%0 Journal Article %T Reappraising the production and transfer of hydrogen atoms from the middle to the upper atmosphere of Mars at times of elevated water vapor %+ PLANETO - LATMOS %+ Space Research Institute of the Russian Academy of Sciences (IKI) %+ Department of Physics [Oxford] %+ School of Physical Sciences [Milton Keynes] %+ Laboratory for Atmospheric and Space Physics [Boulder] (LASP) %A Montmessin, Franck %A Belyaev, Denis, A. %A Lefèvre, Franck %A Alday, Juan %A Vals, Margaux %A Fedorova, Anna, A. %A Korablev, Oleg, I. %A Trokhimovskiy, Alexander %A Chaffin, Michael, S. %A Schneider, Nicholas, M. %Z ExoMars is the space mission of ESA and Roscosmos. The ACS investigation was developed by the Space Research Institute (IKI) in Moscow, and the Laboratoire Atmospheres, Milieux, Observations Spatiales in Guyancourt. The investigation was funded by Roscosmos, the National Centre for Space Studies of France (CNES) and RSF (Russian Science Foundation 20-42-0903). This work was funded by CNES, the Agence Nationale de la Recherche (ANR, PRCI, CE31 AAPG2019, MCUBE project), DIM ACAV labelled by the Ile-de-France region in support for the research (Domaine d’Intérêt Majeur, Astrophysique et Conditions d’Apparition de la Vie). %< avec comité de lecture %@ 2169-9097 %J Journal of Geophysical Research. Planets %I Wiley-Blackwell %V 127 %N 5 %P e2022JE007217 %8 2022 %D 2022 %R 10.1029/2022JE007217 %K Water Vapor %K Mars %K Hydrogen Atoms %K Planetary Atmosphere %K Upper Atmosphere %Z Sciences of the Universe [physics]Journal articles %X Water escape on Mars has recently undergone a paradigm shift with the discovery of unexpected seasonal variations in the population of hydrogen atoms in the exosphere where thermal escape occurs and results in water lost to space. This discovery led to the hypothesis that, contradicting the accepted pathway, atomic hydrogen in the exosphere was not only produced by molecular hydrogen but mostly by high altitude water vapor. Enhanced presence of water at high altitude during southern spring and summer, due to atmospheric warming and intensified transport, favors production of H through photon-induced ion chemistry of water molecules and thus appears to be the main cause of the observed seasonal variability in escaping hydrogen. This hypothesis is supported by the observation of large concentrations of water vapor between 50 km and 150 km during the southern summer solstice and global dust events. Using a simplified yet representative air parcel transport model, we show that in addition to the formation of atomic hydrogen from water photolysis above 80 km, a major fraction of the exospheric hydrogen is formed at altitudes as low as 60 km and is then directly advected to the upper atmosphere. Comparing the injection modes of a variety of events (global dust storm, perihelion periods, regional storm), we conclude that southern spring/summer controls H production and further ascent into the upper atmosphere on the long term with direct implication for water escape. %G English %2 https://insu.hal.science/insu-03660797v2/document %2 https://insu.hal.science/insu-03660797v2/file/JGR%20Planets%20-%202022%20-%20Montmessin%20-%20Reappraising.pdf %L insu-03660797 %U https://insu.hal.science/insu-03660797 %~ INSU %~ CNRS %~ LATMOS %~ UVSQ %~ UNIV-PARIS-SACLAY %~ SORBONNE-UNIVERSITE %~ SORBONNE-UNIV %~ SU-SCIENCES %~ UVSQ-UPSACLAY %~ SU-TI %~ ANR %~ GS-ENGINEERING %~ GS-GEOSCIENCES %~ GS-PHYSIQUE %~ INSTITUT-SCIENCES-LUMIERE %~ ALLIANCE-SU