The composition and evolution of primordial solutions on Mars, with application to other planetary bodies

Geochimica et Cosmochimica Acta, Vol. 68, No. 23, pp. 4993–5008, 2004 Copyright © 2004 Elsevier Ltd Printed in the USA. All rights reserved 0016-7037/04 $30.00 ⫹ .00

doi:10.1016/j.gca.2004.05.036

The composition and evolution of primordial solutions on Mars, with application to other planetary bodies P. L. KING,* D. T. LESCINSKY, and H. W. NESBITT Department of Earth Sciences, University of Western Ontario, London, Ontario N6A 5B7 Canada (Received November 24, 2003; accepted in revised form May 27, 2004)

Abstract—We examine a model for Mars involving bulk primordial solutions (oceans and lakes) that were relatively Mg-rich and SO4/(SO4 ⫹ Cl)-rich. Such solutions could be produced when (1) volatiles leached a planet (or portions of a planet) with an ultramafic-mafic composition in a process called “planetary leaching”; and/or by (2) “impactor leaching” where meteoritic and/or cometary impactor fragments were leached. When Mg-SO4/(SO4 ⫹ Cl)-rich solutions are concentrated, we predict that the following sequence of salts precipitates: phosphates; carbonates; gypsum; epsomite; bloedite; halite; hexahydrite; and, finally bischofite. This sequence is modified slightly if appreciable Fe-, Mg- or Na-carbonates, Fe-sulfates, Mg-phosphate, or other halide salts crystallized before the Mg-Na-sulfate salts, or if HCO3⫹CO3 concentrations vary due to other effects (e.g., atmosphere CO2 levels change). On Mars, the initial primordial solutions would have been relatively salt-rich and water-poor; therefore, the surface solutions formed Mg-Na-SO4-Cl salts (cements, veneers, and dust) and subsurface solutions or ice (solid H2O). This model is supported by the compositions of cements in the regolith on Mars (high Mg, Na, S, and Cl) and geochemical and petrographic evidence that the salts precipitated in the predicted sequence. We suggest that the partial pressure of oxygen was above the hematite-magnetite buffer where Fe3⫹-(hydrous)oxides are stable and SO42- or HSO4- are solutes in any solution. Such a partial pressure of oxygen may have been attained via H2-loss. In contrast, on the Galilean satellites (Europa, Ganymede, and Callisto) surface solutions were relatively water-rich and formed ice, Mg-SO4-rich salts, and solutions, thereby producing surface features dependent on the initial water content and the crystallization path. Unlike the Na-Cl-rich oceans on Earth, the solutions of these planetary bodies likely did not change greatly from their bulk primordial Mg-rich, SO4/(SO4 ⫹ Cl)-rich compositions; hence they did not attain compositions similar to modern seawater. Copyright © 2004 Elsevier Ltd evolution of the ancient martian hydrosphere (primordial surface solutions). Clues to the composition and evolution of the bulk primordial surface solutions as they were concentrated could be held in the martian regolith (silicates ⫹ salts ⫹ oxides; McSween and Harvey, 1998; Warren, 1998; McSween and Keil, 2000). To examine this idea, we need to examine various lines of evidence for the possible components in the bulk system (in this paper referred to as a chemical formula with no electrical charge given; e.g., SO4), so that we can then discuss the ions in solution (solutes, referred to as charged ions; e.g., SO42-) and model possible salts (referred to using words; e.g., sulfate). Analyses from the Viking and Pathfinder missions indicate that the salts inferred to be on the martian surface in soils, cements, rocks, and dust in the atmosphere are Mg-S-rich with significant Na and Cl (Baird et al., 1976; Clark et al., 1976; Toulmin et al., 1977; Gooding, 1978; Clark and Van Hart, 1981; Bell et al., 2000; McSween and Keil, 2000; Brückner et al., 2001; Foley et al., 2001). Recent analyses at Gusev Crater and Meridiani Planum (Landis et al., 2004; Rieder et al., 2004) support these findings. There is no data regarding S- or Cspeciation; however the Viking analyses suggest S6⫹ (Toulmin et al., 1977) and sulfate and carbonate salts have been tentatively identified in the martian dust using infrared spectroscopy (Bell, 1996; Blaney, 2001; Cooper and Mustard, 2002; Bandfield et al., 2003; Christensen et al., 2004). Because sulfate and carbonate salts are relatively soluble (e.g., species derived from

1. INTRODUCTION

The climate history of Mars is poorly constrained, but geomorphic and geochronologic evidence suggests that surface solutions have existed episodically on Mars (Baker, 2001; Jakosky and Phillips, 2001; Masson et al., 2001; Nyquist et al., 2001). Water is the favored solvent in such solutions based on comparative planetology and geomorphic features on Mars (e.g., Jakosky and Phillips, 2001) and new data obtained at the time of writing (European Space Agency and National Aeronautics and Space Administration). Today, Mars contains little or no hydrosphere, although H and OH are bound in the near-surface or subsurface, likely in OH and H2O compounds, and H occurs at low levels in the atmosphere (Krasnopolsky and Feldman, 2001; Feldman et al., 2002; Landis et al., 2004) or as snow patches mantled in dust/sediment (Christensen, 2003). It appears that any martian surface solutions have been largely lost to space through processes such as impact erosion, hydrodynamic escape, and thermal/nonthermal escape and retreat of the residual solutions into the subsurface (Carr, 1996). Furthermore, the cold conditions on Mars today (⬃210 K average) do not allow for long-term solution stability. It is therefore difficult to study the chemistry and compositional

* Author to whom ([email protected]).

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salts with small log K in Table 1), any surface solutions in contact with dust must have been a brine such as “dirty water ice” (Christensen, 2003) or a high salinity brine(s) (Brass, 1980). Such an Mg-S-rich solution would be stable on the surface of Mars and along grain boundaries in rocks, because solutions with these solutes have low-temperature eutectics (Brass, 1980). In sum, the lander and spectral data in combination with phase equilibria data imply brine solutions that may have been intrinsically Mg-S-rich (Fig. 1a and 1b). To explain the high S compositions on Mars, authors have proposed that the S has a fluid source: either high-temperature volcanic gases (“acid fog”), or hydrothermal fluids (Gooding, 1978; Settle, 1979; Morris et al., 1996; Banin et al., 1997; Catling, 1999; Newsom et al., 1999), or low-temperature groundwater that has interacted with S-bearing rocks (Burns, 1988; Burns, 1993). Because these models invoke a separate fluid source for S, this has allowed authors to “decouple” the S salts from the other salts, and instead model S-free brines on the surface of Mars (e.g., Cl-fluids in Knauth and Burt, 2002; HCO3⫹CO3-fluids in Morse and Marion, 1999). These simplified systems, while instructive, may not provide accurate predictions of solution evolution, because on Earth the S-species cannot operate in isolation from other soluble species such as 2Cl-, HCO3-, and CO3 (e.g., Table 1) in either high-temperature systems (Krauskopf and Bird, 1995) or low-temperature evaporite systems (Hardie and Eugster, 1970). In this paper we model the nature and sequence of salts precipitated from multicomponent Mg-S-rich solutions (MgNa-Ca-Fe-SO4-Cl-HCO3⫹CO3-PO4) as possible analogs for the bulk primordial hydrosphere on Mars. We examine several possible bulk primordial solution compositions; discuss the oxygen partial pressure and pH of the solutions; discuss the predicted salt sequence; modification of the salts; and compare our model with remote sensing and meteoritic data for Mars. We then discuss the implications of this model for the hydrospheres on other rocky planetary bodies. First, to better constrain our model, we outline some physical processes that may form primordial solutions. 2. PHYSICAL MODELS FOR FORMING PRIMORDIAL SOLUTIONS ON MARS

Two possible end-member scenarios may result in the development of a primordial hydrosphere on a planetary body (Table 2). One scenario (“planetary leaching”) invokes volatiles, such as C-O-H-S species, sourced from a major reservoir such as the solar nebula during early planetary formation. Such volatiles might be incorporated into the planetary body (e.g., into the mantle and crust), and then due to their volatile nature, they may separate, transporting solutes to the surface crust. Such a scenario could involve leaching of the entire planetary body or portions of the planet (e.g., the crust or a “late planetary veneer”). A second case (“impactor leaching”) involves the introduction of solutes and solvents through cometary and/or meteoritic impactors, likely during the “heavy bombardment” stage (⬃4.5 to, at latest, 3.5 Ga; Chyba, 1990; Owen and Bar-Nun, 1995), resulting in leaching on the planetary surface and near-surface (Table 2). Surface solutions were most abundant during Mars’ early history (Noachian; ⬃4.5 to ⬃3.7 Ga, or at latest ⬃3.5 Ga;

