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The evolution of venus' atmosphere
Planet. Space Sci. 1969, Vol.
17, pp. 1021 to 1028.
Permmon
THE EVOLUTION
Press.
Printed
in Northern
Ireland
OF VENUS’ ATMOSPHERE*
ANNPALM Southwest Center for Advanced Studies, Dallas, Texas 75230, U.S.A. (Received in final form 29 November 1968)
Abstract-Although the atmospheres of the terrestrial planets may have originated by similar mechanisms and duringasimilarepoch, their subsequent developments followeddivergentpaths, partly as the result of different Sun-planet distances. On such a premise a self-consistent model has been devised to explain the present dense CO, atmosphere of Venus. It is considered to have originated from the degassing of the planet’s interior during its early molten phase and to have proceeded in three distinct stages: (1) Gases were released which resembled in composition those of volcanic output. (2) These gases interacted chemically with the lithosphere, relargely in the formation of oxides and carbonates. (3) Eventually the degassing sult’ subs~ 7 ed and the water vapor became depleted by either photodissociation or in-situ mineral oxidation and hydration. The resulting low partial pressures of water vapor and CO, promoted decarbonation which gave rise to the presently observed dense carbon dioxide atmosphere. 1.INTRODUCTION
Since Earth and Venus are closely related planets as revealed by their similar planetary parameters, the very difference of their atmospheres has long posed a challenging problem. The significance of lithospheric chemistry to the evolution of planetary atmospheres has been broached by Urey (1952,1959) in his pioneering work on the origin of the planets. In recent studies Cameron (1963) dismissed the possibly analogous development on Barth and Venus, and Ringwood (1966) suggested that the planets may have accreted from dust clouds that contained different proportions of carbon and hydrogen. By means of free energy calculations Dayhoff et al. (1967) have shown that hydrogen should have been more abundant originally than in the contemporary atmosphere and that the C/O ratio was probably equal to or less than 4. They also concluded that hydrogen and oxygen were lost by escape into space and by chemical interaction with the surface, respectively. To account for the many orders of magnitude difference between Earth and Venusin their water abundance, Sagan (1967) suggested that it was lost by efficient photodissociation followed by escape of hydrogen into space and the removal of oxygen by chemical reactions with reduced compounds in the lower atmosphere or the planetary surface. However, Gold (1964) has expressed doubt concerning the loss of some 300 atmospheres of water from the planet Venus. The inherent difficulty in explaining the origin of Venus’ atmosphere by these models is examined in this study. 2. DESCRIPTION OF MODEL Results of laboratory experiments and geological field studies are invoked to interpret the dissimilarity of the Earth’s and Venus’ atmospheres. It is assumed that the atmosphere of Venus is of secondary origin as is that of the Earth (Rubey, 1955), and three major phases are distinguished in its evolution: 1. During the period of early intense degassing and in the presence of water vapor, carbon dioxide was fixed by chemical interaction with the lithosphere. * Paper presented at the 49th Annual Geophysical Union Meeting April 8-11,1%8, Washington, D.C. 1021
1022
ANN
PALM
2. With the passage of time the major part of the water vapor was lost by photodissociation as hydrogen escaped into space and oxygen diffused downward and oxidized the surface materials. 3. Eventually the initially high degassing rate diminished and low partial pressures of water vapor and carbon dioxide developed. These caused the decomposition of carbonate bearing rocks and allowed carbon dioxide to accumulate to the present dense atmosphere. The underlying hypotheses, based largely on data available in the literature, are : 1. Earth and Venus evolved during the same epoch from similar materials by similar processes. 2. Distinct thermal histories of the planets led to characteristic atmospheres determined by the interaction of effluents with the respective lithospheres. 3. Early in planetary history the degassing rate exceeded the present one by several orders of magnitude. The first hypothesis implies that both Earth and Venus may have been formed from some common cloud of gas and dust as proposed by Schmidt (1944) ; see also Williams and Cremin (1968). The second hypothesis refers to the surface temperature of Venus which has always exceeded that of the Earth; the black-body differential due to closer planet-Sun distance amounts to 50°C. The third hypothesis derives from the assumption that heat dissipation was more intense during the early stages of planetary evolution and that therefore the volatiles initially escaped from the interior at a higher rate. In order to put these events in proper perspective, a time scale has been constructed as shown in Fig. 1. Curve A illustrates the rate of degassing which is assumed to follow a first order kinetics or exponential decay law. The onset and termination of intense degassing are designated tr and t,, respectively, and the concurrent atmospheric changes are shown by curves B. Whereas the correspondence of tr and tZin A and B is significant, the absolute timing remains uncertain. It is noteworthy that during the period of intense degassing no appreciable CO, atmosphere had accumulated because the CO, fraction of the volcanic output reacted chemically with the hot lithosphere by the formation of carbonates which is described in a subsequent paragraph. In contrast, the water vapor that emerged at the surface had built up to a transient atmosphere, but waslost eventually by a series of processes elaborated below. 3. DEGASSING
HISTORY
Because of their parallel developments, Earth and Venus may be composed of the same rock types (bulk composition) that once contained similar quantities of volatiles. During their early histories of melting and resolidification the emitted gases may also have had the same relative composition. Therefore, predominantly water and carbon dioxide emerged at Venus’ surface together with minor quantities of nitrogen, halogen, halides, sulfides and other trace elements. To explain the absence of a hydrosphere Holland (1964) contended that Venus may have lost its water complement during the initial phase of accretion. According to the present model, water was retained initially, but released by subsequent planetary outgassing, and the amount may have been comparable to the fraction in the excess volatiles (Rubey, 1955). In Table 1 these quantities are compared with H,O and CO, in Venus’ contemporary atmosphere as determined recently from Mariner 5 (1967) and Venera 4 (1968) records. In addition, HaO/COa ratios deduced from various sources (Rubey, 1951) are presented since these ratios may shed further light on the abundance of water in Venus’ primitive atmosphere.
