Life detection by atmospheric analysis
Communicated by Lewis D. Kaplan.
Received December 16, 1966.
Originally published in Icarus: International Journal of the Solar System Volume 7, Number 2, September 1967. Copyright © 1967 by Academic Press Inc. Icarus 7, 149-159 (1967).
Living systems maintain themselves In a state of relatively low entropy at the expense of their nonliving environments. We may assume that this general property is common to all life in the solar system. On this assumption, evidence of a large chemical free energy gradient between surface matter and the atmosphere in contact with it is evidence of life. Furthermore, any planetary biota which interacts with its atmosphere will drive that atmosphere to a state of disequilibrium which, if recognized, would also constitute direct evidence of life, provided the extent of the disequilibrium is significantly greater than abiological processes would permit. It is shown that the existence of life on Earth can be inferred from knowledge of the major and trace components of the atmosphere, even in the absence of any knowledge of the nature or extent of the dominant life forms. Knowledge of the composition of the Martian atmosphere may similarly reveal the presence of life there.
There is wide agreement among space scientists whose primary orientation is towards the physical sciences, that experiments to observe the physical properties of the surface and atmosphere of Mars should be given high priority in the Martian exploration program. It is not so widely recognized, however, that these experiments could, in principle, yield useful information to biologists whose primary concern is to determine whether life exists on Mars, and, if so, in what form.
To understand the kind of life that may exist on Mars, information must be obtained about those properties of its surface and atmosphere to which any evolving Martian life must have become adapted. It is not always realized that such information. by providing evidence of effects that cannot be accounted for by abiological processes, could constitute direct and primary evidence of life.
Observations of purely physical properties may provide information supporting the hypothesis that life exists on Mars; this follows directly from a fundamental and highly plausible assumption, that the entropy of living systems is low relative to that of their non-living environments (Lovelock, 1965).
This assumption has two relevant consequences: firstly, that living systems will drive their environments into physical or chemical disequilibrium, recognizable as such if existing data are sufficient to rule out explanations of their state in terms of abiological processes; and, secondly, that there will always exist an entropy gradient between living systems and their nonliving environments. The origin of non-living biogenic material will be revealed by the existence of such a gradient for a time which will vary according to the rate of attainment of equilibrium, as determined by local conditions, but which can be expected to be measurable in units of at least 103 years and thus, probably, be long compared to the lifetimes of the organisms themselves.
Given that the presence of life is characterized by a state of physical and chemical disequilibrium of the system, life plus environment, can essential disequilibria be effectively diagnosed from practical observations? If life exists on Mars, the atmosphere of Mars is likely to be an important part of the environment and its composition should reflect the presence of life forms, on the plausible assumption that they continuously react either directly or indirectly with it. The object of this paper is critically to review the potential biological significance of two types of atmospheric data: those which concern the relationship between the atmosphere and the surface material, as revealed by experiments designed to detect a free energy gradient between the two; and those which concern the concentrations of major and trace components of the atmosphere.
Although the two types of experiments from which such information should be obtainable differ greatly in terms of their implementation, interpretability, and potential biological significance, they both represent ways of implementing a strategy of life detection based on a search for evidence of disequilibria. Both also depend on the assumption that the atmosphere is a relatively homogeneous medium which reflects processes occurring over the whole surface of the planet.
Life detection by means of detection of chemical free energy in surface matter
One of the consequences of the assumption that the entropy of living systems is low relative to that of their nonliving environments is that there will always exist an entropy gradient between the two. After death, the free energy stored in a living system is gradually dissipated into the surrounding environment until, eventually, the matter which once comprised the living system ceases to have a relatively low entropy.
The most likely site for life on a planet is the interface between its surface and the atmosphere where the solid, liquid, and gaseous phases meet at the point of maximum absorption of solar energy, maximum temperature, and where a diversity of raw material, provided by both the relatively mobile atmosphere and the less readily transported surface matter, is available at the same time as the energy necessary for their biotic transformation.
The synthesis of biotic compounds from simple abiotic molecules requires the dissociation of the latter and the build-up of some of the reactive dissociation products into large complex molecules. The dissociation of simple inorganic molecules, such as water and carbon dioxide, not only releases reactive products to be used in the synthesis of biological molecules, but also simple active molecules like oxygen into the atmosphere. These two processes result in the storage of chemical free energy in the biotic molecules, which are large and relatively nonvolatile and hence tend to accumulate at the surface-atmosphere interface. Life, therefore, tends to generate a sharp metastable chemical free energy gradient between the surface and the atmosphere.
