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Instability of a highly reducing atmosphere on the primitive Earth
Precambrian Research, 3 (1976) 463--470
463
© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
INSTABILITY OF A HIGHLY REDUCING ATMOSPHERE ON THE PRIMITIVE EARTH
MIKIO SHIMIZU
Institute of Space and Aeronautical Science, University of Tokyo, Tokyo (Japan) (Received November 6, 1975; revision accepted February 20, 1976)
ABSTRACT Shimizu, M., 1976. Instability of a highly reducing atmosphere on the primitive Earth. Precambrian Res., 3 : 463--470. The eddy diffusion coefficient in the primitive upper atmosphere of highly reducing type (CH4--H2) is estimated to be of the order of l 0 s cm2sec-1 or a little less, by solving the dissipation equation of internal gravity waves. Such a strong atmospheric mixing is clearly inconsistent with the previous assumption by McGovern that photochemical equilibrium was established in the upper atmosphere. By taking into account the mixing effect, the exospheric temperature of the highly reducing atmosphere has been computed to be higher than 1300°K. The result indicates that the hypothetical reducing atmosphere might have disappeared, if it existed, in a short time due to the gravitational escape of hydrogen and, consequently, that an anoxygenous but non-reducing atmosphere might be a more plausible environment for the origin of life.
INTRODUCTION
The existence of a highly reducing atmosphere (H2, CH4, and NH3) on the primitive Earth has frequently been postulated. This assumption is supported by the following indirect evidence: (1) Fermentation, the most primitive form of energy-yielding mechanism in a living system, might start only in the absence of free oxygen. (2) The Urey--Miller experiment suggested that a CH4--NH3-dominant atmosphere was the most suitable environment for the generation of abiotic molecules. (3) The primitive atmosphere might be accreted from the hydrogen-dominated solar nebula. On the other hand, many investigators have suggested that the majority of the carbon in the primitive atmosphere was in an oxidizing state. Indeed, direct evidence, such as an abundance of organic substances in the oldest rocks, has never been found. The critical factor in determining the reducing state of the primitive atmosphere is the availability of free hydrogen. Hydrogen cannot easily
464
escape into space if the exospheric temperature is low. In such a case, a hypothetical CH4--NH3 atmosphere could have persisted long enough to permit chemical evolution. McGovern (1969) has computed the exospheric temperature of a CH4--H2 atmosphere and has obtained low exospheric temperatures less than 800°K in various cases by assuming that products resulting from the photolysis of CH4 would cool the atmosphere efficiently. However, the assumption of photochemical equilibrium adopted in this calculation for obtaining the distribution of the photodissociation products is questionable. It has been known that, in the various cases of planetary atmospheres, the effect of atmospheric mixing in the upper atmosphere has usually destroyed the equilibrium. In this paper the eddy mixing in the primitive atmosphere is discussed and the instability of a hypothetical highly reducing atmosphere on a global scale to the gravitational escape of hydrogen from the top of the atmosphere is postulated. E D D Y D I F F U S I O N IN T H E P R I M I T I V E U P P E R A T M O S P H E R E
