Project Cassini: A Saturn Orbiter/Titan Probe mission proposal

Titan is the only moon in the solar system with a substantial atmosphere. The organic chemistry of its N₂–CH₄ atmosphere may resemble that of the earth's primitive atmosphere before life arose. The investigation of the synthesis of prebiotic molecules in Titan's atmosphere and the atmospheric and surface environments of this planet-sized moon will be the focal point of the Cassini Project proposed to the European Space Agency for an international Saturn Orbiter/Titan Probe mission.     

Gas-Phase Reactions in Extraterrestrial Environments: Laboratory Investigations by Crossed Molecular Beams

We have investigated gas-phase reactions of N(²D) with the most abundant hydrocarbons in the atmosphere of Titan by the crossed molecular beam technique. In all cases, molecular products containing a novel CN bond are formed, thus suggesting possible routes of formation of gas-phase nitriles in the atmosphere of Titan and primordial Earth. The same approach has been recently extended to the study of radical–radical reactions, such as the reaction of atomic oxygen with the CH₃ and C₃H₅ radicals. Products other than those already considered in the modeling of planetary atmospheres and interstellar medium have been identified. Presented at: National Workshop On Astrobiology: Search For Life In The Solar System, Capri, Italy, 26 to 28 October, 2005.     

Organic syntheses from CH4−N2 atmospheres: Implications for Titan

Numerous experiments have already been performed, simulating the evolution of gaseous mixtures containing CH₄ when submitted to energy flux. From their results, it appears that a variety of organic compounds, including unsaturated hydrocarbons and nitriles such as HCN, can be synthesized into noticeable amounts from CH₄–N₂ mixtures. In particular, systematic studies of the influence of the composition of the mixture on the nature and amount of synthesized compounds show that organic volatile nitriles, and particularly cyanoacetylene and cyanogen, are formed only in media rich in nitrogen. Those nitriles have been identified very recently in the atmosphere of Titan, and thus, data from such laboratory experiments may provide important indirect information on the organic chemistry occuring at the periphery of this satellite of Saturn. However, during these experiments, there is a continuous formation and accumulation of molecular hydrogen, which does not occur in the atmosphere of Titan, because of H₂ escape. In order to reassess the data already available from this type of laboratory studies, experiments on CH₄–N₂ atmospheres, with and without H2 escape, have been recently performed. The influence of this parameter on the chemical evolution of the atmosphere and on the nature and relative quantities of organic compounds has been studied.After reviewing these experiments, implications of the obtained results on the organic chemistry at the periphery of Titan are discussed.Paper presented at the 6th College Park Colloquium, October 1981.     

Synthesis of hydrocarbons under presumed prebiotic conditions using high-frequency discharge

Summary Various hydrocarbons were synthesized by high-frequency discharge in a primordial terrestrial model atmosphere. The products were extracted by benzene or methanol and analyzed by GC-MS. The mean carbon chain length of the hydrocarbons formed by the discharge through pure CH₄ gas was less than 6. Benzene was also obtained. Some isomers were obtained for each of the hydrocarbons containing a given number of carbons. When a small amount of C₂H₂ was added to the CH₄, longer chain compounds were formed, as compared with discharge in CH₄ only. However, when the amount of C₂H₂ was increased, unextractable high molecular weight compounds were produced. The amounts of products decreased as the mixing ratio of CO₂ to CH₄ increased. No hydrocarbons were detected when the ratio of CO₂/CH₄ exceeded 1.     