THE EVOLUTION
ii 2
1023
OF VENUS’ ATMOSPHERE
1--_*
to3
z
5
E6 IO2 3 IO4
s
IO'
A
‘I
’
2
TIME,
4
‘2
B. YR
A
20 -
/e
/’ co2 2 rr:
i I
IO-
/ H20
i
0’ ‘I
2
’ TIME,
B
*J’
_-_
‘2
4
l
B. YR
FIQ. 1. TIMESCALEOF ATMOSPH2RIC EVOLUTION ON VENUS. A, rate of degassing of interior; B, rates of decarbonation and of change of temporary vapor atmosphere.
water
With the aid of the values presented in Table 1 early and present degassing rates of water can be distinguished by assuming that the water outgassed in the remote past has largely been lost, but that the water existing now in Venus’ atmosphere is the amount which has emerged since the intense degassing subsided. This fraction has been trapped by the CO, atmosphere which accumulated to its present density by lithological decomposition. It has been estimated that the early rate of degassing, based on the water equivalent in the exThe original cess volatiles, exceeded the current rate by several orders of magnitude. quantity of water that emerged during a period of several billion (log) years depended on the magnitude of the time constants as shown in Table 2. This range of time constants does not appear unreasonable in view of the possible energy sources that have TABLB1. EXCESSVOLATILE3 * Quantity
32 108 51 (1) 18 44
AND ATMOSPHERE OF VENUS
CO*@
H&O
lWg/cm’ 10%01./cm* lo%lol. HlO/CO, by wt. H,O/COI mol.
ON &IU’H
1.8 ::; (2) 2.5 6.1
(3) 4.1 10
Ha0 ?
CO*?
0.009 0.03 0.014 yi
2.1 2.9 1.4 ;6j
37
(5) 301 734
50
* According to Rubey (1955), amount of volatiles not due to chemical weathering. Ratios derived from (1) excess volatilea, (2) volcanic gases, (3) occluded in basalt, (4) occluded in granite, (5) hot springs, (6) H,O complement of excess volatile@ vs. CO, of present 0 9 atmosphere. Recently Kliore and Cain (1968) have estimated pressures of the order of 60 f 10 atm compared with 20 atm used in thii work.
1024
ANN TABLE
2.
hACXION
PALM
OF EARLY
Time constant 7xlWyr
1
0.43 1-o 1.5 2.46
0.90 0.63 0.50 0.33
QUANTITY
OF WATER
Elapsed time, 1O’yr 2 3 0.99 0.86 0.74 O-56
o-999 0.95 0.86 o-70
DEGASSED
4 o-9999 0.98 0.93 0.80
contributed to the early internal heating, such as gravitational contraction, radioactive decay or tidal forces. A present rate of degassing that may not differ from the mean rate overgeologicaltimeispreferred by Gold (1964),inagreementwithRubey’scontention (1951). 4. LITHOSPHERIC
REACTIONS
In this report reactions are examined that may have affected the development of the early atmosphere. A number of chemical reactions that occur between the surface minerals and the contemporary atmosphere have been discussed recently by Mueller (1964). Some typical reactions by which the CO, fraction of the volcanic output may have been tied are described briefly. When lava, steam and gases emerged as a result of igneous activity, CO, was taken up by the surface materials in reactions such as CaO + COaZ CaCO,
(a)
MgSiO, + COs ti MgCO, + SiOs
(b)
Casio,, + NasSiO, + COse
(c)
CaCOa + Na,Si,O,.
The actual carbonation rates depended critically on the composition, temperature and permeability of the surface rocks; at 673”K, for example, type (c) reactions are more rapid than type (a). Characteristic rates of 10la CO, cm-s set-l have been obtained in laboratory experiments (Hyatt ei al., 1958). Assuming that the actual uptake rate was lower by several orders of magnitude and that the degassing rate from the planetary interior proceeded at a rate of about lolo COz cm-s set-l, an appreciable CO, atmosphere never developed during the period of rapid outgassing. It is noteworthy that the uptake of CO, is accelerated by the presence of liquid water or water vapor at elevated temperatures (Richer and Vallet, 1961), conditions that prevailed on Venus in the remote past. Over a period of several billion years some twenty atmospheres of CO,, equivalent to a low estimate of the present atmosphere (Kliore and Cain, 1968), were fixed, and several hundred meters of carbonate bearing rocks may have accumulated. Geochemical evidence for extensive carbonation at volcanic sites has been reported by Collins et al. (1926). They identified early Precambrian deposits of rocks composed of 2-60 per cent carbonates. The results of experiments (Hyatt et al., 1958) which illustrate these types of reactions are reproduced in Fig. 2. Although the uptake of COz is slower than its release, the relative rates do not differ by more than a factor of 10. Under the given experimental conditions of 1000°K and 0.1 at. pressure, CaCO, decomposed at a rate of lo4 g cm-2 min-l. At temperatures around 7OO”K, 2.3 x 104 g CO, cm-2, an amount comparable to Venus’ present atmosphere, may have beenliberated within 104-106yr, depending upon the efficiency of the actual reactions. The thermodynamics (Weeks, 1956) and kinetics of similar reactions have also been studied. The data indicate that these reactions could have occurred on Venus and that, therefore, the contemporary massive COs atmosphere can be explained
THE EVOLUTION OF VENUS’ ATMOSPHERE
1025
I
40 TIME,
HOURS
FIG. 2. CARBONATB RFKTIONS.
