The environment of early Mars and the missing carbonates

David C. FERNANDEZ-REMOLAR1*, Monica SANCHEZ-ROMAN1, Andrew C. HILL1,David GOMEZ-ORTIZ2, Olga Prieto BALLESTEROS1, Christopher S. ROMANEK3,

and Ricardo AMILS1

1Centro de Astrobiologıa (INTA–CSIC), Ctra Ajalvir km 4, Torrejon de Ardoz, 28850 Madrid, Spain2Area de Geologıa, Dpto. de Biologıa y Geologıa, ESCET, Universidad Rey Juan Carlos, C ⁄Tulipan s ⁄n, 29833 Mostoles, Spain

3Department of Earth and Environmental Sciences, University of Kentucky, Lexington, Kentucky 40506, USA*Corresponding author. E-mail: fernandezrd@inta.es

(Received 03 December 2010; revision accepted 08 July 2011)

Abstract–A model is presented in which the aqueous conditions needed to generatephyllosilicate minerals in the absence of carbonates found in the ancient Noachian crust aremaintained by an early CO2-rich atmosphere, that, together with iron (II) oxidation, wouldprevent carbonate formation at the surface. After cessation of the internal magnetic dynamo,a CO2-rich primordial atmosphere was stripped by interactions with the solar wind andsurface conditions evolved from humid to arid, with ground waters partially dissolvingsubsurface carbonate and sulfide minerals to produce acid-sulfate evaporitic deposits in areaswith upwelling ground water. In a subsequent geochemical state (Late Noachian toHesperian), surface and subsurface acidic solutions were neutralized in the subsurfacethrough interaction with basaltic crust, allowing the precipitation of secondary carbonates.This model suggests that, in the early Noachian, the surface waters of Mars maintainedacidity because of a drop in temperature. This would have favored increased dissolution ofCO2 and a reduction in atmospheric pressure. In this scenario, physicochemical conditionsprecluded the formation of surface carbonates, but induced the precipitation of carbonates inthe subsurface.

INTRODUCTION

Recent data from Mars are consistent with theoccurrence of geographically extensive areas of phyllosilicateminerals in Early Noachian cratered terrains (Poulet et al.2005; Bibring et al. 2006; Ehlmann et al. 2008b; Mustardet al. 2008; Wray et al. 2009; Milliken et al. 2010). Thesephyllosilicates have been interpreted to have precipitatedunder neutral to alkaline aqueous conditions as aconsequence of a CO2-depleted atmosphere (Catling1999; Chevrier et al. 2007), despite evidence that suggeststhat CO2 played an essential role in surface geochemicalprocesses during the Noachian (Kahn 1985; Bridges et al.2001; Halevy et al. 2007; Morris et al. 2010).

Phyllosilicates are found in several geologic settings.They are found with impact craters, perhaps fromimpacts excavating preexisting deposits or impact-associated and hydrothermal processes, leading to theformation of phyllosilicate and hydrated silicate minerals

(Newsom and Hagerty 1997; Newsom et al. 1999;Rathbun and Squyres 2002; Bishop et al. 2008; Mustardet al. 2008). In contrast, the layered sulfate-rich depositsthat unconformably overlie the Noachian cratered terrainin Meridiani Planum (Hynek et al. 2003; Arvidson et al.2005; Griffes et al. 2007; Hynek and Phillips 2008;Wiseman et al. 2008) and that are found as internallylayered deposits in Valles Marineris, JuventaeChasma, Terby crater, and Aram Chaos (Gendrin et al.2005; Bishop et al. 2009; Lichtenberg et al. 2010; Ansanet al. 2011) suggest that Late Noachian to Hesperianenvironments were acidic. Therefore, phyllosilicateminerals formed during this time could be in closeassociation with acidic solutions (Squyres et al. 2004a,2004b; Grotzinger et al. 2005; Bibring et al. 2006;Hurowitz et al. 2010) as has been observed in someterrestrial analogs (Story et al. 2010; Fernandez-Remolaret al. 2011). However, others interpret these associationsto indicate less acidic environments (King et al. 2004;

! The Meteoritical Society, 2011.1

Meteoritics & Planetary Science 1–23 (2011)doi: 10.1111/j.1945-5100.2011.01238.x

King and McSween 2005; Murchie et al. 2009). Somegeochemical models suggest that sulfur is the mainacidifying agent on post-Noachian Mars (Halevy et al.2007). However, the relatively low abundances of sulfatesin the Early Noachian does not uniquely explain thelack of carbonate because in Nilli Fossae, there areMg-rich carbonates in terrains that were affected byacidic weathering (Ehlmann et al. 2008a). Nonetheless,the formation of those carbonates may be a localphenomenon and a consequence of water with a higherpH. Furthermore, a recent study by Michalski and Niles(2010) adds a new piece to the carbonate puzzle of Mars;these authors report the occurrence of thick carbonatedeposits below 6 km of basaltic crust at the bottom ofdeep Noachian deposits that have been exposed by abolide impact. Although it is widely accepted that anacidic episode followed the formation of phyllosilicateson Mars, Fan et al. (2008) suggest that acidic conditionswere reached during the Early Noachian as aconsequence of the Tharsis uplift. As a consequence, theacidic minerals coexisted with the Early Noachianenvironments that generated phyllosilicates under milderconditions.

The main goal of this study was to use amultidisciplinary approach––meteorite geochemistry,Mars planetary data, early Earth geochemistry andterrestrial Mars analogs––to synthesize the apparentlycontradictory geochemical and mineralogical datacollected by Mars orbiters and rovers into a coherentgeochemical model. The information we use for ourmodel is included in Supporting Information. Wepropose (following Bibring et al., 2005) that there was aglobal geochemical change from the Noachian toHesperian epochs that involved a fundamental change inpH from a mildly acidic surface geochemistry (pH <4.5) to a strongly acidic surface geochemistry (pH <2.5). This late acidification stage would have promotedthe dissolution of Early Noachian carbonates thatprecipitated in Mars paleosols (Ohmoto et al. 2004;Bullock and Moore 2007) followed by a release of CO2

back to the hydrosphere and atmosphere. Our modeldescribes the geochemical mechanisms that may controlthe surface pH of early Mars and how aqueous systemsthen evolved over time.

GEOCHEMICAL SYNTHESIS FOR THE EARLIESTNOACHIAN: MILD ACIDIFICATION AND

OXIDATION

Geochemical modeling of Mars’ earliest sedimentaryenvironments requires an estimation of the compositionof volatiles in the primordial atmosphere, which is largelydependent on the early differentiation and redox state ofMars’ mantle (McSween 2004; McSween et al. 2009) (see

Supporting Information). The derived atmosphericcompounds would then dissolve in meteoric watersthrough various geochemical pathways governed byphysicochemical parameters (Krasnopolsky 1993). Thesepathways would have generated the first sedimentarymaterials on Mars. The origin of thermodynamicdisequilibrium that produced these chemical sedimentsprobably began with other reactants sourced fromvarious autocatalytic (Lefticariu et al. 2007; Davila et al.2008), photochemical (Konhauser et al. 2007), andradiolytic reactions (Lin et al. 2005).

We make the assumption that on Mars, like onEarth, CO2, SO2 and Fe were critical in controlling thegeochemistry of low temperature systems, including thepH and Eh of aqueous environments (Marion et al.2003, 2008; King and McSween 2005; Tosca et al. 2008).We propose a geochemical model for Mars wherephyllosilicates and iron oxides are formed in the absenceof carbonates at the surface. This model is based on:1. The main atmospheric volatile controlling the

surface and subsurface geochemistry of Mars is CO2,which was sourced from C-rich mantle ventilation(Fig. 1a) during magma ocean solidification beforeapproximately 4.5 Ga ago (Elkins-Tanton 2008;Righter et al. 2008). Balancing the data provided inthe Supporting Information, we consider a 20 baratmosphere as a conservative estimate to construct aviable geochemical model where the partial pressureof CO2 is approximately 5 bars. This is a reasonablelower limit for pCO2 based on recent research(Elkins-Tanton 2008; Hirschmann and Withers2008; Gaillard and Scaillet 2009), which agreeswith previous estimates (Melosh and Vickery 1989;Squyres and Kasting 1994; Gulick et al. 1997; Brainand Jakosky 1998; Carr 1999; Manning et al. 2006).This primordial atmosphere is assumed to havepromoted the high weathering processes that endedin the formation of the extensive deposits ofphyllosilicates.

