The project aims at studying the water escape in Mars' atmosphere, both in present and in past conditions where the orbital parameters of Mars were different from those of today.

 It is currently well known that the climates of Mars, Venus and the Earth have followed different evolutionary paths. The Martian case, with evidences of warmer and wetter climate in the past, is very intriguing. Mars primitive atmosphere may have been quite similar to the current terrestrial atmosphere. Understanding how early Mars evolved into the inhabitable planet can provide clues on the response of terrestrial climate to perturbations induced by natural causes or by human activities. A good knowledge of the paleoclimate and habitability conditions of Mars requires a precise determination of the amount of water lost during its history. Atmospheric escape to space is known to play a key role in shaping the evolution of Mars climate at least in the last 3 billion years. So, understanding its variability in current and past Mars conditions is essential to yield a more precise estimation of Mars past climate conditions. A strong link between Mars' water cycle and hydrogen escape to space has been found recently, revealing a previously unknown effect of the lower atmosphere on atmospheric escape. As the instability of Mars orbital parameters has driven very different water cycles in the past, strong variations of hydrogen (i.e. water) escape during the last ~3 billion years are expected. These variations are neglected by current calculations of total escape during Martian history, which must thus be significantly flawed. CLIMARS will provide an unprecedented self-consistent determination of the total water escape through Martian history, including for the first time these orbital effects and a complete water cycle from the sublimation of the polar caps to its escape from the upper atmosphere.