Spontaneous Mineral-driven electrochemical synthesis of organic matter under Martian conditions

Spontaneous Mineral-driven electrochemical synthesis of organic matter under Martian conditions | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Spontaneous Mineral-driven electrochemical synthesis of organic matter under Martian conditions Chenying Wang, Andrew Steele, Liane Benning, Sangyeon Lee, Richard Wirth, and 10 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8911322/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted You are reading this latest preprint version Abstract Organic molecules preserved in Martian meteorites indicate that reduced carbon can form abiotically. Yet, the processes capable of generating and sustaining such material without biological or external energy inputs remain unresolved. Here, we demonstrate that a purely mineral-based system can spontaneously convert CO2 into complex organic matter through self-sustained electrochemical coupling. By embedding TiO2 and Fe3O4 nanoparticles within an amorphous SiO2 matrix, we construct a mineral assemblage that integrates an anodic phase, a cathodic phase and an electrolyte into a single electrochemical circuit. When immersed in CO2-bearing brine, this mineral junction drives directional electron transfer without applied voltage, illumination or heating. The system reduces CO2 to small oxygenated intermediates (e.g., formate, acetate) and subsequently converts them into aromatic-rich macromolecular carbon that accumulates at Fe-rich mineral interfaces. Nanoscale spectroscopic analyses show that these products closely resemble organic matter preserved in Martian meteorites. Unlike previous abiotic carbon synthesis studies that rely on isolated catalysts or external energy sources, this mineral assemblage sustains spontaneous, dark CO2 reduction driven solely by intrinsic mineral redox disequilibria. These findings identify mineral-driven geo-electrochemistry as a previously unrecognized pathway for abiotic carbon fixation and imply that similar processes could have operated on early Mars, early Earth and other water-bearing planetary bodies, while offering a conceptual framework for low-energy mineral-enabled carbon conversion. Physical sciences/Energy science and technology/Carbon capture and storage Earth and environmental sciences/Planetary science/Geochemistry Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction Organic compounds indigenous to, and preserved within, Martian meteorites provide direct evidence that reduced carbon can form abiotically during aqueous alteration of planetary crusts. 1-6 These organics are consistently associated with Ti-Fe oxides, spinel-group minerals and amorphous silica formed during aqueous alteration 2,4,5 , suggesting a mineralogical control on their synthesis. Proposed mechanisms include serpentinization 6 , carbonation 6 and electrochemical reduction 5 . Electrochemical reduction of CO 2 was inferred from mineral and organic context and texture. Fluid–rock interaction on Mars occurred predominantly within fractures and alteration veins 7,8 , where redox contrasts between coexisting minerals would have been unavoidable 9 . Such environments are inherently electrochemical: minerals with different work functions and redox couples establish local potential gradients when immersed in electrolyte solutions 5 . Nanoscale intergrowths of Fe-rich and Ti-rich oxides embedded in silicate matrices, widely observed in Martian meteorites and alteration products 5 , therefore resemble natural galvanic junctions capable of sustaining directional electron transfer during corrosion and weathering 10 . These mineral interfaces represent a plausible setting for abiotic carbon fixation, but their capacity to drive spontaneous carbon reduction has not been directly demonstrated. Most experimental studies of abiotic carbon synthesis focus either on single “catalysts” usually in hydrothermal settings 11,12 . In contrast, mineral-driven electrochemical coupling offers a fundamentally different operating mode in which intrinsic potential gradients between dissimilar minerals i.e. galvanic processes, rather than temperature or light, provide the driving force for redox reactions 13-15 . While electrochemical reduction of CO 2 is a burgeoning research field for remediation of CO 2 from Earth’s atmosphere, the laboratory systems require external energy input, usually in the form of applied “over-potential (OP)” 16 . Therefore, bringing together observations of electrochemical reduction producing organics on Mars with the question of whether such mineral junctions can sustain carbon reduction under geochemically realistic conditions remains open. Here, we construct a mineral assemblage that reproduces the textural and compositional relationships observed in Martian meteorites as discrete electrodes. By embedding TiO 2 and Fe 3 O 4 nanoparticles within an amorphous SiO 2 matrix, we recreate Ti–Fe spinel–silicate interfaces formed during aqueous alteration of basaltic crust. 17 Experimental observation using both electrochemical, organic chemical, and nano-analytical techniques were then used to establish whether spontaneous galvanic coupling in aqueous brines, and the produced intrinsic electrochemical gradients could drive CO 2 reduction and organic synthesis without any external energy input. Results Spontaneous CO 2 reduction in a coupled mineral galvanic system To evaluate whether mineral junctions can spontaneously drive CO 2 under Mars-like conditions, we constructed a galvanic system composed of TiO 2 ·aSiO 2 and Fe 3 O 4 ·aSiO 2 electrodes immersed in CO 2 -bearing brine. (Extended Data, Fig. 1) Analysis of Martian meteorites revealed the presence of amorphous silica throughout the organic-rich area, and for that reason, we designed the electrodes to encapsulate the oxide phases within a silica matrix 5,6 . Unlike previous abiotic CO 2 reduction studies that employ single catalysts activated by heat, light, or applied voltage, this configuration integrates distinct anodic and cathodic mineral phases. Details of the experimental methods are contained in the supplementary materials. Post experimental analysis of the liquid phase by nuclear magnetic resonance spectroscopy (NMR, Fig. 1) and gas chromatography–mass spectrometry (GC-MS, Fig. 2) show that the coupled TiO 2 ⋅aSiO 2 –Fe 3 O 4 ⋅aSiO 2 contains a suite of soluble and mineral-associated organic compounds not present in the controls. Dominant products ( 1 H-NMR, Fig. 1A) include formate, acetate, acetone and methanol, with product distributions varying systematically with pH. Acidic and neutral conditions (Fig. 1A, data II, III. Extended Data Table. 1) favor small-oxygenated molecules, whereas alkaline conditions (Fig. 1A, data IV) promote the appearance of aromatic species. The emergence of aromatic resonances at 7.4–7.5 ppm in 1 H-NMR data (Fig. 1A curve IV) at high pH is accompanied by increased carbonate and carboxyl signals 18-21 . GC–MS (Fig. 2A) further detects formic acid, acetic acid and benzoic acid in reacted electrolytes, together with aromatic fragments associated with mineral-bound carbon. These compounds are absent from unreacted controls and from experiments conducted without CO 2 . Consistent with GC–MS, 1 H-NMR shows that after CO 2 RR the coupled TiO 2 ·aSiO 2 –Fe 3 O 4 ·aSiO 2 system reproducibly yields acetate, acetone and methanol, whereas the corresponding control solutions contain at most trace acetate and no detectable acetone or methanol (Fig. 1A; Extended Data Table 1). Isotopic labelling with 13 CO 2 (Fig. 1B, 2B, Extended Data Fig. 5) demonstrates that carbon incorporated into formate, acetate and aromatic products derives from the supplied inorganic carbon source: 13 C enrichment is observed in formate and aromatic carbon by 13 C-NMR, and in both formate and acetate by GC–MS. The absence of resolved 13 C-acetate resonances in solution-state 13 C-NMR likely reflects differences in analytical sensitivity and/or spectral overlap under the present acquisition conditions. By contrast, acetone and methanol show no detectable 13 C incorporation by either 13 C-NMR or GC–MS, indicating that their carbon source cannot be unambiguously assigned to the introduced 13 CO 2 in these experiments. Together, these data establish robust CO 2 -to-organics conversion for formate, acetate and aromatic carbon, while constraining acetone and methanol to products whose isotopic provenance remains unresolved. Localization and structure of mineral-associated carbon Building on Prior observation of aromatic macromolecular carbon (MMC) associated with magnetite and silica in Martian Meteorites 2,4,5,6 , we investigated Fe 3 O 4 ⋅aSiO 2 electrodes post-CO 2 reduction using similar techniques used to study the meteorites. Transmission electron microscopy (TEM, Figs. 3A-G, Extended Data Figs. 2-6) and synchrotron-based scanning transmission X-ray microscopy (STXM, Fig.3H) show that organic carbon accumulates preferentially at Fe 3 O 4 –SiO 2 interfaces after reaction (Fig. 3D, Extended Data Figs. 4,6). Non-reacted control samples display sharp magnetite–silica boundaries and lack localized carbon enrichment (Figs. 3B, H, Extended Data Fig.2). In contrast, reacted samples exhibit corrosion textures (Fig 3E and F) and interfacial carbon coatings that extend along oxide–silica boundaries (Fig 3C, D), indicating in situ formation and retention of carbon at mineral junctions (Figs. 3D, H, Extended Data Figs. 2,4,6). High-resolution imaging and electron energy-loss spectroscopy (EELS, Fig 3C and D, Extended Data Fig. 6) identify amorphous carbon layers associated with Fe-rich domains. This spatial confinement demonstrates that carbon is generated and stabilized at magnetite–silica interfaces rather than being uniformly distributed throughout the electrolyte or mineral matrix. The extent and morphology of interfacial carbon vary systematically with pH. At low pH (Extended Data Figs. 3, 6), magnetite remains embedded within amorphous silica and develops moderate porosity, with aromatic carbon retained at the oxide–silica boundary. At neutral pH (Extended Data Fig. 4), increased porosity and carbonate-rich amorphous phases emerge, consistent with partial dissolution and interfacial reorganization. Under alkaline conditions (Extended Data Fig. 5), extensive silica dissolution and magnetite corrosion lead to structural breakdown and partial release of organic products into solution. C K-edge electron energy loss spectroscopy further distinguishes reacted samples from controls (Fig. 3H, Extended Data Table. 