2. A secondary atmosphere (Hirschmann and Withers2008) would develop (Fig. 1a) through a post-4.5 Ga volcanic influx to a crust altered by CO2-mediated reactions, sputtering and impact erosion(Carr 1999), after a 400 Ma episode of declining forthe primordial atmosphere that is consistent withfirst the atmospheric cooling and, second, with thereactivation of the hydrological cycle. In addition,this late evolutionary phase could be followed by anincreased supply of atmospheric SO2 (Gaillard andScaillet 2009), which would favor the production ofmassive quantities of sulfates on the surface of Marsunder acidic (pH < 3) conditions.

3. Solar X-ray and UV irradiation of the atmosphere(Cnossen et al. 2007; Tian et al. 2009) would have

2 D. C. Fernandez-Remolar et al.

probably been (Fig. 1a) a major driving force behinda chemical disequilibrium to produce CO2 from COand CH4, and surface oxidants such as Fe3+, H2O2,O3, O2, and radicals such as HO·, HO2 and O2

) viawater photolysis (Wayne 1991; Krasnopolsky 1993,2006; Yung and DeMore 1999). Different reactionsinvolved in the production of oxidants like H2O2

and O2 (Lefticariu et al. 2007; Davila et al. 2008) orradiolysis (Spinks and Woods 1964; Lin et al. 2005)could support oxidation at the surface. Theoxidation process would be initiated in the crust byelectron transfer from reduced minerals such asfayalite and iron sulfides, as well as dissolved gaseslike CH4, CO, H2S, and H2.

Fig. 1. a) General synthesis for main parameters that rule the geochemical processes discussed in the text displayed as a temporaldiagram, which ranges from Early Noachian to Hesperian; general evolution of Mars is considered as a consequence of changes inthe atmospheric concentration of pCO2 and pSO2 that control climatic conditions and pH of the surface solutions, and theweathering rate (Wr) as a function of CO2 and SO2 and temperature. Moreover, diagram (a) includes main geological episodes andevents, which were associated with changes in the atmospheric thickness and composition of Mars as EUV flux (Tian et al. 2009),Tharsis activity (Phillips et al. 2001), the planetary dynamo collapse (Nimmo and Tanaka 2005), and emergence of fluvial networks(Hynek et al. 2010). b) and c) are simple geochemical diagrams showing the stability of Fe-rich carbonates as a function of fO2,fCO2 and pH at a temperature of 50 "C and [Fe2+] = 10)3.5 M, [SiO2] = 10)4 M, and assuming that S-bearing compounds werenot present in solution. Equilibrium calculations were made at the modern Earth pressure of 1 bar. In diagram (b), siderite stabilityis a function of fCO2 and fO2 at pH approximately 5, which supports siderite stability with a fCO2 > 10)1.7 and fO2 > 10)65.Log f H2 is also shown to evaluate the [H2] that is expected in the atmosphere following the equation H2O fi H2 +1 ⁄ 2ÆO2.Stability of siderite is a function of pH and fCO2 at a fO2 approximately 10)65. Mineral stability parameters suggest that this low-pH carbonate will never form when the pH < 6.4 and fCO2 < 10)3. When fCO2 > 10)3 siderite stability depends on pH, fCO2,and fO2. Assuming an fCO2 approximately 105 for early Mars, 4.5 Ga siderite would precipitate under equilibrium conditions at apH above approximately 2.8. If the pH of surface solutions is dominated by meteoric waters, precipitation will never occur. ForfO2 > 10)40 siderite formation is prevented under any conditions. Estimates of the mineralogical stability of carbonates usingterrestrial-like conditions suggest that siderite should be a common mineral on Mars. However, the mineral stability of carbonatesdepends on atmospheric pressure, which is a driving parameter for CO2 dissolution.

The environment of early Mars and the missing carbonates 3

4. On early Mars (approximately 4.5 Ga), it is assumedin this model that the earliest aqueous environmentswere probably dominated by water masses that wereshallow (Carr 1987; Carr and Wanke 1992; Toscaet al. 2008), which could have resulted in a more orless homogenous water column geochemistry.Shallow water masses on Mars would be consistentwith a low asteroidal input because asteroids andcomets are the largest source of volatiles in thesolar system (Lunine et al. 2003; Albarede 2009),but also with a Mars mantle depleted in volatiles,which prevented a higher release of water to thehydrosphere. Interestingly, the presence of dense andextensive valley networks (Baker 2001; Hynek et al.2010) that met the lowlands of Mars have been agedas Late Noachian to Early Hesperian. This isconsistent with the presence of extensive sulfateoutcrops that demand active hydrological activity(Bishop et al. 2009).

5. A dense primordial atmosphere (approximately20–70 bar) composed mostly of H2O and CO2 woulddrive high alteration rates to produce clays (Elkins-Tanton 2008), and under surface conditions withtemperatures well above 273 K (Forget and promotethe partial neutralization of the meteoric waters thatwould be followed by the release of high quantitiesof SiO2 to the shallow hydrosphere. Interestingly,this excess of silica could have been an importantsource for precipitation of disarranged clays in thoselocations under neutralization of solutions(Fernandez-Remolar et al. 2011).

6. During the phase of phyllosilicate production,both a greater temperature for the crust andmantle associated with a higher geothermal fluxreinforced the greenhouse effect (Wanke et al.1994; Hauck and Phillips 2002) probably maintainedsurface temperatures well above 273 K (Forgetand Pierrehumbert 1997). This would cause highrates of weathering of primary silicate minerals by<4.5 Ga. However, the stability of such dense andprimordial atmosphere over time would have notbeen very high as a result of the solar extremeultraviolet (EUV) flux (Tian et al. 2009). Underthese conditions, the primordial atmosphere mighthave been active during tens of millions of years,which could be enough to promote a globalweathering mediated by CO2.

7. Using Mars planetary data, the main secondarymineralogy on early Mars is clays with some ironoxides, but sulfates and carbonates occur inoutcrops and are very scarce (Ehlmann et al. 2008a).Probably, post-4.5 Ga, carbonates did not formbecause surface conditions did not reach asufficiently high pH (i.e., >4.5) because of the

presence of SO2 (Fairen et al. 2004; Gaillard andScaillet 2009; McCubbin et al. 2009).

8. Including all these geochemical constraints,carbonates could only precipitate in the subsurface inmore neutral and anoxic solutions that would favorthe precipitation of Fe-, Ca- and Mg-carbonates(Fernandez-Remolar et al. 2009a).Under a dense atmosphere, the pCO2 would have

produced acidic surface fluids with a high HCO3)

concentration:

CO2!g" #H2O! H2CO3 !1"

H2CO3 ! HCO$3 #H# !2"

At equilibrium, Reactions 1 and 2 can be expressed as:

K % !HCO$3 " & !H

#" & !pCO2"$1 !3"

where the pH of pure rainwater can be determined as afunction of pCO2 using Reaction 3 (Figs. 1b and 1c):

log!pCO2" % log K$ 2 & pH !4"

Water in equilibrium with a CO2-rich atmospherewould produce mildly acidic conditions (pH 3.5–4)whereby primary silicate minerals would undergo rapidand extensive weathering (Carr 1999). The products ofthis weathering process would include clays andcarbonate minerals, which would theoretically bufferthe pH to less acidic values (pH approximately 6–7),but also remove atmospheric pCO2, and release SiO2 tothe hydrosphere. A lower pCO2 would still have ledto a mineral assemblage dominated by clays andcarbonates that occur at a pH > 4.5, and include theprecipitation of siderite (FeCO3) and a concomitantdecrease in atmospheric pressure. However, the scarcityof carbonates in the oldest terrains of Mars could beexplained by the fact that the pH never rose above 4and that the formation of low-pH carbonates was inhibitedby other geochemical mechanisms like production ofoxidants (see below in this section). Shallow, episodic,well-mixed water bodies (Fairen et al. 2004; Albarede2009; Tosca and Knoll 2009) could maintain acidityand prevent the precipitation of Ca-Mg-carbonates.Although weathering should have decreased pCO2, theconcentration of CO2 would have still been very highand controlled surface solutions to an acidic pH andpreventing carbonate formation on a planetary scale.