Hartmann and Neukum, 2001) and water is the favored major solvent, although the source and absolute amount of total water are difficult to define (Jakosky and Phillips, 2001). There is evidence that both planetary leaching and impactor leaching may have delivered and/or remobilized solutions in the Noachian. Planetary leaching is supported by ancient valley networks, possible lakes and oceans on Mars (Baker, 2001; Jakosky and Phillips, 2001), as well as outflow channels associated with large scale Noachian volcanic events (Phillips et al., 2001). Impactor leaching is supported by cratering densities as well as dynamic and atmospheric calculations (Segura et al., 2002; Lunine et al., 2003) and estimates of meteoritic contributions to the martian soil (Flynn and McKay, 1990). As noted above, surface solutions have been present episodically on the surface of Mars (Baker, 2001; Jakosky and Phillips, 2001; Masson et al., 2001; Nyquist et al., 2001); thus, it is necessary to explore when and how the primordial salts may have been remobilized and (re)deposited on the planetary surface and in the near-surface (also see section 6). Carbonates in the martian meteorite ALH84001 are 3.9 to 4.04 Ga (Rb-Sr and U-Pb dating), and iddingsite alteration in the martian meteorite Lafayette is 0.6 to 0.7 Ga (K-Ar and Rb-Sr) (summarized in Nyquist et al. (2001)). Furthermore, geomorphic evidence indicates recent solution transfer on the surface of Mars (Malin and Edgett, 2000; Malin et al., 2001). Remobilization and (re)deposition are most likely to occur during volcanic/hydrothermal events, dust storms, outflow channels, seasonal sublimation events, or where solutions have percolated along mineral surfaces by capillary action due to ice melting or groundwater transport (via upward or downward solution transport; Landis et al., 2004). Of these processes, only the hightemperature events significantly modify the local chemistry of the hydrosphere on Mars (e.g., hydrothermal alteration haloes), because the low-temperature processes do not significantly change the relative concentrations of components in the martian near-surface or surface. Because these high-temperature processes are similar or identical to the ancient “planetary leaching” events that produced the bulk primordial solutions, we evaluate the evolution of primordial solutions on Mars using model initial bulk compositions. This simplification is not intended to imply that the solutions on Mars would have been uniform or in chemical equilibrium on a planetary scale. 3. BULK COMPOSITION OF PRIMORDIAL SOLUTIONS

Because both planetary leaching and impactor leaching involve solutions interacting with the crust, mantle, and/or chondrite material, both processes produce solutions (primordial hydrosphere) with similar bulk composition. The average surface composition of Mars is close to mafic (Wyatt and McSween, 2002), and because there is no convincing evidence for large-scale plate tectonics (except perhaps pre-⬃3.7 Ga; Stevenson, 2001; Zuber, 2001) much of the martian crust could be mafic. The mantle is assumed to be ultramafic, and chondritic materials are near-ultramafic in composition. Therefore, it is instructive to compare the composition of the Viking and Pathfinder soils to possible primordial solutions that include compositions derived by leaching mafic, ultramafic, and chondritic rocks. We plot these compositions in Figure 1a to show the major anion components (SO4, HCO3 ⫹ CO3, Cl) and

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Table 1. Formulae and equilibrium constants (log K) for minerals in the system: Na-K-Mg-Ca-(Fe)-Cl-SO4-H2O at 25°C and ⬃105 Pa. Mineral name and abbreviation

Formula

Phosphates Whitlockiteb Ca3(PO4)2 Mg-phosphate Mg3(PO4)2 Less soluble carbonates Hydromagnesite Mg5(CO3)(OH)2 · 4H2O Dolomiteb (Dol) (CaCaMg(CO3)2 protodolomite; Ca-Pr) FeCO3 Sideriteb-ss (Sd) Gaylussite Na2Ca(CO3)2 · 5H2O Pirssonite Na2Ca(CO3)2 · 2H2O Calciteb-ss (Cc) CaCO3 MgCO3 Magnesiteb-ss,e (Mag) Nesquehoniteb,e MgCO3 · 3H2O Sulfatesf Polyhaliteb K2Ca2Mg(SO4)4 · 2H2O Syngenite K2Ca(SO4)2 · H2O Eugsterite Na4Ca(SO4)3 · 2H2O Glauberite Na2Ca(SO4)2 CaSO4 · 2H2O Gypsumb,d,e (Gy) Anhydritee CaSO4 e Picromerite (schoenite) K2Mg(SO4)2 · 6H2O Leonitee K2Mg(SO4)2 · 4H2O Aphthitalite NaK3(SO4)2 Na2Mg(SO4)2 · 4H2O Bloediteb,d (Bl) b,d,e Epsomite MgSO4 · 7H2O (Ep) Arcanitee K2SO4 MgSO4 · 6H2O Hexahydritec,d (Hx) Mirabilited,e (Mi) Na2SO4 · 10H2O More soluble carbonates/bicarbonates/other salts Trona Na3(CO3)(HCO3) · 2H2O Natrond,e Na2CO3 · 10H2O Burkeite Na6(SO4)2(CO3) Thermonatritee Na2CO3 · H2O Nahcolite NaHCO3 Kalcinite KHCO3 More soluble sulfates/other salts Mercallite KHSO4 MgSO4 · H2O Kieseritec,d,e (Ki) Starkeyiteb,e MgSO4 · 4H2O Pentahydritee MgSO4 · 5H2O c,d,e Thenardite Na2SO4 (Tn) Kainite KMgClSO4 · 3H2O Chlorides Tachyhydrited-ss CaMg2Cl6 · 12H2O MgCl2 · 6H2O Bischofited-ss,e (Bi) Carnallite KMgCl3 · 6H2O Antarcticited-ss,e CaCl2 · 6H2O Haliteb,d,e (Ha) NaCl Sylvite KCl

Meas. log K

Calc. log Ka

␮oi /RTa

Molality g/kg H2O

⫺28.7 ⫺25.2 ⫺36.47 ⫺10.7 ⫺8.5 ⫺7.5

⫺4.425 ⫺2.38 ⫺2.13 ⫺1.87 ⫺2 ⫺1.25

0.2 ⫺0.3 16.32 4.29 4.1 3.72 1.56 0.894

Ref. for measured data g g

⫺17.08

⫺871.99

⫺9.42 ⫺9.24 ⫺8.40 ⫺7.83 ⫺5.17

⫺1360.5 ⫺1073.1 ⫺455.6 ⫺414.45 ⫺695.3

⫺13.74 ⫺7.45 ⫺5.67 ⫺5.24 ⫺4.58 ⫺4.36 ⫺4.33 ⫺3.98 ⫺3.80 ⫺2.35 ⫺1.88 ⫺1.78 ⫺1.63 ⫺1.23

⫺2282.5 ⫺1164.8 ⫺1751.45 ⫺1047.45 ⫺725.56 ⫺533.73 ⫺1596.1 ⫺1403.97 ⫺1057.05 ⫺1383.6 ⫺1157.83 ⫺532.39 ⫺1061.60 ⫺1471.15

⫺1.04 ⫺0.82 ⫺0.77 0.48 ⫺0.40 0.28

⫺960.38 ⫺1382.78 ⫺1449.4 ⫺518.8 ⫺343.33 ⫺350.06

0.58 ⫺0.12

⫺417.57 ⫺579.80

⫺0.29 ⫺0.19

⫺512.35 ⫺938.2

17.38 4.45 4.33 4.14 1.57 0.90

⫺2015.9 ⫺853.1 ⫺1020.3 ⫺893.65 ⫺154.99 ⫺164.84

h i i g 0.3913 0.1528 1.927 0.8534 0.0153 0.0229 1.959 2.514 1.118 4.004 2.986 0.6927 3.565 1.94

j j j j j j j i,j j i,j i,j i,j i,j i,j

5.619 5.032 4.32 3.687

i,j j j i,j i i i i i i

a Calculated using data from Harvie et al. (1984) and the equation InK ⫽ ⫺⌺␯i(␮oi /RT), where ␯i are stoichiometric coefficients, ␮oi is the chemical potential of the element i, R is the gas constant and T is temperature. b Mineral found in meteorites (Richardson, 1978; Velbel, 1988; Zolensky and McSween, 1988; Brearley and Jones, 1998). Ca-Mg bicarbonates and Fe-bearing sulfates and Ni-bearing salts have also been found in meteorites, but are not included in the table. b-ss: Mineral may be found in meteorites as part of a solid solution series. c Mineral suggested to occur in meteorites, but poorly characterized. d Mineral proposed to exist on the Europa (Kargel et al., 2000). Na-Ca-Mg chloride hydrates have also been suggested for Europa but are not included in the table. d-ss: Mineral proposed to exist on the Europa a solid solution series. e Mineral proposed to exist on Europa (Zolotov and Shock, 2001b). Note that those authors proposed that Europa also contains nonhydrated salts (MgCl2, CaCl2, Na2CO3) and some salts that are not found in nature on Earth, but these are not included in the table due to lack of data. f Burns (1988, 1993), suggested that jarosite may exist on Mars (discussed in section 5.2). g Krauskopf and Bird (1995). h Catling (1999). i Wood (1975). j Spencer (2000).

