Abstract
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 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 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 exchange12,13,14, 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-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 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.
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Data availability
Tables containing all reactions used in the model, including their adopted rate coefficients and computed column rates, are provided in a supplementary PDF file accessible on the journal website. These rates are also accessible in the archived code repository listed below, which also includes our adopted photo cross-sections and all other source data used in our model. Model densities for all species, computed rates for reactions shown in Fig. 2, assumed temperature and escape probabilities and computed photo rates are provided in Excel format in the online version of the paper; this file also includes data for our illustrative water-inventory timelines. Source data are provided with this paper.
Code availability
All model code is available at github.com/emcangi/VenusPhotochemistry. The version of the model used to prepare the manuscript is archived on Zenodo at https://doi.org/10.5281/zenodo.10460004.
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Acknowledgements
M.S.C., E.M.C., B.S.G. and R.D.E. were supported by NASA Solar System Workings grant 80NSSC19K0164 and Planetary Science Early Career Award grant 80NSSC20K1081. E.M.C. was also supported by NASA FINESST award 80NSSC22K1326. M.S.C. and E.M.C. thank M. Landis for helpful discussions about water delivery.
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M.S.C. oversaw the study, performed final model calculations and the photochemical equilibrium calculation and wrote the initial text of the paper. E.M.C. developed the H-bearing and D-bearing photochemical model and nonthermal escape calculation originally used at Mars with a reaction network provided by R.V.Y. and performed initial model calculations for Venus. B.S.G. developed and ran the Monte Carlo model to generate escape probability curves. R.D.E. initially developed the Monte Carlo escape model with support from J.D. and H.G. H.G. performed pilot studies of HCO+-driven loss in the Mars atmosphere. All authors contributed to the interpretation and presentation of model results.
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Extended data figures and tables
Extended Data Fig. 1 Model densities for all species.
The six panels function only to separate species for clarity.
Extended Data Fig. 2 Key photochemical model inputs.
a, Temperature profiles for neutrals, ions and electrons adapted from the inputs in ref. 28. b, Adopted eddy diffusion profile and molecular diffusion coefficients for H and O atoms.
Extended Data Fig. 3 Implications of HCO+-driven loss for Venus ocean scenarios.
a, Escaping H production rates for the two most important processes in our model. b, Schematic water loss history of Venus.
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This file contains Supplementary Methods and Supplementary Tables. Merged PDF containing tables of reactions used in the model, assumed reaction rate coefficients and computed equilibrium model column rates.
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Chaffin, M.S., Cangi, E.M., Gregory, B.S. et al. Venus water loss is dominated by HCO+ dissociative recombination. Nature 629, 307–310 (2024). https://doi.org/10.1038/s41586-024-07261-y
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DOI: https://doi.org/10.1038/s41586-024-07261-y
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