A, rate of CO, uptake; B, rate of decarbonation.
by such processes. In particular, Adamcik and Draper (1963) have calculated the free energy changes for some representative reactions CaCO, + SiO,*
Casio8 + CO,
(a)
MgCOa + SiO,s
MgSiO, + CO,
(b)
FeCO, + #iO,71:
+Fe,SiO, + COB
(c)
and have derived the corresponding equilibrium pressures as functions of temperature shown co* EQUILIRRILJTM PRWSJRJZ, Rtfll T”K 300 400 500 600 ::
@I
(4 9.3 6.8 1.3 4.2 4.8 2.8
x 10-a x lo-’ x 10-l x 10 x 10’
1.8 5.0 5.8 1.3 1.1 5.2
x 10-6 x lo-’ x 10’ x 10’ x 10’
as
(c) 6.3 4.2 2.0 2.6 1.6 5.6
x lO-* x x x x
1w l(r 10’ 10’
These values illustrate the differences in the abundance of carbonates on the planets; accordingly, the surface rocks of Venus do not contain any carbonates. For an open system which exists under actual lithospheric conditions, additional factors determine the stability of the reactants (Danielsson, 1950). Carbon dioxide liberated upon decarbonation can diffuse away from its local source and thus prevents the attainment of its equilibrium partial pressure. At the same time silicates accumulate until all the reactants, carbonate and quartz, are spent. The kinetics of these reactions is also favored under these conditions. 5. LOSS MECHANISM In view of the foregoing considerations CO2 released from the planetary interior should have been removed by chemical interaction with the lithosphere. Therefore, CO, whose optical properties overlap those of water (Berkner and Marshall, 1966) was prevented from radiatively shielding the continued photodissociation of water vapor, and the efficiency with which this temporary potential radiation screen was trapped determined largely the practicability of the proposed model. This aspect has not previously been discussed in the literature. However, due to the rate differentials of input (emission from planetary interior) and output (escape into space) of water, a transient water vapor atmosphere developed as
1026
ANN PALM
shown in Fig. 1. The loss of large quantities of water that emerged at the surface by igneous activity can be explained principally by photodissociation and to a lesser extent by direct interaction with the lithosphere. The photodissociative loss depended largely on the effectiveness of the foliowing processes: (1) photochemical reactions of the transient water vapor atmosphere; (2) exospheric loss of hydrogen; (3) downward diffusion of oxygen; and (4) oxidation of the surface materials. They are described only briefly in this report to illustrate the loss mechanisms and to assure the self-consistency of the proposed model. The relevant photochemical reactions (Bates, 1954) and rate coefficients (Keneshea, 1967) are H,O+hv+H+OH
JH = 10-5 r&c-r
OH+hv+H+O
JH’
OH+H+Hs+O
koH = 1.5 x 10-11 d(T)
exp ( -4/RZ’)cmS
OH+O-+H+O,
koo = l-5 x IO-l1 d(T)
exp (-3~~~)crns
OH + OH+H,O
+ 0
(1) (la)
kH2 = 1.5 x lo-l1 1/(T) exp (--4/W)
se@ se&
cm3 see-r
(2) (3) (4)
O+O+M*O,+M
k. = 3 x lO++ cm6 set-l
t5a)
O+O-+O,+hv
kor = 10-a cm3 set-r
(5b)
OH+H+M-+HsO+M
kH1 = 103’ cm6 set-l
(6)
03+hu-+O+0
Jo = lo-s set-1
(7)
O+Oa+M+Os+M
k oe = 6.5 x lo-34 ems set-l.
(8)
Many other minor reactions (Bates and Nicolet, 1950) may contribute to the chemosphere but are omitted because of their low probab~ty of occurrence. Reactions (5a) and (6) increase asps with (6) being slower than @a); they take place at lower altitudes, atp w 1W mb, while photodissociation occurs at optically thin levels of p m lo-4 mb, where the absorption cross-sections for water exceeds the collision cross-section. The loss rate, approximated by use of reactions (l), (S), (6) and (7),
UH30) ‘v
J2,[kotO)“W) -
2JotO31- kdII)(oH)t~)
indicates that the limiting step is the r~ombination of oxygen. This rate is orders of magnitude smaller than that of photodissociation of water; moreover, oxygen has to accumulate to some 0.5 cm before it acts as a radiation shield slowing down further water dissociation. Another important factor which accounts for the effective depletion of water vapor is the mass transport of oxygen away from the water dissociation level and its uptake by the lithosphere. Although the probability of formation at the photodissociation layer kept the ozone concentration low according to reaction (8), ozone that formed near the surface enhanced the removal of oxygen because of its ready oxidation of the surface rocks. The life cycle of the main constituents assumed the following pattern sources Sinks
Ha0 planetary interior photodissociation
H photodissociation planetary escape
0 photodissociation assimilation at the lithosphere.
