Oxidative Polymerization of Organic Compounds in the Presence of Iron-Bearing Minerals: Implications for Organic Preservation on Mars

Exploring for evidence of life on Mars involves detecting organic signals that may reflect the prebiotic processes which led to the emergence of life or the molecular remnants of biology. For detection at the present day, organic signals, whether abiotic or biological in origin, must have survived over periods that extend to billions of years. The terrestrial fossil organic record indicates that polymerization enhances preservation over time. One polymer-forming process that is observed on Earth, but may be especially relevant to Mars, is oxidative polymerization, where organic compounds are assembled into polymers or macromolecules by reaction with oxidants. On Earth, oxidative polymerization commonly uses atmospheric oxygen, but mineral oxidants can also promote the reaction. Oxidized iron can act as a polymerizing agent for organic compounds and is widespread on the surface of Mars. For instance, the iron oxyhydroxide goethite (α-FeO(OH)) has been detected on Mars by spectrometers carried by orbiters and rovers. This study examines whether Fe(III)-bearing minerals under Mars-relevant conditions can promote oxidative coupling of simple aromatic compounds. Our results show that in environments representative of the Martian near surface, the ferric/iron(III) content of goethite causes oxidative coupling of 2-naphthol (C₁₀H₈O) to its dimer, 1,1′-bi-2-naphthol (BINOL) and, under certain conditions, to larger fused-ring aromatic products. Recognition of the effective polymerization of organic compounds in the presence of a mineral that is common on Mars provides direction for the ongoing search for preserved Martian organic matter when selection of landing sites or sampling locations is taking place.