2) 29,30 . Control electrodes lack detectable aromatic signatures, whereas reacted electrodes display pronounced spectral features corresponding to aromatic, carboxyl and carbonate functional groups. These spectroscopic fingerprints 29,30 (Fig. 3H) closely resemble those reported for organic matter preserved in Martian meteorites (green trace Fig 3H), indicating that the experimentally generated carbon shares key chemical characteristics with Martian meteoritic organics. Together, these observations establish that mineral interfaces act as both reactive sites and stabilizing hosts for newly formed organic matter. The preferential localization of aromatic-rich carbon at Fe 3 O 4 –aSiO 2 boundaries mirrors the association between amorphous silica and aromatic carbon observed in altered Martian meteorites. Electronic properties and band alignment of TiO 2 ⋅aSiO 2 –Fe 3 O 4 ⋅aSiO 2 pairs The liquid and solid phase analyses confirm that the TiO 2 ⋅aSiO 2 -Fe 3 O 4 ⋅aSiO 2 acts as a galvanic pair system that can spontaneously reduce CO 2 to formate, acetate and aromatics across a wide pH range. To elucidate the origin of this activity and to confirm the reaction’s validity and the intrinsic driving force behind spontaneous CO 2 RR, we examined the electronic properties and band alignment of the two electrodes using Mott-Schottky and Cyclic Voltammetry analyses. Mott–Schottky analysis (Fig. 4A-C) shows that TiO 2 ⋅aSiO 2 behaves as an n-type semiconductor, whereas Fe 3 O 4 ⋅aSiO 2 exhibits p-type character across the investigated pH range. Linear fits yield flat band potentials that define the relative positions of the semiconductor band edges and their systematic pH-dependent shifts. These data establish the energetic landscape of the Ti-Fe-silicate pair and constrain the alignment of its electronic states relative to solution redox couples and therefore confirm whether the two minerals could establish a natural potential difference when coupled. In simple terms, this analysis tells us whether the two minerals can naturally behave like a tiny battery when placed in water. Independent open circuit potential measurements demonstrate that Fe 3 O 4 ⋅aSiO 2 exhibits a lower equilibrium potential than TiO 2 ⋅aSiO 2 under identical electrolyte conditions, indicating that Fe 3 O 4 ⋅aSiO 2 acts as the anodic phase and TiO 2 ⋅aSiO 2 as the cathodic phase when the two electrodes are electrically coupled. Upon immersion in electrolyte, the p-type Fe 3 O 4 ⋅aSiO 2 and n-type TiO 2 ⋅aSiO 2 therefore establish a mineral junction that supports spontaneous galvanic behavior, with Fe 2+ oxidation at the Fe-rich phase supplying electrons that are transferred to the Ti-rich phase, where they drive CO 2 reduction. 31 The magnitude of the built-in potential and the resulting charge-transfer characteristics vary with pH. At low pH (Fig. 4A), large band offsets generate a strong internal electric field and pronounced galvanic coupling 31,33 , favoring efficient electron flow between the two mineral phases. At neutral pH (Fig. 4B), reduced band separation enhances interfacial charge-transfer kinetics 34 , consistent with changes in the organics produced in the liquid-phase analyses. Under alkaline conditions (Fig. 4C), the resulting galvanic potential, together with increased interfacial conductivity, enables multi-electron transfer and yields the highest activity 35 , accompanied by enhanced formation of more structurally complex products. Cyclic voltammetry recorded under galvanic coupling conditions (Fig. 4D-F) shows enhanced cathodic currents in CO 2 -saturated brine relative to N 2 -saturated electrolyte across the investigated pH range. The negative-going onset potentials in cathodic current under CO 2 indicate that the coupled mineral system generates sufficient intrinsic electrochemical bias to drive carbon reduction without external bias or illumination. Notably, these electrochemical responses are not reproduced by either mineral phase alone under identical conditions (Extended Data Fig. 7). TiO 2 ⋅aSiO 2 in isolation produces only low yields of small-oxygenated products, whereas Fe 3 O 4 ⋅aSiO 2 exhibits limited CO 2 reduction activity but efficiently transforms CO 2 reduction intermediates into more reduced and structurally complex organics. These observations indicate that the full reaction sequence requires both phases operating in an electrically coupled configuration. Coupled mineral behavior enables spontaneous carbon synthesis The complementary electronic properties of the two minerals define the direction of electron flow and the sites of redox reactions. The conduction band of TiO 2 ⋅aSiO 2 lies at a more negative potential than the CO 2 redox couple, enabling thermodynamically favorable CO 2 reduction on TiO 2 ⋅aSiO 2 (Fig. 5). 36,37 Oxidation of Fe 2+ at the Fe 3 O 4 ⋅aSiO 2 phase supplies electrons that are transferred to the TiO 2 ⋅aSiO 2 phase, where they drive the reduction of dissolved CO 2 . The resulting products accumulate at mineral–silica interfaces and in solution, forming both small-oxygenated molecules and aromatic-rich carbon. This coupled electrochemical–mineralogical behavior provides a mechanistic link between mineral galvanic processes and abiotic organic synthesis under planetary conditions. Control experiments using single-phase electrodes (supplementary doc 1.2) confirm that neither mineral alone can reproduce the full product suite. TiO 2 ⋅aSiO 2 reduces CO 2 only to small-oxygenated products (Extended Data Fig. 7A), whereas Fe 3 O 4 ⋅SiO 2 shows limited CO 2 reduction activity but efficiently converts CO 2 -reduction intermediates into more reduced and structurally complex organics, including aromatic species (Extended Data Fig. 7B, C). Together, the coupled mineral system functions as an integrated electrochemical reactor in which primary reduction and secondary upgrading occur within the same mineralogical framework. Discussion This study establishes mineral-driven electrochemical coupling between two different minerals or even different phases of the same mineral as a previously unrecognized pathway for abiotic carbon fixation. This identifies galvanic mineral assemblages as natural electrochemical reactors that drive abiotic carbon fixation. By demonstrating that purely mineral assemblages can spontaneously convert CO 2 into complex organic matter without hydrothermal conditions or applied external energy input, we provide a mechanistic link between planetary mineralogy, electrochemistry and organic synthesis. In contrast to previous abiotic CO 2 reduction studies that rely on single catalysts, 41 our results (Fig. 5) show that a coupled mineral assemblage can function as a complete electrochemical circuit in which electron flow is generated internally by mineral redox disequilibria and directly coupled to CO 2 reduction. This identifies galvanic mineral assemblages as natural electrochemical reactors that drive abiotic carbon fixation. The coupled TiO 2 ⋅aSiO 2 –Fe 3 O 4 ⋅aSiO 2 system exhibits clear functional differentiation between its two mineral components. CO 2 reduction occurs predominantly at the TiO 2 ⋅aSiO 2 surface, while Fe 3 O 4 ⋅aSiO 2 functions as the electron source through Fe 2+ oxidation, generating small-oxygenated intermediates such as formate and acetate, together with additional oxygenated products including acetone. Whereas Fe 3 O 4 ⋅aSiO 2 promotes secondary surface reactions that convert these intermediates into more reduced and structurally complex aromatic carbon. Single-electrode control experiments show that neither mineral alone can reproduce the full product suite, establishing that organic synthesis emerges from their coupled behavior rather than from the activity of either phase in isolation as in typically studied abiotic synthesis pathways. 42 Such division of labor is characteristic of electrochemical systems but has rarely been demonstrated in purely mineral assemblages and demonstrates that geo-electrochemical reactions supply energy to reactions in the environment. The spontaneous operation of this system is explained by the complementary electronic properties of the two minerals. TiO 2 ⋅aSiO 2 consistently exhibits n-type character with a conduction band edge sufficiently negative to drive carbon reduction (Fig. 4,5). Their junction therefore establishes a built-in electric field that directs electrons from the Fe-rich phase toward the Ti-rich phase, generating potential differences that required for CO 2 reduction. This intrinsic driving force enables the coupled mineral system to operate spontaneously under ambient conditions, in contrast to single-phase electrodes that require large applied OPs to achieve comparable reactivity. 43 The architecture of the TiO 2 ·aSiO 2 –Fe 3 O 4 ·aSiO 2 system mirrors key features of Martian alteration environments. Martian meteorites and rover observations document widespread co-occurrence of Fe–Ti oxides, spinel-group minerals and amorphous silica formed during aqueous alteration of basaltic crust. The exsolution lamellae seen in Martian spinels are capable of operating as nano-scale electrodes in a galvanic system, showing that when immersed in CO 2 -bearing brines, such assemblages would naturally establish mineral junctions analogous to the anodic–cathodic pairing constructed here. Our design therefore represents a mineralogical analogue of fracture- and vein-hosted alteration textures, as well as areas of zonation within single minerals on Mars, in which redox disequilibria between coexisting phases could sustain electrochemical reactions during fluid–rock interaction. Nanoscale imaging and spectroscopy (Fig. 3) show that organic carbon is spatially confined to magnetite–silica interfaces, linking carbon synthesis directly to mineral junctions. The emergence of aromatic carbon exclusively in reacted samples and its absence in controls demonstrate that these compounds are generated in situ rather than inherited from contamination. The close correspondence between experimentally produced aromatics and organic matter preserved in Martian meteorites suggests that similar mineral-driven electrochemical processes could have operated during aqueous alteration of Martian basalts. The pH-dependent evolution of mineral textures (Fig. 1-3) further constrains the environmental conditions under which such