THE
EVOLUTION
OF VENUS’
ATMOSPHERE
1027
Hence, the combined effects of low recombination probability of 0 and the downward diffusion and convection of 0, favor the photodissociatively initiated depletion of water vapor. Once free hydrogen formed, it was lost by thermal escape and possibly by the ionizing action of the solar wind, but the importance of the solar wind diminished at higher exospheric temperatures due to shorter escape times and increasing scale heights. Besides photodissociation, ionization contributed to a complex chemical kinetics and to a rise in the temperature of the exosphere. Corresponding to a possible exospheric temperature of 700°K the thermal escape flux, F = nH/r, is estimated as 6.7 x 10ro H cm-2 set-l byassuming that the particle density, n = 108 H cm 3, the scale height, H = 6.7 x 107 cm and the escape time, T = 105 sec. Although this is considerably less than the 10r3 photons cm-s set-l in the critical wavelength region, 1600 A < iz < 2000 A, some 1oaRH cm-2 may have escaped over a period of 45 x lo9 yr. The oxygen atoms separated out by diffusion toward the surface which provided an efficient sink in such reactions as the conversion of fayalite to magnetite 3Fe$iO,
+ 0, -P 2F%O, + 3Si0,
and numerous other reactions involving ferrous, manganous and titanous ions or sulfides. Over a period of several billion years some 105 g 0 cm-2 could have been assimilated as fresh surfaces were continually being supplied by erosion, deformation and most sign&antly by lavas and ashes poured out from the upper mantle. According to Verhoogen (1946), since the Cambrian, about 2 x 104 g cm-2 of lava have been added to the Earth’s crust, and by tripling this value to correct for submarine ejecta, 5-l x 106g cm-2 may have emerged in 4.5 x 109 yr at an output rate of lOA g cm-2 yr-l. Since volcanic activity was undoubtedly more intense in the early stages of planetary evolution, a 50 times higher rate may have caused the deposition of some 73 km thick layer, or the equivalent of the crust. If similar extrusions had occurred on Venus, a 9 per cent concentration of magnetite in the volcanic accession could then have accounted for the uptake of some 1W 0 cm-2, comparing well with the values given in Table 1. 6. ASSESSMENT
OF THE MODEL.
The feasibility of the proposed model rests largely on the relative rates of individual processes. The early rate of degassing, R,, is most uncertain. It may have been about 10ro CO, cm-2 se+ attended by some 2 x 10rl Ha0 cm-2 se& and should have exceeded the current slow rate, R,, by one to three orders of magnitude. According to laboratory studies (Hyatt et al., 1958) CO, could have been taken up by the surface rocks at a rate, &, of 10r3 CO, cmh2 se& and released again at a rate, R,, of 1014CO, cm-2 se&. Due to the inhomogeneity of the surface materials and the limited abundance of carbonate forming minerals, the efficiency of the actual process was 10-s or lower. Nevertheless, the uptake of CO, may have exceeded its release from the planetary interior, producing a primitive atmosphere that was optically thin with respect to CO,. By a similar line of reasoning it is concluded that the early atmosphere was optically thin also with respect to 0,. Both 0 and 0, are assumed to have diffused toward the surface where they interacted chemically with the lithosphere R,. In the absence of an appreciable radiation shield, the water vapor photodissociation, R , most likely proceeded at a faster rate than the escape rate of
1028
ANN PALM
hydrogen, R,. The entire set of reaction rates whose relative magnitudes determine the feasibility of the model can then be formulated as follows
IOR,,< R, < NFR,; R, < Rd; 4 > R, -cR, and R, > R,. A corollary of these developments is a high oxide content of the crustal rocks and a lack of carbonates. The possible equivalence of atmospheric CO, on Venus and the sedimentary carbonates on Earth has already been noted by Sagan (1962). The abundance of hydrated minerals may be considerable since not only the oxides but also micas and amphibole type minerals are stable at the prevailing conditions today (Mueller, 1964). Moreover, igneous activity may have been at least as extensive as on Earth. In conclusion it should be noted that, provided the three underlying hypotheses are acceptable, the existence of a dense atmosphere containing principally CO, and little H,O, in contrast to the relatively thin atmosphere on Earth, can be attributed to the distinct chemical interactions of volcanic output and lithosphere. Acknowledgements-Thanks are due to Dr. M. J. Holdaway for discussions of relevant geochemical processes and Dr. Adamcik for a set of calculated pressure data. This work was supported by the National Aeronautics and Space Administration under Grant No. NGL 44-004-001.
