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Lithospheric and Atmospheric Interaction on the Planet Venus VLADISLAV P. VOLKOV V.I. Vernadskiy Institute of Geochemistry and Analytical Chemistry ABSTRACT A host of interesting problems related to the probability of a global process of chemical interaction of the Venusian atmosphere with that planet's surface material has emerged in the wake of flights by the Soviet space probes, "Venera4, -5, -6, and -7" (1967-70~. It was disclosed during these flights that the temperature of Venus' surface attains 750 K, pressure is approximately 90 aim., and CO2 constitutes 97% of the atmosphere. We shall explore several of these issues which were discussed in the pioneering works of Mueller (1963, 1969) and Lewis (1968, 1970~: Is Venus' troposphere in a state of chemical equilibrium? Can we assume that the chemical composition of the troposphere is buffered by the minerals of surface rock? . What are the scales and mechanisms involved as exogenic processes take place? 1b what degree is the composition of cloud particles tied to the process of lithospheric-atmospheric interaction? We have succeeded in resolving a number of these problems over the past 20 years. At the same time, critical issues such as the chemical constituents of the near-surface layer of Venus' atmosphere, cloud particle chemistry, and the mineralogy of iron and sulfur in surface rock obviously cannot be definitely resolved until further landing craft will have been sent to the surface of Venus. Several research projects have been conducted in the USSR and the United States, which used physical-chemical and thermodynamic methods 218

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AMERICAN AND SOVIET RESEARCH 219 for computing multi-component systems. These projects have helped us to understand the particularity of the natural process occurring on the surface of Venus (Lewis and Kreimendahl 1980, Barsukov et al. 1980; Volkov et al. 1986; and Zolotov 1985~. Factual material from the studies of the atmosphere and surface of Venus, gathered with the "Venera" series spacecraft and during the "Pioneer Venus" mission, can be used to compare our view of the distribution, chemical composition, and physical properties of products of lithospheric- atmospheric interaction on Venus. MANIFESTATION OF EXOGENIC PROCESSES USING PHOTOGEOLOGICAL DATA The following conclusions on the nature and scope of exogenic pro- cesses were made after the probes "Venera-15" and "Venera-16" finished mapping Venus: The present surface relief of Venus was formed as a result of the combined processes of crater formation, volcanism, and tectonic activity; The rate of renewal of Venus' relief is estimated to take a million years for the first several centimeters (in the last three billion years), as compared to hundreds of meters on Earth and the first several meters on Mars (Nikolaeva et al. 1986~; There is no evidence of exogenic processes on a global scale, such as lunar regolith; There are no traces of fluvial or aeolian processes having occurred on a scale that matches the resolution of the radar images (one kilometer). At the same time, microscale exogenic processes have been quite clearly manifested. TV-panoramas from "Venera-9" and "Venera-10" recorded three types of processes: the formation of cracks; degradation with the emergence of desert aeolian weathering ridges; and corrosion, akin to porous aeolian or chemical weathering. The "Venera-13" and "Venera-14" images show laminated formations which have been interpreted (F-loren- skiy et al. 1982) as aeolian-sedimentation rock. Their formation can be described as a cycle: weathering transportdeposition lithiphication weathering. . . Experiments to estimate such physical properties of surface rock on Venus as porosity, and carrying capacity such as "Venera-13" and "Venera- 14" (Kemurdgian et al. 1983) confirmed the existence of loose, porous bedrock. Loose, porous bedrock with an estimated thickness of 10 cen- timeters exists at the landing site of the Soviet "Venera-13" and "Venera-14" probes. The question of their geological nature is still unanswered: Are they products of chemical weathering or aeolian activity?

