Towards a new climatology of Martian water vapor column abundance derived from NOMAD LNO nadir full mission observations

Water vapor (H₂O) is a highly variable minor constituent of the Martian atmosphere. Through its well characterized seasonal cycle, it controls, together with CO₂ and dust cycles, the present Mars climate. In particular, H₂O plays a crucial role in the cloud formation and therefore can significantly impact the Mars radiative budget. In terms of photochemistry, it also affects the oxidizing capacity of the Martian atmosphere by providing OH radicals via its photolysis.

Since the end of 1970s, H₂O vertical distribution and column abundance have been observed by several space-borne infrared instruments operating either in solar occultation, limb, and/or nadir viewing modes (e.g. Viking 1-2/MAWD, Mars Global Surveyor/TES, Mars Express/PFS and SPICAM, TGO/NOMAD and ACS; see [1] and references therein). Based on these observational data sets and in combination with Global Circulation Models (GCM; see e.g. [2] and [3]), the seasonal and latitudinal variability of the water vapor abundance is now mostly understood.

Here we present a new climatology of Martian water vapor column density derived from NOMAD LNO dayside nadir observations. LNO (Limb Nadir and solar Occultation) is one of the two echelle grating infrared spectrometers installed on the NOMAD (Nadir and Occultation for MArs Discovery) instrument aboard the ESA ExoMars Trace Gas Orbiter (TGO). In these spectrometers, the echelle grating is combined with an Acousto-Optic Tunable Filter (AOTF) for the spectral window selection [4]. Since April 2018, LNO primarily measures H₂O and CO in the Martian atmosphere from the nadir viewing mode. However, despite its high scientific value (more than 7 years of globally distributed observations), the LNO dataset has been largely underexploited since only the first Martian year of the mission has been analysed so far (see [5] and [6]).

The LNO nadir H₂O vertical column densities over the full mission period are retrieved through an Optimal Estimation approach [7] using the ASIMUT-ALVL radiative transfer tool [8]. ASIMUT-ALVL is applied separately to LNO reflectance factor spectra from three diffraction orders around the 2.6 µm water absorption band: orders 167 (3754-3784 cm^-1), 168 (3776-3806 cm^-1), and 169 (3799-3829 cm^-1). Altitude, pressure, temperature, H₂O, CO₂, aerosol dust, and water ice a priori vertical profiles are extracted at the spectra locations from the GEM-Mars GCM [3]. Emissivity spectra are taken from a home-made climatology based on [9] and [10]. Scattering by dust and water ice particles is taken into account in the forward simulations by using the LIDORT radiative transfer model [11] included in the ASIMUT-ALVL tool. A new evaluation of the LNO AOTF function and the temperature dependence of its central position has been carried out and these new calibration data are used in our H₂O retrieval.

In this presentation, we will show the retrieval results over the full mission period and investigate the impact of the ASIMUT-ALVL settings on the retrieved H₂O vertical column densities, the consistency between the three selected diffraction orders, and the level of agreement with other co-located observational data sets (LNO, SPICAM, and EMM-HOP/EMIRS from [5], [12], and [13],  respectively). The effect of the new calibration data on the LNO water vapor retrievals will be also discussed. In the last part of the presentation, the H₂O seasonal, inter-annual, and latitudinal patterns will be assessed over the full mission period.

Acknowledgements

The NOMAD experiment is led by the Royal Belgian Institute for Space Aeronomy (IASB-BIRA) with co-PI teams from Spain (IAA-CSIC), Italy (INAF-IAPS) and the United Kingdom (Open University). This project acknowledges funding by: the Belgian Science Policy Office (BELSPO) with the financial and contractual coordination by the ESA Prodex Office (PEA 4000103401, 4000121493, 4000140753, 4000140863); by the Spanish Ministry of Science and Innovation (MCIU) and European funds (grants PGC2018-101836-B-I00 and ESP2017-87143-R; MINECO/FEDER), from the Severo Ochoa (CEX2021-001131-S) and from MCIN/AEI/10.13039/501100011033 (grants PID2022-137579NB-I00, RTI2018-100920-J-I00 and PID2022-141216NB-I00); by the UK Space Agency (grants ST/V002295/1, ST/V005332/1, ST/X006549/1, ST/Y000234/1 and ST/R003025/1); and by the Italian Space Agency (grant 2018-2-HH.0).

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