In this sense, an excess of CO2 from volcanicemission of volatiles (Phillips et al. 2001; Head andWilson 2011) would induce acidification at a global scale.This is reported on Earth at midocean ridges (Fairenet al. 2004; Orr et al. 2005; Kuffner et al. 2008), at thepoint where high-pressure CO2 enters the ocean (Nihouset al. 1994):

4 D. C. Fernandez-Remolar et al.

CO2 # CO2$3 #H2O ! 2 &HCO$

3 #H# !5"

The acidification effect driven by carbon dioxide canalso be evaluated by combining Equations 1–4 with thewater dissociation, assuming that the aqueous solutionsare at equilibrium with siderite (see Loaiciga 2006), whichwould be the first carbonate to precipitate from mildlyacidic fluids (Ohmoto et al. 2004). The result is apolynomial equation dependent on geochemicalparameters such as temperature (T), pH, pressure of CO2

(pCO2) and equilibrium constants (see Millero 1995;Caldeira andWickett 2005; Loaiciga 2006):

a!pCO2;T" & !H#"4 # !H#"3 # b!pCO2;T" & !H#"# c!pCO2;T" % 0

!6"

where a, b, and c (Loaiciga 2006) are also parametersdepending on pCO2, equilibrium constants andtemperature (Fig. 2). By fixing constants a and b inEquation 6, it is possible to evaluate the combined effectthat pCO2 and temperature exert on the pH whensolutions of an aqueous system are in equilibrium withsiderite. The pH-T-pCO2 plots in Fig. 2 show decreasingpH at constant pCO2, and as temperature moderates, pHdecreases. However, the geochemical estimations forpCO2 have equilibrium constants calculated for a 1 baratmosphere. On the contrary, during the early planetarystages for both rocky bodies, it is expected that highpressure primitive atmospheres (Squyres and Kasting1994; Zahnle et al. 2007) would have favored CO2

dissolution and carbonate dissociation (Millero 1982,2007; Duan and Sun 2003; Broecker 2005), as well asvery high alteration rates of silicates (Brantley 2005).Some theoretical calculations have estimated that themaximum CO2 solubility in modern oceans increaseslinearly as a function of water depth such that thesolubility increases by 1 bar every 10 m of water depth(Teng et al. 1996; Duan and Sun 2003; Duan et al. 2006).This linear relationship is maintained up to a pressure of30 bars (approximately 300 m depth) at a constanttemperature <20 "C. Using this terrestrial data, wemodel the pH effects of CO2 dissolution in shallow watermasses on early Mars (Fig. 3a). Assuming anatmospheric pressure of 20 bars at the end of the coolingof the magma ocean (Elkins-Tanton 2008) surface waterswould allow the dissolution of up to 0.15 moles of CO2

per liter (fCO2 approximately 4.2), which would inhibitcarbonate formation below a pH of 3.5 (Fig. 3b). Acidicconditions would also be reinforced by the subaqueousintroduction of CO2-saturated solutions from submarinehydrothermal systems, which occurs in Earth’s oceanstoday and in the past (Shitashima 1998).

Surface acidification would not prevent othercarbonate minerals forming on Mars’ surface; Fe-rich

carbonate phases like siderite can form at a pH < 4.5depending on pCO2 and redox conditions (Ohmoto 1996;Ohmoto et al. 2004). Interestingly, noting the occurrenceof ferric nontronites, Chevrier et al. (2007) providedthermodynamic support for the prevalence of anoxidizing environment on Mars’ surface that wouldinhibit siderite formation. On Earth, this carbonateoccurs in mildly acidic conditions (pH approximately 4–5),but is very susceptible to oxygen at current Earthconcentrations (Ohmoto 1996; Ohmoto et al. 2004).Under neutral to alkaline conditions, Fe3+-clays, oxides,hydroxides or oxyhydroxides would form rapidlybecause the oxidation of Fe2+ at Eh <)0.3 V is fast(Singer and Summ 1970). However, in this case, theproduction of clays should be accompanied by themassive precipitation of carbonates with a very diversecomposition bearing Fe, Mg, and Ca. Therefore, theformation of ferric clays and the inhibition of carbonateprecipitation occur primarily in oxidizing and acidicenvironments (Eh > 0, pH < 4.5).

There are several mechanisms that can promotethe increase of redox conditions, the subsequentoxidation of Fe2+ and the final inhibition ofsiderite. One of the most reasonable processes for earlyMars could be direct UV irradiation due to thehigh EUV from the flux at this time (Braterman et al.1983; Francois 1987; Konhauser et al. 2007; Tian et al.2009):

2 & Fe2#!aq" # 2 &H# # h & v! 2 & Fe3#!aq" #H2!g" !7"

4.5 Ga M

ars conditions

pH = K·(PCO ), T = 100 ºC 2

pH = K·(PCO ), T = 0 ºC 2

Fig. 2. Diagram showing the relationship between pH, T, andpCO2 in a solution in equilibrium with siderite, which resultsfrom solving Equation 6 expressed in the main text (seeLoaiciga 2006). It shows that siderite buffering solutions lowertheir pH as pCO2 increases, at temperatures 0–60 "C. Above60 "C, the pH slightly increases, as CO2 solubility decreases.

The environment of early Mars and the missing carbonates 5

and the production of oxidizing molecular species in theatmosphere, in surface or subsurface environments bydifferent radiolytic pathways (Burns 1993; Krasnopolsky1993; Lin et al. 2005; Bullock and Moore 2007;Lefticariu et al. 2007; Davila et al. 2008). Photochemicalradiolysis of the oxidized crust would have generatedradicals or highly reactive oxidants like H2O2 and O2

from CO2, CO, and H2O. The oxidation of the ferrouscation by peroxides follows the established well-knownFenton reaction pathway (Equation 8) (Kieber et al.

2003), where reaction rates are higher under acidicconditions (Kremer 2003; De Souza et al. 2006; Junget al. 2009):

Fe#2 #H2O2 ! Fe#3 #OH$ # &OH !8"As higher oxidation rates in the photochemical

reactions of Fe+2 (Equation 7) are also expected underacidic conditions (Braterman et al. 1983), acidicconditions mediated by atmospheric fCO2 may haveplayed an essential role in maintaining redox conditions

P (bar)

7.2

5.2

4.2

4.7

-810

-410

-210

-110

3.16

3.20

3.25

3.31

3.40

3.55

<7.00

Cs pH

oT = 50 CoT = 20 C

a

b

Fig. 3. CO2 solubility (CS) as a function of pressure. a) CO2 solubility in a NaCl marine-like solution (Teng et al. 1996; Duanet al. 2006) versus surface pressure at temperatures of 20 and 50 "C. CO2 solubility increases with increasing pressure andtemperature. The diagram shows that CO2 solubility depends on pressure and temperature in the sense that CO2 is linearlyincreased as pressures are augmented and T is decreased. As a consequence, cold and high-pressurized environments as the deepoceans currently have lower pH and more inorganic carbon in solution. Correlation between CO2 solubility and pH is shown onthe right hand axis. b) Shows how pH decreases as CO2 pressure increases.

