Ancient Martian brines left bromine rich fingerprints in jarosite minerals

by Riko Seibo
Tokyo, Japan (SPX) Dec 14, 2025

Jarosite, a sulfate mineral widely detected on Mars, records interaction between acidic, oxidizing waters and the planet's surface rocks, but the halogen chemistry of those fluids has been difficult to reconstruct. New experimental work examines how bromine and chlorine enter jarosite under Mars-like conditions, using them as tracers of fluid evolution and environmental conditions.

Researchers from the Institute of Geology and Geophysics, Chinese Academy of Sciences, and Chengdu University of Technology synthesized potassium-and sodium-jarosite to study how halogens partition into the mineral structure. Their study, published in the journal Planet, provides experimental evidence that jarosite preferentially captures bromine, especially under cold near-surface conditions proposed for early Mars.

To simulate plausible Martian environments, the team produced jarosite via two routes: low-temperature Fe2+ oxidation at 25 C and hydrothermal Fe3+ hydrolysis at 140 C. They then combined X-ray diffraction, Raman spectroscopy, and X-ray fluorescence to quantify halogen uptake and determine where bromine and chlorine substitute within the jarosite crystal lattice.

The experiments show a consistent trend in which bromide is favored over chloride during incorporation into jarosite. In potassium jarosite formed at 25 C, solid-liquid partition ratios for bromide reached values up to 18, while sodium jarosite remained halogen-poor under all tested conditions. The identity of the alkali cation at the A-site controls this behavior, with K-bearing jarosite accommodating far more halogen substitution than its Na analog.

Multiple lines of evidence, including contraction of lattice parameters, changes in Raman O-H stretching bands, and stoichiometric balances, indicate that bromine and chlorine mainly replace structural hydroxyl groups rather than occupying interlayer sites. This substitution leads to measurable lattice contraction, most pronounced in bromine-enriched potassium jarosite formed at low temperature. Formation temperature exerts a major control, with low temperatures enhancing bromine uptake through slower crystallization and defect-mediated trapping, while higher-temperature hydrothermal conditions yield more crystalline but halogen-poor jarosite.

These findings align with previous work on bromine-enriched evaporitic double salts produced under Martian-relevant conditions, indicating that jarosite functions as a selective bromine sink in cold, acidic brines similar to those inferred for early Mars. The study therefore links jarosite composition, formation temperature, and halogen capture in a single geochemical framework.

For planetary science, bromine-rich jarosite, especially potassium-dominated varieties, emerges as a potential indicator of low-temperature, chemically evolved brines and more persistent aqueous activity on ancient Mars. Jarosite thus serves both as a record of past environments and as an active phase in halogen cycling on planetary surfaces.

These results are expected to guide future Mars Sample Return analyses, where detailed study of sulfate minerals could clarify the chemistry, timing, and evolution of Martian waters. Laboratory synthesis tied to orbital and in situ observations strengthens efforts to reconstruct the history of water, habitability, and chemical evolution on the Red Planet.

Research Report:Halogen partitioning and structural incorporation in K-and Na-jarosite: Experimental insights under Mars-relevant conditions