History of Martian volatiles: Implications for organic synthesis

ICARUS 15, 279 303 (1971)

History of Martian Volatiles: Implications for Organic Synthesis ~ F R A S E R P. FANALE Lunar and Planetary Sciences Section, Jet Propulsion Laboratory California Institute of Technology, Pasadena, California 91103 Received December 18, 1970; revised March 25, 197l A theoretical reconstruction of the history of Martian volatiles indicates that Mars probably possessed a substantial reducing atmosphere at the outset of its history and that its present tenuous and more oxidized atmosphere is the result of extensive chemical evolution. As a consequence, it is probable that Martian atmospheric chemical conditions, now hostile with respect to abiotic organic synthesis in the gas phase, were initially favorable. Evidence indicating the chronology and degradational history of Martian surface features, surface mineralogy, bulk volatile content, internal mass distribution, andt~lerm~l history suggests that Mars catastrophically developed a substantial reducing atmosphere as the result of rapid accretion. This atmosphere probably persisted--despite the direct and indirect effects of hydrogen escape--for a geologically short time interval during, and immediately following, Martian accretion. That was the only portion of Martian history when the atmospheric environment could have been chemically suited for organic synthesis in the gas phase. Subsequent gradual degassing of the Martian interior throughout Martian history could not sustain a reducing atmosphere due to the low intensity of planet-wide orogenic activity and the short atmospheric mean residence time of hydrogen on Mars. During the post-accretion history of Mars, the combined effects of planetary hydrogen escape, solar-wind sweeping, and reineorporation of volatiles into the Martian surface produced and maintained the present atmosphere. I. [NTRODUCTION

This study reconstructs theoretically the chemical evolution of Martian surface volatiles and considers whether the Martian surface environment was ever chemically suited for the abiotic origination of life. The assumption is made t h a t life must be indigenous to a planet. In this context, the chemical evolution of Martian volatiles emerges as the link between Martian planetology and possible Martian biology. Thus, an understanding of the history of Martian volatiles is the key to evaluating the past suitability of the Martian environment for the origination of life. : This paper presents the results of one phase of research carried out at the J e t Propulsion Laboratory, California Institute of Technology, under Contract No. NASV-100, sponsored by the National Aeronautics and Space Administration. 10

Major questions concerning possible Martian biology include: 1. Were conditions at the Martian surface ever chemically suited for the spontaneous origination of life ? 2. Did life, in fact, originate and propagate on Mars as a result? 3. Is life likely to be detected on Mars by a landed instrument package? An a f r m a t i v e answer to any of these questions virtually demands an aifirmative answer to the preceding one(s). Hence, it follows t h a t the most basic theoretical question and the one most likely to be answered in the affirmative is the first. Although the mechanisms by which life originated on Earth are still the subject of intensive investigation and debate, it is widely acknowledged t h a t development of a substantial reducing atmosphere is 279

2~0

F R A S E g ~P. F A N A L E

a [)i'erequisite for spontaneous atmosi)heric organic synthesis. It is clear that the a-rob Martian atmosphere, consisting essentially of (?O._, plus possibly a small chemically inert comltonent , hardly constitutes a fa,vorable ehenfical setting for organic synthesis. Furthermore, the general al)l)earance of the Martian surface seems t<> indicate that it is relatively old and pristine compared to that of the Earth. ()n Earth, even in the absence of life, one might suspect that extensive volcanisn| an
hiologieal implications are concerned. This is particularly true of the gross cratere
HISTORY OF 1VIARTIAN VOLATILES

and possibly necessary motivation is the search for Martian life. Decisions concerning the form t h a t this search should take must be based, in part, upon what Martian exploration has thus far indicated concerning the possibility of finding evidence of life. Known present Martian surface and atmospheric properties intuitively suggest great pessimism. The evidence and arguments presented in this study tend to mitigate t h a t pessimism in t h a t they give reason to believe that, despite present conditions on Mars, the Martian surface environment initially constituted a setting chemically conducive to abiotie organic synthesis in its atmosphere. ~I. MARTIAN SURFACE P R O P E R T I E S

A. Chronology of Surface Features Indirect evidence concerning the history of Martian volatiles is provided by studies of Martian surface morphology and even by certain Apollo lunar results. The cratered appearance of the Martian surface may be interpreted as indicating a quiet orogenic history for Mars because the oldest portion of the surface, including the largest craters, appears to be primordial. The age of the most primitive portion of the surface is probably about 4 × l09 years, based on morphological studies (Leighton et al., 1969). The morphology of Martian craters larger than a few kilometers is different from t h a t of smaller Martian craters-suggesting differences in origin and history. According to L. Soderblom (personal communication), the former tend to be flat-bottomed, lack central peaks, and appears degraded, whereas the latter are round-bottomed and exhibit more definite raised rims. One explanation of the differences between these two populations is t h a t the large craters are relic from accretion and experienced a discrete episode of erosional degradation t h a t preceded the formation of the smaller, younger craters. No radiometric ages on samples of known Martian origin are available. However, Apollo l l and 12 lunar geochronological results, together with theoretical consider-

281

ations, suggest t h a t lunar surface chronology may have important implications for Mars. Apollo 11 rock samples from Mare Tranquilitatis yielded conventional Rb-Sr, U - T h - P b , and K - A r ages of about 3.7 x l09 years. Exotic rock fragments found in both the Apollo l l and 12 suites yielded conventional radiometric ages of 4.4 x 109 and 4.5 × l09 years, respectively (Albee et al., 1970 ; Tatsumoto and Rosholt. 1970; Turner, 1970). Some Apollo l l dust and breccia samples yielded concordant Pb ages of about 4.6 × l09 years (Silver, 1970; Tatsumoto and Rosholt, 1970), while Rb-Sr analysis of Apollo 11 lunar soil gave a model age of 4.5 f 109 years (Albee et al., 1970). The U and Th contents of most Apollo 11 and 12 samples analyzed are about 50 times those exhibited bv normal chondrites or other meteoritic material. It would appear that large reservoirs of magma, at least 50 times larger than the reservoir effectively sampled by Apollo 11, were mobilized on the Moon over four billion years ago. Heat from decay of very long-lived nuclides, such as U 2as, U 23~, Th 2a2, and K 4°, cannot be invoked to supply the energy for this melting. Such heating is much more likely to have resulted directly from the energy released by lunar accretion, fission, or capture process. Assuming the Moon accreted as an independent body, and t h a t Mars and the Moon had similar accretion intervals, it follows that much more intensive heating of each increment of accreting material would have resulted from the lunar accretion process had it continued to the Martian mass (e.g., see Ringwood, 1966). Alternatively, if lunar heating from decay of relic short-lived products of nucleosynthesis was responsible, then Mars would probably have been similarly affected. Thus, surface morphological similarity between Mars and the Moon may imply a fairly quiescent overall orogenic history for Mars. However, this similarity is also at least compatible with the possible occurrence of an initial episode of extensive melting of Martian material and the consequent development of a surface volatile inventory capable of producing an

2N *)

F R A S E R P. F A N A L E

initial episode of erosional degradation. E v i d e n c e t h a t such early heating of Martian material did, in fact, actually (r<'cur will be presented. B. Surface Mineralogy Evidence pertaining to Martian surface mineralogy, along with other evidence, t)laces constraints on the initial elemental composition of Martian surface v(rlatiles, and implies t h a t this composition mnst have been greatly modifieuht satisflv these "'remote" analyses. High U, Th, an(l 14, contents tbr the Apolh~ 11 returned samples d e m o n s t r a t e extensive differentiation (r[' mare material. In the Martian case. ext~ensive basaltic mineralogy also seems likely, t h o u g h there is some evidence for alteration. B u t there is n<) evidence vet fr<)m slrectral d a t a t h a t in(ticates ;(he degree of (tifferentiation. Also. whether the material t h a t has been "()xi(tize
oxide could explain the observed spectrum. In s u m m a r y , there is evidence for the existence o f " b a s a l t i c " and possibly altered r<)ck at the Martian surface. This is an imt)ortant observation, since, as will be shown, it indicates t h a t the initial degree of oxidation <)f Martian juvenile gas is likely to have heen quite different from t h a t exhibited hy the present Martian atmosphere. III. BULK I)ROPERTIES OF MAle=s: IMPLI('ATIONS

FOR T H E H I S T ( ) R Y \r()I,ATII,ES

()F ITS

A. Pla,~etary I'olalile Contempt and C o m po~it io ~ l)espite the fact t h a t no material awdlaMe for analysis is known to have originated (m Mars, indirect evidence exists t h a t suggests similarity hetween p l a n e t a r y bulk volatile contents an(l COml)osition tbr Martian and Earth m aterial. A procedure often used to infer (lifterences in planetary outgassing and atmospheric histories is comparison of planetary atmospherie compositions. But objections to this procedure are always raised: How (Io we known t h a t differences in planetary atmospheric compositions result fron| differences in outgassing and atmospheric history as opposed to innate bulk differences in the volatile contents of the l)articles t h a t accreted to f'orm the respective planets ( There is some basis tor such objections, since there is evidence for modification of solar system material in the dispersed circumsolar ch) ud prior to i)lanetary accretion. In addition to evi(tence ti'om chemical and isotopic studies of meteoritic material (Wood, 1963, Anders, 196S), there are astronomical observations t h a t SUl)port this view. The density of Mercury is ahout 5.2 g c m :~ (Ash et al., 1967) much higher than the uncoml)ressed density of the earth (~4.o ~o cm :~). Thus, a significantly hi~her bulk metal-to-silicate ratio rs indieate
283

ttISTORXr OF MARTIA:N VOLATILES

suggesting t h a t the possess similar metal/ silicate ratios. The early sun has been suggested as a likely energy source for intensive early circumsolar cloud "metamorphism" (Hayashi, 1961 ; Anders, 1968), which would explain the apparent decrease in metal-to-silicate ratio with increasing heliocentric distance in the inner portion of the preplanetary cloud. The effect of intense heating on volatile contents of particles could have been profound. Hence, one would not expect t h a t the volatile inventory of the particles that formed Mercury resembled that of the particles t h a t formed the Earth. The possibility of differences in volatile content among the remaining terrestrial planets must therefore be considered. However, evidence exists from meteorite studies t h a t suggests such hypothetical cloud metamorphism did not produce large differences in volatile content between the preplanetary dust in the vicinity of Earth and that in the vicinity of Mars. Surveyor chemical analyses of both maria (Turkevich et al., 1969) and highland (Turkevich el al., 1968) lunar sites strongly indicate t h a t chondrites are not of lunar surface origin, regardless of whether or not the Moon is chondritic in bulk. Orbit dynamic studies support this point of view (Wetherill, 1967) and indicate that chondrites originate at least at an asteroidal distance in the circumsolar ch)ud. Also, normal chondrites are the most primitive Earthlike material available. Hence, any observed similarities between normal chondrites and Earth rocks arc also likely to be shared by bulk Martian material. It is, therefore, significant that average analyses of gas released from mafic rocks and normal chondroites, the starting materials of which are known to have formed in widely separated portions of the solar system, exhibit great compositional similarity. This similarity is shown in Table I where the composition of gas released by in vacuo fusion of terrestrial rocks and t h a t from normal chondrites is compared. Note t h a t the elemental composition ( C : O : H : N ) and degree of oxidation of the dry gas released fl'om mafic terrestrial rocks and from meteorites are similar. Other factors