6 D. C. Fernandez-Remolar et al.

above 0 V on early Mars. Moreover, oxidation of Fe+2

to Fe3+ may have reinforced acidification by simplehydroxylation of the ferric cation or hydrolysis(Fernandez-Remolar et al. 2008):

Fe3# #H2O! Fe!OH"2# #H# !9"Interestingly, the weathering of Fe-Mg bearing

primary silicates in the crust, fueled by a highatmospheric fCO2, would leach high amounts of Fe2+

into surface waters. The higher the alteration rate, thegreater the production of Fe2+ which would be availablefor oxidation in surface environments that wouldpromote the geochemical coupling between ironhydrolysis and CO2 dissolution. As a result, acomplicated imbalance in the production of ferricoxidants and its reduction by some other reactions, someof them mediated by photochemistry (De Souza et al.2006; Gammons et al. 2008), drove the redox conditionswhere the ferric iron mediated the inhibition of siderite.

Although the planetary data support the inhibitionof carbonate formation on the surface of early Mars, thesubsurface conditions may have favored its precipitation(Ehlmann et al. 2008a) with Fe+2, Ca2+, and Mg2+

cations (Fig. 4), as suggested by carbonate found in theMartian meteorites (King et al. 2004). Carbonateprecipitation may have occurred in a multistage processinvolving neutralization of acidic solutions and reactionof all oxidizing agents with the basaltic crust that wouldnot be replenished by photochemistry on the surface butalso with the reaction of highly reducing compoundsgenerated in the crust like H2S, CO, CH4, and H2 as seenduring serpentinization and pyritization (Drobner et al.1990; Sleep et al. 2004).

A similar process of neutralization and reductionmay occur inside sedimentary bodies where allochemicalcomponents (e.g., clasts of ferromagnesian mineralogies)would react with the CO2-rich connate fluids, as hasbeen reported in different terrestrial environments(Hall et al. 2004; Larsen 2008). This mechanism wouldproduce iron carbonate minerals in association withphyllosilicates that originated in other environments(e.g., hydrothermalism, weathering) and latertransported to the subsurface by fluvial activity.

GEOCHEMICAL SYNTHESIS OF THE LATENOACHIAN TO HESPERIAN GEOCHEMICAL

SURFACE ENVIRONMENT: INCREASED SO2 ANDOXIDANTS

The co-occurrence of Fe-Mg-Ca sulfates with oxidesin Late Noachian terrains (Bishop et al. 2009;Lichtenberg et al. 2010) requires the generation ofoxidants and acidic chemicals (Hurowitz et al. 2010).On Earth, acidic sulfates and oxides are commonly

formed in association with sulfide ore bodies andgeothermal ⁄volcanic centers (King and McSween 2005;Yen et al. 2008), where sulfur is released as SO2 and H2Sinto solution under acidic conditions. Both types ofsystems are linked to volcanic complexes governed by themantle redox state and magma composition (Arculus1985; Herd 2008). It has been calculated that thequantity of sulfur in silicate melt on Mars is 3–4 timeshigher than on Earth (see Gaillard and Scaillet 2009 fora complete discussion). During the Late Noachian,changes in the oxidation state of the mantle increased theoxygen fugacity of the magma, moving the geochemicalsystem toward the fayalite-magnesite-quartz (FMQ)buffer state (see Supporting Information). Under theseconditions, iron is removed from the system as FeO,which is one of the mechanisms to increase the sulfurcontent of a silicate melt and is an essential step towardthe release of S in the gas phase (Gaillard and Scaillet2009).

On Earth, there are three main pathways to transfersulfur into acidic surface waters (1) photochemicaloxidation of S-bearing gases in the atmosphere and laterrainout (Seinfeld and Pandis 2006); (2) release of acidichydrothermal solutions from volcanic centers (Gaillardand Scaillet 2009; McCubbin et al. 2009); and (3)weathering of sulfide minerals via meteoric solutions(Burns 1988; Zolotov and Shock 2005). Each one ofthese three mechanisms could be part of a long-termtrend that begins with magmatic sources and ends withsulfide weathering (Tornos 2006; Fernandez-Remolaret al. 2008; King and McLennan 2010).

The photooxidation of S-bearing gases to producesulfuric compounds has been described as a complexchemical process occurring inside planetary atmospheres(Wong et al. 2003) that involves sulfur, sulfur monoxide,and sulfites as transitional chemicals (Larson et al. 1978;Halevy et al. 2007; Johnson et al. 2008):

H2S#O2 ! SO2 #H2 !10"

SO2 #O!O;O2;O3;OH"! SO3 !11"

SO2 #H2O2 ! H2SO4 !12"

SO2 #H2O! H2SO4 !13"

During episodes of volcanic activity, fO2 is an agentthat induces oxidation and acidification of hydrothermalsolutions enriched in S-bearing volatiles (Lueth et al.2005; Craw 2006). In this context, the existence ofjarosite and hematite in fluid inclusions (McCubbin et al.2009) suggests that the hydrothermal fluids involvedin their formation were well above the limit (logfO2 = )37) to oxidize Fe2+ and S2) at 200 "C.Although both Fe2+ and S2) oxidation can occur inmaterials affected by hydrothermal activity, the

The environment of early Mars and the missing carbonates 7

Fig. 4. Geochemical cartoon of Early Noachian Mars. The geochemistry is dominated by a high pressure atmosphere and a lowgeothermal gradient, which are expected at the end of magma ocean cooling. a) A CO2-rich atmosphere (Hirschmann and Withers2008; Gaillard and Scaillet 2009) would provide meteoric acidification and soda-like acidic water masses. Surface temperatureswould be highly driven by an extreme greenhouse effect and the release of geothermal heat, which would in turn lead to highweathering rates. High temperatures at and below the surface would also inhibit the formation of strong geochemical gradientsbetween water bodies on the surface and beneath the surface. Massive phyllosilicate formation would be expected under thesegeochemical constraints. b) T-log fO2 diagram tracing the diagram showing changes in the relationship between IW (iron-wustite)and FMQ (fayalite-magnetite-quartz) buffers for the early Noachian; in this situation, mantle chemistry might have favoredthe production of CO2 in different proportions depending on what carbon bearing volatiles, like CO and CH4, were present.In hydrothermal areas with a higher oxidant input, the redox conditions could vary from the IW buffer to the FMQ buffer. c) pH–Eh diagram showing the mineral stability in different geological areas (surface, underwater, and subsurface) of early Mars; acidicweathering plus oxidation would inhibit carbonate formation on Mars’ surface; however, acidic and oxic solutions could beneutralized in the subsurface once the groundwater reacts with a basaltic Mars crust. As a consequence, carbonates couldeventually form in the subsurface.

8 D. C. Fernandez-Remolar et al.

oxidation of ferrous to ferric iron on Mars requireshighly oxidizing conditions, i.e., ‡ log fO2 of )37(Fig. 5) when assuming a temperature of 200 "C forhydrothermal fluids (McCubbin et al. 2009). As aconsequence, the magmatic chamber would require alower supply of oxidants to induce a log fO2 < )40,which is enough to oxidize sulfur and produce acidicsolutions. However, for a temperature of 200 "C, a logfO2 > )10 would be the minimum required to oxidizeferrous to ferric iron and thus form ferric mineralogiesin the fluid inclusions, which suggests that thehydrothermal fluids that produced jarosite and hematitewere highly oxidizing. Therefore, it is possible thatmeteoric waters during the Hesperian were sufficientlyoxidized to cause an increase in the oxidation state ofmagmatic sources and release acidic fluids with a highoxidizing power.

The meteoric weathering of sulfide ore bodies occursin the upper parts of hydrothermal systems exposed tometeoric fluids (Burns 1988). Although sulfide oxidationmight have been an active process during the entirethermal episode that drove the formation of the LateNoachian-Hesperian sulfate deposits, it would havebecome the dominant mechanism during the last stagesof sulfate formation. Thus, once volcanism ended,reaction with the basaltic crust of Mars could haveconsumed the remaining oxidizing and acidifying agentsthat were generated during the peak of volcanic activity(Phillips et al. 2001). Sulfide oxidation by reaction withatmospheric oxygen might have occurred via thefollowing reaction pathway:

FeS# 2:25 &O2 # 2:5 &H2O! H2SO4 # Fe!OH"3 !14"

Iron oxidation under low-pH conditions is a veryslow process when the oxidizing agent is oxygen(Braterman et al. 1983). However, the kinetics offerrous iron oxidation by some photochemical andradiolytically derived oxidants like H2O2 (Equation 7)are increased when pH decreases. Weathering of Marssulfide ore bodies was idealized by Burns (1988) tooccur in the subsurface when the concentration of otheroxidants like oxygen are low (Sato 1960; Jennings et al.2000):

FeS# 4:5 &H2O2 ! H2SO4 # Fe!OH"3 # 2 &H2O !15"

Obviously, this reaction requires the generation offerric iron at the surface by the photochemical reactionsshown in Equations 7 and 8.

