THE ANCIENT MARTIAN ATMOSPHERE AND CONDITIONS FOR EVAPORITE AND HEMATITE FORMATION: A MINERALIZATION PATHWAY MODEL FROM CARBONATE TO HEMATITE

We hypothesize that the Martian atmosphere in the Noachian epoch was warmer and wetter and maintained a denser CO₂-H₂O atmosphere that enabled paleolake and lacustrine landscape playas to form evaporites through precipitation and groundwater capillary action. We propose a model where a carbonate to hematite mineralization pathway involves precipitation and Fe-rich groundwater infiltration into the soil, where evaporites form concentric rings in basins as observed on Earth. As evaporation takes place siderite (FeCO₃) precipitates first, followed by magnesium calcite, hydromagnesite (Mg₅(CO₃)₄(OH)₂∙4H₂O), gypsum (CaSO₄·2H₂O), and highly soluble salts. We show that the most common alteration product of siderite in these conditions is soluble hydrous ferric oxide. This phase can diagenetically alter to less soluble iron oxide minerals such as hematite (Fe₂O₃), goethite (FeO∙OH), and magnetite (Fe₃O₄), leaving minimal and possibly undetectable traces of precursor carbonates. Through this mineralization pathway siderite oxidizes and CO₂ is released from the regolith, producing a more acidic chemical weathering environment for subsequent formation of phyllosilicates and jarosite (KFe₃(SO₄)₂(OH)₆ from the basalt parent rock. Siderite diagenesis to hydrous ferric oxides may indicate evaporative conditions, with chemical exchange between the atmosphere and Martian regolith enabling this mineral sequence. We hypothesize that the small platy gray hematite concretions (blueberries) found in stratigraphic layers in Meridiani Planum and Eagle Crater were formed by carbonate mineral burial diagenesis from evaporite deposits with a mineralization pathway either directly to hematite or via goethite. This alteration mechanism differs from a conventional explanation in that siderite is a precursor mineral and may account for the lack of carbonates observed and is an alternative to direct hematite precipitation from oxygenated Fe-rich water. This implies that the large equatorial region containing Sinus Meridiani and Arabia Terra was underlain by an extended water table and/or experienced periodic precipitation events. We encourage the use of siderite and its solid solution series between magnesite and rhodochrosite in spectral analysis, as they may occur as residual minerals at low weight percentages.