Martian volatiles: Their degassing history and geochemical fate

x c ~ a u s 28, 179-202 (1976)

Martian Volatiles: Their Degassing History and Geochemical Fate F. P. F A N A L E Space Sc/enoeo D / v / s / ~ , Jet Propu/s/on Laboratory,

California Institute of Technology, 4800 Oak Grove Drive, Pasadena, California 91103 Received September 24, 1975; revised October 27, 1975 Observations of Mars and cosmochemical considerations imply that the total inventory of degassed volatiles on Mars is 102 to 103 times that present in Mars' atmosphere and polar caps. The degassed volatiles have been physically and chemically incorporated into a layer of unconsolidated surface rubble (a "megaregolith") up to 2kin thick. Tentative lines of evidence suggest a high concentration (~5 g/cm 2) of 4°Ar in the atmosphere of Mars. I f correct, this would be consistent with a degassing model for Mars in which the Martian "surface" volatile inventory is presumed identical to that of Earth but scaled to Mars' smaller mass and surface area. The implied inventory would be: (4°Ar) = 4g/cm 2, ( H 2 0 ) = l x 10Sg/cm 2, ( C 0 2 ) = 7 x 103g/cm 2, ( N 2 ) = 3 x 102g/era 2, ( C l ) = 2 x 103g/cm 2, and (S) = 2 x 102g/cm 2. Such a model is useful for testing, but differences in composition and planetary energy history may be anticipated between Mars and Earth on theoretical grounds. Also, the model demands huge regolith sinks for the volatiles listed. I f the regolith were in physical equilibrium with the atmosphere, as much as 2 x 104g/cm 2 of H20 could be stored in it as hard-frozen permafrost, or 5 x 104g/ cm 2 if equilibrium with the atmosphere were inhibited. Spectral measurements of Martian regolith material and laboratory measurement of weathering kinetics on simulated regolith material suggest large amounts of hydrated iron oxides and clay minerals exist in the regolith ; the amount of chemically bound H20 could be from 1 x 104 to 4 x 104g/cm 2. I n an Earth-analogous model, a 2kra mixed regolith must contain the following concentrations of other volatile-containing compounds by weight: carbonates = 1.5°/0, nitrates = 0.3%, chlorides = 0.6°/0, and sulfates--0.1°/0. Such concentrations would be undetectable by current Earth-based spectral reflectance measurements, and (except the nitrates) formation of the "required" amounts of these compounds could result from interaction of adsorbed H20 and ice with primary silicates expected on Mars. Most of the CO2 could be physically adsorbed on the regolith. Thus, maximum amounts of H20 and other volatiles which could be stored in the Mars regolith are marginally compatible with those required by an Earthanalogous model, although a lower atmospheric 4°At concentration and regolith volatile inventory would be easier to reconcile with observational constraints. Differences in the ratios of H20 and other volatiles to 4°Ar between surface volatiles on the real Mars and on an Earth-analogous Mars could result from and reflect differences in bulk composition and time history of degassing between Mars and Earth. Models relating Viking-observable parameters, e.g., (4°Ar) and (36At), to the time history and overall intensity of Mars degassing are given. B U L K VOLATILE AND SEMI-VOLATILE

a b o u t 5 g / c m ~, is s u g g e s t e d b y Mars 6 r e s u l t s (Moroz, 1974), a n d b y d i f f e r e n c e s between measured and predicted Martian exospheric temperatures (Levine and R e i g l e r , 1974). A h i g h 4 ° A r a b u n d a n c e , u p

CONTENTS OF MARS AND EARTH A n 4 ° A r : C 0 2 r a t i o for t h e M a r t i a n a t m o s p h e r e o f a b o u t 0.3, e q u i v a l e n t t o Copyright ~ 1976 by Academic Press, Inc. All rights of reproduction in any form reserved. Printed in Great Britain

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to 25%, would not be in conflict with sulfide-rich outer core of the Earth would interferometric measurements (Kaplan, explain the "chondritic coincidence" be1975), It is therefore appropriate to con- tween heat production in a hypothetical sider the implications of the 4°At content chondritic earth and the observed heat of the Martian atmosphere--which should flow from the Earth. It would also explain be definitively measured by the Viking the virtual equality of heat flow between entry probe and lander--for Martian vola- continents and ocean basins as well as tile history. other geophysical observations (Stacey, To assess these implications, some rela- 1975) and petrological evidence (Goettel, tionship between the bulk K content of 1975). Thus it seems likely t h a t the Earth's Mars and its bulk content of chemically K content is similar to t h a t of normal active volatile constituents must be estab- chondrites and this is even more likely to be lished. In fact, some assumptions concern- true of Mars (Lewis, 1972). ing the bulk volatile composition of Mars The thermochemical fractionation model are implicit in all attempts to interpret of Lewis (1972, 1974) also predicts that Martian atmospheric data in terms of de- Mars, owing to accumulation within the gassing history. On the basis of physical tremolite field, should have a bulk H20 models of the preplanetary nebula content of ~0.3%0, which coincides with (Cameron, 1963, 1973), Lewis (1972, 1974) reported H20 concentrations for normal has constructed models of planetary forma- chondrites [see data of Wiik (1956) and of tion which adequately explain differences Mason (1962)]. We should bear in mind, in uncompressed density among most however, t h a t the reported HzO concentraplanetary objects as resulting from dif- tions in normal chondrites may be someferences in the temperature and pressure what too high because no clear identificaat which solids forming these objects tion of terrestrial water in these objects is ceased to condense from or, in some possible. The Earth's oceans correspond models, reequilibrate with, the nebula to somewhat less than 0.1 of this "chondgas. Lewis' models predict t h a t the Earth ritic" concentration of H20 in the Earth, should not be depleted in K relative to the and if the concentration of H~O in oceanic chondritic meteorites or Mars. On the basalts of~0.5% (Moore, 1970) is typical of other hand, Larimer (1971) has argued the mantle as a whole, the Earth would the Earth is depleted in K, Na, and Rb have a H20 content similar to t h a t of both relative to chondrites. Also, Wasserburg chondrites and Mars. Since oceanic basalts et al. (1964) point out t h a t terrestrial are probably richer in HzO than the mantle crustal rocks consistently exhibit lower as a whole, the Earth's H20 content may K/U ratios than chondrites. In my opinion, be somewhat less than t h a t of Mars, but the balance of the evidence favors a chond- definitely not an order of magnitude less. l~tic K content for both the Earth and Other arguments suggesting t h a t the (more certainly) Mars: First, geochemical Earth and Mars have similar bulk HzO evidence suggests t h a t virtually all ele- contents will be presented in a later section. ments, including nonradiogenic rare gases, In summary, it seems reasonable to other than the alkali metals are present in assume t h a t the K content of both the the Earth in abundances comparable to Earth and Mars is similar to t h a t of normal those in chondrites (Larimer, 1971). I t chondrites, t h a t Mars possesses a H20 seems unlikely t h a t the alkali metals would content similar to t h a t of normal chondbe uniquely depleted. Second, despite the rites, and t h a t the water content of the pseudoisotopic behavior of U and K in Earth is equal to or somewhat less than t h a t ordinary igneous differentiation, there is of Mars. evidence t h a t extensive concentration of A SIMPLE EARTH-ANALOGOUSMARS K, but not of U, in a sulfide-rich outer DEGASSING MODEL Earth core and lower mantle is quite likely Given the above postulates, we first on geochemical grounds (e.g., see Lewis, 1971). Finally, concentration of K in a examine the consequences for our estimates

MARTIAN VOLATILES

of the Martian surface volatile inventory if the reported high atmospheric 4°Ar content proves to be correct. Projected inventories of outgassed H, C, N, Cl, S, and 4°Ar (which may be expressed as equivalent H,O, CO z, N 2, etc.) can be calculated for an utterly Earth-analogous Mars. An utterly Earth-analogous Mars is here defined as a hypothetical body which currently has in its surface and atmosphere an inventory of degassed volatiles identical to that of Earth, b u t scaled to Mars' smaller mass and surface area. A similar approach has been suggested by Owen (1974) for the interpretation of rare gas data to be obtained by the Viking landed spacecraft. Given the many probable differences between the bulk compositions and energy histories of Mars and the Earth discussed here, an Earth-analogous Mars model seems unlikely to be accurate; nevertheless, it serves as a useful starting point to which existing and forthcoming data on Mars can be referenced. Specifically, these hypothetical Martian surface volatile inventories are derived by scaling the Earth's " R u b e y excess volatile inventory" (Rubey, 1951) to the mass and surface area of Mars. The R u b e y inventory provides the best estimates of the quantities of HzO, COz, and other volatiles released from the Earth's interior. It is imperfect for our purposes, not only because of the possibly somewhat higher bulk content of HzO and other volatiles of Mars, b u t also because it does not take into account possible reincorporation of volatiles into the Earth's mantle. It is unfortunate that we cannot develop a model in which the Earth is ignored and only chondritic values are used since, as shown above, there are reasons for believing that the chemical kinship between Mars and normal chondrites is greater than between Mars and the Earth. The utterly Earth-like Mars model is a practical compromise dictated by our inability to estimate, a priori, the fractionation factors that will accompany partial transfer of the bulk volatile inventory to a planetary surface. The preceding discussion shows, however, that the K and volatile contents of the Earth, Mars, and ehondrites are unlikely

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to be widely disparate, which suggests that our forced choice of a scaled empirical comparison between Mars and the Earth at least provides a reasonable starting hypothesis. The values for 4°Ar, HzO, C02, N2, C1, and S in the Earth's " R u b e y " surface volatile inventory are 4.8 x 1019, 1.7 × l024, 9.1 × 1022,4.2 × 1021,3.0 × 1022, and2.2 × 10:lg, respectively (Rubey, 1951). Scaling these values to both Mars' smaller mass (0.107 that of the Earth) and surface area (0.28 that of Earth) we obtain for Mars the following volatile concentrations for the surface and atmosphere of an Earthanalogous Mars: (4°Ar)= 4g/cm 2, (HzO) = 1 . 3 × 10~g/cm 2, ( C 0 2 ) = 7 × 103g/cm 2, (Nz) = 3 × 102g/cm 2, (CI) = 2 × 103g/cm 2, and (S) = 2 × 102g/cm 2. Support for the usefulness of the utterly Earth-like Mars model for testing purposes is provided by 4°Ar content given above (4g/cm2). Note that this value, which agrees with that suggested by the Mars 6 observations (~5g/cm 2) within the uncertainty of the latter, is directly obtained by simply scaling the Earth's 4°Ar content to the mass and surface area of Mars. Thus, if the Mars 6 observations are correct, the model requires no arbitrary normalization whatever. Moreover, it is generally consistent with what is known of the bulk volatile contents of the Earth and Mars as discussed in the previous sections. However, the simple Earth-analogous model gets into somewhat deeper waters (so to speak) when a sink must be found for the vast quantities of H zO, CO z, and other volatiles listed above. Since these inventories are orders-of-magnitude greater than the possible Mars atmosphere and surface polar cap inventories of these volatiles, storage in a "hidden" reservoir is required. For reasons which will be outlined, I consider physical and chemical reincorporation into a deep regolith to be the most likely geochemical fate of most of the "missing" volatile inventory. However, I also present evidence that the amounts of chemically active volatiles that are "required" to be stored by the simple Earthanalogous degassing model may be falsely high.