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220 PLANETARY SCIENCES There is no direct evidence of the existence of aeolian forms as yet. No global aeolian, martian-Wpe structures were revealed in the area mapped by the radars of the "Venera-15" and"Venera-16" probes. Nor do any of the four TV-panoramas show aeolian forms. Experimental simulation (Greely et al. 1984) demonstrated that in an atmosphere of CO2, with a pressure of approximately 100 atm. and wind speed of up to 3 m/see-i, signs of rippling occur when saltation of particles of up to 75 Am in diameter takes place. Theoretical estimates of the threshold rates of the separation of particles of varying dimensions produced similar results. Dust fraction, transported as suspension, will have a diameter of < 30 ~m. Scrupulous investigation of the TV-panoramas, incorporating data from measurements of the optical properties of the near-surface atmo- sphere, have demonstrated that the formation of dust clouds from the aerodynamic landing of Soviet probes is a reality. It is considered that the nature of particle behavior during wind activity on Venus is similar to the sorting of material at the bottom of the ocean at a depth of about 1000 meters. Let us sum up the information on exogenic processes that was gen- erated by research on the morphology and properties of the surface of Venus: The rate of exogenic processing of the Venusian surface relief Is extremely low; the morphology of the ancient (0.5 to one billion years) relief has been excellently preserved; physical weathering (the equivalent of terrestrial, geological pro- cesses) has not been found: there is no aqueous water, living matter, or climatic contrasts; Regolith-like forms of relief are not developed; Aeolian activity on present-day Venus does not lead to the forma- tion of global forms which can be differentiated on radar maps; The television images show traces of chemical weathering in the form of rock corrosion and degradation. The findings from X-ray-fluorescent analyses on the Soviet "Venera- 13" and "Venera-14" and "Vega-2" probes and K, U. and Th determinations on the Soviet"Venera-S, -9, and -10," and "Vega-l, and -2" probes (Surkov 1985) have given us information regarding the chemical nature of the surface rock. It is merely important for this paper to note that all of this rock belongs to the basalt group and contains almost 10 times more sulfur than their terrestrial equivalents.

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AMERICAN AND SOV7ET RESEARCH THE CHEMICAL COMPOSITION AND A CHEMICAL MODEL OF THE TROPOSPHERE OF VENUS 221 It became clear, following the flight by the Soviet "Venera4" probe in 1967, that CO2 accounts for 97% of the Venusian troposphere, N2 is approximately 3%, and the remaining constituents account for appron- mately 0.1% (by volume). Unfortunately, we lack instrumental data on chemical composition at elevations below 20 kilometers. This creates con- siderable difficulty as we attempt to understand the chemical processes at the boundary between the atmosphere and the surface. The troposphere of Venus can be seen as a homogenous, well mixed, gaseous envelope for the major constituents (CO2 and N2) and the inert gases. It is clear that complex relationships exist between the physical (turbulent mixing, and horizontal and vertical planetary circulation of gas masses) and chemical (condensation and vaporization of cloud particles, gas-phase reactions, and gas-mineral types of interaction) processes in the atmosphere which lead to the existence of vertical and horizontal gradients of microconstituent concentrations (H2O, SO2 and CO; see Figure 1~. Venus' high surface temperature can be regarded as a factor which enhances the chemical interaction of the atmosphere with surface rock and, as a consequence, yields a dependency of the atmosphere's composition on heterogenic chemical reactions at the atmosphere-surface boundary. Mueller (1964) proposed 25 years ago that three zones may exist in the vertical profile of Venus' atmosphere, depending on the predominance of varying types of chemical processes: . The zone of thermochemical reactions in which the composition of the atmosphere is buffered by surface rock minerals; The zone of "frozen" chemical equilibrium, where the composition of gases corresponds to their equilibrium ratios in the near-surface layer of the troposphere; The zone of photochemical reactions in the upper atmosphere. According to this model, chemical reactions at the planet's surface take place amid a constant influx of reactive matter from the crust reservoir, as geological and tectonic activity also occur. Using the principle of global chemical quasiequilibrium in the atmosphere-crust system, we can apply thermodynamic computations to estimate the equilibrium concentrations of atmospheric gases that are not accessible to direct measurements. Lewis (1970) obtained more complete data on calculations of the chemical composition of the near-surface atmosphere; he took into account the results of the atmospheric analyses performed by the Soviet "Venera-4, -5 and Hi" probes. Unlike Mueller (1964), he only considered chemical equilibrium at the atmosphere-surface boundary Table 1~. Both of the