It is well established that eddy diffusion is the most important factor in determining the distribution of photodissociation products in the upper atmosphere of planets. A current theory for the generation mechanism of eddy diffusion is that the eddies are created by the dissipation of largeamplitude internal gravity waves randomly excited in the lower atmosphere. The lower atmosphere of the primitive Earth should have been so turbulent (Shimizu, 1974a) that there were a number of excitation sources for the internal gravity waves. We have already computed the wave-induced eddydiffusion coefficient in the atmospheres of Jupiter and Venus (Shimizu, 1974b), following the method developed by Midgley and Liemohn (1966). The same procedure has been adopted here to compute the eddy-diffusion coefficients in both the CH4 and the H2 atmospheres. Parameters adopted are 200°K for the atmospheric temperature, 988 cm sec -2 for the gravitational acceleration, 1.439 and 1.40 for Cp/C v in the cases of H2 and CH4, respectively, and the Prandtle number as unity. The results are shown in Figs.1 and 2. By assuming that the dimensions of eddies are similar to that of the present atmosphere (10--20 km for the vertical wavelength and 100 km for the horizontal wavelength), the most likely value for the eddy-diffusion coefficients in the hypothetical reducing atmosphere would appear to be of the order of 108 cm2sec-1 in the case of the H2 atmosphere and a little less in the case of the CH4 atmosphere. McGovern (1969) assumed that photochemical equilibrium was established by computing the distribution of CH4 dissociation products in the thermospheres of CH4--H2 atmospheres. As has been argued above, however, the upper atmosphere should be strongly mixed. Acetylene and other photolysis products of methane in the upper atmosphere might be rapidly diluted, without chemically reacting, in spite of large reaction-rate coefficients among these hydrocarbons. A weaker infrared radiator, CH4, may be the main con-
465
10~ Earth (H2)
.lOOkm
10~
v
10~
10
100
1,000
~ (km) Fig.1. E d d y d i f f u s i o n c o e f f i c i e n t s in t h e p r i m i t i v e u p p e r a t m o s p h e r e (H: case) as a funct i o n of h o r i z o n t a l w a v e l e n g t h kx a n d vertical w a v e l e n g t h ?'z o f t h e eddies.
constituent of the upper atmosphere, at least at its initial phase of chemical evolution. EXOSPHERIC TEMPERATURE OF THE HYPOTHETICAL REDUCING ATMOSPHERE
The exospheric temperature of an atmosphere may be determined by solving the heat-conduction equation d
dz (
T)+Q-L=O
(1)
as usual, where K is the thermal conduction coefficient of the atmosphere, T the temperature, Q the heating-source function, and L the cooling-rate function. McGovern (1969) computed the cooling rate by assuming that the infrared cooling of the photolysis products of methane was most efficient. However, the dissociation of CH4 might effectively be prevented by the strong atmospheric mixing and the rate might be overestimated. Furthermore, he assumed
466
1°9F Earth (CH,)
E
lOlL IO
Fig.2. E d d y d i f f u s i o n c o e f f i c i e n t s in t h e primitive u p p e r a t m o s p h e r e (CH 4 case) as a funct i o n o f h x a n d h z o f t h e eddies.
that the dependence of de-excitation rates of various hydrocarbons on temperature was in the form of exp(-A/T) but, in reality, it should have the form of exp(-B/Tl$). There are some other drawbacks in his work besides this: the heating efficiency was taken to be 0.25, b u t in fact it is much larger (Henry and McElroy, 1969; Strobel and Smith, 1973). The solar UV flux at solar activity minimum was adopted b u t the flux at maximum should be taken. (It is important to note that the exospheric temperature and thus the escaping rate of hydrogen are very sensitive functions of the solar flux. The total a m o u n t of escaping hydrogen over the whole phase of solar activity may almost be determined by the integration of the escape flux over its maximum periods.) This underestimation of the heating rate m a y partly be compensated for by his adoption of Hinteregger's old flux (overestimated by a factor of 1.5) and by his neglect of the low luminosity of the primordial sun (70% of the present one, 4.5 • 109 years ago (Sagan, 1972), although unexpectedly low flux of the observed solar neutrinos made this theoretical value still dubious). Overall underestimation of heating rate may reach a factor of a b o u t two,
467