processes can operate 44,45 , with alkaline conditions favoring aromatic formation and indicating progressive carbon–carbon coupling and reduction. Moderate porosity and interfacial carbon retention at acidic and neutral pH favor the stabilization of organic products at oxide–silica boundaries 46,47 , whereas strong alkalinity promotes mineral dissolution and the release of products into solution. These observations are consistent with scenarios in which carbon fixation occurs locally at mineral interfaces, but the resulting labile organic carbon can subsequently be redistributed by evolving fluid chemistry. The association of more refractory aromatic carbon with alteration textures in Martian meteorites is therefore consistent with episodic mineral-driven synthesis coupled to changing aqueous conditions. More broadly, mineral-driven electrochemical synthesis provides a unifying framework linking corrosion, redox gradients, and organic carbon formation within planetary crusts. Similar Ti- and Fe-bearing assemblages occur in altered terrestrial basalts and are expected in the rocky interiors of icy ocean worlds 48,49 , implying that this mechanism is not restricted to Mars. Mineral junctions capable of sustaining galvanic coupling may thus represent a general geochemical pathway for abiotic carbon fixation wherever igneous minerals interact with CO₂-bearing brines. Beyond its planetary implications, this mechanism also redefines the conceptual landscape of electrochemical carbon reduction on Earth. Modern CO₂ reduction strategies are dominated by externally powered systems optimized for efficiency and selectivity 50 . In contrast, mineral-driven electrochemistry operates through intrinsic potential gradients generated by natural materials, offering a model for low-energy carbon conversion that does not require engineered catalysts or applied bias. Although such systems are not optimized for industrial throughput, they demonstrate that carbon fixation can proceed under minimal energetic constraints when appropriate mineral assemblages are present. Together, these results establish mineral-driven electrochemical coupling as a geochemically realistic and previously underappreciated mode of abiotic carbon synthesis. By directly linking semiconductor properties, nanoscale mineral textures, and organic product formation, this study provides a mechanistic explanation for the co-occurrence of Ti–Fe spinels, amorphous silica, and aromatic carbon in Martian meteorites and suggests that abiotic organic synthesis is an inherent consequence of fluid–rock interactions in redox-stratified environments on early Earth, Mars, and any other water-bearing planetary bodies. Materials and Methods Thin Film Electrode Synthesis Magnetite-amorphous-silica thin‐film electrodes (Fe 3 O 4 ·aSiO 2 ) were synthesized by dispersing 150 mg of nano-magnetite powder (Fisher Scientific) in 20 mL of deionized water containing 500 µL of sodium silicate solution (“water glass”, Sigma-Aldrich). The suspension was sonicated for 15 min to achieve a uniform dispersion and then drop-cast onto fluorine-doped tin oxide (FTO) glass slides (conductive substrates, Sigma-Aldrich). Coated slides were dried and cured at 75 °C for 48 hrs to promote adhesion and formation of a stable amorphous silicate matrix. The resulting Fe 3 O 4 ·aSiO 2 films consist of magnetite nanoparticles embedded within an a-SiO 2 framework and serve as the Fe-rich “spinel” endmember in the galvanic system. Titanium-amorphous-silica thin-film electrodes (TiO 2 ·aSiO 2 ) were prepared by dispersing 150 mg of TiO 2 nano powder (Sigma-Aldrich) in 20 mL of deionized water with 500 µL of sodium silicate solution, followed by 15 min sonication. The suspension was drop-cast onto FTO slides and treated identically (75 °C, 48 hrs) to yield TiO 2 nanoparticles encapsulated in an amorphous silica matrix. These electrodes represent the Ti-rich spinel domains. Analytical Techniques Electrochemical reactions were carried out using the coupled TiO 2 ⋅aSiO 2 –Fe 3 O 4 ⋅aSiO 2 mineral system under CO 2 -saturated aqueous conditions (Fig.S1). Product analysis and electrochemical measurements were performed on samples collected after reaction. All experiments were repeated independently four times with similar results (n = 4). Proton ( 1 H) and Carbon ( 13 C) - Nuclear Magnetic Resonance spectroscopy ( 1 H- and 13 C- NMR) : Liquid-phase CO 2 RR products were identified and quantified using 1 H- and 13 C-NMR. Spectra were acquired on a Bruker 600 MHz (14.1 T) AVIII NMR spectrometer under ambient conditions, using standard pulse sequences with water suppression, a 5 s relaxation delay, and no spinning. For each sample, 500 µL of electrolyte was mixed with 65 µL D 2 O (ACROS Organics) and 35 µL TMSP-d 4 (3-(trimethylsilyl) propionic acid-d 4 , Sigma-Aldrich) as an internal standard (58 µM for 1 H-NMR; 20 mM for 13 C-NMR). TMSP-d 4 was used for both chemical-shift referencing (0.00 ppm) and quantitative integration. (n=4) Headspace Solid-Phase Microextraction coupled with Gas Chromatography-Mass Spectrometry (HS-SPME-GC-MS): Volatile and semi-volatile organics were analyzed by headspace solid-phase microextraction coupled with gas chromatography-mass spectrometry (HS-SPME-GC-MS) on an Agilent NW7590 GC-MS instrument using helium as the carrier gas and a 44 mins runtime. Electrolyte samples were acidified to pH ~1 with HCl, and 1 mL aliquots were transferred to 10 mL vials. AC-WR-95/PDMS fiber was exposed to the headspace at 80 °C for 1 h, then desorbed in the GC inlet at 260 °C for 10 min (n=4). Standard solutions of formic acid (7 mM), acetic acid (100 µM), phenol (100 µM), and saturated benzoic acid were run under identical conditions to determine retention times and identify corresponding peaks in CO 2 RR samples (Fig.S7). Electrochemical characterization measurements (Gamry Interface 1000E Potentiostat/Galvanostat/ZRA Instrument): Electrochemical characterization was performed primarily in a three-electrode configuration using either Fe 3 O 4 ·aSiO 2 or TiO 2 ·aSiO 2 as the working electrode (geometric area 1.0 cm²), a saturated calomel reference electrode (SCE), and a Platinum counter electrode. Cyclic voltammetry (CV) was carried out from 0.0 to -1.2 V vs SCE at a scan rate of 50 mV s -1 with 1 mV step size (5 consecutive cycles, 2 s equilibration at the initial potential). Measurements were performed under N 2 and CO 2 at pH 2, 7, and 12 to determine CO 2 RR onset potentials, open-circuit potentials (OCP), and current density enhancement under CO 2 relative to N 2 (n=4). Mott–Schottky (MS) was conducted with a 0.05 V potential step, 10 mV AC amplitude, and 3000 Hz (Fe 3 O 4 ·aSiO 2 ) /100 Hz (TiO 2 ·aSiO 2 ) frequency, using an electrode area of 1 cm 2 . For Fe 3 O 4 ·aSiO 2 /TiO 2 ·aSiO 2 , Mott Schottky plots of C -2 vs potential were used to extract flat-band potentials (U FB ) and carrier densities. (n=4) All potentials are reported versus SCE unless otherwise stated. Ultra-Violent-Via Spectroscopy for Band Gap and Electron Affinity Analysis: Diffuse-reflectance UV-Vis spectra of Fe 3 O 4 ·aSiO 2 and TiO 2 ·aSiO 2 films were acquired on a PerkinElmer Lambda 950 UV-Vis-NIR spectrophotometer with a 150 mm integrating sphere over 200-900 nm. Tauc plot analysis (Fig.S11) of Kubelka-Munk-transformed spectra yielded direct and indirect band gaps of Fe 3 O 4 ·aSiO 2 and TiO 2 ·aSiO 2 (n=4). Transmission Electron Microscopy (TEM): Focused ion beam (FIB) sections of Fe 3 O 4 ·aSiO 2 electrodes were prepared and examined using a TECNAI F20 X-Twin TEM at 200 kV (Deutsches GeoForschungsZentrum, Potsdam), equipped with an EDAX EDS system, Gatan TRIDIEM imaging filter, and HAADF detector. Electron energy-loss spectroscopy (EELS) was performed in diffraction mode with 0.1 eV per pixel energy dispersion and 620 mm camera length to probe C K-edge features (e.g., ~285 and 290 eV). Spectra were analyzed using Digital Micrograph. Scanning Transmission X-ray Microscopy (STXM): STXM measurements at the C 1s and N 1s absorption edges were carried out at Diamond Light Source (beamline I08, UK) on ultrathin FIB-prepared sections identified by TEM. Image stacks were collected with ~50 nm spatial resolution and 10 ms dwell time per pixel. Data were processed using MANTiS and aXis2000 to generate C K-edge XANES spectra and chemical-speciation maps. Declarations Acknowledgements This work is supported by a NASA Astrobiology ICAR Program grant to K.L.R. and A.S., and by the Carnegie Institution of Washington. L.G.B., R.W. and A.S. acknowledge financial support from the Helmholtz Recruiting Initiative (I-044-16-01) awarded to L.G.B. J.P.H.P. acknowledges partial funding from the GFZ Discovery Fund (P-032-45-002). Beamtime at Diamond Light Source I08 was provided under proposals MG-36247 and MG-33333. Author contributions C.W. designed the experiments, synthesized materials, conducted electrochemistry, NMR, GC–MS and microscopy analyses, and wrote the manuscript. A.S. supervised the project, TEM, EELS, STXM measurements and contributed to interpretation. L.G.B., R.W. and A.Sch. performed TEM and EELS. B.K. and L.C.C.H. conducted STXM measurements. S.L. and W.L. assisted with sample measurements. M.J.H.P., K.L.R., A.P., G.C. and T.A.S. contributed to planetary interpretation. All authors discussed results and contributed to manuscript editing. Competing interests The authors declare no competing interests. References Udry, A., Howarth, G. H., Herd, C. D. K., Day, J. M. D., Lapen, T. J. & Filiberto, J. What Martian meteorites reveal about the interior and surface of Mars. J. Geophys. Res. 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Peaks corresponding to acetate, acetone, methanol, formate and aromatic proton\u003csup\u003e18-21\u003c/sup\u003e. (B) \u003csup\u003e13\u003c/sup\u003eC-NMR spectra highlighting carbon incorporation and chemical-shift evolution across pH values in \u003csup\u003e13\u003c/sup\u003eCO\u003cimg width=\"6\" height=\"15\" src=\"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAkAAAAXCAMAAADuk5XJAAAAAXNSR0IArs4c6QAAAD9QTFRFAAAAAAAAADpmADqQAGa2Oma2OpDbZgAAZjqQZpDbZrb/kDoAkGY6tmYAtmY6tv//2////7Zm/9u2//+2///bAfJVzgAAAAF0Uk5TAEDm2GYAAAAJcEhZcwAAFiUAABYlAUlSJPAAAAAZdEVYdFNvZnR3YXJlAE1pY3Jvc29mdCBPZmZpY2V/7TVxAAAAO0lEQVQYV2NgGExAiIeRkYkf6CIRbk4GIXYWmNt4YSxhdi6oGB8bjMGKzhBkFmAQ5hBgYBBmZwQCIA8AP7wBZuL1Cr4AAAAASUVORK5CYII=\"/\u003e\u0026nbsp;experiments; labelled formate and aromatic carbons appear at high pH.