influencing rock gas composition, especially the degree of oxidation, will be discussed. Another demonstration of similarity between Earth and chondrite volatiles is possible from comparison of chondritic and terrestrial non radiogenic rare gas inventories, as shown in Fig. 1. Since occluded rare gas is easily lost from solid material on heating, a thermal metamorphic event affecting other volatile concentrations in preplanetary material would have affected rare gas concentrations as well. Figure 1 is based upon the working hypothesis that the Earth is thoroughly outgassed with respect to nonradiogenic Ne, Ar, and Kr. The initial bulk nonradiogenic rare gas inventories of terrestrial material cannot be less than the atmospheric inventories divided by the mass of the Earth since (1) the N e : A r : K r ratios contraindicate a major solar wind contribution to terrestrial atmosphere Ar and Kr, and (2) the Earth cannot be more than 100% outgassed. Normal chondrites are completely degassed in the laboratory when the nonradiogenic rare gas contents are determined. Hence, the data in Fig. 1 strongly suggest t h a t average terrestrial material possessed, at the time of its accretion, an initial rare gas inventory roughly comparable to that of normal chondritic material. The comparatively low Xe abundance in the Earth's atmosphere (Fig. 1) is possibly the result of inefficient Xe degassing fYom the Earth, or preferential Xe readsorption 10 -7

/

10-8

10 -9

i0 -I0 Ne

Ar

20

40

kr

Xe

10 - l l

60

8D

190

120

140

MASS

F~(;. 1. P r i m o r d i a l r a r e g a s c o n c e n t r a t i o n in n o r m a l chondrites c o m p a r e d with inferred values for E a r t h . C h o n d r i t i e v a l u e s a r e th(~ a v o r a g e c o n t e n t s for t e n n o r m a l c h o n d r i t e s r e p o r t e d t)y S i g n e r (1964). E a r t h v a l u e s a r e i n f e r r e d h ' o m atmospheric concentrations and assumptions d i s c u s s e d in t h e t e x t .

3

3 3

'2 4

41

5tP

(F)

Ullramafics (Av 3)

4

64 26

((:)

Normal ehondites (Av I0)

10:1

3

3

40 54

(S)

"Marie plutonics (Av 3)

FllSi()ll itt ~'(1C~¢0

1.7

I. 5

6

54 36

((')

""[)ialmse a n d I)asalts'" (Av 1o)

93 5

25o ('

5

42 48 5

""I~;eh)w r~'d heat ....

7

i; 87

Hod heat"

(Ias e v o l v e d f r o m L - g r o u p c h o n d r i t e h(mmst('a(I (C)

H e a t i n g itl eaem~

Estimated theoretically: assuming equilibrium

I~]~TIMATEN ~t

- .3

1

"2 9O 7

15:1

ti :~

55 36

M e t a l l i c F(, p r e s e n t in Metallic Fe equilibrium : absent : fv(,(,zmg fl*e(,zillg point 1200 (' l)Oinl :: 1050 (?

\VITH THFORETI('AI~

a The~ wetical e s t i m a t e s a s s u m e d e q u i l i b r i u m w i t h m e l t s o f g e n e r a l l y b a s a l t i c c o m p o s i t i o n . A n a l y s e s r e p o r t e d b y (i!hamberlin ( 1 9 0 9 ) (('1 : E m m o n d s (l !it|4) (E): a n d Sh(~pherd I1938) (S) f - r g a s released on i~, vac~o h e a t i n g a n d fusion o f m e t ( , . r i t e s a n d t o r r e s t r i a l rocks.

('O N., CH4 H.,O: d r y gas

H.,

(!O.2

V,)latih' constituent

1

( ~ O M P O S [ T I O N O F T H E ] ) R Y (~AF, P O R T I O N O F ]~O('K I~rOLAT|LEN : ( ' O M P A R ] S O N

TABLF

>

>

m

>

~J2

4-~

t~

HISTORY OF MARTIA:N VOLATILES

285

additional information on Martian outgassing history. These observations have been generally interpreted to indicate a cool history for Mars relative to the Earth, and a low degree of differentiation and level of orogenic activity for Mars. They include the observations by MacDonahl (1962) that the secular acceleration and figure of Mars imply that the planet is cool and strong. Estimates of the moment of inertia factor, C/Mr", have suggested that Mars is largely homogeneous, the increase of density with depth being not much more than would be expected from normal compression in a chemically homogeneous object. As shown in Fig. 2, the extrapolated zero-pressure density of the material best satisfying the approximately homogeneous model considered by Kovach and Anderson (1965) for a possible Mars radius of 3306 km (a value widely accepted at that time) was about 3.8--almost the "terrestrial" value. This, if interpreted freely in compositional terms, would indicate about 25% metal, or "core material" by mass (not necessarily as an inner core), analogous to the Earth, as shown in Fig. 3. Lyttleton (1965) contends this material need not be Fe Ni alloy in either planet, and that, instead, the Earth's core-mantle boundary itself may represent a pressure-induced

on sediments, but discussion of supporting evidence for this is beyond the scope of this paper. Despite what seem to be dramatic differences in magnitude and composition between the Earth's surface volatile inventory and t h a t of Mars, it seems unlikely t h a t these differences could be the result of inherent initial differences between the volatile content and composition of the solid material that accreted to form each planet. Otherwise, it is unlikely t h a t the major volatiles released by in vacuo heating of normal chondrites and Earth rocks would be as similar in composition as indicated in Table I, and t h a t similarities between the contents of nonradiogenic rare gas nuclides in normal chondrites and the Earth as a whole would be as great as indicated in Fig. 1. Thus, even though Martian solid material has not yet been analyzed, the hypothesis t h a t similarities exist between the bulk terrestrial and Martian volatile contents and compositions has an experimental basis, and is not based upon simplicity of hypothesis alone.

B. Internal Mass Distribution of Mars Earth-based observations of the bulk physical properties of Mars have yielded

10 kb

S #MARS (K&A) coE 5 - - - - -

Z

//.--AVERAGF H GROUP // CHONDRITE (p= 3.66) ~ \ r~ // [18% METALL C Fe] (U) ~ : 1 1 ~-"OLD" MARS // ~.,~---"NEW" MARS I// / - - AVERAGE L GROUP -~ ~ --1/// CHONDRITE (# = 3.51) /~""~.~ ~ ,n [7VoMETALLIC Fe](U)

Z

g

T ] T'-R, = 3306 krn (K&A)

Z-Rb4 " 3394krn(K) -0.5

- 1I.0

_11.5

I°gl0 ( M//M )

~-P~V~kb (K&A COMPRESSION, _2.01~ -R = :J394) FELDSPATHIC PYROXENITE (R) AT 10 kb

FIG. 2. R e l a t i o n b e t w e e n m e a n d e n s i t y , u n e o m p r e s s e d d e n s i t y , a n d m a s s e x p r e s s e d as t h e r a t i o of o b j e c t m a s s to E a r t h m a s s a c c o r d i n g to K o v a c h a n d A n d e r s o n (1965) (K & A). T h e c u r v e labeled " o l d M a r s " c o r r e s p o n d s to t h e h y p o t h e t i c a l u n c o m p r e s s i o n of a M a r s h a v i n g a r a d i u s e q u a l to t h e f o r m e r l y a c c e p t e d v a l u e of 3306 kin. A h y p o t h e t i c a l c u r v e is also d r a w n for t h e " n e w " r a d i u s d e t e r m i n e d b y K l i o r e et al. (1969) (K). T h e (essentially) u n c o m p r e s s e d d e n s i t i e s of f e l d s p a t h i c p y r o x e n i t e a c c o r d i n g to R i n g w o o d (1966) (R), a n d a v e r a g e s for H - a n d L - G r o u p c h o n d r i t e s , acc o r d i n g to U r e y a n d Craig (1953) (U) are c o m p a r e d w i t h h y p o t h e t i c a l u n c o m p r e s s e d M a r t i a n d e n s i t i e s a t t h e r i g h t of t h e g r a p h .

:~,Nli

F R A S E R P. F A N A L E

3~

/Z

/ / '-OL2--. . . . . . . . . . . . .

4

/

:

/ o=

2b

/ .OROUP(u,o/ / / /

< E :2

,0p

/

'Nfi'W '

o 3.4

X5 k GROUP {U)

M

3.6

i

3.8

4.0

UNCOMP2ESSED DENSIIY

Fro. 3. Uneompressed density vs. % metallic ir, m fin' eosmieally important materials. An empirical correlation between % metallic iron may be obtained from actual cosmically abundant materials such as (H) and (L) group ehondrites (Urey, 1960), and the Earth, based on seismie data and theoretical estimates of Earth uncompressed density from Hingw()od (1966) (R) (the higher values) and Kovaeh and Anderson. (1965) (K & A) (the lower values). The intersection of this empil'ieally determine([ lira' with the vertical bars representing lhe infi,rre(I (fl'om Fig. 2) "'ohl" and "nt~w'" uncompressed densities ,)f" Mars indicates tim corresp(mding inferred percentages of metalli(. phase in Mat's. The possit)le range (4' the value M~.I3:lm, indicating the rati() of Martian tort, mass t() total Martian mass (l>,indm', 1969) is als(~ s h ( ) w I I .

phase change. Nevertheless, the belief t h a t Mars possesses a significant metallic phase, b u t has not f o r m e d a core, has for y e a r s c o n s t i t u t e d the main a r g u m e n t for a cool M a r t i a n interior history. Mariner IV, VI, a n d V I I o b s e r v a t i o n s h a v e n e g a t e d t h a t a r g u m e n t by indicating a Martian radius of 3394 k m (Kliore el al., 1969). I f this radius were used in conjunction with K o v a e h a n d A n d e r s o n ' s model, the e x t r a p o l a t e d zero-pressure density for Mars would be a b o u t 3.5 (Fig. 2). Figures 2 a n d 3 show the relationship between the