even if we use a moderate estimate of heating rate by taking the day--night average, which was also not adopted in his paper. Rasool and McGovern (1966) claimed that a hydrogen atmosphere might have a low exospheric temperature due to its high conductivity. However, the large-scale height of the upper atmosphere should increase the exospheric temperature contrary to their expectation (Gross, 1972; Shimizu, 1974a). In our computation, all these points are corrected: the heating efficiencies are taken to be 0.86 and 0.65 for H2 and CH4, respectively (it should be noted that the contribution from the solar Lyman alpha line is essential for methane), and the day--night average is adopted. The adopted cross sections and spectrum of the solar flux are from Hudson (1971) and from Donnelly and Pope (1973). The deactivation probabilities of CH4 1--0 transition at 7.7 pm by the collision with CH4 and H2 are obtained by fitting to the experimental results of Yardley et al. (1970) and Eucken and Ayber (1940), respectively, under the assumption of 7ufa temperature dependence. The thermal conductivities of H2 and CH4 can be written as hH~ = 1050 X/ T and ~2H4 = 210 x/ T, respectively. For the mixture of gases, the conductivity is averagecl so that 1/X = 1/XH: + 1/XCH 4
The mesopause temperature and density are fixed at 150°K and 1012 cm -3, respectively. A slight variation of these two parameters does not change the subsequent conclusions. 10,000 |:.1
5,000
If = 2
? 2,000 E.~ 1,000
a~
500
20(3
~
1L0 li5 2J0 Z~ 9'5 100 H2 in Total Gas I%) Fig.3. Exospheric temperatures of a CH4--H 2 atmosphere as a f u n c t i o n of percentage of H 2 for various heating rates ( F = 1 as a s t a n d a r d ; see text).
468
The results of the computations are shown in Fig.3. The curve with the index F = I corresponds to the plausible heating rate discussed above. The exospheric temperature of the primitive atmosphere in such a case is 1400°K for the CH4 atmosphere and it increases very sharply as the mixing ratio of H2 increases. Since the exospheric temperature of the 100% H2 atmosphere for F = 1 is so high and since it is not computable by our present program, the exospheric temperatures for smaller F's are also calculated to see the general tendency of the curves. If the temperature is over 900°K, the decay time of the primitive atmosphere is less than 104 years, perhaps too short for the formation of a living system. In the above computation, the effect of eddy diffusion on the thermal structure of the upper atmosphere has been neglected. The eddy diffusion works to reduce and to increase the exospheric temperature, by the eddy transport of heat and by the heating associated with the atmospheric mixing, respectively. The effect of eddy transport was investigated in the case of the Martian ionosphere and was found to be unimportant (Shimizu, 1973). If the situation occurs similarly in the case of the primitive atmosphere of the Earth, the computed exospheric temperature may give a lower limit. Some time after the escape of a large amount of H~, the composition of the primitive atmosphere became richer in complex hydrocarbons, such as C2H2, although not as many as McGovern assumed. The exospheric temperature might, however, still be higher than 900°K in this case if we correct the underestimation of heating rate by McGovern pointed out above, and in particular if we do not adopt the (dubious) small value for the primordial solar flux. DISCUSSION AND CONCLUSIONS
In addition to the two interesting characteristics, (1) high exospheric temperature and (2} large eddy-diffusion coefficient, the hypothetical reducing atmosphere has another important feature due to its main constituent, CH4. This molecule is known in laboratory experiments to leave precipitable polymers in its ultraviolet photolysis. The dissociation products of CH4 in the upper atmosphere are transported (or diluted} to the lower levels by eddy diffusion. At the lower levels, various kinds of reactions may occur depending on the temperature and pressure of the levels: the reverse reaction of CH2 to CH4 (perhaps the most rapid reaction due to the presence of H2), the formation of C2H2, etc., and the formation of precipitable polymers. The processes are irreversible, as Strobel (1973) pointed out in the Jovian case, due to the existence of precipitable substances. In the Jovian case, the amount of polymer formed during 4.5 Ga may still be negligible compared with the total amount of CH4. On the other hand, the CH4 amount of the primitive atmosphere might be much smaller. Its exospheric temperature might be so high that the determining factor of CH4 disappearance due to its dissociation into precipitable polymers (or 'coal tar' fallen on the surface)