Exobiology on Titan

With a dense N₂-CH₄ atmosphere rich in organics, both in gas and aerosol phases, and with the possible presence of hydrocarbons oceans on its surface, Titan, the largest satellite of Saturn, appears as a natural laboratory to study chemical evolution toward complex organic systems, in a planetary environment and over a long time scale. Thanks to many analogies with planet Earth, it provides a unique way to look at the various physical and chemical processes, and their couplings which may have been involved in terrestrial prebiotic chemistry. Indeed, analogies with the Earth have a limit since Titan's temperatures are much lower than on the Earth and since liquid water is totally absent. However, from that aspect, Titan also serves as a reference laboratory worth studying — indirectly — the role of liquid water in exobiology. The Cassini-Huygens mission currently developed by NASA and ESA will send an orbiter around Saturn and Titan and a probe in Titan's atmosphere. This mission which will be launched in 1997 for an expected arrival in 2004, offers a unique opportunity to study in detail extra-terrestrial, not life-controled, organic processes, and consequently it will have significant implications in the fields of exobiology and the origins of life.This paper is dedicated to the memory of Cyril PONNAMPERUMA who largely contributed to the development of the scientific fields of Chemical Evolution and Exobiology.     

Kinetics and mechanism of alkali-induced decomposition of 2-alkoxyethylcobaloximes

Kinetics of the base-induced decomposition of five 2-alkoxyethyl(aquo)cobaloximes, ROCH₂CH₂- Co(D₂H₂)OH₂ (R = C₆H₅, CF₃CH₂, CH₃, CH₃CH₂, (CH₃)₂CH), have been studied manometrically in aqueous base, ionic strength 1.0 M (KC1) at 25.0± 0.1 °C under an argon atmosphere. For the complexes with good leaving group alkoxide substituents (R = C₆H₅ and CF₃CH₂) the reactions are first- order in cobaloxime and first-order in hydroxide ion and produce stoichiometric amounts of ethylene and leaving group alcohol (ROH). NMR observation of decomposing solutions and workup of cobalt chelate products show that the reaction is initiated by hydroxide ion attack on an equatorial quaternary carbon leading to formation of an altered cobal- oxime product in which one of the Schiff's base linkages has become hydrated. For the remainer of the complexes the yield of ethylene is less than stoichiometric and pH-dependent, and the ethylene evolving reaction is second-order in hydroxide ion activity. The yield-limiting side reaction is shown to be base-catalyzed formation of a base-stable but photolabile alkoxyethylcobaloxime analog in which a Schiff's base linkage of the chelate has become hydrated, β-Elimination to form alkyl vinyl ethers was not observed for any of the alkoxyethylcobal- oximes. The second-order dependence of ethylene formation on hydroxide ion activity for R = CH₃, CH₂CH₃, and CH(CH₃)₂ is discussed at some length, but is not well understood at present.     

Protein modeling and molecular dynamics simulation of the two novel surfactant proteins SP-G and SP-H

The prospects of a control for a novel gallium nitride pseudo-halide vapor phase epitaxy (PHVPE) with HCN were thoroughly analyzed for hydrocarbons–NH₃–Ga gas phase on the basis of quantum chemical investigation with DFT (B3LYP, B3LYP with D3 empirical correction on dispersion interaction) and ab-initio (CASSCF, coupled clusters, and multireference configuration interaction including MRCI+Q) methods. The computational screening of reactions for different hydrocarbons (CH₄, C₂H₆, C₃H₈, C₂H₄, and C₂H₂) as readily available carbon precursors for HCN formation, potential chemical transport agents, and for controlled carbon doping of deposited GaN was carried out with the B3LYP method in conjunction with basis sets up to aug-cc-pVTZ. The gas phase intermediates for the reactions in the Ga-hydrocarbon systems were predicted at different theory levels. The located π-complexes Ga…C₂H₂ and Ga…C₂H₄ were studied to determine a probable catalytic activity in reactions with NH₃. A limited influence of the carbon-containing atmosphere was exhibited for the carbon doping of GaN crystal in the conventional GaN chemical vapor deposition (CVD) process with hydrocarbons injected in the gas phase. Our results provide a basis for experimental studies of GaN crystal growth with C₂H₄ and C₂H₂ as auxiliary carbon reagents for the Ga-NH₃ and Ga-C-NH₃ CVD systems and prerequisites for reactor design to enhance and control the PHVPE process through the HCN synthesis.     

