Prélude à la mission Venus Express : étude de l'atmosphère par spectro-imagerie infrarouge

Abstract : Since the discovery of the night-side infrared emission from Venus in 1983 by Allen and Crawford, which may be observed in a few spectral windows, its origin and scientific value has been fully recognised. The carbon dioxide opacity is low enough within some narrow atmospheric windows so that the thermal radiation from the deep (25-40 km) and hot (400-500 K) layers can reach an external observer. These emissions, in particular from the spectral interval extending from 2.2 until 2.5 µm, provide a unique investigation vector to study the tropospheric layers located beneath the thick cloudy layers, in particular in terms of chemical composition thanks to some absorption bands of minor components whose vertical profiles could be measured, such as CO, OCS, H2O and SO2. Studying local variations of these minor species can give a lot of information about the dynamics of the lower atmosphere as well as the understanding of its chemistry - including the sulphur cycle - and the putative geological activity of the planet (by detecting water vapour release from volcanoes).
The beginning of the Venus Express space mission, the first one dedicated to this planet since the discovery of the nightside infrared emission, and especially the expected data from the spectro-imaging device VIRTIS have led to some preliminary work, among which this thesis. Having acquired some spectra with the SpeX instrument at the Infrared Telescope Facility in Hawaii, we obtained data similar in spectral resolution (R ~ 2000) to those expected from the VIRTIS instrument. These observations were conducted during three campaigns during favourable quadrature seasons in Februrary 2003, August 2004 and November 2005. Then, the numerical radiative transfer model developped by Bruno Bézard and simulating infrared thermal spectra enabled us to develop algorithms which can quickly derive various atmospheric parametres (mixing ratios of a minor species at a specific altitude, vertical gradients, isotopic ratio of water vapour, lower cloud opacity) from a given observed spectrum.
We then present the results, such as the confirmation of the suspected enhancement in carbon monoxide towards middle and high latitudes. Anti-correlated variations of carbonyl sulfide have also been noticed. A related variability of the verical gradients of these two species has been measured too. In addition, we provide a qualitative dynamical interpretation to these results in terms of general vertical circulation. More precise constraints on the mean abundance of water vapour and its isotopic ratio have also been derived.
Finally, the last part of this report deals with the analysis of the first data received from VIRTIS by similar methods, which demonstrates their applicability and brings confirmation of some previous results. It also deals with the future prospects, combining spectrscopic studies of minor components with dynamical interpretations of their variations thanks to general circulation models.
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Emmanuel Marcq. Prélude à la mission Venus Express : étude de l'atmosphère par spectro-imagerie infrarouge. Astrophysique [astro-ph]. Université Paris-Diderot - Paris VII, 2006. Français. ⟨tel-00126105⟩

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