Fe3+-mediated oxidation kinetics on Mars could beslower than on Earth, which would seemingly contradictthe existence of geographically extensive sulfate deposits.However, if most of the oxidizing power generatedduring the Late Noachian-Hesperian was sourced by

–100 –90 –80 –70 –60 –50 –40 –30 –20 –10 00

50

100

150

200

250

300

log f O2(g)

T(°

C)

SO4

2-

FeSO4+

H2S(aq)

HSO4-

Pyrite

–100 –90 –80 –70 –60 –50 –40 –30 –20 –10 0 100

50

100

150

200

250

300

log f O2(g)

T(°

C)

Fe2(SO4)3

FeSO4

Hematite

Melanterite

Pyrite

Dia

gram

Fe++

, P=

L-V

cur v

e,a

[ma i

n]=

10– 2

,a[H

2O]

=1,

f[S

O2(

g)]

=10

–1,p

H=

2

–100 –90 –80 –70 –60 –50 –40 –30 –20 –10 0 100

50

100

150

200

250

300

log f O2(g)

T(°

C)

SO4--

H2S(aq)

HSO4-

SH2S(g)

Dia

gram

SO

2(g),

P=

L-V

c ur v

e,f[

mai

n]=

10–1

,a[H

2O]

=1,

pH=

4

Fig. 5. Stability of chemical species as a function of log fO2(g)and T ("C) in an acidic solution sourced from hydrothermalfluids (pH = 2, fSO2 = 10)3, [Fe2+] = 10)2, which wouldprovide ideal conditions to form jarosite. These geochemicalconditions would favor hydrothermal acidic solutions(McCubbin et al. 2009).

The environment of early Mars and the missing carbonates 9

acidic and oxidizing hydrothermal fluids, then slowreaction kinetics could be overcome. These oxidizingand acidic compounds could then weather igneousmineralogies, including sulfide orebodies, to a partial orcomplete neutralization of the reactive solutions. Thisprocess may have been recorded as a compositionalchange from ferric to ferrous sulfates at Aram Chaos(Lichtenberg et al. 2010).

GEOCHEMICAL MODEL FOR THEMINERALOGICAL DISTRIBUTION ON MARS

The geochemical constraints discussed in theprevious section can be used to create a geological modelfor the mineral distribution in Mars’ crust. It includesmodeling of pH-Eh surface to subsurface gradientsresulting from the neutralization and reduction of surfacefluids as they react with Mars’ silicate crust. The modelincludes current understanding of the mineralogicaldiversity of Mars, including silicates, sulfates, andcarbonates found in some Mars meteorites (Bridges et al.2001). It is based on the main assertion that early Marsexperienced two acidic stages (1) Early Noachian CO2

acidification and mild oxidation under a high pressureatmosphere compatible with the global production ofphyllosilicates, followed by; (2) a stage of strongeracidification and oxidation mediated by hydrothermallysourced sulfur-bearing solutions, associated with a SO2-rich oxidizing atmosphere, which promoted theformation of sulfates and ferric oxides. The model iscompatible with the formation of phyllosilicates andsulfates on Mars’ surface, as well as carbonates andsulfides in underground regions.

Such a geological scenario is based on terrestrialobservations that have been made in the Rıo Tinto Marsanalog (Fernandez-Remolar et al. 2008). An issue here isthat Rıo Tinto is not hosted by basalts, and it is wellknown that pristine starting mineralogy has a strongeffect on resultant fluid chemistry and hence alterationproducts (e.g., King et al. 2004; Tosca et al. 2005). Inthis acidic system, the main Mars mineralogies like ironoxides, sulfates, phyllosilicates, and carbonates occuralong a pH–Eh weathering gradient ranging from thevery oxidizing and acidic conditions on the surface tostrongly reducing and quasi-neutral conditions in thesubsurface (Fig. 6a). At Rıo Tinto, the mineral reactionand formation processes are activated by the inflow ofstrongly acidic brines (pH approximately 2, Ehapproximately 500 mV) enriched in Fe3+ and HSO4

)

into the hydrothermal complex via faults. The acidicwaters leach the hydrothermal wall-rock (K-Na-feldspars),neutralizing the oxidants and acidic compounds thatdrive the production of sulfates, phyllosilicates, and ironoxides (Fig. 6b). Once the pH rises above 3.5–4, oxides

Fig. 6. Geochemical and mineralogical changes along a pH–Ehgradient in a terrestrial subsurface dominated by reactions withoxidizing and acidifying compounds in contact with volcanicbasement rock (Rıo Tinto data recovered during 2005 and 2006field campaigns). a) Scatterplot of the pH and Eh of subsurfacesolutions at different depths. The regression line (R2

approximately 0.7) shows an inverse correlation; from oxidizingand acidic solutions at the surface evolving to neutral andanoxic solutions at depth. b) pH–Eh mineral stability diagramconstructed from field and laboratory data, as well asthermodynamic modeling by using the Act program included inthe GWB software suite. Three different geochemical gradientsfor real and theoretical cases have been added to observe changesin mineral stability by varying the pH and Eh in the subsurface.(1) the Rıo Tinto terrestrial analog. (2) pH–Eh gradient mediatedby Early Noachian crust altered by CO2. (3) SO2-rich acidicfluids leaching the Late Noachian-Hesperian basement. (2) and(3) were obtained using the Xt1 program included in the GBWsoftware suite, as shown in Figs. 7a and 7b. All three pH-Ehgradients show the same trend, from acidic and oxic mineralogieson the top left of the figure (surface environments), to neutraland anoxic in the bottom right of the figure (anoxic subsurface).

10 D. C. Fernandez-Remolar et al.

and phyllosilicates are the main alteration products.Carbonates and sulfides occur in the deepest regionswhere the pH is about 5–7 (Fernandez-Remolar et al.2009a). Periodic variations in the redox state and pH ofthe subsurface solutions, as sampled from three RıoTinto boreholes (Fernandez-Remolar et al. 2008, 2009a),suggest that there are repeated cycles of formation anddissolution of sulfides and carbonates. The coexistence ofcarbonates and phyllosilicates, and the formation of sulfidesin solutions are probably due to the influx of seasonalrainfall, reaction rates (kinetics), and assemblages thatare in thermodynamic disequilibrium. Oxidation of thehydrothermal sulfides to sulfate and ferric iron decreasesthe pH and produces acidic solutions that migratedownwards.

When the pH and Eh of Rıo Tinto water samples,which were collected at different depths, are plotted ona pH–Eh phase diagram, they follow a regression linefrom Fe-sulfate to phyllosilicates to carbonate(Fig. 6b). A homologous mineralogical pathway can bealso achieved by CO2–HSO4

) weathering of a basaltic-like crust composed of fayalite, forsterite, anorthite,and siderite (Figs. 5b, 6a, and 6b; Table 1). The results

shown in Fig. 7 were obtained using the Xt1 program(Bethke 2008), assuming an early Noachian weatheringof a forsterite-fayalite-anorthite crust by CO2 (Fig. 6a),and a late Noachian alteration of the same earlyNoachian crust that includes siderite and is mediatedby the acidic speciation of SO2 (Fig. 6b). Moreover,plots of carbonate alkalinity versus depth showincrements comparable to pH increases (Figs. 6a and6b).