\u003csup\u003e22-25\u003c/sup\u003e (C) Quantified concentrations (\u003cimg width=\"8\" height=\"15\" src=\"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAwAAAAXCAMAAAAIul6NAAAAAXNSR0IArs4c6QAAAFFQTFRFAAAAAAAAAAA6AABmADpmADqQAGa2OgAAOgA6Ojo6Oma2OpDbZgAAZpCQZpDbZrb/kDoAkNv/tmYAtmY6tv//25A62////7Zm/9uQ//+2///bRdrkzAAAAAF0Uk5TAEDm2GYAAAAJcEhZcwAAFiUAABYlAUlSJPAAAAAZdEVYdFNvZnR3YXJlAE1pY3Jvc29mdCBPZmZpY2V/7TVxAAAAYklEQVQoU81OSRKAIAxL3arigqBS+f9DBaozPsHc0iwN8ANckwGibc+8JdAKXFNXdh3VBggn+TWUy2tw9Z6JcIoIfyKeSgSO1uhnNrLk/EDNFi2N6Y/0pUYRtEbxdCqxH+UGmbUETGLvRSoAAAAASUVORK5CYII=\"/\u003eM) of major liquid-phase organics determined from \u003csup\u003e1\u003c/sup\u003eH-NMR integration for TiO\u003csub\u003e2\u003c/sub\u003e⋅aSiO\u003csub\u003e2\u003c/sub\u003e–Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e⋅aSiO\u003csub\u003e2\u003c/sub\u003e galvanic CO\u003csub\u003e2\u003c/sub\u003eRR.\u003c/p\u003e","description":"","filename":"1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8911322/v1/ecfcc3606bfb03a3afdbd8da.jpg"},{"id":105152213,"identity":"99ad19f5-6ab9-4b8b-bc22-0ca342264a0b","added_by":"auto","created_at":"2026-03-22 15:41:17","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":79184,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003e\u003cstrong\u003eGC–MS spectra of CO\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cem\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e\u003cstrong\u003eRR products under different pH conditions.\u003c/strong\u003e\u003c/em\u003e\u003cem\u003e (A) GC–MS chromatograms of electrolyte solutions before and after 2 h of CO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eRR showing the distribution of acetic acid, formic acid and benzoic acid. (B) \u003c/em\u003e\u003csup\u003e\u003cem\u003e13\u003c/em\u003e\u003c/sup\u003e\u003cem\u003eC-labelling experiments reveal slightly shorter retention times and a +1 amu shift per carbon in \u003c/em\u003e\u003csup\u003e\u003cem\u003e13\u003c/em\u003e\u003c/sup\u003e\u003cem\u003eC-labelled acetic and formic acids, consistent with previous isotope studies.\u003c/em\u003e\u003csup\u003e\u003cem\u003e26\u003c/em\u003e\u003c/sup\u003e\u003cem\u003e.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8911322/v1/f33145d2771528b3c64ac703.jpg"},{"id":105563813,"identity":"ceee9f26-9736-4c73-b0d7-5b670f55047f","added_by":"auto","created_at":"2026-03-27 12:47:54","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":90871,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003e\u003cstrong\u003eTEM images of Fe\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cem\u003e\u003cstrong\u003e3\u003c/strong\u003e\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e\u003cstrong\u003eO\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cem\u003e\u003cstrong\u003e4\u003c/strong\u003e\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e\u003cstrong\u003e·aSiO\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cem\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e\u003cstrong\u003e electrodes before (A-B) and after (C-F) CO\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cem\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e\u003cstrong\u003eRR.\u003c/strong\u003e\u003c/em\u003e\u003cem\u003e A) Control sample (untreated electrode), displaying a well-preserved Magnetite (Mt) distribution within the silica matrix. B) Control sample at higher magnification: Magnetite (Mt) structure shows uncorroded sharp edges (white arrows) within a retained silica (Si) framework containing some void space (V). C) Image of a single magnetite cluster after CO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eRR, showing silica (light grey), and darker magnetite, within the center of the magnetite nanoparticles, Box 1 is an area mapped by EELS across the C1S edge (Extended Data Fig. 6D), Box 2, an area investigated by high-resolution imaging, shown in E and F (scale bar 5 nm). D) Carbon distribution (285.0 eV)\u003c/em\u003e\u003csup\u003e\u003cem\u003e27,28\u003c/em\u003e\u003c/sup\u003e\u003cem\u003e within the hollow center of the features in B and C. E) High-resolution image of Box 2, the magnetite silica interface reveals a triangular/dentate appearance to the edges of the magnetite nano-particles (white arrows), F) High-resolution image of amorphous silica is surrounded by the irregular surface of magnetite. G) Selected area diffraction from the area shown in F. H) STXM analysis reveals the formation and distribution of organic carbon species on the Fe\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eO\u003c/em\u003e\u003csub\u003e\u003cem\u003e4\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e·aSiO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e electrode before and after CO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eRR\u003c/em\u003e\u003csup\u003e\u003cem\u003e29,30\u003c/em\u003e\u003c/sup\u003e\u003cem\u003e.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8911322/v1/aba017bcd1b1ca0a800b3660.jpg"},{"id":105563811,"identity":"b7665193-e828-4c84-b768-106b2febf0f0","added_by":"auto","created_at":"2026-03-27 12:47:54","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":173766,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003epH-dependent band alignment and CO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eRR behavior of TiO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e⋅aSiO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e and Fe\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e3\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e4\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e⋅aSiO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e electrodes.\u003c/strong\u003e (A–C) Mott–Schottky (MS) plots of the capacitance (C\u003csup\u003e-2\u003c/sup\u003e) versus potential (vs saturated calomel electrode, SCE) for TiO\u003csub\u003e2\u003c/sub\u003e⋅aSiO\u003csub\u003e2\u003c/sub\u003e and Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e⋅aSiO\u003csub\u003e2\u003c/sub\u003e electrodes in synthetic seawater at pH 2 (A), pH 7 (B) and pH 12 (C). (D–F) Cyclic voltammograms (CV) of the TiO\u003csub\u003e2\u003c/sub\u003e⋅aSiO\u003csub\u003e2\u003c/sub\u003e at pH 2 (D), pH 7 (E) and pH 12 (F) in N\u003csub\u003e2\u003c/sub\u003e-saturated (black) and CO\u003csub\u003e2\u003c/sub\u003e-saturated (red) electrolyte.\u003c/p\u003e","description":"","filename":"4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8911322/v1/a8ff492764d0bb1228903545.jpg"},{"id":105563259,"identity":"c2b12897-f100-4edc-a6cd-d9a1121c8382","added_by":"auto","created_at":"2026-03-27 12:46:33","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":115475,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003e\u003cstrong\u003epH-dependent band alignment and intrinsic driving force for galvanic CO\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cem\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e\u003cstrong\u003e reduction on TiO\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cem\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e\u003cstrong\u003e⋅aSiO\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cem\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e-\u003c/em\u003e\u003cem\u003e\u003cstrong\u003eFe\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cem\u003e\u003cstrong\u003e3\u003c/strong\u003e\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e\u003cstrong\u003eO\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cem\u003e\u003cstrong\u003e4\u003c/strong\u003e\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e\u003cstrong\u003e·aSiO\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cem\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e\u003cstrong\u003e pairs.\u003c/strong\u003e\u003c/em\u003e\u003cem\u003e Schematic energy diagram showing the conduction band (CB) and valence band (VB) edges of Fe\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eO\u003c/em\u003e\u003csub\u003e\u003cem\u003e4\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e, Fe\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eO\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e, TiO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e·aSiO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e and Fe\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eO\u003c/em\u003e\u003csub\u003e\u003cem\u003e4\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e·aSiO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e at pH 2, 7 and 12. Energies are referenced both as electrochemical potentials (U vs. SHE, left axis) and electronic energies (eV, right axis). (p-type) Fe\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eO\u003c/em\u003e\u003csub\u003e\u003cem\u003e4\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e·aSiO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e and (n-type) TiO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e·aSiO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e; band-gap values are indicated inside each box. Horizontal dashed lines mark the Fe\u003c/em\u003e\u003csup\u003e\u003cem\u003e3+\u003c/em\u003e\u003c/sup\u003e\u003cem\u003e/Fe\u003c/em\u003e\u003csup\u003e\u003cem\u003e2+\u003c/em\u003e\u003c/sup\u003e\u003cem\u003e redox couple and the pH-dependent CO₂/CO₂RR potentials. The built-in potential between Fe\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eO\u003c/em\u003e\u003csub\u003e\u003cem\u003e4\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e·aSiO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e and TiO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e·aSiO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e, together with their electron affinity (E\u003c/em\u003e\u003csub\u003e\u003cem\u003eea\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e), drives electron flow from Fe\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eO\u003c/em\u003e\u003csub\u003e\u003cem\u003e4\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e·aSiO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e to TiO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e·aSiO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e and enables spontaneous dark CO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e reduction to organic products in the galvanic cell.