1. Swedish Institute of Space Physics, S-98128 Kiruna, Sweden
2. Centre d'Étude Spatiale des Rayonnements, BP-44346, F-31028 Tolouse, France
3. IGPP, University of California, Los Angeles, California 90095, USA
4. Space Research Institute, Austrian Academy of Science, A-8042 Graz, Austria
5. Institute of Space and Astronautical Science, 3-1-1 Yoshinodai, Sagamihara 229-8510, Japan
6. Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK
7. University of Arizona, Tucson, Arizona 85721, USA
8. University of Wales Aberystwyth, Penglais, Aberystwyth, Ceredigion SY23 3BZ, UK
9. Department of Physics, West Virginia University, Morgantown, West Virginia 26506-6315, USA
10. Department of Physical Sciences, University of Helsinki, Box 64, 00014 Helsinki, Finland
11. Finnish Meteorological Institute, Box 503, FIN-00101 Helsinki, Finland
12. Space Physics Research Laboratory, University of Michigan, Ann Arbor, Michigan 48109-2143, USA
13. Max-Planck-Institut für Sonnensystemforschung, Max-Planck-Str. 2, D-37191 Katlenburg-Lindau, Germany
14. Space Science Laboratory, University of California, Berkeley, California 94720-7450, USA
15. Space Technology Ireland, National University of Ireland, Maynooth, Co. Kildare, Ireland
16. Instituto di Fisica dello Spazio Interplanetari, I-00133 Rome, Italy
17. Applied Physics Laboratory, Johns Hopkins University, Laurel, Maryland 20723-6099, USA
18. KFKI Research Institute for Particle and Nuclear Physics, PO Box 49, H-1525 Budapest 114, Hungary
19. Southwest Research Institute, San Antonio, Texas 78228-0510, USA
20. University of Bern, Physikalisches Institut, CH-3012 Bern, Switzerland

Correspondence to: S. Barabash¹ Correspondence and requests for materials should be addressed to S.B. (Email: stas@irf.se).

Abstract

Venus, unlike Earth, is an extremely dry planet although both began with similar masses, distances from the Sun, and presumably water inventories. The high deuterium-to-hydrogen ratio in the venusian atmosphere relative to Earth's also indicates that the atmosphere has undergone significantly different evolution over the age of the Solar System¹. Present-day thermal escape is low for all atmospheric species. However, hydrogen can escape by means of collisions with hot atoms from ionospheric photochemistry², and although the bulk of O and O₂ are gravitationally bound, heavy ions have been observed to escape³ through interaction with the solar wind. Nevertheless, their relative rates of escape, spatial distribution, and composition could not be determined from these previous measurements. Here we report Venus Express measurements showing that the dominant escaping ions are O⁺, He⁺ and H⁺. The escaping ions leave Venus through the plasma sheet (a central portion of the plasma wake) and in a boundary layer of the induced magnetosphere. The escape rate ratios are Q(H⁺)/Q(O⁺) = 1.9; Q(He⁺)/Q(O⁺) = 0.07. The first of these implies that the escape of H⁺ and O⁺, together with the estimated escape of neutral hydrogen and oxygen, currently takes place near the stoichometric ratio corresponding to water.

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