The integration of geochemical data recovered inselected terrestrial environments and geochemicalmodeling can shed light on the missing carbonates onMars and the known distribution of Mars mineralogies inthe Early Noachian to Hesperian. During the EarlyNoachian, the weathering of the crust mediated bymeteoric and surface waters enriched in CO2 promotedthe formation of phyllosilicates on the surface of Mars(Fig. 7c). Nontronites and iron oxides could have formedunder slightly oxidizing conditions mediated byphotochemistry and radiolysis, carbonates would form inthe shallow crust regions (Figs. 7a and 7b) and sulfateswould not precipitate due to the chemical state ofMars’ mantle (Gaillard and Scaillet 2009). In the Late

Table 1. Physical and chemical parameters used for modeling the geochemical gradients on Early Mars (see Figs. 6and 7) using the Xt1 software (Bethke 2008). Geochemical components are based on the Mars solutionconcentrations shown by Catling (1999). Iron concentration for Late Noachian has been increased to the lowersaturation levels found in acidic systems (see Fernandez-Remolar et al., 2004, table 1). Gas fugacities for CO2 havebeen fixed to 5 and 0.1 (Carr 1999) to simulate changes in the distribution from Early to Late Noachian in theC-bearing volatile inventory of Mars. Oxygen fugacities of 10)55 and 10)30 correspond to redox potential of 0.22 Vand 0.62 V for Early and Late Noachian ages, respectively.

Parameters Early NoachianLateNoachian-Hesperian

Hydrogeological parameters Surface temperature ("C) 50 0Depth (m) 1000 1000Discharge (m yr)1) 4 4Porosity (vol%) 25 25Diffusion coefficient (cm2 s)1) 10)6 10)6

Crust composition (vol%) Fayalite 35 35Forsterite 15 15Anorthite 25 25Siderite – 10

Hydrochemistry of surfacewaters (mol L)1)

pH 3.5 1.5O2(g) 10)55 10)30

CO2(g) 5 0.1SO4

2) – 2 · 10)5

Mg2+ 4 · 10)5

Cl) 7 · 10)5

K+ 2.5 · 10)5

Al3+ 3.5 · 10)5

SiO2(aq) 1.5 · 10)5

Ca2+ 5 · 10)5

Na+ 4.3 · 10)5

Fe2+ ⁄Fe3+ 3.5 · 10)5 3 · 10)3

The environment of early Mars and the missing carbonates 11

Noachian-Hesperian, HSO4)-rich fluids promoted the

precipitation of sulfates and iron oxides at the surface,the formation of clays and the dissolution of EarlyNoachian carbonates that formed in the shallowsubsurface. Carbonate could then precipitate in the deepersubsurface when both the pH and Eh of groundwatersallowed for a higher alkalinity and carbonateoversaturation (Fig. 7c).

The formation of authigenic mineralogies infillingthe porosity of the Noachian aqueous sediments of Marswould explain the close association of Fe-rich smectiteswith clays that formed from the weathering of primaryAl-silicates, such as kaolinite. In this case, carbonateswould begin to precipitate toward the base ofsedimentary bodies where meteoric waters would beclose to neutral and also reduced.

3.5

4.0

4.5

5.0

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Depth (m)

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0.00

0.05

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0.15

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bona

te a

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inity

as

CaC

O /m

g!kg

)3

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inity

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)3

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-0.34

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)

-0.3

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0 8

Surface

Subsurface (< 50-100 m)

Deep subsurface (>100 m)

Early Noachian (~ 4.5 Ga)

neutralizationCO2

-

HCO3

pH

-0.34

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)

-0.3

1.1

0.0

0 8

Surface

Subsurface (< 50-100 m)

Deep subsurface (>100 m)

neutralizationCO2

-

HCO3

Late Noachian (~ 3.8-3.7 Ga)

a

b

c

pHpH

0 200 400 600 800 1000

0 200 400 600 800 10000 200 400 600 800 1000

Fig. 7. pH, Eh, and alkalinity against depth diagrams resulting from modeling a weathering process on a Mars mafic crust.Main geochemical parameters obtained in models on basement alteration using the Xt1 software. Theoretical modelingresults in pH increases and Eh decreases in direct relationship with the rise in alkalinity of the subsurface. a) pH, Eh, andalkalinity after modeling alteration mediated by an Early Noachian CO2-enriched solution on a 1000 m basaltic crust for aMars composed by fayalite, forsterite, and anorthite at a temperature of 50 "C (Table 1). b) pH, Eh, and alkalinity for theLate Noachian acidic alteration of the same type of crust modeled in (a) with the addition of siderite to simulate carbonateprecipitation in the subsurface at a temperature of 0 "C. c) geochemical conditions inferred from the terrestrial data andcomputer modeling of the surface and subsurface areas of Mars during the Early Noachian and the Late Noachian-Hesperian.

12 D. C. Fernandez-Remolar et al.

GEOCHEMICAL EVOLUTION OF AQUEOUSSYSTEMS ON MARS

We postulate that large-scale carbonate formationwas prevented on Mars by two acidic stages that wereseparated by a transitional evolutionary stage.

Stage I: Early Noachian

The first stage of Mars planetary evolution waswarm and wet and drove crustal weathering under mildlyacidic and oxidizing conditions. A CO2-rich, denseatmosphere (Carr 1999) formed by cometary input andprimordial volcanic emissions by 4.5 Ga (Elkins-Tanton2008) warmed the surface by introducing greenhousegases (Squyres and Kasting 1994). Mars’ surface wouldhave also been heated geothermally due to the high heatflux (Squyres and Kasting 1994; Hauck and Phillips2002). Acidification would have resulted from the dense,CO2-rich atmosphere reinforced by the photochemicaland radiolytical oxidation of dissolved Fe+2 (Fairenet al. 2004; Coogan and Cullen 2009) sourced fromhydrothermal activity and crustal leaching. During thisstage, the internal Mars dynamo (Williams and Nimmo2004; Nimmo and Tanaka 2005) was probably operatingand CO2 was present in sufficient abundance to reach thetriple-point temperature at the surface. Photochemicalreactions with volcanic gases, such as H2O, CO2, NO2,SO2, probably formed highly reactive oxidants such asH2O2, HO2, O3, O2, and H2CO3, which would haveenhanced crustal weathering (see Fig. 6a). The presenceof Fe3+-bearing nontronites on the surface of Mars(Chevrier et al. 2007) suggests an Eh >150 mV at thesurface and Chevrier and Keck (2009) have estimated theredox potential to oscillate between 800 and 30 mV for apH range of 4–8. The absence of early Noachian sulfatessupports the occurrence of silicate weathering under aCO2-rich atmosphere relatively impoverished in SO2,

which would prevent the formation of massive sulfatedeposits during this time. Phyllosilicates would havebeen distributed on the surface, and carbonates andsulfides could have formed in subsurface environments inthe presence of more neutral and anoxic waters. Towardthe end of this first stage, rates of sulfide and subsurfacecarbonate accumulation might have decreased as theplanetary dynamo began to shutdown (Nimmo andTanaka 2005; Solomon et al. 2005).

Middle-Late Noachian Transition

The transition from a CO2- to SO2-dominated surfacegeochemistry probably occurred as Mars’ planetarydynamo began to collapse, which would have caused a netloss by gradual escape of the atmosphere (Nimmo and

Tanaka 2005; Solomon et al. 2005; Chassefiere et al.2007). According to Tian et al. (2009), this primordialatmosphere would have been thinned by the high solarEUV fluxes that eroded the primordial Mars atmospherefrom pre-Noachian to mid-Noachian times. As a result,the CO2-rich atmosphere rapidly collapsed whichpromoted a cooling at planetary scale and the emergenceof a glacial era. Periglacial structures that formed duringthis stage are covered by sulfates and suggest that thesurface temperature was less than 0 "C, which is consistentwith a thin atmosphere. Liquid solutions would bemaintained by salt oversaturation that decreases themelting point of solutions as has been recently proposedby Fairen et al. (2009) and Fairen (2010). A decrease involatiles combined with other processes such as impactcratering and sputtering would have thinned theatmosphere (Carr 1999; Jakosky and Phillips 2001;Jakosky et al. 2005). The solubility of CO2 would haveincreased as temperatures dropped, thereby sequesteringmore CO2 in Mars’ hydrosphere, and possibly inclathrates (Miller and Smythe 1970). As a result, the pH ofthe surface waters would remain acidic (Fig. 8).

