Abstract Jarosite-group minerals are ubiquitously observed in Martian sulfate-bearing deposits and serve as diagnostic proxies for past acidic aqueous environments. Proposed formation mechanisms include evaporation of acidic saline waters, magmatic vapor alteration, subglacial weathering, and microbially mediated Fe2+ oxidation. Among these, microbial formation has drawn astrobiological interest. To assess the effects of Martian atmospheric composition, we constructed a laboratory atmospheric simulation system and examined the Fe2+ oxidation and mineralization capacity of the acidophilic iron-oxidizing bacterium Acidithiobacillus ferrooxidans under O2-depleted, CO2-rich conditions. Biomineralization experiments were performed across four CO2/O2 gas treatments, with initial dissolved O2 (DO) saturation values of 14.6%, 8.0%, 3.7%, and 1.4%, respectively. Fe2+ oxidation and jarosite precipitation were markedly delayed in the two lower-DO-saturation treatments, whereas biomineralization occurred in all inoculated treatments. XRD, SEM, EDS, and FTIR analyses revealed time-dependent mineralogical and compositional changes in the jarosite-group precipitates, with NH4-related features becoming more apparent during later mineralization. Collectively, these findings demonstrate that microbial jarosite formation is viable under CO2-rich, O2-limited aqueous conditions, thereby lending empirical support to the potential for biological contributions to Martian jarosite deposits. The observed mineralogical and compositional features are not uniquely diagnostic of a biological origin, making the products difficult to distinguish from abiotic counterparts and suggesting that reliable identification of biosignatures in Martian jarosite will require converging evidence from multiple independent lines of investigation.
Abstract Perchlorate salts are widespread on Mars and are commonly regarded as a potential barrier to organic preservation because ultraviolet (UV) irradiation may activate perchlorate photochemistry and generate destructive oxychlorine species. Despite this, multiple rover missions have identified indigenous organic compounds in Martian sediments, indicating that preservation pathways remain incompletely understood. Previous work showed that Fe(III)-rich minerals such as goethite can promote oxidative coupling of aromatic organic compounds under Mars-analog conditions, producing higher molecular weight products that should be more resistant to degradation. Here, we examine whether UV-irradiated perchlorate suppresses mineral-mediated oxidative polymerization of 2-naphthol adsorbed onto goethite under those same conditions. Across organic loadings of 0.5 to 10 wt %, oxidative coupling proceeded in the presence of magnesium perchlorate, yielding dimers and higher-order oligomers. Yields were not measurably suppressed on experimentally relevant hour time scales. Our results provide experimental constraints on the competition between perchlorate photochemistry and mineral surface oxidative coupling under dry, Mars-analog UV conditions. This may represent a potential preservation mechanism in perchlorate-bearing, iron-rich Martian sediments.
Abstract Perchlorates have been detected by multiple Mars exploration missions, and their possible formation mechanisms have been widely investigated as well. However, their stability associated with Martian dust activity remains poorly understood. On present-day Mars, frequent dust storm activity may trigger electrostatic discharge (ESD) processes, thereby driving active chlorine chemistry at the Martian surface and atmosphere. ESD may be an important process influencing the stability of (per)chlorates on present-day Mars. Here, we exposed NaClO3 and NaClO4 powders to glow-discharge ESD under simulated present-day Martian conditions. Infrared spectra indicate that ESD induces chemical transformation between chlorate (ClO3–) and perchlorate (ClO4–). In situ gas measurements further show the release of reactive chlorine-bearing gaseous species, including ClO2 and Cl2. Ion chromatography of gas-absorption solutions detected dissolved Cl–, further supporting chlorine mobilization from solid (per)chlorate during the ESD process. These results suggest that ESD may promote redox interconversion among chlorine species with different oxidation states. This process deepens our understanding of an active chlorine recycling pathway operating on present-day Mars.
Abstract Photocatalytic Mn2+(aq) oxidation on Fe oxide surfaces involves coupled interfacial reactions and photochemical processes and contributes to Fe–Mn redox cycling in Earth’s surface and near-surface environments. Al substitution is ubiquitous in natural Fe oxides, yet its effects on photochemical Mn2+(aq) oxidation and subsequent Mn(III/IV) oxide formation remain poorly understood. Here, we combined photochemical experiments with synchrotron-based characterization of Al-substituted goethite and hematite (0–5 mol % Al) to investigate Fe oxide-catalyzed Mn2+(aq) oxidation. Mn2+(aq) oxidation kinetics revealed contrasting effects of Al substitution between the two Fe oxide phases, with increasing Al substitution progressively suppressing Mn2+(aq) oxidation on goethite while enhancing it on hematite. At these relatively low Al substitution levels, changes in structure and band gap were minor, whereas surface charge exhibited contrasting trends between the two Fe oxide phases. These results suggest that phase-dependent mineral-ion interactions at Fe oxide surfaces may play a more important role than changes in bulk structure or intrinsic optical properties in regulating photocatalytic Mn2+(aq) oxidation at low impurity levels. Al substitution also suppressed Mn(III/IV) oxide formation on goethite, whereas Mn(III/IV) oxide products persisted on hematite but exhibited markedly altered diffraction features. Together, these observations demonstrate that Al substitution regulates both Mn2+(aq) oxidation and Mn(III/IV) oxide formation in a Fe oxide phase-dependent manner. Using Al as a representative impurity, this study provides a framework for understanding how common impurities in natural Fe oxides regulate photochemical Mn oxidation and, in turn, influence coupled Fe–Mn geochemical cycling.
Lignin-derived compounds (LDCs) are major contributors to soil organic carbon and play a critical role in the global biogeochemical cycle of carbon. However, the reliable identification of LDCs in complex environmental media remains a major analytical challenge. In addition, owing to lignin's heterogeneous, cross-linked polymeric structure, many LDCs are poorly soluble and exhibit low ionization efficiency in mass spectrometry, substantially hindering their identification. To overcome these analytical hurdles, we developed an advanced analytical pipeline integrating click reaction with high-performance liquid chromatography-high-resolution tandem mass spectrometry (HPLC-HRMS/MS) and nontargeted metabolomic analysis, enabling the systematic screening and putative identification of LDCs, including species that are otherwise poorly ionizable or insoluble. Our study screened LDCs by reacting extractions of azide-tagged lignin polymers with dibenzocyclooctyne-polyethylene glycol acid (DBCO-PEG1). We employed two complementary analytical pathways: (i) targeted mass difference screening of azide-tagging and DBCO-PEG1 adduct, and (ii) diagnostic MS/MS fingerprint analysis based on characteristic fragment ions of DBCO-PEG1. Using this strategy, we screened 17 high-confidence triplet candidates of LDCs and additional putative candidates identified via MS/MS fingerprinting, spanning a broad chemical space. By integrating click reaction with HPLC-HRMS/MS and nontargeted metabolomic analysis, this study established a framework for tagging and screening analytically inaccessible ("dark") organic compounds. Further structural characterization of candidate compounds and the application of this approach to incubation studies provides a pathway toward improved molecular identification of LDCs crucial to soil organic carbon persistence.
