Global Distribution of Atmospheric Formaldehyde Deposition Correlated with Water Vapor on a Warm Early Mars

Abstract On early Mars, transient warm periods during the late Noachian and early Hesperian were likely triggered by episodic inputs of reducing greenhouse gases, such as H 2 . In such a weakly reducing atmosphere, formaldehyde (H 2 CO), a precursor for the abiotic formation of sugars via the formose reaction, can be effectively produced. While this pathway is a candidate source for Martian organic matter, previous 1D photochemical modeling could not predict the spatial distribution of organic synthesis. In this study, we developed a lookup table of H 2 CO production based on key atmospheric variables using a photochemical model and coupled it with a global climate model. We calculated the global distribution of the atmospheric deposition fluxes of H 2 CO under a 2 bar CO 2 atmosphere containing 6% H 2 with obliquity of 40°. Our results reveal a strong positive correlation between atmospheric H 2 CO deposition and H 2 O vapor abundance, driven by both the enhanced photochemical production of H 2 CO through H 2 O photolysis and the efficient scavenging by precipitation in humid regions. Significant atmospheric deposition of H 2 CO is observed over water bodies, including the northern ocean. Furthermore, atmospheric deposition hotspots emerge around topographic highs, such as the Tharsis and Elysium mons, due to orographic precipitation. We suggest that basins adjacent to persistently humid regions, which likely experienced H 2 CO accumulation and subsequent organic synthesis, could be potential landing sites for future missions to investigate ancient chemical evolution and whether it led to the origin of life.