469
and H2 (which escaped instantaneously to space) is the supply rate of the solar ultraviolet photons. If the CH4 surface pressure of the primitive atmosphere was 1 arm, its dissociation time may easily be estimated to be at the most 107 years, by assuming its photodissociation cross section to be larger than 10 -24 cm 2 in the wavelength region shorter than 1700 A. The C2H2 atmosphere might disappear in a much shorter time, since the cross section of C2H2 is large in the longer wavelength region. In summary a highly reducing atmosphere on the primitive Earth, if it existed, might have been photodissociated to leave 'coal tar' behind in t o o short a time for a living system to be formed in it. The previous optimistic conclusion as to the durability of a highly reducing atmosphere on the primitive Earth should be cautiously reevaluated. The existence of a global anoxygenous b u t non-reducing atmosphere on the primitive Earth has already been suggested by R u b e y (1951), Abelson (1966), and others on the basis of geological considerations. Abelson (1966) and others have actually succeeded in producing organic precursor molecules in a gas, simulating such anoxygenous atmospheres. Evidences for nitrate and sulphate respiration between fermentation and oxygen respiration has been obtained by Egami and his collaborators since 1957 and Egami {1974) has advocated the possibility of early biological evolution in an anoxygenous atmosphere on this biochemical basis. The results of our aeronomical computations presented in this paper are in accordance with these theories. A recent analysis of the observational data of the cometary atmospheres has revealed that C, N, and O may be present in the solar nebula in the form of CO, N2, and H:O, respectively (Shimizu, 1976a, 1976b). Anders et al. (1973) showed in a laboratory experiment that organic substances could be formed by Fischer--Tropsch-type catalytic reactions of water-bearing minerals and magnetite in a gas simulating the solar nebula. These minerals might have been able to condense at the orbit of the Earth at the formation of the solar system (Lewis, 1974). The organic c o m p o u n d s might have reached the surface of the primitive Earth and could have been abiotic precursors of the living system. Furthermore, this mechanism does not break the constraint imposed on the primitive Earth in this paper.
REFERENCES Abelson, P.H., 1966. Chemical events on the primitive Earth. Proc. NAS U.S.A., 55: 1366. Anders, E., Hayatsu, R. and Studier, M.H., 1973. Organic compounds in meteorites. Science, 182: 781. Donnelly, R.F. and Pope, J.H., 1973. The 1--300 A solar flux for a moderate level of solar activity for use in modeling the ionosphere and upper atmosphere. NOAA Tech. Rep., ERL 276-SEL 25. Egami, F., 1974. Inorganic type of fermentation and anaerobic respirations in the evolution of energy yielding metabolisms. Orgins Life, 5: 405.
