Iron Geochemical Cycling on Mars: A Temporal and Planetary Perspective

Introduction:

Iron plays a pivotal role in shaping Mars’ surface processes, influencing its climate evolution, and affecting its potential for habitability. Over the past decade, data from Mars exploration missions have significantly deepened our understanding of iron geochemical cycling -- the set of processes through which iron transitions between different oxidation states and mineral forms within the Martian surface environment. These transformations are closely linked to Martian hydrological activity, atmospheric changes, and redox conditions that may have supported life. This review synthesizes key advances in our knowledge of iron cycling on Mars from recent years. It examines the primary sources and mineral forms of iron, traces the temporal evolution of iron cycling across geological epochs, explores its environmental and climatic implications, and reviews mechanistic insights gained from experimental and modeling studies. The paper also discusses unresolved scientific debates and methodological challenges, providing perspectives to guide future study.

 

Sources and forms of Fe on Mars

Iron inventory on Mars primarily originates from mafic and ultramafic igneous rocks formed through planetary differentiation and volcanic activity. Over time, these iron-bearing silicates underwent aqueous alteration, giving rise to a diverse suite of secondary minerals. These include iron oxides (e.g., hematite, magnetite), hydroxides (e.g., goethite, ferrihydrite), sulfates (e.g., jarosite), carbonates (e.g., siderite), and iron-rich phyllosilicates (e.g., nontronite) [1,2]. Notable new discoveries from the rover missions include siderite-rich layers in Gale Crater, suggesting CO₂ sequestration in ancient Martian lakes [3] and at Jezero Crater, Perseverance identified coarse-grained olivine-rich igneous rocks and serpentinized fragments, indicative of hydrothermal activity [4]. Furthermore, spectral analyses now suggest that Martian dust is dominated not by crystalline hematite, but by ferrihydrite, an amorphous iron oxyhydroxide typically formed under low-temperature aqueous conditions [2].

 

Temporal evolution of Fe cycling

Iron cycling on Mars closely mirrors the planetary transition from early wet or icy conditions to the cold, arid environment observed today. During the Noachian period (4.1-3.7 Ga), iron was likely highly mobile in the form of Fe²⁺ within neutral to mildly acidic aqueous environments. Evidence from Gale Crater suggests the presence of redox-stratified lake systems, characterized by magnetite and ferrous phyllosilicates at depth, with more oxidized iron phases near the surface [5]. Ferrihydrite-rich sediments may also have formed under these conditions. In the Hesperian epoch (~3.7-3.0 Ga), increasingly oxidizing and acidic conditions favored the formation of iron sulfates, such as jarosite. The detection of high-Mn oxides by the Curiosity rover has been interpreted as evidence for transiently oxygen-rich episodes[6], though alternative oxidants, such as chlorates or UV-driven photochemical processes, remain plausible [7]. During the Amazonian period (~3.0 Ga to present), extreme cold and aridity severely limited aqueous alteration. Iron cycling during this time has been dominated by the oxidation of surface-exposed Fe²⁺ minerals and the wind-driven redistribution of iron-rich dust.

 

Climatic and habitability implications

Iron minerals function as valuable environmental proxies on Mars. For example, ferrihydrite is indicative of cold, aqueous conditions, while jarosite and other sulfates point to acidic, evaporative environments. The occurrence of siderite implies near-neutral pH and elevated CO₂ levels, suggesting a more temperate early climate [3]. Beyond environmental reconstruction, iron redox cycling may have supported microbial metabolisms. Nitrate-dependent Fe²⁺ oxidation has been proposed as an energetically favorable pathway in early Martian lakes, which likely contained both dissolved Fe²⁺ and nitrate [8]. Moreover, oxidized iron minerals may have served as long-term sinks for oxygen, helping to buffer the planetary atmospheric composition and influence climate evolution. In the present day, iron-bearing dust continues to affect Mars energy balance by modulating solar radiation and atmospheric dynamics.

 

Future directions

Iron cycling on Mars has not only recorded environmental transitions but has also actively shaped them. Processes such as serpentinization likely contributed to abiotic hydrogen production and may be linked to episodic methane detections. At the same time, extensive Fe oxidation may have consumed significant amounts of atmospheric O₂, hindering its long-term accumulation. Upcoming missions, including Mars sample return, will enable detailed laboratory analyses of iron speciation and isotopic composition, including isotopic signatures of other elements, critical for constraining the timing of redox transitions and evaluating potential biosignatures. Key open questions remain: When and how did the Fe oxidation occur on Mars? Did Mars undergo a global oxidation event? How deeply did oxidation penetrate the crust? Answering these questions will require integrated approaches that combine planetary missions, laboratory experiments, and advanced geochemical modeling.

 

Acknowledgements: This research was supported by the National Natural Science Foundation of China (Grant Nos. 42441803, 4237304).

 

References

[1] Fraeman, A. A. et al. (2020). J. Geophys. Res. Planets, 125, e2020JE006527.

[2] Valantinas, A. et al. (2025). Nat. Commun., 16, 1712.

[3] Tutolo, B. M. et al. (2025). Science, 370, 270-274.

[4] Farley, K. A. et al. (2022). Science, 377, 1321-1327.

[5] Hurowitz, J. A. et al. (2017). Science, 356, eaah6849.

[6] Lanza, N. L. et al. (2016). Geophys. Res. Lett., 43, 7398-7407.

[7] Mitra, K. et al. (2022). J. Geophys. Res. Planets, 127, e2021JE007067.

[8] Bryce, C. et al. (2018). Front. Microbiol., 9, 513.