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PHYSICAL STORAGE OF VOLATILES IN THE MARTIAN REGOLITH

To assess the efficacy of the regolith as a storage bin for Mars volatiles, we must first estimate the depth of t h a t regolith. Some theoretical considerations and some observations suggest t h a t a thick blanket of unconsolidated material (at least 1-2km deep) may exist on Mars (Chapman et al., 1969) analogous to the megaregolith in the nonmare regions on the Moon (Hartmann, 1973). Some observational evidence for exceedingly thick blankets of unconsolidated material has also recently been discussed by Malin (1975). R. S. Saunders of the J e t Propulsion Laboratory has pointed out to me that a case for such a thick megaregolith can also be based on the crater distribution as follows. The ratio of ejecta volume to crater radius may be estimated by the relationship (McGetchin et al., 1973) Vr = 0.88R 2"72. (1) Estimating, thus, the ejecta exclusively from specific heavily cratered regions [regions 1, 24, 29, and 32 of Jones (1974)] we find 5 × 106km 3 of material has been ejected over a region with an area of approximately 107km 2. Thus a regolith of ~500m thickness is to be expected based on these observations alone. Even these regions are significantly less cratered than the lunar highlands, so m a n y times this amount of fragmented material must have been ejected here and elsewhere on Mars. In addition, the basin component of the ejecta from Argyre, Isidis, and Hellas probably exceeds all the ejecta from the smaller craters. It does not seem unreasonable, therefore, to postulate an initial 2km thick layer of unconsolidated material on Mars. Although this may be considered to be analogous to the observed "megaregolith" on the Moon (Hartmann, 1973), the material may be much finer on Mars (Malin, 1975). The estimate of 2km is an upper limit to the present thickness of unconsolidated material, since it is safe to assume t h a t a considerable amount of igneous material has been injected into the regolith and has partly reconsolidated it regionally. However, in certain regions on Mars,

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there is morphological evidence t h a t original thick ( > l k m ) blankets of very fine material are still present (Malin, 1975). In this connection, note t h a t much of the chemical interaction between atmosphere and regolith material probably occurred before global igneous episodes, at least before the Tharsis volcanism. The latter, and other episodes of regional igneous activity, could have aided considerably in altering regolith material if large amounts of ground ice were already present in intimate mixture with comminuted igneous material (see below). The problem is now to postulate physical and chemical mechanisms whereby enormous amounts of surface volatiles (especially HzO ) could be stored in the Martian regolith even the postulated 2km megaregolith--without violating current observational data on Mars. The first problem to be attacked is the most difficult: namely, finding a sink for what amounts to over a kilometer of water. First let us consider the possible extent of storage of H~O as ice in the regolith. The possible occurrence of hard-frozen permafrost on Mars has been discussed by Lederberg and Sagan (1962), Leighton and Murray (1966), Wade and DeWys (1968), Anderson et al. (1967), and others. Also, morphological features resembling those characteristic of Alaskan thermokarst terrain have been identified in Mariner images of the Martian "fretted" terrain (Gatto and Anderson, 1975). One way to estimate the possible distribution of ground ice in the postulated thick layer of unconsolidated material on Mars is to assume that during the history of Mars, all ground ice in the regolith must have achieved physical equilibrium with the base of the atmosphere. Thus one may postulate t h a t the lower atmosphere has been a ready sink for H20 from (subliming) ground ice at any depth. An approximation of mean or subsurface temperatures on Mars as a function of latitude t h a t is good enough for present purposes can be obtained from the model of Leighton and Murray (1966). These are tabulated, as a function of latitude, in Table I. It seems reasonable to assume, in keeping with the assumption t h a t the

MARTIAN VOLATILES

ground ice is in equilibrium with the lower atmosphere, t h a t the base of the hardfrozen permafrost (where the pores are filled with H20 ice) will be located at the point where the thermal gradient within the regolith raises the subsurface temperature a few meters below the surface, which I equate to the mean annual temperature, 203°K. This is the temperature at which H20 ice is in equilibrium with a Pine of 2 × 10-3gm. This PH20, in turn, is the basal PH2O which would result if there were 10/~m of precipitable H20 well mixed in the lowermost 2km of the atmosphere. Both higher and lower values for the H20 content of the atmospheric column have been reported (e.g., see Tull, 1970) and 10~tm seems a good value for the present nominal model. The depth Z to the base of the ground ice may thus be estimated as Z ~ (T s -

TM)(K/Q),

(2)

where T s = the sublimation temperature, chosen as 203°K, T M = the mean annual temperature, from Table I, K = the soil

conductivity, and Q = the heat flow. This is, in essence, the equation for the flow of heat through a slab, wherein the thermal gradient is (T M - Ts)/Z. For the soil conductivity I use a value of 8 x 104ergcm -l see -1 °C-l , which is appropriate for a hardfrozen limonitic soil of high ice content (Clark, 1966). The greatest uncertainty is the thermal conductivity. Since the stability of a postulated hard permafrost layer is questioned here, it would seem appropriate to use the given value e l K . However, ice-free soils tend to have lower conductivities. This would result in a higher thermal gradient and a thinner ground ice lens. The heat flow can be estimated by assuming t h a t Mars, in bulk, has approximately the same K, U, and Th concentrations as chondrites (Lewis, 1972, 1974). The present heat production rate in Mars, based on this assumption, is 4.4 × 10~9erg/sec, which, for present purposes, I equate to the surface heat loss. The surface area of Mars is 1.4 × 10~Scm2, and therefore, Q % 30erg cm-2sec -~. Substitution in (2) yields Z = 4 . 2 x 1 0 5 - 2 . 6 × 103T M.

TABLE I CALCULATION OF GROUND ICE DISTRIBUTION" FOR AN EQUILIBRIUM MODELa Latitude

TM

0° 10°N/S 20°N/S 30°N/S 40°N/S 50°N/S 60°N 60°S 70°N 70°S 80°N 80°S 90°N 90°S

2.25 222 K 218 K 215 K 205K 185 K 170K 173K 155K 170K 146K 163 K 142K 160 K

Z (km) 0 0 0 0 0

0.4 0.8 0.7 1.2 0.8 1.4 1.0 1.5 1.0

° F o r each l a t i t u d e of Mars (column 1 ), a m e a n a n n u a l t e m p e r a t u r e is given (column 2) as derived from the model of Leighton and Murray (1966). I n c o l u m n B, d e p t h s to t h e base o f h a r d frozen p e r m a f r o s t are g i v e n in a c c o r d a n c e w i t h t h e r e l a t i o n s h i p Z = 4 . 2 × l0 s - 2 . 6 x l0 a T M, as d e r i v e d in t h e t e x t . 7

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(3)

Values for Z as a function of latitude obtained from (3) are also given in Table I, and a section showing the ground ice distribution t h a t would be generated by such a model is shown in Fig. 1. No ground ice is expected in equilibrium at latitudes less than 40 ° , but global ice lenses are expected beginning at +40 ° and thickening toward the poles, where a maximum thickness of 1-2km is predicted. The depth to the top of the "equilibrium" lenses is about the depth at which the maximum temperature is ~200K, which is generally < l m . The H20 content of some terrestrial permafrost exceeds 50%0 by volume (e.g., see Gatto and Anderson, 1975), although a global average of 25% might be more reasonable. On the basis of Fig. l, it therefore seems possible t h a t as much as 2 × 104g/cm 2 (or 1 × 104 for a 25% ice content in the lenses) of H20 could be stored as ground ice, which is only a factor of 6 to 12 less than that required to explain t h e " missing" H20.

