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 updates 20 . 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 scenarios 23 . Design limitations prevented past Venus missions from measuring both HCO + and the escaping hydrogen produced by its recombination; future spacecraft measurements are imperative. Water loss to space late in Venus history is shown to be more active than previously thought, with unmeasured HCO + dissociative recombination dominating present-day H loss.