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8911322/v1/9b93b76c899e70db019f4af1.jpg"},{"id":108181478,"identity":"858c73bf-1208-44d8-8e3e-99383192bf1d","added_by":"auto","created_at":"2026-04-30 08:58:40","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1073649,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8911322/v1/d257376f-96f6-4550-8c9d-635439ef1202.pdf"},{"id":105563402,"identity":"9197f732-630d-4157-8210-e1a07ad36cd6","added_by":"auto","created_at":"2026-03-27 12:46:53","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":34637356,"visible":true,"origin":"","legend":"cover letter","description":"","filename":"extendeddatanature.docx","url":"https://assets-eu.researchsquare.com/files/rs-8911322/v1/c228c87b528603a4d8ebaa5c.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Spontaneous Mineral-driven electrochemical synthesis of organic matter under Martian conditions","fulltext":[{"header":"Introduction","content":"\u003cp\u003eOrganic compounds indigenous to, and preserved within, Martian meteorites provide direct evidence that reduced carbon can form abiotically during aqueous alteration of planetary crusts.\u003csup\u003e1-6\u003c/sup\u003e These organics are consistently associated with Ti-Fe oxides, spinel-group minerals and amorphous silica formed during aqueous alteration\u003csup\u003e2,4,5\u003c/sup\u003e, suggesting a mineralogical control on their synthesis. Proposed mechanisms include serpentinization\u003csup\u003e6\u003c/sup\u003e, carbonation\u003csup\u003e6\u003c/sup\u003e and electrochemical reduction\u003csup\u003e5\u003c/sup\u003e. Electrochemical reduction of CO\u003csub\u003e2\u003c/sub\u003e was inferred from mineral and organic context and texture.\u003c/p\u003e\n\u003cp\u003eFluid\u0026ndash;rock interaction on Mars occurred predominantly within fractures and alteration veins\u003csup\u003e7,8\u003c/sup\u003e, where redox contrasts between coexisting minerals would have been unavoidable\u003csup\u003e9\u003c/sup\u003e. Such environments are inherently electrochemical: minerals with different work functions and redox couples establish local potential gradients when immersed in electrolyte solutions\u003csup\u003e5\u003c/sup\u003e. Nanoscale intergrowths of Fe-rich and Ti-rich oxides embedded in silicate matrices, widely observed in Martian meteorites and alteration products\u003csup\u003e5\u003c/sup\u003e, therefore resemble natural galvanic junctions capable of sustaining directional electron transfer during corrosion and weathering\u003csup\u003e10\u003c/sup\u003e. These mineral interfaces represent a plausible setting for abiotic carbon fixation, but their capacity to drive spontaneous carbon reduction has not been directly demonstrated.\u003c/p\u003e\n\u003cp\u003eMost experimental studies of abiotic carbon synthesis focus either on single \u0026ldquo;catalysts\u0026rdquo; usually in hydrothermal settings\u003csup\u003e11,12\u003c/sup\u003e. In contrast, mineral-driven electrochemical coupling offers a fundamentally different operating mode in which intrinsic potential gradients between dissimilar minerals i.e. galvanic processes, rather than temperature or light, provide the driving force for redox reactions\u003csup\u003e13-15\u003c/sup\u003e. While electrochemical reduction of CO\u003csub\u003e2\u003c/sub\u003e is a burgeoning research field for remediation of CO\u003csub\u003e2\u003c/sub\u003e from Earth\u0026rsquo;s atmosphere, the laboratory systems require external energy input, usually in the form of applied \u0026ldquo;over-potential (OP)\u0026rdquo;\u003csup\u003e16\u003c/sup\u003e. Therefore, bringing together observations of electrochemical reduction producing organics on Mars with the question of whether such mineral junctions can sustain carbon reduction under geochemically realistic conditions remains open.\u003c/p\u003e\n\u003cp\u003eHere, we construct a mineral assemblage that reproduces the textural and compositional relationships observed in Martian meteorites as discrete electrodes. By embedding TiO\u003csub\u003e2\u003c/sub\u003e and Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e nanoparticles within an amorphous SiO\u003csub\u003e2\u003c/sub\u003e matrix, we recreate Ti\u0026ndash;Fe spinel\u0026ndash;silicate interfaces formed during aqueous alteration of basaltic crust.\u003csup\u003e17\u003c/sup\u003e Experimental observation using both electrochemical, organic chemical, and nano-analytical techniques were then used to establish whether spontaneous galvanic coupling in aqueous brines, and the produced intrinsic electrochemical gradients could drive CO\u003csub\u003e2\u003c/sub\u003e reduction and organic synthesis without any external energy input.\u0026nbsp;\u003c/p\u003e"},{"header":"Results","content":"\u003ch2\u003eSpontaneous CO\u003csub\u003e2\u003c/sub\u003e reduction in a coupled mineral galvanic system\u003c/h2\u003e\n\u003cp\u003eTo evaluate whether mineral junctions can spontaneously drive CO\u003csub\u003e2\u003c/sub\u003e under Mars-like conditions, we constructed a galvanic system composed of TiO\u003csub\u003e2\u003c/sub\u003e·aSiO\u003csub\u003e2\u003c/sub\u003e and Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e·aSiO\u003csub\u003e2\u003c/sub\u003e electrodes immersed in CO\u003csub\u003e2\u003c/sub\u003e-bearing brine. (Extended Data, Fig. 1) Analysis of Martian meteorites revealed the presence of amorphous silica throughout the organic-rich area, and for that reason, we designed the electrodes to encapsulate the oxide phases within a silica matrix\u003csup\u003e5,6\u003c/sup\u003e. Unlike previous abiotic CO\u003csub\u003e2\u003c/sub\u003e reduction studies that employ single catalysts activated by heat, light, or applied voltage, this configuration integrates distinct anodic and cathodic mineral phases. Details of the experimental methods are contained in the supplementary materials.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003ePost experimental analysis of the liquid phase by nuclear magnetic resonance spectroscopy (NMR, Fig. 1) and gas chromatography–mass spectrometry (GC-MS, Fig. 2) show that the coupled TiO\u003csub\u003e2\u003c/sub\u003e⋅aSiO\u003csub\u003e2\u003c/sub\u003e–Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e⋅aSiO\u003csub\u003e2\u003c/sub\u003e contains a suite of soluble and mineral-associated organic compounds not present in the controls. Dominant products (\u003csup\u003e1\u003c/sup\u003eH-NMR, Fig. 1A) include formate, acetate, acetone and methanol, with product distributions varying systematically with pH. Acidic and neutral conditions (Fig. 1A, data II, III. Extended Data Table. 1) favor small-oxygenated molecules, whereas alkaline conditions (Fig. 1A, data IV) promote the appearance of aromatic species. The emergence of aromatic resonances at 7.4–7.5 ppm in \u003csup\u003e1\u003c/sup\u003eH-NMR data (Fig. 1A curve IV) at high pH is accompanied by increased carbonate and carboxyl signals\u003csup\u003e18-21\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eGC–MS (Fig. 2A) further detects formic acid, acetic acid and benzoic acid in reacted electrolytes, together with aromatic fragments associated with mineral-bound carbon. These compounds are absent from unreacted controls and from experiments conducted without CO\u003csub\u003e2\u003c/sub\u003e. Consistent with GC–MS, \u003csup\u003e1\u003c/sup\u003eH-NMR shows that after CO\u003csub\u003e2\u003c/sub\u003eRR the coupled TiO\u003csub\u003e2\u003c/sub\u003e·aSiO\u003csub\u003e2\u003c/sub\u003e–Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e·aSiO\u003csub\u003e2\u003c/sub\u003e system reproducibly yields acetate, acetone and methanol, whereas the corresponding control solutions contain at most trace acetate and no detectable acetone or methanol (Fig. 1A; Extended Data Table 1).\u003c/p\u003e\n\u003cp\u003eIsotopic labelling with \u003csup\u003e13\u003c/sup\u003eCO\u003csub\u003e2\u003c/sub\u003e (Fig. 1B, 2B, Extended Data Fig. 5) demonstrates that carbon incorporated into formate, acetate and aromatic products derives from the supplied inorganic carbon source: \u003csup\u003e13\u003c/sup\u003eC enrichment is observed in formate and aromatic carbon by \u003csup\u003e13\u003c/sup\u003eC-NMR, and in both formate and acetate by GC–MS. The absence of resolved \u003csup\u003e13\u003c/sup\u003eC-acetate resonances in solution-state \u003csup\u003e13\u003c/sup\u003eC-NMR likely reflects differences in analytical sensitivity and/or spectral overlap under the present acquisition conditions. By contrast, acetone and methanol show no detectable \u003csup\u003e13\u003c/sup\u003eC incorporation by either \u003csup\u003e13\u003c/sup\u003eC-NMR or GC–MS, indicating that their carbon source cannot be unambiguously assigned to the introduced \u003csup\u003e13\u003c/sup\u003eCO\u003csub\u003e2\u003c/sub\u003e in these experiments. Together, these data establish robust CO\u003csub\u003e2\u003c/sub\u003e-to-organics conversion for formate, acetate and aromatic carbon, while constraining acetone and methanol to products whose isotopic provenance remains unresolved.\u003c/p\u003e\n\u003ch2\u003eLocalization and structure of mineral-associated carbon\u003c/h2\u003e\n\u003cp\u003eBuilding on Prior observation of aromatic macromolecular carbon (MMC) associated with magnetite and silica in Martian Meteorites\u003csup\u003e2,4,5,6\u003c/sup\u003e, we investigated Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e⋅aSiO\u003csub\u003e2\u003c/sub\u003e electrodes post-CO\u003csub\u003e2\u003c/sub\u003e reduction using similar techniques used to study the meteorites. Transmission electron microscopy (TEM, Figs. 3A-G, Extended Data Figs. 2-6) and synchrotron-based scanning transmission X-ray microscopy (STXM, Fig.3H) show that organic carbon accumulates preferentially at Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e–SiO\u003csub\u003e2\u003c/sub\u003e interfaces after reaction (Fig. 3D, Extended Data Figs. 4,6). Non-reacted control samples display sharp magnetite–silica boundaries and lack localized carbon enrichment (Figs. 3B, H, Extended Data Fig.2). In contrast, reacted samples exhibit corrosion textures (Fig 3E and F) and interfacial carbon coatings that extend along oxide–silica boundaries (Fig 3C, D), indicating in situ formation and retention of carbon at mineral junctions (Figs. 3D, H, Extended Data Figs. 2,4,6).\u003c/p\u003e\n\u003cp\u003eHigh-resolution imaging and electron energy-loss spectroscopy (EELS, Fig 3C and D, Extended Data Fig. 6) identify amorphous carbon layers associated with Fe-rich domains. This spatial confinement demonstrates that carbon is generated and stabilized at magnetite–silica interfaces rather than being uniformly distributed throughout the electrolyte or mineral matrix.