Stable carbon isotope evolution of formaldehyde on early Mars

Organic matter in the Martian sediments may provide a key to understanding the prebiotic chemistry and habitability of early Mars. The Curiosity rover has measured highly variable and ¹³C-depleted carbon isotopic values in early Martian organic matter whose origin is uncertain. One hypothesis suggests the deposition of simple organic molecules generated from ¹³C-depleted CO derived from CO₂ photochemical reduction in the atmosphere. Here, we present a coupled photochemistry-climate evolution model incorporating carbon isotope fractionation processes induced by CO₂ photolysis, carbon escape, and volcanic outgassing in an early Martian atmosphere of 0.5-2 bar, composed mainly of CO₂, CO, and H₂ to track the evolution of the carbon isotopic composition of C-bearing species. The calculated carbon isotopic ratio in formaldehyde (H₂CO) can be highly depleted in ¹³C due to CO₂-photolysis-induced fractionation and is variable with changes in atmospheric CO/CO₂ ratio, surface pressure, albedo, and H₂ outgassing rate. Conversely, CO₂ becomes enriched in ¹³C, as estimated from the carbonates preserved in ALH84001 meteorite. Complex organic matter formed by the polymerization of such H₂CO could explain the strong depletion in ¹³C observed in the Martian organic matter. Mixing with other sources of organic matter would account for its unique variable carbon isotopic values.