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[EN] Mars is a vast and complex world. It is a terrestrial planet with a reddish appearance, due to a surface mostly covered by ferric oxide dust and rocks. Mars has a faint atmosphere mainly composed of carbon dioxide (CO2), with atmospheric circulation and weather patters, like Earth.It has, however, remarkable diurnal oscillation of winds, due to a considerable thermal excursion. Despite the importance of the thermosphere, for instance, to the atmospheric escape to space, this is maybe the less known region of the Martian atmosphere. Most of the information we have of these altitudes comes from very disperse and unconnected sources. Among them,a few in situ profiles taken during the descent of some missions, like Viking 1 and 2, the aerobraking manoeuvres by the Mars Global Surveyor and Mars Reconnaissance Orbiter spacecrafts, the SPICAM instrument on board Mars Express, and the instruments on board the MAVEN mission. These measurements allowed to obtain density and temperature profiles, and to study the seasonal and geographical variabilities of the thermosphere. According to these observations and their numerical simulations, the thermosphere of Mars is a complex and dynamic region, strongly coupled to lower layers. Concretely, the effects caused by the dust storms and the temperature variability in the low atmosphere are propagated upwards up to the thermosphere. To understand this region, it is therefore necessary a global view of the atmosphere, from its interactions with the surface, to the exchanges of species with the exosphere. The thermospheric data previously described have a limited temporal and geographical coverage. Some important issues, like the influence of solar activity, are difficult to understand from the available data. Most of them, except aerobraking measurements, concentrate in the night side of the planet, leaving the diurnal thermosphere almost unexplored. It is in the dayside thermosphere where the strongest infrared atmospheric non-thermal emissions are produced. These infrared emissions offer an interesting possibility for remote sounding at these heights in all terrestrial planets. There are indeed thermospheric observations of Mars in the infrared, but they have not been sufficiently exploited so far, due to the complexity of the physical interpretation and the numeric difficulty of the required mathematical inversion. These observations were acquired by two instruments on board Mars Express, OMEGA and PFS. Their analysis is expected to provide a wider and deeper understanding of the dayside thermosphere at the maximum sensitivity altitudes., [EN]In the Group of Terrestrial Planetary Atmospheres (GAPT, for its Spanish acronym) at the Instituto de Astrofı́sica de Andalucı́a (IAA), a large experience on non-thermal atmospheric emissions, physical models for the Mars atmosphere, and tools for the inversion of such emissions are available. Molecular species, like CO2 , produce strong non-thermal emissions in the infrared in the higher layers of the atmosphere. At those altitudes, the density is so low and molecular collisions are so rare that local thermodynamic equilibrium (LTE) conditions no longer apply. The departure from LTE typically occurs in the diurnal hemisphere when such species are excited by solar radiation in the rotational-vibrational bands in the near and medium infrared (between 1 and 10 um). The emissions produced contain information on the densities of the emitting species, and therefore contribute to the extraction of density and temperature profiles in the higher atmosphere. Some difficulties arise with this type of observations. First, the emission of these tenuous layers is low, so the observation in limb geometry, where the emission of a large atmospheric path is integrated on the detector, is extremely helpful. Besides, inherent difficulties arise when dealing with non-LTE conditions, as this approximation is not valid. This issue is solved by the use of inversion codes including a non-LTE model in the forward calculation (fundamental tool of the inverse problem). Finally, the lack of local measurements of the atmospheric magnitudes involved, needed to start and guide the retrieval, is overcame by the assumption of a priori conditions predicted by 3-D numerical simulations by state-of-the-art General Circulation Models of Mars. We analysed limb infrared CO2 emissions, in the region around 4.3 um, obtained by OMEGA in the daylight thermosphere of Mars, in order to infer information on fundamental atmospheric parameters, like density and temperature. These emissions are caused by CO2 fluorescence of solar radiation, and the investigation needs to take into account non-LTE conditions. We performed a radiometric calibration on the data provided by OMEGA, cleaned the available spectra, including the use of clustering techniques, and generated radiance vertical profiles for each orbital dataset. The distribution and geometry of the spectra acquired by OMEGA are highly heterogeneous, leading to very different projections in the limb of the Martian atmosphere. For this reason, a series of geometric criteria was established in order to allow for an easier and consistent comparison among the results of the retrievals., [EN]Once the radiance vertical profiles were generated, we applied a non-LTE retrieval scheme based on a extensively validated scheme working for Earth, which we adapted to Martian conditions. In this work we present information on the inversion set up, and a discussion on the retrieved CO2 density profiles. A total of 742 profiles were formed from the 47 OMEGA orbits with limb observations previously selected. The convergence rate achieved considering the entire dataset was 94%, which is considered as very satisfactory. From the retrieved CO2 densities, we derived temperature profiles, assuming hydrostatic equilibrium. For this, we made use of an algorithm developed for that task. For 60% of the orbits analysed we found a minimum in the temperature profile at 140–150 km, indicating a thermosphere colder than that of the model used, the LMD-MGCM. On the opposite side, a thermosphere warmer than that predicted by the model was obtained in 30% of the orbits. An extensive sensitivity study of the retrieval scheme was also carried out. We found that, in general, the uncertainty due to the instrumental Gain calibration and that caused by the retrieval noise error itself are of primary importance, while the influence of the temperatures in the reference atmosphere used as a priori, provided by our General Circulation Model (GCM), is minor. According to our study, CO2 profiles can be derived with a precision of around 20% and a vertical resolution of around 15 km between 120 and 160 km tangent altitude. Finally, we compared the density and temperature profiles obtained to the predictions of the LMD-MGCM and to the results recently provided by other instruments studying the Martian thermosphere. In general, no clear correlation of the data-model discrepancies obtained with any temporal or spatial dimension is observed, neither from a global study nor when a more homogeneous subset of OMEGA observations, i.e., at constrained geolocation, is analysed. There is one exception, the solar zenith angle, which affects the atmospheric emission. Most observations from other instruments, like in situ or remote measurements by NGIMS and by IUVS (both on board MAVEN), respectively, have uncertainties of the order of those presented in this work. The results from these experiments also bring to light important differences when compared to the LMD-MGCM or other General Circulation Models. This global comparison with numeric simulations indicates an atmospheric variability in line with that found in our OMEGA data. This result points to the necessity of validation of global models at thermospheric altitudes. The thermosphere of Mars is, indeed, a complex and dynamic region., [ES] Marte es un mundo vasto y complejo. Un planeta rojizo, debido a su superficie, ampliamente cubierta por polvo y rocas de óxido férrico. Marte tiene una atmósfera tenue, principalmente compuesta por dióxido de carbono (CO2), con circulación atmosférica y patrones climáticos, como la Tierra. Sin embargo, tiene destacadas oscilaciones diurnas de viento, debido a una excursión térmica considerable. Las oscilaciones tienen un efecto en todas las capas de la atmósfera, y ejercen una influencia apreciable sobre el resto de la circulación atmosférica global de Marte. En la ultima década, varias misiones han viajado al Planeta Rojo y algunas más han sido aprobadas para ser lanzadas en los próximos años. Dos de estas misiones han sido diseñadas, construidas y operadas por/desde Europa, y son especialmente relevantes para esta Tesis: Mars Express y ExoMars, This work was conducted as part of the project UPWARDS-633127 under the European Union's Horizon 2020 research and innovation Programme. The IAA team was supported by the Spanish Ministry of Economy, Industry and Competitiveness and by FEDER funds under grant ESP2015-65064-C2-1-P (MINECO/FEDER)