 million years ago, Metazoans made an appearance.

 

PRIMITIVE EARTH, MARS TODAY, AND THE POSSIBLE ORIGIN OF LIFE ON MARS

 

Having examined the conditions under which life probably originated on Earth, we shall approach the question of whether life may have arisen independently on Mars. From the present Martian environment, as revealed by Mariner 9, inferences can be made regarding both past and present conditions on Mars and comparisons can be made with primitive Earth. Several similarities are immediately apparent:

 

Geology

 

Many dynamic geologic processes which probably influenced biological evolution on Earth are now known to be occurring on Mars, in sharp contrast to the moon where geologic evolution did not advance as far (50). Mars has, as mentioned earlier , undergone geochemical differentiation making a variety of biologically important light elements more readily available at the surface. Recent, possibly still active volcanoes exist on Mars, providing various gases, heat, and, possibly, a medium for the catalysis and synthesis of precursor organic compounds. The promise of volcanic areas for biogenesis has been stressed (72, 77) and studies of Antarctic volcanoes (78) have demonstrated that fumaroles provide favorable conditions for existing life. Generally speaking, then, Mars is an actively evolving planet supporting geologic processes similar to those on primitive Earth.

 

Temperature, Pressure, Liquid Water

 

It has been demonstrated (63) that Earth’s oceans, as well as its atmosphere, probably accumulated gradually, implying that there was probably not a deep global ocean on primitive Earth. It appears possible that only isolated bodies of water were present when life arose. This situation is comparable to that which existed on Mars, which clearly possessed streams in various equatorial locations. This, in turn, implies higher surface pressures and, perhaps, temperatures in the past more closely resembling conditions of primitive Earth.

 

Atmospheric Composition

 

The present atmosphere of Mars is almost entirely CO2 although CO, H2O vapor, O2 and O3 have been detected in trace amounts. Except for the absence of N2, this composition is similar to that inferred from geologic evidence for the early Earth (Phase II), with subsequent modification toward increasing CO2 concentrations although Mars may not have devolatilized to the extent of Earth (53). Since Venus is now known to have an atmosphere composed largely of CO2 (79), it appears that the terrestrial-type planets physically able to retain gases have had similar atmospheric evolutionary paths. This implies that the Martian atmosphere, like Earth’s, is secondary and perhaps of volcanic origin and may initially have consisted of H2, H2O, CO and CO2, a reducing mixture analogous to Phase II. Hydrogen and water would have been lost from Mars because of the lower gravity although volcanic outgassing would replenish water. Carbon monoxide and methane (if originally present) would gradually be replaced by carbon dioxide, as postulated for Earth (63).

 

The net result would be that Mars today represents an atmosphere which progressed through a terrestrial Phase II stage and continued to advance by these mechanisms until today’s stage consisting almost entirely of CO2 was reached. The absence of oxygen, while it may reflect the stage of evolutionary development of life, does not preclude the origin of life and, indeed, may be essential for it. Nor does the absence of nitrogen gas preclude the existence of surface nitrogen compounds available for metabolism, although suitable nitrogen cycles must be postulated.

 

Ultraviolet Radiation

 

As mentioned earlier, ultraviolet radiation strikes the surface of Mars almost unattenuated. Although detrimental to advanced forms of terrestrial life, UV radiation may actually be considered a prerequisite for the formation of life because it provides the greatest amount of available energy required for production of organic molecules from reducing gas mixtures. The high UV flux on Mars today is analogous to that on Primitive Earth before development of atmospheric shielding such as the dense ozone shield of Phase IV. Terrestrial life must have arisen in the presence of greater surface UV radiation than incident on Mars because of the Earth’s proximity to the sun. That UV radiation may not be as harmful to primitive life forms as sometimes supposed is a conclusion receiving some experimental support (41, 42). Alternatively, oceans or areas of volcanic fluidization. both of which existed on primitive Earth and may have existed on Mars, could have provided shields against UV radiation. The dust clouds on Mars cited previously would also afford shielding.

 

These similarities between the Martian and terrestrial environments when terrestrial life arose make it not unlikely that life may have arisen, or is now in the process of arising, independently on Mars. The chief limiting factors may be the low abundance of liquid water and evidence for a former reducing atmosphere even though the latter may not be necessary. It seems, therefore, that the stage was at one time set, or is now set, for the origin of life on Mars.