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Fig. 1. Triangular diagrams showing A) anions and B) cations that are most abundant in primordial solutions. Terrestrial continental groundwaters from mafic rocks (basalt and gabbro) and ultramafic rocks (peridotite and serpentine) are also plotted using data from White et al. (1964) and Nesbitt and Bricker (1978). We omitted groundwaters from ocean islands (seawater contamination), groundwaters adjacent to ore deposits (unusual compositions), and groundwaters with extreme pH (⬎11) because the latter solutions are saturated in Mg-bearing silicate and oxide minerals (see the detailed discussion in Nesbitt and Bricker (1978)). The calculated solutions extracted from various chondrite types (H, CV, CM, CI) and a model bulk silicate earth (BSE) (Zolotov and Shock, 2001b) fall close to the field for groundwaters derived from ultramafic rocks, indicating that the groundwaters from ultramafic rocks may be a good analog for solutions derived from such compositions. Plotted on this diagram are leachates of Murchison (ML) and Orgueil (OL) carbonaceous chondrites (Fanale et al., 1977; Fanale et al., 2001). Note that the leachate of the Orgueil meteorite is relatively Mg-rich, and because analyses did not include Cl-, we illustrate a range of reasonable SO4/(Cl2 ⫹ SO4) concentrations based on the analysis deficit. The CO32- and HCO3- amounts have not been measured for the leachates, and therefore, a range of values (lines and fields) are shown in Figure 1a. The “chondrite salt” composition (Kargel et al., 2000) is also shown; because they gave values of (Na ⫹ K) mass %, we converted their values to moles assuming that the amount of K is insignificant. We also show the Viking soil analyses recalculated by McSween and Keil (2000). These analyses did not include the alkali elements, and so we used estimates for these concentrations of Na2O ⫽ 2.1% and K2O ⫽ 0.6% (from McSween and Keil, 2000). The Pathfinder soil compositions (B ⫽ Brückner et al., 2001; F ⫽ Foley et al., 2001) are plotted and have compositions similar to the “chondrite salts” and the calculated Europan ocean (Zolotov and Shock, 2001b).

Figure 1b to show the cation components (Na ⫹ K, Mg, Ca). These are also the major anions and cations found in groundwaters that form on igneous rocks on Earth (Garrels, 1967). Because many of the compositions do not have well con-

strained HCO3 ⫹ CO3 we show some solutions as lines or fields representing a range of possible HCO3 ⫹ CO3 contents (Fig. 1a). We assume that all S is SO4 and this assumption is discussed in section 3.3. The reported compositions of the

Table 2. Possible processes to produce solutions of different compositions on planetary bodies. 1. Production of primordial solutions a) Planetary leaching: The planet or part of the planet was leached as it cooled (high T). b) Impactor leaching: Comet and/or meteorite impactors were leached at the planet’s surface (low T). c) Combined planetary & impactor leaching 2. Evolution of primordial solutions (inorganic processes) a) Planetary bodies with limited plate tectonics: b) Planetary bodies with plate tectonics: Solution evolution depends on its initial bulk composition Solutes and solvents are modified at mid-ocean ridges, (including the salt:water ratio) and crystallization sequence. during formation and chemical fractionation of crust and the hydrologic cycle (weathering, river input etc.). 3. Solution remobilization/solute transport Glacial and hydrologic processes; alteration; comet and/or meteorite impacts, volcanic and magmatic processes, metamorphic and hydrothermal processes; aeolian processes. 4. Solution evolution (organic processes) Organically-mediated reef building, sulfate reduction, hydrocarbon production, sediment production etc.

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Pathfinder soils are slightly different depending on the calibration (Fig. 1a and 1b; Brückner et al., 2001; Foley et al., 2001), but these differences are not critical to our general model. 3.1. Leachates of Ultramafic Rocks and Chondrites Minerals found in ultramafic rocks such as olivine, pyroxenes, and sulfides are common in martian meteorites and chondrites, and olivine has been detected on Mars with infrared spectroscopy remotely and using landers (Christensen et al., 2004). Solutions that leach ultramafic compositions contain solutes derived from the most readily weathered primary minerals (olivine and sulfides). On Earth, such groundwaters are generally Mg-rich and Fe-poor compositions (discussed in detail in section 3.4). The cation ratios for continental groundwaters from ultramafic rocks are similar to calculated leachates of rocks with similar mineralogy, such as chondritic meteorites (H-, CV-, CM- CI-meteorites) and model bulk silicate earth (BSE; Zolotov and Shock, 2001a) shown in Figure 1b. The anions in the continental groundwaters are dominated by HCO3⫹CO3, and generally the SO4/(SO4⫹Cl) is high (Fig. 1a). There is limited data on the compositions of leachates from chondritic meteorite “falls”, but those measured to date are Mg-Na-Ca-rich with high SO4/(SO4⫹Cl) (Fig. 1a and 1b) and the leachates yielded high salinity solutions (Fanale et al., 1977; Fanale et al., 1998; Fanale et al., 2001). In the case of meteorite leachates, the soluble species are likely derived from the primary minerals and also from a variety of soluble MgNa-(K)-Ca-HCO3⫹ CO3-SO4-Cl-salts (e.g., salts with small log K in Table 1). Because leachate compositions may depend on kinetic and temperature effects and the physical pathway of the leaching solution (section 7.2), the “chondrite salt” composition calculated by Kargel (1991) may better reflect potential primordial solution compositions (Kargel, 1991; Hogenboom et al., 1995; Fanale et al., 1998). Relative to the ultramafic rocks and the chondrite leachates, the “chondrite salt” composition includes more Ca and HCO3 ⫹ CO3 because it accounts for CaCO3 (a less soluble salt; Table 1). It is unknown whether the more soluble salts (e.g., Mg-SO4rich salts) in chondritic meteorites form in terrestrial and/or extraterrestrial environments, because sulfate “blooms” have been observed in museum samples, presumably due to atmospheric water vapor diffusing in grain boundaries, then dissolving and remobilizing extraterrestrial soluble sulfate salts and/or altering primary minerals such as sulfides (Gounelle and Zolensky, 2001; section 6, below). Nonetheless, the observation that soluble sulfate salts can be mobilized on Earth is evidence that ultramafic rocks and chondritic meteorites may produce saline Mg-Na-(K)-Ca-HCO3⫹ CO3-SO4-Cl-solutions that precipitate highly soluble salts. 3.2. Leachates of Mafic Rocks Solutions that leach rocks of mafic compositions are Ca- and (Na ⫹ K)-rich relative to solutions derived from ultramafic rocks, as shown by the compositions of continental groundwaters (Fig. 1b). This is largely due to weathering of the more alkali-rich minerals (plagioclase and pyroxene) and basaltic glass. Leachates of mafic rocks have been investigated exper-

Fig. 2. Diagram showing the different fields for the different sulfur species, modified from Nesbitt (1984). The gray box indicates the log of the partial pressure of oxygen (log (PO2)) values for conditions suggested for the martian mantle, one to four log units below the quartz-fayalite-magnetite buffer (QFM-1 to QFM-4). As discussed in the text, at these log (PO2) values the stable sulfur species are H2S or HS- for the entire pH range. The dotted line indicates the hematitemagnetite equilibrium condition (Nesbitt, 1984). The diagram was prepared at 25°C and ⬃105 Pa, and as temperature is 2-lowered, the oxygen fugacity buffers, reactions, and SO42--HS- and SO4 -H2S boundaries have absolute log (PO2) values that are lower (e.g., see analogous diagrams in Gooding (1978) and Marion et al. (2003)); thus it is best to discuss the solution conditions relative to the buffers. Because Fe3⫹ (hydrous) oxides are observed on Mars along with sulfate salts, we propose that the martian solutions existed in the SO42- or HSO4- fields with log (PO2) values between ⫺69 (hematite field) and ⫺5 (the current martian atmosphere).

imentally also with application to Mars (Banin et al., 1997; Moore and Bullock, 1999; Golden et al., 2000; Golden et al., 2003; Tosca et al., 2003). 3.3. Oxidation State of the Primordial Fluids The primordial leaching solutions on Mars likely contained C-O-H-S-N-halogen species commonly found in volcanic gases and thought to be present in the early history of planetary bodies (Garrels and Mackenzie, 1971). These dissolved or gaseous species would have added to the reactivity of the leaching solutions and increased the solute contents, particularly if leaching occurred at high temperatures. To evaluate these processes in more detail it is necessary to first examine models for the oxygen partial pressure of these solutions; a high 2oxygen partial pressure will favor oxidized species (e.g., SO4 ), while low oxygen partial pressure favors reduced species (e.g., H2S). The oxygen partial pressure (PO2) was moderately low in Mars’ early history based on the calculated PO2 of the martian mantle (maximum oxygen fugacity of one to four log units below the quartz-fayalite-magnetite buffer; Ghosal et al., 1998; Herd et al., 2002 and references therein; Fig. 2). Low PO2 conditions were proposed for the early Earth (Garrels and Mackenzie, 1971) and likely existed on early Mars also. Under

