Analysis of complex mixtures recovered from space missions statistical approach to the study of Titan atmosphere analogues (tholins)

To study Titan, the largest moon of Saturn, laboratory simulation experiments have been performed to obtain analogues of Titan's aerosols (named tholins) using different energy sources. Tholins, which have been demonstrated to represent aerosols in Titan's haze layers, are a complex mixture, resulting from the chemical evolution of several hydrocarbons and nitriles. Their chromatographic analysis yields complex chromatograms, which require the use of mathematical procedures to extract from them all the information they contain. Two different chemometric approaches (the Fourier analysis approach and the statistical model of peak overlapping) have been successfully applied to pyrolysis-GC-MS chromatogram of a tholin sample. Fundamental information on the mixture's chemical composition (number of components, m) and on the separation system performance (separation efficiency, sigma) can be easily estimated: the excellent correspondence between the data calculated by the two independent procedures proves the reliability of the statistical approaches in characterizing a tholin chromatogram. Moreover, the plot of autocorrelation function contains, in a simplified form, all the information on the retention pattern: retention recursivities can be easily singled out and related to specific molecular structure variations. Therefore, the autocorrelation function (ACF) plot constitutes a simplified fingerprint of the pyrolysis products of tholins, which can be used as a powerful tool to characterize a tholin sample.

Chemical and optical behaviour of tholins, laboratory analogues of Titan aerosols

Since 1997, after having identified for the first time C4N2 (the only molecule detected on Titan and undetected in the laboratory at this date) in a simulated atmosphere of Titan, our group intended to determine several properties (including optical behavior) of laboratory analogues of Titan's tholins. This article summarizes the results obtained in the frame of that program (observation by microscopy, solubility in hydrocarbons and nitriles, chemical composition, and optical behavior in the 200-900nm range), and finally investigates the following items: what are the key questions still remaining?; how to answer them?

Production of nitrogen oxides by lightning and coronae discharges in simulated early Earth, Venus and Mars environments

We present measurements for the production of nitrogen oxides (NO and N2O) in CO2-N2 mixtures that simulate different stages of the evolution of the atmospheres of the Earth, Venus and Mars. The nitrogen fixation rates by two different types of electrical discharges, namely lightning and coronae, were studied over a wide range in CO2 and N2 mixing ratios. Nitric oxide (NO) is formed with a maximum energy yield estimated to be ~1.3 x 10(16) molecule J-1 at 80% CO2 and ~1.3 x 10(14) molecule J-1 at 50% CO2 for lightning and coronae discharges, respectively. Nitrous oxide (N2O) is only formed by coronae discharge with a maximum energy yield estimated to be ~1.2 x 10(13) molecule J-1 at 50% CO2. The pronounced difference in NO production in lightning and coronae discharges and the lack of formation of N2O in lightning indicate that the physics and chemistry involved in nitrogen fixation differs substantially in these two forms of electric energy.

Production of hydrocarbons and nitriles by electrical processes in Titan's atmosphere

Although lightning has not been observed in Titan's atmosphere, the presence of methane rain in the troposphere suggests the possibility of electrical activity in the form of corona and/or lightning discharges. Here we examine the chemical effects of these electrical processes on a Titan simulated atmosphere composed of CH4 in N2 at various mixing ratios. Corona discharges were simulated in two different experimental arrays. For the detection of reactive intermediates we used a mass spectrometer to study the main positive ions arising by bombarding low-energy electrons from a hot filament into low-pressure methane. The final stable products, generated by applying a high voltage in a coaxial reactor with either positive or negative polarity, were separated and detected by gas chromatography-Fourier transform infrared spectroscopy and electron impact mass spectrometry (GC-FTIR-MS). Lightning discharges were simulated by a hot and dense plasma generated by a Nd-YAG laser and the final products were separated and detected by GC-FTIR-MS. Corona discharges produce linear and branched hydrocarbons as well as nitriles whereas lightning discharges generate mainly unsaturated hydrocarbons and nitriles. Lightning discharges are about 2 orders of magnitude more efficient in product formation than corona discharges.

Possible contribution of different energy sources to the production of organics in Titan's atmosphere

A quantitative comparison of the products arising from the irradiation of a Titan's simulated atmosphere is presented. The energy sources used represent some of the main events that occur in the satellite's atmosphere. All of the compounds identified are classified in the hydrocarbon and nitrile chemical families. Almost all of the detected compounds in Titan's atmosphere are produced by one or more energy sources. The compounds with the highest energy yields include the C2 hydrocarbons, methanonitrile and ethanonitrile. The possibility of using some of the produced organics as tracer compounds during the Huygens descend to identify the leading energy form in the different atmospheric levels remains open.

