The rainout of formaldehyde favors a formose-based origin of life in shallow ponds

Formaldehyde (CH ₂ O) is a key building block in prebiotic chemistry, and is central for sugar synthesis via the formose reaction. Atmospheric photochemistry can produce substantial amounts of CH ₂ O that can then be delivered to the ocean or ponds via rainout. To assess the potential for atmospheric CH ₂ O to trigger the formose reaction in subaerial water bodies, we couple photochemical simulations with rainout and aqueous modeling. Our model results demonstrate that the photochemical production of CH ₂ O is confined to similar atmospheric mixing ratios (X _{CH 2 O} ≈10 ^-10 ) and rainout fluxes (Φ _{CH 2 O} ≈3.5×10 ⁸ cm ^-2 s ^-1 ) for a broad range of atmospheric and environmental compositions. Yet, its accumulation in aqueous bodies is fundamentally limited by the Cannizzaro reaction. This competitive pathway rapidly depletes formaldehyde with increasing pH, preventing the steady-state aqueous concentrations (i.e., [CH ₂ O]≥0.01 mol/L) required for sugar synthesis in global oceans. As a result, a global ocean on early Earth was unfavorable for a sustained formaldehyde-driven sugar chemistry. In contrast, shallow water bodies such as volcanic hot spring pools and shallow ponds with large surface-area-to-volume ratios can reach the required concentrations for moderately alkaline pH values. Additional sources of CH ₂ O, such as exogenous delivery and lightning, cannot substantially alter these constraints as photochemistry remains the dominant source of CH ₂ O by orders of magnitude. Our results suggest that the origin of life via formose-like pathways is likely restricted to localized, shallow-water environments on early Earth, a potentially habitable early Venus, and rocky planets in general.