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We will investigate the impact of day-night temperature and compositional gradients at the Martian terminator on the retrieval of vertical profiles of ozone obtained from NOMAD-UVIS solar occultations. Rapid variations in species concentration at the terminator have the potential to cause asymmetries in the species distributions along the line of sight of a solar occultation experiment. Ozone, in particular, displays steep gradients across the terminator of Mars due to photolysis [1]. Nowadays, most of the retrieval algorithms for solar and stellar occultations rely on the assumption of a spherically symmetrical atmosphere. However, photo-chemically induced variations near sunrise/sunset conditions need to be taken into account in the retrieval process in order to prevent inaccuracies.NOMAD (Nadir and Occultation for MArs Discovery) is a spectrometer composed of 3 channels: 1) a solar occultation channel (SO) operating in the infrared (2.3-4.3 μm); 2) a second infrared channel LNO (2.3-3.8 μm) capable of doing nadir, as well as solar occultation and limb; and 3) an ultraviolet/visible channel UVIS (200-650 nm) that can work in the three observation modes [2,3]. The UVIS channel has a spectral resolution ozone and aerosols [4,5,6].Since the beginning of operations, on 21 April 2018, NOMAD-UVIS acquired more than 8000 solar occultations with an almost complete coverage of the planet.NOMAD-UVIS spectra are simulated using the line-by-line radiative transfer code ASIMUT-ALVL developed at IASB-BIRA [7]. In a preliminary study based on SPICAM-UV solar occultations (see [8]), ASIMUT was modified to take into account the atmospheric composition and structure at the day-night terminator. As input for ASIMUT, we used gradients predicted by the 3D GEM-Mars v4 Global Circulation Model (GCM) [9,10]. References[1] Lefèvre, F., Bertaux, J.L., Clancy, R. T., Encrenaz, T., Fast, K., Forget, F., Lebonnois, S., Montmessin, F., Perrier, S., Aug. 2008. Heterogeneous chemistry in the atmosphere of Mars. Nature 454, 971–975.[2] Vandaele, A.C., et al., Planetary and Space Science, Vol. 119, pp. 233–249, 2015. [3] Neefs, E., et al., Applied Optics, Vol. 54 (28), pp. 8494-8520, 2015.[4] M.R. Patel et al., In: Appl. Opt. 56.10 (2017), pp. 2771–2782. DOI: 10.1364/AO.56.002771. [5] M.R. Patel et al., In: JGR (Planets), Vol. 126, Is. 11, 2021.[6] Khayat, Alain S. J., et al., In: JGR (Planets), Vol. 126, Is. 11, 2021.[7] Vandaele, A.C., et al., JGR, 2008. 113 doi:10.1029/2008JE003140.[8] Piccialli, A., Icarus, submitted.[9] Neary, L., and F. Daerden (2018), Icarus, 300, 458–476, doi:10.1016/j.icarus.2017.09.028.[10] Daerden et al., 2019, Icarus 326, https://doi.org/10.1016/j.icarus.2019.02.030 |