Perhaps the simplest experiment in life detection, therefore, is to look for a chemical free energy difference between Martian surface matter and atmosphere. A promising experiment consists of equally heating two aliquots of surface material, one in an inert gas such as argon or nitrogen, the other in the Martian atmosphere, and recording the temperature of each sample. A combustion reaction between the sample and its own atmosphere would produce an excess of heat, whereas heat changes due to chemical transitions in the surface matter would be equal in both samples.
The occurrence of such a combustion would be detected by a difference in temperature between the sample heated in the atmosphere and that heated in the inert gas It the planet were to have a reducing atmosphere, a lesser but still observable evolution of heat could be expected from the reduction of the more oxidizable metastable biotic molecules. If the constituents of the atmosphere were known, additional information could be obtained by noting the temperature at which the combustion occurred and by examining the volatile pyrolysis products,.
It is debatable whether such an experiment is subject to false positives of any sort except, of course, those resulting from machine errors or inadvertent contamination of the equipment before launching For example, sulfide rocks will burn on heating in air although their ignition temperature is different from that of organic matter. On Earth, however, at least some sulfide rocks are thought to be of biological origin. It would seem that if the fundamental assumption on which this experiment is based (that life can be defined in terms of its entropy reducing function) is valid, evidence of chemical free energy gradients between surface matter and atmosphere should always be construed as prima facie evidence of life.
Experiments designed to identify and determine quantitatively the major and trace components of the Martian atmosphere may be recommended on many grounds. The utility of such measurements to scientific programs not primarily concerned with Martian biology need not be stressed. They are, however, essential for the satisfactory interpretation of analyses of surface material, bacteriological experiments, and other experiments designed to demonstrate the existence of Martian life forms by their direct observation.
The assumption that the atmosphere of Mars, like that of Earth, is relatively homogeneous implies that such atmospheric experiments are not likely to be site-limited. The probability of such an experiment being performed near an active volcano and, hence, of results being recorded that could not be extrapolated to other locations, is negligible. Moreover, such a situation would be recognized from other available information and the results of the experiment could still be usefully interpreted. For tlicsc reasons, attempts to analyze the Martian atmosphere deserve high priority. The primary disadvantage that may be urged against such experiments as part of a biological program is that they arc unlikely to provide direct or relatively conclusive evidence of life on Mars.
As already mentioned, one might, on very plausible theoretical grounds, suppose that life forms in continuous interaction with the atmosphere would drive it into chemical or physical disequilibrium, recognizable as such if observations of its state were sufficiently complete. The claim that an analysis of the atmosphere of Mars can provide information which can be interpreted as indicative of the presence of life forms deserves critical examination. Analysis of the Martian atmosphere cannot be advocated as a good means of detecting life if terrestrial atmospheric information does not provide good evidence for the existence of terrestrial life. If, however, a good case can be made that terrestrial life can be demonstrated from the analysis of the terrestrial atmosphere, analysis of the Martian atmosphere ought to receive very high priority as an exobiological experiment and care should be exercised to ensure that the experiments are designed to provide the greatest amount of potentially significant biological information.
The arguments presented above suggest that atmospheric evidence of life will take the form of an observable free energy gradient which is anomalous in that it cannot be accounted for in terms of the abiological processes which could be expected to occur on a planet devoid of life. For example, the simultaneous presence of two gases which, like hydrogen and oxygen, are chemically incompatible, is evidence of such a gradient. Whether it is an anomalous gradient is, however, somewhat less easy to determine. Such a determination requires a model of the abiological steady-state atmosphere which takes into account those departures from simple thermodynamic equilibrium which can be attributed to the action of the solar flux and related atmospheric transport phenomena. Any constituents whose concentrations are not accounted for by such a model could then be construed as evidence of life.
Unfortunately, the current status of aeronomy is such that we must seriously question any proposed model of an abiological steady-state atmosphere. At this moment, therefore, one cannot clearly identify all those atmospheric constituents whose concentrations can be accounted for only on the hypothesis that life exists on Earth. But the question is not whether one can identify all the atmospheric constituents whose observed concentrations are incompatible with the hypothesis that there is no life on Earth. The question is rather, can at least one constituent be identified, for it is clear that if one cannot be explained on abiological grounds, the whole atmosphere of which it is a part can be said to depart from abiological equilibrium, and, therefore, provide evidence of life.