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222 By 90 LLJ ~ 70 An LL ~ 50 US > o m LLJ 30 PLANETARY SCIENCES N2 ~ CO2 | night HE SO2 HCI `` night / I / CO day I // day 2,'1 s8 10 i,,, }~1//: ' ~ i 2 _ _ _ 3 I / COS 1/ 1 1/ . 1/ 1 1 1 1 1 1 1, i 10-5 10-3 10-1 10 103 105 RELATIVE CONTENT BY VOLUME, PARTS PER MILLION FIGURE 1 A schematic vertical cross-section of the troposphere of Venus. It shows the distribution of macro- and microconstituents based on data from measurements per- formed by the "Venera" series and the "Pioneer Venus" probes. 1: microconstituents; 2: macroconstituents; 3: data requiring refinement. above models used the existence of chemical equilibrium throughout the troposphere, to its upper cloud boundary. The literature has been discussing Urey's (1951) hypothesis for quite some time. He proposed "Wollastonite" equilibrium as a mechanism for buffering PCO2 in the global, equilibrium atmosphere-crust system (Mueller 1963; Vinogradov and Volkov 1971; Lewis and Kreimendahl 1980~: CaC03 + SiO2 ~ CaSiO3 + CO2 calcite quartz Wollastonite The thermodynamic calculations performed in these studies demonstrated that the mineral association of calcite-quartz-Vollastonite on the surface of Venus can buffer Pcog (~ ~ bar) at a temperature of 742 K This is Estuary

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AMERICAN AND SOVIET RESEARCH TABLE 1 Chemical Models of Venus' Troposphere 223 Quasi ne Zone of "Frozen" Authors of Chemical Chemical Initial Equilibrium Equilibrium Data R. Mueller Troposphere + Troposphere to an Spectroscopic measurements 1963, 1969 lithosphere altitude of 80 km* of CO, HCI, HE, and H2O J. Lewis 1970 Troposphere + " " Spectroscopic measurements surface rock of CO, HCI, HF; H2O (t'Venera-5 & -6" data) Florenskiy " " Troposphere to lower Chemical analysis of the et al. 1976 cloud boundary atmosphere on Soviet "Venera 4 & -10" probes Khodakovskiy " " Near-surface Chemical analysis of the et al. troposphere atmosphere on Soviet "Venera 1979 -11 & -12" probes & "Pioneer Venus" Krasnopol'skiy and Parshev 1979 .. .. Troposphere to an altitude of 60 km$* .. .. Supper boundary of the cloud layer **According to Krasnopol'skiy and Parshev (1979): to the "zone of photochemical reactions" commensurate with the surface conditions. However, interpretation of the multisystem computations has shown that carbonates are unstable. Yet, the high concentration of SO2 in the troposphere is one of the determining factors of this process (Zolotov 1985; Volkov et al. 1986~. Consequently, "Wollastonite" equilibrium can scarcely be seen as the basis for a chemical model of Venus' atmosphere. F"lorenskii e! al. (1976) developed the idea in 1976 that there may be chemical equilibrium in the subcloud portion of the troposphere. The lower atmosphere was divided into three zones: The stratosphere with an upper layer of clouds, which is the zone of photochemical processes; The main cloud layer zone, where photochemical (above) and thermochemical (below) processes compete; The portion of the troposphere below the cloud base, which is the zone where thermochemical equilibria are predominant. This model brought us to a closer understanding of the Venusian troposphere as a complex, predominantly nonequilibrious system, even though numerical estimates of microconstituent concentrations (primarily SO2) departed greatly from the actual values Cable 2~.