 

POSSIBLE LIFE FORMS ON MARS

 

If the origin of life on Mars proceeded as on Earth, then early terrestrial life forms characteristic of anaerobic Phases II or III may be indicative of those anticipated on Mars. On primeval Earth, the principle mode of metabolism of the first life forms is thought to have been anaerobic fermentation of abiogenically formed organic compounds. Current theory holds that as these heterotrophs gradually consumed the available organic energy sources, autotrophs appeared. The most likely primitive terrestrial organisms are the anaerobic fermenters, anaerobic respirers and anaerobic photosynthesizers, none of which evolve or utilize oxygen. Clostridia, sulfate reducers, methane bacteria and photosynthetic bacteria are especially interesting. Oxygen-evolving blue-green algae are also considered primitive, but probably appeared somewhat later (Phase IV) followed by oxygen-utilizing organisms.

 

Evidence for the early existence of Desulfovibrio, an anaerobic sulfate-reducing heterotroph, has been obtained from isotopic ratios of sulfur (80). Desulfovibrio fractionates 32S and 34S during sulfate reduction, preferentially utilizing 32S. Since rocks formed 2-3.5 billion years ago are enriched in their 32S content over the juvenile sulfur found in meteorites and volcanoes, the early existence of sulfate reducers is implied.

 

Sulfate reducers, as well as methane forming bacteria, are often found in deep oil wells. The association of these bacteria with petroleum implies their early ability to have utilized hydrocarbons (72).

 

A recent study (81) has appeared in which the structure of bacterial ferredoxins has been examined. Ferredoxins are primitive metalloproteins, believed to have played a significant role in the development of fermentative bacteria. Clostridia contain the simplest known ferredoxin, with a molecular weight of about 6000. The simplicity of structure is compatible with spontaneous formation under primitive conditions. Spontaneous polymerization of activated amino acids using a montmorillonite clay catalyst can produce polypeptides up to 4000 in molecular weight (73). The ferredoxin active site is thought to contain only iron and sulfur, both of which spontaneously add to the polypeptide chain under anaerobic conditions. The six amino acids found in the Murchison meteorite constitute 64 percent of the amino acid content of ferredoxin. Such evidence dates Clostridia early in the evolutionary scheme.

 

Desulfovibrio, methane bacteria and Clostridia all utilize simple organic compounds as carbon and energy sources. Methane forming bacteria, for example, can grow with formate as the sole carbon source (26). However, all three anaerobic respirers have been reported to exist as autotrophs (82) and to derive energy by the oxidation of molecular hydrogen utilizing carbon dioxide as sole carbon source according to the following oxidative schemes:

 

H2 + SO4  ð  H2S

 

H2 + CO2  ð  CH4

 

H2 + CO2  ð  CH3COOH

 

As such, this group of organisms becomes quite interesting as candidates for early terrestrial life. In fact, we suggest that the earliest autotrophs may have been the same early heterotrophs which simply evolved alternate biochemical pathways to obtain energy and carbon. This hypothesis provides a logical continuum between heterotrophs and autotrophs.

 

An interesting speculation for Martian biochemistry is to consider the terrestrial autotroph, Bacillus oligocarbophilus, which derives energy through the oxidation of CO to CO2. It has been postulated (83, 84) that such a pathway is not impossible for chemosynthetic Martian organisms if coupled to sulfur reduction as has been reported for cell-free extracts of Desulfovibrio (85).

 

Terrestrial anaerobic autotrophs which do not evolve oxygen are the photosynthetic purple sulfur bacteria and the green bacteria which capture the sun’s energy according to the following equations (86):

 

CO2 + H2S + H2O  ð  (CH2O) + H2SO4

 

CO2 + H2A  ð  (CH2O) + H2O + A

 

On the basis of structural relationships of ferredoxins, it has been proposed (81) that evolution proceeded as follows: anaerobic fermentative bacteria ð green photosynthetic bacteria ð red photosynthetic bacteria ð algae ð higher plants. This sequence represents the order of increasing ferredoxin complexity and closely resembles a sequence which would be deduced as above from metabolic considerations in relation to atmospheric composition. Other evidence that bacterial photosynthesis is primitive and preceded algal and plant oxygen-evolving photosynthesis is derived from a recent study (87) of Precambrian stromatolites in Yellowstone National Park. These early stromatolites appear to result from photosynthetic bacteria, implying that bacterial photosynthesis preceded algal oxygen-evolving photosynthesis and that oxygen may not have appeared until later than previously supposed.

 

Photosynthetic purple sulfur bacteria could have formed an early biogenic sulfur cycle by coupling with sulfate reduction by Desulfovibrio which require excess sulfate. It is of interest that the photosynthetic purple bacteria possess the only known pathway (80) in chemoautotrophs for the conversion of chemical energy into high energy phosphate without utilizing oxidative phosphorylation or photophosphorylation.