Chemically active components of the Earth's atmosphere
|Gas||Ratio by volume||Remarks|
|N2||0.78||Mixed in troposphere|
|O2||0.21||Mixed in troposphere|
|CO2||3 x 10-4||Mixed in troposphere|
|H2O||10-6 to 10-2||Dissociates in mesosphere|
|O3||10-7 to 10-8||Peak in stratosphere|
|CH4||1.5 x 10-6||Dissociates in stratosphere|
|N2O||2.5 x 10-7||Dissociates in stratosphere|
|H2||5 x 10-7||Dissociates in thermosphere|
|CO||5 x 10-8||Variable, industrial as well as atmospheric origin|
|NO, NO2||5 x 10-10 to 2 x 10-8|
Table I, from the data of Nicolet (1964), shows the principal and trace constituents of the atmosphere. A cursory examination of this table reveals that although the atmosphere contains large quantities of oxygen and is, therefore, an oxidizing atmosphere, it also contains appreciable quantities of gases which are characteristic of reducing atmospheres, namely CH4, CO, and H2. The simultaneous presence of methane and oxygen can only, it is believed, be explained on the grounds that a terrestrial biota exists. The observed concentrations of CO and H2 may also be indicative of the presence of life, as may that of N20. The arguments supporting the contention that these constituents have biological import are outlined below.
The strongest atmospheric evidence for a terrestrial biota is atmospheric methane which is present to the extent of 1.5 parts per million. Although methane is unique among hydrocarbons in its stability and reluctance to react with 02 and other oxidizing gases in the lower atmosphere, it nevertheless cannot persist indefinitely. Methane will oxidize at stratospheric levels by reaction with atomic oxygen (Bates and Witherspoon, 1952; Bates and Nicolet 1965). The identity of the final products of methane oxidation is not clear; the most probable first step appears to be
CH4 + 0 -> CH2 + H2OThe methylene radical is highly reactive and could undergo further oxidation leading to most or all of the following stable final products: H20, CO2 CO, and H2. Although the details of the fate of methane in the atmosphere may be obscure there can be no doubt that it is oxidized and that its oxidation is an irreversible process. The converse reaction leading to the formation of methane in the atmosphere is highly improbable. To form methane from the carbon and hydrogen compounds of the atmosphere requires a sequence of reactions involving at least four steps. The intermediate compounds in the sequence towards methane must all be partially oxidized organic compounds or free radicals. The probability that any of these, once formed, will encounter the rare molecule necessary for the next step before reaction with the abundant oxygen or decomposition by UV radiation is vanishingly small. The probability that all four sequential steps will occur is, therefore, virtually zero.
For these reasons the oxidation of methane in the atmosphere may be concluded to be irreversible. The presence of methane, therefore, implies that it is being produced at the surface, but the arguments against its abiological production in the atmosphere also apply to the likelihood of its abiological production at the surface. Although it is conceivable that some methane can be abiologically produced at the surface, for example, by reaction of iron carbides with water, this would be very rare and could not possibly be expected even to give rise to atmospheric concentrations many orders less than exist. Even if, for example, the oxidation of methane were so slow as to result in a residence time of 106 years, the corresponding production rate required to maintain the current concentration would have to be 106 molecules cm-2sec-1.
Methane can, of course, be produced in a reducing atmosphere at high temperatures and there now seems little doubt that methane was at one time a major constituent of the early terrestrial reducing atmosphere. Methane could not have persisted in the atmosphere during its transformation to an oxidizing atmosphere, nor could it have remained isolated from the atmosphere during the long period since oxygen became a primary constituent, now to reappear as an outgassed product. Methane could not survive the temperature, pressure, and chemical conditions deep in the Earth.
This does not necessarily apply to Mars or other planets, where, so far as is known, temperature and pressure conditions within the planet may not be comparable with those of Earth. Of course, if the atmosphere of Mars is partially reduced, as now seems likely (Connes, Connes, and Kaplan, 1966), the presence of methane as a constituent cannot necessarily be invoked in support of the hypothesis of its biological genesis unless it is known, for example, that photochemical processes on Mars effect its rapid removal and the prevailing surface conditions preclude its abiological replacement.