More ground ice could well exist at latitudes <40 °, since all of Mars' subsurface to

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" ZONES WHEREHARD-FROZEN PERMAFROSTCOULD EXIST IN EQUILIBRIUM WITH PRESENTATMOSPHERE

FIe. I. The possible distribution of hard-frozen regolith permafrost on Mars is shown. Arguments given in the text suggest that a regolith of unconsolidated material as much as 2 km thick may exist on Mars. Within this regolith, hard-frozen permafrost eould occur anywhere above the 273 K isotherm which, in the figure, is that calculated on the basis of the surface mean annual temperature as a timetion of latitude (Table I), and an internal thermal gradient of 40°/km. The thermal gradient assumed is that for a Mars with bulk chondritic [4°K] and ~U] concentrations, and a regolith conductivity (K) of 8 × 104ergcm -i sec -1K -l (a reasonable value for hard-frozen permafrost). ]f the pores in the regolith are assumed to be largely unfilled, then K would be lower and the 273 K isotherm could occur at a more shallow position in the regolith. The existence of such permafrost at latitudes <+_40° depends on the ability of the overlying regolith to prevent preexisting ground ice from subliming into the atmosphere (see text). However, in the "equilibrium" zones, as shown, ground ice could exist even if physical equilibrium were achieved with the present lower atmosphere; the total amount of ground ice in the Martian regolith could be as high as 5 × 10~kma, or 4 × 104g/cm 2 averaged over the surface of Mars.

g r e a t d e p t h is (technically) " p e r m a f r o s t , " or p e r m a n e n t l y frozen g r o u n d ; b u t such g r o u n d ice could not exist in equilibrium with t h e a t m o s p h e r e . T h e existence of extensive g r o u n d ice a t latitudes ~<±40 ° m i g h t suggest t h a t the ice was f o r m e d u n d e r early conditions of surface t e m p e r a t u r e a n d a t m o s p h e r i c PH2O which differed considerably f r o m present conditions (e.g., see Fanale, 197 la), a n d t h e n was sealed off f r o m s u b s e q u e n t equilibration w i t h the a t m o s p h e r e . Alternatively, g r o u n d ice a t low latitudes m i g h t reflect the global or regional outgassing of H 2 0 as t h e result of relatively recent internal m a g m a t i c a c t i v i t y , t o g e t h e r with t h e inability of the resulting " t r a p p e d " ice to equilibrate with t h e a t m o s p h e r e . Smoluchowski (1968) and others h a v e discussed the ability of overlying regolith m a t e r i a l to p r e v e n t sublimation of an existing ice l a y e r o v e r geologic periods of time. F r o m his results it a p p e a r s plausible t h a t a sufficiently fine-grained, thick, a n d c o m p a c t e d regolith could allow an initially t h i c k u n d e r l y i n g ice layer to s u r v i v e for periods o f 109 y r or more. I n t h e e x t r e m e case, therefore, a hard-frozen 2 k m

regolith with an ice c o n t e n t of, say, 25% b y v o l u m e could possibly h a v e s u r v i v e d for the h i s t o r y of Mars a n d could contain as m u c h as 7 × 107km 3 (5 x 104g/cm 2) of ice. I n Fig. l, a lower d e p t h for hard-frozen p e r m a f r o s t a t latitudes < ± 4 0 ° is also suggested. This results from t h e f a c t t h a t t h e M a r t i a n internal t h e r m a l g r a d i e n t would raise the t e m p e r a t u r e to o v e r 2 7 3 K within the p o s t u l a t e d thick regolith, so t h a t only liquid w a t e r could exist a t g r e a t d e p t h in low latitudes. T h e position of this lower b o u n d as shown in Fig. l is based on t h e a s s u m p t i o n t h a t the c o n d u c t i v i t y is t h a t of h a r d frozen soil. I f the overlying regolith in low latitudes has a low m o i s t u r e c o n t e n t to a considerable depth, t h e n it will h a v e a lower c o n d u c t i v i t y t h a n assumed, resulting in a shallower position for the 2 7 3 K isot h e r m in the regolith. Regardless of w h e t h e r the equilibrium or nonequilibrium model is valid, a substantial fraction of the w a t e r t h a t m u s t be dispensed with in t h e " E a r t h - a n a l o g u o u s " or " h i g h A r " Mars degassing model could simply be stored as g r o u n d ice. L a b o r a t o r y a n d theoretical studies

MARTIAN VOLATILES

suggest that physical adsorption is likely to be an important process on Mars, particularly with regard to phenomena related to atmosphere-regolith exchange of volatiles (Fanale and Cannon, 1971, 1974; Flaser and Goody, 1976). However, it appears, from results of Fanale and Cannon (1974), that only about 3 × 103g/ cm 2 of HzO could be physically adsorbed on the free surfaces of even a 2km thick regolith, or only ~ 1 % of the "required" amount. Physical adsorption could, however, account for a significant percentage of the "missing" CO2, since, if equilibrium with the atmosphere were achieved at 200K, then I × 103g/cm 2 of CO2 would be contained in a 2km regolith (Fanale and Cannon, 1974). If the entire regolith were considered to be at a temperature of 160K, valid only for regions >±60 ° latitude, then as much as 4 × 103g/cm 2 of CO 2 might be adsorbed on a 2km regolith (Fanale and Cannon, 1974). In contrast, if the surface reservoir of frozen CO 2 postulated by Murray and Malin existed, it would be expected to contain only 15--90g/em z o f CO z, averaged over the Martian surface. Thus, the adsorbed CO z is expected to be less than the total stored CO z "required" by the simple Earth-analogous model, but possibly a substantial fraction of that amount. CHEMICALLY BOUND VOLATH,ES IN THE REGOLITH

Although the occurrence of large amounts of ground ice in the Martian regolith is of intrinsic interest, it would appear that, to satisfy a fully Earth-like Mars degassing model, it is necessary to postulate extensive occurrence of volatilecontaining mineral phases in the Martian regolith. This is in perfect harmony with most data concerning the mineralogy of the Martian regolith. Among the leading candidates for important volatile-containing phases are limonite (an assemblage of hydrated iron oxide and other minerals) and montmorillonite (a clay mineral). The Martian surface is ochre-colored. The visible spectrum of Mars bears little resemblance to spectra of unaltered basalt

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(Adams, 1968). Over the years, several workers have suggested that limonite was a principal constituent of the surface (e.g., see Dollfus, 1957; Sagan eta/., 1965). The visible spectrum of Mars can be well matched by ground basalt that has been altered by leaching and deposition by a dilute HN03 solution, yielding a brownochre residue (Adams, 1968). The approximate spectral match extends to the nearinfrared region (Fanale, unpublished data), but the mineralogical composition of the resulting crust has not been established. H u n t et al. (1973) tentatively identified montmorillonite as the dominant component of the dust during the planet-wide 1971 dust storm, on the basis of the midinfrared transmission spectrum of the dust as inferred from Mariner 9 I R I S results. Of course this identification, even if correct, would not establish montmorillonite as an important constituent of the regolith, since grain abrasion and dust levitation processes could greatly enrich montmorillonite in the dust (vs the bulk regolith). Clearly, what is needed is evidence pertaining to the bulk bound HzO content of the regolith. While spectral reflectance studies of the Martian surface are not ideally suited to this purpose, they still seem to be the best available source of information. McCord et al. (1971) showed that a spectrum obtained b y ratioing the visible and near-infrared spectrum of light areas of Mars to that of the dark areas measured through the same tellurie air path has deftnite residual 1.4 and 1.95/~mH20 bands in the light areas. This clearly means that HzO is present in the light-area topmost regolith in significant quantities, b u t does not preclude the presence of significant amounts of water in the dark-area topmost regolith as well. Some observations suggest that the water bands might be deeper but for "opaque masking"; Johnson and Fanale (1973) showed that a few percent of an anhydrous opaque component intimately mixed with the hydrated clay mineral montmorillonite can practically eliminate the otherwise exceedingly deep HzO bands normally exhibited in its spectrum. This reservation apparently does not apply as strongly to another recent

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F.P. FANALE

observation, that of the 3~m band as observed in high-altitude measurement of Mars' spectrum by Houck et al. (1973). In the spectrum of the Orgueil meteorite, the 3 ~m band is prominent despite eom plete suppression of the 1.4 and 1.95tzm bands by the process discussed by Johnson and Fanale (Salisbury, personal communication). It is obviously difficult to transform these observations into quantitative estimates of the amount of chemically bound H20 in the regolith. Houek et al. (1973) interpreted the depth of the 3~m band as suggesting a water content of between 0.3 and 3.0% H20 by weight in the regolith. This estimate involves several modeling assumptions and the measurement does not distinguish between physically and chemically bound H20. However, the result is consistent with reeent reflectance measurements by Huguenin et al. (1974a), who deduced a goethite content of about 5% in the topmost regolith of the bright areas from reflectance data. Moreover, laboratory studies have been reported which simulate the conditions of exposure of primary igneous minerals such as olivine and pyroxene. These studies indicate that under condition of uv illumination, atmospheric 02, adsorbed H20, and eolian abrasion, characteristic of the Martian surface, these ferrosilicates alter at rates of 10-1"5±l'Stzmyr-I (Huguenin, 1974). Therefore a nominal thickness of 100m of goethite and montmorillonite could be produced during Martian history. The upper limit would be 3km, which corresponds to the entire regolith postulated here. Results of current laboratory studies favor estimates near the upper limit (R. Huguenin, personal communication). The formula for goethite is Fe203.H20 and that for montmorillonite is usually given as (Mg, Ca)O-Al203.5SiO2.nH20. However, Anderson (1967) points out that n is less than 1.0 in equilibrium with ice at T < -30°C. The absolute upper limit on the amount of chemically bound water in even the 2 km regolith postulated here is 4 × 104g/cm z, corresponding to a loose regolith consisting of pure goethite. The observations of

Houck et al. suggest a lesser upper limit of ~ l x 104g/cm ~. However, this is not absolutely rigid since the results of Huguenin et al. suggest that the surface assemblage at any time is controlled by the precise conditions at the surface (uv illumination, etc.) and that intensive dehydroxylation reactions which take place at the very surface during the Martian day may be reversed at night and below the surface. As stated above, the laboratory measurements of weathering rates suggest that an entire deep regolith of weathered material may have been produced in geological time. Siever (1974), has pointed out that such surface weathering on Mars may be severely inhibited by the protection of unaltered materials by a thin coating of weathering products. Estimates of the abrasion rate on Mars (Sagan, 1973) suggest that it is at least 102 times the highest chemical weathering rate postulated by Huguenin (1974). Obviously, these estimates would have to be erroneously too high by the same factor for the removal of weathered rinds to be a rate-limiting factor. The presence of basalt in the dark areas, indicated b y the presence of a clearly identifiable 1.0/~m band in their reflectance spectrum (McCord et al., 1971), argue strongly against effective protection of exposed material by weathered crusts or rinds of weathering products. Hence, exposure does not appear to be a rate-limiting process for weathering. Note that there is also some evidence that the dark areas tend to have steep slopes. This, together with the spectral reflectance data, suggests a model in which material is continually eroded from dark, sloping areas of unaltered basaltic rocks and deposited in flatter, brighter areas which serve as sinks for weathering products. A quantitative model of the kinetics of weathering and transport of material on Mars is currently being developed by Huguenin (personal communication). By the same token, however, the presence of substantial amounts of exposed igneous rock on Mars indicates that much of the original 2km regolith on Mars postulated here may have been reconstructed. One of the side effects of such reconstruction may possibly be the altera-