470 Eucken, A. and Ayber, S., 1940. Die Stobanregung Intermolekulare Schwingungen in Gasen und Gasmischungen VI Schallabsorptions und Dispersion Messungen an CH4, COS und ihren Mischungen mit Zusatzgasen. Z. Phys. Chem., B46: 195. Gross, S.H., 1972. On the exospheric temperatures of hydrogen dominated planetary atmospheres. J. Atmos. Sci., 29: 214. Henry, R.J. and McElroy, M.B., 1969. The absorption of extreme ultraviolet radiation by Jupiter's upper atmosphere. J. Atmos. Sci., 26: 912. Hudson, R.D., 1971. Critical review of ultraviolet photoabsorption cross sections for molecules of astrophysical and aeronomic interest. Rev. Geophys., 9: 305. Lewis, J.S., 1974. The temperature gradient in the solar nebula. Science, 186: 440. McGovern, W.E., 1969. The primitive Earth: thermal models of the upper atmosphere for a methane-dominated environment. J. Atmos. Sci., 26: 623. Midgley, J.E. and Liemohn, H.B., 1966. Gravity waves in a realistic atmosphere. J. Geophys. Res., 71: 3729. Rasool, S.I. and McGovern, W.E., 1966. Primitive atmosphere of the Earth. Nature, 212: 1225. Rubey, W.W., 1951. Geological history of sea water. Bull. Geol. Soc. Am., 62: 1111. Sagan, C., 1972. Earth and Mars: evolution of atmosphere and surface temperatures. Science, 177: 52. Shimizu, M., 1973. Atmospheric mixing in the upper atmospheres of Mars and Venus. J. Geophys. Res., 78: 6780. Shimizu, M., 1974a. Molten Earth and the origin of prebiological molecules. Origins Life, 6: 15. Shimizu, M., 1974b. Atmospheric mixing in the upper atmospheres of Jupiter and Venus. J. Geophys. Res., 79: 5311. Shimizu, M., 1976a. Neutral temperature of cometary atmospheres. The study of Comets. NASA Sp--393, p. 763. Shimizu, M., 1976b. The structure of cometary atmospheres. II, Ion distribution. Astrophys. Space Sci., in press. Strobel, D.F., 1973. The photochemistry of hydrocarbons in the Jovian atmosphere. J. Atmos. Sci., 30: 489. Strobel, D.F. and Smith, G.R., 1973. On the temperature of the Jovian thermosphere. J. Atmos. Sci., 30: 718. Yardley, J.T., Fertig, M.N. and Moore, C.B., 1970. Vibrational deactivation in methane mixture. J. Chem. Phys., 52: 1450.
463
© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
INSTABILITY OF A HIGHLY REDUCING ATMOSPHERE ON THE PRIMITIVE EARTH
MIKIO SHIMIZU
Institute of Space and Aeronautical Science, University of Tokyo, Tokyo (Japan) (Received November 6, 1975; revision accepted February 20, 1976)
ABSTRACT Shimizu, M., 1976. Instability of a highly reducing atmosphere on the primitive Earth. Precambrian Res., 3 : 463--470. The eddy diffusion coefficient in the primitive upper atmosphere of highly reducing type (CH4--H2) is estimated to be of the order of l 0 s cm2sec-1 or a little less, by solving the dissipation equation of internal gravity waves. Such a strong atmospheric mixing is clearly inconsistent with the previous assumption by McGovern that photochemical equilibrium was established in the upper atmosphere. By taking into account the mixing effect, the exospheric temperature of the highly reducing atmosphere has been computed to be higher than 1300°K. The result indicates that the hypothetical reducing atmosphere might have disappeared, if it existed, in a short time due to the gravitational escape of hydrogen and, consequently, that an anoxygenous but non-reducing atmosphere might be a more plausible environment for the origin of life.
INTRODUCTION
The existence of a highly reducing atmosphere (H2, CH4, and NH3) on the primitive Earth has frequently been postulated. This assumption is supported by the following indirect evidence: (1) Fermentation, the most primitive form of energy-yielding mechanism in a living system, might start only in the absence of free oxygen. (2) The Urey--Miller experiment suggested that a CH4--NH3-dominant atmosphere was the most suitable environment for the generation of abiotic molecules. (3) The primitive atmosphere might be accreted from the hydrogen-dominated solar nebula. On the other hand, many investigators have suggested that the majority of the carbon in the primitive atmosphere was in an oxidizing state. Indeed, direct evidence, such as an abundance of organic substances in the oldest rocks, has never been found. The critical factor in determining the reducing state of the primitive atmosphere is the availability of free hydrogen. Hydrogen cannot easily
464