The fact that 21 vol. % of the Earth's atmosphere is oxygen is not in itself conclusive evidence that life exists here, although many in attempting to account for this concentration have argued an important biological role from knowledge that oxygen is biologically photosynthesized (Hutchinson, 1954). Recognizing that the abiological production of oxygen through upper atmosphere photolysis of water is self-regulating (Urey, 1959) and proceeds at a very low rate after accumulation of as little as 0.1% of the present atmospheric level, Berkner and Marshall (1964) conclude that the biota was responsible for increasing oxygen concentration to its present level.
However, the present concentration of oxygen does not support the conclusion that an extant biota is presently producing oxygen. There is no evidence that the biota is effecting a net increase in atmospheric oxygen and it can be argued that biotic consumption of respiratory oxygen may account for all that is photosynthetically liberated. Thus, while the current oxygen concentration provides some evidence for the past existence of an oxygen-producing biota, it does not by itself imply the existence of a contemporary biota.
The biological significance of atmospheric oxygen as evidence of the presence of a contemporary biota lies in the discrepancy between its probable rate of abiological production and the rate at which it is apparently being removed from the atmosphere.
That the terrestrial atmosphere was at one time a reducing atmosphere seems well established, but the nature of the mechanisms responsible for the loss of hydrogen and the consequent transformation to an oxidizing atmosphere is uncertain. Photolytic decomposition of H2O with escape of the hydrogen undoubtedly played a significant role in this transformation, although ! there is disagreement as to its extent. A good survey of this controversy, up to 1954, is presented by Hutchinson (1954).1 The disagreement is not important. The fact is that this is the only abiological mechanism offered to account for the loss of hydrogen and the release of O2. This process is still going on, though at a lower rate than during the early history of the Earth, because the high concentration of O2 limits the efficiency of the photolytic process (Urey, 1959). The present estimate, based on a current loss of 2.5 x 107 atoms of hydrogen cm-2sec-1 corresponds to a total oxygen production of approximately 1.2 x 107 atoms cm-2sec-1 (Nicolet, 1964). It is clear that any evidence which supports the conclusion that oxygen is being removed much more rapidly from the atmosphere than it is photolytically produced requires an additional oxygen production mechanism.
The lifetime of methane toward oxidation by monatomic oxygen
|Altitude (km)||Lifetime (sec)|
|30||1 x 108|
|40||2 x 106|
|50||8 x 106|
From the data of Bates and Nicolet (1965).
In the preceding section the simultaneous presence of methane and oxygen in the atmosphere was shown to imply a biological methane production mechanism. It also implies a consumption of oxygen, since oxygen reacts with methane to form oxides of carbon and hydrogen. The lifetime of methane is very long in the lower atmosphere, but above the tropopause it is oxidized by 0 and H02. Table II, from Bates and Nicolet (1965), shows the lifetime of methane towards oxidation by 0 in the stratosphere in the region from 30 to 50 km. Above 70 km the lifetime is much shorter (Bates and Witherspoon, 1952).
Consequently, the rate of oxidation of methane is dependent on the speed with which it is transported to regions above about 30 km, which is accomplished by diffusion and by large-scale motions of the atmosphere, the latter being the more effective. Since the characteristic times associated with these motions are not known, a realistic estimate of the methane oxidation rate cannot be made. However, it seems unlikely that these motions could be so slow as to result in a methane residence time of more than 1000 years. The abundance of methane is approximately 3.2 x 1019 molecules per square cm atmosphere column. A 1000-year residence time, therefore, corresponds to an oxidation rate on the order of 109 molecules cm-2sec-1, which implies a consumption of not less than three times as many oxygen atoms, assuming that the final reaction products are CO and H2O. The observed concentration of methane and the assumption that its residence time is not greater than 1000 years therefore lead to the conclusion that oxygen must be produced at a rate not less than about 100 times that estimated for photolytic decomposition.
Unless the rate of photolytic decomposition of H2O has been very seriously underestimated, these considerations imply that there must be additional sources of atmospheric oxygen. We conclude that these sources are probably biological in nature because aeronomists have failed to postulate alternative abiological oxygen production processes to account for the transition from reducing to oxidizing conditions.
