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.