Water Vapor on Mars: A Refined Climatology and Constraints on the Near‐Surface Concentration Enabled by Synergistic Retrievals.

With the utilization of a novel synergistic approach, we constrain the vertical distribution of water vapor on Mars with measurements from nadir‐pointing instruments. Water vapor column abundances were retrieved simultaneously with 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. We show that by exploiting a spectral synergy retrieval approach, we obtain more accurate water vapor column abundances compared to when only one instrument is used, providing a new and highly robust reference climatology from Mars Express. We present a composite global data set covering all seasons and latitudes, assembled from colocated observations sampled from seven Martian years. The synergy also 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 single spectral interval. Special attention is given to the north polar region, with extra focus on the sublimation of the seasonal polar cap during the late spring and summer seasons. Column abundances from the Mars Climate Database were found to be significantly higher than synergistically retrieved values, especially in the summer Northern Hemisphere. Deviations between synergy and model in both magnitude and meridional variation of the vertical confinement were also discovered, suggesting that certain aspects of the transport and dynamics of water vapor are not fully captured by current models. Plain Language Summary: Water vapor plays an important role in the weather and climate on Mars, even though little of it remains today. The behavior of water vapor has been studied for decades, yet how water vapor varies with altitude, especially close to the surface, remains an open question. In this study, we use measurements from two instruments on the Mars Express satellite to learn about the near‐surface water vapor. By combining measurements from the SPICAM and PFS spectrometers, a composite full‐year climatology is assembled. We measure the total amount of water vapor with great accuracy, and also obtain information about the vertical distribution. The north polar cap is studied in detail during early summer, when part of the polar ice cap sublimates into water vapor and is transported south. The results are compared to model data from the Mars Climate Database, and significant differences between the observations and the model are identified. The total water content is found to be smaller than model estimates, while observations indicate that more water than expected is confined near the surface. This suggests that some aspects of the atmospheric transport processes are not currently fully understood. Key Points: Using a spectral synergy retrieval method on nadir observations from SPICAM and PFS to obtain a highly accurate water vapor climatologyThe synergy method is sensitive to the vertical distribution of H2O, and can distinguish near‐surface water from the rest of the columnDiscrepancies in meridional and seasonal behavior of vertical confinement are revealed between the synergy and the Mars Climate Database [ABSTRACT FROM AUTHOR]