Venus water loss is dominated by HCO+ dissociative recombination.

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Despite its Earth-like size and source material1,2, Venus is extremely dry3,4, indicating near-total water loss to space by means of hydrogen outflow from an ancient, steam-dominated atmosphere5,6. Such hydrodynamic escapelikely 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 GEL5,7. To complete Venus water loss, and to produce the observed bulk atmospheric enrichment in deuterium of about 120 times Earth8,9, nonthermal H escape mechanisms still operating today are required10,11. Early studies identified these as resonant charge exchange12-14, hot oxygen impact15,16 and ion outflow17,18, establishing a consensus view of H escape10,19 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-dayvolcanic water outgassing and/orimpactor 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 water21,22 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.