\u003c/p\u003e\n\u003cp\u003eThe extent and morphology of interfacial carbon vary systematically with pH. At low pH (Extended Data Figs. 3, 6), magnetite remains embedded within amorphous silica and develops moderate porosity, with aromatic carbon retained at the oxide–silica boundary. At neutral pH (Extended Data Fig. 4), increased porosity and carbonate-rich amorphous phases emerge, consistent with partial dissolution and interfacial reorganization. Under alkaline conditions (Extended Data Fig. 5), extensive silica dissolution and magnetite corrosion lead to structural breakdown and partial release of organic products into solution.\u003c/p\u003e\n\u003cp\u003eC K-edge electron energy loss spectroscopy further distinguishes reacted samples from controls (Fig. 3H, Extended Data Table. 2)\u003csup\u003e29,30\u003c/sup\u003e. Control electrodes lack detectable aromatic signatures, whereas reacted electrodes display pronounced spectral features corresponding to aromatic, carboxyl and carbonate functional groups. These spectroscopic fingerprints\u003csup\u003e29,30\u003c/sup\u003e (Fig. 3H) closely resemble those reported for organic matter preserved in Martian meteorites (green trace Fig 3H), indicating that the experimentally generated carbon shares key chemical characteristics with Martian meteoritic organics.\u003c/p\u003e\n\u003cp\u003eTogether, these observations establish that mineral interfaces act as both reactive sites and stabilizing hosts for newly formed organic matter. The preferential localization of aromatic-rich carbon at Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e–aSiO\u003csub\u003e2\u003c/sub\u003e boundaries mirrors the association between amorphous silica and aromatic carbon observed in altered Martian meteorites.\u003c/p\u003e\n\u003ch2\u003eElectronic properties and band alignment of TiO\u003csub\u003e2\u003c/sub\u003e⋅aSiO\u003csub\u003e2\u003c/sub\u003e–Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e⋅aSiO\u003csub\u003e2\u003c/sub\u003e pairs\u003c/h2\u003e\n\u003cp\u003eThe liquid and solid phase analyses confirm that the TiO\u003csub\u003e2\u003c/sub\u003e⋅aSiO\u003csub\u003e2\u003c/sub\u003e-Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e⋅aSiO\u003csub\u003e2\u003c/sub\u003e acts as a galvanic pair system that can spontaneously reduce CO\u003csub\u003e2\u003c/sub\u003e to formate, acetate and aromatics across a wide pH range. To elucidate the origin of this activity and to confirm the reaction’s validity and the intrinsic driving force behind spontaneous CO\u003csub\u003e2\u003c/sub\u003eRR, we examined the electronic properties and band alignment of the two electrodes using Mott-Schottky and Cyclic Voltammetry analyses. Mott–Schottky analysis (Fig. 4A-C) shows that TiO\u003csub\u003e2\u003c/sub\u003e⋅aSiO\u003csub\u003e2\u003c/sub\u003e behaves as an n-type semiconductor, whereas Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e⋅aSiO\u003csub\u003e2\u003c/sub\u003e exhibits p-type character across the investigated pH range. Linear fits yield flat band potentials that define the relative positions of the semiconductor band edges and their systematic pH-dependent shifts. These data establish the energetic landscape of the Ti-Fe-silicate pair and constrain the alignment of its electronic states relative to solution redox couples and therefore confirm whether the two minerals could establish a natural potential difference when coupled. In simple terms, this analysis tells us whether the two minerals can naturally behave like a tiny battery when placed in water. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003eIndependent open circuit potential measurements demonstrate that Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e⋅aSiO\u003csub\u003e2\u003c/sub\u003e exhibits a lower equilibrium potential than TiO\u003csub\u003e2\u003c/sub\u003e⋅aSiO\u003csub\u003e2\u003c/sub\u003e under identical electrolyte conditions, indicating that Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e⋅aSiO\u003csub\u003e2\u003c/sub\u003e acts as the anodic phase and TiO\u003csub\u003e2\u003c/sub\u003e⋅aSiO\u003csub\u003e2\u003c/sub\u003e as the cathodic phase when the two electrodes are electrically coupled. Upon immersion in electrolyte, the p-type Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e⋅aSiO\u003csub\u003e2\u003c/sub\u003e and n-type TiO\u003csub\u003e2\u003c/sub\u003e⋅aSiO\u003csub\u003e2\u003c/sub\u003e therefore establish a mineral junction that supports spontaneous galvanic behavior, with Fe\u003csup\u003e2+\u003c/sup\u003e oxidation at the Fe-rich phase supplying electrons that are transferred to the Ti-rich phase, where they drive CO\u003csub\u003e2\u003c/sub\u003e reduction.\u003csup\u003e31\u003c/sup\u003e\u003c/p\u003e\n\u003cp\u003eThe magnitude of the built-in potential and the resulting charge-transfer characteristics vary with pH. At low pH (Fig. 4A), large band offsets generate a strong internal electric field and pronounced galvanic coupling\u003csup\u003e31,33\u003c/sup\u003e, favoring efficient electron flow between the two mineral phases.\u0026nbsp;\u0026nbsp;At neutral pH (Fig. 4B), reduced band separation enhances interfacial charge-transfer kinetics\u003csup\u003e34\u003c/sup\u003e, consistent with changes in the organics produced in the liquid-phase analyses. Under alkaline conditions (Fig. 4C), the resulting galvanic potential, together with increased interfacial conductivity, enables multi-electron transfer and yields the highest activity\u003csup\u003e35\u003c/sup\u003e, accompanied by enhanced formation of more structurally complex products.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eCyclic voltammetry recorded under galvanic coupling conditions (Fig. 4D-F) shows enhanced cathodic currents in CO\u003csub\u003e2\u003c/sub\u003e-saturated brine relative to N\u003csub\u003e2\u003c/sub\u003e-saturated electrolyte across the investigated pH range. The negative-going onset potentials in cathodic current under CO\u003csub\u003e2\u003c/sub\u003e indicate that the coupled mineral system generates sufficient intrinsic electrochemical bias to drive carbon reduction without external bias or illumination.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eNotably, these electrochemical responses are not reproduced by either mineral phase alone under identical conditions (Extended Data Fig. 7). TiO\u003csub\u003e2\u003c/sub\u003e⋅aSiO\u003csub\u003e2\u003c/sub\u003e in isolation produces only low yields of small-oxygenated products, whereas Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e⋅aSiO\u003csub\u003e2\u003c/sub\u003e exhibits limited CO\u003csub\u003e2\u003c/sub\u003e reduction activity but efficiently transforms CO\u003csub\u003e2\u003c/sub\u003e reduction intermediates into more reduced and structurally complex organics. These observations indicate that the full reaction sequence requires both phases operating in an electrically coupled configuration.\u0026nbsp;\u003c/p\u003e\n\u003ch2\u003eCoupled mineral behavior enables spontaneous carbon synthesis\u003c/h2\u003e\n\u003cp\u003eThe complementary electronic properties of the two minerals define the direction of electron flow and the sites of redox reactions. The conduction band of TiO\u003csub\u003e2\u003c/sub\u003e⋅aSiO\u003csub\u003e2\u003c/sub\u003e lies at a more negative potential than the CO\u003csub\u003e2\u003c/sub\u003e redox couple, enabling thermodynamically favorable CO\u003csub\u003e2\u003c/sub\u003e reduction on TiO\u003csub\u003e2\u003c/sub\u003e⋅aSiO\u003csub\u003e2\u003c/sub\u003e (Fig. 5).\u003csup\u003e36,37\u003c/sup\u003e Oxidation of Fe\u003csup\u003e2+\u003c/sup\u003e at the Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e⋅aSiO\u003csub\u003e2\u003c/sub\u003e phase supplies electrons that are transferred to the TiO\u003csub\u003e2\u003c/sub\u003e⋅aSiO\u003csub\u003e2\u003c/sub\u003e phase, where they drive the reduction of dissolved CO\u003csub\u003e2\u003c/sub\u003e.\u0026nbsp;The resulting products accumulate at mineral–silica interfaces and in solution, forming both small-oxygenated molecules and aromatic-rich carbon. This coupled electrochemical–mineralogical behavior provides a mechanistic link between mineral galvanic processes and abiotic organic synthesis under planetary conditions.\u003c/p\u003e\n\u003cp\u003eControl experiments using single-phase electrodes (supplementary doc 1.2) confirm that neither mineral alone can reproduce the full product suite. TiO\u003csub\u003e2\u003c/sub\u003e⋅aSiO\u003csub\u003e2\u003c/sub\u003e reduces CO\u003csub\u003e2\u003c/sub\u003e only to small-oxygenated products (Extended Data Fig. 7A), whereas Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e⋅SiO\u003csub\u003e2\u003c/sub\u003e shows limited CO\u003csub\u003e2\u003c/sub\u003e reduction activity but efficiently converts CO\u003csub\u003e2\u003c/sub\u003e-reduction intermediates into more reduced and structurally complex organics, including aromatic species (Extended Data Fig. 7B, C). Together, the coupled mineral system functions as an integrated electrochemical reactor in which primary reduction and secondary upgrading occur within the same mineralogical framework.\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eThis study establishes mineral-driven electrochemical coupling between two different minerals or even different phases of the same mineral as a previously unrecognized pathway for abiotic carbon fixation. This identifies galvanic mineral assemblages as natural electrochemical reactors that drive abiotic carbon fixation. By demonstrating that purely mineral assemblages can spontaneously convert CO\u003csub\u003e2\u003c/sub\u003e into complex organic matter without hydrothermal conditions or applied external energy input, we provide a mechanistic link between planetary mineralogy, electrochemistry and organic synthesis. In contrast to previous abiotic CO\u003csub\u003e2\u003c/sub\u003e reduction studies that rely on single catalysts,\u003csup\u003e41\u003c/sup\u003e our results (Fig. 5) show that a coupled mineral assemblage can function as a complete electrochemical circuit in which electron flow is generated internally by mineral redox disequilibria and directly coupled to CO\u003csub\u003e2\u003c/sub\u003e reduction. This identifies galvanic mineral assemblages as natural electrochemical reactors that drive abiotic carbon fixation.