Water Vapor and HCl Vertical Distribution in Mars as Measured by TGO/NOMAD Solar Occultations

NOMAD [1] (Nadir and Occultation for MArs Discovery) is a multi-channel spectrometer onboard the ExoMars 2016 Trace Gas Orbiter (TGO), operating from Martian orbit since April 2018. Among other two, the Solar Occultation (SO) channel covers the infrared (IR) spectrum from 2.3 to 4.3 µm (2320 to 4350 cm⁻¹). The design of NOMAD SO allows for a vertical sampling of typically 1 km. Its high spectral resolution (λ/∆λ∼17000) and its relatively high signal to noise ratio (∼2500), make this instrument suitable for the detection of trace species in the Martin atmosphere such as water vapor (H₂O) or hydrogen chloride (HCl).

Here we present vertical profiles of H₂O and HCl obtained during six continuous Earth years of NOMAD SO observations. The retrievals have been performed with an inversion scheme combining pairs of diffraction orders in the case of water vapor, following up and improving several previous studies [2]. In the case of HCl, we used multiple detector bins, retrieving an independent vertical profile form each bin in order to obtain robust detection of this species. This set up allowed us sounding water vapor up to about 120 km altitude and HCl up to 60 km. This study presents the most extended data set of water vapor measurements from the NOMAD instrument to date, and an ambitious data set of HCl observations. Covering three full and consecutive Martian Years, observations from April 2018 to December 2023 were analyzed, making a total of more than 7000 H₂O and more than 2500 HCl vertical profiles ranging from the perihelion of Mars Year (MY) 34 to the aphelion of MY 37. We show consistent seasonal and latitudinal water vapor patterns, with H₂O systematically being more vertically extended during the perihelion season than during the aphelion. In addition, we present an analysis of the water vapor local time variability, confirming overall larger abundances during the evenings than during mornings, and an extensive comparison of our NOMAD results with other water vapor data sets from TGO and with the Mars Planetary Climate Model (MPCM), applying clustering analysis techniques to water vapor vertical profiles for the first time on Mars. Regarding HCl, although until now considered to be a negligible compound in the Martian atmosphere [3, 4], it has been detected systematically by two instruments onboard TGO: the Atmospheric Chemistry Suite (ACS) [5] and more recently NOMAD [6]. Here we present the latest HCl vertical profiles and the seasonal variability of this species from a climatological point of view, revealing possible links with water vapor and dust.

References:
[1] Vandaele, A. C. et al. 2018, Space Science Reviews 214, 1–47. https://doi.org/10.1007/s11214-018-0517-2
[2] Brines, A. et al. 2023, Journal of Geophysical Research: Planets 128, e2022JE007273. https://doi.org/10.1029/2022JE007273
[3] Hartogh, P et al. 2010, Astronomy & Astrophysics 521, L49. https://doi.org/10.1051/0004-6361/201015160
[4] Villanueva, G. et al. 2013, Icarus 223, 11–27. https://doi.org/10.1016/j.icarus.2012.11.013
[5] Korablev, O. et al. 2021, Science Advances 7, eabe4386. https://doi.org/10.1126/sciadv.abe4386
[6] Aoki, S. et al. 2021, Geophysical Research Letters 48, e2021GL092506. https://doi.org/10.1029/2021GL092506