Tertiary phosphine complexes of nickel(0) and nickel(I). New dinitrogen complexes of nickel(0)

The reduction of NiX₂(PCy₃)₂ (X = Cl, Br; PCy₃ = tricyclohexylphosphine) in toluene with sodium sand under argon affords [NiX(PCy₃)₂]₂ or Ni(PCy₃)₃. In the same way starting from NiX₂P₂ [X = Cl, Br; P = P(C₂H₅)₃, P(CH₂CH₂CH₂CH₃)₃, P(C₂H₅)₂ C₆H₅] the tetracoordinate Ni(0) complexes NiP₄ are obtained. These give NiP₃(N₂) under nitrogen. The electronic spectra of Ni(0) and Ni(I) complexes, both in the solid state and solution, are reported.     

Review and Latest Results of Laboratory Investigations of Titan's Aerosols

Titan, the largest satellite of Saturn, has an atmosphere chiefly made up of N₂ and CH₄, and including many organics. This atmosphere also partly consists of hazes and aerosol particles which shroud the surface of this satellite, giving it a reddish appearance. The aerosols observed in Titan's atmosphere are thought to be synthesized at high altitudes (>300 km) and fall to the surface. Varying with temperature profiles, condensation phenomena take place in the lower atmosphere, about 100 km below. These solid particles, often called tholins, have been currently investigated for many years by laboratory scientists and physics modellers. This paper assesses past research and results in different fields (elemental composition, optical constants, pyrolysis, particle size), highlighting interests and questions aroused by these studies. It also presents the latest results and advances, and concludes with existing problems and future pathways.     

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.Presented at the 2nd ISSOL Meeting and the 5th ICOL in Kyoto, 5–10 April, 1977.     

Biomass fast pyrolysis in a fluidized bed reactor under N2, CO2, CO, CH4 and H2 atmospheres

Biomass fast pyrolysis is one of the most promising technologies for biomass utilization. In order to increase its economic potential, pyrolysis gas is usually recycled to serve as carrier gas. In this study, biomass fast pyrolysis was carried out in a fluidized bed reactor using various main pyrolysis gas components, namely N₂, CO₂, CO, CH₄ and H₂, as carrier gases. The atmosphere effects on product yields and oil fraction compositions were investigated. Results show that CO atmosphere gave the lowest liquid yield (49.6%) compared to highest 58.7% obtained with CH₄. CO and H₂ atmospheres converted more oxygen into CO₂ and H₂O, respectively. GC/MS analysis of the liquid products shows that CO and CO₂ atmospheres produced less methoxy-containing compounds and more monofunctional phenols. The higher heating value of the obtained bio-oil under N₂ atmosphere is only 17.8 MJ/kg, while that under CO and H₂ atmospheres increased to 23.7 and 24.4 MJ/kg, respectively.     

Biomass Pyrolysis in an Argon/Hydrogen Plasma Reactor

This paper presents the experimental results of biomass pyrolysis in a laboratory argon/hydrogen plasma reactor. The samples tested were wood and rice husk. The gaseous product was found to contain mainly H₂, CO, C₂H₂ and CH₄. The conversion of carbon and oxygen from the biomass feed to gaseous product can reach up to 79 % and 72 %, respectively. The results indicate that plasma pyrolysis of biomass may be a useful way for gaseous fuel production.     

Denitrification, Acetylene Reduction, and Methane Metabolism in Lake Sediment Exposed to Acetylene

Samples of sediment from Lake St. George, Ontario, Canada, were incubated in the laboratory under an initially aerobic gas phase and under anaerobic conditions. In the absence of added nitrate (NO₃⁻) there was O₂-dependent production of nitrous oxide (N₂O), which was inhibited by acetylene (C₂H₂) and by nitrapyrin, suggesting that coupled nitrification-denitrification was responsible. Denitrification of added NO₃⁻ was almost as rapid under an aerobic gas phase as under anaerobic conditions. The N₂O that accumulated persisted in the presence of 0.4 atm of C₂H₂, but was gradually reduced by some sediment samples at lower C₂H₂ concentrations. Low rates of C₂H₂ reduction were observed in the dark, were maximal at 0.2 atm of C₂H₂, and were decreased in the presence of O₂, NO₃⁻, or both. High rates of light-dependent C₂H₂ reduction occurred under anaerobic conditions. Predictably, methane (CH₄) production, which occurred only under anaerobiosis, was delayed by added NO₃⁻ and inhibited by C₂H₂. Consumption of added CH₄ occurred only under aerobic conditions and was inhibited by C₂H₂.     

