Simulation Experimentale et Modélisation Appliquées aux Phénomènes Organiques dans l'Environnement Cométaire
Comets are objects that should have kept a lot of key
information about the formation of our solar system, they may have also
imported water and organic molecules to Earth which could have played a
key role in life’s origin (Oro and Cosmovici, 1997). In the Oort Cloud
or the Kuiper Belt, comets have been held in the furthest and coldest regions
of the solar system and thus may be composed of the most pristine material
we can access. After collisions or gravitational perturbation they leave
their original orbit, for a more eccentric trajectory that drives them
closer to the sun.
Although more than twenty molecules have already been detected in the gaseous phase, we note that there is no direct information on the chemical composition of the nucleus. In order to get further indications, experimental simulations on cometary ice analogs have been carried out. They are based on Greenberg’s model of cometary nuclei which suggests that comets are made of interstellar dust (Greenberg, 1982). Molecules formed in molecular clouds condense on silicate cores and are irradiated by UV and/or galactic cosmic rays in the interstellar medium. The laboratory experiments simulate these processes. They lead to the formation of a mantle of complex refractory organics, called "Yellow Stuff" that should embed grains in the interstellar medium. Afterward, when the cloud collapses to form a new planetary system, the grains accrete together with their organic mantle. Looking at the list of organic compounds which have been identified in the experimental simulations, one can expect that many more molecules than those already detected should also be present on cometary nuclei. Among them, prebiotic compounds like Hexamethylenetetramine (C6H12N4) (Bernstein et al., 1995) and other amino-acids precursors (Kobayashi et al., 1995). Nevertheless, most of them, with high molecular weight, will never be detected in the gaseous phase because their vapor pressures are too low. Only degradation products of those molecules, when they are heated or irradiated by solar UV photons, could be detected by remote observations.
The daughter molecules produced by such a mechanism should present an extended source in the coma. It is the case of formaldehyde (H2CO) in comet Halley (Meier et al., 1993), and in comet Hale-Bopp. Since Polyoxymethylene (POM), a polymer of H2CO, has been tentatively detected by (Huebner et al., 1989) in comet Halley, it is often evoked as a parent molecule of formaldehyde to explain its extended source (Biver, 1997; Greenberg and Li, 1998; Meier et al., 1993). But physico-chemical data to model this phenomenon are lacking and it isn’t yet possible to conclude whether POM is a good candidate or not.
To study photodegradation of solid molecules in cometary environments, we have developed a reactor in which high molecular weight compounds are irradiated at 122 or 147 nm under vacuum. This is the experience SEMAPHORE Cometaire. The experimental setup can be connected to a mass spectrometer or to the large simulation chamber of LISA for IR and UV analysis. Thus photodegradation products are quantitatively detected and one can go back to their quantum yield of production. These results have direct application to the study of extended sources and to establish correlations between the composition of the nucleus and the coma. Our first application is Polyoxymethylene.
About Semaphore :
Cottin, H., S. Bachir, F. Raulin, and M.C. Gazeau, Photodegradation
of Hexamethylenetetramine by VUV and its relevance for CN and HCN extended
sources in comets, Advances in Space Research, in Press.
Cottin, H., M.C. Gazeau, P. Chaquin, Y. Bénilan, and F. Raulin, Experimental and theoretical studies on the gas/solid/gas transformation cycle in extraterrestrial environments, Journal of Geophysical Research (Planets), in press.
Cottin, H., M.C. Gazeau, Y. Bénilan, and F. Raulin, Polyoxymethylene as parent molecule for the formaldehyde extended source in comet Halley, The Astrophysical Journal, 556 (1), 417-420, 2001.
Cottin, H., M.C. Gazeau, J.F. Doussin, and F. Raulin, S.E.M.A.Ph.Or.E COMETAIRE, a tool for the study of the photochemical decomposition of probable cometary large organic molecules. First application : Polyoxymethylene, Physic and Chemistry of the Earth, 24 (C) (5), 597-602, 1999.
Cottin, H., M.C. Gazeau, J.F. Doussin, and F. Raulin, An experimental study of the photodegradation of polyoxymethylene at 122, 147 and 193 nm, Journal of photochemistry and photobiology, 135 (A : Chemistry), 53-64, 2000.
Bernstein, M.P., S.A. Sandford, L.J. Allamandola, S. Chang, and M.A.
Scharberg, Organic Compounds Produced By Photolysis of Realistic Interstellar
and Cometary Ice Analogs Containing Methanol, The Astrophysical Journal,
454, 327-344, 1995.
Biver, N., Molécules Mères Cométaires : Observations et Modélisations, Doctorat thesis, Paris VII, 1997.
Cottin, H., M.C. Gazeau, J.F. Doussin, and F. Raulin, S.E.M.A.Ph.Or.E COMETAIRE, a tool for the study of the photochemical decomposition of probable cometary large organic molecules. First application : Polyoxymethylene., Physic and Chemistry of the Earth, in press.
Greenberg, J.M., What are comets made of - A model based on interstellar dust, in Comets, edited by L.L. Wilkening, pp. 131-163, University of Arizona Press, Tucson, 1982.
Greenberg, J.M., and A. Li, From interstellar dust to comets : the extended CO source in comet Halley, Astronomy and Astrophysics, 332, 374-384, 1998.
Huebner, W.F., D.C. Boice, and A. Korth, Halley's polymeric organic molecules, Advances in Space Research, 9 (2), 29-34, 1989.
Kobayashi, K., T. Kasamatsu, T. Kaneko, J. Koike, T. Oshima, T. Saito, T. Yamamoto, and H. Yanagawa, Formation of amino acid precursors in cometary ice environments by cosmic radiation, Advances in Space Research, 16 (2), (2)21-(2)26, 1995.
Meier, R., P. Eberhardt, D. Krankowsky, and R.R. Hodges, The extended formaldehyde source in comet P/Halley, Astronomy and Astrophysics, 277, 677-691, 1993.
Oro, J., and C.B. Cosmovici, Comets and Life on the primitive Earth, in Astronomical and Biochemical Origins and the Search for Life in the Universe, edited by C.B. Cosmovici, S. Bowyer, and D. Werthimer, pp. 97-120, 1997.