Impact of gradients at the Martian terminator on the retrieval of ozone from TGO/NOMAD-UVIS
Authors: Piccialli Arianna , Vandaele Ann Carine , Neary Lori , Willame Yannick , Aoki Shohei , Trompet Loic , Depiesse Cedric , Viscardy Sebastian , Daerden Frank , Erwin Justin , Thomas Ian R. , Ristic Bojan , Mason Jon P. , Patel Manish , Khayat Alain , Wolff Mike , Bellucci Giancarlo , Lopez-Moreno Jose Juan 
Published: March 2022

&lt;p&gt;We will investigate the impact of &lt;strong&gt;day-night temperature and compositional gradients&lt;/strong&gt; at the Martian terminator on the retrieval of vertical profiles of &lt;strong&gt;ozone&lt;/strong&gt; obtained from&lt;strong&gt; NOMAD-UVIS solar occultations&lt;/strong&gt;.&amp;#160;&lt;/p&gt;&lt;p&gt;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.&lt;/p&gt;&lt;p&gt;&lt;strong&gt;NOMAD&lt;/strong&gt; (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 &amp;#956;m); 2) a second infrared channel LNO (2.3-3.8 &amp;#956;m) capable of doing nadir, as well as solar occultation and limb; and 3) an ultraviolet/visible channel &lt;strong&gt;UVIS &lt;/strong&gt;(200-650 nm) that can work in the three observation modes [2,3].&amp;#160;&lt;/p&gt;&lt;p&gt;The UVIS channel has a spectral resolution &lt;1.5 nm. In the solar occultation mode, it is mainly devoted to study the climatology of&lt;strong&gt; ozone &lt;/strong&gt;and &lt;strong&gt;aerosols &lt;/strong&gt;[4,5,6].&lt;/p&gt;&lt;p&gt;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.&lt;/p&gt;&lt;p&gt;NOMAD-UVIS spectra are simulated using the line-by-line radiative transfer code &lt;strong&gt;ASIMUT-ALVL &lt;/strong&gt;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 &lt;strong&gt;GEM-Mars v4 Global Circulation Model&lt;/strong&gt; (GCM) [9,10].&amp;#160;&lt;/p&gt;&lt;p&gt;&lt;strong&gt;References&lt;/strong&gt;&lt;br&gt;[1] Lef&amp;#232;vre, 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&amp;#8211;975.&lt;br&gt;[2] Vandaele, A.C., et al., Planetary and Space Science, Vol. 119, pp. 233&amp;#8211;249, 2015.&amp;#160;&lt;br&gt;[3] Neefs, E., et al., Applied Optics, Vol. 54 (28), pp. 8494-8520, 2015.&lt;br&gt;[4] M.R. Patel et al., In: Appl. Opt. 56.10 (2017), pp. 2771&amp;#8211;2782. DOI: 10.1364/AO.56.002771.&amp;#160;&lt;br&gt;[5] M.R. Patel et al., In: JGR (Planets), Vol. 126, Is. 11, 2021.&lt;br&gt;[6] Khayat, Alain S. J., et al., In: JGR (Planets), Vol. 126, Is. 11, 2021.&lt;br&gt;[7] Vandaele, A.C., et al., JGR, 2008. 113 doi:10.1029/2008JE003140.&lt;br&gt;[8] Piccialli, A., Icarus, submitted.&lt;br&gt;[9] Neary, L., and F. Daerden (2018), Icarus, 300, 458&amp;#8211;476, doi:10.1016/j.icarus.2017.09.028.&lt;br&gt;[10] Daerden et al., 2019, Icarus 326, https://doi.org/10.1016/j.icarus.2019.02.030&lt;/p&gt;