Alain Bossard

Abundance of organic compounds photochemically produced in the atmospheres of the outer planets

Abstract Organic photochemical syntheses in the Jovian atmosphere were simulated by irradiating, at 147 nm, gaseous mixtures of methane and ammonia with varying quantities of hydrogen. An excess of H 2 did not eliminate organic synthesis but did affect the yields quantitatively and qualitatively.

The escape of molecular hydrogen and the synthesis of organic nitriles in planetary atmospheres

What is the influence of hydrogen escape from the atmosphere of small planetary bodies on the synthesis of organic molecules in that atmosphere? To answer this question, laboratory experiments have been performed to study the evolution of different reducing model atmospheres submitted to electrical discharges, with and without the simulation of H2 escape. A study of mixtures of nitrogen and methane shows a very strong effect of H2 escape on the formation of organic nitriles, the only nitrogen containing organics detected in the gas phase. These are HCN, CH (triple bond) C-CN, (CN)2, CH2=CH-CN, CH3CN and CH3CH2CN. The yield of synthesis of most of these compounds is noticeably increased, up to several orders of magnitude, when hydrogen escape is simulated. The escape of H2 from the atmosphere of the primitive Earth may have played a crucial role in the formation of reactive organic molecules such as CH (triple bond) C-CN or (CN)2, which can be considered as important prebiotic precursors. These experimental results may also explain extant data concerning the nature and relative abundance of organics present in the atmosphere of Titan, a planetary satellite which may be an ideal model within our solar system for the study of organic cosmochemistry and exobiology.

Gas phase synthesis of organophosphorus compounds and the atmosphere of the giant planets

Abstract Experimental studies were carried out on the evolution of CH 4 PH 3 , CH 4 PH 3 H 2 , CH 4 CH 3 PH 2 , and C 2 H 6 PH 3 mixtures submitted to spark discharges, and of CH 4 PH 3 mixtures submitted to 147-nm UV irradiation. In all these experiments, the only P-organics detected in gas phase are alkylphophines, but with a number of C atoms not higher than that of the starting hydrocarbon. In the spark discharge experiments, the main products obtained from CH 4 PH 3 are H 2 , CH 3 PH 2 , and C 2 H 6 or P 2 H 4 , depending on the initial mole fraction (low or high, respectively) of PH 3 . Methylphosphine appears much more stable than phosphine. Its rate of formation relative to PH 3 increases dramatically when the initial mole fraction of PH 3 is decreased, and decreases markedly with the addition of large amounts of H 2 . In the UV experiments, the main volatile products are H 2 and P 2 H 4 ; the production of organics is very low. Discussion of the mechanisms involved shows the importance of the reaction CH 3 + PH 2 + M → CH 3 PH 2 + M in the synthesis of methylphosphine. In the spark discharge experiments, the insertion reaction CH 2 + PH 3 + M → CH 3 PH 2 + M cannot be excluded. On the contrary, results of the UV experiments show that the insertion reaction of PH ∗ into CH 4 is negligible. The main reactions of PH ∗ are collisional quenching by PH 3 or CH 4 and energy-transfer-induced decomposition of PH 3 : PH ∗ + PH 3 → PH + PH 2 + H . The former must play the most important role, its rate constant appearing approximately 2 and 10 times higher than the rate constant of the reactions PH ∗ + PH 3 → PH + PH 3 and PH ∗ + CH 4 → PH + CH 4 , respectively. Implications of these results for Saturns atmosphere are presented.

Far UV irradiation of model prebiotic atmospheres.

