Stable carbon isotope evolution of formaldehyde and implications for the origin of early Martian organic matter

Organic matter in early Martian sediments may provide a key to understanding prebiotic chemistry and habitability of Mars. The Curiosity rover has reported highly variable and ¹³C-depleted carbon isotope ratios of the organic matter in the early Martian sediments (House et al. 2022). However, its origin is unknown. One of the potential sources is the deposition of formaldehyde (H₂CO), formed by photochemical reduction of CO₂ in the atmosphere (Ueno et al., 2022). Here, we develop a coupled photochemistry-climate evolution model incorporating carbon isotope fractionation induced by CO₂ photolysis, C escape, and volcanic outgassing. This model calculates the evolution of carbon isotope composition in H₂CO in the early Martian atmosphere, beginning with an atmosphere composed of mantle-derived CO, CO₂, and H₂. The calculated evolution of carbon isotopic ratio in H₂CO covers the extensive range observed in Martian organic matter, with its minimum δ¹³C value of ~-200‰. The calculated δ¹³C value of CO₂ at approximately 4 Ga corresponds with those estimated from the analysis of the ALH 84001 meteorite. Our findings suggest that some organic matter in early Martian sediments could have originated from photochemically produced H₂CO. If Mars was frozen but episodically warm in the late Noachian, surface ice containing the atmospheric H₂CO (δ¹³C < -140‰) may have dissolved into Martian water bodies, leading to diverse carbon isotopic ratios in the resulting organic matter.