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe coupled TiO\u003csub\u003e2\u003c/sub\u003e⋅aSiO\u003csub\u003e2\u003c/sub\u003e–Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e⋅aSiO\u003csub\u003e2\u003c/sub\u003e system exhibits clear functional differentiation between its two mineral components. CO\u003csub\u003e2\u003c/sub\u003e reduction occurs predominantly at the TiO\u003csub\u003e2\u003c/sub\u003e⋅aSiO\u003csub\u003e2\u003c/sub\u003e surface, while Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e⋅aSiO\u003csub\u003e2\u003c/sub\u003e functions as the electron source through Fe\u003csup\u003e2+\u003c/sup\u003e oxidation, generating small-oxygenated intermediates such as formate and acetate, together with additional oxygenated products including acetone. Whereas Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e⋅aSiO\u003csub\u003e2\u003c/sub\u003e promotes secondary surface reactions that convert these intermediates into more reduced and structurally complex aromatic carbon. Single-electrode control experiments show that neither mineral alone can reproduce the full product suite, establishing that organic synthesis emerges from their coupled behavior rather than from the activity of either phase in isolation as in typically studied abiotic synthesis pathways.\u003csup\u003e42\u003c/sup\u003e Such division of labor is characteristic of electrochemical systems but has rarely been demonstrated in purely mineral assemblages and demonstrates that geo-electrochemical reactions supply energy to reactions in the environment.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe spontaneous operation of this system is explained by the complementary electronic properties of the two minerals. TiO\u003csub\u003e2\u003c/sub\u003e⋅aSiO\u003csub\u003e2\u003c/sub\u003e consistently exhibits n-type character with a conduction band edge sufficiently negative to drive carbon reduction (Fig. 4,5). Their junction therefore establishes a built-in electric field that directs electrons from the Fe-rich phase toward the Ti-rich phase, generating potential differences that required for CO\u003csub\u003e2\u003c/sub\u003e reduction. This intrinsic driving force enables the coupled mineral system to operate spontaneously under ambient conditions, in contrast to single-phase electrodes that require large applied OPs to achieve comparable reactivity.\u003csup\u003e43\u003c/sup\u003e\u003c/p\u003e\n\u003cp\u003eThe architecture of the TiO\u003csub\u003e2\u003c/sub\u003e·aSiO\u003csub\u003e2\u003c/sub\u003e–Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e·aSiO\u003csub\u003e2\u003c/sub\u003e system mirrors key features of Martian alteration environments. Martian meteorites and rover observations document widespread co-occurrence of Fe–Ti oxides, spinel-group minerals and amorphous silica formed during aqueous alteration of basaltic crust. The exsolution lamellae seen in Martian spinels are capable of operating as nano-scale electrodes in a galvanic system, showing that when immersed in CO\u003csub\u003e2\u003c/sub\u003e-bearing brines, such assemblages would naturally establish mineral junctions analogous to the anodic–cathodic pairing constructed here. Our design therefore represents a mineralogical analogue of fracture- and vein-hosted alteration textures, as well as areas of zonation within single minerals on Mars, in which redox disequilibria between coexisting phases could sustain electrochemical reactions during fluid–rock interaction.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eNanoscale imaging and spectroscopy (Fig. 3) show that organic carbon is spatially confined to magnetite–silica interfaces, linking carbon synthesis directly to mineral junctions. The emergence of aromatic carbon exclusively in reacted samples and its absence in controls demonstrate that these compounds are generated in situ rather than inherited from contamination. The close correspondence between experimentally produced aromatics and organic matter preserved in Martian meteorites suggests that similar mineral-driven electrochemical processes could have operated during aqueous alteration of Martian basalts.\u003c/p\u003e\n\u003cp\u003eThe pH-dependent evolution of mineral textures (Fig. 1-3) further constrains the environmental conditions under which such processes can operate\u003csup\u003e44,45\u003c/sup\u003e,\u0026nbsp;with alkaline conditions favoring aromatic formation and indicating progressive carbon–carbon coupling and reduction. Moderate porosity and interfacial carbon retention at acidic and neutral pH favor the stabilization of organic products at oxide–silica boundaries\u003csup\u003e46,47\u003c/sup\u003e, whereas strong alkalinity promotes mineral dissolution and the release of products into solution. These observations are consistent with scenarios in which carbon fixation occurs locally at mineral interfaces, but the resulting labile organic carbon can subsequently be redistributed by evolving fluid chemistry. The association of more refractory aromatic carbon with alteration textures in Martian meteorites is therefore consistent with episodic mineral-driven synthesis coupled to changing aqueous conditions.\u003c/p\u003e\n\u003cp\u003eMore broadly, mineral-driven electrochemical synthesis provides a unifying framework linking corrosion, redox gradients, and organic carbon formation within planetary crusts. Similar Ti- and Fe-bearing assemblages occur in altered terrestrial basalts and are expected in the rocky interiors of icy ocean worlds\u003csup\u003e48,49\u003c/sup\u003e, implying that this mechanism is not restricted to Mars. Mineral junctions capable of sustaining galvanic coupling may thus represent a general geochemical pathway for abiotic carbon fixation wherever igneous minerals interact with CO₂-bearing brines.\u003c/p\u003e\n\u003cp\u003eBeyond its planetary implications, this mechanism also redefines the conceptual landscape of electrochemical carbon reduction on Earth. Modern CO₂ reduction strategies are dominated by externally powered systems optimized for efficiency and selectivity\u003csup\u003e50\u003c/sup\u003e. In contrast, mineral-driven electrochemistry operates through intrinsic potential gradients generated by natural materials, offering a model for low-energy carbon conversion that does not require engineered catalysts or applied bias. Although such systems are not optimized for industrial throughput, they demonstrate that carbon fixation can proceed under minimal energetic constraints when appropriate mineral assemblages are present.\u003c/p\u003e\n\u003cp\u003eTogether, these results establish mineral-driven electrochemical coupling as a geochemically realistic and previously underappreciated mode of abiotic carbon synthesis. By directly linking semiconductor properties, nanoscale mineral textures, and organic product formation, this study provides a mechanistic explanation for the co-occurrence of Ti–Fe spinels, amorphous silica, and aromatic carbon in Martian meteorites and suggests that abiotic organic synthesis is an inherent consequence of fluid–rock interactions in redox-stratified environments on early Earth, Mars, and any other water-bearing planetary bodies.\u003c/p\u003e"},{"header":"Materials and Methods","content":"\u003ch2\u003eThin Film Electrode Synthesis\u003c/h2\u003e\n\u003cp\u003eMagnetite-amorphous-silica thin‐film electrodes (Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e\u0026middot;aSiO\u003csub\u003e2\u003c/sub\u003e) were synthesized by dispersing 150 mg of nano-magnetite powder (Fisher Scientific) in 20 mL of deionized water containing 500 \u0026micro;L of sodium silicate solution (\u0026ldquo;water glass\u0026rdquo;, Sigma-Aldrich). The suspension was sonicated for 15 min to achieve a uniform dispersion and then drop-cast onto fluorine-doped tin oxide (FTO) glass slides (conductive substrates, Sigma-Aldrich). Coated slides were dried and cured at 75 \u0026deg;C for 48 hrs to promote adhesion and formation of a stable amorphous silicate matrix. The resulting Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e\u0026middot;aSiO\u003csub\u003e2\u003c/sub\u003e films consist of magnetite nanoparticles embedded within an a-SiO\u003csub\u003e2\u003c/sub\u003e framework and serve as the Fe-rich \u0026ldquo;spinel\u0026rdquo; endmember in the galvanic system. Titanium-amorphous-silica thin-film electrodes (TiO\u003csub\u003e2\u003c/sub\u003e\u0026middot;aSiO\u003csub\u003e2\u003c/sub\u003e) were prepared by dispersing 150 mg of TiO\u003csub\u003e2\u003c/sub\u003e nano powder (Sigma-Aldrich) in 20 mL of deionized water with 500 \u0026micro;L of sodium silicate solution, followed by 15 min sonication. The suspension was drop-cast onto FTO slides and treated identically (75 \u0026deg;C, 48 hrs) to yield TiO\u003csub\u003e2\u003c/sub\u003e nanoparticles encapsulated in an amorphous silica matrix. These electrodes represent the Ti-rich spinel domains.\u003cins cite=\"mailto:Andrew%20Steele\" datetime=\"2025-09-08T18:40\"\u003e\u0026nbsp;\u003c/ins\u003e\u003c/p\u003e\n\u003ch2\u003eAnalytical Techniques\u003c/h2\u003e\n\u003cp\u003eElectrochemical reactions were carried out using the coupled TiO\u003csub\u003e2\u003c/sub\u003e\u0026sdot;aSiO\u003csub\u003e2\u003c/sub\u003e\u0026ndash;Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e\u0026sdot;aSiO\u003csub\u003e2\u003c/sub\u003e mineral system under CO\u003csub\u003e2\u003c/sub\u003e-saturated aqueous conditions (Fig.S1). Product analysis and electrochemical measurements were performed on samples collected after reaction. All experiments were repeated independently four times with similar results (n = 4).