escape into space if the exospheric temperature is low. In such a case, a hypothetical CH4--NH3 atmosphere could have persisted long enough to permit chemical evolution. McGovern (1969) has computed the exospheric temperature of a CH4--H2 atmosphere and has obtained low exospheric temperatures less than 800°K in various cases by assuming that products resulting from the photolysis of CH4 would cool the atmosphere efficiently. However, the assumption of photochemical equilibrium adopted in this calculation for obtaining the distribution of the photodissociation products is questionable. It has been known that, in the various cases of planetary atmospheres, the effect of atmospheric mixing in the upper atmosphere has usually destroyed the equilibrium. In this paper the eddy mixing in the primitive atmosphere is discussed and the instability of a hypothetical highly reducing atmosphere on a global scale to the gravitational escape of hydrogen from the top of the atmosphere is postulated. E D D Y D I F F U S I O N IN T H E P R I M I T I V E U P P E R A T M O S P H E R E
It is well established that eddy diffusion is the most important factor in determining the distribution of photodissociation products in the upper atmosphere of planets. A current theory for the generation mechanism of eddy diffusion is that the eddies are created by the dissipation of largeamplitude internal gravity waves randomly excited in the lower atmosphere. The lower atmosphere of the primitive Earth should have been so turbulent (Shimizu, 1974a) that there were a number of excitation sources for the internal gravity waves. We have already computed the wave-induced eddydiffusion coefficient in the atmospheres of Jupiter and Venus (Shimizu, 1974b), following the method developed by Midgley and Liemohn (1966). The same procedure has been adopted here to compute the eddy-diffusion coefficients in both the CH4 and the H2 atmospheres. Parameters adopted are 200°K for the atmospheric temperature, 988 cm sec -2 for the gravitational acceleration, 1.439 and 1.40 for Cp/C v in the cases of H2 and CH4, respectively, and the Prandtle number as unity. The results are shown in Figs.1 and 2. By assuming that the dimensions of eddies are similar to that of the present atmosphere (10--20 km for the vertical wavelength and 100 km for the horizontal wavelength), the most likely value for the eddy-diffusion coefficients in the hypothetical reducing atmosphere would appear to be of the order of 108 cm2sec-1 in the case of the H2 atmosphere and a little less in the case of the CH4 atmosphere. McGovern (1969) assumed that photochemical equilibrium was established by computing the distribution of CH4 dissociation products in the thermospheres of CH4--H2 atmospheres. As has been argued above, however, the upper atmosphere should be strongly mixed. Acetylene and other photolysis products of methane in the upper atmosphere might be rapidly diluted, without chemically reacting, in spite of large reaction-rate coefficients among these hydrocarbons. A weaker infrared radiator, CH4, may be the main con-
465
10~ Earth (H2)
.lOOkm
10~
v
10~
10
100
1,000
~ (km) Fig.1. E d d y d i f f u s i o n c o e f f i c i e n t s in t h e p r i m i t i v e u p p e r a t m o s p h e r e (H: case) as a funct i o n of h o r i z o n t a l w a v e l e n g t h kx a n d vertical w a v e l e n g t h ?'z o f t h e eddies.
constituent of the upper atmosphere, at least at its initial phase of chemical evolution. EXOSPHERIC TEMPERATURE OF THE HYPOTHETICAL REDUCING ATMOSPHERE
The exospheric temperature of an atmosphere may be determined by solving the heat-conduction equation d
dz (
T)+Q-L=O
(1)
as usual, where K is the thermal conduction coefficient of the atmosphere, T the temperature, Q the heating-source function, and L the cooling-rate function. McGovern (1969) computed the cooling rate by assuming that the infrared cooling of the photolysis products of methane was most efficient. However, the dissociation of CH4 might effectively be prevented by the strong atmospheric mixing and the rate might be overestimated. Furthermore, he assumed
466
1°9F Earth (CH,)
E
lOlL IO
Fig.2. E d d y d i f f u s i o n c o e f f i c i e n t s in t h e primitive u p p e r a t m o s p h e r e (CH 4 case) as a funct i o n o f h x a n d h z o f t h e eddies.