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such PO2 conditions, the martian ultramafic-mafic crust contained sulfides (Fig. 2), as observed in the martian meteorites (McSween and Treiman, 1998), and leaching sulfides under 2these conditions would produce H2S and HS- species, not SO4 species, in solution (Fig. 2; for details see Nesbitt (1984)). However, the evidence for Fe3⫹ (hydrous) oxides on the martian surface (Burns, 1993; Bell et al., 2000; Christensen et al., 2000; Morris et al., 2000; Christensen et al., 2001) indicates that the surface log (PO2) conditions were at least ⫺69 (Fe2O3 field), which is ⬃7 log units higher than the mantle conditions 2(Fig. 2). At such PO2 conditions, SO4 and HSO4 are stable (Fig. 2), which is consistent with sulfate salts observed in the dust and cements (see above; Bandfield et al., 2003) and an (unknown) oxidizing agent in the regolith (Klein et al., 1976). Constraints on the upper log (PO2) value are based on the current martian atmosphere value of ⫺5. The mechanism for oxidation is likely H2-loss. Significant H2-loss on Mars is indicated by the high D/H value of the martian atmosphere relative to Earth and relative to the martian mantle (McKeegan and Leshin, 2001). Hydrogen-loss may drive up oxygen fugacity via reactions such as dehydrogenation of the atmosphere, which in turn may promote oxidation of Fe in silicates and other solids (e.g., martian amphiboles; King et al., 1999). Thermodynamic modeling in the Ca-Mg-Fe-Na-KSi-Al-C-S-Cl-H2O system shows that H2-loss, as expected on Mars during high-temperature alteration, will result in oxidation of C-O-H-S species (Zolotov and Shock, 2001b; Zolotov and Shock, 2003) and low-temperature alteration will be similar. 3.4. Reasons for Modeling Salts and Solutions in an Fe-Poor System Aqueous Fe2⫹ (Fe2⫹ aq ) is supplied to solution by weathering Fe-bearing phases (pyrite, glass, olivine, pyroxene), either in the presence of O2-H2O-CO2 gas (Gooding, 1978) or aqueous fluids (Burns, 1988; Burns, 1993; Catling, 1999; Marion et al., 2003). Despite the liberation of Fe during weathering, most continental groundwaters from both ultramafic and mafic rocks have extremely low concentrations of total Fe in solution (e.g., White et al., 1964; Nesbitt and Bricker, 1978). This is in common with solutions derived from leaching chondrites (Fanale et al., 1977; Fanale et al., 2001), calculated “chondrite salts” (Kargel et al., 2000), and experimental Mars analog solutions (Moore and Bullock, 1999; Golden et al., 2000; Golden et al., 2003). Low Fe2⫹ aq concentrations in solution are related to the high precipitation rates for Fe3⫹ (hydrous) oxides, even at relatively low PO2 (Fig. 2; also see Burns, 1993; Marion et al., 2003). For example, hematite is observed on the surface of Mars (Christensen et al., 2000; Christensen et al., 2001) and it precipitates at log (PO2) greater than ⫺69. Ferrihydrite (Fe(OH)3) is present in martian meteorites (Bridges et al., 2001), and it also precipitates in the SO42- and HSO4 fields, dependent on pH and the activity of Fe2⫹ in solution (see Marion et al., 2003). Precipitation of such Fe3⫹ (hydrous) oxides will lower the Fe concentration in the martian solutions. For this reason, we model an Fe-poor solution system and limit our discussion of Febearing salts to relatively insoluble FeCO3 (siderite; see section 5.1) and Fe3⫹-sulfates (section 5.1).

3.5. Summary of the Bulk Composition of the Martian Solutions Taken together, the thermodynamic considerations and minerals identified on Mars indicate that the solutions contained oxidized species (SO42-, CO32- ⫹ HCO3-) and precipitated sulfates and carbonates. Because the surface salts are oxidized, we speculate that H2-loss influenced the oxidation state of the solutions and crystallized salts and oxides early in Mars’ history. The Pathfinder soils and recalculated Viking soils (McSween and Keil, 2000; Brückner et al., 2001; Foley et al., 2001) have Ca-Mg-Na cation ratios similar to calculated “chondrite salt” composition (Fig. 1a and 1b) and they are intermediate to the compositions of groundwaters from ultramafic and mafic rocks, the calculated Europan ocean and chondrite leachates (Fig. 1b). Both Viking and Pathfinder soil results have relatively SO4/ (SO4 ⫹ Cl)-rich compositions, although the Brückner et al. (2001) data are most SO4/(SO4 ⫹ Cl)-rich. Such Mg-rich, SO4/(SO4 ⫹ Cl)-rich end-member solutions can be explained by our physical model (Table 2, section 2). The oxidized nature of the S- and C-species can be explained by H2-loss from a neutral to basic solution and is consistent with thermodynamic models and minerals identified on Mars. Below we examine the evolution of solutions and salts in two model end-member bulk primordial martian hydrospheres: (1) Mg-Na-SO4-Cl solutions, and (2) Mg-Na-Ca-Fe-HCO3⫹CO3SO4-Cl-PO4 solutions. We predict the nature and sequence of salts precipitated and their effect on the composition of the solution during equilibrium removal of water (e.g., evaporation or freezing). This model allows us to gain insight into the formation processes affecting the near surface of Mars and other planetary bodies. 4. MODEL 1: EVOLUTION OF Mg-Na-SO4-Cl-RICH BULK PRIMORDIAL SURFACE SOLUTIONS

The evolution of Mg-SO4/(SO4 ⫹ Cl)-rich solutions, such as those derived by leaching ultramafic rocks and/or chondrites, can be evaluated using the Mg-Na-SO4-Cl phase diagram (Hardie and Eugster, 1970; our Fig. 3). This diagram is appropriate for modeling the martian hydrosphere because it includes the major species in the Viking and Pathfinder soils, chondrite leachates (calculated and experimental) and “chondrite salt” compositions (Fig. 3). Significantly, the soil compositions are similar to calculated Europan ocean compositions (based on altering ultramafic rocks) and also close to chondritic leachate and “chondrite salt” compositions, indicating that the bulk martian surface solutions were indeed Mg-SO4/(SO4 ⫹ Cl)-rich and Na-Cl-poor (Fig. 1a and 1b). The ultramafic and mafic groundwaters are not plotted on this diagram because they contain considerable HCO3 ⫹ CO3; the evolution of these end-member solutions is discussed in section 5.1. The Mg-Na-SO4-Cl phase diagram (Fig. 3) was prepared at 25°C and ⬃105 Pa, but can be applied to colder, lower pressure planetary bodies because the phase topology is qualitatively correct (⬃-5 to 45°C, Fig. 3 of Kargel (1991)) and the phases modeled are similar to those used in thermodynamic models at low temperature (although those models used a different bulk composition and did not include bloedite so that the predicted

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Fig. 3. Phase diagram for the Mg-Na-SO4-Cl system at 25°C and ⬃105 Pa showing stable salt assemblages and solution compositions (Hardie and Eugster, 1970). Salts discussed in the text include epsomite (Ep), hexahedrite (Hx), kieserite (Ki), bischofite (Bi), halite (Ha), bloedite (Bl), mirabilite (Mi), and thenardite (Tn). Other salts are listed in Table 1. Fig. 3a shows calculated compositions of solutions extracted from chondrites and the bulk silicate earth (Zolotov and Shock, 2001b). Compositions of leachates from carbonaceous chondrites are from Orgueil (OL; Fanale et al., 1977), Murchison (ML; Fanale et al., 1998; Fanale et al., 2001), and “chondrite salt” (Kargel et al., 2000). The Pathfinder soil compositions (salts ⫹ silicates ⫹ oxides) are shown using calibrations from Brückner et al. (2001); Pathfinder-B) and Foley et al. (2001); Pathfinder-F). The solution evolution paths are discussed in detail in the text. The Cl- content of the Orgueil leachate was calculated as described in Figure 1. Figure 3b shows the predicted solution evolution path of the Orgueil leachate (OL to #6 and #7) as discussed in the text. Analyses of the leachate from Nakhla (Sawyer et al., 2000) are also shown along with potential paths for evolution of the Nakhla solutions (see text) from a hypothetical starting composition a, which is close to the compositions of leachates, chondrite salts, and the martian soils. The solution pathways to points b-g and beyond are discussed in detail in the text. Vectors in compositional space for gypsum precipitation (Gy.) and Na-carbonate precipitation (Na) are also shown from the hypothetical starting composition, a.

