Spectroscopic Characterization of Green Rust and its Implications for Mars Astrobiology

English Abstract:

One mineral that may be particularly relevant to prebiotic chemistry on the early Earth, early Mars, and other wet rocky worlds is Green Rust (GR), a double layered Fe(II,III) oxyhydroxide with a hydroxylated brucite structure. Green rust forms readily in natural environments, and would have comprised a large fraction of chimneys precipitated at submarine alkaline hydrothermal vents on the early Earth where it has been proposed that metabolism emerged. As a double layer hydroxide GR has many interesting and useful properties: it can concentrate a variety of organic and phosphorus species in its interlayers, preserving them against degradation, and it is a versatile redox catalyst for reactions that might drive the emergence of life; e.g., the reduction of nitrate to ammonium. It is likely that GR and related minerals are still present on Mars today. If so, they may preserve biosignatures or tell us about the early Martian environment and its potential to give rise to life. To address these science questions and the challenges associated with the analysis of GR, we are simulating its formation in ancient hydrothermal systems and developing methods to detect, analyze, process, and preserve/cache these delicate minerals – particularly using spectroscopic technologies that will fly on upcoming Mars missions. GR was synthesized anaerobically in laboratory systems using chloride or carbonate iron salts, and the resulting precipitate analyzed laser Raman spectroscopy time-series. A total of 20 spectra were recorded at 5-min intervals. By the end of the experiment the initial green rust was completely oxidized into maghemite and ferrihydrite. The laser power at the sample surface was kept below <1mW in order to prevent thermally-induced oxidation. Quantitative analysis of the spectral time-series allowed us to constrain the kinetics of green rust oxidation and determine intermediate oxidation products, including goethite and hematite. Future experiments will study the stabilization of GR by trace components such as phosphates, and its ability to drive other proto-metabolic reactions and sequester organic molecules.