that the dependence of de-excitation rates of various hydrocarbons on temperature was in the form of exp(-A/T) but, in reality, it should have the form of exp(-B/Tl$). There are some other drawbacks in his work besides this: the heating efficiency was taken to be 0.25, b u t in fact it is much larger (Henry and McElroy, 1969; Strobel and Smith, 1973). The solar UV flux at solar activity minimum was adopted b u t the flux at maximum should be taken. (It is important to note that the exospheric temperature and thus the escaping rate of hydrogen are very sensitive functions of the solar flux. The total a m o u n t of escaping hydrogen over the whole phase of solar activity may almost be determined by the integration of the escape flux over its maximum periods.) This underestimation of the heating rate m a y partly be compensated for by his adoption of Hinteregger's old flux (overestimated by a factor of 1.5) and by his neglect of the low luminosity of the primordial sun (70% of the present one, 4.5 • 109 years ago (Sagan, 1972), although unexpectedly low flux of the observed solar neutrinos made this theoretical value still dubious). Overall underestimation of heating rate may reach a factor of a b o u t two,
467
even if we use a moderate estimate of heating rate by taking the day--night average, which was also not adopted in his paper. Rasool and McGovern (1966) claimed that a hydrogen atmosphere might have a low exospheric temperature due to its high conductivity. However, the large-scale height of the upper atmosphere should increase the exospheric temperature contrary to their expectation (Gross, 1972; Shimizu, 1974a). In our computation, all these points are corrected: the heating efficiencies are taken to be 0.86 and 0.65 for H2 and CH4, respectively (it should be noted that the contribution from the solar Lyman alpha line is essential for methane), and the day--night average is adopted. The adopted cross sections and spectrum of the solar flux are from Hudson (1971) and from Donnelly and Pope (1973). The deactivation probabilities of CH4 1--0 transition at 7.7 pm by the collision with CH4 and H2 are obtained by fitting to the experimental results of Yardley et al. (1970) and Eucken and Ayber (1940), respectively, under the assumption of 7ufa temperature dependence. The thermal conductivities of H2 and CH4 can be written as hH~ = 1050 X/ T and ~2H4 = 210 x/ T, respectively. For the mixture of gases, the conductivity is averagecl so that 1/X = 1/XH: + 1/XCH 4
The mesopause temperature and density are fixed at 150°K and 1012 cm -3, respectively. A slight variation of these two parameters does not change the subsequent conclusions. 10,000 |:.1
5,000
If = 2
? 2,000 E.~ 1,000
a~
500
20(3
~
1L0 li5 2J0 Z~ 9'5 100 H2 in Total Gas I%) Fig.3. Exospheric temperatures of a CH4--H 2 atmosphere as a f u n c t i o n of percentage of H 2 for various heating rates ( F = 1 as a s t a n d a r d ; see text).
468
The results of the computations are shown in Fig.3. The curve with the index F = I corresponds to the plausible heating rate discussed above. The exospheric temperature of the primitive atmosphere in such a case is 1400°K for the CH4 atmosphere and it increases very sharply as the mixing ratio of H2 increases. Since the exospheric temperature of the 100% H2 atmosphere for F = 1 is so high and since it is not computable by our present program, the exospheric temperatures for smaller F's are also calculated to see the general tendency of the curves. If the temperature is over 900°K, the decay time of the primitive atmosphere is less than 104 years, perhaps too short for the formation of a living system. In the above computation, the effect of eddy diffusion on the thermal structure of the upper atmosphere has been neglected. The eddy diffusion works to reduce and to increase the exospheric temperature, by the eddy transport of heat and by the heating associated with the atmospheric mixing, respectively. The effect of eddy transport was investigated in the case of the Martian ionosphere and was found to be unimportant (Shimizu, 1973). If the situation occurs similarly in the case of the primitive atmosphere of the Earth, the computed exospheric temperature may give a lower limit. Some time after the escape of a large amount of H~, the composition of the primitive atmosphere became richer in complex hydrocarbons, such as C2H2, although not as many as McGovern assumed. The exospheric temperature might, however, still be higher than 900°K in this case if we correct the underestimation of heating rate by McGovern pointed out above, and in particular if we do not adopt the (dubious) small value for the primordial solar flux. DISCUSSION AND CONCLUSIONS
