Abstract : This PhD work is an experimental and theoretical study of the cometary organic refractory component contribution to the gaseous phase of comets. We present the reasons that explain the major interest arousing from these objects, mainly from the point of view of the organic chemistry which is one of the most complex ever encountered in the solar system (except on the Earth).
From observations and experimental simulations we have made an inventory of all molecules present or suspected to be present in comets. This list has been the starting point of the selection process of chromatographic columns for the COSAC experiment on board the Rosetta mission. Our bibliographic work points out that a noticeable amount of the organic compounds of comets consists of high molecular weight molecules that don't sublime in the coma, but cover the dust particles emitted from the nucleus. We then put forward the hypothesis that refractory compounds might be degraded in the cometary environment by heat, UV and charged particles processes. We have built up an experimental set-up in order to study two of these mechanisms : photo and thermal degradation.
The S.E.M.A.Ph.Or.E cométaire experiment allows to irradiate solid molecules in the far UV at a controlled temperature. We have studied the photodegradation of polyoxymethylene and shown that this polymer was degraded by UV of wavelengths under 190 nm mainly into H2CO and CO. We have also identified other photodegradation products : CO2, HCOOH, CH3OH, CH3OCHO, CH3OCH2OCH3 and C3H6O3. We have estimated for most of them their quantum yield of production and have proposed chemical mechanisms to explain their formation from the polymer. The thermal degradation rate (activation barrier, frequency factor) has also been measured.
Then, we have been able to calculate the density profile of formaldehyde in the comet Halley, if polyoxymethylene is degraded by photo and thermal degradation, in order to estimate if this polymer is a good candidate to explain the extended source of H2CO. The first mechanism requires a dust production much higher than observed, but thermal degradation allows a good adjustment of our model to observations for cometary grains made of about 7 % of POM and warmed to a temperature of 350 K in the coma. Thus, we propose for the first time a mechanism which explains the formaldehyde extended source in realistic conditions of the cometary environment.
We also present our first results obtained with another molecule which could also be present on comet and contribute to the CN extended source : hexamethylenetetramine. The photodegradation of this molecule is not very efficient, and we only detect a little production of HCN. The comparison of these results with previous works shows that it is important to consider interaction between the solid compound and the products of water photochemistry.