' n e w " and "'old" theoretical uncoml)ressed densities of Mars (corresl)onding to r 3394 km and r 3306 kin, respectively) a n d the a v e r a g e densities of H and I, (Iroup ehondrites (Urey and Craig, 1953). Also plotted on Fig. 3 is the " p e r m i s s i b l e " range of the p e r c e n t a g e of t o t a l Martian mass r e p r e s e n t e d b y the present Martian (!ore mass inferred b y Binder (1969) based on the theoretical analysis of Mariner IX" data. An a p p r o x i m a t e empirical relationship between percent of metallic phase and density for cosmically a b u n d a n t nmteriM is indicated b y the diagonal dashed line in Fig. 3. N o t e t h a t , according to the correlation between p e r c e n t metallic p h a s e and density suggested b y the m e t e o r i t e points, Mars would contain a b o u t 2,5% metallic Fe if its radius were 3306 kin, b u t only 8% if its radius is 3394 kin. B u t according to Binder's analysis, the p e r c e n t a g e of the Martian mass r e p r e s e n t e d by the core lies hetween 2.7 and 4.9%. Thus, while Mars was once t h o u g h t to be metal-rich ( 2 5 - 3 0 % m e t a l phase) and essentially undifferentiated, it now a p p e a r s in contrast, t h a t Mars m a y be quite metalpoor( S% metallic phase) a n d at least p a r t l y d i f f e r e n t i a t e d - - w i t h at least half of all its available metallic phase eoncentrated in a core. Considering the low strength of the, Martian g r a v i t a t i o n M field, it a p p e a r s that instead of d e m a n d i n g a cool t h e r m a l history, the surmised internal mass distribution of Mars indicates a period of'internal melting. F u r t h e r m o r e , the absence of a s u b s t a n t i a l Martian m a g n e t i c field despite the rapid r o t a t i o n of Mars might suggest t h a t Mars lacks a s u b s t a n t i a l molten zone at present, p e r h a p s indicating c o m p a r a tively early occurrence of this melting. This reconciles the early c o n c e n t r a t i o n of U in the o u t e r m o s t portion of the Moon (and s u b s e q u e n t h m a r quieseenee) with a possibly parallel Martian t h e r m a l history which was formerly, and falsely, constrained so as not to allow extensive internal melting at a n y t i m e during the history of Mars. T o g e t h e r with independent evidence eoncerning the t h e r m a l history of M a r t i a n m a t e r i a l (see n e x t section), the internal mass distribution of Mars has i m p o r t a n t implications for the t e m p o r a l

HISTORY OF MARTIAN VOLATILES

history of supply of volatiles to the Martian surface.

IV.

THERMAL HISTORY OF MARTIAN MATERIAL

Most thermal models for the E a r t h have been based upon factors such as the amount of present surface heat flow, the absence of a substantial molten zone in the mantle, the concentrations of radioactive nuclides in meteorites and certain terrestrial rocks, the presence of a substantially molten E a rt h core, and so on. Recently, a new model for the Ear th's thermal history has been advanced by Hanks and Anderson (1969). In their model, an additional constraint is satisfied, namely, t h a t core formation must precede the emplacement of the oldest known rock possessing remanent magnetism. As a consequence, t h e y conclude t h a t the E a r t h accreted in an interval of 500,000 years or less. They also discuss the consequences of this reasoning for Martian thermal history. A simliar accretion interval (i.e., length of time for accretion) for Mars as for the E a r t h does not necessarily follow, but seems a reasonable assumption. Also, any given short accretion time would have much less severe thermal consequences for Mars than E a r t h because of the difference in mass. I f the Ear th contained even the highest allowable bulk content of U, Th, and K, it must have accreted in less than 500,000 years to have produced a molten core and remanent magnetism in a 2.7 billion-yearold rock. I t is reasonable to suppose t h a t a short accretion interval for E a r t h would suggest a short accretion interval for Mars as well. I f Mars accreted in 500,000 years of less and has chondritic abundances of U, Th, and K, it would have become molten throughout within 4.6 billion years (refer to the models of Hanks and Anderson). Yet, unlike the Earth, Mars is not presumed to be extensively molten at the present time because of the absence of a significant magnetic field, despite a rotation rate comparable to t h a t of the Earth. The explanation proposed by Hanks and Anderson for this somewhat para-

"-)87

doxical situation is t h a t U, Th, and K contents of both the E a r t h and Mars are much lower than previously suspected, and both the E a r t h and Mars accreted extremely rapidly. This would seem to be the only simultaneous solution for the terrestrial and Martian thermal models, excepting those models t hat (1) melt Mars throughout, (2) fail to provide the E a r t h with a magnetic field by 2.7 billion-years ago, or (3) demand t hat the E a r t h and Mars accrete in very different lengths of time. The most important feature of this analysis of Martian thermal history is that, even for Mars, with its low gravitational field, fairly high surface temperatures are achieved during the latter part of the accretion process. Martian surface temperatures up to 900°K are predicted to result from the accretion of the outer part of Mars for Martian accretion times similar to their "most acceptable" Earth accretion times of about 2 × 10 ~ years (Hanks and Anderson, 1969). Outer temperatures of 900°K correspond to the minimum thermal effect of Martian accretion. The Hanks-Anderson analysis is constrained to preclude extensive planetary melting of Mars at any time in the past; in fact, according to the conclusions given above, extensive internal melting of Mars at some time in the past. is not only permitted, but suggested by Mariner data on the mean radius and figure of Mars. The maximum thermal effect attained, if all gravitational energy were trapped in the interior and none reradiated to space (accretion in less than about l0 w years), would raise the outermost portion of Mars to temperatures in excess of 10,000°K. It is also significant t h a t analysis of accretion dynamics in the solar nebula suggests much shorter accretion times than given by Hanks and Anderson for the terrestrial planets (A. G. W. Cameron, personal communication), and temperatures in excess of 2 x 10S°K for the outer portion of Mars. The actual thermal effect of accretion on outermost Martian material was probably intermediate between these " m i n i m u m " and " m a x i m u m " effects.

28~

F R A S E R P. F A N A L E

In addition, the effect of accretion on the thermal history of individual particles forming Mars may be much greater than that on the entire planetary body. For example, in the ease of the "most likely'" Martian model of Hanks and An(lerson, where initial interior temperatures at the cessation of accretion do not exceed about 900~K, Martian material was l)robably heated to much higher temperatures during accretion, but most of the heat was reradiated to space before burial. Only that portion of the heat which was I)uried is of critical importance tot the sut)sequent thermal history <)f Mars, since ii is to the initial temperature profile (I)uried at the eessarion of accretion) that the subsequent increments of heat generate(l in the interior of that i)lanet by radioactive decay must be added. But for a('eretion outgassing of Martian material, the time-temperature history experienced by each radial increment of ad(]ed material is more critical regardless of what proportion of that heat is buried and what proportion reradiated to space. It shouhl be n<>ted, for example, that impacts at 5.o kin/see (the minimum impact velocity on Mars) and at higher velocity generally result in projectile ineandeseenee and the t)roduction of abundant glass in basaltic targets (I). Gault, personal communication). Furthermore, although each inerement of material probably ext)erienees only (me such beating episode as a projectile, it may experience several subsequent heating episodes as a target. Finally, once the pressure of the incipient Martian atmosi)here reaehed the millibar range, beating and ablation of particles on atmospheric entry would be the major mechanism of degassing (Ringwood, 1966). As a result,
V. (IHEMICAL EVOLUTION {)F MARTIAN V()LATII,ES

A. Initial Ele'menlal ('omlu>,s'iti(m The elemental eonlposition of a planet's atmosphere plays a critical role in deter mining whether a planet's surface environment is ehenfieally suitable for organic synthesis. Upon release to the atmosphere. the molecular composition of juvenile gas. ~wiginally in equilibrium with a magma, will change greatly as reequilibration occurs under surface conditions of muel~ lower temperature and pressure than in the magma. However, an atmosphere with a.n elemental composition initially hostile to organic synthesis (e.g., one that is prohibitively oxygen-rich or carbon-poor) is apt to remain so unless some interaction oeeurs between atmospheric gas and its surroundings, which changes the elemental composition of the atmosphere. Three examples of such interactions are (1) hy
HISTORY OF MARTIAN VOLATILES

Holland (1962), who point out t h a t the most critical melt parameters controlling the oxygen fugacity are the ratio Fe3+/Fe 2+ and the melt freezing temperature. The degree of dissociation of HeO and C02 is essentially a function of temperature only, not total pressure. This is so because, although their dissociation would produce an increase in number of moles in a closed gaseous system, the O.~ produced reacts quantitatively with Fe e+ in the magma so that the number of moles of gas in the system is essentially independent of the degree of H20 and CO.~dissociation. At the melting temperature of rocks, H~O and COe are significantly dissociated. But PO., in the gas phase cannot exceed the very low oxygen pressures demanded by the Fe2+/Fe a+ ratio of residual basaltic melts. Near the melt freezing point, this oxygen pressure may be as low at 10-1~ atm (see discussion below). As a consequence, He and CO produced by HeO and C02 dissociation reside in the gas phase, but the stoichiometrically equivalent amount of Oe produced by such dissociation cannot. The interaction between gas and magma can be represented approximately in terms of partial pressures as follows: 2He + Oe ~- 2HeO, K (Pro°)2 _ (pm)O(po~),

(1)

2C0 + Oe ~ 2C0._,, (Pco2) e .z

K., _ - -

(Pco)

,

(2)

(Po,)

2Fe203 m ~ 4FeO0) + ()2<,), K3 = (FeO)t(P°~) (Fe.~03)"

(3)

Thus, Po~ relates the degree of reduction of gas and t h a t of the melt: (Fe"0-3)-') K = P()~ (FeO)4 a Y

Melt (Pmo) 2 1 (PH2) e K 1

(Peon) ~ 1 (Pco) 2 K2

Gas Phase : = Atmosphere + ) oceans + sediments . + loss to space

(4)

289

Many factors are important in determining Po2 at the presumed conditions of gasmagma separation (which will here be equated to melt freezing), and these factors may vary widely from magma to magma, even at constant total melt composition. The history of the volatiles themselves, e.g., their possible assimilation from or loss to the environment of emplacement (country rock, sea water, or atmosphere), is crucial. This may determine the path of crystallization of the melt, hence the temperature at which the last residual liquid freezes as well as Fe2+/Fe ~+ and total iron content of t h a t liquid (Kennedy, 1955). Since the melt parameter (FeO)4/(Fce03) 2essentially determines the equilibrium oxygen fugacity at any given temperature during crystallization, it follows t h a t H2/H.,O and CO/COx can also vary widely depending on the circumstances of crystallization. Experimental results used here to estimate the initial elemental composition of juvenile volatiles are those of Roeder and Osborne (1966), obtained for the system MgO FeO-F%Os-CaAleSi.,O SiOe. The composition of this system approximates t h a t of natural basaltic rocks. One important conclusion reached by Roeder and Osborne is that, "The course of fractional crystallization of liquids in the system at constant total bulk composition (constant oxygen content, as in a closed system) is toward low oxygen partial pressures and ferrogabbroic liquids. The residual liquid in the latter instance approaches the composition, 45 SiO.,, 3S iron oxide, 6 CaO, and 11 A1._)O~,where it is in equilibrium with anorthite, fayalite, magnetite, and tridymitc at 1050°C and an oxygen partial pressure of about 10-11 a t m . " The equilibrium chemical composition of the gas phase can be calculated for the freezing point, when the volatiles cease to be controlled by the Po~ demanded by an infinite amount of magma, and instead become effectively infinite relative to the amount of liquid silicate. This transition point is depicted in Fig. 4, which relates magma chemistry to atmospheric chemistry. The composition of gas frozen out in equilibrium with basaltic magma under these conditions (T = 1050°C, Po~ = 10-al

~!)()

FRASER [', FANALE SIAC, L tl

S 1a,OE I HIGH T O X Y O [ h FLJL~pIi'd G

FREEZiNC POINT OF MELI

ELEMED. TAL COMPOSITION OF GAS UNCONSERVED: Oc C ~ N FLOW TO A N D FbiC,M [HL M.AGM*',

"'

! }~

!

