[P-CG18] Planetary Magnetosphere, Ionosphere, and Atmosphere

[PCG18-11] Variation of CO/CO₂ profile in the Martian middle atmosphere observed by TGO/NOMAD

*Nao Yoshida¹, Hiromu Nakagawa¹, Shohei Aoki^2,3, Justin Erwin², Ann Carine Vandaele², Isao Murata¹, Ian Thomas², Frank Daerden², Lori Neary², Shungo Koyama¹, Naoki Terada¹, Yasumasa Kasaba¹, Bojan Ristic², Manish R Patel⁴, Giancarlo Bellucci⁵, Jose-Juan Lopez-Moreno⁶ (1.Dep. Geophysics Graduate School of Science Tohoku University, 2.Royal Belgian Institute for Space Aeronomy, BIRA-IASB, 3.LPAP, STAR Institute, Université de Liège, 4.School of Physical Sciences, The Open University, 5.Instituto di Astrofisica e Planetologia Spaziali (IAPS/INAF), 6.Instituto de Astrofisica de Andalucia (IAA/CSIC))

Keywords:Trace Gas, Mars, IR observation, CO, Eddy diffusion, Photochemistry

The upper atmosphere is the reservoir for atmospheric escape to space. The composition in the Martian upper atmosphere is mainly controlled by the solar Extreme-Ultraviolet (EUV) irradiance [e.g., Fox et al., 1996; Bougher et al., 2015; Krasnopolsky et al., 2002] but also varies with the altitude of homopause [Yoshida et al., 2020]. Homopause is the compositional boundary above which the molecular diffusion coefficient becomes larger than the eddy diffusion coefficient. On Mars, it is inferred that homopause altitude varies between 60 and 140 km altitudes with season, latitude, and local time [Jakosky et al., 2017; Slipski et al., 2018; Yoshida et al., 2020]. The substantial variation of homopause altitude suggests the influence of the lower atmospheric processes on the upper atmosphere. The inflation and contraction of the lower atmosphere are suggested as the main source of variations of homopause altitude. Besides, the gravity waves, planetary-scale waves, thermal tides, and global circulation are suggested to affect the homopause location [Imamura et al., 2016; Leovy et al., 1982; González-Galindo et al., 2009]. Since the variability of atmospheric composition study has been conducted by Mars Volatile Evolution spacecraft which measures the atmospheric composition in the upper atmosphere, variability of the lower-middle atmospheric composition has not acquired. To understand the dynamical coupling between the lower and the upper atmospheres, we use the Nadir and Occultation for MArs Discovery (NOMAD) instrument aboard Trace Gas Orbiter (TGO). NOMAD solar occultation is designed the combination of the Acousto Optical Turnable Filter and echelle grating [Neefs et al., 2015; Thomas et al., 2016]. NOMAD solar occultation operates in the wavelength range of 2.2 - 4.3 μm with a high-resolution (0.15 – 0.22 cm^-1) [Vandaele et al., 2018]. It provides us CO and CO₂ spectrum below 100 km and 180 km altitudes, respectively.

The retrieved column mixing ratio of CO has the seasonal variation, which increases at the wintertime polar regions and decreases at the summertime polar regions [Smith et al., 2009; 2018; Krasnopolsky, 2007]. Seasonal variation of CO mixing ratio is controlled by CO₂ sublimation and condensation. The general circulation model including atmospheric chemistry has suggested that the vertical structure of CO mixing ratio increases along with altitude above 60 km [Daerden et al., 2018; Lefevre et al., 2008]. The CO mixing ratio is constant in the lower atmosphere because time of vertical mixing is shorter than the CO lifetime of ~6 years [Krasnopolsky, 2007]. However, the lifetime of CO becomes comparable to the time of vertical mixing around 60 km due to the production by CO₂photodissociation when a linear decrease in the eddy diffusion coefficient (e.g., Izakov, 1978) is assumed. These studies suggest that both photochemistry and eddy diffusion are important in determining the CO/CO₂ ratio profiles.

In this study, we applied the equivalent width technique [Chamberlain and Hunten, 1987; Krasnopolsky, 1986] to derive a new set of the column densities of CO and CO₂, respectively, with the observed atmospheric transmittance spectra by NOMAD solar occultation. The CO (2-0) band and the CO₂ (21102 – 00001) band are carefully selected for retrievals due to the contribution of nearby and central orders [cf. Liuzzi et al., 2019]. It is noted that CO₂ absorption cross sections, we use, have high sensitivity to the background temperature. We found that the retrieved CO/CO₂ ratio between 60 and 90 km increases with altitudes as predicted by the models. The seasonal variation of observed CO/CO₂ profile will be discussed. For interpretation, 1D photochemical model is compared newly obtained CO/CO₂ profiles, especially in order to infer the eddy diffusion coefficient in the lower atmosphere on Mars.