ADAMCIK,J. A. and DRAPER, A. L. (1963). Planet. Space Sci. l&1303. BATES,D. R. and NICOLET,M. (1950). J. geophys. Res. 55, 301. BATF&D. R. (1954). The Earrh OSu Planet (Ed. G. P. KUIPER), pp. 567-583 and 591-592. The University of Chicago Press, Illinois. BERKNER,L. V. and -L, L. C. (1966). J. atmos. Sci. 23, 133. CAMERON,A. G. W. (1963). Zcarus 2,249. CouINs, W. H., Qumrcx, T. T. and TxiOMSoN, E. (1926). Michipiwten Iron Ranges. pp. 31-32. Can. Geol. Survey Memoir. 147, Ottawa. DANIELSON,A. (1950). Geochim. Cosmochim. Acta 1, 55. DAYHOPP, M. O., ECK, R. V., Lrrpr~corr, E. R. and SATAN, C. (1967). Science 155, 556. CbLD, T. (1964). The Origin and Evolution of Atmospheres and Oceans (Eds. P. J. Brancaxio and A. G. W. Cameron), pp. 249-255. Wiley, New York. HOUND, H. D. (1964). The Origin and Evolution of Atmospheres and Oceans (F&J. P. J. Brancazio and A. G. W. Cameron), pp. 97-99. Wiley, New York. HYATI,E. P., CUTLER, I. B. and WADSWORTH,M. E. (1958). Am. Ceramic Sot. J. 40,70. KEWHEA, T. J. (1967). AFCZU-67-0221, Paper No. 263, pp. 5-12. A.F.C.R.L. Bedford, Massachusetts. KLIORE, A. and CAIN, D. L. (1967). J. atmos. Sci. 25,549. Mariner 5 (1967). Rep. in Science 158,1665-1690. MUELLER,R. F. (1964). Icarus 3,285. RICHIXR,A. and VALLET,P. (1961). Compt. rend. 252,178O. RINGWOOD, A. E. (1966). Geochim. Cosmochim. Acta 30,41. RIJBEY, W. W. (1951). Geol. Sot. Am. Bull. 62, 1137. RUBBY,W. W. (1955). Geol. Sot. Am. Spec. Paper 62,631. SAOAN, C. (1962). Icarus 1,151. SAGAN,C. (1967). Origins of the Atmospheres of the Earth and Planets, in International Dictionary of Geophysics (Eds. S. K. Runcom et al.), pp. 97-104. Pergamon Press, Oxford. Scnhnwr, 0. Y. (1944). Dokl. Akad. Nauk, S.S.S.R. 45,229. UREY,H. C. (1952). The Planets. p. 149. Yale University Press, New Haven. UREY, H. C. (1959). The atmospheres of the planets. Handbook of Physics. Vol. 52, pp. 383-393. Springer, Berlin. Venera 4 (1968). Rep. in Science 159,1228-1230. VE~I~EN, J. (1946). Am. J. Sci. 244,745. Wetnrs, W. F. (1956). J. Geol. 64,245. WIUIAMS, I. P. and C~EMIN, A. W. (1968). Q. J. R. u.rtr. Sot. $40.
17, pp. 1021 to 1028.
Permmon
THE EVOLUTION
Press.
Printed
in Northern
Ireland
OF VENUS’ ATMOSPHERE*
ANNPALM Southwest Center for Advanced Studies, Dallas, Texas 75230, U.S.A. (Received in final form 29 November 1968)
Abstract-Although the atmospheres of the terrestrial planets may have originated by similar mechanisms and duringasimilarepoch, their subsequent developments followeddivergentpaths, partly as the result of different Sun-planet distances. On such a premise a self-consistent model has been devised to explain the present dense CO, atmosphere of Venus. It is considered to have originated from the degassing of the planet’s interior during its early molten phase and to have proceeded in three distinct stages: (1) Gases were released which resembled in composition those of volcanic output. (2) These gases interacted chemically with the lithosphere, relargely in the formation of oxides and carbonates. (3) Eventually the degassing sult’ subs~ 7 ed and the water vapor became depleted by either photodissociation or in-situ mineral oxidation and hydration. The resulting low partial pressures of water vapor and CO, promoted decarbonation which gave rise to the presently observed dense carbon dioxide atmosphere. 1.INTRODUCTION
Since Earth and Venus are closely related planets as revealed by their similar planetary parameters, the very difference of their atmospheres has long posed a challenging problem. The significance of lithospheric chemistry to the evolution of planetary atmospheres has been broached by Urey (1952,1959) in his pioneering work on the origin of the planets. In recent studies Cameron (1963) dismissed the possibly analogous development on Barth and Venus, and Ringwood (1966) suggested that the planets may have accreted from dust clouds that contained different proportions of carbon and hydrogen. By means of free energy calculations Dayhoff et al. (1967) have shown that hydrogen should have been more abundant originally than in the contemporary atmosphere and that the C/O ratio was probably equal to or less than 4. They also concluded that hydrogen and oxygen were lost by escape into space and by chemical interaction with the surface, respectively. To account for the many orders of magnitude difference between Earth and Venusin their water abundance, Sagan (1967) suggested that it was lost by efficient photodissociation followed by escape of hydrogen into space and the removal of oxygen by chemical reactions with reduced compounds in the lower atmosphere or the planetary surface. However, Gold (1964) has expressed doubt concerning the loss of some 300 atmospheres of water from the planet Venus. The inherent difficulty in explaining the origin of Venus’ atmosphere by these models is examined in this study. 2. DESCRIPTION OF MODEL Results of laboratory experiments and geological field studies are invoked to interpret the dissimilarity of the Earth’s and Venus’ atmospheres. It is assumed that the atmosphere of Venus is of secondary origin as is that of the Earth (Rubey, 1955), and three major phases are distinguished in its evolution: 1. During the period of early intense degassing and in the presence of water vapor, carbon dioxide was fixed by chemical interaction with the lithosphere. * Paper presented at the 49th Annual Geophysical Union Meeting April 8-11,1%8, Washington, D.C. 1021
1022
ANN
PALM
