Constraints on water vapor content in the lower atmosphere of Mars

By the utilisation of a novel synergistic approach, we constrain the vertical distribution of water vapor on Mars with nadir-pointing instruments. Water vapor column abundances were retrieved simultaneously from PFS (sensing the thermal infrared range) and SPICAM (sensing the near-infrared range) on Mars Express, yielding distinct yet complementary sensitivity to different parts of the atmospheric column. This approach offers a way to study the vertical partitioning of water, which has remained out of the scope of nadir observations made by single instruments covering a specific spectral range. Here we present a global dataset covering all seasons and latitudes, with measurements taken over 8 Martian years. Special focus is given to the high-latitude regions in spring and summer during the sublimation of the seasonal polar cap. A significant deviance was discovered between the retrieved vertical distribution water vapor, and the predicted profile from the Mars Climate Database, which is used as prior. We also show that by exploiting a synergistic retrieval approach, we obtain more accurate water vapor column abundances compared to when only one instrument is used. The synergistic retrieval has a tendency to obtain a lower total column abundance compared to the prior, albeit with a stronger vertical partitioning. This indicates a more prominent low-altitude layer, with more water vapor contained close to the surface than predicted by models.