Magmatic and impact-induced gas-solid reactions controlled the early evolution of the martian surface and atmosphere

There are many outstanding questions about the evolution of the surface and atmosphere of Mars. How and why did Mars's surface and atmosphere lose most of their H₂O? Why is the martian dust relatively uniform in composition? Why are surface S contents high and carbonate minerals rare? Why do abundant Fe-oxides/(oxy)hydroxides commonly co-exist with reduced Fe-sulfides? Why are S and halogens locked in soluble sulfate and (oxy-)halide salts?

We propose that high temperature gas-solid reactions in Mars's early history provide a framework that explains the planet's broad surface and atmospheric features. Hot gas-solid reactions are common on Earth, and are recognized on present-day Venus, occurring in: volcanic gas plumes, the (sub-)surface of volcanoes, and impact events. Recent experimental, thermochemical and field data show that hot gases react rapidly and efficiently with common silicate minerals and glasses to produce predictable products. These reactions occur over both temporal and lateral scales that demonstrably influence the evolution of a planet.

The primary crust of Mars was produced through long-lived basaltic volcanism that released magmatic gases. Analogous with measured/modelled terrestrial gases and modelled lunar gases, the martian gases include oxidized and reduced H-O-S-C-N-P-Cl-Br-K-Na-Fe-Zn-Ni species. Recent experiments show that martian magmatic gases condense to form Fe-oxides, Fe-sulfides, sulfur, metals and alkali halides. The minerals produced by high temperature gas-rock chemisorption reactions include soluble Ca-, Na-, Mg-(hydrated) sulfates, Na- and Ca-(oxy)-halogen salts, and less soluble Fe-O(-H) minerals, sulfides and (amorphous) Al-Si-O(-H-+) phases. These are the very mineral groups widely distributed on the martian surface. Impacts into this material (+ices) causes further gas-solid reactions when fine particles and gases are produced and react. The reacted material is homogenised across the surface and redistributed via sedimentary processes.

We suggest that gas-solid reactions significantly contributed to the present martian atmosphere by effectively "scrubbing" it of reactive gases such as H₂O, SO₂, S₂ and halogens. Concomitantly, Mars's early atmosphere largely retained CO₂ because reaction rates for carbonation are slow in the absence of H₂O.