Stable carbon isotope evolution of formaldehyde on early Mars

Area of Science:

• Astrobiology
• Planetary Science
• Geochemistry

Background:

• Organic matter in Martian sediments is key to understanding early Mars' habitability and prebiotic chemistry.
• Curiosity rover data reveals highly variable, 13C-depleted carbon isotopes in Martian organic matter of uncertain origin.
• A leading hypothesis suggests simple organic molecules formed from 13C-depleted carbon monoxide (CO) derived from atmospheric carbon dioxide (CO2) photolysis.

Purpose of the Study:

• To model the evolution of carbon isotopic composition in early Mars' atmosphere.
• To investigate the role of CO2 photolysis, carbon escape, and volcanic outgassing in carbon isotope fractionation.
• To explain the origin of 13C-depleted organic matter observed on Mars.

Main Methods:

• Developed a coupled photochemistry-climate evolution model for early Mars.
• Incorporated carbon isotope fractionation processes including CO2 photolysis and volcanic outgassing.
• Tracked the evolution of carbon isotopic composition in carbon-bearing species under early Martian atmospheric conditions (0.5-2 bar CO2, CO, H2).

Main Results:

• Photochemical reduction of CO2 can generate highly 13C-depleted formaldehyde (H2CO).
• Carbon isotopic ratios in H2CO are sensitive to atmospheric CO/CO2 ratio, surface pressure, albedo, and H2 outgassing.
• CO2 becomes enriched in 13C, consistent with meteorite data (ALH84001).

Conclusions:

• Polymerization of photochemically produced H2CO can explain the 13C-depleted Martian organic matter.
• Variability in carbon isotopes suggests mixing with other organic matter sources.
• The findings support a plausible abiotic pathway for organic matter formation on early Mars.