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eProton (\u003csup\u003e1\u003c/sup\u003eH) and Carbon (\u003csup\u003e13\u003c/sup\u003eC) - Nuclear Magnetic Resonance spectroscopy (\u003csup\u003e1\u003c/sup\u003eH- and \u003csup\u003e13\u003c/sup\u003eC- NMR)\u003c/strong\u003e\u003cstrong\u003e:\u003c/strong\u003e Liquid-phase CO\u003csub\u003e2\u003c/sub\u003eRR products were identified and quantified using \u003csup\u003e1\u003c/sup\u003eH- and \u003csup\u003e13\u003c/sup\u003eC-NMR. Spectra were acquired on a Bruker 600 MHz (14.1 T) AVIII NMR spectrometer under ambient conditions, using standard pulse sequences with water suppression, a 5 s relaxation delay, and no spinning. For each sample, 500 \u0026micro;L of electrolyte was mixed with 65 \u0026micro;L D\u003csub\u003e2\u003c/sub\u003eO (ACROS Organics) and 35 \u0026micro;L TMSP-d\u003csub\u003e4\u003c/sub\u003e (3-(trimethylsilyl) propionic acid-d\u003csub\u003e4\u003c/sub\u003e, Sigma-Aldrich) as an internal standard (58 \u0026micro;M for \u003csup\u003e1\u003c/sup\u003eH-NMR; 20 mM for \u003csup\u003e13\u003c/sup\u003eC-NMR). TMSP-d\u003csub\u003e4\u003c/sub\u003e was used for both chemical-shift referencing (0.00 ppm) and quantitative integration. (n=4)\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eHeadspace Solid-Phase Microextraction coupled with Gas Chromatography-Mass Spectrometry (HS-SPME-GC-MS):\u003c/strong\u003e Volatile and semi-volatile organics were analyzed by headspace solid-phase microextraction coupled with gas chromatography-mass spectrometry (HS-SPME-GC-MS) on an Agilent NW7590 GC-MS instrument using helium as the carrier gas and a 44 mins runtime. Electrolyte samples were acidified to pH ~1 with HCl, and 1 mL aliquots were transferred to 10 mL vials. AC-WR-95/PDMS fiber was exposed to the headspace at 80 \u0026deg;C for 1 h, then desorbed in the GC inlet at 260 \u0026deg;C for 10 min (n=4). Standard solutions of formic acid (7 mM), acetic acid (100 \u0026micro;M), phenol (100 \u0026micro;M), and saturated benzoic acid were run under identical conditions to determine retention times and identify corresponding peaks in CO\u003csub\u003e2\u003c/sub\u003eRR samples (Fig.S7).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eElectrochemical characterization measurements (Gamry Interface 1000E Potentiostat/Galvanostat/ZRA Instrument):\u003c/strong\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eElectrochemical characterization was performed primarily in a three-electrode configuration using either Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e\u0026middot;aSiO\u003csub\u003e2\u003c/sub\u003e or TiO\u003csub\u003e2\u003c/sub\u003e\u0026middot;aSiO\u003csub\u003e2\u003c/sub\u003e as the working electrode (geometric area 1.0 cm\u0026sup2;), a saturated calomel reference electrode (SCE), and a Platinum counter electrode. Cyclic voltammetry (CV) was carried out from 0.0 to -1.2 V vs SCE at a scan rate of 50 mV s\u003csup\u003e-1\u003c/sup\u003e with 1 mV step size (5 consecutive cycles, 2 s equilibration at the initial potential). Measurements were performed under N\u003csub\u003e2\u003c/sub\u003e and CO\u003csub\u003e2\u003c/sub\u003e at pH 2, 7, and 12 to determine CO\u003csub\u003e2\u003c/sub\u003eRR onset potentials, open-circuit potentials (OCP), and current density enhancement under CO\u003csub\u003e2\u003c/sub\u003e relative to N\u003csub\u003e2\u0026nbsp;\u003c/sub\u003e(n=4). Mott\u0026ndash;Schottky (MS) was conducted with a 0.05 V potential step, 10 mV AC amplitude, and 3000 Hz (Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e\u0026middot;aSiO\u003csub\u003e2\u003c/sub\u003e) /100 Hz (TiO\u003csub\u003e2\u003c/sub\u003e\u0026middot;aSiO\u003csub\u003e2\u003c/sub\u003e) frequency, using an electrode area of 1 cm\u003csup\u003e2\u003c/sup\u003e. For Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e\u0026middot;aSiO\u003csub\u003e2\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e\u0026middot;aSiO\u003csub\u003e2\u003c/sub\u003e, Mott Schottky plots of C\u003csup\u003e-2\u003c/sup\u003e vs potential were used to extract flat-band potentials (U\u003csub\u003eFB\u003c/sub\u003e) and carrier densities. (n=4) All potentials are reported versus SCE unless otherwise stated.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eUltra-Violent-Via Spectroscopy for Band Gap and Electron Affinity Analysis:\u003c/strong\u003e Diffuse-reflectance UV-Vis spectra of Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e\u0026middot;aSiO\u003csub\u003e2\u003c/sub\u003e and TiO\u003csub\u003e2\u003c/sub\u003e\u0026middot;aSiO\u003csub\u003e2\u003c/sub\u003e films were acquired on a PerkinElmer Lambda 950 UV-Vis-NIR spectrophotometer with a 150 mm integrating sphere over 200-900 nm. Tauc plot analysis (Fig.S11) of Kubelka-Munk-transformed spectra yielded direct and indirect band gaps of Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e\u0026middot;aSiO\u003csub\u003e2\u003c/sub\u003e and TiO\u003csub\u003e2\u003c/sub\u003e\u0026middot;aSiO\u003csub\u003e2\u003c/sub\u003e (n=4).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTransmission Electron Microscopy (TEM):\u0026nbsp;\u003c/strong\u003eFocused ion beam (FIB) sections of Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e\u0026middot;aSiO\u003csub\u003e2\u003c/sub\u003e electrodes were prepared and examined using a TECNAI F20 X-Twin TEM at 200 kV (Deutsches GeoForschungsZentrum, Potsdam), equipped with an EDAX EDS system, Gatan TRIDIEM imaging filter, and HAADF detector. Electron energy-loss spectroscopy (EELS) was performed in diffraction mode with 0.1 eV per pixel energy dispersion and 620 mm camera length to probe C K-edge features (e.g., ~285 and 290 eV). Spectra were analyzed using Digital Micrograph.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eScanning Transmission X-ray Microscopy (STXM):\u003c/strong\u003e STXM measurements at the C 1s and N 1s absorption edges were carried out at Diamond Light Source (beamline I08, UK) on ultrathin FIB-prepared sections identified by TEM. Image stacks were collected with ~50 nm spatial resolution and 10 ms dwell time per pixel. Data were processed using MANTiS and aXis2000 to generate C K-edge XANES spectra and chemical-speciation maps.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work is supported by a NASA Astrobiology ICAR Program grant to K.L.R. and A.S., and by the Carnegie Institution of Washington. L.G.B., R.W. and A.S. acknowledge financial support from the Helmholtz Recruiting Initiative (I-044-16-01) awarded to L.G.B. J.P.H.P. acknowledges partial funding from the GFZ Discovery Fund (P-032-45-002). Beamtime at Diamond Light Source I08 was provided under proposals MG-36247 and MG-33333.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u003c/strong\u003eC.W. designed the experiments, synthesized materials, conducted electrochemistry, NMR, GC\u0026ndash;MS and microscopy analyses, and wrote the manuscript. A.S. supervised the project, TEM, EELS, STXM measurements and contributed to interpretation. L.G.B., R.W. and A.Sch. performed TEM and EELS. B.K. and L.C.C.H. conducted STXM measurements. S.L. and W.L. assisted with sample measurements. M.J.H.P., K.L.R., A.P., G.C. and T.A.S. contributed to planetary interpretation. All authors discussed results and contributed to manuscript editing.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eUdry, A., Howarth, G. H., Herd, C. D. K., Day, J. M. D., Lapen, T. J. \u0026amp; Filiberto, J. What Martian meteorites reveal about the interior and surface of Mars. \u003cem\u003eJ. Geophys. Res. Planets\u003c/em\u003e \u003cstrong\u003e125\u003c/strong\u003e, e2020JE006523 (2020). \u003cu\u003ehttps://doi.org/10.1029/2020JE006523\u003c/u\u003e \u003c/li\u003e\n\u003cli\u003eSchmitt-Kopplin, P., Matzka, M., Ruf, A., Menez, B., Chennaoui Aoudjehane, H., Harir, M., Lucio, M., Hertzog, J., Hertkorn, N., Gougeon, R. D., Hoffmann, V., Hinman, N. W., Ferri\u0026egrave;re, L., Greshake, A., Gabelica, Z., Trif, L. \u0026amp; Steele, A. 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Sci.\u003c/em\u003e \u003cstrong\u003e15\u003c/strong\u003e, d2ee00472k (2022). https://doi.org/10.1039/D2EE00472K\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-8911322/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8911322/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"Organic molecules preserved in Martian meteorites indicate that reduced carbon can form abiotically. Yet, the processes capable of generating and sustaining such material without biological or external energy inputs remain unresolved. Here, we demonstrate that a purely mineral-based system can spontaneously convert CO2 into complex organic matter through self-sustained electrochemical coupling. By embedding TiO2 and Fe3O4 nanoparticles within an amorphous SiO2 matrix, we construct a mineral assemblage that integrates an anodic phase, a cathodic phase and an electrolyte into a single electrochemical circuit. When immersed in CO2-bearing brine, this mineral junction drives directional electron transfer without applied voltage, illumination or heating. The system reduces CO2 to small oxygenated intermediates (e.g., formate, acetate) and subsequently converts them into aromatic-rich macromolecular carbon that accumulates at Fe-rich mineral interfaces. Nanoscale spectroscopic analyses show that these products closely resemble organic matter preserved in Martian meteorites. Unlike previous abiotic carbon synthesis studies that rely on isolated catalysts or external energy sources, this mineral assemblage sustains spontaneous, dark CO2 reduction driven solely by intrinsic mineral redox disequilibria. These findings identify mineral-driven geo-electrochemistry as a previously unrecognized pathway for abiotic carbon fixation and imply that similar processes could have operated on early Mars, early Earth and other water-bearing planetary bodies, while offering a conceptual framework for low-energy mineral-enabled carbon conversion.","manuscriptTitle":"Spontaneous Mineral-driven electrochemical synthesis of organic matter under Martian conditions","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-03-22 15:41:12","doi":"10.21203/rs.3.rs-8911322/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-geoscience","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"ngeo","sideBox":"Learn more about [Nature Geoscience](http://www.nature.com/ngeo/)","snPcode":"","submissionUrl":"","title":"Nature Geoscience","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Research","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"95d64ac2-d894-48dd-ac49-b78246344f4c","owner":[],"postedDate":"March 22nd, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":64807411,"name":"Physical sciences/Energy science and technology/Carbon capture and storage"},{"id":64807412,"name":"Earth and environmental sciences/Planetary science/Geochemistry"}],"tags":[],"updatedAt":"2026-04-28T16:43:02+00:00","versionOfRecord":[],"versionCreatedAt":"2026-03-22 15:41:12","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8911322","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8911322","identity":"rs-8911322","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}