The environment of early Mars and the missing carbonates

Abstract

Abstract-A model is presented in which the aqueous conditions needed to generate phyllosilicate minerals in the absence of carbonates found in the ancient Noachian crust are maintained by an early CO 2 -rich atmosphere, that, together with iron (II) oxidation, would prevent carbonate formation at the surface. After cessation of the internal magnetic dynamo, a CO 2 -rich primordial atmosphere was stripped by interactions with the solar wind and surface conditions evolved from humid to arid, with ground waters partially dissolving subsurface carbonate and sulfide minerals to produce acid-sulfate evaporitic deposits in areas with upwelling ground water. In a subsequent geochemical state (Late Noachian to Hesperian), surface and subsurface acidic solutions were neutralized in the subsurface through interaction with basaltic crust, allowing the precipitation of secondary carbonates. This model suggests that, in the early Noachian, the surface waters of Mars maintained acidity because of a drop in temperature. This would have favored increased dissolution of CO 2 and a reduction in atmospheric pressure. In this scenario, physicochemical conditions precluded the formation of surface carbonates, but induced the precipitation of carbonates in the subsurface.

Compositional investigation of the proposed chloride‐bearing materials on Mars using near‐infrared orbital data from OMEGA/MEx

“…While carbonates have not been detected by OMEGA so far [ Jouglet et al , 2007], they have been remotely observed as excavated materials only with CRISM higher spatial resolution data [ Ehlmann et al , 2008; Michalski et al , 2012]. It has been suggested that the rarity of carbonates on Mars could be explained by the absence of an Earth‐like carbonate cycle during early Mars [ Ehlmann et al , 2008; McLennan and Grotzinger , 2008], and/or later dissolution under exposure to acid conditions [ Chevrier et al , 2007; Fernández‐Remolar et al , 2011]. On the other hand, hydrated sulfates have been identified on Mars in extensive and diverse deposits by OMEGA and CRISM [e.g., Gendrin et al , 2005; Langevin et al , 2005; Mangold et al , 2008; Murchie et al , 2009].…”

Orbital evidence for more widespread carbonate‐bearing rocks on Mars

“…One possible fate of calcium carbonates weathered under such acidic conditions is formation of bassanite (CaSO 4 •½H 2 O) [ Vaniman et al ., ], which has indeed been found at the base of the ancient clay‐rich Mawrth Vallis layered phyllosilicate‐rich deposit [ Wray et al ., ]. The fate of any carbon liberated by weathering of putative massive deposits is unclear, although one possibility is reprecipitation at greater depths where fluids became neutralized through interaction with the basaltic crust [ Fernández‐Remolar et al ., ]. Alternatively, experimental and analog studies suggest mere surficial weathering of carbonates (formation of a sulfate coating) [ Clark et al ., ; Cloutis et al ., ], which could mask their spectral signature and thus would avoid the “missing carbon” problem.…”

Dioctahedral Phyllosilicates Versus Zeolites and Carbonates Versus Zeolites Competitions as Constraints to Understanding Early Mars Alteration Conditions

“…Hence, laboratory experiments (Dehouck et al, ; Dehouck et al, ; Peretyazhko et al, ) and/or geochemical simulation (Catalano, ; Chevrier, Poulet, & Bibring, ; Fernández‐Remolar et al, ; Griffith & Shock, ; Liu & Catalano, ) have been devoted to better understand their formation. In particular, due to the Fe‐rich Martian crust (compared to the Earth's) and the current oxidizing conditions, redox effects on the formation of Fe,Mg‐smectites have been studied under conceivable early Martian environmental conditions.…”