Computational screening of ZIFs for CO2 separations

Using molecular simulations, we studied a diverse collection of zeolite–imidazolate frameworks (ZIFs) to evaluate their performances in adsorption- and membrane-based gas separations. Molecular simulations were performed for both single-component gases (CH₄, CO₂, H₂ and N₂) and binary gas mixtures (CO₂/CH₄, CO₂/N₂, CO₂/H₂ and CH₄/H₂) to predict the intrinsic and mixture selectivities of ZIFs. These two selectivities were compared to discuss the importance of multi-component mixture effects on making predictions about the separation performance of a material. Gas separation performances of ZIFs were compared with other nanoporous materials and our results showed that several ZIFs can outperform well-known zeolites and metal–organic frameworks in CO₂ separations. Several other properties of ZIFs such as gas permeability, working capacity and sorbent selection parameter were computed to identify the most promising materials in adsorption- and membrane-based separation of CO₂/CH₄, CO₂/N₂, CO₂/H₂ and CH₄/H₂.     

N-substituted ethylcarbamate complexes of thorium(IV) and lanthanum(III) nitrates

N-substituted ethylcarbamates form with thorium nitrate the complexes Th(NO₃)₄·3RHNC(O)OC₂H₅ (where R = CH₃, C₂H₅, C₆H₅(CH₃)CH) and with lanthanum nitrate the complexes La(NO₃)₃· 2RR′NC(O)OC₂H₅·3H₂O (where R = CH₃, C₂H₅, C₆H₅(CH₃)CH; R′ = H and R = CH₃, C₆H₅; R′ = C₂H₅ or R = R′ = CH₃). In addition the anhydrous La(NO₃)₃·3(C₂H₅)₂NC(O)OC₂H₅ has been isolated. From the IR spectra it is deduced that the carbamates coordinate the metal through the carbonyl oxygen atom and that the nitrato groups act as chelated ligands. ¹H nmr spectral data of the complexes are reported and discussed.     

Synthesis and study on telluronium salts based on 1-oxa-4-organo-4-telluracyclohexane cation

A new series of heterocyclic telluronium salts (C₄H₈OTeRX: R=CH₃CH₂, CH₂CHCH₂, CH₃, C₄H₉, X=I; R=CH₂CHCH₂, X=Br; R=CH₃, X=ClO₄; R=CH₃, CH₃CH₂, C₆H₅, X=BPh₄) have been prepared. Conductivity measurements in dimethylsulphoxide (DMSO) and N,N-dimethytl- formamide (DMF) have shown that there is an ion pair interaction between the anion and the tellurium cation.¹H NMR studies showed that there is no reaction between the solute and the solvent. All compounds are stable in solution in DMSO. Infra-red spectra are reported and discussed.     

Study on the photochemical reaction of HCN and its polymer products relating to primary chemical evolution

The photochemical reaction of HCN at 184.9 nm is studied in the gas phase. (CN)₂, H₂, CH₄, NH₃, N₂H₄, C₂H₆, and CH₃NH₂ are identified as gas phase products, and a reaction mechanism is proposed. HCN polymers^** are also obtained as solid reaction products, and their structure is investigated by Infrared Spectroscopy, UV-Visible Spectroscopy, Mass Spectrometry, and Amino acid Analysis. The process and nature of the formation of the polymers are discussed.     