FUNCTION OF [ /kiND Ic. Fe ~: O N L Y

'~ [ O SUPPLY H A N D C IN EXCESS OF H 2 0 , CO-2 MIXTLJRf

PO 2 A F U t & _ I I O N O f k R O Z M . OUT ELD'4Ei'- TAL CC)MPOSITION, P q t . D T Ok VOLATH~q MOLES CF SLLICArL LIC~ULD A N D SOLIDI : It.~OLES OF

,t.'OLES Q'F SILICA[E LIQUID AND SC'I. IDI (MOLTS OF

C A S~

"LLEMED4TAL COMPOSLTION OF VOLATtLEq IS COb
i

POp ESSLNTtALLY A

I:OLL:

COOLING ON :~LJI'I:AC E

J IIOLL:

I O tSA~E OCLANS Ai'db CIt4- ICH ATMOSPHEPL B[ FORk EXCLgS

it'¢DROGEN LOS [

I:i(:. 4. t / o l e of" intr(!ri()r e h ( , l [ i i s t r y

in atlllilS-

1)hi'rio a b i o i i e organio synt|iosis.

atilt) inay be calculated fron, relations (I), (2), and (3), above, and from the assunip,ion t h a t C.:N:H in the gas corresponds a p p r o x i m a t e l y t(, average vahies ibr gas released from marie rocks and normal ehondritic meteorites, as given in Table 1. The restllting eomt)osition of dry gas expe(;ted in equilibrium with ordinary basalt at its fi'eezing point is givel, in Table 1. If metMlie Fe was present in ileal'stirfaee Martian magmas, gaseous effusions were even more reducing t h a n those expected for o r d i n a r y basMtie inagnms, as pointed out, fbr the ease of the I)rinlitive Earth, bv Holland (1962). If" nietallie Fe is i)resent ill equilibrium, the low degree of oxidation results in a high d r y gas-to-water ratio (+3: l). The dr 5' gas nloleeular eoniposition e x p e c t e d in e(luilibriuni with basaltic ntagmas containing metallic Fe is also given in Table 1. The conlposition of gases irreversil)lv released by impact fusion and heating ()r at n/osl)herie ablation of individual t)lanetesimals during Martian aeeretion may also be estimated. The cornices,,ion of /ases irreversibly release(1 from norinal (diondrites as the result of i~ racuo heating has been measured. These are Mso shown in TM)le 1. Many of these d a t a art, old, and siinilar nmasurements should be pertbrmed using modern techniques. However, there is considerable sintilarity between the analyses of (!hamberlin (1.(tlLt)), Shepherd (193~), and l~]mmen(Is (1!)64). This, together with the fact thai these a/r(i( '

with theoretical estimates based on et)mputed equilibrium composition at the freezing point, loaves little d o u b t as to the validity of the analytical d a t a in Table I. The elemental eonlpositions of the dry <>as nloleeular assemblages given in Table I are ldotted in Fig. 5. Also indicated on Fig. 5 are eompositional regions (liseussed by Eel< el al. (1966) who perfornmd equilibriuin calculations for the system C 0 H. ()f parth.u]ar interest is the relationshit) between the p a t h followed by the hdtial Martian atmospheric mass balance and two "'thresholds" fbr organie synthesis al 300:1/, and I atln pressure described by Eek et al. These are the "o×idizing threshold." indicated by the H.,O (!()., tie line. and a second threshold represented by the CH~ ('()., tie-line. Below the oxidizing line (region 1), sonie free 0.> is present and all Ol'ganie (!ompollllds are oxidized. Above this line (in region 2), traees of m a n y o r g a n i e s appears in addition to (t()~, ('.(),

REMC VAt

n 7 .

R[MOV/~L--

OX IDI Z If'4 C> THRESHOLD 7-

/ / H C

I"1(:. 5. E h ' n l e n t a l e . m p o , q t i . n ,>r mass balance, qd' .juvenih' d r y gas. A dial)ases a n d basalts: B matic p l u t o n s (Nhephord, 1938) (Av 3): (' ultvmnafic rocks ( E n t m o n d s , 1964) ( A \ 3): I) eh(mdrites ( ( ' h a m b e r l i n . 19091 (Av lO); E eomp,Jsiti~m e a h ' u l a h , d t~ he itt ¢'quilibrium with an o r d i n a r y basaltic nuuglna at its fret'zing p
ll~" t ] l ( '

t)~kth t~tken t)y t h u ( ' : H : ( )
~lkll'tittll

~lt,lllosphtw(,

mass ~/s

lhu rust,It of' 'nit ial w a t e r r'enioval (by c o n ( l e n s a IMli) m i d - v e n t u a l h y d r o g e n loss (by ex(Jspll(,rie ~'suall~' ) .

ItISTOI~¥

OF MARTIAN

H20 and C H 4. In the region above the tie line, region 3, equilibrium formation of large proportions of polycyclic aromatics occurs, according to Eck. Based on arguments presented earlier, the total volatile assemblage as initially supplied to the Martian surface resembled that released by in vacuo heating of terrestrial marie rocks on normal chondrites and predicted to be in equilibrium with basaltic rocks at their freezing points. Hence, it initially consisted of a mixture of water vapor and a dry gas portion of composition similar to those plotted on Fig. 5. Water removal (see Fig. 5) therefore would have resulted in an initial atmospheric composition close to those of the plotted points (A to E) on the figure. Eventually, all of the subsequent "drift" on the Martian atmospheric C O-H mass balance can be attributed to the direct and indirect effects of exospheric escape of hydrogen. The chemical effects of hydrogen exospheric loss have been discussed by Urey (1952). As will be shown later, it is unlikely that the free hydrogen mean residence time in the Martian atmosphere exceeds about 2000 years. The rate of Martian atmosphere oxidation by hydrogen escape also depended strongly on the temperature and pressure at which atmospheric gas-gas equilibrium was determined, and is the subject of a later section. As shown below, one of the major effects of hydrogen escape is the eventual conversion of all atmospheric carbon to CO.,. Up to 30t~ precipitable water vapor has been observed in the Martian atmosphere (Schorn, 1970). Continuing dissociation of this atmospheric water, coupled with hydrogen escape and supply of new atmospheric water from the interior or from surface water or ice, served as an oxygen source. This process, which may be termed "hydrogen driving," may be schematically represented as follows: CH4-CO 2

VOLAT1LES

2ql

It is unlikely t h a t only enough free oxygen was produced by water dissociation to oxidize the excess atmospheric carbon. Surface oxidation, discussed earlier, is likely to have resulted from an additional 02 production from dissociation of' small amounts of atmospheric H20 and concurrent hydrogen escape. This is so because, as shown by Kennedy (1955), virtually no common igneous rocks can exist in chemical equilibrium with free oxygen pressures in excess of about 10 ~ atmospheres at temperatures below" about 1000°C. It may be concluded t h a t the elemental composition of the Martian atmosphere has changed drastically throughout Martian history. Initially, the Martian atmosphere possessed a reducing composition which was chemically suited to abiotic organic synthesis. The entire discrepancy between the hypothetical initial elemental composition of the Martian atmosphere and that presently observed may be attributable to the effects of exospherie hydrogen escape (a process known to occur on Mars), as may possible surface oxidation. B. Variables Influencing Martian Atmospheric Evolution

Short- and long-term changes in molecular composition experienced by released surface volatiles on primitive Mars following their separation from Martian magmas and infalling solid matter will now be evaluated. These changes played an important role in determining the chemical suitability of the early Martian atmosphere for synthesis of organic molecules. The results of Abelson (1966) indicate t h a t a methane-rich atmosphere per se is not an absolute prerequisite for organic synthesis in the gas phase. On the other band, a reducing atmosphere is. A methanerich atmosphere would strongly favor the probability of organic synthesis, while the chemistry of one in which almost all carbon was present in the form of CO., (as in the present Martian atmosphere) would be hostile. Also, it will be shown that, owing to the low Martian gravitational field, methane formation is the most likely

2,1t2

F R A S E R 17. F A N A L E

mechanism by which a reducing atmosphere on Mars could have been rendered metastable against the effects of hydrogen exospheric escape for a geologieallv significant period of time. Thus, the question is not only w h e t h e r Martian atmospheric chemical evolution resulted in the production of a reducing a t m o s p h e r e at any time during Martian history, but, if so, how h)ng such an a t m o s p h e r e might have persisted. l)uring release of volatiles from a m a g m a on E a r t h , three e n v i r o n m e n t a l wu'iables change drastically and produce corresponding changes in the initial juvenile molecular assemblage. T h e y are : (l) t e m p e r a t u r e , (2) total pressure, and t3) oxygen pressure. On primitive Mars, i n s t a n t a n e o u s changes were probably induced primarily by the first two, as on lhe early abiotic E a r t h . Changes also e v e n t u a l l y results from removal of some constituents initially introdu ted as j u venile gas. Henc, e, on Mars, atmospheri(, evolution may be considered as the result of continuing chemical reequilibration which has occurred in an "open intermediate reservoir," i.e., one i n t e r m e d i a t e between an(I open to, outer space, to which hydrogen has been lost, the p l a n e t a r y surface to which volatiles, especially water. has been lost, and the interior (and infalling solid matter) fl'om which volatiles have I)een supplied. To (,onstruet a model which incJudes the effects of such volatile depletion on Martian atmospheric evolution and to make semi(luantitative evaluation of the d y n a m i c i)hysieal and chemical equilibriun~ in this eo m plex system tractable, several si mpli(ving assumptions will 1)e made. Such assumptions are also necessitated t)y the fact t h a t the most primitive history is being ('onsi(lexe(I for a system, the in'e.s'enl structure, and kinetics of which are r a t h e r sl)eculative. It will be assumed t h a t g a s gas chemical equilibrium always obtaine(I in the primitive Martian atmosphere, an(t was established with a time e<)nstant shorter than t h a t for hydrogen loss to space. It will also be arhitrarilv assumed t h a t a h y p o t h e t i c a l M~rtian a t m o s p h e r e possessing a C H 4 / C ( ) ~ ratio greater t h a n

unity would have been chemically conducive to organic synthesis, although the converse is p r o b a b l y not true. As a first step, the evolution of the Martian atmosphere as an a t m o s p h e r e hypothetieall 5" closed to hydrogen escape will be considered.