The fact that oxygen is being temporarily removed from the atmosphere at a greater rate than it is abiologically produced does not support the conclusion that the terrestrial biota is responsible for a net increment in the supply of O2. The oxygen produced by the biota is consumed in the oxidation of biotic products. We are not concerned here with the origin of the atmosphere nor with the details of the biological mechanisms which govern oxygen concentration, but with the fact that the present atmosphere is evidence of a present biota.
Carbon Monoxide, Hydrogen, and Nitrous Oxide
C0 and H2 are gases which are characteristic of a reducing atmosphere. Their mere presence in an oxidizing atmosphere is, therefore, suggestive. But they do not provide good evidence for life, because they are the products of reversible dissociation reactions involving other constituents of the atmosphere, H2O, CO2, and oxygen. This means that, in all probability, these gases will always be present in certain amounts in atmospheres containing the former compounds, as a consequence of abiological processes. The critical question is whether the observed concentrations can be accounted for on this basis or whether they arc excessive.
The relevant aeronomical arguments are involved and inconclusive. Both CO and H2 are produced in the upper atmosphere by photodissociation of CO2 and H2O. To estimate the rate of production the concentration of the parent molecules must be known, together with the rates at which these are replenished in the production zone by the relevant atmospheric transport mechanisms. Both CO and H2 are removed by oxidation in regions below their production zones, the rates of removal in these being dependent upon the physical and chemical conditions and, in particular, on the concentrations of the relevant reactive molecular species such as OH, HO2, and O and on the temperature and pressure.
The overall oxidation rate is clearly dependent, in addition, upon the rate at which CO and H2 are transported downward to the removal zones. Without fairly accurate information on which to base estimates of turnover rates, the concentrations which would be expected under abiological conditions cannot be specified. At present, such information is lacking, especially in regard to vertical transport phenomena. It is, therefore, not possible to arrive at firm estimates of the abiological steady-state concentrations of CO and H2.
In spite of these uncertainties, the concentration of CO appears to be somewhat excessive. Bates and Witherspoon (1952) report that the observed concentration of CO is difficult to account for solely in terms of upper atmosphere photodissociation of CO2. Similarly, the analyses of Hesstvedt (1965) indicate the difficulty of accounting for the abundance of hydrogen in terms of its downward transport from the region of its photochemical production.
N20 also appears to be a somewhat anomalous constituent, but not because of any inconsistency about its presence in an oxidizing atmosphere. The possible biological significance of N2O is that its lifetime in the atmosphere is short because it is readily destroyed by comparatively long wavelength UV radiation. Goody and Walshaw (1953) calculated the removal rate to be in the order of 1011 molecules cm-2 sec-1.
Several authors (Cadle, 1964; Adel, 1951; Goody and Walshaw, 1953; Harteck and Dondes, 1954; and Schiff, 1964) have considered possible abiological atmospheric reactions capable of generating N20 and it is not yet clear whether any of these reactions can account for a production at the required level. All of the proposed reactions take place in the stratosphere or higher. The question of the biotic significance of these gases (and possibly others) could perhaps be decided if information about their abundance profiles were available. These profiles would aid in determining where the gas is produced and where it is removed.
The significance of an abundance profile may be illustrated by a highly simplified example of a hypothetical compound AO which is assumed to photolyze in the upper atmosphere to A + 0. It is also assumed that the reverse reaction will occur readily with oxygen atoms, but not with 02. The atmosphere can be divided into four principal regions: an upper region where photolysis is complete; the next highest region where A can recombine with oxygen atoms; a transport zone; and the lowest region where A is uniformly distributed and where no atmospheric reactions occur.
Figure 1 shows these regions together with three hypothetical abundance profiles. If substance A is neither produced nor removed at the surface, there will be no change in concentration below the recombination zone and its abundance profile will I resemble profile (1). If, however, the substance is produced at the surface, its profile will resemble (2), which shows a concentration in the lower atmosphere exceeding that in the transport zone. The converse will be true of substances which are removed at the surface and which show a concentration minimum in the mixed zone [see profile (3)].
Figure 1: Atmospheric distribution of a hypothetical molecule (A), the product of the photochemical dissociation of the oxide (AO) in the upper atmosphere: (1) no reaction at the surface; (2) production at the surface; (3) removal at the surface.
The biotic significance of surface production and removal reactions on Earth is that the portion of the solar flux capable of effecting chemical reactions among the constituents of the atmosphere does not reach the surface of Earth. Consequently, any chemical reactions occurring at the surface capable of maintaining detectable concentration gradients like profile (2) or (3) in Fig. 1 can only be attributed to the action of a biota.