Ethylene is involved in the autochemotropism of Phycomyces

The sporangiophore of the fungus Phycomyces blakesleeanus has the property of growing away from a barrier which is few mm from the growing zone of the sporangiophore (avoidance or autochemotropic response). A model has been published (Cohen, R.J., Jan, N.Y., Matricon, J., Delbrück, M.: J. Gen. Physiol. 66, 67–95 (1975)). To explain the avoidance response which postulates that the sporangiophore emits and readsorbs a volatile growth-promoting effector (gas X) and that the barrier modifies the effector distribution by acting as an aerodynamic obstacle, causing a higher concentration of gas X on the side of the sporangiophore closer to the barrier. From this model we deduced three properties of the gas X. Of the several gases tested (N₂, CO₂, CH₄, C₂H₂, C₂H₄, C₂H₆) only ethylene (C₂H₄) had all these three properties, a finding which suggests that it has a role in the avoidance response (autochemotropism).Abbreviation Spph Sporangiophore     

Emission of hydrogen from deep and shallow freshwater environments

In-situ partial pressures of hydrogen in anoxic profundal lake sediments reached values of up to 5 Pa which were more than 5 orders of magnitude lower than the partial pressures of methane. Analysis of gas bubbles collected from anoxic submerged paddy soil showed H₂ partial pressures in the range of 1.8 ± 1.3 Pa being ca. 4 orders of magnitude lower than the CH₄ partial pressures. H₂ emission rates, on the other hand, were less than 3 orders of magnitude lower than the CH₄ emission rates indicating that H₂ and CH₄ were oxidized to a different extent in the rhizosphere of the soil before they reached the atmosphere, or that H₂ was produced by the plants. More than 70% of the emitted H₂ reached the atmosphere via plant-mediated flux. The rest was emitted via ebullition from the anoxic soil and, in addition, was produced in the paddy water. A significant amount of H₂ was indeed found to be produced in the water under conditions where thallic algae and submerged parts of the rice plants produced oxygen by photosynthesis. Very little H₂ was emitted via molecular diffusion through the paddy water; in addition, this amount was less than expected from the degree of supersaturation and the diffusional emission rate of CH₄ indicating a relatively high rate of H₂ consumption in the surface film of the paddy water. The total H₂ source strength of rice paddies and other freshwater environments was estimated to be less than 1 Tg yr^-1, being negligible in the atmospheric budget of H₂.     

Regulation of growth in stem sections of deep-water rice

Submergence in water greatly stimulates internodal elongation in excised stem sections of deep-water rice (Oryza sativa L. cv. Habiganj Aman II) and inhibits growth of leaf blades and leaf sheaths. The highest rates of internodal growth have been observed in continuous light. Very little growth occurs in submerged sections kept in darkness or incubated under N₂ in the light. The effect of submergence on the growth of deep-water rice is, at least in part, mediated by C₂H₄, which accumulates in the air spaces of submerged sections. This accumulation results from increased C₂H₄ synthesis in the internodes of submerged sections and reduced diffusion of C₂H₄ from the tissue into the water. Increased C₂H₄ levels accelerate internodal elongation and inhibit the growth of leaves. Compounds capable of changing the rate of C₂H₄ synthesis, namely aminoethoxyvinylglycine, an inhibitor of C₂H₄ synthesis, and 1-aminocyclopropane-1-carboxylic acid, the immediate, precursor of C₂H₄, have opposite effects on growth of internodes and leaves. The enhancement of internodal elongation by C₂H₄ is particularly pronounced in an atmosphere of high CO₂ and low O₂. The increase in C₂H₄ synthesis in internodes of submerged sections is primarily triggered by reduced atmospheric concentrations of O₂. The rate of C₂H₄ evolution by internodes isolated from stem sections and incubated in an atmosphere of low O₂ is up to four times greater than that of isolated internodes incubated in air. In contrast, C₂H₄ evolution from the leaves is reduced under hypoxic conditions. The effect of submergence on growth of stem sections of deep-water rice can be mimicked by exposing non-submerged sections to a gas mixture which is similar to the gaseous atmosphere in the internodal lacunae of submerged sections, namely 3% O₂, 6% CO₂, 91% N₂ (by vol.) and 1 l l^-1 C₂H₄. Our results indicate that growth responses obtained with isolated rice stem sections are similar to those of intact deep-water rice plants.Abbreviations ACC 1-aminocyclopropane-1-carboxylic acid - AVG aminoethoxyvinylglycine