(L Hypothetical Martian Atmospheric Evolution without Hydrogen Escape I f the pri mitive Martian atmosphere was effectively a closed system with respect to hydrogen exospheric escape, Mars would probably have developed all atmosphere ehemieally conducive to abiotie organic synthesis. An i m p o r t a n t equilibrium in the primit,ive atmospheric reservoir would have been : ('()~

4H,, ~-(~H4 + 2H.,(), K 4 = ( l ' (,u,,)(t >m,))

(4)

( t)H~)I(P.(,~)

While other atmospheric e(tuilibria wet'(. i m p o r t a n t , consideration of this reaction is convenient, since: 1. The e(luilibriunl constant contains an expression t h a t directly reflects the chenfieal state of atmospheric carbon. 2. An upper limit can I)e set on one variable (Pmo) based on the assumption of liquid-gas equilibrium. 3. As a result, the chemical state of atmospheric carbon can be directly expresse(1 as a highly sensitive function of the one variable (Pa:) t h a t can be directly relate(1 (next section) to the halance hetween volatile supply from the interior and exospherie escape----and t h e r e b y to l)lanetary physieal properties of Mars. 4. Methane synthesis, as will be shown, is the major mechanism whereby a reducing elemental composition could have been "l)roteeted '' against tile effects of hydrogen escape from Mars. Specifically, if the Martian a t m o s p h e r e were a closed system witil respect to hydrogen escape, cooling and reequilibralion would have resulted in conversion of most atnmspheI'ic carbon, originally supplied as (~().,, to methane. At 25(!.

HISTORY OF MAI~TIAN VOLATILES

K 4 T 1020, at 225°C, K 4 ~ 1 0 s, and at 374°C (the critical temperature of water), K4 ~ 10< Thus, in a closed system, cooling and reequilibration of a gaseous molecular assemblage similar to those observed to be released by in vacuo heating of normal chondritic or even basaltic material (Table I) would have resulted in the production of a methane-rich atmosphere owing to the change in the equilibrium constant, K,, with falling temperature. Also, water removal on cooling is important. The effect of water vapor removed on cooling may be represented as follows: 3(He)g +

(CO)g

present Martian atmosphere are about 227°C (500°K), and are observed above 150 K m (Kliore et al., 1969). At 227°C, K , is much lower (1 × 10s). Furthermore. theoretically Pu~o could, at 227°C, be as great as 30 atmospheres, but water would probably be cold-trapped by condensation somewhere at temperatures at or below 25°C, since the present equilibrium blackbody temperature at the Martian heliocentric distance is 210°K (Glasstone, 1968). I f water were still cold-trapped at 25°(;I. then even at a high effective temperature of gas-gas equilibrium, the PH~O would

(CH~)g

.

2,(}3

+

(H.O)g Liquid H_,O

4(H2)g + (CO2)g

,

)- (CH,)g

The excess hydrogen, produced at high temperatures by H20 dissociation and Fe e+ oxidation, cannot have escaped the hypothetically closed system during cooling, nor can it have been used in the water-gas reaction, He ÷ COz = CO + H20, since lower temperatures favor the production of even more C02 and H e at the expense of H oO and CO. Instead, virtually all hydrogen supplied to the surface would, on cooling in equilibrium, have combined with carbon to form methane. From the equilibrium expression above, Pcm

K4

constant at any given T At 25°C, K4 ~ 1020 and, assuming t h a t Pmo equals the saturation pressure in equilibrium with liquid water (3 x 10-~arm), then CH4 = CO2 when P m = 2 × 10-° atm. This is the "required" minimum equilibrium hydrogen pressure at 25°C for most of the carbon to be present in the form of methane. Note that, if equilibrium with liquid water is presumed, CH~/CO., is proportional to (Pn,) 4 at constant T. The highest temperatures prevalent in the

+

2(HzO)g

still equal 3 × 10-2, and thus, if g a s ~ a s equilibrium were established at 227°C, Pc., ( p . , ) , . [ 1 × 10s ] Pc()= ~= L(3 x

lO-::2)"J

and, for CH4/COo = l, PH, ("required") would equal 2 × l0 -:~ a t m - - a factor of 10 a higher than if atmospheric chemical equilibrium was "frozen-in" at 25°C. Thus, the temperature at which gas-gas equilibrium is presumed to have been effectively "frozen-in" is important, but complete conversion of hydrogen to methane would have occurred even if equilibrium were "frozen-in" at the highest temperature presently observed in the Martian atmosphere. I t was shown earlier t h a t material presently comprising the outermost portion of Mars was probably heated to temperatures in excess of 500°C for geologically significant periods of time, and to much higher temperatures for short periods of time during the accretion process. Under these conditions, chondritic meteorites typically yield in excess of 1.0 ccstp/g of indigenous dry gas of composition similar to those in Table I, according to the results of a number of investigators, summarized by Krinov (1960).

2,[}4

FRASER P. FANALE

Even if one assumes (conservatively) t h a t only the o u t e r m o s t 100 k m of ac,creting Martian material were so affected, the total a m o u n t of evolved gas would exceed about (3.5g/cma)(l ;- 10;)cm)(1 ccstl)/tr~, •,~,J 3.5 >: lO r ccstp/cnl "e, erresponding to a high m e t h a n e abundance, until i n t e r m e d i a t e or low temperature atmospheric equilibrium was atta.ined. i). Hydrogen Escape and the Acc~mulatio~z of a Reducing Abno,s'phere o,~ Mars P l a n e t a r y escape of hy
cloud as tim possible source of "'any ()riginal methane ammonia mixture" in the Martian atmosphere, but cites the paucity of K r and Xe in tile E a r t h ' s atmosphere as evidence against such direct retention. He points out t h a t Mars would be even less effective t h a n E a r t h in retaining primary volatiles fl'om the secreting gas dust cloud and t h a t " d u r i n g the process of its forma tion when gravitational attraction was even less, it couht not have retained m e t h a n e . " Again, the reference is to a possible p r i m a r y atmosphere. He states: "Mars with its low mass loses hydrogen quickly .. 2' because the escape velocity from Mars is 5.0 km/sec while t h a t fl'om the E a r t h is 11.2 kin/see. Here, it, is c
HISTORY OF MARTIA:N VOLATILES r2

KNOWN

I

CONSTANT AT GIVEN T

ARBITRARILY

/ F "~

DEFIN~-DAS UNITY

~ OUTFLUX INFLUX (SEETEXT)

]?z(.~. 6. Example of relationship between physical dynamics of p l a n e t a r y degassing and exospherie loss to atmospheric chemical equilibrium. M.R.T. = effective mean residence time. T = total effective influx/outflux. $

h y d r o g e n p r e s s u r e (for CH4 => CO2) u n d e r any given set of equilibrium conditions, and the Martian atmospheric hydrogen mean residence time. From this, and from the composition of juvenile dry gas (Table I), a m i n i m u m r a t e o f v o l a t i l e s u p p l y f r o m o r r e e q u i l i b r a t i o n w i t h t h e i n t e r i o r t h a t is "required" for sustenance of such an atmosphere may be estimated. In Table II, these rates are given for several sets of hypothetical equilibrium conditions. They

295

are also compared, for perspective, with the hypothetical continuing volatile supply rate that would have been capable of supplying the Earth's total surface volatile i n v e n t o r y a t a u n i f o r m r a t e o v e r 4.5 b i l l i o n y e a r s ( R u b e y , 1951). T h i s r a t e is a b o u t 1 x 10 -7 a r m p e r y e a r ( i n c l u d i n g H , , 0 ) , w h i c h , f o r c o n v e n i e n c e , is r e f e r r e d t o in T a b l e I I as o n e " R u b e y . " H e r e , t h i s u n i t is u s e d for c o m p a r a t i v e p u r p o s e s o n l y , a n d i t s use d o e s n o t p r e s u p p o s e t h a t t h e E a r t h ' s s u r f a c e v o l a t i l e s w e r e , in f a c t , supplied at a uniform rate throughout Earth history. The estimated rates given (in T a b l e I I ) a r e s t r o n g l y d e p e n d e n t on assumptions concerning the temperature at which atmospheric e q u i l i b r i u m is established and that at which exospheric escape takes place. Preliminary analysis of results from spacecraft observations of Mars by Mariner VI and VII has suggested that the average temperature at the base o f t h e M a r t i a n e x o s p h e r e is p r e s e n t l y close t o 5 0 0 ° K (227°C) ( K l i o r e et al., 1969). This temperature, according to the classical thermal escape treatment of S p i t z e r (1952), w o u l d r e s u l t in a m e a n r e s i d e n c e t i m e o f h y d r o g e n in t h e M a r t i a n atmosphere of about 1.5 × l03 y e a r s b e c a u s e o f t h e low M a r t i a n g r a v i t a t i o n a l field ( ~ 1 / 3 G).

TABLE II M I N I M U M V O L A T I L E S U P P L Y R A T E S F O R A C C U M U L A T I O N OF A M E T H A N E - R I c H

Atmosphere

Exospheric temperature (°C)

ATMOSPHERE

Minimum rate of total Hydrogen juvenile volatile supply mean residence Equilibration (for CH4/CO2 ~ 1) time temperature . . . . (yr) (°C) I f PH2O = p~oc I f PH20 - t)~ '

Primitive to present Mars

227

1.5 X 10 a

227 25

Prim irive Earth

700

2

374 227

x 107

~ 1 X 102 Rubeys a .~0.1R ~0.1R ~ 1 x 10 - 2 R

~ 3 × l03 R ~ 0.1 R ~ 1 x 102R ~0.3R

Note: Exospherie telnperatures are based (for Mars) on Mariner VI and V I I results and (for the primitive Earth) theoretical estimates. Hydrogen atmospheric mean residence times are calculated. Temperatures of atmospheric equilibration are assumed, and are those at which gas-gas atmospheric chemical equilibrium is assumed to have "frozen-in" (Te). Pv = saturation vapor pressure of water; t)vTe = saturation vapor pressure of water at effective g a s ~ a s equilibration temperature. a 1 R u b e y (R) ~ 1 x 10 -7 a t m / y r (see text).