Continuing chemical reactions between the atmosphere and the solid surface are prevented by the extreme slowness of diffusion processes in the solid state. Even a liquid surface such as the sea will be characterized by relatively slow chemical reactions and, under abiological conditions, liquid surfaces should be in equilibrium with the atmosphere. On Earth, diffusion of gases from below the surface is quantitatively insignificant and hence incapable of maintaining concentration gradients that could be confused with those indicative of life processes.
Review of evidence
In the preceding sections an attempt has j been made to demonstrate the existence of a terrestrial biota from evidence consisting wholly or largely of the composition of the atmosphere. The significance of this demonstration lies in the possibility of drawing comparable biological inferences from information about the atmospheres of other planets. It is, therefore, useful to review the kinds of information used above to support these conclusions.
The first and strongest argument is that based on the observation of atmospheric methane, which undergoes irreversible reaction with oxygen, in conjunction with a large concentration of atmospheric O2. It is a chemical argument which requires almost no knowledge of atmospheric physics for its validity. If the concentrations of either methane or oxygen were less than they are, the argument would still hold.
The second argument is also based on the observation of methane and oxygen, but requires, in addition, an assumption about the effects of large-scale movements of the atmosphere in transporting methane, together with estimates of the rate of photolytic production of oxygen and of the lifetime of methane in the stratosphere. These two estimates are derived from fairly detailed and sophisticated models of the atmosphere and, implicit in them, fairly detailed knowledge of its physical structure. To the extent that the second argument implies far more knowledge of atmospheric processes, it is much weaker than the first. If methane were less abundant, the second argument could not be supported.
The last three constituents considered. CO, H2, and N2O, do not provide unequivocal evidence of life; they are just suggestive. The biological significance these constituents can now be said to have seems to depend almost entirely upon rather sophisticated and involved aeronomical reasoning, for the details of which the reader is referred to the literature already cited. If additional atmospheric measurements in the form of concentration profiles of CH4, CO, H2, and N2O were available, much of the ambiguity might be removed. Some of the inconclusiveness of aeronomical arguments may be due in part to the fact that aeronomists need not base conclusions about the origin of atmospheric gases on atmospheric measurements alone. In the case of the Earth, at least, there is considerable information on the production of atmospheric "contaminants" by the biota. Table III presents some recent estimates of the biotic production of some of the gases considered above, together with their atmospheric abundance.
Biological significance of extraterrestrial atmospherical analysis
The object of this inquiry into the biological significance of the terrestrial atmosphere has been to determine whether analysis of the atmosphere could be viewed as a life detection experiment, with particular reference to Mars. The following conclusions seem to follow from the preceding discussion:
- Not only is the presence of a terrestrial biota reflected in the composition of the atmosphere; it can also be inferred almost entirely from atmospheric measurements alone. In particular, no assumptions about the nature and biochemistry of the biota are necessary for such an interpretation.
- The biological significance of an atmospheric mixture lies in the relative concentrations of a variety of constituents and not wholly in the presence or absence of any single one of them. Such a mixture is biologically significant if it represents a departure from a predictable abiological steady state. The strongest evidence is the simultaneous presence of two gases, like methane and oxygen, capable of undergoing . irreversible reaction; even with gases which can be expected to occur under abiological conditions, departures from the abundances to be anticipated will be of biological significance.
- The presence of even very minor constituents may be very significant; consequently, every attempt should be made to provide as complete an analysis of the atmosphere as possible.
By analogy with Earth, Martian life, if present, may affect Mars' atmosphere by giving rise to anomalous concentrations of compounds whose presence could be expected as a result of abiological reactions and / or by concentration in the atmosphere of compounds incompatible with other constituents.
Estimated abiological and biological production of selected atmospheric components contrasted with atmospheric abundance
(molecules cm-2 column)
(molecules cm-2 yr-1)
(molecules cm-2 yr-1)
|Oxygen||4.5 x 1024||1.9 x 1017 a||3.2 x 1020 b|
|Carbon dioxide||6.4 x 1021||-||3.2 x 1020 b|
|Methane||3.2 x 1019||nil||2 x 1018 c|
|Hydrogen||1.0 x 1019||-||1.0 x 1015 c, d|
|Nitrous oxide||5.0 x 1018||1.0 x 1016 to 1.0 x 1018||5.0 x 1017 e|
|Carbon monoxide||1.0 x 1018||1.0 x 1016 g||8.0 x 1017 g|
a Nicolet, 1964. b Leith, 1963. c Koyama, 1963. d Does not include hydrogen of industrial origin. e Goody and Walshaw, 1953. f Cadle, 1964. g Bates and Witherspoon, 1952.
