Exploring Fe-bearing Amorphous Materials in Mars-Analog Samples: Implications for Future Returned Sample Interpretation

Martian sedimentary rocks, sediments, and surface soils contain abundant amorphous and poorly crystalline materials. The formation, preservation, subsequent modification, and stability of these amorphous phases are widely regarded as sensitive indicators of past climatic conditions and near-surface geological environments on Mars. Among the various candidates proposed to account for the amorphous components, Fe-bearing materials represent a particularly important yet underexplored class. Secondary Fe-rich alteration products (e.g., sulfates, phyllosilicates, hydroxides, and carbonates) are prone to strong hydrolysis, resistance to evaporation-driven crystallization, and nanophase formation, all of which favor the development and retention of amorphous or weakly ordered states.

Here, we present results from recent simulation experiments and natural analogue samples from Mars-analog systems. Our investigation integrates three representative systems. (1) Experiments on Fe-rich olivine serpentinization reveal the precipitation of low-crystallinity Fe-Si-rich phyllosilicate materials from derived solutions, highlighting a coupled Fe oxidation and Si redistribution pathway during low-temperature hydrothermal water-rock interaction. Nanophase magnetite is observed to nucleate on the surfaces of these Fe-Si phases. (2) Evaporation and photooxidation of Fe-sulfate solutions produce Fe-sulfate gels and associated assemblages of Fe sulfates and Fe(III) oxides with elevated amorphous components, reflecting redox transformation under acidic, highly oxidizing, and low-water-activity conditions analogous to Martian sulfate-rich environments. (3) Recent work on diagenetically modified jarosite-bearing materials from terrestrial acid-sulfate settings documents distinctive patchy and crustal microtextures, associations with abundant amorphous or poorly crystalline phases. These textures potentially record microscale co-precipitation, dissolution-reprecipitation, and late-stage diagenetic overprinting within acidic sulfate systems.

Across all systems, microstructural, mineralogical, and spectroscopic observations indicate that Fe-bearing amorphous materials are not incidental by-products but integral components of Mars-like alteration pathways, sensitive to fluid chemistry, redox state, and water availability. We propose that such materials represent transient yet information-rich phases on Mars, capable of preserving signatures of aqueous conditions and oxidative processes that may remain cryptic in crystalline assemblages alone.