salt sequence is slightly different; Zolotov and Shock, 2001a). Furthermore, pressure effects are negligible in the MgSO4-H2O system from atmospheric pressure to 400 MPa (Hogenboom et al., 1995), thus we assume that they are negligible at the lower

pressures on Mars for the systems of interest. The Mg-NaSO4-Cl phase diagram presented in Figure 3 is electrically neutral (charge-balanced) and includes the major components present, and thus provides more information than previously

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published salt-water ternary phase diagrams (Fanale et al., 1977; Brass, 1980; Kargel, 1991). We assume that when Mg-SO4/(SO4 ⫹ Cl)-rich solutions are progressively concentrated as water is removed by evaporation or freezing, that the salts crystallize in a sequence predicted by equilibrium considerations. Although kinetic factors may produce nonequilibrium salt assemblages, the study of terrestrial closed basin brines demonstrates that most evaporite sequences are successfully predicted from equilibrium modeling and equilibrium-based phase diagrams. We also assume that equilibrium considerations are followed for the system when water is removed by freezing. It is unclear whether freezing experiments in this system have produced equilibrium salt assemblages where sufficient water was removed to result in supersaturation of salts, because in those studies the salt compositions were not quantitatively analyzed (Herut et al., 1990; Fanale et al., 2001). However, as we show below, our predictions are qualitatively consistent with infrared spectra of freezing experiments in the Mg-Na-SO4-CO3 system (⬍130 K and ⬍10-8 torr, McCord et al., 2002), further suggesting that the phase diagram topology is similar at cold, low pressure conditions. Because all potential starting compositions plot in the epsomite (Ep) field, all solutions will precipitate Ep first if concentrated (Fig. 3a). Removal of this salt causes the solution to evolve towards point A, where bloedite (Bl) crystallizes with Ep. The solution composition then follows the Ep-Bl eutectic curve until halite (Ha) precipitates at the peritectic (P1). The bulk compositions of all solutions are such that Ha is the last salt to join the precipitation sequence, provided all previously precipitated salts are available to react with the solution of P1 composition. If instead the solution is isolated from all previously precipitated salts, it will continue to evolve to another peritectic (P2: Ep ⫹ hexahedrite (Hx) ⫹ Ha), and finally to the eutectic (E) where Ha ⫹ Hx ⫹ bischofite (Bi) crystallize. These salt assemblages (Ep ⫹ Bl ⫹ Ha/Bi) probably existed on the martian surface (Baird et al., 1976; Clark et al., 1976; Brass, 1980; Banin et al., 1992; McSween and Keil, 2000), although Ep may now be found in a less hydrated form (e.g., Hx or kieserite, Ki) because the water vapor pressure on Mars is low (section 6). 5. MODEL 2: EVOLUTION OF Mg-Na-Ca-(Fe)-HCO3ⴙCO3SO4-Cl-PO4 BULK PRIMORDIAL SURFACE SOLUTIONS

As discussed above, it is likely that the martian primordial solutions also contained Ca, Fe, HCO3 ⫹ CO3, and PO4 (Fig. 1a and 1 b). Thus, the salt evolution of the martian primordial solutions is best described by the Mg-Na-Ca-(Fe)-HCO3 ⫹ CO3-SO4-Cl-PO4 system where it is probable that Ca and HCO3⫹CO3 are significant constituents and Al, Fe, and PO4 are minor constituents, by analogy with the chemistry of groundwaters from mafic continental rocks (Fig. 1a and 1 b). The behavior of Fe has already been discussed (section 3.4; Burns, 1993; Marion et al., 2003). Using the Mg-Na-Ca-HCO3 ⫹ CO3-SO4-Cl system, we can predict the salt crystallization sequence with solubility products (Table 1): less soluble carbonate, sulfate, and phosphate salts will crystallize early, and more soluble sulfate, carbonate and chloride salts will crystallize later.

5.1. Effects of Crystallizing Less Soluble Carbonate, Phosphate, and Sulfate Salts Equilibrium crystallization of the Mg-Na-Ca-HCO3⫹CO3SO4-Cl solutions can be evaluated using both Ca-Mg-HCO3SO4 and Mg-Na-SO4-Cl phase diagrams (Figs. 3 and 4). There are extremely limited data on the Mg-Na-Ca-HCO3-SO4-Cl system at the low pressures and temperatures on Mars, in particular there is little data on Mg-CO3 salts (Marion, 2001), and thus, it is difficult to evaluate whether it is valid to extrapolate the 25°C, ⬃105 Pa phase diagram to martian conditions. We assume that the general topology of the diagram is not greatly affected (similar to our arguments in section 4). The HCO3/(HCO3⫹SO4) of the martian primordial solutions are poorly constrained, therefore we show the “chondrite salt” composition (point B in Fig. 4) as an analog for solutions derived from ultramafic rocks. However, if the Noachian martian atmosphere had a high partial pressure of CO2, the HCO3 in solution would be higher (close to A, groundwaters from mafic rocks in Fig. 4; see section 5.3). If leaching occurs at high temperatures then Ca and HCO3 would tend to go into solution (closer to point B). In both cases, the initial solution compositions would first crystallize calcite (Cc.), thus this range in starting composition does not materially affect the salt precipitation sequence. As the solution composition evolves away from the Cc. composition, Mg/Ca increases; therefore the calcite becomes more Mg-rich (Mg-Cc). The solution will, therefore, follow a curved path that begins with a slope close to the Cc. trajectory (solid line) and becomes steeper as more Mg-Cc. is removed. Removal of calcite (Cc.-Mg-Cc. solid solution) will result in the solution evolving to more Mg-rich compositions until Ca-protodolomite (Ca-Pr.) precipitates along with Mg-Cc., lowering the HCO3 content of the solution until P1, where gypsum (Gy.) joins the assemblage. When Mg-Cc. is consumed, the solution composition follows the Gy. ⫹ Ca-Pr. curve to P2; Gy and magnesite (Mag.) then crystallize until the assemblage is joined by Ep at E. The details of the peritectic curves (Cc. ⫹ L1 3 Ca-Pr. ⫹ L2 and Ca-Pr ⫹ L3 3 Mag. ⫹ L4, where L is a liquid composition) are slightly more complicated due to Mg-Ca solid solution (Nesbitt, 1990). If HCO3/ (HCO3⫹SO4) was initially very low, then the first salt to crystallize would be Gy until the solution reaches P1, followed by Gy. ⫹ Ca-Pr. to P2; Gy ⫹ Mag. to E where Gy ⫹ Mag. ⫹ Ep crystallize. Iron-rich carbonates are found in martian meteorites (summary in Bridges et al., 2001), and such carbonates are expected based on the log K value for siderite (FeCO3; Table 1). Because Fe2⫹ concentrations are probably low (section 3.4) and Fe-rich carbonate is not very soluble, it will precipitate early in the Cc.-Mg-Cc. solid solution sequence, resulting in a small reduction of HCO3/(HCO3⫹SO4). Overall, equilibrium crystallization of Mg-Ca-HCO3-SO4 solutions will result in Mg- and SO4/(SO4 ⫹ Cl)-rich solution compositions with epsomite stable, so long as Mg is in excess of dissolved phosphate or bicarbonate. This condition is met in most terrestrial solutions and is likely true on Mars because large quantities of Mg-phosphates and (hydro)magnesite are not observed in martian meteorites. Magnesite was tentatively identified as a possible carbonate mineral by thermal infrared spectroscopy (Bandfield et al., 2003); however, the magnesite

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Fig. 4. HCO3/(HCO3 ⫹ SO4) vs. Mg/(Mg ⫹ Ca) phase diagram for the carbonate system (modified after Nesbitt (1990)). The phase diagram was prepared based on data at 25°C and ⬃105 Pa, and we assume that temperature and pressure do not change the general topology of the phase relations. The epsomite field has been expanded for clarity. We have plotted groundwater compositions from mafic continental areas in area A (White et al., 1964) and “chondrite salt” at point B (Kargel et al., 2000). These two starting solutions (and any starting solutions in between) would follow the same equilibrium crystallization sequence: Mg-Cc.; Mg-Cc. ⫹ Ca-Pr.; Gy. ⫹ Ca-Pr.; Gy. ⫹ Mag.; and finally Gy. ⫹ Mag. ⫹ Ep. Note that this diagram does not explicitly show the solutions derived from chondrites (Zolotov and Shock, 2001b), and groundwaters derived from ultramafic rocks (Fig. 1). These have such high Mg/(Mg ⫹ Ca) values that they would evolve first to P2 after crystallizing either Ca-Pr., Mag. or Gy, dependent on their initial HCO3/(SO4 ⫹ HCO3) content.