MARTIAN VOLATILES

tion of preexisting unaltered regolith materials by liquid H20 generated at depth by the intrusion of igneous rocks into hard-frozen permafrost. The occurrence of such a process seems reasonable and may constitute another significant source of weathered material. Treatment of its effects is beyond the scope of the paper, but they are treated elsewhere (Soderblom and Wener, in preparation). In summary, the existence of ~1 × 104g/cm 2 of H20 as bound H20 in hydrated iron oxide and clays in the Martian regolith seems in good accord with the data and as much as 4 × 104 g/cm z could be present. The upper limit of 4 × 104g/cm 2 is given in Table II, where it is compared with the value of 1.3 × 105g/ cm 2 t h a t is required by the simple Earthanalogous model. Table II (column 4) lists the concentrations(in weight %) ofvolatiles in carbonates, nitrates, chlorides, and sulfates which would be "required" for an Earth-analogous model if chemical recombination in a well-mixed 2 km regolith were the only significant geochemical sink for the corresponding amounts of CO 2, N 2, Ci, and S as listed in column 2. The "required" concentrations of carbonates, nitrates, chlorides, and sulfates are 1.5, 0.3, 0.6, and 0.1%, respectively. The presence of such small quantities cannot be ruled out by currently available spectral reflectance data. The formation of carbonates could occur as the result of the interaction of adsorbed, interfacial, capillary, or liquid H20 with silicates. The formation of carbonates would be expected to occur at the surface of Mars even under present conditions (Huguenin, 1974b). The partial pressure of CO 2 on Mars is 10 times t h a t on Earth. Any liquid water in equilibrium with a Pco2 of 5 × 10 -3 bars would have a pH of 7.0, and rapid erosion of exposed silicates and production of carbonates would result. Even when equilibrium between the water and the deposited carbonates resulted, the pH would be only 7.3, as opposed to the pH of sea water, which is 8.2 (e.g., see Rubey, 1951). Thus, erosion and carbonate deposition would be expected to continue efficiently if any liquid water achieved equilibrium with the atmos-

187

phere or subsurface pore CO 2 on Mars. It is important to note, however, t h a t both carbonates and sulfates could be formed on Mars without the presence of liquid H20 at the surface or even in the subsurface. This means that measurement of small amounts of carbonates, hydrated iron oxides, and salts in the Martian regolith will not constitute a demonstration of the prior existence of liquid H20 on the surface or the subsurface of Mars, although, in the author's opinion, at least the latter may reasonably be expected on other grounds. Perhaps the most likely mechanism for the formation of nitrates on Mars is the formation of dilute solutions of nitric acid in capillary, interfacial, adsorbed, and ground water. Lewis and Randall (1923) pointed out that a 0.1 molal HNO3 solution in water would be in equilibrium with the PN, and Po2 at the base of Earth's atmosphere. They pointed out that if a suitable catalyst were involved, and there were no biological recycling, all the Earth's atmospheric 02 could quite possibly be removed as HNO 3 in the oceans and, of course, this would result in nitrate precipitation. However, Mars currently has both a much lower PN, and Po~ than the Earth. Because the N2:CO 2 ratio for the Martian atmosphere is probably lower than that in the Earth's " R u b e y " volatile inventory (Rubey, 1951), Brinkman (1971) and McElroy (1972) suggested that escape of nitrogen by nonthermal mechanisms may have been an important process on Mars. In view of the fact t h a t the high 4°Ar abundance, if correct, would now seem to demand escape of about 3 x 102g/cm 2 of N 2, and also some depletion mechanism for larger quantities of other constituents, it seems appropriate to look instead to chemical recombination processes as the major mechanism for depletion. Despite the fact t h a t PN~ and Po~ are currently very low on Mars, the nitrogen pressure might have been buffered at higher levels in the Martian past by nitrate formation. The availability of rock surfaces exposed to intensive uv illumination might allow more favorable catalysis than on Earth. The possibility t h a t high-energy cosmic rays may play a significant role in breaking down the N z

ground ice goethite/elay physically adsorbed C02 and carbonates nitrates

chlorides

sulfates

1.3 x 105

7 x 103

3 × 102

2 x 10 3

2 x 102

CO2

N2

CI

S

Possible sink

H20

Volatile

A m o u n t to be disposed of in Earth-analogous Mars model (g/cm 2)

~2 x 102 (0.04% of regolith)

~ 2 x 10 3 (0.6% of regolith)

~3 x 102 (0. ! % of regolith)

~7 x 103 (~<1.5% of regolith)

~5 x 104 (max) ~ 4 x 10" (max)

A m o u n t in sink (g/cm 2) ( w t % assuming 2kin regolith)

M a x i m u m plausible sink is similar to t h a t roquired b y earth-analogous model Occurrence of b o t h forms expected ; listed concentrations allowed within observational constraints P r o d u c t i o n mechanisms controversial, but concentration allowed within observational constraints P r o d u c t i o n e x p e c t e d ; concentration allowed within observational constraints Production expected ; concentration allowed within observational constraints

C o m m e n t on possible sinks

CAPACITY OF POSSIBLE SINKS FOR VOLATILES IN THE MARTIAN REOOLITH CONTRAST~ID WITH "REQUIREMENTS" OF AN EARTH-ANALOGOUS DEGASSlNO MODEL

TABLE II

oo oo

MARTIAN VOLATILES

molecule has been suggested to the author by Y. L. Yung. Finally, there is no paucity of oxygen atoms available for this process. McElroy (1972) points out that even at the present escape rate, the number of hydrogen atoms that would have escaped from Mars in geologic time is ~10 zs cm -z. Therefore, a comparable number of oxygen atoms would be produced by this process alone. The Earth-analogous model requires ~2 × 103g/cm 2 of oxygen for carbonate formation and ~1 × 102g/cm 2 for nitrate formation (Table II). The amount of oxygen freed by hydrogen escape is about 6 × 10Zg/cm 2, nearly equal to the amount needed for carbonate and nitrate formation. Moreover, although McElroy (1972) postulates that the freed Oz also escapes, Huguenin (1976) has argued that the regolith may represent a massive sink for the excess O z. This suggests that surface catalysis of oxidation reactions could play an important role in accounting for the imbalance between H and O production from H20 dissociation and hydrogen exospheric escape. Therefore, although the precise mechanism for nitrate formation on Mars cannot be specified or the presence of nitrates authenticated by observations, there appear to be plausible mechanisms for substantial removal of N z from the atmosphere and for formation of regolith nitrates. An alternative view has recently been expressed to the author by R. Prinn, who believes that there are no reasonable mechanisms whereby substantial quantities of nitrates could be manufactured on Mars and that the paucity of nitrogen in the Martian atmosphere is better attributed to an initial bulk deficiency of Mars in nitrog e n - - a hypothesis based on cosmochemical arguments (Prinn and Lewis, 1976). Formation of salts (e.g., sulfates and chlorides) by contact between all forms of water and silicates in the Martian regolith is also a likely process, which does not require large stable bodies of surface H20. Formation of salts occurred in the parent body of the Orgueil meteorite, presumably a small, cold, airless body, to such an extent that the meteorite consists of 15% epsomite: MgS04" 7HzO (DuFresne and Anders, 1962). DuFresne and Anders postu-

189

lated the formation of epsomite by liquid HzO under permafrost cover. A large number of other salts, including carbonates, have been identified in other carbonaceous chondrites (e.g., see Mason, 1962). Also, several lines of observational evidence and theoretical arguments suggest the presence of salts on the surface of Io (Fanale et al., 1974), the innermost Galilean satellite of Jupiter, which is a lunar-sized body with a surface pressure of <10 -8 bars and a surface temperature of ~140K. Salt formation does not appear to require large standing bodies of liquid water on the surface of objects. Mechanisms for salt formation on Mars and the role of salt formation in Martian weathering processes have also been discussed by Malin (1974). In summary, it has been shown that the formation of carbonates, nitrates, sulfates, and other salts in the Martian regolith could plausibly be postulated to occur even under present Martian surface and nearsurface conditions. Moreover, the concentrations of these compounds that would have to be postulated in a well-stirred 2km regolith in order for it to serve as the total sink for the postulated amounts of CO z, N z, S, and Cl released in the simple Earth-analogous model are low enough (<2%) that their presence would not be detectable from visible and near-infrared spectral observations. The maximum amount of chemically bound HzO that could be present is ~4 × 104g/cm 2, which is similar to the maximum amount of water that could plausibly be present as ground ice (~5 × 104g/cm2). This analysis does not consider the possible variety of conditions that may have prevailed during early Martian history. The suggestion has been made that Mars possessed a substantial reducing atmosphere for a small portion of its earliest history (Fanale, 1971a). While weathering conditions under such circumstances are even more difficult to describe quantitatively than those at present, the possible occurrence of substantial atmosphere--regolith chemical interaction during such an epoch should not be overlooked. Still, the water-storage problem remains as a possibly serious problem for the Earth-analogous Mars model. Again, it

190

F.P. FANALE

should be emphasized that the model for the occurrence of large quantities of water ice and the arguments for the occurrence of other volatile-containing compounds on Mars are of great interest in themselves even if they fail to validate a strictly Earthanalogous degassing model on a quantitative basis or if the 4°Ar content of the Martian atmosphere should prove to be much less than 5g/cm 2.