~!)6

F R A S E R P. F A N A L E

A s indicated above, if a t m o s p h e r i c equilibrium is achieved at 25"(', then a methane-rich atmosphere would be c h a r a c t e r i z e d b y a h y d r o g e n pressure of at least 2 × 10 ~ a t m . This implies a h y d r o g e n outflux o f a t l e a s t (2 × 10 - a a t m ) / ( 1 . 5 / l0 a years) ~ 10 ~'~ a t m / y r . F r o m earlier (lis~ cussion of volatile composition, the volume percent excess of free hydrogen in the volatile influx would be between a b o u t 6(~ and 3 % (Table I), d e p e n d i n g on w h e t h e r or not metallic iron was in equilibrium with melt s u p p l y i n g the volatiles. In the least f a v o r a b l e case, (l >. 10!~)/(3 x 10-")_~ 3 ".: l0 -~ a t m / y r of t o t a l volatiles m u s t be supplied and, in the m o s t favorable, (l >: 10-~a)/((i y I() -1) ~ 2 × 10 ~ at, m/yr (Table 1I). In other words, the minimum degassing r a t e require(t in order fl)r a u n i f o r m l y degassing Mars to stabilize an e(luilibrium methane-rich atmosphere against p l a n e t a r y escape of hydrogen would h a v e p r o d u c e d a t o t a l M a r t i a n 1)ressure of 10 100 bars if" continued for 4.5 billion years. Yet, the present h)w a t m o s p h e r i c pressure a n d cratered surface m o r p h o l o g y of Mars b o t h suggest a much lower level of conti'rminq orogeny an(t (legassing for Mars t h a n for the E a r t h . In order for such a high continuing degassing rate on Mars (about I/4 t h a t suggested by 1~ubey for the F~arth) to be reconciled with the present t o t a l Martian at nmsl)heric i)ressure (~5 rob) it wouhl be necessary to hyl)othesize t h a t all m a j o r Martian volatiles h a v e atmosi)heric mean residence t i m e s on the order of 10 ~;y e a r s or less. Thus. continuing and geoh)gically " u n i t b r m " Martian outgassing associated with Martian orogenic a n d volcanic a c t i v i t y t h r o u g h o u t the history of t h a t t)lanet couhl trot h a v c resulted in the generation an(l s u s t e n a n c e o[" a, r e d u ( i n g a t m o s p h e r e . E i t h e r Mars never develol)ed such an a t m o s p h e r e , or its d e v e l o p n m n t was cont i n g e n t upon a n d limited to a period during which the " l e v e l " or rate of Martian outgassing was orders of m a g n i t u d e higher t h a n t h a t which characterized Mars during t,he r e m a i n d e r ()fits history. It a p p e a r s t h a t ( t e v e l o p m e n t of a Martian a t m o s p h e r e conducive to organie synthesis, if" it occurred at all, m u s t h a v e been eaused 1)3"

a process which was intrinsically p h m e t a r y in scope and e a t a s t r o p h i e ill nature, such as p l a n e t a r y accretion. T h e r e m a i n i n g question is whether even accretion (legassing of Mars could h a v e ereated a m e t h a n e - r i c h a t m o s p h e r e despite the effects of hydrogen escape ; and, if so, how long such a m c t a s t a b l e a t m o s p h e r e m i g h t have been sustained. It m a y I)e shown t h a t even under c o m p a r a t i v e l y m f f a v o r a b l e conditions of atmospheri(. equilibration, the (legassing rate associated with accretion wouM have st|lti(~ett t() generate and sustain a reducing at mosphere despite hydrogen escape. ,ks calculated earlier, at. an effective t e m l ) e r a t u r e of atmosi)heric equilibration of 227~'(!, the highest l)rescnt a m b i e n t a t m o s p h e r i c t c m i~erature on Mars, a methanc-rieh a t m o s p h e r e would be characterized b y a hydrogen t)ressure of a b o u t '2 > 10 :~ a t m . With an exospheric t e m p e r a t u r e of about 500°K, this pressure would correspond to a net h y d r o g e n flux frona Mars of a b o u t 1 ~ !0 ~; a t m / y r , which, in equilibrium. "'requires" a t o t a l volatile flux from the interior of from 5 ~ 10 i to 3 -.~ 10 ~, a t m / y r . As mentioned, there is evidene(, suggesting t h a t the Martian accretion t i m e was geologically short, p r o b a b l y between 1 . 1()a and 5 :. 10 '~' years, and t h a t a t o t a l volatiht i n v e n t o r y e(tuivalent to several a t m o s p h e r e s was l)robably released during the accretion interval. This implies a volatile release r a t e t h a t was g r e a t e r t h a n 3:. 10 r, a t m / y r , or 300 R u b e y s , hence sufficient to comt)ensate for exospherie loss during accretion even if a t m o s p h e r i c reequilibration oecurred at 22T:'(L As shown above, the hydrogen t)ressurc in equilibriunt with a met haneri(;h a t m o s p h e r e is three orders of magnit u d e h)wer at 2 5 ' ( ' t h a n at, 227°(I. Hen(e. the volatile release rate a c c o m p a n y i n g Martian accretion would h a v c been several orders of m a g n i t u d e higher t h a n required for s u s t e n a n c e of a reducing a t m o s p h e r e if t h a t a t m o s p h e r e was able to effectively equilibrate at 25~(!. The h y d r o g e n loss r a t e from the planet m a y also be r o u g h l y e(luated to a net m e t h a n e destruction r a t e following the cessation of accretion. Thus. under the

Ill.STORY OF MARTIAN ¥OLATILES

most favorable circumstances, hydrogen loss/methane destruction rates might be as low as l0 -9 atm/yr and would allow survival of an initial methane-rich Martian atmosphere for periods on the order of l0 s years. On the other hand, if hydrogen supply and atmospheric chemical equilibrium are effectively determined at high temperature, comparable, say, to the present Martian exospheric temperature of about 500°K, it would seem unreasonable to expect a methane-rich atmosphere catastrophically generated during accretion to have survived for periods in excess of 105 years, i.e., significantly beyond the termination of accretion itself. However, one important special mechanism by which a reducing atmosphere on Mars might have more effectively stabilized itself against hydrogen loss following accretion is that discussed for the case of the primitive Earth atmosphere by Rasool and McGovern (1966). They suggested that a high abundance of hydrogen and methane in the primitive Earth's atmosphere might have lowered the exospheric temperature, possibly by a factor of three, largely because of the high thermal conductivity of hydrogen. In keeping with this hypothesis, an exospheric temperature of 700°C is assumed for the comparative calculations, the results of which are given in Table II. The fact that the Venus atmosphere possesses an exospheric temperature of only 700°K (Stewart, 1966), while that of the present earth is about 2000°K, also indirectly suggests a lower exospheric temperature for the primitive Earth. An analogous hypothetical lowering of the Martian exospheric temperature might have had the effect of dramatically increasing the effective hydrogen mean residence time in the Martian atmosphere over those given in Table II and, consequently, might have enhanced the ability of a primitive reducing Martian atmosphere to stabilize itself against hydrogen loss. A speculative possibility is that accretion of solar hydrogen might have stabilized the early Martian atmosphere against hydrogen escape. A solar-wind flux more than 100 times the present flux would have been required.

297

Thus, a highly simplified, but quantitative treatment of the dynamic equilibrium between orogenic outgassing and exospheric hydrogen escape suggests that Mars could never have developed a methane-rich atmosphere as a result of geologically gradual, continuous or "uniform" outgassing throughout its history. This conclusion is directly based upon the pressure dependence of methanesynthesizing reactions and the observation that, owing to its low gravitational field, comparatively low rate of outgassing, and exospheric temperature, Mars would at no time have developed a high enough surface hydrogen pressure to favor methane synthesis. On the other hand, consideration of the probable nature of Martian accretion degassing suggests that Mars catastrophically developed a substantial reducing atmosphere on accretion, and that the elemental composition and total pressure would have allowed conversion of almost, all the free hydrogen to methane if atmospheric equilibrium were achieved at low temperatures. As a consequence, an early atmosphere was developed on Mars which favored organic synthesis, and in which the specific rate of methane destruction may have been relatively low because of the low equivalent free hydrogen pressure. Once catastrophically generated, such an atmosphere under favorable circumstances might have stabilized itself against methane loss for a period of from 10 ~ to lO s years. Subsequently, Mars could not have generated or sustained a highly reducing atmosphere. E. Evolution to Present Atmospheric Cow,position and Pressure It might seem that a hypothesis involving the early existence of a much more massive and reducing early Martian atmosphere is in conflict with observations of the total pressure and composition of the present Martian atmosphere, and with the cratered appearance of the surface. However, evidence is available demonstrating the continuing operation of processes that may have been capable of transforming the Martian atmosphere from its hypothetical initial composition and pressure

2!)N

F R A S E g P. F A N A L E

its l)resently observable COml)osition and |)ressure in 4.5 y lo" years. (!onsider, first, the ultimate effects of hydrogen escape. Methane, water, an(] all hydrogencontaining <'ompounds are ultimately ,testroye(I as the result of hydrogen eseal)e, as are some compounds t h a t do n(/t contain hv(h'ogen, such as carbon monoxide. This I)ro('ess, which m a y be t e r m e d " h y d r o g e n driving," was shown schematically earlier in this paper. It shouht be eml)hasize(l t h a t (I()._>, along with possible excess ()., and e(msequent surface oxidation, is the ultimate p r o d u c t of this process. H y d r o g e n driving must prewdl, and material balance dictates t h a t the atmosphere must become greatly depleted in hv(h'o,'en relative to the calculated source comllosition simply because hydrogen, mfi(tuely is being removed f'rom t h e • system Since Ct ( ). (IH 4, and H,,() are d e s t r o y e d in the hydrogen driving i)roeess, planetary atmospheres are, in principle, metastable with respect to these constituents, insofar as t h e y are metastabh, with respect to retention of hydrogen itself. The composition (essentially ('().z plus, presmnably, a small inert component,) t h a t presently characterizes the Martian a t m o s p h e r e may, theretbre, be regarded as tile inevitable result of the hydrogen driving process---as m a y possible Martian surface o x i d a t i o n - - a n d .not as a reflection o f the initial elemental mass |)alance o r degree of oxidation of p l a n e t a r y surface volatiles (see Fig. 5 and discussion ()f elemental composition). ()ther t)rocesses m a y have strongly del)leted the initial Martian atmosl)here in volatile sllecies of intermediate mass also. Because of the virtual absence of a Martian m a / n e t i e fiehl, solar-wind sweeping may be an e x t r e m e l y i m p o r t a n t |)recess t'()r Martian volatile removal. ,I. F. (lmm (personal communication) roughly estimated a total o x y g e n loss rate froni tile planet of a b o u t 2.7 : 107 Clll 2 see-I from solar-w ind sweeping. I f (l(),a is the supplier, and its dissociation is not the rate-limiting atoll, a total ('O., inventory of l)erhaps 20 times the present atmospheric inventory has been d e s t r o y e d in 4.5 < 1(1:~ years by this />recess. F u r t h e r m o r e . this estimate to

may be far too h)w, since it is based on the

pre,~'e~t sohu'-wind flux, and it, is generally t h o u g h t t h a t the solar-wind flux was much higher during the early history of the solar system. This is an i m p o r t a n t , hut conjectural l>ossibility. The failm'e of Mariner ~r'l alia VII to detect N.. ill the Martian a t m o s p h e r e (Barth c/ may be tied u 1) in the ll()l'th polar cap or adsorbed on the Martian soil ( F'anale and (lannon, 1970). If the N:(! ratio in the Martian atmosphere is also fimnd to fall within the broad limits of this (list ributigous situation obtains with resl)ect to tile t)iologically crucial question ()f t h e Martian water budget. Based on arguments l)resente(1 earlier, it seems highly improbable t h a t the bulk planetary inventory of volatiles oeehided or <'heroic ally comhine(I in Mart|all solid material could t)c strikingly different fr(lm that exhibited I)5" terrestrial igneous rocks (,r