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224 PLANETARY SCIENCES TABLE 2 Chemical Composition of the Venusian Near-Surface Troposphere from Computational Data (Relative Levels of Microconsiituents by Volume). Gas 1 2 3 4 5 6 CO 2.104 5.l0-S l.7.l0-s l.5.l0-S 7.210~ l.7.l0-s H2O 5.104 3.2.104 2.l0-s 2~104 2.l0-s 2.10-5 so2 3.10-7 8.lo-6 1.3.104 1.3.104 1.3.104 1.3.104 H2S 5.lo-6 1.2~10-6 52.lo-8 3.10-7 8~10-9 8.l0-s COS s.lo-s 3.2-10-S 2.3-10-S 2.l0-s 3.lo-6 4elo-s s2 2~10-8 4 10-8 1.8~10-? 10-7 1 3-10-8 2~10-8-8~10-7 H2 7.10-7 10-7 2.410-9 2.lo-8 10-9 2.5.l0-S O2 8 10-26 10-24 10-23 10-23 1.8.10-S Notes: The underlined figures are initial data of measurements on space probes or ground- based facilities; 1: Mueller (1969); 2: Lewis (1970); 3: Khodakovskiy et al. (1979); 4: Krasnopol'skiy and Parshev (1979); 5: Zolotov (1985). Column 6 tabulates data of measurements made an He "Venera" and "Pioneer Venus" probe senes; no measurements were performed below an altitude of 20 kilometers (Figure 1). Following measurements of the chemical composition of the tropo- sphere by the Soviet "Venera-ll and -12" probes and the "Pioneer Venus" probe, the computational and experimental values of microconstituent con- centrations were compared. Khodakovskii et al. (1979) and Krasnopol'skii and Parshev (1979) concurrently and independently proposed models (Ta- ble 2). These models were the first to compare gas-phase reaction rates with troposphere mixing rates. These consequences were generated: The troposphere is generally in nonequilibrium, with the exception of the near-surface layer with a thickness of the first kilometer, where the highest temperatures are dominant. However, the processes of het- erogeneous catalysis at the atmosphere-surface boundary may favor the establishment of chemical equilibrium in relation to certain constituents; The chemical composition of the microconstituents in the vertical cross-section below the cloud base region of the troposphere does not vary: it corresponds to the "frozen" equilibrium at the atmosphere-surface boundary (T = 735 K; P = 90 atm). The principal of "frozen" equilibrium was applied in order to theoret- ically estimate the chemical composition of cloud particles; this enables us to better understand the sulfur and chlorine cycles in the atmosphere-crust system (Volkov 1983; VoLkov e! al. 1986~. The lack of instrumental determinations of microconstituents in the

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AMERICAN AND SOVIET RESEARCH 225 troposphere at altitudes below 20 kilometers prevents us from solving the critical problem of the ratio of gaseous sulfur: H2S ~ COS ~ SO2 (Lewis 1970) or H2S + COS < SO2 (Khodakovskii et al. 1~79; see Table 2~. Furthermore, gas chromatographic determination of oxygen by the Soviet "Venera-13, and -14" probes cannot be reconciled with the concurrent pres- ence in these same samples of 80 ppm H2S and 40 ppm COS (see VoLkov and Khodakovskii 1984 for greater detail). The only original attempt to experimentally estimate the oxidation-reduction regime on Venus' surface, using a "Kontrast" detector on the Soviet "Venera-13, and -14" probes (Florenskii et al. 1983) pointed to the presence in the near-surface layer of the troposphere of a reducing agent (CO). However, it does not give us a clear-cut solution to the oxygen problem. The results from estimations of the chemical composition of the tro- posphere and the nature of the processes occurring in its near-surface layer can be summarized in three conclusions: (1) Chemical equilibrium in the troposphere of Venus has generally not been reached. (2) The vertical gradients of SO2, H2O and CO concentrations are a function of the competition between physical and chemical processes in the troposphere. (3) The near-surface troposphere can be seen as a layer in a state of "frozen" chemical equilibrium. Unfortunately, we have yet to resolve the question of the oxidation- reduction regime on Venus' surface, as well as the problem of the existence of free oxygen in the troposphere. THE MINERAL COMPOSITION OF SURFACE ROCK ON VENUS Many investigations have attempted to estimate the possible mineral associations on the surface of Venus using chemical thermodynamic meth- ods. Mueller published the first such study as a component of the afore- mentioned chemical model of the atmosphere (Mueller 1963) and obtained the following results: Temperature and pressure on Venus' surface are consistent with silicate-carbonate equilibrium, and carbon is bound in the rock in COCOS form; Oxygen partial pressure is buffered by Fe-containing minerals; Graphite and the native metals are not stable; Nitrogen is not bound in the condensed phases;

