Martian water vapor vertical profiles from solar occultation measurements by NOMAD onboard TGO/ExoMars: H2O-Temperature retrievals with the IAA-KOPRA forward model

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 μm (2320 to 4350 cm^-1). An Acousto-Optical Tunable Filter (AOTF) is used to select different spectral windows (with a width that varies from 20 to 35 cm^-1) corresponding to the desired diffraction orders to be used during the atmospheric scan. The SO channel has a sample rate of ∼1 s, that allows a vertical sampling of ∼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^-1) and 168 (3775-3805 cm^-1), taken simultaneously during different solar occultation scans within the first year of measurements. This combination of diffraction orders allowed us to explore the distribution of the water vapor at atmospheric tangent altitudes from ∼10 km up to about ∼100 km and in different atmospheric conditions, but also to study its spatial and seasonal variability.

Here we present the water vapor vertical profiles of a subset of the solar occultations observed during the first year of TGO/ExoMars, including data taken during the 2018 Global Dust Storm (GDS) and during the 2019 local dust storm. Similar studies have been done by [2] and [3], showing that dust storms allow water vapor to reach higher altitudes in the atmosphere. Also, thanks to this phenomenon, escape of atomic hydrogen has a relevant role in the planetary evolution, as [4] shows. The data presented here have been analyzed with pre-processing and cleaning tools developed entirely at the IAA, and then, have been inverted using a state-of-the-art retrieval scheme [5]. Our method allows us to use consistent temperature profiles during the inversion i.e., obtained from inversion of other diffraction orders’ measurements of the same scan (see companion contributions to this conference [6], [7], [8]). This impose the limitation to use a dataset where CO₂ orders and H₂O orders have been observed simultaneously. Also, synthetic spectra have been generated mimicking the NOMAD SO behavior as a first step of a comprehensive error analysis. Preliminary results will also be presented, showing water vapor vertical profiles and estimations of hydrogen escape. Comparisons with results of other groups in the NOMAD team will be shown [9], [10].

Acknowledgments
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 Astrofísica de Andalucía (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.

References
[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).
[2] Aoki, S. et al. Water vapor vertical profiles on Mars in dust storms observed by TGO/NOMAD. Journal of Geophysical Research: Planets (2019).
[3] 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).
[4] Chaffin, M., Deighan, J., Schneider, N. & Stewart, A. Elevated atmospheric escape of atomic hydrogen from Mars induced by high-altitude water. Nature geoscience 10, 174-178 (2017).
[5] Jurado Navarro, A. A. et al. Retrieval of CO₂ and collisional parameters from the MIPAS spectra in the earth atmosphere (2016).
[6] López-Valverde, M. A. et al. CO₂ and Temperature vertical profiles in the Martian atmosphere from solar occultation measurements at 2.7 μm by instruments NOMAD and ACS on board the Exomars Trace Gas Orbiter. EPSC (2021).
[7] Modak, A. et al. Retrieval of Martian CO vertical profiles from NOMAD solar occultation measurements. EPSC (2021).
[8] Stolzenbach, A. et al. Vertical profiles of Martian aerosols nature and distribution parameters retrievals from NOMAD-SO. EPSC (2021).
[9] Aoki, S. et al. Water vapor vertical distributions on Mars: Results from three years of TGO/NOMAD science operations (EPSC2021-153). EPSC (2021).
[10] Villanueva, G. L. et al. Water heavily fractionated as it ascends on Mars as revealed by ExoMars/NOMAD. Science Advances 7, eabc8843 (2021).