New experiments reveal jarosite’s selective bromine capture under Mars-like conditions, offering a fresh lens to decode the Red Planet’s aqueous past and halogen cycling.
Jarosite, a sulfate mineral widely detected across the Martian surface, has long been regarded as a key indicator of past aqueous activity. Its occurrence reflects oxidizing, acidic waters that once interacted with Martian rocks, yet the detailed chemistry of those ancient fluids, particularly their halogen inventory, has remained difficult to reconstruct. Halogens such as bromine and chlorine are sensitive tracers of fluid evolution and environmental conditions, but the mechanisms by which they become incorporated into jarosite under Mars-relevant conditions have been poorly constrained.
In a new experimental study published in Planet (2025, 1(1)), researchers from the Institute of Geology and Geophysics, Chinese Academy of Sciences, and Chengdu University of Technology systematically investigated halogen partitioning behavior and structural incorporation mechanisms in potassium- and sodium-endmember jarosites. Their work, “Halogen partitioning and structural incorporation in K- and Na-jarosite: Experimental insights under Mars-relevant conditions,” provides the first comprehensive experimental evidence showing how bromine is preferentially captured by jarosite, particularly under cold, near-surface conditions analogous to those on ancient Mars.
To simulate plausible Martian aqueous environments, the team synthesized jarosite through two pathways: low-temperature Fe²⁺ oxidation at 25°C, and hydrothermal Fe³⁺ hydrolysis at 140°C. Integrated chemical, crystallographic, and spectroscopic analyses, including X-ray diffraction, Raman spectroscopy, and X-ray fluorescence, were used to quantify halogen uptake and identify substitution sites within the jarosite structure.
The results reveal a clear and robust trend: bromine is strongly favored over chlorine for incorporation into jarosite, with solid–liquid partition ratios for Br⁻ reaching values as high as 18 in potassium jarosite formed at 25°C. By contrast, sodium jarosite incorporated only trace halogens under all conditions, remaining persistently bromine- and chlorine-poor. This pronounced selectivity is governed not only by the halogen species but also by the identity of the alkali cation at the jarosite A-site, with K⁺-bearing jarosite exhibiting a substantially higher capacity for halogen substitution than its Na⁺ analog.
Mechanistically, combined evidence, from lattice parameter contraction, attenuation of Raman O–H stretching bands, and stoichiometric constraints, indicates that bromine and chlorine primarily substitute for structural hydroxyl groups rather than occupying interlayer positions. This substitution produces measurable lattice contraction, most strongly expressed in low-temperature K-jarosite enriched in bromine. The study further demonstrates that formation temperature exerts a major control on halogen incorporation: low temperatures promote bromine uptake through slower crystallization and defect-mediated trapping, whereas higher-temperature hydrothermal conditions yield more crystalline but halogen-poor jarosite. These findings complement prior observations of bromine enrichment in Martian-relevant evaporitic double salts, reinforcing the view that jarosite can act as a selective bromine sink in cold, acidic brines similar to those inferred for early Mars.
The implications of this work extend broadly across planetary science and future Mars exploration. By establishing a mechanistic link between jarosite composition, formation temperature, and halogen capture, the study provides a new geochemical framework for interpreting sulfate mineral assemblages on Mars. Notably, bromine-enriched jarosite, particularly potassium-dominated varieties, may indicate formation in low-temperature, chemically evolved brines, conditions conducive to more persistent aqueous activity on ancient Mars. The results also highlight jarosite’s dual role on planetary surfaces: not only as a paleoenvironmental archive, but also as an active participant in halogen cycling.
These insights will be especially valuable for upcoming Mars Sample Return missions, where high-resolution analyses of sulfate minerals could reveal the chemistry, timing, and evolution of Martian waters with unprecedented clarity.
Published in peer-review journal Planet, this work advances our understanding of water-rock interactions under Mars-like conditions and underscores the diagnostic potential of halogen signatures in sulfate minerals. By bridging laboratory synthesis with planetary observations, the study strengthens global efforts to reconstruct the history of water, habitability, and chemical evolution on the Red Planet.

DOI：10.15302/planet.2025.25003