Spectroscopie infrarouge cométaire : analyse des observations de la comète 9P/Tempel 1 obtenues avec le télescope spatial Spitzer lors de l'évènement Deep Impact

Abstract

Comets contain the most primitive icy material from the epoch of Solar System formation. Their composition may potentially be unchanged since their accretion in the protoplanetary disk. Studying them informs us about the physical and chemical processes of planet formation. When comets approach the Sun, the nucleus surrounds itself with an atmosphere called coma consisting of dust and products of ice sublimations. NASA selected the comet 9P/Tempel 1 as the target for the Deep Impact event. It is the only spatial mission, to this day, to have examined material from the interior of a cometary nucleus resulting from a planned collision that occurred on the 4th of July 2005. During this thesis, I study the ejecta created by this event which allowed me to 1. Analyze the activity of comet 9P/Tempel 1 and the properties of its coma before and after the impact and 2. Determine the dust-to-ice ratio in the deep layers of the nucleus. To perform this research, I developed numerical models to interpret infrared spectroscopic data from the Spitzer Space Telescope before and after the impact. The Spitzer spectra, between 5. 2--13. 2 µm enable us to study the fluorescence emission of the v₂ vibrational band of water at 6. 4 µm and the thermal emission of the dust. The temporal evolution of the continuum was analyzed using a dust thermal model which considers two size distributions and two grain compositions : amorphous carbon and intimate silicate- carbon mistures. The temperature of grains was derived from the radiative equilibrium and the absorption coefficients was calculated by using Mie theory. The free parameters of the size distribution were constrained for the dust ejecta and for the ambient coma dust which allow us to deduce the mass of the ejecta in the field of view. The study of these data suggests that a significant number of small grains were released during the impact and that grains split up during their expansion in the coma. The total mass of the injecta range from (0. 5--2. 1) x 10⁶ kg for sizes 0. 1--100µm, which is in good agreement with other values published in the literature. The temporal evolution of the dust ejecta emission within the Spitzer field of view was interpreted by a time-dependent model which simulates the development of the dust cloud and takes into account the dynamics of the grains. The velocity law for each grain size was constrained by the model. The water emission was extracted from the Spitzer spectra and the water columns within the Spitzer extraction aperture were inferred using a fluorescence excitation model. The pre-impact spatial distribution of water molecules allowed to determine the water production rate for the ambient coma of the comet 9P/Tempel 1, equal to 4. 7 x 10²⁷ molecules s-1. The temporal evolution of the number of water molecules within the FOV, investigated utilizing a time-dependent water model, allowed to deduce the mass of water injected by the impact equal to (7. 4 +/- 1. 5) x 10⁶ kg. This temporal evolution brings to light that sustained production of water molecules occured after impact from sublimating icy grains. A model of sublimation of icy outflowing grains was developed to analyze the sustained production of water molecules after the impact. Two approaches, corresponding to a dense and rarefied medium, were used to account for the dynamics of water molecules escaping from grains (pure ice on including impurities) in the ambient flow. This analysis of data brings to light the presence of pure ice grain in the injecta. The mass of ice deducted by the model for pure ice grains for size 0. 1--1 µm is estimated to Mice > 4. 7 x 10⁶ kg. This study about water and dust in the injecta leads to a dust-to-ice ratio 4. 7 x 10⁶ kg. Cette étude sur l'eau et la poussière dans les éjectas conduit à un rapport poussière/glace < 0. 03. Ce résultat, mis en comparaison avec le rapport poussière/gaz ~ 1 normalement mesuré dans les atmosphères cométaires, suggère la présence d'une quantité importante de glace sous la surface du noyau de comète 9P/Tempel 1