IIISTOl¢g OF MARTIAN VOLATILES

the total earth surface volatile i n v e n t o r y . Hence, the site of "missing" w a t e r on Mars remains a puzzle, especially since the (thin) Martian polar caps now appear to consist primarily of COo (Leighton et al.,

19(~.q). A possible clue to the solution lies in the observation t h a t the " d r y " Martian atmosphere appears, in fact, to be s a t u r a t e d with water. A n o r t h e r n hemispheric average of "25 p~ precipitable" water was r e c e n t l y observed b y Schorn et al. (1969). This would, depending on relative vertical distribution of w a t e r and CO., in the atmosphere, be a b o u t equivalent to a basal atmospheric pressure of a b o u t 3 × l0 -~ ram. This is a b o u t equal to the s a t u r a t i o n pressure at 225°K (by no means the lowest surface t e m p e r a t u r e on the Martian disc). Hence, it appears there is no reasonable physical means b y which the Martian a t m o s p h e r e could be much more charged with w a t e r v a p o r t h a n it is observed to be, and t h a t near-surface e n v i r o n m e n t s on Mars m a y be as s a t u r a t e d with w a t e r as basal atmospheric air in a tropical rain forest on E a r t h . Nor is this c o m m e n t entirely a specious one: certain crucial properties of the atmosphere-soil system, such as the n u m b e r of monolayers of w a t e r physieMly adsorbed on soil, and hence the n u m b e r of grams of water adsorbed in equilibrium on Martian soil per unit internal surface area, are essentially dei)endent upon the relative w a t e r pressure, and much less so on the absolute P m o or the t e m p e r a t u r e t a k e n b y themselves. Thus, the e x t r e m e l y smM1 a m o u n t of w a t e r observed in the Martian a t m o s p h e r e may, nonetheless, be close to saturation v a l u e - in equilibrium with a reservoir of adsorbed H.)O and ice in the Martian soil on which no extensive q u a n t i t a t i v e u p p e r limit can presently be set from the available atmosl)heric data. .) 2 I n t h i s c o n t e x t , it is i n t e r e s t i n g to n o t e t h a t , in e> (Pco:)/(l)co.,)s, w h e r e P s ~ t h e s a t u r a t i o n pressure. I n fact, m o s t of t h e t i m e t h e r e l a t i v e

299

F u r t h e r m o r e , the small a m o u n t of w a t e r t h a t is present as water v a p o r in the i n t e r m e d i a t e atmospheric reservoir is continually subject to dissociation into two products, hydrogen and oxygen, both of which p r o b a b l y have uniquely short atmospheric mean residence times. The present hydrogen atmospheric mean residence time on Mars is unlikely to exceed l04 years (Spitzer, 1952) if the t e m p e r a t u r e at the base of the Martian exosphere is a b o u t 500°K as observed b y Kliore et al. (1969). Also, free oxygen cannot exist at. pressures t h a t would presently be detectable in the Martian a t m o s p h e r e if it were in equilibrium with the Fe2+/Fe :~+ ratio of' a n y solid igneous material known to be of significant a b u n d a n c e in the solar system, i.e., terrestrial, lunar, or meteoritic matter, at t e m p e r a t u r e s equal to or less t h a n the melting t e m p e r a t u r e s of these materials (e.g., see K e n n e d y , 1948). The precise mechanism b y which such a t m o s p h e r e surface chemicM equilibrium might be i m p l e m e n t e d is not known. B u t it is clear from the above reasoning, and from evidence for surface oxidation mentioned earlier, t h a t the Martian a t m o s p h e r e c~nnot be regarded as a closed chemical system with respect to free oxygen a n y more t h a n it can for free hydrogen or, hence, water. The combined effects of exospheric escape, solar-wind sweeping, and interaction with the surface could have modified the composition and t o t a l pressure of ~n originally methane-rich and much more massive early Martian atmosphere, such as hypothesized here, so as to have produced the presently observable composition and pressure. Hence, the hypothetical early Martian atmospheric conditions, suggested here on the basis of other evidence, can also be reconciled with present observations of Martian atmosphere composition and pressure.

F. Biological Implications of tt~e Cratered Morphology The concept, presented herein, of a d y n a m i c Martian a t m o s p h e r e t h a t is an p r e s s u r e o f w a t e r on M a r s is p r o b a b l y a b o u t 100 t i m e s t h e r e l a t i v e CO2 p r e s s u r e n e a r t h e M a r t i a n equator.

300

F R A S E R P. I r A ~ A L E

open system with respect to most of its ('onstituents on a geological time scale, is also completely compatible with the cratered Martian surface morphoh)gy. In ta('t, the preceding a r g u m e n t s suggest a r e i n t e r p r e t a t i o n of the gross a p p e a r a n c e of the Martian surface--.l)hotographed by Mariners IV, VI, and VII at least as far as its biological implications are concerned. There is a t e n d e n c y tbr us to be 16d t)y the Martian morphology to regard Mars as an orogenically inactive, or " d e a d " planet, and to suspect intuitively t h a t a comparatively " s t a t i c " surface implies a " s t a t i c " atnmsphere as well. ()i'ogeni(' activity certainly causes gas effusion, and the ~cneration of an ~tmosphere d<)es a p p e a r to I)e an absolute prerequisite for sI)ontaneous abiotic organic synthesis in p l a n e t a r y environments. Hen~,c.(.,, the gener~dlv, old appe'~ranee of the Martian ,~;urface has suggested to m a n y a lower 1)robability of Martian life t h a n if the surface had resembled the orogenically <,ontorted surface of a denuded E a r t h . Assertions hv I)oth scientists and nonscientists to this effect have appeared repe.'-~tedly in the press. However, q u a n t i t a t i v e aspects of Martian volatile chemical evolution considered here strongly suggest t h a t given any total Martian effused volatile inventory, the most biologically "favoral)le" circumstances for its relea.se, as far as the chemistry of the r e c t a - e n v i r o n m e n t was concerned, would have been geologically catastrophic release on accretion. In fact. as shown earlier, accretion release is the only mechanism of Martian outgassing t h a t could have resulted in the develop,nent of ~ substantial an(t chemically reducing atmosphere. F u r t h e r m o r e , the cratered Martian surface has 1)een shown to he compatible with extensive early melting, as in the lunar case. As a conse(luence, the "'hmar" or "dea(l" at)peara n t e of the Martian surf~ce suggests a history t h a t is not significantly less comi)atit)le with the possible oc(mrrenee of spontaneous organic synthesis t h a n t h a t which would t)e suggested if 31ars, with its 1)resent atmospheric i)ressure and composition, exhibited a more ";active" sllrfa(~e.

\' I. (!()NCLV,~I()N,~

Theoretical reconstruction of the chendca] evolution of Martian volatiles indicates t h a t the Martian surface an(l atmospheric e n v i r o n m e n t , now chemically hostile with respe(.t to abiotic organi(, synthesis in the gas phase, is likely to have been initially favorable. Evidence indicating the thermal and a.ccretional history of Martian and E~rth material suggests t h a t considerable early heating of Martian material resulted from rapid p l a n e t a r y accretion. The hypothesis t h a t Mat's experienced considerable early t)lanetary differentiation as a consequence is compatible with present knowledge of the Martian internal mass distribution. The observation t h a t at least part of the Martian surface appears to be chronologic~dly l)rimordial indicates "skewing" of this planetary differentiation (and a c c o m p a n y ing degassing) tow~rd earliest Martian history. Early Martian degassing is also indicated bv morphologieal evidence fin' an initi-d episode of surface degradation m d q u e in Martian history. The original elemental composition and magnitude of vo]atiles supplied to the M~rtian surface can 1)e estimated fronl ~malysis of gas released bv i~ ~acuo heating of meteorites and terrestrial rocks and t]'om equilibrium calculations. ()n either basis. the estimated elemental composition and a m o u n t s of gas initially released to the Martian surface as the result of the intbrred early t h e r m a l history of Martian material is grossly incompatible with t h a t presently observed in the Martian atmosphere. Instead, it appears likely t h a t a substantial and highly reducing volatile assemblage was supI)lied to the Martian surface as the result of accretion heating. If the Martian atmosphere constituted a closed system which t h r o u g h o u t its history lost, only water ti'om its gaseous assemblage, cooling and reequilibration woukl have resulted in a methane-rich atmosphere. The present Martian atmosf)here is tenuous and oxidized, providing a (.heroical setting unconduc'ive to abiotie organic synthesis. However, this reflects the extensive chemical evolution t h a t Martian surface volatile in-

HISTORY OF MARTIAN VOLATILES

ventory has undergone since its supply to the surface--not its original condition. This evolution is primarily the result of the exospheric escape of hydrogen, atmosphere-surface interaction, and solar-wind sweeping. Continuing supply of juvenile volatiles to the surface and atmosphere-interior reequilibration on Mars could not keep pace with the effects of these modifying processes, owing largely to the low rate of continuing Martian orogeny and the high rate at which certain of these modifying processes (especially hydrogen loss) can occur on Mars. However, catastrophic supply of volatiles to the Martian surface associated with accretion occurred rapidly enough to temporarily mitigate the effects of the "modifying processes" alluded to above. As a consequence, a metastable reducing and methane-rich atmosphere is likely to have accumulated and persisted for up to l0 s years at the outset of Martian history. This period represented the only time that the Martian environment was chemicMly suited for abiotic organic synthesis in the atmosphere, and hence was a uniquely favorable opportunity for the spontaneous origination of life on Mars. ACKNOWLEDGMENTS The author would like to express appreciation to Professor Norman H. Horowitz for the many stimulating discussions which, in fact, instigated this study. The author is also grateful to Douglas B. Nash, J am es E. Conel, Lewis D. Kaplan, Professor Don L. Anderson, Wesley Huntress, Jr., and R a y m o n d T. Greet for detailed reviews of the manuscript and m a n y helpful suggestions. REFERENCES ABELSON, P. H. (1965). Abiotic synthesis in the Martian environment. Prec. U.S. N a t l . Acad. Sci. 54, 1490-1497. ABELSON, P. H. 0966). Chemical events on the primitive Earth. Prec. U.S. N a t l . Acad. Sci. 55, 1365. ADAMS, J. B., CONEL, J. E., DUNNE, A., FANALE, F. P., HOLSTROM, G. B., AND LOOMIS, A. L. (1969). Strategy for scientific exploration of the terrestrial planets. Rev. Geophys. 7, 3, 623-661. ADAMS,J. B. (1968). Lunar and Martian surfaces : Petrologic significance of absorption bands in the near infrared. Science 159, 1453-1455.