EXOSPHERIC ESCAPE AS A ~¢IASSIVE VOLATILE SINK

The possible role that volatile sinks other than regolith storage may have played in Martian atmospheric history should be considered--especially exospheric escape (McElroy, 1972). Estimates of mass loss from the Martian atmosphere as the result of solar wind interactions range from 3 to 8 g/sec (Coultier et al., 1974; see also Michel, 1971). The higher value corresponds to <1 g/cm 2 in geological time and is clearly negligible in the context of the "high At" or Earth-analogous models. More intensive solar wind sweeping during early solarsystem history is frequently invoked to explain the absence of any "solar" component in the Earth's atmosphere (Brown, 1957). Even recently, Siever (1974) categorically stated " . . . volatile outgassed from the Earth's interior contributed the secondary atmosphere after the earlier primitive atmosphere was swept away by the solar wind." But the fact that the Earth's atmosphere retained the full inventory of nonradiogenic rare gas corresponding to a chondritie Earth, together with arguments based on rare gas elemental and isotopic composition, argues strongly against the effectiveness of such a process, at least on Earth (Fanale, 1971b). Therefore, unless and until quantitative models offering the possibility of massive nonthermal escape from Mars' atmosphere are forthcoming, it seems more reasonable to consider regolith storage as the dominant sink for degassed Martian volatile constituents of mass >4 for those models incorporating extensive Martian degassing.

EFFECT OF THE TIME HISTORY OF J~)EGASSII~G ON THE "SURFACE" VOLATILE INVENTORY

The degassing history of planetary objects is largely dependent on planetary energetics. There are a variety of planetary energy sources, each of which could be dominant, depending on the conditions under which the object formed: Accretionary or impact heating, compression, decay of radio-nuclides, exchange of high and low density phases in the gravitational field, and possibly induction heating all could play the major role in supplying the energy for degassing. Each of these processes has its own time history. For example, heat from accretion of planetesimals can only be "buried" in a planet at the very outset of its history, because only rapid accretion (in periods of time ,~l m.y.) will allow a substantial percentage of the gravitational energy to be buried before it is reradiated to space from the incipient planet's surface (Hanks and Anderson, 1969). A similar constraint exists on surface-imposed induction heating, since this must be driven by the intense solar wind associated with a T Tauri phase of the early sun (Sonnett et al., 1970). In contrast, energy from decay of the long-lived nuclides such as 4°K, 235U, 238U, and 232Th is simply not available in sufficient quantities to cause planetary differentiation in an initially cold and homogeneous object until many hundreds of millions of years have elapsed. Thus, in principle at least, a bimodal thermal history, as reflected in the time-temperature history of planetary interiors, is possible at certain depths. If those depths represent a potential source region for generation of surface magmas, then the surface expression of volcanism and, hence, degassing, may be chronologically bimodal as well. There are several widespread misconceptions concerning planetary degassing that must be dealt with before an attempt is made to interpret the tentatively observed 4°Ar content of the Martian atmosphere and indications of volatiles in the regolith in terms of Martian degassing history. First, although rapid accretion results in the burial of planetary material at elevated

MARTIAN VOLATILES

temperatures, it is not necessary in order to assure a significant episode of catastrophic early degassing. This is an important consideration for Mars, since Weidenschilling (1974) has recently calculated that it may have taken Mars up to 200m.y. or longer to accrete. Fanale (1971b) has pointed out, on the basis of the experimental work of Gault and Heitowit (1963), that the individual increments added to at least the outer portion of Mars must have experienced considerable degassing and melting, even if accretion was not rapid. This is so because impacts into comminuted materials at velocities greater than about 5km/sec can result in projectile incandescence and sintering of a target mass greater than the projectile mass. Additionally, it should be noted that each bit of added material experiences a history both as a projectile and, later, as a target. Even when buried, heating continues as the result of cumulative shear resulting from seismic waves from larger impacts. Ablation in an incipient atmosphere could also play a role in degassing. From these considerations and meteorite degassing studies, Fanale (1971a) concluded that it would be difficult for Mars to have accreted without development of a reducing atmosphere at least two to three orders of magnitude more massive than the total volatile inventory represented in its present atmosphere and caps, and that most of that initial volatile inventory was subsequently physically or chemically reincorporated within the Martian regolith. In the case of the Earth, the situation is more clear-cut. Fanale (1971b) has presented numerous arguments, based on the Pb isotopic history of Earth, rare gas studies, and other lines of evidence, which indicate that the Earth experienced very intensive outgassing as the direct result of accretion. It should be noted, however, that the accretion energy per gram is much greater for the Earth than for Mars (Hanks and Anderson, 1969), that the accretion interval was probably much shorter for the Earth than for Mars (Weidenschilling, 1974), and that the onset of differentiation in the Earth could have been partly triggered b y compression, regardless of the accretion

191

interval (J. S. Lewis, personal communication). Thus, although there are theoretical grounds for believing that at least the outer portion of the accreting Mars was subjected to intensive degassing on accretion, there are also grounds for believing that the Earth experienced more intensive global degassing on accretion. These possibly great differences must be kept in mind when attempting to evaluate the strictly Earth-analogous Mars degassing model. Another widespread misconception is that the presence of substantial quantities of 4°Ar in a planetary atmosphere constitutes evidence against catastrophic initial degassing. It is true that a high 4°Ar content is a direct indication of degassing during later planetary history as distinguished from degassing that occurred at the outset of planetary history, since virtually no 4°Ar existed in the interior at the outset. However, the occurrence of subsequent, continuing degassing of 4°Ar does not in itself preclude the occurrence of catastrophic early degassing (or even the supply of most of a planet's surface volatile inventory during its accretion), as demonstrated for the case of the Earth (Fanale, 1971b). Moreover, it should be obvious that, all other things being equal, subsequent or continuous degassing could only be aided by the same energy history which would cause catastrophic initial differentiation and degassing because the latter would both raise initial temperatures at each depth in the planet and generally concentrate radionuclides (at least U and Th) in its outer portions where surface magmatism is instigated. Another erroneous procedure is to compare directly the degassing efficiency for 4°Ar to that for H20 and other volatiles without taking into account this fundamental difference: 4eAr, 4He, and other radiogenic gases have a "generation function" (the exponential decay of 4°K) on which the transfer function (degassing of the interior) is superimposed, whereas HzO and other volatiles do not. This consideration may prove to be important if the Martian atmosphere should be found to contain ~5g/cm 2 of 4°Ar while at the same

192

F . P . FANALE

time the Mars regolith is found to lack the enormous i n v e n t o r y of stored volatiles required b y an Earth-analogous model. I t is t r u e t h a t such a difficulty could be a t t r i b u t e d simply to bulk compositional differences between the two planets. However, this approach can be rejected for the following reason. I n the section on bulk composition of the E a r t h and Mars, it was shown t h a t the most widely accepted nebula condensation model and, in fact, almost a n y reasonable model for thermochemical fractionation in the nebula, would predict t h a t Mars should have at least as high a bulk H 2 0 : K ratio as the E a r t h (e.g., see Lewis, 1972, 1974). This would obviously amplify the difficulty mentioned above, not diminish it. To illustrate the difference between the degassing of 4°Ar and other volatile constituents, let us first consider the continuous or uniformitarian degassing of a planet. The simplest possible way to represent such degassing is to assume the specific intensity of degassing is represented b y a constant, a, in which case the p e r t i n e n t equations for H 2 0 and 4°Ar (Turekian, 1964) would be

d[HeO]t/dt =

and

-~,~o[H20]t,

[He0], = [H20]l e -~t,

(4) (5)

where [HzO]t equals the a m o u n t of water in the planet's interior at time t, and [H20]l equals the initial, or bulk, H 2 0 concentration in the planet. However, owing to the fact t h a t the 4°At m u s t be generated b y 4°K d e c a y to be available for degassing, the corresponding equations for a°Ar are

d[4°Ar],/dt= A[KJ~e-xt -

~A~[4°Ar] (6)

if [K], = [4°K],[R/(1 + R)],

(7)

where 4°K l = initial bulk [4°K] of planet; R = the " b r a n c h i n g " ratio which gives the relative probability of the (Argon-producing} electron capture mode of decay to the fl- mode of dec%v, which produces 4°Ca; and A is the d e c a y constant for 4°K. Thus the equivalent of (5) for 4°Ar is [4°Ar], = ,~[K],(~A,

-

,~)-' (e -~t' - e-~'"). (8)

f

f

thrill / / / /

/ /11///

76

f E

~RS~" 4o

// /OTAL 4OArGENERATION /- / IN THEthRTHANDI~RS /// IF BOTHHAVEC ~ I T I C O~

I/

//

AMOUNTOF4OAr IN I EARTH'SATMOSPHERE 10

,, ,) ATMOSPHERE(TENTATIVE) I

TIME, b.y.

Fro. 2. Generation of 4°Ar in a hypothetical chondritic Earth (left ordinate) and Mars (right ordinate) as a function of time. The amount of argon plotted on each ordinate is the total amount of 4eAr which would be generated in each body (regardless of whether it was degassed or re* mained in the interior). On each curve is indicated the amount of argon currently in the atmosphere of each planet. The amount of argon postulated for the Martian atmosphere is only tentatively assigned based on observations and arguments cited in the text.

Figure 2 shows the buildup of total 4°At (interior plus atmosphere) with time for a h y p o t h e t i c a l chondritic E a r t h and Mars. Also shown are the a m o u n t of 4°Ar k n o w n to be in the E a r t h ' s a t m o s p h e r e and the (maximum) a m o u n t t h o u g h t to be present in the atmosphere of Mars. T h r e e points m a y be drawn from this figure: (1) The percentage of total p l a n e t a r y 4°Ar t h a t is in the a t m o s p h e r e of b o t h planets is, in fact, r a t h e r low, if the preceding discussion of their bulk compositions is essentially correct. I n o t h e r words, regardless of w h e t h e r the Mars 6 observations and aeronomic a r g u m e n t s which suggest the possible presence of a substantial a m o u n t of 4°Ar in the Martian atmosphere are correct, there are good theoretical grounds for postulating 4°Ar as at least an

MARTIAN VOLATILES 1.0 R A ~ IN OCEANIC ~BASALTSiWO~E.