Life detection by atmospheric analysis might be argued to be more appropriate to planets which, like the Earth, have a high density of living matter at or near the surface, but inapplicable to planets on which the biota, if present, is likely to be very thin. But the ratio of atmospheric mass to surface area is 30 times less for Mars than for Earth, so that the quantity of living matter required to produce comparable effects could be much less. Furthermore, the removal of one biologically produced component could, conceivably, be dependent upon the simultaneous presence of another similarly produced. If the removal rate is concentration-dependent, an easily measured steady-state concentration can occur even when the production rate is very low.
A more fundamental point is that the ability of a biota to control its environment and drive it to a state of low entropy is not a function of the density of that biota; it follows from the tendency of living organisms to continue to grow and develop until the supply of available raw materials establishes a limit. Consequently, the detectability of a planetary biota by means of atmospheric compositional analysis is not necessarily limited by the density of the biota.
A life detection experiment based on analysis of the Martian atmosphere should be designed to measure the concentrations of as many of the components of the atmosphere as possible, including both the major and trace components. In view of what is not known of the Martian atmosphere or surface, it would be unwise at this stage to prejudice the experiment by guessing what may be the more significant constituents to measure. The significance of any single compound can only be determined after the analytical information has been obtained and interpreted. This requires at least a minimum of supporting information to make possible the specification of the conditions under which these components would be expected to react.
Some of this auxiliary information, such as the solar spectrum and the temperature at the surface, are already available; the state of oxidation of the surface and the radiation flux at the surface may be available during the early Voyager missions. This latter information may not be necessary for the unequivocal interpretation of the biological significance of the atmospheric measurements, but it would be of value in confirming any conclusions. If, as is possible, the Martian atmosphere is poised between an oxidizing and a reducing state knowledge of the oxidation state of the surface would be of great value.
Information regarding isotopic concentrations may also be of importance. In particular, where the same element occurs in two or more atmospheric compounds, the relative amounts of its isotopes would provide valuable information on the nature of the reactions between them. The well known ability of living organisms to fractionate isotopes of the same elements provides a means of distinguishing biotic and abiotic reactions; thus, evidence of clear isotopic selectivity might in some circumstances be suggestive evidence of biotic transformation.
Science is divided artificially into a spectrum of disciplines ranging from Physics to Biology. The language, thought, and experimental approaches of each have been conditioned by the historical course of their development and by the nature of the experimental evidence available for consideration. It is inevitable that Biology and associated disciplines such as Biochemistry are geocentric in nature if only because of the lack of any experimental experience other than that on Earth.
By a similar process of development that branch of Physics dealing with astronomy is essentially exocentric in approach, even to the point of a blindness to biological phenomena at the Earth's surface.
Hitherto almost all experiments proposed for extraterrestrial life detection have arisen from an essentially biological background and have sought to recognize life elsewhere in terms of well known laboratory procedures on Earth such as microbiological growth experiments and the analytical procedures of Biochemistry. Apart from the overwhelming geocentric bias of these approaches, the mechanics of transporting a biochemical laboratory to another planet are expensive and technically of the greatest difficulty.
The purpose of this paper has been to draw attention to the obvious fact that the phenomenon of life is not arbitrarily limited to effects occurring in the realm of "Life Sciences". Because of this the presence of life on other planets may be recognized from comparatively simple measurements of their physical environment and superficial chemical composition. Such measurements may even be possible by astronomical observations from the Earth and, if successful in detecting a biota would serve to guide achievement of a more detailed understanding of its nature through more complex approaches such as those of the biological sciences.
We thank Dr. L. D. Kaplan and Dr. N. H. Horowitz of the Jet Propulsion Laboratory and Mr. G. B. Thomas of Hamilton Standard for the expert advice they freely and generously provided. We are especially grateful to Lord Rothschild F.R.S. for his encouragement and support during the preparation of this paper. This work was supported by contract NASw 871 between the National Aeronautics & Space Administration and Hamilton Standard Division of United Aircraft Corporation.
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