Abstract
Since April 2018, the Nadir and Occultation for Mars Discovery (NOMAD) instrument on board ESA’s Trace Gas Orbiter has been continuously recording measurements of the atmosphere of Mars. The main goal was to confirm the presence of methane, a possible indicator of life onMars. From the measurements of NOMAD, no sign of methane has yet been found but many other constituents of the atmosphere of Mars can be derived.
This work focuses on the deduction of vertical profiles (slices of the atmosphere for a range of altitudes) of carbon dioxide density and temperature from the Solar Occultation (SO) channel of NOMAD. This channel comprises an echelle grating for the light dispersion coupled with an acousto-optic tunable filter. Particular attention has been dedicated to the spectral calibration.
The profiles of carbon dioxide density and temperature are necessary for the deduction of the density of other species such as water and carbon monoxide. In remote sensing, this deduction is called a "retrieval" and is an inverse problem, involving the regularisation of the retrieved profiles.
The regularisation is performed in this work with the Tikhonov method and the best fine-tuning of this regularisation is achieved with the Expected Error Estimation method.
Up to 1848 profiles of carbon dioxide density and temperature have been retrieved from April, 21st 2018 until December 26th 2022. Carbon dioxide density profiles are directly retrieved from the spectra of NOMAD-SO while temperature is retrieved from the latter and the hydrostatic
equilibrium equation.
This manuscript contains the seasonal, latitudinal, longitudinal, and diurnal variations of CO2 density and temperature at the terminator of Mars, focusing on the mesosphere (extending from 50 to 100 km). Specific trends have been identified, such as the CO2 seasonal cycle, the variation with Hadley circulation cells, the polar warmings, and warm and cold pockets. The latter sometimes bear the presence of CO2 ice clouds and their locations are reported. Some warm layers are present in the Northern hemisphere at both dawn and dusk and have particularly high amplitudes in the Southern hemisphere at dawn but are absent in the Southern hemisphere at dusk. Strong longitudinal variations are also reported. They are probably related to atmospheric tides. This dataset covers two Martian years and the trends are very similar between those two years.
| Date of Award | 12 Dec 2023 |
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| Original language | English |
| Awarding Institution |
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| Sponsors | European Space Agency (ESA Retired) |
| Supervisor | Muriel Lepere (Supervisor), Ann Carine Vandaele ICPAE (Supervisor), Luc Henrard (President), Miguel Dhyne (Jury), Jean-Claude Gérard (Jury) & Miguel Ángel López-Valverde (Jury) |