II Z

RANG[ IN NO'AM. ~CHONORI~$ ~11!. 1 ~ I z

0.1

I

P,.

~

MOOEL h ONLy CONTINUGJS, l~ OROqR DEGASSINGOF HzO ~ EARTH OF 90%OF EARTH'SSURFACEHzO, FOU.OW~O BY 1StORDERD~GASSING ,FOFAI~I~,Ngi1N~tllfa '

o

I e 10"13

H

IF ~0% OF EARTH'S -SURFACEX20 SUPPLIED .ACCRETION i 1 x 10"12

.HzO

] I x 10"ll

a40Ar

@.<~, II I X 10"lO

Fzc,. 3. Curve 1 (Model I) gives t h e relationship between the bulk H 2 0 c o n t e n t of the E a r t h a n d the degassing c o n s t a n t [= in E q n . (4)] t h a t m u s t be postulated for the E a r t h ' s H 2 0 for the h y p o t h e t i c a l case where (1) all the E a r t h ' s surface H 2 0 m u s t be supplied by continuous, first-order degassing t h r o u g h o u t E a r t h history (see text), and (2) an a m o u n t of H 2 0 equal to the t o t a l actual i n v e n t o r y of degassed H 2 0 on E a r t h (Rubey, 1951) m u s t be supplied o v e r a period of 4.5 x 109 AE. This a m o u n t is 1.7 x 1024gH20. C u r v e 2 (Model II) also shows the relationship between t h e bulk HzO and a, the degassing c o n s t a n t for E a r t h ' s surface water, b u t for a different set of assumptions. As in model I, it is required (I) t h a t the E a r t h ' s surface H 2 0 inv e n t o r y (1.7 × 1024gH20) be supplied in 4.SAE. H o w e v e r , in t h e case of Model II, it is also ass u m e d t h a t (2) the solid material t h a t formed the E a r t h condensed and accreted to form the earth within a few million years, and t h a t degassing of 9 0 % of t h e current E a r t h ' s surface H 2 0 invent o r y was supplied at t h a t time, and t h a t (3) the remaining 10% was supplied by continuous, first-order degassing. The general class of models represented by Model I I is vastly preferable to Model I. One reason is that, as shown by point A on the figure, e v e n if it is p o s t u l a t e d t h a t HzO is degassed as efficiently as "OAr during the continuous phase of degassing, it is still necessary to postulate t h a t the E a r t h is as rich in HzO as the m o s t H20-rich normal chondrites. N e i t h e r of these conditions is acceptable (see text). Moreover, models similar to Model I I are more in keeping with o t h e r evidence concerning the hist o r y of E a r t h degassing (Fanale, 1971b; S c h w a r t z m a n , 1973a, b; Ozima, 1975).

important component. This is why almost three decades ago, Brown (1949) stated, on the basis of theoretical arguments similar to those outlined above, "In the

193

case of Mars, it might well be that Argon is the major atmospheric constituent." (2) Figure 2 shows that, in principle, all the 40Ar in the atmospheres of the Earth and Mars could have been generated in the first 200m.y. of their histories if both were chondritic and formed 4.5AE ago. (3) Ifdegassing e r a chondritic Earth and Mars were modeled by first-order degassing of a chondritic object (Turekian, 1964) and if the higher values of ~5g/cm 2 for 4°Ar in the Martian atmosphere are correct, then it may be shown by solving (8) that aAr = 2.8 × 10-11yr -I (see Turekian, 1964) and ~ r % 3 . 1 × 10-11yr-l; i.e., it would indicate that the degassing efficiency of Mars and the Earth for Argon are exceedingly similar. EARTH DEGASSINO H I S T O R Y : IMPLICATIONS FOR MARS

The question is: Taking these factors into consideration, how can the Ar degassing efficiency in either object be related to the degassing efficiency for HzO? Obviously, the problem is complicated b y two considerations: First, the efficiency of degassing (=) of a rare gas and a chemically active compound like HzO could be exceedingly different (Larimer, 1971 ). Second, as pointed out above, Argon has a generation function, whereas HzO does not. I propose that, if Mars should prove to have a "surface" 4°Ar:HzO higher than that of Earth, then this would not reasonably be attributable to differences in bulk composition for reasons given earlier, b u t it can be explained simply by taking into account the two considerations just mentioned. To demonstrate the importance of the 4°Ar generation function in producing differences in degassing history between HzO and 4°Ar, let us consider the case of the Earth. Figure 3, Curve I, shows a plot of the weight percent of HzO in the bulk Earth which one needs to postulate in order for an amount of H20 equal to the mass of the Earth's surface HzO (oceans) to be degassed in 4.Sb.y. given any value for the degassing constant, =. Obviously, this relationship holds only for degassing

194

F. P, FANALE

histories which can be adequately modeled b y the (uniformitarian) first-order degassing relationship discussed above. The difficulty with such a model of Earth degassing soon becomes apparent. In continuous degassing, one would normally assume that aH2o < aAr, there is no reason why H20, which is so soluble in silicate melts and diffuses so much more slowly through crystal lattices than a rare gas does, should be more efficiently degassed than Ar. Point A on Fig. 3 shows the value of the bulk H 2 0 content the Earth would have to possess to degas its oceans in 4.5b.y. by continuous degassing alone if aH2o = aAr. Note that this set of conditions requires that the Earth possess, in bulk, a higher H 2 0 content than the average normal chondrite--too high an H20 content, according to the Lewis (1972, ]974) models--even if the Earth had degassed water as efficiently as Ar during its history. Since this latter condition is also unacceptable, it means that continuous degassing models cannot simultaneously allow a reasonable ratio of the degassing constants (aH2O:aAr) and a reasonable bulk H 2 0 content for the Earth. For example, if H 2 0 is degassed 10% as efficiently as Ar, a more reasonable possibility than equal degassing efficiencies, Curve I indicates that the Earth would have to possess about 2% H20 by weight, or at least ten times as much H20 as normal ehondrites! The above difficulty has also been discussed by Larimer (1971). Larimer assumed that "the fraction of H 2 0 released (from the Earth during its degassing history) is unlikely to be greater than that of Ar, since Ar, unlike H20, does not take part in mineral formation." Larimer used this assumption to derive an upper limit on the K content of the Earth and on this basis he suggested that the Earth may be significantly depleted in K and other alkali metals relative to chondrites. As can be seen in Fig. 3, Curve I, Larimer's postulate immediately solves the dilemma, at least in an analytical sense. A lower postulated K content for the Earth would mean a higher postulated ~ for Ar, given the amount of 4°Ar in the Earth's atmosphere. In turn, this would move point A to the right on Curve I and would

require an equal or lower bulk H20 for the Earth than for chondrites. Although the above solution "works" analytically, I find it an extremely awkward one and, in fact, unacceptable from a eosmochemicai point of view. There has not been any physical mechanism suggested in any model of planetary formation which would uniquely deplete the Earth, in bulk, in semivolatiles, such as the alkali metals, relative to chondrites while at the same time leaving the Earth with close to chondritic bulk concentrations of elements both more refractory and more volatile (e.g. the nonradiogenic rare gases) than the alkali metals. Since the latter set of conditions is supported by a wide range of geochemical observations (e.g. Lewis, 1972, 1974; Fanale, 197lb; Larimer, 1971), I reject the former (unique alkali metal depletion) as being eosmochemically unrealistic. Alternatively, I propose that, even though it is unreasonable to postulate more efficient degassing of H20 than Ar during the "continuous" phase of Earth degassing history, extensive catastrophic degassing occurred at the very outset of Earth history at a time when the Earth, or Earthforming material, contained relatively little 4°Ar. There is abundant evidence that the Earth underwent an intensive and catastrophic episode of degassing when it formed (Fanale, 1971b). Recently, this conclusion has been confirmed by quantitative models based on the abundances of 4°Ar and 36Ar in the Earth's atmosphere as well as other geochemical data (Schwartzman, 1973a, 1973b; Ozima, 1975). Although all the evidence for the importance of catastrophic early degassing of the Earth will not be repeated here, it should be noted that the Pb isotopic evolution of the Earth (Patterson and Tatsumoto, 1964) implies the concentration of at least half of the Earth's U in the outermost 100km or so of the Earth during the first 100m.y. of Earth history. Accumulation of heat from decay of the long-lived nuclides ~35U, 23sU, 232Th, and 4°K in the first 100m.y. could not supply nearly enough heat for such a process, which further pinpoints the time of the differentiation at the time of accretion, when

MARTIAN VOLATILES

the maximum amount of gravitational energy was available. Other sources of energy such as might be derived from the early Sun or from adiabatic compression in the Earth would also be associated with the earliest phases of Earth history. As pointed out by Fanale (1971 b), any process that would cause such efficient migration of an element like U must have involved thorough melting of the Earth. This suggests t h a t most, if not all, the water in the Earth's oceans was first supplied at the outset of Earth history. I f we merely postulate t h a t 90% or more the HzO in the Earth's oceans was so supplied, and t h a t 10°/0 or less was subsequently supplied by a process t h a t can be adequately described by the first-order degassing model, then the problem posed by Curve I in Fig. 3 may be eliminated. Curve II on Fig. 3 shows the relationship between ~H20 and the bulk H20 content of the Earth which would prevail if 90% of the water in the Earth's oceans was catastrophically supplied at the outset of Earth history. Curve II shows that even with a continuous degassing constant for HzO t h a t is from ×3 to x l 0 lower than t h a t for Ar, the Earth's oceans can still be supplied without postulating an unreasonably high bulk HzO content for the Earth ; i.e., a bulk HzO content greater than t h a t of normal chondrites. Along Curve II a "zone of interest" is shown which I consider to contain the preferred combinations of bulk HzO for the Earth and a~ "°. Values for a bulk H20 content for the Earth that are somewhat, but not drastically, lower than those for chondrites are "preferred" since Lewis' (1972) model predicts t h a t normal chondrites themselves acereted at only 1.2AU. Neither the "zone of interest" nor Curve II is to be considered as a unique or a restrictive solution which precludes all other possible Earth degassing models. For example, another set of solutions could be postulated which correspond to 95% catastrophic initial supply of the Earth's oceans, and so on. However, unless the evidence and arguments put forth in this paper are seriously in error, solutions lying close to Curve I are definitely unacceptable.