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226 PLANETARY SCIENCES A number of chlorine- and fluorine-containing minerals are stable at the surface. Lewis (1970) calculated 64 mineral equilibria in order to estimate P and T on the surface before the probes performed these measurements. One out of three proposed options for the P and T values was in satisfactory agreement with the actual values obtained a year later. Lewis yielded the following, additional forecast estimates: Surface rock contains H2O molecules bound in the form of tremo- lite; . sulfur is bound in the cloud layer in the form of mercury sulfides. Carbonyle-sulfide is the dominant form in which sulfur is found in the troposphere. This prediction proved only partially true: sulfur is actually the main component of cloud particles, but the latter consist primarily of H2SO4. A series of studies to calculate mineral composition was conducted in 1979-83 at the VI. Vernadskiy Institute using the computation of the phase ratios in multicomponent, gaseous systems, modeling the atmosphere/sur- face-rock system. The computations were based on troposphere chemical analysis data from the Soviet "Venera" series of probes, "Pioneer Venus," thermodynamic constants of about 150 phases, the chemical components of terrestrial magmatic rock, and the results of x-ray-fluorescent analysis of rock at three probe landing sites ("Venera-13," "Venera-14," and "Vega- 2"~. Compilation of this material can be found in VoLkov et al. (1986~. It should be stressed that three important predictive conclusions were made before the first data on the chemical composition of Venus' bedrock were obtained: Sulfur may be bound as sulfates (CaS04) and/or sulfides (FeS2), and its concentration greatly exceeds known sulfur levels in terrestrial equivalents; . Water-containing minerals are unstable; Carbonates are unstable; Magnetite Fe3O4 must be a widespread constituent both as primary and as altered bedrock These conclusions were generally confirmed, albeit with some refine- ment, by comparing them with X-ray-fluorescent analyses at "Venera-13," "Venera-14" and "Vega-2" landing sites, and by further, more detailed theoretical investigations (Zolotov 1985, 1989~. Sulfur at the surface probe landing sites, if we judge from the data of additional, postflight calibration investigations (Surkov et al. 1985), is in an anhydrite form (Cased. Sulfur content may serve as a measure of convergence to the state of chemical equilibrium relative to SO2 in the