2. With the passage of time the major part of the water vapor was lost by photodissociation as hydrogen escaped into space and oxygen diffused downward and oxidized the surface materials. 3. Eventually the initially high degassing rate diminished and low partial pressures of water vapor and carbon dioxide developed. These caused the decomposition of carbonate bearing rocks and allowed carbon dioxide to accumulate to the present dense atmosphere. The underlying hypotheses, based largely on data available in the literature, are : 1. Earth and Venus evolved during the same epoch from similar materials by similar processes. 2. Distinct thermal histories of the planets led to characteristic atmospheres determined by the interaction of effluents with the respective lithospheres. 3. Early in planetary history the degassing rate exceeded the present one by several orders of magnitude. The first hypothesis implies that both Earth and Venus may have been formed from some common cloud of gas and dust as proposed by Schmidt (1944) ; see also Williams and Cremin (1968). The second hypothesis refers to the surface temperature of Venus which has always exceeded that of the Earth; the black-body differential due to closer planet-Sun distance amounts to 50°C. The third hypothesis derives from the assumption that heat dissipation was more intense during the early stages of planetary evolution and that therefore the volatiles initially escaped from the interior at a higher rate. In order to put these events in proper perspective, a time scale has been constructed as shown in Fig. 1. Curve A illustrates the rate of degassing which is assumed to follow a first order kinetics or exponential decay law. The onset and termination of intense degassing are designated tr and t,, respectively, and the concurrent atmospheric changes are shown by curves B. Whereas the correspondence of tr and tZin A and B is significant, the absolute timing remains uncertain. It is noteworthy that during the period of intense degassing no appreciable CO, atmosphere had accumulated because the CO, fraction of the volcanic output reacted chemically with the hot lithosphere by the formation of carbonates which is described in a subsequent paragraph. In contrast, the water vapor that emerged at the surface had built up to a transient atmosphere, but waslost eventually by a series of processes elaborated below. 3. DEGASSING
HISTORY
Because of their parallel developments, Earth and Venus may be composed of the same rock types (bulk composition) that once contained similar quantities of volatiles. During their early histories of melting and resolidification the emitted gases may also have had the same relative composition. Therefore, predominantly water and carbon dioxide emerged at Venus’ surface together with minor quantities of nitrogen, halogen, halides, sulfides and other trace elements. To explain the absence of a hydrosphere Holland (1964) contended that Venus may have lost its water complement during the initial phase of accretion. According to the present model, water was retained initially, but released by subsequent planetary outgassing, and the amount may have been comparable to the fraction in the excess volatiles (Rubey, 1955). In Table 1 these quantities are compared with H,O and CO, in Venus’ contemporary atmosphere as determined recently from Mariner 5 (1967) and Venera 4 (1968) records. In addition, HaO/COa ratios deduced from various sources (Rubey, 1951) are presented since these ratios may shed further light on the abundance of water in Venus’ primitive atmosphere.
THE EVOLUTION
ii 2
1023
OF VENUS’ ATMOSPHERE
1--_*
to3
z
5
E6 IO2 3 IO4
s
IO'
A
‘I
’
2
TIME,
4
‘2
B. YR
A
20 -
/e
/’ co2 2 rr:
i I
IO-
/ H20
i
0’ ‘I
2
’ TIME,
B
*J’
_-_
‘2
4
l
B. YR
FIQ. 1. TIMESCALEOF ATMOSPH2RIC EVOLUTION ON VENUS. A, rate of degassing of interior; B, rates of decarbonation and of change of temporary vapor atmosphere.
water
With the aid of the values presented in Table 1 early and present degassing rates of water can be distinguished by assuming that the water outgassed in the remote past has largely been lost, but that the water existing now in Venus’ atmosphere is the amount which has emerged since the intense degassing subsided. This fraction has been trapped by the CO, atmosphere which accumulated to its present density by lithological decomposition. It has been estimated that the early rate of degassing, based on the water equivalent in the exThe original cess volatiles, exceeded the current rate by several orders of magnitude. quantity of water that emerged during a period of several billion (log) years depended on the magnitude of the time constants as shown in Table 2. This range of time constants does not appear unreasonable in view of the possible energy sources that have TABLB1. EXCESSVOLATILE3 * Quantity
32 108 51 (1) 18 44
AND ATMOSPHERE OF VENUS
CO*@
H&O
lWg/cm’ 10%01./cm* lo%lol. HlO/CO, by wt. H,O/COI mol.
ON &IU’H
1.8 ::; (2) 2.5 6.1
(3) 4.1 10
Ha0 ?
CO*?
0.009 0.03 0.014 yi
2.1 2.9 1.4 ;6j
37
(5) 301 734
50
* According to Rubey (1955), amount of volatiles not due to chemical weathering. Ratios derived from (1) excess volatilea, (2) volcanic gases, (3) occluded in basalt, (4) occluded in granite, (5) hot springs, (6) H,O complement of excess volatile@ vs. CO, of present 0 9 atmosphere. Recently Kliore and Cain (1968) have estimated pressures of the order of 60 f 10 atm compared with 20 atm used in thii work.
1024
ANN TABLE
2.
hACXION
PALM
OF EARLY
Time constant 7xlWyr
1
0.43 1-o 1.5 2.46
0.90 0.63 0.50 0.33
QUANTITY
OF WATER
Elapsed time, 1O’yr 2 3 0.99 0.86 0.74 O-56
o-999 0.95 0.86 o-70
DEGASSED
4 o-9999 0.98 0.93 0.80
contributed to the early internal heating, such as gravitational contraction, radioactive decay or tidal forces. A present rate of degassing that may not differ from the mean rate overgeologicaltimeispreferred by Gold (1964),inagreementwithRubey’scontention (1951). 4. LITHOSPHERIC
REACTIONS
In this report reactions are examined that may have affected the development of the early atmosphere. A number of chemical reactions that occur between the surface minerals and the contemporary atmosphere have been discussed recently by Mueller (1964). Some typical reactions by which the CO, fraction of the volcanic output may have been tied are described briefly. When lava, steam and gases emerged as a result of igneous activity, CO, was taken up by the surface materials in reactions such as CaO + COaZ CaCO,
(a)
MgSiO, + COs ti MgCO, + SiOs
(b)
Casio,, + NasSiO, + COse
(c)
CaCOa + Na,Si,O,.
