A spectral synergy method to retrieve martian water vapor column-abundance and vertical distribution applied to Mars Express SPICAM and PFS nadir measurements

Up to now, all attempts to retrieve martian water vapor from nadir observations have focused their analysis on a single spectral domain and comparison between the results of various experiments showed the difficulty to reconcile the water vapor datasets together. Inspired by a methodology recently developed for the analysis of Earth observations, a spectral synergy approach has been tested on the water vapor extraction from the measurements of two instruments onboard Mars Express. These instruments cover near-infrared (NIR) and thermal infrared (TIR) domains within which water vapor possesses diagnostic absorption/emission signatures. Since the two instruments have operated concomitantly around Mars, co-located measurements could be selected and processed to create a dataset exploitable by synergy. The synergy relies on a Bayesian inference algorithm to find the best fitting values of water vapor and other parameters (e.g. atmospheric and surface temperature, dust opacity) simultaneously for both NIR and TIR intervals. Results demonstrate that synergy augments the information content about water vapor by up to > 50% compared to traditional non-synergistic methods. Not only does the synergy provide an unbiased estimate of the total column abundance of H2O but it also provides a correspondingly unbiased estimate of H2O abundance in the first 5 km of the boundary layer, an atmospheric region long remained unexplored although being known for hosting key mechanisms controlling the fate of volatiles on Mars. Using synergy, the scientific return of nadir observations of current and future missions at Mars (such as the ExoMars Trace Gas Orbiter) can be fully optimized.