Date of Award
Doctor of Philosophy (PhD)
As the nearest planetary neighbor with potential earlier habitable conditions, Mars is replete with minerals that hold clues to its past chemistry and evolution of the aqueous systems. Oxidized iron (Fe) and manganese (Mn) minerals on Mars are geochemical markers of such environments in which they formed and record past pH, redox conditions, and intensity of water-rock interaction. Fe and Mn oxides can therefore be used to reconstruct past Martian environmental conditions from settings where aqueous fluids were active. Various processes that form Fe and Mn oxide minerals on Mars have been previously proposed, including chemical oxidation using molecular oxygen (O2) and photo-oxidation by ultraviolet (UV) rays. However, the effectiveness of O2 and UV as oxidants on Mars might be adversely affected due to the acidic fluid chemistry generally expected under Mars-relevant conditions. Oxyhalogens have a high thermodynamic oxidation potential, wide distribution, high preservation potential, and the ability to form dense brines that may percolate deeply into the Martian subsurface. However, their role as potential Fe and Mn oxidants on Mars has been largely unexplored. In my dissertation, I studied the formation of Fe and Mn oxide minerals by oxyhalogens [(per)chlorate & bromate] under Mars-relevant conditions.In chapter 2, I show that perchlorate is an ineffective Fe oxidant on Mars, whereas chlorate can oxidize dissolved Fe(II) in in Mars‐relevant fluids at both near‐neutral and acidic pH conditions on timescales of weeks at room temperature conditions. Diverse Fe(III)-bearing phases are produced and their mineralogy is controlled by the fluid chemistry. I developed a kinetic rate law model that showed that the rate of Fe(II) oxidation by chlorate is orders of magnitude higher than by O2 or via UV-photooxidation. The results demonstrate that chlorate could be an important Fe(II) oxidant on Mars, especially in acidic fluids in which other oxidants (O2 or UV light) become less effective. In chapter 3, I calculated the capacity of chlorate to oxidize ferrous iron and conducted experiments in different ratios of Fe(II) and chlorate in Mars-relevant fluids at room temperature. The results show that chlorate present on the surface of Mars is expected to be fully available to react with Fe(II) and that 1 wt.% chlorate can produce approximately 6-12 wt.% Fe(III) or mixed valence mineral products. The mineral products are a function of fluid type (chloride and sulfate), pH, and the rate of Fe(II) oxidation. The results showed close agreement to the previously parameterized kinetic model and confirmed the applicability of the rate law model to accurately predict the rate of Fe(II) oxidation by chlorate in diverse systems. Here I show that chlorate can provide a substantial oxidizing capacity and produce all iron oxides found on Mars either directly or through diagenesis. In chapter 4, I show that chlorate can oxidize Fe(II) and produce Fe(III)-bearing minerals at temperatures as low as 0ºC in Mars-relevant fluids and accurately follow the kinetic rate law model developed earlier. The Fe(III)-bearing phases produced are determined by the fluid type, initial pH, reactant concentration, and temperature. Akaganeite, an Fe(III) oxyhydroxide mineral found on Mars, was favored at low temperature thus demonstrating the substantial effect of temperature on the Fe(III) mineral formation on Mars. In chapter 5, I show that bromate can oxidize dissolved Mn(II) in homogeneous systems and produce the Mn(III/IV) oxide nsutite in Mars-relevant fluids. While chlorate alone was not found to be an effective Mn(II) oxidant, mixed chlorate-bromate systems, with molar similar to those found on Mars, could prove to be an effective Mn(II) oxidant. Given the thermodynamic unfavorability of homogeneous and heterogeneous Mn(II) oxidation by O2 below pH ~7 and the absence of any other plausible Mn(II) oxidant on Mars, I argue that bromate is the likely Mn(II) oxidant on Mars. The presence of Mn(III/IV) oxides on Mars therefore points to an active halogen cycle on early Mars. In this dissertation, the importance of oxyhalogen species (chlorate and bromate) in serving as important geochemical oxidants of Fe(II) and Mn(II) in Mars relevant fluids at temperatures as low as 0ºC is discussed. The results demonstrate the effectiveness of chlorate and bromate to form common Fe(III)- and Mn(III/IV)-oxides in myriad Mars-relevant fluid conditions. The kinetic rate law model developed is being tested and verified at different geochemical conditions and could be confidently used to simulate Fe(II) oxidation by chlorate in diverse Mars-relevant fluids. The dissertation demonstrates the likelihood of oxyhalogen species to be important Fe(II) and Mn(II) oxidants on Mars in the past as well as in the present.
Chair and Committee
Jeffrey G. Catalano
Raymond E. Arvidson
Mitra, Kaushik, "Iron and Manganese Oxidation by Oxyhalogen Species: Implications for Paleoenvironmental Reconstruction on Mars" (2021). Arts & Sciences Electronic Theses and Dissertations. 2574.
Available for download on Tuesday, November 15, 2022