The actual carbonation rates depended critically on the composition, temperature and permeability of the surface rocks; at 673”K, for example, type (c) reactions are more rapid than type (a). Characteristic rates of 10la CO, cm-s set-l have been obtained in laboratory experiments (Hyatt ei al., 1958). Assuming that the actual uptake rate was lower by several orders of magnitude and that the degassing rate from the planetary interior proceeded at a rate of about lolo COz cm-s set-l, an appreciable CO, atmosphere never developed during the period of rapid outgassing. It is noteworthy that the uptake of CO, is accelerated by the presence of liquid water or water vapor at elevated temperatures (Richer and Vallet, 1961), conditions that prevailed on Venus in the remote past. Over a period of several billion years some twenty atmospheres of CO,, equivalent to a low estimate of the present atmosphere (Kliore and Cain, 1968), were fixed, and several hundred meters of carbonate bearing rocks may have accumulated. Geochemical evidence for extensive carbonation at volcanic sites has been reported by Collins et al. (1926). They identified early Precambrian deposits of rocks composed of 2-60 per cent carbonates. The results of experiments (Hyatt et al., 1958) which illustrate these types of reactions are reproduced in Fig. 2. Although the uptake of COz is slower than its release, the relative rates do not differ by more than a factor of 10. Under the given experimental conditions of 1000°K and 0.1 at. pressure, CaCO, decomposed at a rate of lo4 g cm-2 min-l. At temperatures around 7OO”K, 2.3 x 104 g CO, cm-2, an amount comparable to Venus’ present atmosphere, may have beenliberated within 104-106yr, depending upon the efficiency of the actual reactions. The thermodynamics (Weeks, 1956) and kinetics of similar reactions have also been studied. The data indicate that these reactions could have occurred on Venus and that, therefore, the contemporary massive COs atmosphere can be explained
THE EVOLUTION OF VENUS’ ATMOSPHERE
1025
I
40 TIME,
HOURS
FIG. 2. CARBONATB RFKTIONS.
A, rate of CO, uptake; B, rate of decarbonation.
by such processes. In particular, Adamcik and Draper (1963) have calculated the free energy changes for some representative reactions CaCO, + SiO,*
Casio8 + CO,
(a)
MgCOa + SiO,s
MgSiO, + CO,
(b)
FeCO, + #iO,71:
+Fe,SiO, + COB
(c)
and have derived the corresponding equilibrium pressures as functions of temperature shown co* EQUILIRRILJTM PRWSJRJZ, Rtfll T”K 300 400 500 600 ::
@I
(4 9.3 6.8 1.3 4.2 4.8 2.8
x 10-a x lo-’ x 10-l x 10 x 10’
1.8 5.0 5.8 1.3 1.1 5.2
x 10-6 x lo-’ x 10’ x 10’ x 10’
as
(c) 6.3 4.2 2.0 2.6 1.6 5.6
x lO-* x x x x
1w l(r 10’ 10’
These values illustrate the differences in the abundance of carbonates on the planets; accordingly, the surface rocks of Venus do not contain any carbonates. For an open system which exists under actual lithospheric conditions, additional factors determine the stability of the reactants (Danielsson, 1950). Carbon dioxide liberated upon decarbonation can diffuse away from its local source and thus prevents the attainment of its equilibrium partial pressure. At the same time silicates accumulate until all the reactants, carbonate and quartz, are spent. The kinetics of these reactions is also favored under these conditions. 5. LOSS MECHANISM In view of the foregoing considerations CO2 released from the planetary interior should have been removed by chemical interaction with the lithosphere. Therefore, CO, whose optical properties overlap those of water (Berkner and Marshall, 1966) was prevented from radiatively shielding the continued photodissociation of water vapor, and the efficiency with which this temporary potential radiation screen was trapped determined largely the practicability of the proposed model. This aspect has not previously been discussed in the literature. However, due to the rate differentials of input (emission from planetary interior) and output (escape into space) of water, a transient water vapor atmosphere developed as
1026
ANN PALM
shown in Fig. 1. The loss of large quantities of water that emerged at the surface by igneous activity can be explained principally by photodissociation and to a lesser extent by direct interaction with the lithosphere. The photodissociative loss depended largely on the effectiveness of the foliowing processes: (1) photochemical reactions of the transient water vapor atmosphere; (2) exospheric loss of hydrogen; (3) downward diffusion of oxygen; and (4) oxidation of the surface materials. They are described only briefly in this report to illustrate the loss mechanisms and to assure the self-consistency of the proposed model. The relevant photochemical reactions (Bates, 1954) and rate coefficients (Keneshea, 1967) are H,O+hv+H+OH
JH = 10-5 r&c-r
OH+hv+H+O
JH’
OH+H+Hs+O
koH = 1.5 x 10-11 d(T)
exp ( -4/RZ’)cmS
OH+O-+H+O,
koo = l-5 x IO-l1 d(T)
exp (-3~~~)crns
OH + OH+H,O
+ 0
(1) (la)
kH2 = 1.5 x lo-l1 1/(T) exp (--4/W)
se@ se&
cm3 see-r
(2) (3) (4)
O+O+M*O,+M
k. = 3 x lO++ cm6 set-l
t5a)
O+O-+O,+hv
kor = 10-a cm3 set-r
(5b)
OH+H+M-+HsO+M
kH1 = 103’ cm6 set-l
(6)
03+hu-+O+0
Jo = lo-s set-1
(7)
O+Oa+M+Os+M
k oe = 6.5 x lo-34 ems set-l.
