Experimentally investigating polycyclic aromatic hydrocarbon transport in planetary hydrothermal systems: relevance for Earth and Mars

Polycyclic aromatic hydrocarbons (PAHs) are a group of organic compounds that are ubiquitous in terrestrial and aquatic environments on Earth, and have been detected throughout our solar system. The sources of PAH formation are not always well understood as these structures are often convoluted within insoluble organic matter and can be formed via abiotic processes (particularly heat). Hydrothermal systems are a potential PAH source on Earth and other planets due to their thermal regimes and ability to synthesize, preserve, and transport organic matter. On Earth this means that PAHs (which are persistent organic pollutants) can spread and contaminate surrounding environments; and for planetary science, they can contribute to organic molecular complexity of planetary bodies. In this study, we seek to validate the mobility of two commonly studied three ring PAHs, phenanthrene and anthracene, using a continuous-flow reactor containing a Mars regolith simulant. We used nuclear magnetic resonance spectroscopy and ion chromatography to analyze the liquid samples, and visible near infrared spectroscopy and Fourier transform infrared spectroscopy to analyze the regolith samples. Our results demonstrate that phenanthrene undergoes more degradation than anthracene in MilliQ water and synthetic hydrothermal fluid, and this result occurs more quickly with flow than under stagnant conditions. These findings demonstrate that PAHs can be transported beyond the original deposition site even at ambient temperatures and could represent possible PAH degradation mechanisms that occurred on Mars and Earth. Thus, these experimental results also identify conditions directly relevant to PAH remediation on Earth. Conditions such as low-temperature flow, iron-bearing minerals, and moderate ionic strength promote surface-mediated oxidation and adsorption, implying passive or nature-based remediations could enhance PAH attenuation in engineered systems.