fit may represent a nonunique solution (as those authors stated) and an extensive set of Ca-Mg carbonates were not examined. Because the precipitation of carbonates leads to a solution that is enriched in Mg and SO4 components, then the Mg-NaSO4-Cl diagram (Fig. 3) is appropriate for modeling these martian solutions once the carbonates and phosphates have crystallized. We assume a starting composition near point a is representative of a martian primordial solution (near other possible compositions). If there is early crystallization of MgPO4 salts or Ca-Mg-rich carbonates (e.g., Mg-Cc., hydromagnesite and other salts listed in Table 1), the solution will decrease in Mg/(Na2 ⫹ Mg) value as it is concentrated and those Mg-bearing minerals crystallize (points b - g, Fig. 3b). If sulfate salt crystallization commences at point b or c (Fig. 3b), Ep or Bl respectively will be first to crystallize until the solution composition intersects the Ep-Bl join, after which it evolves along the join towards P1. If sulfate salt crystallization commences at point d (Fig. 3b), the solution will first precipitate Bl, which will be joined by Ha ⫹ Bl and terminate its evolution at the Ep ⫹ Ha ⫹ Hx peritectic (P1). Analogous crystallization paths can be constructed from the more Na-rich solutions (points e, f and g to P4, P3, P1). 5.2. Effects of Crystallizing Other Carbonate, Bicarbonate, Sulfate, and Chloride Salts Crystallization of more soluble Na carbonates, bicarbonates and other salts (listed in Table 1) will drive solution compositions towards higher Mg/(Na2 ⫹ Mg). For example, for an

initial solution at point a, the solution composition would follow the Na arrow in Figure 3b, resulting in few changes to the salt assemblages crystallized. Because there is currently no quantitative evidence of Na-rich sulfate salts in martian meteorites (although it can not be ruled out because thenardite has been identified qualitatively in the carbonaceous chondrite Murray; King and King, 1981), the martian salts probably crystallized from solutions with relatively high Mg/(Na2 ⫹ Mg) (i.e., not points e– g in Fig. 3b). Crystallization of other sulfates (e.g., K2SO4, FeSO4, CaSO4, NiSO4, ZnSO4, Cu1–2SO4 and related hydrates) will only slightly deplete the solution in SO4/(Cl2 ⫹ SO4) (Gy. trend from point a in Fig. 3b) due to the low concentrations of K, Fe, Ca, and Ni relative to Mg. Precipitation of Al-SO4 salts such as alunogen (Al2(SO4)3.17H2O) (Banin et al., 1997) and alkali-Al sulfates is unlikely, given the low concentrations of Al in the martian solutions (e.g., solutions derived from basaltic material without glass; Tosca et al., 2003). Jarosite (KFe3(SO4)2(OH)6), a relatively insoluble sulfate that undergoes hydrolysis, has been proposed to exist on Mars (Burns, 1988; Burns, 1993; Morris et al., 1996). However, Fe-sulfates are uncommon in martian meteorites (Bridges et al., 2001), and they tend to form in acidic conditions that are difficult to reconcile with the presence of carbonates and lack of weathering (section 6). Furthermore, large quantities of jarosite are unlikely to form due to low Fe concentrations (section 3.4) and/or low K concentrations in solutions, in contrast to areas such as Hawaii where K is available from seawater aerosols (cf.

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Morris et al., 1996; Golden et al., 2003). If K is the limiting component, then other ferric hydroxysulfate minerals may precipitate (e.g., melantorite; see a summary of other ferric hydroxysulfate minerals in Alpers et al. (2000)). Alternately, other jarosite-like minerals may precipitate; for example Ferich sulfates with different univalent cations (e.g., Na⫹, NH4⫹, H3O⫹) or different divalent cations (e.g., Ni2⫹, Cu2⫹, Zn2⫹). Better constraints on the thermodynamic properties of such minerals would aid modeling the precipitation of Fe-rich sulfates. Crystallization of other chlorides and chloride hydrates (e.g., KCl, CaCl2.6H2O, and other chloride salts; Table 1) is unlikely to enrich the solution in SO4/(Cl2 ⫹ SO4) (opposite of the Gy. trend in Fig. 3b) due to low concentrations of the cations. The effect of other halogens (e.g., Br) is currently difficult to constrain, but is likely to be better defined after data from the recent Mars missions has been processed. In summary, early crystallization of Ca-Mg-carbonates and phosphates causes SO4 and Cl to increase in residual solutions, resulting in the precipitation of Mg-(Na)-SO4-rich salts and CaSO4-(hydrate) salts. This sequence leads to solutions with higher Cl contents and possibly crystallization of Na-, Mgchloride salts. 5.3. Effects of Changing CO2 Levels on the Primordial Fluids If the martian atmosphere had a high partial pressure of CO2, (PCO2), the concentration of HCO3 ⫹ CO3 in solution would be high and the solution would tend to be more acidic (e.g., 2CO2 ⫹ H2O 3 H⫹ ⫹ HCO3- 3 2H⫹ ⫹ CO3 ), dependent on other acid-base weathering reactions involving Mg-Fe silicates (e.g., olivine and pyroxenes), Ca-Mg-Al silicates (e.g., plagioclase and pyroxenes), sulfides and carbonates. Acidic solutions, derived from a high PCO2, would tend to dissolve carbonate salts (e.g., CaCO3 ⫹ H⫹ 3 Ca2⫹ ⫹ HCO3-) and inhibit the precipitation of Fe3⫹ (hydrous) oxides (Burns, 1993). Because carbonate salts and Fe3⫹ (hydrous) oxides are found on the martian surface (Christensen et al., 2000; Christensen et al., 2001; Bandfield et al., 2003; Christensen et al., 2004) and in martian meteorites (Bridges et al., 2001), the pH of the solutions when these minerals precipitated was likely near neutral to basic, and the HCO3 ⫹ CO3 and PCO2 were not exceedingly high. Related arguments can be made if significant CO2 is sequestered in clathrates, which are significant phases in cold conditions on Earth (e.g., Miller, 1969). When clathrates crystallize, the concentration of HCO3 ⫹ CO3 in solution will decrease and the solution will tend towards neutral or basic pH, dependent on other acid-base weathering reactions. 6. MODIFICATION OF THE PREDICTED SALTS AND WEATHERING ON THE MARTIAN SURFACE

As noted above, the salts on Mars’ surface may have been remobilized or (re)deposited over time. In particular, the highly soluble salts (Table 1) are easily remobilized by extremely low water concentrations (sulfates; Gounelle and Zolensky, 2001) and may also be remobilized by low water concentrations at low temperatures (Mg-carbonates; Jull et al., 1988).

It is also possible that highly hydrated salts on the martian surface may have lost water via thermal or radiolytic water loss and/or ion and electron bombardment, resulting in structural disorder (McCord et al., 2002). Thus, salts such as Ep may now be found in a less hydrated form (e.g., Hx or kieserite, Ki), and Gy may be found as anhydrite because the water vapor pressure on Mars is low (Clark, 1978) and salts with high water contents are not necessarily thermodynamically stable (Gooding, 1978; Vaniman et al., 2004). Nonetheless, we contend that the overall bulk composition of the salts in the martian regolith is likely representative of the solutes in the bulk primordial hydrosphere for the following reasons: (1) processes leading to remobilization and solute transport (Table 2) result in the same compositions as those predicted by extracting solutions from mafic, ultramafic, or chondritic rocks (section 3); (2) the rate of soil production on the extremely cold surface of Mars is likely very slow and could be on the order of 1 m per billion years, including meteoritic input (Flynn and McKay, 1990); therefore, the cold regolith on Mars is likely superposed on older Noachian zones (Christensen, 2003); (3) several lines of evidence suggest that limited weathering has occurred on the martian surface since the Noachian. For instance, a restricted degree of weathering is suggested to occur at the Viking and Pathfinder sites based on the bulk geochemical analyses (Bell et al., 2000; McLennan, 2000; McSween and Keil, 2000). Also, limited weathering is inferred for the martian meteorites because there is stable isotopic disequilibrium between salts, other alteration minerals and the high-temperature silicates (review in Bogard et al., 2001). While it is debatable whether clay minerals are present on Mars (Wyatt and McSween, 2002), if they are present then Ca-Mg-Al-silicates probably have been weathered, resulting in neutralization of any highly acidic solutions (Garrels, 1967) that would further limit weathering. All things considered, although the primordial salts have probably been remobilized and (re)deposited on the martian surface, their bulk composition likely reflects the bulk primordial hydrosphere composition. Lateral and vertical variations in solution distribution due to the location(s) of paleo-solutions (oceans or lakes) are expected, but the general sequence of salt precipitation predicted above is plausible on a global scale. 7. COMPARISON OF THE MODEL WITH MARTIAN METEORITE DATA

7.1. SNC Meteorite Salt Data The salts in the SNC (Shergottite-Nakhlite-Chassignite) or “martian” meteorites also support our model and indicate that the predicted salts may be stable on the surface of Mars. The SNC meteorites contain the sequentially crystallized salts: Ca-Mg-phosphate, Fe-Ca-Mg-carbonate, Ca-sulfate-(hydrate) through Na-Mg-(Ni)-sulfate, along with minor chlorides and nitrates (McSween and Treiman, 1998; Bridges et al., 2001). Previous workers have proposed that the salts on Mars are stratified with Mg-Na-SO4-rich salts at the surface and CaSO4(hydrate) salts and Ca-Fe-Mg-CO3 salts at depth (Gooding, 1978; Banin et al., 1997; Warren, 1998). This sequence could have been produced by evolution of a bulk primordial Mg-SO4/ (SO4 ⫹ Cl)-rich solution. As the solution was concentrated it crystallized carbonates and phosphates first (at depth, or the carbonates settled from the surface solution analogous to pro-