Moreover, in a declining atmosphere, the accumulationof atmospheric oxidants by photochemical processeswould partially buffer the input of reducing volatiles ofvolcanic origin. Given that the volatile concentration inthe atmosphere was decreased by the negative imbalanceresulting from the decreasing of volcanic activity and thehigher EUV fluxes by <4.5 Ga (Gaillard and Scaillet2009; Tian et al. 2009), the oxidant flux to the subsurfacewould have increased with time as the oxidation state ofMars’ mantle increased. This would also induce thedevelopment of acidic geothermal systems that wereactive during the Late Noachian (Yen et al. 2008).

Stage II: Late Noachian to Hesperian

The second stage involved a strongly acidic andoxidizing geochemistry that produced geographicallyextensive sulfate deposits, enhanced by the weathering ofprimary or secondary sulfides. The change to adominantly sulfur geochemistry probably occurred inthree ways (1) through a rapid increase in volcanicactivity dominated by the exhalation of SO2 (Carr 1999;Williams et al. 2009; Head and Wilson 2011; Robbinset al. 2011), which would have started when Tharsis wasgrowing during the mid Noachian (Phillips et al. 2001);(2) through a dramatic increase in the rate of meteoriticbombardment (Segura et al. 2002), although thishypothesis does not fit the impact flux over time(Hartmann and Neukum 2001; Ivanov and Head 2001),and seems inconsistent with the age of the networkvalleys (Hynek et al. 2010); and (3) through a decreasingon the solar EUV flux at the end of the Noachian (Tian

The environment of early Mars and the missing carbonates 13

et al. 2009). Sulfuric acid would then have beenproduced by photochemical reactions of SO2, the crustalweathering of sulfides, and acidic hydrothermal activityand ferric iron hydrolysis (Braterman et al. 1983).

The introduction of huge quantities of volatiles intoa CO2-exhausted Late Noachian Mars atmosphere dueto the decreasing activity of the dynamo, which has shutoff well before the Late Noachian (Nimmo and Tanaka2005), shifted the chemical balance of the atmosphere tothe massive production of oxidants and acidifyingcompounds. Thermal isolation of the magmatic sourcespromoted by the frozen crust of Mars may have favoredan increase in thermal activity (Kargel et al. 2007) in thisLate Noachian episode, as well as the oxidizing state ofthe magma (Head and Wilson 2011). It is probable thathydrogen peroxide would have been formed inphotochemical reactions involving CO2 and H2O, whichwould have led to a vigorous surface weathering regime(Hoffman and Edwards 1975). The interaction of theoxidizing solutions with the sulfide–carbonate depositswould promote carbonate dissolution and reprecipitationin deeper regions of the Mars crust after neutralizationand reduction of the incoming fluids. As a result,carbonates may still be present in the subsurface (Michalskiand Niles 2010) of Mars where redox conditions werefavorable.

The acidic-oxidizing Late Noachian event wouldhave rapidly dissolved all carbonate minerals formed inthe subsurface and sediments by weathering the EarlyNoachian crust, returning CO2 back to the atmosphereand hydrosphere, and possibly reprecipitating carbonatesin deeper regions of the subsurface. Under theseconditions, old phyllosilicate deposits may have persisted(Altheide et al. 2010). Moreover, newly generated clays(Wang et al. 2006; Schmidt et al. 2009; Story et al. 2010)like kaolinite, illite, and nontronite would be expected tooccur by acidic leaching of the igneous basement(Fernandez-Remolar et al. 2009b).

ASTROBIOLOGICAL IMPLICATIONS

The absence of surface carbonates may be the key tounderstanding, or even discovering, life on Mars. Theprecipitation of some carbonates on Earth’s surface isrelated to microbial activity through CO2 uptake byautotrophs coupled to the organic decomposition byheterotrophic living forms (e.g., Vasconcelos andMcKenzie 1997; Castanier et al. 2000; Ehrlich 2002;Fraiser and Corsetti 2003; Vasconcelos et al. 2006),which release CO2 back to the environment in the formof HCO3

). In this sense, these carbonates are biomineralproducts, which can become valuable biosignatures inparticular circumstances. Biominerals are consideredfossils of metabolism and comprise a powerful tool to

a

b

c

Temperature

Pressure

Fig. 8. Estimation of CO2 dissolution in (a) the Marshydrosphere from the Early Noachian to the Middle Hesperianusing the data of Duan and Sun (2003) in water with 35&salinity. Solubility changes account for a P–T temporalgradient (b) that would start at T = 50 "C and P = 20 barsfor the Early Noachian, and end at T = 0 "C and P = 0.5bars (Carr 1999). Under these changing P–T conditions, the pHis maintained at approximately 3.5 (c) assuming that fCO2

evolved from 5 to 0.5 bars over the same timespan.

14 D. C. Fernandez-Remolar et al.

understand evolutionary processes (Vasconcelos et al.2006). An example is Archean stromatolitic carbonatesthat are associated with the earliest evidence of microbiallife in shallow oceans (Allwood et al. 2006). Hence, thesedimentary record is the only evidence we have of lifeon Earth for the first seven-eighths of the planet’sexistence. Apparently, a massive CO2 uptake bystromatolite-building microbial communities in an oceanoversaturated with CO2 would change the waterchemistry to a more neutral pH. In this way, carbonatescan be considered a symptom of life on Earth throughcarbon fixing in the form of biomass and biominerals.

Using this reasoning, the absence of carbonates onthe surface of Mars can be a consequence of threedifferent situations: (1) Mars has never had any history oflife because there is no evidence of surface carbonates, or(2) sulfur and iron bacteria promoted the production ofcarbonates in the Mars subsurface in the same way thathas been observed on Earth (Fernandez-Remolar et al.2008, 2009b). The second option is as probable as the firstgiven that Early Mars had a similar habitability to that ofEarly Earth with regard to availability of water, energysources, and chemical disequilibrium (Gaidos et al. 2005;Hoehler 2007; Des Marais et al. 2008). This is the case forsulfur and iron compounds that played an essential rolein the geochemistry of Mars and have been available forautotrophic microbes for billions of years. In someconditions, Fe and S bacteria could use sulfates and ferriciron as electron acceptors to produce carbonates byorganic decomposition (Monetti and Scranton 1992;Labes and Schonheit 2001). In the Mars subsurface,carbonate precipitation mediated by microbial activitycould occur when two conditions are met. The firstcondition would be the neutralization of the surfacesolutions entering an aquifer where precipitation wouldbe expected at a pH > 5 (Ohmoto et al. 2004). Thiswould be followed by a second condition regarding theconcentration of ions like Fe, Ca, and Mg by microbes(Ahimou et al. 2002).

Modern Earth surface carbonate environments,which share similarities with Early Earth environments,provide important insights into the physical and chemicalprocesses that may have operated on Early Mars (e.g.,Vasconcelos and McKenzie 1997; van Lith et al. 2002,2003; Vasconcelos et al. 2006; Sanchez-Roman et al.2008a). In addition, biological characteristics must beconsidered in carbonate formation processes, whichinclude biosphere and geosphere interactions (Vasconceloset al. 2006). Recent studies reveal that there is amicrobial factor in the formation of carbonates and otherauthigenic minerals (e.g., Vasconcelos et al. 1995;Knorre and Krumbein 2000; Ehrlich 2002; Sanchez-Roman et al. 2008b). Most of these studies are basedon field observations and validated by laboratory

experiments. Microbes induce mineral formation bybreaking the thermodynamic barriers to inorganicmineral precipitation.

