Discoveries on the vertical distribution of water vapor in Mars' atmosphere as observed by the SPICAM-IR spectrometer

The vertical distribution of water vapor in the atmosphere of Mars is a crucial, and sometimes the only, diagnostics of several important processes for the hydrological cycle of the planet. Up to now, however, water vapor vertical profiles have been directly observed only by ISM/Phobos, with very limited spatial and temporal coverage (Rodin et al. 1997). Our current knowledge relies above all on General Circulation Models, which employ strong assumptions. The H2O vertical profile is considered to be mainly controlled by atmospheric temperature, through saturation physics, and it is usually schematized as a step function: well-mixed below the saturation height and abruptly declining to zero above it. In this work we test the current vertical knowledge of water vapor by analyzing the SPICAM dataset. The SPICAM spectrometer onboard Mars Express is perfectly suited to study the vertical structure of Mars’ atmosphere, thanks to its solar and stellar occultation mode of observation both in IR and UV (Lebonnois et al. 2006, Fedorova et al. 2009). It has been monitoring the planet for more than three Martian years, providing a series of measurements unprecedented for quality and spatial and temporal coverage. In particular, solar occultations in the IR channel allows the simultaneous retrieval of H2O, CO2 and aerosols atmospheric profiles. The evolution and behavior of the vertical distribution of gaseous H2O for a whole Martian year will be shown, and the discrepancies with GCM predictions will be highlighted. Particular attention will be devoted to describe and explain the behavior regarding the saturation state of the atmosphere. The interaction between the water and dust cycles will be examined, thanks to the simultaneous observations of water and aerosols. The observations trace a consistent picture of the vertical behavior of water vapor. They point towards a new paradigm, which stresses the importance of water-dust interactions with respect to the role of atmospheric thermal structure. References: Fedorova A. et al - Icarus, vol. 200, p. 96-117 (2009) Lebonnois S. et al. - JGR, vol. 111 E9 (2006) Rodin A. et al. - Icarus, vol. 125, p. 212-229 (1997)