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cesses sometimes observed in salt lakes on Earth), depleting the initially Fe- and Ca-poor surface solution in Fe and Ca. As carbonates crystallized, they became zoned from Fe- to Ca- to Mg-rich compositions (review in Bridges et al. (2001), also see Catling (1999)). The observed correlations of Mg/(Ca ⫹ Mg) with increasingly heavier ␦18O values and increasing ␦13C values are consistent with solution concentration (review in Bridges et al. (2001)). Sulfur isotope systematics are also instructive: ␦34S data for Nakhlites also suggest interaction with oxidized surface fluids percolating downward (Greenwood et al., 2000), as do large 17O excesses and large 33S depletions (Farquhar and Thiemens, 2000; Farquhar et al., 2000). One feature of many of the martian meteorites is the relatively low abundance of sulfates and chlorides (McSween and Treiman, 1998; Bridges and Grady, 2000). If the martian meteorites had salt compositions in the same ratio as the Viking and Pathfinder soils, then the expected SO4/(SO4 ⫹ Cl) is ⬃80% (Fig. 1a), which if precipitated, results in a sulfate: chloride salt mass ratio of ⬃11:1. This prediction may be tested if SO4 and Cl are the limiting components in forming sulfate and chloride salts. However, the prediction may not be informative because it is not known whether the salts in the meteorites precipitated from a solution composition recorded on the martian surface by the landers, or whether the meteorite salts crystallized from a more evolved composition (e.g., Fig. 3a and 3b). Furthermore, the sulfates are so hygroscopic that they may be easily removed or remobilized during sample preparation (e.g., sample cutting and thin section preparation), or may migrate along grain boundaries during storage (Velbel, 1988; Gounelle and Zolensky, 2001). 7.2. Nakhla Meteorite Leachate Data If the soluble salts of the Nakhla meteorite are homogeneously distributed, and if solution access to the samples is similar, experimental leachates should yield similar compositions. However, the two leachate solutions of Sawyer et al. (2000) differ substantially in their proportions of the components Na and Mg, and Cl and SO4, thus demonstrating the heterogeneity of the Nakhla samples leached. The heterogeneity is expected because the meteorite has undergone efflorescence and/or differential leaching of salts (Gooding et al., 1991), and the leachates of Sawyer et al. (2000) indicate such a scenario. One Nakhla leachate sample plots in the halite field and the other in the bloedite field (Fig. 3b), demonstrating that the solutions have leached different proportions of salts from the meteorite. The Na-Cl-rich composition of Nakhla interior (a) sample of Figure 3b demonstrates that it has dissolved primarily Na⫹ and Cl-, with only minor amounts of Mg2⫹ and/or SO42- in solution. By contrast, Nakhla interior (b) sample (Fig. 3b) has dissolved large amounts of Mg2⫹ and SO42, and lesser amounts of Na⫹ and Cl-. The Nakhla interior (b) sample does not have the extreme Na and Cl ratios found in terrestrial seawater (Figs. 1 and 4b). In fact, the composition of the Nakhla interior (b) leachate (Fig. 3b) can be achieved without dissolving any NaCl; if MgCl2 and NaSO4 salts were dissolved in the leachate in a ⬃1: 1 ratio, the resulting leachate would yield the Nakhla interior (b) solution composition. These

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leachate data demonstrate that the salt content in the meteorite is heterogeneous. Thus, all data and calculations contained herein, except the Nakhla (a) leachate, support a Mg-rich, SO4/(SO4 ⫹ Cl)-rich solution. 8. IMPLICATIONS FOR THE HYDROSPHERES OF ROCKY PLANETARY BODIES

Water is thought to have been distributed heterogeneously in the primordial solar system, with water enrichment at Jupiter due to the formation of ice (the “snow line”; Cyr et al., 1998). Dynamic models predict that water enrichment in the outer asteroid belt and on Earth results from the inward drift of icy particles and/or water delivery from comets and/or asteroids (e.g., Cyr et al., 1998; Delsemme, 1999; Morbidelli et al., 2000). We now examine how varying the bulk water content on planetary bodies will affect brine and salt evolution. We propose that the hydrospheres on Mars, Europa, Ganymede, and Callisto had Mg-SO4/(SO4 ⫹ Cl)-rich bulk primordial solute compositions with varying water contents, and that the solutions evolved in an analogous manner upon concentration. The idea that these planetary bodies have MgSO4/(SO4 ⫹ Cl)-rich solutions derived from leaching a bulk chondrite (ultramafic) composition has been examined in detail by various authors (Kargel, 1991; Kargel et al., 2000; Zolotov and Shock, 2001a). As we show above for Mars, these planetary hydrospheres could also have been derived by leaching ultramafic-mafic rocks. The surfaces of Europa, Ganymede, and Callisto are thought to contain hydrated Mg-Na-SO4-HCO3⫹CO3 salts, solutions, and ice (solid H2O) (McCord et al., 1998b; McCord et al., 2001). Possible species include epsomite and natron (McCord et al., 2001), or more hydrated MgSO4 minerals (Dalton et al., 2004), H2SO4 hydrate (Carlson et al., 1999), or CO2 and SO2, which may be products of irradiation of non-ice materials (Cooper et al., 2001). We refer to this surface as a “shell” (salts, solution and ice). If the bulk primordial solutions of these satellites were Na-Mg-SO4/(SO4 ⫹ Cl)-rich, then the most likely salts crystallized would be epsomite and bloedite (between A and P1 on Fig. 3a), not epsomite and natron because they are not thermodynamically favored (Marion, 2001). As described for Mars, if the solutions were concentrated to P1 (Fig. 3a), then Ha would crystallize and Bi would ultimately crystallize (at E, Fig. 3a). If the solutions were CO3-rich, then Mg or Na carbonates might also precipitate. Because the initial bulk composition was Mg-SO4/(SO4 ⫹ Cl)-rich, it is unlikely that compositions with extreme Cl/(Cl ⫹ SO4) similar to seawater on Earth were attained through concentration processes on these planetary bodies. We further hypothesize that other rocky planetary bodies, such as asteroids, have undergone similar processes (Fredriksson and Kerridge, 1988; Hogenboom et al., 1995). The idea that these solutions were present on C-type asteroids is consistent with the sequence of salt assemblages observed in carbonaceous chondrites (Richardson, 1978; Zolensky and McSween, 1988; Endre␤ and Bischoff, 1996; Brearley and Jones, 1998). Carbonates in the Ivuna meteorite are typically zoned from Cato Mg-rich compositions (Endre␤ and Bischoff, 1996), as our model predicts (Fig. 4). Also, the Orgueil Na-Mg-Ni-sulfates show decreasing SO4 (Fredriksson and Kerridge, 1988) and

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Fig. 5. Schematic phase diagram for salt (e.g., Mg-SO4- rich salt) and water. The crystallization pathways for water-rich and water-poor solutions are discussed in the text. Briefly, if a water-rich solution is cooled, ice will form at the surface first, beginning at point I, with final salt and ice crystallization at E. If a water-poor solution is cooled, salt will form at the surface first, beginning at point S, and again with final salt and ice crystallization at E. These scenarios could explain small-scale ice-rich and salt-rich portions of Europa, Ganymede, and Callisto. The surface of Mars may have been produced either from solutions with low initial bulk water content or from water-rich solutions that lost water (e.g., via evaporation or freezing) and became water-poor (path A to B). Note that solution density decreases as water content increases.

increasing Mg/(Na2 ⫹ Mg) contents (qualitative analysis; Fredriksson and Kerridge, 1988), consistent with our predicted solution evolution path (OL to #6 to #7 in Fig. 3b). The Orgueil sulfate #7 (Fig. 3b) was crystallized from a solution that was somewhat more evolved, having crystallized both epsomite and bloedite. The salts on icy planetesimals could vary vertically and laterally dependent on salt solubility and the relative densities of salt and ice solution (Kargel, 1991; Kargel et al., 2000). For instance, Europa’s surface is Mg-SO4/(SO4 ⫹ Cl)-rich with icy areas, while 30% to 50% of Ganymede’s surface is ice-rich (with some areas almost pure ice), and Callisto likely has less ice (McCord et al., 1998a; McCord et al., 1999). We propose that these differences might additionally result from variations in surface temperatures and differences in the water contents of the bulk primordial solutions. For instance, if water-rich solution is cooled, ice will form at the surface first (as in Arctic areas on Earth). This scenario is shown schematically for a hypothetical salt (e.g., Mg-SO4-rich salt) and water system (Fig. 5). At point I (the ice liquidus), ice crystallizes and it is effectively removed from the system by floating, producing ice at the surface. The ice could act as a thermal blanket, preventing much additional cooling, or if temperature drops further, the underlying solution will evolve (becoming more dense) towards the eutectic (E) composition, eventually to crystallize salt and ice from the eutectic solution.