UV light has been the most important energy source on the primitive Earth. However, very few experiments have been performed to test directly the possible role of this energy source on the chemical evolution of the primitive atmosphere, mainly on account of experimental difficulties. Experiments are generally performed with other excitations, mainly electric discharge, and it is frequently assumed that UV irradiation would give similar results.As theoretical considerations make this assumption questionable, direct experimental controls have been undertaken: Model primitive atmospheres have been submitted to 147 nm UV light and the gaseous phase has been analysed. Preliminary qualitative results concerning CH4−NH3 atmospheres are reported.Irradiation of pure CH4 gives rise to the synthesis of a large number of hydrocarbons, mainly saturated hydrocarbons but including also unsaturated ones as, C2H2, C2H4, C3H6, C3H4. These insaturated hydrocarbons are synthetized at a very low rate when ammonia is present in the medium.Irradiations of CH4−NH3 mixtures give rise, in addition to hydrocarbons, to important amounts of HCN (about 0.1%) and to lesser amounts of CH3CN and C2H5CN. No unsaturated nitriles such as acrylonitrile and cyanoacetylene have been detected. Search for amines is in progress.These results evidence that UV irradiation may contribute largely to synthesis of HCN in CH4−NH3 atmospheres and, consequently to the synthesis of many biochemical compounds that can be derivated from HCN. However, synthesis of other compounds, such as pyrimidines, which can derivate from other nitriles, such as cyanoacetylene, cannot be initiated only by UV light, contrary to electric discharges. In addition, if electric discharges are very efficient for synthesis of nitriles in CH4−N2 atmospheres, there is not yet evidence that UV light is able to do so.

Organic synthesis from reducing models of the atmosphere of the primitive earth with UV light and electric discharges

SummaryThe purpose of this paper is to compare the role of UV light and of electric discharges, the two most important sources of energy on the primitive earth, in the synthesis of organic compounds out of a reducing model of that atmosphere. Since Millers experiments in 1953, most of the experimental simulations have been performed with electric discharges, and it has been assumed that UV radiations would give similar results.In order to check this assumption we have performed both experimental simulations in our laboratory. Experimental results indicate that this assumption was wrong in a large extent. Our four main conclusions are:1.Unlike electric discharges, UV light is not an efficient source for producing unsaturated carbon chains.2.UV light is efficient for producing nitriles in CH4-NH3 mixtures when the mole fraction of NH3 is very low while electric discharges need a higher mole fraction of NH3.3.UV light is not able to produce nitriles from CH4-N2 mixtures while electric discharges produce important quantities of diversified nitriles from these mixtures.4.UV light is not very efficient for producing aldehydes from CH4-H2O model atmosphere, electric discharges seem to be able to produce them more efficiently.

Organic syntheses in gas phase and chemical evolution in planetary atmospheres

Atmospheric chemistry may be one of the important pathways to the synthesis of organic compounds in a planetary periphery. Depending on the nature of the carbon source (CH4, CO or CO2), the main composition of the atmosphere, and the respective roles of the various energy sources, is it possible, and to what extent, to produce organics? What kind of gaseous mixture is the most favourable to prebiotic organic syntheses? How far can the results of laboratory works be extrapolated to the case of planetary atmospheres? These questions are discussed, on the basis of several available laboratory data, and by considering the main atmospheric composition of the planets of the solar system, and the list of organic compounds which have already been detected in their atmospheres.

Far UV Photolysis of Methane-Water Gaseous Mixtures and the Prebiotic Synthesis of Aldehydes

Far UV photolysis of model reducing atmosphere (CH4-H2O) has been performed in order to test the possibilities of organic synthesis on the primitive earth. The only detected organic products are hydrocarbons, ketones and formaldehyde. The results are compared with those of a previous study which evidenced the synthesis of aldehydes, alcohols and ketones. The prebiotic significance of these results are discussed.

Gas phase synthesis of organophosphorous compounds and the atmosphere of the giant planets

In the spark discharge experiments, the main products obtained from CH4-PH3 are H2, CHIPH2 and C2H6 or P2H4, depending on the initial mole fraction (low or high, respectively) of PHI. Methylphosphine appears much more stable than phosphine. Its yieid of formation relatively to PH3 increases drastically when the initial mole fraction of PH3 is decreased, and decreases strongly with the addition of large amounts of H2. In the UV experiments, the main volatile products are H2 and P2H4; the production of organics is very low.