301

ADUEVSKY, V. S., MAROV, M. Y. A., AND ROZHDESTVENSKY,M. K. (1970). Heat transfe," in the Venus atmosphere. J . A t m . Sci. 27, 4, 561. ALBEE, A. L., et al. (1970). Ages, irradiation history and chemical composition of hmar rocks from the Sea of Tranquillity. Neietwe 167, 3918, 463. ANDERS, E. (1968). Chemical processes in the early solar system as inferred from meteorites. Accounts Chem. Res. 289. ANDERSON, D. L., SAM)~m, C., AND ,IoRDON, T. (1971). Composition and evolution of the mantle and core. Science 171, 3976, 1103 -1113. ASH, E., et al. (1967). Astronomical constants and planetary ephemerides deduced from radar and optical observations. Astron. J . 72, 338-350. BARTH, C. A., FASTIE, W. G., HERD, (!. W.. PEARCE, J. B., KELLEY, K. K., STEVCART,A. l., T~O~AS, G. E., ANDERSON, G. P., AND RAPER, O. F. (1969). Mariner 6: Ultraviolet spectrum of Mars' upper atmosphere. Science 165, 3897, 1004. BINDER, A. (1969). Internal structure of Mars. J . Geophys. Res. 74, 12, 3110. CttAMBERLIN,12~.T. (1909). "The Gases in Rocks," p. 14. Carnegie Inst. of Washington, Pub. No. 106. DALGARNO, A., AND McELRoY, M. B. (1970). Mars: Is nitrogen present?. Science 170, 3954, 167-168. ECK, 1%. V., LIPPENCOTT, E. R., DAYHOFF, M. O., AND PRATT, Y. T. (1966). Thermodynamic equilibrium and the inorganic origin of organic compounds. Science 153, 3736, 628-633. EMMONDS, R. C. (1964). Granites by recrystallization. A m e r . J . Sci. 262, 561. FANALE, F. P., AND CANNON, W. A. (1970). Adsorption on the Martian regolith: Implications for the Martian volatile budget and diurnal brightening. N a t u r e , in press. GLASSTONE, S. (1968). "The Book of Mars." NASA SP-179, Pub. Scientific and Technical Information Division, Office of Technology Utilization, National Aeronautics and Space Administration. HANKS, T. C., AND ANDERSON, D. L. (1969). The early thermal history of the Earth. Phys. E a r t h Planet. Interiors 2, 1, 19-29. HAYASHI, L. (1961). Stellar evolution in early phases of gravitational contraction. Publ. Astron. Soc. J a p a n 13,450-452. HOLLAND, H. D. (1962). Model for the evolution of the E a r t h ' s atmosphere. I n "Petrologic Studies (Buddington Memorial Volume)," p. 447. Geol. See. Amer.

}102

I.'IIASEP~ 1'. YANAI,I,;

N. H. (1971). Photoeatalyti(~ pl'()dueti()l, (,f"

SI1A@:R. L. T. (1970). U r a n i u n i - t h o r i u m - l e a ( I is()tope reh~t,i(ms in ] i l l i a r m a t e r i M s . Ncie~,:e

()l'galii(' (~onlpo/In(is fi'(lin (:() lul(l He() in il silliulated Martian atln(is][)iler(,, l>roeeeditlff,s . qf tke National A c a d e m y <4 ,','eie,cex 68, 3.

167, 3918, 468. SPvrzEi~, C., J R. (1052). The terrestrial atlnosl)her( , ab,)v(, 300 kin. I n "'The A t m o s p h e r e s of

574 578. KI~ZNNI.ZDY, (~. (!. (1948). Equilibriuin b(,tweell v()h~tiles a,n(I iron ()xides in igiie()us rooks. Amer. J . ,~'ci. 246, 529 549. lqEN NEDY, (I. (!. (1955). Some aSl)eet,s of th(, I'oh' of" w a t e r in rock melts, ht "(',rust ()f th(, E a r t h " (Poldervarat. A., Ed.). (b,ol. S()(!. Anlev., Speeial Paper 62.

t h e E a r t h and tqanets,'" 2nd ed. ((l. P. Kuip(w, Ed.), pp. 21 l 247. U n i v . o f (!hicago Press, Cliieag(i, lllin()is. SICEWAR'I', I / . ~V. (1066). Interl)rc, t a t i o n (>[" } l a r i l i e r 5 0-lid Vel/el'tt 4 da,ta O11 t h e IIp[)el' atln()sph(q'e (if ~t7(~tlllS. J . :4 tlII.O,S'. ~%'C7.25> 578.

HUBBARD, ,I. S., HARDY, ,]. P.. AND HORO\\'ITZ,

|'~LIORE, A.,

FJFLDBO,

(I.,

NEIDEL,

l{.,

AND

HASOOL, S. I. (1969). Mariner (i and 7: Ha(li() oeeultlit, ioll n/ea,sllreln(qits ()f til(, atmosph(,r(, (d'Mars. Ncience 166, 1303.

KOVACIt, H. L., ANI) ANDERSON, |). L. (19(15). The int(!ri()rs of the terrestrial plailets. J . ¢;eopkys. Res. 70, 2873 2882. K mNOV, E. L. (1960). "Principh,s ()fMet(~()rities.'" l)ergamon l)ress, L()ll(|on. LEIGltTON. l)~. 1-}., HOROV¢ITZ, N. H., MURRAY, ]{. (!., SHARP, 1{.. l)., HEt~.HIMAN. A. H . , YOl N(;,

A. T., S~n'rtf, 13. A., I)AVI~ZS. M. E., AXD Lb:ROY, ('. B. (1969). Ma, r i n e r IV l ) h o t o g r a p h y ()I" Mars : Initild reslllLs. Neie,ce 166, 49 67. ]A(TTLETON, 11. A. (10t15). ()li th(' internal slvueture ()f the phuiet Mal's. M o , t D l y Notices Roy. Astr(m. Nee. 120, 21 39. MA('I)ONALI), ( I . . | . F. (1962). ()n the interllal ('(mstituti()ll ()f tile iiinev phm('ts, ,/. (;eopkys. lies. 67, 2945 2974. J{AS()(JL, S. 1., AND M(J(IJOVERN, \V. E. (191i11). P r i m i t i v e a t n i o s p h e r e ()f' t,h(, E a r t h . N a t , r e

5067, 1225. l / i x ( ; w o o D , A. E. (19(111). Chemical (wolutiol, ()1" th(, terrestrial planets. GeoclHm. (%smockim. Acta 30, 41 104. |{.OEDI~]R, i). L.. AND OSBORNE, E.

F. (1961i).

E x p e r i m e n t a l d a t a for tim system MgO-FeOI"eeOa Ca AI,,SiiOs-Si02 mid their petroh)gie ilnplieatiolis. A mer. J. Nci. 264, 428 480. Ill'BE'C, Vf. H. (1951). (leologie history (if" sea water. Bull. Geol. Nee. A met. 62, l 1 l l. S('HOaN, R. A., I"aaMEm C. P., AND LITTLE, S. ,1. (1969). High dispersion spectr()scopie st udi(,s ()1" Mars I11. l)relimilmr\" results ()f 1968 69 wafer-vapor studies, l e a r , s l l , 3, 28:{ 280, NHEPttERD, E. S. (1938). The gases in reeks a n(t some related problems. A m e r . .I. ,S'ci. 5th ,S'er. 35A, 311.

StoNER, P. (1964). Primordial rare gases in meteorit(~s. I n " T h e Origin and Evoluti(m ()f Atnmspheres a n d Oceans" (P. 13raneazio, mid .~.

(I.

\V.

( J/~Illel'()II,

Wiley, N~w Y~)rk.

I~](1S.), p p .

183-190.

TATSUMOTO, M.. AND HOSHOLT. J. N. (1070). Ages ()f the Moon: An isotopic study ()f U-Th-Pb systematies Nc'ience 167, :1018, 461.

()f

lunl0A'

slUilllles.

ri'IiRKIqVICtt, z~x. L., FRANZ(lliOTE, E. ,l.. el (l[.

(1968). Chemical analysis ()f the Moon at th(, Survey()r VIII lmiding sit(' : P r e l i m i n a r y results. ,S'eie~we 162, 117 118. TURKlqVI(~II, A. L.. FRANZGROTE, E. ,1., et al. (10(39). (:heuiieal con|posit, ion ()[" tim l u n a r Sllrf~{ee ill llcl/tre T r a i l ( l l l i | l i t l t t u s . Ncie,ce 165,

2 7 7 279. Tvmxl,:R, (I. (1970). Argon-40/Argon-30 datiiig of lililtll' rook samples. Scierme 167, 3918, 4(i(i. UiCEY, H. (!. (1952). " T h e Planets." Yale Univ. Press. N(+w Haven, (',onn. [h~t.:v, H. (!., AXD (~RAI(G H. (1953). Thi'c(.nltJ()siti()n i)l" the sl()ne lnet, eorites m i d the, o r i g i n ()f the nlet('()l•itl~S. (ieocki.m. (*o,~'m,oeki,~. Acta 4,

36 82. \VETH ~:lti LL, ('. (1967). Collisions in the aster()ida [ b(,It. J . (;eophgs. Ren. 72, 2429. Wool), ,I. A. (1963). ()n the origin of eh()ndruh,s and ehon(h'ites. Icarus 2, 152 180. ADDENDUM A D D E D 29 J u n e , Recently

1.q71

Hubbard,

et al. (1971), h a v e H.,(). m i d ((')_., a r e i r r a d i a t e d w i t h u l t r a v i o l e t l i g h t in t h e p r e s e n c e o f soil, o r g a n i c c o m p o u n d s ( t e n t a t i v e l y i d e n t i f i e d as f o r m a l d e hyde, aeetaldehyde, and glycolic acid) are t b r m e d w h i c h a r c r e c o v e r a b l e f r o m t h e soil. S u c h a g~s m i x t u r e c o u l d b e s t b e d e s c r i b e d as " s l i g h t l y r e d u c i n g " . Surface effects. t h a t a p p e a r to be c r u c i a l i n t h e i r e x p e r i ment. include the occurrence of gas-gas r e a c t i o n s a t soil s u r f a c e s , a n d s t o r a g e o f o r g m f i c p r o d u cts o n s u r f a c e s . I n t h e p r e s e n t work, the evahlation of the suitability of the Martian atmosphere for abiotic organic synthesis refer to the chemical conduciven e s s o f the g a s p h a s e t o s u c h s y n t h e s i s .

r e p o r t e d t h a t w h e n m i x t u r e s ot ( . ) .

HISTORY" OF MARTIAN

i t should also be m e n t i o n e d t h a t Anderson et al. (1971) h a v e r e c e n t l y p o i n t e d o u t t h a t Binder (1969) used a zeropressure d e n s i t y of 8 g / c m 3 for the M a r t i a n core in his analysis as discussed in this t e x t , a n d t h a t this value is g r e a t e r t h a n the u n c o m p r e s s e d d e n s i t y of the e a r t h ' s

VOLATILES

303

core. I f it is assumed, instead, t h a t tile d e n s i t y of the M a r t i a n core is b r a c k e t e d b y the d e n s i t y of the e a r t h ' s core a n d b y the d e n s i t y of the euteetie mix in the s y s t e m F e - F e S at 30kilobars, t h e n the M a r t i a n core mass can he as m u c h as 9-15 p e r c e n t of the t o t a l mass of Mars.