This crystallization path could explain both the ice-rich and salt-rich areas on Europa, Ganymede, and Callisto. If instead, a solution was water-poor, then a different crystallization path will be followed. When a water-poor solution is concentrated, salts will crystallize first (point S on the salt liquidus; Fig. 5). On Earth, when salts crystallize they commonly nucleate at the surface and may trap atmospheric gases as inclusions forming stable surface crusts of salts with overall reduced density relative to the density of an inclusion-free salt (e.g., salt lakes with surface salt crystals). As the temperature continues to drop, the solution composition trends towards E (the solution becomes less dense), and at temperatures lower than E, the salt and ice crystallize in eutectic proportions. For example, a water-poor solution may produce an Mg-SO4-rich salt at the surface. Water-poor planetary bodies could result from either initially low bulk water contents or due to H2O-loss (path A to B; Fig. 5) through freezing, sublimation, and/or other processes such as H2-loss. To summarize, the surface composition of a planetary shell is determined not only by the densities and solubilities of the phases present in the shell, but also by the crystallization sequence and initial solution composition. 9. COMPARISON BETWEEN THE EVOLUTION OF SOLUTIONS ON PLANETARY BODIES

On Earth, modern seawater has been modified from its primordial composition (Table 2) by the formation and chem-

Primordial solutions on Mars and other planetary bodies

ical fractionation of the continental crust (which is approximately andesitic in composition; Taylor and McLennan, 1985), hydrothermal brine flux at midocean ridges, and the hydrologic cycle (e.g., weathering, river input.) (Hardie, 1996; Lowenstein et al., 2001). These inorganic processes were supplemented by organic processes that resulted in solvents and solutes being sequestered in some cases and liberated in others (Table 2). In contrast to Earth, the primordial solutions on Mars would not have been affected to the same extent by these processes because: 1) the crust is likely mafic in composition and weathering is limited; 2) midocean ridge hydrothermal brines may have a limited role in changing surface solution compositions, for example by reducing sulfate to sulfides; 3) solutions have existed episodically on the cold martian surface, resulting in a limited hydrologic cycle; and, 4) it is uncertain whether organic processes have a role on Mars, but any role is minor compared with present-day Earth, thus biologic sulfate reduction and biologic carbonate formation are less important. Overall, there are fundamental differences between Earth and Mars indicating that the martian primordial solutions would not follow Earthbased chemical pathways. It is likely that the compositional evolution of the primordial solutions on Mars and the icy Galilean satellites was influenced by the initial water content of the Mg-SO4/(SO4 ⫹ Cl)-rich bulk primordial solution and the crystallization path. On Mars, the salts (Ep ⫹ Bl ⫹ Ha/Bi) have been preserved as cements, veneers, and dust, and there is evidence for near-surface groundwater solutions. The martian solutions have evolved to more Cl/(SO4⫹Cl)-rich compositions, but have not produced Na-Cl-rich solution compositions, like terrestrial seawater. The surface shells of Europa, Ganymede, and Callisto were derived by leaching of carbonaceous chondrite materials (Kargel et al., 2000). On those planetary bodies, the bulk primordial hydrosphere may have had a similar composition, but the water content may have been higher than Mars (Fig. 5). In contrast to Earth, the surface solutions on Mars, the asteroids, and the icy Galilean satellites did not evolve substantially from their Mgrich, SO4/(SO4 ⫹ Cl)-rich bulk primordial compositions, and it is unlikely that compositions with extreme Cl/(SO4 ⫹ Cl) similar to seawater on Earth were attained through concentration processes on these planetary bodies. The relative lack of compositional evolution of these surface solutions is likely related to the absence of a heterogeneous crust and upper mantle (lack of plate tectonics), lack of sulfate reduction mechanisms (hydrothermal brine flux and biologic sulfate reduction), and relatively low temperatures. The timing of the salt precipitation event(s) on the different planetary bodies is difficult to constrain. The salts on the asteroidal bodies (chondritic meteorites) were likely present in the early solar system based on 53Mn-53Cr chronometry (Endre␤ et al., 1994), model Rb-Sr ages (Zolensky et al., 1999), and Ar-Ar and I-Xe analyses (Whitby et al., 2000) of chondritic meteorites. It is possible that H2O-loss from the asteroids occurred within 2 million years of the oldest known solar system minerals (Whitby et al., 2000), although there is also evidence for episodic salt precipitation events (Endre␤ and Bischoff, 1996). As noted above, waters on Mars also existed episodically, and the water content was likely relatively low (Fig. 5). In contrast, the Galilean satellites likely had higher water contents (Fig. 5) due to their proximity to Jupiter and the

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“snow line,” where water is thought to be enriched in the solar system (e.g., Cyr et al., 1998). In conclusion, the processes influencing the chemical evolution of solutions on Mars and the Galilean satellites differed greatly from those influencing the chemical evolution of oceans on Earth. Because one ingredient for life is the composition of the primordial surface solutions, it essential to understand the evolution of these solutions on other planetary bodies (e.g., Kargel et al., 2000). Acknowledgments—This research was supported by NSERC grants to P. L. King, H. W. Nesbitt, and D. T. Lescinsky. We thank G. M. Young, R. Wogelius, H. McSween, J. Whitby, L. Hardie, R. Macdonald, B. Marty, L. Mansinha, and anonymous reviewers for helpful comments on the manuscript. P. L. King and D. T. Lescinsky thank P. Brown, B.W. Chappell, J. H. Fink, J.R. Holloway, and P.R. Christensen for introducing them to planetary geology. Associate editor: R. Wogelius REFERENCES Alpers C. N., Jambor J. L., and Nordstrom D. K. (2000) Sulfate Minerals: Crystallography, Geochemistry and Environmental Significance. In Reviews in Mineralogy Volume 40, Mineralogical Society of America. Baird A. K., Toulmin P. III, Clark B. C., Rose H. J. Jr., Keil K., Christian R. P., and Gooding J. L. (1976) Mineralogical and petrographic implications of Viking geochemical results from Mars: interim report. Science 194, 1288 –1293. Baker V. R. (2001) Water and the martian landscape. Nature 412, 228 –236. Bandfield J. L., Glotch T. D., and Christensen P. R. (2003) Spectroscopic identification of carbonate minerals in the martian dust. Science 301, 1084 –1087. Banin A., Clark B. C. and Wänke H. (1992) Surface chemistry and mineralogy. In Mars (ed. H. H. Kieffer et al.), pp. 594 – 625, University of Arizona Press. Banin A., Han F. X., Kan I., and Cicelsky A. (1997) Acidic volatiles and the Mars soil. J. Geophys. Res. 102, 13341–13356. Bell J. F. III (1996) Iron, sulfate, carbonate, and hydrated minerals on Mars. In Mineral Spectroscopy: A Tribute to Roger G. Burns (ed. M. D. Dyar et al.), Special Publication Volume 5, pp. 359 –380, Geochemical Society. Bell J. F. III, McSween H. Y., Crisp J. A., Morris R. V., Murchie S. L., Bridges N. T., Johnson J. R., Br. D. T., Golombek M. P., Moore H. J., Ghosh A., Bishop J. L., Anderson R. C., Brückner J., Economou T., Greenwood J. P., Gunnlaugsson H. P., Hargraves R. M., Hviid S., Knusen J. M., Madsen M. B., Reid R., Rieder R., and Soderblom L. (2000) Mineralogic and compositional properties of Martian soil and dust: results from Mars Pathfinder. J. Geophys. Res. 105, 1721–1755. Blaney D. L. (2001) The 4.5 um sulfate absorption feature on Mars and its relationship to formation environment. Lunar Planet. Sci. XXXII, 1919 (abstract). Bogard D. D., Clayton R. N., Marti K., Owen T., and Turner G. (2001) Martian volatiles: isotopic composition, origin and evolution. Space Sci. Rev. 96, 425– 458. Brass G. W. (1980) Stability of brines on Mars. Icarus 42, 20 –28. Brearley A. J. and Jones R. H. (1998) Chondritic meteorites. In Planetary Materials (ed. J. J. Papike), Reviews in Mineralogy, Vol. 36, pp. 3-01–3-398, Mineralogical Society of America. Bridges J. C. and Grady M. M. (2000) Evaporite mineral assemblages in the nakhlite (martian) meteorites. Earth Planet. Sci. Lett. 176, 267–279. Bridges J. C., Catling D. C., Saxton J. M., Swindle T. D., Lyon I. C., and Grady M. M. (2001) Alteration assemblages in martian meteorites: implications for near-surface processes. Space Sci. Rev. 96, 365–392.

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