Ultraviolet Photooxidation of Carbonate-Bearing Aqueous Ferrous Iron Within Open and Closed Experimental Systems—Implications for Sunlit Surface Water Bodies on Early Mars

Ultraviolet (UV) photooxidation has been considered a viable pathway to oxidize aqueous ferrous iron expected to have been present in early Martian surface waters. This reaction has been proposed to have created iron mineral deposits in Meridiani Planum as well as Gale Crater and would have affected chemical conditions such as pH and dissolved iron concentrations. Previous experimental work has not focused on the influence of atmospheric CO₂ on aqueous ferrous iron photooxidation. Atmospheric CO₂ would have been an important factor since climate models require high pCO₂ (in addition to other greenhouse species) to stabilize liquid water on the ancient surface environment of Mars. In this work, we conducted two types of irradiation experiments under anoxic conditions, with solutions containing ∼100 ppm (1–2 mM) iron and 0–35 mM dissolved inorganic carbon (DIC) at initial pH varying between 3 and 8. Utilizing two experimental setups revealed the importance of DIC within these solutions in determining the fate of iron. When the solutions experience CO₂ degassing, pH increases in a predictable manner and iron is removed from solution and precipitates as carbonate green rust and magnetite. When DIC remains in solution in the absence of degassing effects, the pH changes and iron losses were minimized. Under these conditions, UV- induced iron photooxidation does not occur. These results imply that under the assumption of a 1 bar CO₂ atmosphere, ferrous iron in surface environments would not have been photooxidized and would have remained as a stable species in solution in the absence of other oxidants.