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.
References :
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.