Mars water vapor vertical distributions from the troposphere to the mesosphere from NOMAD Solar Occultation for Martian Years 34 and 35
Authors: Brines Adrian , Lopez Valverde Miguel Ángel , Stolzenbach Aurélien , Modak Ashim , Funke Bernd , Gonzalez Galindo Francisco , Lopez Moreno Jose Juan , Aoki Shohei , Vandaele Ann Carine , Daerden Frank , Thomas Ian , Erwin Justin , Trompet Loïc , Ristic Bojan , Villanueva Gerónimo Luis , Liuzzi Giuliano , Patel Manish , Bellucci Giancarlo 
Published: September 2022

&lt;p&gt;&amp;#160; &amp;#160; NOMAD [1] (Nadir and Occultation for MArs Discovery) is a multi-channel spectrometer onboard the ExoMars 2016 Trace Gas Orbiter (TGO), which began its observations in April 2018. The Solar Occultation (SO) channel has a spectral range coverage from 2.3 to 4.3 &amp;#956;m (2320 to 4350 cm&lt;sup&gt;-1&lt;/sup&gt;), where numerous ro-vibrational bands of H2O are located. An Acousto-Optical Tunable Filter (AOTF) is used to select different spectral windows (with a typical width from 20 to 35 cm&lt;sup&gt;-1&lt;/sup&gt;) corresponding to the desired diffraction orders to be used during an atmospheric scan. The SO channel has a sample rate of &amp;#8764;1 s, that allows a vertical sampling of typically 1 km. In order to obtain as much information as possible from the Martian atmosphere, for this study we have analyzed data from diffraction orders 134 (3011-3035 cm&lt;sup&gt;-1&lt;/sup&gt;), 136 (3056-3081 cm&lt;sup&gt;-1&lt;/sup&gt;), 168 (3775-3805 cm&lt;sup&gt;-1&lt;/sup&gt;) and 169 (3798-3828 cm&lt;sup&gt;-1&lt;/sup&gt;), taken during different solar occultation scans since the beginning of the science operations phase of the TGO/ExoMars mission. This combination of diffraction orders allowed us to obtain an optimal mapping of the distribution of the water vapor at altitudes from &amp;#8764;10 km up to about &amp;#8764;100 km and in different atmospheric conditions, and hence, permitting the study of its spatial and seasonal variability.&lt;/p&gt;&lt;p&gt;The data presented here have been analyzed with pre-processing and cleaning tools developed entirely at the IAA (see companion contributions to this conference [2-4]), and then, inverted using a state-of-the-art retrieval scheme [5]. For the inversion, we have introduced a novel strategy of combining (when coincident in the same solar occultation) pairs of NOMAD orders (in contrast to a post-processing strategy), using orders 134 or 136 for the lower atmosphere, commonly below 60 km, and orders 168 or 169 for the upper atmosphere. This combination procedure allows us to exclude heavily saturated absorption lines present at low altitudes in orders 168-169, which contain strong absorption lines, permitting the exploitation of these orders for the analysis of the atmosphere up to 100 km. Here we present and discuss detailed maps showing the vertical distribution of the water vapor abundance of a subset of the solar occultations observed during the first two years of TGO/ExoMars. This research confirms some findings also obtained by other teams within the NOMAD consortium [6-8] and ACS instrument [9], showing that dust storms allow water vapor to reach higher altitudes in the atmosphere. This is important for the escape of atomic hydrogen [10] and therefore, for the long-term evolution of the Martian atmosphere. Detailed studies on the characterization of the Martian hygropause and saturation events are also presented.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&lt;strong&gt;Acknowledgments&lt;/strong&gt;&lt;br&gt;The IAA/CSIC team acknowledges financial support from the State Agency for Research of the Spanish MCIU through the 'Center of Excellence Severo Ochoa' award for the Instituto de Astrof&amp;#237;sica de Andaluc&amp;#237;a (SEV- 2017-0709) and funding by grant PGC2018-101836-B-100 (MCIU/AEI/FEDER, EU). ExoMars is a space mission of the European Space Agency (ESA) and Roscosmos. The NOMAD experiment is led by the Royal Belgian Institute for Space Aeronomy (IASB-BIRA), assisted by Co-PI teams from Spain (IAA-CSIC), Italy (INAF-IAPS), and the United Kingdom (Open University). US investigators were supported by the National Aeronautics and Space Administration.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&lt;strong&gt;References&lt;/strong&gt;&lt;br&gt;1. Vandaele, A. C. et al. NOMAD, an integrated suite of three spectrometers for the ExoMars Trace Gas mission: technical description, science objectives and expected performance. Space Science Reviews 214, 1-47 (2018).&lt;br&gt;2. L&amp;#243;pez-Valverde, M. A. et al. Temperature and density profiles from the two solar occultation instruments NOMAD and ACS during a full Martian Year. EPSC (2022).&lt;br&gt;3. Modak, A. et al. Mapping of Martian CO from NOMAD solar occultation measurements for MY35 and 36. EPSC (2022).&lt;br&gt;4. Stolzenbach, A. et al. Composition and size of Martian aerosols as seen in the IR by NOMAD-SO experiment onboard TGO. EPSC (2022).&lt;br&gt;5. Jurado Navarro, A. A. et al. Retrieval of CO2 and collisional parameters from the MIPAS spectra in the earth atmosphere (2016).&lt;br&gt;6. Aoki, S et al. Water vapor vertical profiles on Mars in dust storms observed by TGO/NOMAD. Journal of Geophysical Research: Planets (2019).&lt;br&gt;7. Aoki, S. et al. Global vertical distribution of water vapor on Mars: Results from 3.5 years of ExoMarsTGO/NOMAD science operations. submitted (2022).&lt;br&gt;8. Villanueva, G. L. et al. Water heavily fractionated as it ascends on Mars as revealed by ExoMars/NOMAD. Science Advances 7, eabc8843 (2021).&lt;br&gt;9. Fedorova, A. A. et al. Stormy water on Mars: The distribution and saturation of atmospheric water during the dusty season. Science 367, 297-300 (2020).&lt;br&gt;10. Chaffin, M., Deighan, J, Schneider, N. &amp; Stewart, A. Elevated atmospheric escape of atomic hydrogen from Mars induced by high-altitude water. Nature geoscience 10, 174-178 (2017).&lt;/p&gt;