The occurrence of Fe-rich carbonates in thesubsurface of Rıo Tinto indicates mildly acidic to neutralpH (approximately 5–7) and somewhat reducing (Eh < 0)conditions (Fernandez-Remolar et al. 2008). The presenceof sulfate-reducing bacteria and iron-reducing bacteriathat oxidize organic matter (or inorganic compounds)(Warthmann et al. 2000; Fernandez-Remolar et al. 2008)would induce the precipitation of carbonate minerals inthe subsurface by increasing the dissolved inorganiccarbon in the aqueous system. Indeed, the neutralizationof subsurface solutions is promoted by microorganisms(iron and sulfate reducers), thus, the potential exists thatthose carbonates were microbiologically mediated. In fact,the metabolic activity of sulfate-reducing bacteria,anaerobic degradation of organic matter, results in anelevated pH (from acidic to neutral) and leads to aconcentration of ions (Fe, Ca, Mg) and bicarbonate ions,and the nucleation of the carbonate microcrystals. SRBcan induce significant amounts of carbonate precipitation,implying a major microbial contribution to the carbonatesedimentary budget (Warthmann et al. 2000; van Lithet al. 2002).

During Earth’s early history, anaerobic bacteria mayhave been particularly important for Fe-rich carbonateproduction on the surface because anoxic conditions weremore prevalent than today. However, the environment onearly Mars suggests that these anoxic conditionsfavorable for carbonate production existed in thesubsurface. In this way, there could be huge quantities ofsubsurface carbonates that are escaping remote detection(Michalski and Niles 2010). The existence of carbonateglobules in ancient Mars meteorites like ALH 84001(Niles et al. 2009) could be a small sample of thecarbonate factory in cryptic (sub-)regions of the planet.

The understanding of biochemical, biological, andgeological interactions, as one unique system, can be animportant step to better understand the origin of thesignals created by early life on Earth (and Mars). Thepossible relationship between diverse microbial processesand associated authigenic minerals could providean important step to trace microbial evolution on Marsas well as on Earth. For example, geochemical,mineralogical, and microbiological characteristics of theRıo Tinto area support its classification as an importantfossil environment, particularly as an analog for Marsand Archean environments, which are dominated byacid–sulfate systems. In this way, the precipitation ofmineralogies, such as carbonates, in acidic environments(Fernandez-Remolar et al. 2009b), which is unexpected,could become valuable knowledge in tracing microbialactivity on other planetary bodies. This does not mean

The environment of early Mars and the missing carbonates 15

that carbonates are unquestionable biomarkers, but itsoccurrence in acidic sediments or subsurface regions ofMars could open a new possibility in searching forextinct or extant life.

CONCLUSIONS

Using planetary data, meteorite chemistry (discussedin the Supporting Information), and data from modernterrestrial analogs, it is possible to establish a model toexplain why the surface of early Mars is devoid ofcarbonates. The surface geochemical environment wasnot favorable for carbonate production, but carbonatescould have formed in the shallow subsurface to deepersubsurface areas. A synthesis of geochemical data fromthe Noachian and Hesperian epochs reveals some of themain evolutionary stages that may explain the differencesbetween the carbon cycle on Earth and Mars (Fig. 9).The three stages, that defined the Earth, and somemissed in Mars, are (1) simple weathering of the crust bymeteoric waters at <4.5 Ga; (2) emergence of an activebiosphere that ‘‘pumped-out’’ CO2 and mediated the

production of carbonates starting at approximately3.5 Ga; and (3) the development of plate tectonics andmidocean ridges that actively buffer excess ions andother molecules supplied by fluvial systems. However, allthese mechanisms may have not been operating togetheron Mars; on the contrary, there is evidence only for themost primitive mechanism of buffering CO2 and pH thatis a simple chemical reaction between meteoric watersand the silicate crust (Wyatt and McSween 2002). In thiscontext, the buffering would be incomplete to producean excess of different compounds in the hydrosphere(Maynard 1976) that would have a pH below 7.Furthermore, a shallow hydrosphere for Mars, asopposed to Earth, would facilitate the equilibrationbetween the water masses and the CO2-rich atmosphere.

Under these circumstances, the solutions would bebuffered to acidic conditions except in those areas wherethe atmospheric supply of CO2 would be graduallyremoved to form HCO3

) and precipitate carbonates.Although Mars could surely have had some deep basinsto favor the precipitation of carbonates, it is in thesubsurface and the sedimentary bodies where carbonates

Fig. 9. Idealized diagram showing the main evolutionary steps that were successively involved in the regulation of CO2 and pH inEarth and Mars. There are strong differences between both planets; whereas Mars only had one step to remove CO2 from thehydrosphere through simple continental weathering, Earth coupled two novel powerful pumps corresponding to an emergingbiosphere and the development of Middle Ocean Ridges (MOR) spreading centers. Under these circumstances, Mars had by<4.5 Ga a shallow primordial hydrosphere with an excess of ions under equilibrium with a CO2-rich atmosphere that sourceacidic meteoric waters. This mechanism prevented the formation of early carbonates on the surface, although subsurfaceprecipitation and sediment cementation can be expected. Later on, the fast and global cooling by loss of the planetary dynamopromoted the formation of a CO2-enriched cryosphere that was a source of transient and denser atmospheres during punctuatedepisodes of post-Noachian volcanisms with acidic geochemistry. Nahklite MIL 03346 is displayed in the diagram to trace back theage of the acidic episodes on Mars (McCubbin et al. 2009).

16 D. C. Fernandez-Remolar et al.

and authigenic phyllosilicates could precipitate escapingfrom the more acidic conditions of the meteoricsolutions. Moreover, the development of a shallowhydrosphere fed on an aggressive weathering of thecrystalline crust would favor the oxidation of dissolvedcations like Fe+2 that can be oxidized to Fe+3 throughsome oxidants produced in the atmosphere byphotochemistry (see Reactions 7 and 8) (Konhauseret al. 2007; Coogan and Cullen 2009). The production ofoxidants would also prevent the formation of siderite,the most suitable phase for slightly acidic conditions, butunstable under Eh > 0.

Planetary data suggest that Mars never reached thenext two essential steps to become an Earth. The collapseof an active hydrosphere in the planetary surfaceprobably prevented not only the emergence of a surfacebiosphere but also the development of plate tectonicsthat is an efficient global system for hydrochemicalregulation of pH and dissolved compounds sourced onthe continents. Once the magnetic dynamo collapsed by4.0 Ga (Nimmo and Tanaka 2005), the fast coolingpromoted that the CO2 exceeding cryosphere formedcarbon dioxide clathrates (Miller and Smythe 1970). Itcould be a source of greenhousing volatiles for transientatmospheres developed by thermal episodes punctuatedover the post-Noachian times. Such thermal events wereassociated with the release of acidic solutions controlledby the geochemistry of the sulfur and iron underwhich the carbonate formation would be inhibited.Alternatively, there are very few outcrops dated to theEarly Noachian on Mars, it can also be postulated thatcarbonates are not recognized due to lack of preservationof these materials.

Neither the geological record, nor the geochemicaldata are against a cryptic biosphere that could interactwith the different geochemical cycles of Mars. Theemergence of life under an acidic hydrosphere (Russelland Hall 1997; Maheen et al. 2010) and its migration tocryptic planetary regions could have been possible. Thedeepening biosphere would also move down to thesubsurface the production of carbonates. Therefore, ifthis hypothesis is correct, the detection of carbonates inMars subsurface rocks should be considered as a targetto identify those areas of interest for looking for life onMars.

Acknowledgments—This work is supported by researchprojects AYA2009-1168 and CGL2009-08227-E ⁄BTEfrom the Spanish Ministry of Research and Innovation.We are very grateful to Prof. Raymond Arvidson,Dr. Richard Morris, Dr. Goro Komatsu, WladyAltermann, and Dr. Pennelope King who have been agreat source of ideas and support, as well as to Dr. Brian

Hynek and Dr. Alberto Fairen, who greatly improvedthe manuscript by their suggestions and comments.

Editorial Handling—Dr. Michael Zolensky

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