New 1.38-µm Water Vapor Band Spectroscopy for a CO2 Atmosphere: H2O Measurements in the Martian Atmosphere in the SPICAM/MEX and ACS NIR/TGO Experiments

The H2O 1.38-µm band has been used to measure water vapor in the Martian atmosphere since the MAWD (Mars Atmospheric Water Detector) experiment on Viking-1 and -2. Currently, two experiments in orbit around Mars—SPICAM IR (SPectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars InfraRed) on Mars Express and ACS NIR (Atmospheric Chemistry Suite Near InfraRed) on the Trace Gas Orbiter—are measuring water vapor in this spectral range. The spectrometers provide monitoring of the seasonal cycle of the column water vapor abundance and its vertical distribution over several Martian years. The HITRAN (High Resolution Transmission) database was used as a spectroscopic information for water vapor retrievals in these experiments. To take into account the broadening of water vapor lines in the carbon dioxide atmosphere of Mars, a scaling factor of 1.7 was adopted relative to the broadening by air, presented in HITRAN. This could lead to systematic uncertainty in the results, even despite the low pressure in the Martian atmosphere. Recent laboratory measurements of the broadening of water vapor lines in CO2 for the lines of three vibrational bands ν1 + ν3, 2ν2 + ν3, and 2ν1 in the spectral region of 6760–7430 cm–1 have improved the spectroscopic parameters for the carbon dioxide atmosphere. We performed water vapor retrievals with new spectroscopy at 1.38 µm for SPICAM IR nadir measurements and ACS NIR occultation measurements. In the case of SPICAM IR, changes due to spectroscopy were below the sensitivity of the instrument due to the low resolution and signal-to-noise ratio. For the ACS NIR, which is a high resolution spectrometer, the new spectroscopy resulted in systematic deviations of 2–5% depending on the height in the atmosphere, exceeding the random errors of the instrument.