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AMERICAN AND SOWET RESEARCH 227 atmosphere-crust system (Lewis and Prinn 1984; VoLkov et al. 1986~. It may be possible that rock with a maximum level of sulfur (1.9 mast %, "Vega-2") were in contact with the atmosphere longer than the bedrock at the landing sites of the Soviet "Venera-13" and "Venera-14" probes. In his 1985 study, Zolotov conducted thermodynamic assessments of carbonate stability depending on the concentration of SO2, since a reaction such as: CaC03 + 1.5SO2 ~ CaSO4 + CO2 + 0.25S2 takes place in Venus surface conditions free of kinetic constraints. As it turned out, the presence of SO2 in quantities exceeding 1 ppm excludes the existence of calcite and dolomite. However, magnesite (MgCO3), as a product of the alteration of pure forsterite, MgSiO4 (Folk), may be stable at altitudes of 1.5 to eight kilometers. Zolotov demonstrated in this same study (1985) that hematite (Fe2O3) may even be stable at an altitude of more than 1.5 kilometers (Figure 2), in addition to magnetite (Fe3O4) (the product of water vapor-driven oxidation of Fe-containing silicates, CO2 and SON. Hematite stability is apparently confirmed by the results obtained from interpreting the surface color on the TV images from "Venera-13 and -14" (Shkuratov e! al. 19~7~. Nevertheless, in their 1980 study Lewis and Kreimendahl retain the conclusion regarding calcite (CaC03) and wustite (Fe O) stability, while allowing for the prevalence of H2S and COS over SO2 in conditions of total chemical equilibrium at the surface-atmosphere boundary. They come to the same logical conclusion that in this case, the surface rock of Venus' crust is characterized by an extremely low degree of oxidation (Fe+3/Fe+2 at one to two orders lower than the terrestrial value). Strictly speaking, the ultimate solution to the problem of the oxidation of Venus' crust has not been found due to the lack of instrumental data. In 1975, WaLker (1975) drew attention to the possible dependence of the mineral constituents of Venus' surface on the hypsometric level. The pressure (= 65 atm.) and temperature (= 100 K) gradients are actually so great that they may alter the composition of the phases of rock during their exogenous cycle, that is, under the influence of aeolian transport. If we take into account the fact that our knowledge of Venus' mineralogy does not go beyond the framework of theoretical forecasting, the factor of "hypsometric control" must still be considered hypothetical Let us summarize the theoretical investigations of the chemical inter- action of Venus' rock with its atmosphere. Alteration of the composition of Venus' basalts during interaction with the aunosphere is highly probable;

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228 T. K 690 705 720 735 750 765 PLANETARY SCIENCES : -25 -24 -23 -22 -21 -20 IgPO2 H,KM +6 +4 +2 o -2 -4 FIGURE 2 Estimates of the oxidation-reduction regime in the troposphere and on the surface of Venus from data produced lay measurements (1,2,5) and computations (3,4~. (Zolotov 1985~. 1. CO2 = CO + 2O2 (CcO2 = 96.53to; Coo = 20 ppm). 2. SO2 = 2S2 + O2 (CSO2 = 130 . 185 ppm; CS2 = 20 ppb). 3. 3Fe2O3 = 2Fe3O4 + 2O2 (buffer HM). 4. 2Fe304+ 3SiO2 = 3Fe2SiO4 + O2. S. "Kontrast" detector (Florenskiy et al. 1983~. Apparently, the primary outcome stemming from this interaction will be the sink of sulfur in the crust as anhydrite (CaSO4) and/or iron sulfides (FeS and FeS2~; The existence of carbonates (besides MgCO3), free carbon and nitrogen compounds on the surface of Venus is thermodynamically prohib- ited; The lack of complete factual data prevents our making a clear-cut conclusion as to the stability of water-bearing minerals and the degree of oxidation of the Venusian crust. THE CYCLES OF VOLATILE COMPONENTS Interpretation of data on Venus' atmospheric chemistry, and in par- ticular, consideration of the photochemical processes in the stratosphere (Krasnopolskii 1982; Yung and De More 1982) demonstrated that nitrogen and carbon cycles are completed in the atmosphere. The H2O cycle poses more problems, since we are not yet clear on the vertical profile of H2O concentrations in the near-surface atmosphere. Clearly, sulfur is the only volatile element on Venus which, in the

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AMERICAN AND SOVIET RESEARCH h, KM 80 - 50 - 30 229 catalysis , _ _ _ _ _ _ , , LCOz ~ MARCO, +O:1 ~ hv ~~'x At/ ~ 1 S(lS812. hv ~ ~ ~ ~ C 1 h`, ~ I ! tH2+~+CO I `~ +CO2 I |SO2, H2S, COS| ~ Weathering crust ~ ~ W~ ~_FeSiO3 ~, FeS2 ~CaAl2si2O-8- ~:CaSO8 /// 1 -- - 2 3 4 - -5 ~ i ~ ~ ~ /// FIGURE 3 Diagram of the cycles of C02, sulfur and chlorine in the Venusian atmosphere 1: chlorine cycle; 2: CO2 cycle; 3: rapid sulfur cycle; 4: slow sulfur cycle; 5: sulfur flux into the crust. contemporary geological epoch, participates in the Cyclical mass exchange between the atmosphere and the crust. Sulfur's behavior as a constituent of the cloud layer essentially determines its structure and dynamics. Three cycles (Figure 3) have been discerned, depending on the rates at which these processes unfold (Lewis and Prinn 1984~. The rapid cycle takes place in the stratosphere and the clouds and sets the stage for the photochemical emergence and thermal destruction of sulfuric acid aerosols. The residence time for the SO2 molecule is estimated