195

MARS DEGASSING. MODELS

The importance of the preceding for Mars degassing models is that, even if the bulk 4°K:HzO ratios of Mars and Earth were similar, greatly different surface 4°Ar:HzO ratios in the surface volatiles could result exclusively from differences in the time history of degassing. With this in mind, it may now be shown that reasonable degassing models may be constructed for Mars which reconcile even the highest reported value for 4°Ar (~5g/em z) with observational and theoretical constraints on the amounts of regolith-stored HzO. Specific inferences concerning differences between the time histories or relative intensities of Earth and Mars degassing are not yet justified, owing to the great uncertainty in both the 4°Ar content of the Martian atmosphere and the HzO content of the Martian regolith, not to mention our lack of certainty concerning bulk compositional differences between the two objects. Nonetheless, it is instructive to represent qualitatively how differences in the time history and intensity of degassing between Mars and the Earth might be reflected in the Martian atmospheric 4°Ar content and surface, or regolith, H20 content. This is shown in Fig. 4. Also plotted is the (tentatively assigned) 4°Ar content of the Martian atmosphere as deduced from Mars 6 observations. On the ordinate is plotted the value for the total HzO content of the Martian "megaregolith," including both ground ice and hydrated minerals, which may be reasonably assigned on the basis of the preceding discussion without exceeding observational or theoretical constraints. From Fig. 4 it is clear that even the high value of the Mars atmospheric 4°Ar content which has been inferred from the Mars 6 data can be reconciled with the possible H:O content of the Martian regolith. This is true even for the Earth-analogous Mars degassing model. I f the 4°Ar content is determined to be as high as suggested by the Mars 6 observations, but evidence is also found t h a t the Mars surface volatile inventory cannot be as high as suggested here, then this would suggest generally "later" degassing for Mars, compared

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F1o. 4. The 4°Ar content of the Martian atmosphere and the projected H20 content of the Martian regolith are shown as qualitative functions of the time history and intensity of Martian degasaing (as compared with the time history and intensity of Earth degassing). The Mars symbol indicates the anticipated values for 4°Ar and H20 for a hypothetical Mars which has experienced a degassing history exactly analogous to that of Earth. A lower HzO:4°Ar for Mars' surface volatiles relative to the Earth's surface volatile inventory (Rubey, 195I) could result from either. (1) a much longer or more delayed period of accretion for Mars than for Earth, or (2) a greater role of continual or later degaasing (relative to catastrophic aecretional degasaing) for Mars compared to Earth. The 4°Ar content of the Martian atmosphere which has been tentatively inferred from Mars 6 results (Moroz, 1974) is also plotted, as is an estimate of the H20 content of the Mars' regolith which may be reasonably postulated based on arguments presented in the text. with the E a r t h . The alternative possibility - - t h a t such a set of observations could be explained b y a difference in the bulk 4 ° K : H 2 0 ratios of Mars and E a r t h - - m a y be dismissed, on the basis of arguments given earlier. " L a t e r degassing," however, m a y have either of the following meanings. (1) Mars experienced its accretion as a p l a n e t a r y object, and concurrent degassing, after a greater "waiting period," subsequent to the condensation of the dust from which it e v e n t u a l l y formed, t h a n did the Earth. (2) F o r Mars, accretional degassing was less i m p o r t a n t relative to continuing later degassing t h a n for the E a r t h . The effects t h a t the preceding two possible differences in the time history of degassing of the two planets could have

on their surface 4°Ar: HzO ratios are also illustrated b y Figs. 2 and 3, respectively. Weidensehilling (1974) has suggested on theoretical grounds t h a t the ratio of t h e accretion intervals of the E a r t h and Mars m a y have been a b o u t 1 : 30. Absolute values for the accretion intervals of 10 and 300m.y. for the two objects would not be inconsistent with Weidenschilling's analysis. I n Fig. 2 it m a y be seen t h a t such a difference in accretion chronology could result in the E a r t h ' s a t m o s p h e r e receiving a v e r y small contribution of 4°Ar during the accretion process, while t h a t of Mars could receive most of its 4°Ar during accretion. Figure 3 illustrates the relationship between the time history of degassing, the relative ease of 4°Ar and HzO degassing, and the resulting surface 4°At: HzO ratio for E a r t h . I t m a y be seen that, if it were not recognized t h a t the E a r t h experienced an episode of catastrophic initial degassing, t h e n bulk compositional constraints (shown on the ordinate o f Fig. 3) would lead to the conclusion t h a t the E a r t h either degassed Ar and HzO with similar ettlciency t h r o u g h o u t its history, or conrains more HzO t h a n chondrites. N e i t h e r conclusion is reasonable. F u r t h e r , the observed E a r t h surface H 2 0 : 4°Ar is higher t h a n t h a t which should be anticipated for Mars unless Mars experienced a degassing history fully as d o m i n a t e d b y catastrophic initial degassing as the Earth's. Although a definitive m e a s u r e m e n t o f the 4°Ar c o n t e n t of the Martian a t m o s p h e r e is likely to result from the forthcoming Viking 1976 landed mass spectrometer, observations which allow direct and reliable assessment of the total a m o u n t of H zO and other volatiles which have been incorp o r a t e d into the regolith column are unlikely to be available for some time. However, some conclusions concerning the a p p r o x i m a t e i n v e n t o r y of stored chemically active volatiles in the regolith m a y be inferred indirectly from the 36Ar content o f the atmosphere. The n e x t section discusses Viking results which m a y be highly indicative of Mars's degassing history, and especially the usefulness of the 36Ar:COz and 4°Ar:COz ratios in simultaneously suggesting b o t h the overall intensity and

~IARTIAN VOLATILES

the time history of Mars degassing. It should be borne in mind t h a t the preceding qualitative relationships between the time history of degassing and the H20:4°Ar ratio apply as well to 36Ar: 4°Ar in the sense that, like H20, 36Ar is not generated by radioactive decay and hence is present from the outset of Martian history. Two outstanding differences between 36Ar and H20, however, are t h a t the total inventory of degassed a6Ar is relatively easy to determine and t h a t 36Ar, unlike H20 , is likely to be degassed about as easily as 4°Ar. FUTURE TESTING OF MARS VOLATILE HISTORY MODELS

It has been shown that the 3°Ar:4°Ar ratio together with other evidence may serve as a constraint on models of Earth degassing history (Fanale, 1971b; Ozima, 1975). Owen (1974) has pointed out that a similar significance would attach to such a measurement on Mars. In this issue Owen (1976) and Levine (1976) offer further discussion of the interpretation of the 4°Ar abundance in terms of Martian degassing history. To the degree t h a t the bulk 36Ar content of Mars can be estimated, the 36Ar content or 36Ar:CO2 mixing ratio would indeed place some constraints on the total, or time-integrated amount of Martian degassing. Moreover, since 3~Ar shares the generation function of H20 and the other major volatiles (i.e., it has no generation function at all) 4°Ar: 36Ar will have implications for the temporal history of Mars degassing as well. Again, the value of the measurement is dependent upon the degree to which the hulk 36Ar content of Mars can be deduced on theoretical grounds. Unfortunately, cosmochemical considerations and studies of meteorites suggest only t h a t the 36Ar content of Mars is likely to fall in the range from 1.0 × 10 -8 to 1.0 × 10-~ecSTP/g, which is the range in all types of chondrites (Signer, 1964). This is equivalent to a total planetary 36Ar inventory of from 8 × 10 -3 to 8 × 10-1g/ cm 2. Since the total 4°Ar inventory of a chondritic Mars would be 56g/cm 2, it follows that Mars in total will have an 4°Ar:3~Ar ratio of between 70 and 7000.

197

I f Mars degassed in the continuous, firstorder mode only, then 4°Ar:3~Ar in the atmosphere would be between 50 and 5000 (the slight difference in range results from the 4°Ar generation function). I f a value of <50 were observed, it would constitute unambiguous evidence of the relatively greater importance of catastrophic initial degassing. Within the range of 50 to 5000, interpretations would depend on the rather dubious certainty with which the bulk 36Ar content can be said to be known. As far as the overall (time-integrated) effectiveness of Mars degassing is concerned, the reasoning might run as follows. The Earth has in its atmosphere alone an 3°Ar inventory equivalent to slightly more than 1.0 × 10-SceSTP of 36Ar per gram of total Earth mass. This is within the range exhibited by chondrites. These observations together with our inability to predict the relative degassing efficiencies of Ar and CO 2 degassing for any object on an a priori physical basis suggests once again t h a t the simple Earth-analogous model may be of tactical use. In this model, as discussed earlier, all volatiles are degassed from Mars not only in the same ratios as on Earth, but in the same absolute amounts, only scaled to Mars' smaller mass and surface area. In this case the 36Ar:CO2 supply ratio in degassed volatiles on Mars would be the same as in the Earth's " R u b e y " surface volatile inventory, or 36Ar:CO3 T 1.5 × 10 -~. However, in the single Earth-analogous model, a quantity of CO 2 (see Table II) 500 times greater than the present atmospheric inventory, and amounting to 7.0 × 103g/ cm 2 is postulated to have first been degassed and then sequestered in the regolith as adsorbed CO2, carbonates, elathrates, etc. Therefore, since 36Ar would merely accumulate in the Martian atmosphere, the 36Ar:C02 mixing ratio in the present atmosphere would now be approximately ! . 5 × 10 -4× 5.0× 1 0 2 = 8 × 10-4 . Again, the most reasonable range would seem to be between this value and one an order of magnitude higher, allowing for the 36Ar content of Mars to be an order of magnitude higher than the bulk 36Ar content of the Earth as deduced from its