(8)
Many other minor reactions (Bates and Nicolet, 1950) may contribute to the chemosphere but are omitted because of their low probab~ty of occurrence. Reactions (5a) and (6) increase asps with (6) being slower than @a); they take place at lower altitudes, atp w 1W mb, while photodissociation occurs at optically thin levels of p m lo-4 mb, where the absorption cross-sections for water exceeds the collision cross-section. The loss rate, approximated by use of reactions (l), (S), (6) and (7),
UH30) ‘v
J2,[kotO)“W) -
2JotO31- kdII)(oH)t~)
indicates that the limiting step is the r~ombination of oxygen. This rate is orders of magnitude smaller than that of photodissociation of water; moreover, oxygen has to accumulate to some 0.5 cm before it acts as a radiation shield slowing down further water dissociation. Another important factor which accounts for the effective depletion of water vapor is the mass transport of oxygen away from the water dissociation level and its uptake by the lithosphere. Although the probability of formation at the photodissociation layer kept the ozone concentration low according to reaction (8), ozone that formed near the surface enhanced the removal of oxygen because of its ready oxidation of the surface rocks. The life cycle of the main constituents assumed the following pattern sources Sinks
Ha0 planetary interior photodissociation
H photodissociation planetary escape
0 photodissociation assimilation at the lithosphere.
THE
EVOLUTION
OF VENUS’
ATMOSPHERE
1027
Hence, the combined effects of low recombination probability of 0 and the downward diffusion and convection of 0, favor the photodissociatively initiated depletion of water vapor. Once free hydrogen formed, it was lost by thermal escape and possibly by the ionizing action of the solar wind, but the importance of the solar wind diminished at higher exospheric temperatures due to shorter escape times and increasing scale heights. Besides photodissociation, ionization contributed to a complex chemical kinetics and to a rise in the temperature of the exosphere. Corresponding to a possible exospheric temperature of 700°K the thermal escape flux, F = nH/r, is estimated as 6.7 x 10ro H cm-2 set-l byassuming that the particle density, n = 108 H cm 3, the scale height, H = 6.7 x 107 cm and the escape time, T = 105 sec. Although this is considerably less than the 10r3 photons cm-s set-l in the critical wavelength region, 1600 A < iz < 2000 A, some 1oaRH cm-2 may have escaped over a period of 45 x lo9 yr. The oxygen atoms separated out by diffusion toward the surface which provided an efficient sink in such reactions as the conversion of fayalite to magnetite 3Fe$iO,
+ 0, -P 2F%O, + 3Si0,
and numerous other reactions involving ferrous, manganous and titanous ions or sulfides. Over a period of several billion years some 105 g 0 cm-2 could have been assimilated as fresh surfaces were continually being supplied by erosion, deformation and most sign&antly by lavas and ashes poured out from the upper mantle. According to Verhoogen (1946), since the Cambrian, about 2 x 104 g cm-2 of lava have been added to the Earth’s crust, and by tripling this value to correct for submarine ejecta, 5-l x 106g cm-2 may have emerged in 4.5 x 109 yr at an output rate of lOA g cm-2 yr-l. Since volcanic activity was undoubtedly more intense in the early stages of planetary evolution, a 50 times higher rate may have caused the deposition of some 73 km thick layer, or the equivalent of the crust. If similar extrusions had occurred on Venus, a 9 per cent concentration of magnetite in the volcanic accession could then have accounted for the uptake of some 1W 0 cm-2, comparing well with the values given in Table 1. 6. ASSESSMENT
OF THE MODEL.
The feasibility of the proposed model rests largely on the relative rates of individual processes. The early rate of degassing, R,, is most uncertain. It may have been about 10ro CO, cm-2 se+ attended by some 2 x 10rl Ha0 cm-2 se& and should have exceeded the current slow rate, R,, by one to three orders of magnitude. According to laboratory studies (Hyatt et al., 1958) CO, could have been taken up by the surface rocks at a rate, &, of 10r3 CO, cmh2 se& and released again at a rate, R,, of 1014CO, cm-2 se&. Due to the inhomogeneity of the surface materials and the limited abundance of carbonate forming minerals, the efficiency of the actual process was 10-s or lower. Nevertheless, the uptake of CO, may have exceeded its release from the planetary interior, producing a primitive atmosphere that was optically thin with respect to CO,. By a similar line of reasoning it is concluded that the early atmosphere was optically thin also with respect to 0,. Both 0 and 0, are assumed to have diffused toward the surface where they interacted chemically with the lithosphere R,. In the absence of an appreciable radiation shield, the water vapor photodissociation, R , most likely proceeded at a faster rate than the escape rate of
1028
ANN PALM
hydrogen, R,. The entire set of reaction rates whose relative magnitudes determine the feasibility of the model can then be formulated as follows
IOR,,< R, < NFR,; R, < Rd; 4 > R, -cR, and R, > R,. A corollary of these developments is a high oxide content of the crustal rocks and a lack of carbonates. The possible equivalence of atmospheric CO, on Venus and the sedimentary carbonates on Earth has already been noted by Sagan (1962). The abundance of hydrated minerals may be considerable since not only the oxides but also micas and amphibole type minerals are stable at the prevailing conditions today (Mueller, 1964). Moreover, igneous activity may have been at least as extensive as on Earth. In conclusion it should be noted that, provided the three underlying hypotheses are acceptable, the existence of a dense atmosphere containing principally CO, and little H,O, in contrast to the relatively thin atmosphere on Earth, can be attributed to the distinct chemical interactions of volcanic output and lithosphere. Acknowledgements-Thanks are due to Dr. M. J. Holdaway for discussions of relevant geochemical processes and Dr. Adamcik for a set of calculated pressure data. This work was supported by the National Aeronautics and Space Administration under Grant No. NGL 44-004-001.
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