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230 PLANETARY SCIENCES TABLE 3 Mineral Composition of Venusian Surface Rock Based on Theoretical Assessments (Secondary Minerals) 1 2 3 Carbon in CaCO3 Carbonates Ca and Mg Cow) iS unstable Carbonates are unstable MgCO, (?) H2O in amphiboles H2O in amphiboles Amphiboles (?) and micas Fe2O,, Fe3O4 FeO, Fe3O4 where Xcos + XH25 > XSO2 Fe304. Fe2O3 (?) where XCOS + XH2S < XSO2 Sulfur in sulfides of Fe and anhydatc (CaS04) Predominantly CaS04, sulfides of Fe are stable Nitrogen-beanug minerals are unstable Chlonne- and fluonde-beanng minerals are stable (fluonte/apatite?) 1. Mueller 1963, 1969 2. Lewis 197~, Lewis and Kreimendshl 1980 3. Khodalcovskiy c' al. 1978; Volicov 1983; Zolotav 1989. to be from several hours to several years. Two alternative scenarios are proposed in Table 4. It is difficult to select between the two because of the lack of experimental data on the rates of certain photochemical reactions. The slow atmospheric cycle is most likely a function of photochemical and thermodynamic reactions in the lower atmosphere which lead to the existence of reduced forms: H2S and COS and elementary sulfur. Ap- parently, the stratosphere is the region of H2S and COS flux: they either photodissociate there or are oxidized by molecular oxygen to SO3. The time span of sulfur molecules in the cycle is estimated to be several dozen years (Lewis and Prinn 1984~. The mass exchange between Venus' crust and its atmosphere is carried out in a "geological" sulfur cycle. The source of sulfur is crust matter which produces sulfur-bearing gases through both volcanism and the interaction of minerals with atmospheric gases, such as FeS2 with CO2, H2O, and CO. These gases repeatedly participate in photo- and thermodynamic pro- cesses in the atmosphere. The rapid atmospheric Cycle brings about the long-term existence of a cloud cover made up of condensed H2SO4 parti- cles. The competition of photo- and thermochemical reactions in the slow cycle apparently support the existence of SO2 as the dominant form of sulfur in the atmosphere. An excess of SO2 compared with its equilibrium concentration in the atmosphere-crust system create an SO2 flux in the form of sulfates in surface rock. Two factors determine the scales and rates of flux:

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AMERICAN AND SOVIET RESEARCH TABLE 4 Sulfur Cycles on Venus 231 Cycle Timeframe I. Fast cycle (stratosphere and cloud layer) (Winick and Stewart 1980) (Krasnopol'skiy 1982) CO2 + he ~ CO + O SO2+ hv ~ SO + O < 10 yrs. SO2 + O + M ~ SO3 + M SO + O + M ~ SO2 + M (OH, HO2 are catalysers) SO2 + O + M ~ SO3 + M SO3 + H2O ~ H2SO4 (Sol) SO3 + H2O ~ H2SO4 (Sol) II. Slow cycle (lower atmosphere and cloud layer) SO3 + 4CO ~ COS + 3SO2 SO3 + H2 + 3CO ~ H2S + 3CO2 e > 10 yrs. COS + hv ~ CO + S H2S + hv ~ HS + H COS + 1-5O2 ~ SO3 + CO H2S + 1.502 ~ SO3 + H2 m. Geological Cycle CaSiO3, CaAl2Si2O8 + SO2 ~ CaSO4 ~ 106 yrs. FeSiO3, Fe3O4 + COS (H2S) ~ FeS(FeS2) The time frame for a cycle to run its course depends on: 1) Mineral gas reaction rates on the planet's surface 2) Length of time during which mineral particles are in contact with the atmosphere such as surface relief renewal rates . surface; The rate of heterogeneous mineral = gas reactions on the planet's The residence span in which mineral particle are in contact with the atmosphere, for example, the surface relief renewal rate. The completing of the "geological" cycle probably occurs as the altered surface rock (rich CaS04) is resmelted in the deep regions of the crust. Attenuated volcanic and tectonic activity on Venus ultimately reduces the thickness of the cloud layer because sulfur is fixed in the crust and depleted in the atmospheric reservoir. GENERAL CONCLUSIONS We can cite at least four firmly established facts that determine the existence of the chemical interaction of Venus' atmosphere with its surface rock. These are:

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232 PL4NETARY SCIENCES Loosely porous rock on the planet's surface is developed; massive rock display traces of corrosion and degradation; Inhere is no global regolith; aeolian transport on a limited scale is supported by weak winds in the near-surface atmosphere; The troposphere contains reactive gases (microconstituents): SO2, H2O, CO, and others;- Venus' basalts contain one to 1.5 more orders of sulfur than their terrestrial equivalents. We can make the following conclusions based on our interpretation of the entire set of observational data: (1) The processes of lithospheric-atmospheric interaction substantially alter primary basalts and subject them to chemical weathering. The scale of this process cannot be estimated; (2) The troposphere is generally not in a state of chemical equilibrium with the surface rock and the chemical composition of the near-surface layer may correspond to a "frozen" equilibrium which is buffered by the minerals. (3) Sulfur is in a state of cyclical mass exchange between the atmo- sphere and the crust. (4) Nitrogen and oxygen in the crust's rock do not form stable phases. Their cycles become completed in the atmosphere. REFERENCES Barsukov, V.L, V.P. Volkov, and I.L. Khodakovsky. 1982. The crest of Venus: theoretical models of chemical and mineral composition. J. Geophys. Res. 87(suppl. Part 1~:A3- A9. Florenskiy, K.P., A.T. Bazilevskiy, and AS. Selivanov. 1982. Panoramas of the landing sites of "Venera-13" and "Venera-14" (a preliminary analysis). Astron. Vestnik. 16(3):131-138. Florenskiy, K.P., V.P. Volkov., and O.V. Nikolaeva. 1976. Towards a geochemical model of the troposphere of Venus. Geokhimiya 8: 1135-1150. Florenskiy, IMP., O.V. Nikolaeva, V.P. Volkov, et al. 1983. On the oxidation-reduction conditions on the surface of Venus, based on data from the "Kontrast" geochemical detector deployed on the "Venera-13" and "Venera-14" probes. Kosmich. issled. 21~3~:351-354. Greely, R., J. Iverson, R. Leach, et al. Windblown sand on Venus: preliminary results of laboratory simulations. Icarus 57~1~:112-124. Kemurdjian, A.L., P.N. Brodskiy, V.V. Gromov, et al. 1983. Preliminary results in determining the physical and mechanical properties of bedrock by the Soviet automated "Venera-13" and "Venera-14" probes. Kosmich. issled. 21~3~:323-330. Khodakovskiy, I.L., V.P. Volkov, Yu.I. Sidorov, et al. 1978. The mineralogical composition of bedrock, and the processes of hydration and oxidation of the outer envelope of the planet Venus (a preliminary forecast). Geokhimiya 12:1821-1835. Khodakovskiy, I.L., V.P. Volkov, Yu.I. Sidorov, et al. 1979. A geochemical model of the troposphere and crust of the planet Venus, based on new data. Geokhimiya 12:1748-1758. Krasnopol'skiy, V.^ 1982. Photochemistry of the atmospheres of Mars and Venus. Nauka, Moscow.

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