Venus water loss is dominated by HCO⁺ dissociative recombination

Despite its Earth-like size and source material^1,2, Venus is extremely dry^3,4, indicating near-total water loss to space by means of hydrogen outflow from an ancient, steam-dominated atmosphere^5,6. Such hydrodynamic escape likely removed most of an initial Earth-like 3-km global equivalent layer (GEL) of water but cannot deplete the atmosphere to the observed 3-cm GEL because it shuts down below about 10-100 m GEL^5,7. To complete Venus water loss, and to produce the observed bulk atmospheric enrichment in deuterium of about 120 times Earth^8,9, nonthermal H escape mechanisms still operating today are required^10,11. Early studies identified these as resonant charge exchange^12-14, hot oxygen impact^15,16 and ion outflow^17,18, establishing a consensus view of H escape^10,19 that has since received only minimal updates²⁰. Here we show that this consensus omits the most important present-day H loss process, HCO⁺ dissociative recombination. This process nearly doubles the Venus H escape rate and, consequently, doubles the amount of present-day volcanic water outgassing and/or impactor infall required to maintain a steady-state atmospheric water abundance. These higher loss rates resolve long-standing difficulties in simultaneously explaining the measured abundance and isotope ratio of Venusian water^21,22 and would enable faster desiccation in the wake of speculative late ocean scenarios²³. Design limitations prevented past Venus missions from measuring both HCO⁺ and the escaping hydrogen produced by its recombination; future spacecraft measurements are imperative.