Highway to Space: the Direct Connection between the Lower and the Upper Atmosphere of Mars sheds a New Light on the History of Water. F. Montmessin 1 , F. Lefèvre 1 , O. Korablev 2,3 , A. Fedorova 2,3 , J.-L. Bertaux 1 , J.-Y. Chaufray 1 , M. Chaffin 3 , N. Schneider 3 , L. Maltagliati 1 , A. Määttänen 1 , A.V. Trokhimovsky 2,3 , 1 CNRS LATMOS, 11 bd d’Alembert, 78280 Guyancourt, France, 2 IKI, Space Science Institute, Moscow, Russia, 3 MIPT, Dolgoprudnyi, Russia, 4 LASP, Boulder, Colorado. Introduction: The SPICAM experiment onboard Mars Express has accumulated over the last decade a wealth of observations that has permitted a detailed characterization of the atmospheric composition and activity from the near-surface to above the exosphere. Here, we present a synthesis of the observations col- lected to date in order to assemble a single, coherent picture of the Martian atmosphere specifically adress- ing the issue of water decomposition into its lighter component (hydrogen) that can escape to space. In doing so, we propose a different angle for the long- term evolution of water and hydrogen on Mars. Observations Modes: SPICAM is dual ultraviolet (110 to 320 nm)-infrared (1 to 1.7 µm) spectrometer specifically designed to retrieve the major and minor species abundances of the Martian atmosphere [1]. SPICAM has the distinct capability of observing with a variety of geometrical configurations; monitoring the column-integrated abundances of ozone, water vapor as well as aerosols in a nadir-looking mode (Figure 1), characterizing their vertical distribution in either stellar or solar occultation modes so as to constrain their pres- ence above typically 10 km up to 140 km of altitude (for CO 2 ) [2,3,4]. In a dedicated limb staring mode, SPICAM can infer the density of hydrogen atoms from 200 up to 10 000 km of altitude while using the reso- nantly scattered solar photons at the Lyman-alpha emission line of Hydrogen [5,6] Figure 1: The multi-annual monitoring made by SPICAM for ozone, water vapeur, aerosols and molec- ular oxygen emission from nadir observations. Datasets: Since the beginning of its operations at Mars, SPICAM has performed several thousands of stellar and solar occultations and has provided a multi- annual tracking in nadir mode (Figure 1). The study presented here focuses solely on the datasets character- izing the vertical distribution of the species of interest. Water vapor profiles. The study presented in [7] follows for the first time the evolution of water vapor profiles during a full martian year. 120 profiles, ob- tained by SPICAM were retrieved that cover the north- ern spring-summer season and the southern spring of Mars Year (MY) 29. The seasonal evolution of water vapor mixing ratio vertical distribution reveals a strong dynamism, especially during southern spring, that is not predicted by models. The measured profiles exhibit often abrupt temporal variations and a great variety of shapes, with the frequent presence of detached layers (Figure 2). The water vapor vertical distribution is more reactive than expected to regional perturbations, which can propagate rapidly through the atmosphere, create abrupt water vapor and aerosol upsurges and influence the large-scale vertical evolution of these two constituents. This phenomenon has been observed sev- eral times during MY29. Figure 2: (from [7]). A collection of water vapor profiles obtained by SPICAM during MY29 showing the prominent presence of detached layers up to 80km. The polar ozone dichotomy. At low-to-mid lati- tudes, martian ozone is distributed vertically in two main layers, a near-surface layer and a layer at an alti- tude between 30 and 60 km. In [8] evidence is reported for the existence of a previously overlooked ozone layer that emerges in the southern polar night at 40–60 km in altitude, with no counterpart observed at the north pole (Figure 3). Comparisons with global climate simulations for Mars indicate that this layer forms as a result of the large-scale transport of oxygen-rich air