Beyond Brines: Iron-Based Ionic Liquids as Persistent Non-Aqueous Solvents on Mars

Perchlorate and chloride brines, while capable of transient liquid stability on Mars, rapidly lose their aqueous component under Martian pressure. Ionic liquids' negligible vapor pressures enable indefinite persistence, and they function without water activity constraints or chaotropic stress. Here we present the novel concept of iron-based ionic liquids as a complementary class of Mars-relevant solvent systems.

We synthesized iron-based imidazolium ionic liquids and we showed that (i) month-long exposure to simulated Mars surface conditions (600 Pa, CO₂) produces negligible mass loss (<0.1%), and (ii) they exhibit glass transitions near −65°C, bulk melting points of 7–19°C, and thermal stability exceeding 300°C. Notably, both CO₂ dissolution and confinement within nanoporous matrices, conditions directly relevant to Mars, are known to substantially depress melting points in imidazolium-based ionic liquids. We will present results from ongoing experiments examining how these factors influence the phase behavior of our iron-based compounds, with implications for their liquid stability range under Martian surface conditions. We performed Raman spectroscopy at 532 nm and confirmed tetrahedral iron-halide anion formation. Based on Raman data, we established diagnostic fingerprints for in situ detection.

The geochemical precursors required for iron-based ionic liquid formation (i.e., iron oxides, chlorides, bromides, sulfates, and organic molecules including chloromethane) have all been detected on Mars. Whether iron-based ionic liquids can support biochemical processes or preserve biosignatures remains unexplored, but their capacity for solvating polar molecules, negligible volatility, and potentially extended liquid range under Mars-relevant conditions motivate systematic investigation. We propose that ionic liquids represent an underexplored component of Mars solvent chemistry detectable by current instrumentation (i.e., SuperCam instrument aboard Perseverance).