198

FANALE

F.P.

a t m o s p h e r i c 36Ar c o n t e n t alone. O f course, t h e a l t e r n a t i v e model is one in which not m u c h storage of m a j o r volatiles has occurred on Mars. I n this case, m o s t of t h e COs ever degassed would reside in t h e present a t m o s p h e r e a n d caps, with t h e result t h a t 4°Ar:CO2 in the a t m o s p h e r e would equal t h e s u p p l y ratio. Such a model m i g h t be t e r m e d t h e " o s t e n s i b l e " Mars a t m o s phere model, since it proposes t h a t essentially all the CO 2 ever degassed is still visible or obvious. Similar a r g u m e n t s a p p l y to 2°Ne : COs, a n d in f a c t similar values would seem to be expected, f r o m t h e p l a n e t a r y Ne c o n t e n t of meteorites (e.g. see Signer, 1964). Also, if Z°Ne:36Ar ~ 1.0, t h e n this, t o g e t h e r w i t h o t h e r information, would argue against a d o m i n a n t c o n t r i b u t i o n to a6Ar f r o m accretion of the solar wind, which has a m u c h higher 2°Ne:36Ar ratio (and

also a higher 2°Ne: S2Ne ratio) t h a n does t h e p l a n e t a r y p r i m o r d i a l c o m p o n e n t of meteoritic rare gas or the E a r t h ' s a t m o s p h e r e . T o m a k e the q u a l i t a t i v e aspects of t h e preceding discussion easier to visualize, I h a v e c o n s t r u c t e d a d i a g r a m (Fig. 5) which relates b o t h t h e t o t a l a m o u n t a n d t i m e of Mars degassing to t h e mixing ratio of 4°Ar and 36Ar (and 2°Ne) to CO s in t h e M a r t i a n a t m o s p h e r e . I h a v e refrained f r o m " b o x i n g in" t h e models, owing p r i m a r i l y to the g r e a t u n c e r t a i n t y in the b u l k 36Ar concent r a t i o n of Mars, as well as o t h e r p a r a meters. T h e ability of t h e Viking landed Gas C h r o m a t o g r a p h Mass S p e c t r o m e t e r t o detect small a m o u n t s of volatile-containing phases in the M a r t i a n topsoil has been extensively discussed elsewhere (Anderson etal., 1973). I t should be noted)_however,

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FIe. 5. Expected values of 36Ar:COz and 4°At:CO2 are given for several Mars degassing models, characterized by different total amounts of degassing and times of degassing. For a given overall intensity of Mars degassing, the later that degassing occurs, the further toward the right the Martian atmosphere point should fall. "Later degassing," however, could refer either to (1) accretion of Mars later in solar-system history than accretion of Earth, or (2) a greater relative role for degassing throughout Martian history, as opposed to accretional degassing, than for Earth. The a6Ar content of the Martian atmosphere is primarily dependent on the overall intensity, rather than the time history, of Mars degassing. Only qualitative trends are indicated; lines of demarcation arc not drawn, owing to several uncertainties in the assumptions. The greatest of these is the expected bulk 36Ar content of Mars.

MARTIAN VOLATILES

t h a t particular interest attaches to the possible presence of nitrates in the Martian soil since, as indicated earlier, it requires more ingenuity to propose mechanisms for nitrate formation on Mars than equivalent mechanisms for formation of carbonates, sulfates, and chlorides. Within the limitation imposed by the local nature of the sampling, detection of substantial amounts of any of the volatile-containing phases discussed in the text will be crucial for testing of degassing models. These limitations will probably seem less severe if both landers detect roughly similar concentrations of such compounds. From the preceding discussion it is also clear t h a t formulation of a specific quantitative model for Mars degassing and volatile storage requires at least one extensive parameter: namely, the depth of unconsolidated regolith material. Some insights into the problem may be provided by results of the Viking seismic experiment.

CONCLUSION8

I. Observational evidence and theoretical arguments suggest t h a t the total amount of H20, C02, N2, and other volatiles supplied to the Martian atmosphere throughout its history exceeds l × 104g/ em 2 ; i.e., it is at least 100 times the volatile inventory which is known to be present in the Mars atmosphere and polar caps. Furthermore, this evidence suggests that the major mechanism of subsequent storage (or the geochemical fate) of this large volatile inventor)' was chemical and physical reincorporation into the Martian regolith. These lines of evidence and arguments include : (a) Spectral reflectance studies coupled with laboratory studies of weathering kinetics under Martian conditions suggest t h a t the Martian regolith contains a large proportion of weathering products, including hydrated iron oxides and clay minerals. (b) Estimates based on the crater distribution on Mars suggest that the

199

thickness of that unconsolidated regolith over much of the Martian surface may be as great as 2 km. (c) Calculation of the expected distribution of hard-frozen permafrost in this "megaregolith" suggests that hardfrozen permafrost may contain 1 × 107 to 5 × 107km 3 of H20 ice, or as much as 4 x 104g/cm 2 averaged over the surface of Mars. (d) Two rather tentative but independent lines of evidence--Mars 6 observations and aeronomic argum e n t s - s u g g e s t s that the Martian atmosphere may contain from l0 to 40% 4°Ar. Theoretical arguments (which place a lower bound on the ratio of H.,O and other volatiles to K in Mars as a whole) suggest that if the 4°Ar content indeed proves to be that high, then at least 1 × 104g/cm 2 of H20 and other volatiles were probably released during Mars history. 2. The above arguments, together with observations of the Earth's surface volatile inventory (present in the oceans and sedimentary rocks), suggest that one tactically useful approach to understanding Martian volatile history is to calculate the inventory of all surface volatiles t h a t would be present on Mars if the Martian inventories were exactly equivalent to those present on the Earth, but scaled to Mars' smaller mass and surface area. This correctly predicts the "observed" amount of 4°Ar (~5g/cm 2) but also demands that the Martian regolith contain the following quantities of volatiles: (H20) = 1 × 10~g/ cm 2, (CO2) = 7 × 103g/em 2, (N2) = 3 × l02 g/cm 2, (C1) = 2 × 103g/cm 2, and (S) = 12 × 102g/em -'. These quantities, if stored in volatile-containing phases in a 2 km mixed regolith, would correspond to the following concentrations: carbonates = 1.5%, nitrates = 0.3%, chlorides = 0.6%, and sulfates < 0.1%. Physical chemical considerations and laboratory studies of weathering processes indicate t h a t concentrations this low could have been formed on Mars by interaction of ice, adsorbed water, and atmospheric gases with primary igneous

200

F . P . FANALE

minerals in 4 . 5 A E even if conditions on Mars were never greatly different from present conditions. Moreover, such low concentrations cannot y e t be ruled out on the basis of available visible and infrared reflectance d a t a on Mars. However, the problem of HzO storage proves more severe. I t is difficult to rationalize the presence of more t h a n 5 × 104g/cm z of H 2 0 as g r o u n d ice in the regolith, and m u c h lower a m o u n t s m a y be present. While a m o u n t s of chemically b o u n d H 2 0 as great as 4 × 104g/cm z could be postulated to exist in the regolith, estimates closer to 1 × 104g/cm z are perhaps more justifiable. Thus, the m a x i m u m a m o u n t s o f HzO and o t h e r volatiles t h a t could be stored in the Martian regolith are marginally compatible with the a m o u n t s required b y the " E a r t h - a n a l o g o u s " or high 4°Ar model. In other words, such a model cannot be rejected on the grounds t h a t there is no suitable storage place for the large volatile i n v e n t o r y which would be implied. 3. I f the 4°Ar content of the Martian atmosphere proves to be as high as NSg/cm z, as suggested b y the Mars 6 observations, b u t the stored regolith inventories of other volatiles are found to be much lower t h a n " r e q u i r e d " b y an Earth-analogous degassing model, then several explanations of the discrepancy m a y be considered : The postulate t h a t the bulk p l a n e t a r y ratios of HzO and other volatiles to 4°K were lower in Mars t h a n in the E a r t h m u s t be rejected. Reasonable models for nebula condensation and planet a r y formation which are consistent with other solar system d a t a predict t h a t these ratios are even higher for Mars t h a n the E a r t h , which exacerbates r a t h e r t h a n relieves the HzO storage problem. Alternatively, it m a y be shown t h a t the discrepa n c y can be explained as resulting from differences in the time history o f degassing between the two planets. The relationship between the time history and i n t e n s i t y of degassing of Mars and E a r t h , and the a m o u n t s of 4°Ar, HzO, and 36At in their surface volatile inventories m a y be incorp o r a t e d in q u a n t i t a t i v e Mars and E a r t h degassing models (Figs. 3-5). The possible implications of future atmospheric and

surface measurements on Mars, including anticipated rare gas m e a s u r e m e n t s b y t h e landed Viking mass spectrometer, for the mass a n d time of release of the Martian surface volatile i n v e n t o r y m a y be preassessed using these models. ACKNOWLEDGMENTS This paper prosents the results of one phase of research carried out at the J e t Propulsion Laboratory, California Institute of Technology, under Contract NAS 7-100, sponsored by the National Aeronautics and Space Administration. The author is grateful to R. L. Huguenin, J. S. Lewis, M. Malin, R. S. Saunders, and D. M. Anderson for helpful discussions. REFERENCES ADAMS,J. B. (1968). Lunar and Martian surfaces : Petrologic significance absorption bands in the near infrared. Science 159, 1453-1455. ANDERSON, D. M., BIEMAI~'N,K., ORGEL, L. E., ORe, J., OWEN, T., SHULMAN, G. P., TOULMIN,

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