In addition to the two interesting characteristics, (1) high exospheric temperature and (2} large eddy-diffusion coefficient, the hypothetical reducing atmosphere has another important feature due to its main constituent, CH4. This molecule is known in laboratory experiments to leave precipitable polymers in its ultraviolet photolysis. The dissociation products of CH4 in the upper atmosphere are transported (or diluted} to the lower levels by eddy diffusion. At the lower levels, various kinds of reactions may occur depending on the temperature and pressure of the levels: the reverse reaction of CH2 to CH4 (perhaps the most rapid reaction due to the presence of H2), the formation of C2H2, etc., and the formation of precipitable polymers. The processes are irreversible, as Strobel (1973) pointed out in the Jovian case, due to the existence of precipitable substances. In the Jovian case, the amount of polymer formed during 4.5 Ga may still be negligible compared with the total amount of CH4. On the other hand, the CH4 amount of the primitive atmosphere might be much smaller. Its exospheric temperature might be so high that the determining factor of CH4 disappearance due to its dissociation into precipitable polymers (or 'coal tar' fallen on the surface)
469
and H2 (which escaped instantaneously to space) is the supply rate of the solar ultraviolet photons. If the CH4 surface pressure of the primitive atmosphere was 1 arm, its dissociation time may easily be estimated to be at the most 107 years, by assuming its photodissociation cross section to be larger than 10 -24 cm 2 in the wavelength region shorter than 1700 A. The C2H2 atmosphere might disappear in a much shorter time, since the cross section of C2H2 is large in the longer wavelength region. In summary a highly reducing atmosphere on the primitive Earth, if it existed, might have been photodissociated to leave 'coal tar' behind in t o o short a time for a living system to be formed in it. The previous optimistic conclusion as to the durability of a highly reducing atmosphere on the primitive Earth should be cautiously reevaluated. The existence of a global anoxygenous b u t non-reducing atmosphere on the primitive Earth has already been suggested by R u b e y (1951), Abelson (1966), and others on the basis of geological considerations. Abelson (1966) and others have actually succeeded in producing organic precursor molecules in a gas, simulating such anoxygenous atmospheres. Evidences for nitrate and sulphate respiration between fermentation and oxygen respiration has been obtained by Egami and his collaborators since 1957 and Egami {1974) has advocated the possibility of early biological evolution in an anoxygenous atmosphere on this biochemical basis. The results of our aeronomical computations presented in this paper are in accordance with these theories. A recent analysis of the observational data of the cometary atmospheres has revealed that C, N, and O may be present in the solar nebula in the form of CO, N2, and H:O, respectively (Shimizu, 1976a, 1976b). Anders et al. (1973) showed in a laboratory experiment that organic substances could be formed by Fischer--Tropsch-type catalytic reactions of water-bearing minerals and magnetite in a gas simulating the solar nebula. These minerals might have been able to condense at the orbit of the Earth at the formation of the solar system (Lewis, 1974). The organic c o m p o u n d s might have reached the surface of the primitive Earth and could have been abiotic precursors of the living system. Furthermore, this mechanism does not break the constraint imposed on the primitive Earth in this paper.
REFERENCES Abelson, P.H., 1966. Chemical events on the primitive Earth. Proc. NAS U.S.A., 55: 1366. Anders, E., Hayatsu, R. and Studier, M.H., 1973. Organic compounds in meteorites. Science, 182: 781. Donnelly, R.F. and Pope, J.H., 1973. The 1--300 A solar flux for a moderate level of solar activity for use in modeling the ionosphere and upper atmosphere. NOAA Tech. Rep., ERL 276-SEL 25. Egami, F., 1974. Inorganic type of fermentation and anaerobic respirations in the evolution of energy yielding metabolisms. Orgins Life, 5: 405.
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