 

As discussed in a previous section, by analogy with Earth the results of Mariner 9 increase the probability that life may at one time have evolved on Mars. If atmospheric conditions at the times life formed on Earth and Mars were similar, then it does not seem unreasonable that primeval Martian life may have biochemically resembled some of the primeval terrestrial organisms discussed above. However, the current environments of the two planets show differences which have suggested to some that, if life has evolved on Mars, advanced forms may have progressed in a direction quite different from our own. While this may be true, we wish to point out that advanced terrestrial life forms have not evolved to the exclusion of all primitive life forms, severe environmental changes notwithstanding. Well-adapted primitive bacterial types (and certain metaphyta and metazoa types) have persisted. Thus, it is not unreasonable to assume that certain primitive Martian types may still be present on Mars even if advanced forms exist. The adaptability of terrestrial life to its wide variety of habitats encourages the hypothesis that, if life did evolve on Mars, it may well have adapted to the present environment. It is extremely interesting to note that bacterial adaptations to hostile terrestrial environments are often accomplished not only by advanced organisms but by the primitive sulfate reducers, methane bacteria and blue-green algae, furthering the possibility that, if life evolved on Mars, it exists there today.

 

THE VIKING MISSION

 

Our present view of Mars locates it within the possible extent of our sun’s life zone. The most severe and, possibly, the only critical environmental constraint is the availability of liquid water. It is possible to postulate evolutionary development of organisms with water-husbanding capabilities, or the existence of physical mechanisms to provide liquid water transitorially. The principal unknown historical fact concerning the evolution of materials of biological interest is whether or not a reducing atmosphere formerly existed. However, we are no longer certain that the planetary production of organics can be accomplished only in a reducing atmosphere. In all, it seems quite likely that, if the approach to the General Theory of Biology is correct, Mars will yield an interesting array of organics or biochemicals whether or not a reducing atmosphere ever existed. And the possibility of living organisms is sufficiently high to render the search for them exciting.

 

The complement of experiments in the forthcoming Viking ‘75 Mission is an appropriate one. Water abundance and distribution and atmospheric composition will be determined to improve our understanding of the vital environmental factors. Organic compounds present on the surface and trace atmospheric constituents near the surface will be identified by mass spectrometric analysis (88). The Viking life detection experiments seem especially well chosen in view of the preceding discussion of metabolic possibilities on Mars. Anaerobic fermenting heterotrophs are sought by the labeled release experiment (42) which places 14C-labeled organic substrates on Martian soil and monitors for biological evolution of 14CO2. The substrates selected are formate (a substrate for methane bacteria and also widely used by other bacteria) , glycine and dl-alanine (present in the Murchison meteorite and products of Urey-Miller type reactions), dl-lactate (a substrate for terrestrial anaerobic fermenters and for Desulfovibrio), and glycolic acid (a widely utilized terrestrial substrate readily formed under synthetic Martian conditions). The pyrolytic release experiment (89) is designed to detect fixation of carbon dioxide or carbon monoxide into an organic fraction, thereby seeking Martian autotrophs. The dependence of any such carbon fixation on light will be determined to indicate whether photosynthetic pathways are extant. The influence of exposure to water vapor will also be determined. Finally, the gas exchange experiment (90) will provide Martian soil with a complex organic nutrient and monitor by gas chromatography and mass spectrometry the resulting changes in composition and concentration of gaseous components. This experiment can theoretically detect gaseous steps in metabolic pathways in addition to those sought by labeled release and pyrolytic release whether or not they have a terrestrial counterpart. And, of course, the imaging experiment may determine the presence of macroscopic life by direct observation or may produce significant, indirect evidence.

 

CONCLUSIONS

 

The Mariner 9 data have rekindled biological interest in Mars. A reevaluation of that planet has markedly improved the probability that it harbors life. Accordingly, the Viking Mission biology experiments will be eagerly watched. Detection of living organisms would constitute an event of unparalleled scientific significance constituting a major advance toward proving the General Theory of Biology.

 

Should the life detection experiments yield negative results, prospects for a meaningful test of the biology theory will remain. The theory will be refined or revised depending on the diversity and complexity of possible life precursor compounds found. Based on analytical results, attempts will be made to deduce whether the bioevolutionary process is in progress or whether it was halted at a certain stage and, if so, why. If a paucity of biologically significant compounds are found, the General Theory of Biology will be returned to the synthesis laboratory. In any event, Viking seems destined to establish a major benchmark in biology.

 

ACKNOWLEDGMENTS

 

This paper results from the authors’ participation in the Mariner 9 Infrared Interferometer Spectrometer (IRIS) experiment in which they were supported by the Goddard Space Flight Center of National Aeronautics and Space Administration. The authors are indebted to Drs. R. Hanel and J. Pearl of the Goddard Space Flight Center, Dr. R. Young of NASA Headquarters, Dr. R. MacElroy of the Ames Research Center, and Dr. J. R. Schrot of Biospherics for helpful review of the paper.

 

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