Experimental simulation of early Martian volcanic lightning

A mixture of possible Martian volcanic gases were reproduced and irradiated by a high-energy infrared laser to reproduce the effects of lightning on the production of prebiotic molecules. The analysis of products were performed by a gas chromatograph interfaced in parallel with a FTIR-detector and a quadrupole mass spectrometer equipped with an electron impact and chemical ionization modes. The main products identified were hydrocarbons and an uncharacterized yellow film deposit. Preliminary results indicate the presence of hydrogen cyanide among the resultant compounds.

Transport of extraterrestrial biomolecules to the Earth: problem of thermal stability

The idea of extraterrestrial delivery of organic matter to the early Earth is especially attractive at present and is strongly supported by the detection of a large variety of organic compounds, including amino acids and nucleobases, in carbonaceous chondrites. Whether these compounds can be delivered by other space bodies is unclear and depends primarily on capability of the biomolecules to survive high temperatures during atmospheric deceleration and impacts to the terrestrial surface. In the present study we estimated survivability of simple amino acids (alpha-aminoisobutyric acid, L-alanine, L-valine and L-leucine), purines (adenine and guanine) and pyrimidines (uracil and cytosine) under rapid heating to temperatures of 400 to 1000 degrees C under N2 or CO2 atmosphere. We have found that most of the compounds studied cannot survive the temperatures substantially higher than 700 degrees C; however at 500-600 degrees C, the recovery can be at a per cent level (or even 10%-level for adenine, uracil, alanine, and valine). Implications of the data for extraterrestrial delivery of the biomolecules are discussed.

A conceptual design for cosmo-biology experiments in Earth's Orbit

A conceptual design was developed for a cosmo-biology experiment. It is intended to expose simulated interstellar ice materials deposited on dust grains to the space environment. The experimental system consists of a cryogenic system to keep solidified gas sample, and an optical device to select and amplify the ultraviolet part of the solar light for irradiation. By this approach, the long lasting chemical evolution of icy species could be examined in a much shorter time of exposure by amplification of light intensity. The removal of light at longer wavelength, which is ineffective to induce photochemical reactions, reduces the heat load to the cryogenic system that holds solidified reactants including CO as a constituent species of interstellar materials. Other major hardware components were also defined in order to achieve the scientific objectives of this experiment. Those are a cold trap maintained at liquid nitrogen temperature to prevent the contamination of the sample during the exposure, a mechanism to exchange multiple samples, and a system to perform bake-out of the sample exposure chamber. This experiment system is proposed as a candidate payload implemented on the exposed facility of Japanese Experiment Module on International Space Station.

Corona chemistry in Titan

The atmosphere of Titan is constantly bombarded by galactic cosmic rays and Saturnian magnetospheric electrons causing the formation of free electrons and primary ions, which are then stabilized by ion cluster formation and charging of aerosols. These charged particles accumulate in drops in cloud regions of the troposphere. Their abundance can substantially increase by friction, fragmentation or collisions during convective activity. Charge separation occurs with help of convection and gravitational settling leading to development of electric fields within the cloud and between the cloud and the ground. Neutralization of these charged particles leads to corona discharges which are characterized by low current densities. We have therefore, experimentally studied the corona discharge of a simulated Titan's atmosphere (10% methane and 2% argon in nitrogen) at 500 Torr and 298 K by GC-FTIR-MS techniques. The main products have been identified as hydrocarbons (ethane, ethyne, ethene, propane, propene+propyne, cyclopropane, butane, 2-methylpropane, 2-methylpropene, n-butane, 2-butene, 2,2-dimethylpropane, 2-methylbutane, 2-methylbutene, n-pentane, 2,2-dimethylbutane, 2-methylpentane, 3-methylpentane, n-hexane, 2,2-dimethylhexane, 2,2-dimethylpentane, 2,2,3-trimethylbutane, 2,3-dimethylpentane and n-heptane), nitriles (hydrogen cyanide, cyanogen, ethanenitrile, propanenitrile, 2-methylpropanenitrile and butanenitrile) and a highly branched hydrocarbon deposit. We present the trends of hydrocarbons and nitriles formation as a function of discharge time in an ample interval and have derived their initial yields of formation. The results clearly demonstrate that a complex organic chemistry can be initiated by corona processes in the lower atmosphere. Although photochemistry and charged particle chemistry occurring in the stratosphere can account for many of the observed hydrocarbon species in Titan, the predicted abundance of ethene is too low by a factor of 10 to 40. While some ethene will be produced by charged-particle chemistry, the production of ethene by corona processes and its subsequent diffusion into the stratosphere appears to be an adequate source. Because little UV penetrates to the lower atmosphere to destroy the molecules formed there, the corona-produced species may be long-lived and contribute significantly to the composition of the lower atmosphere and surface.

Behavior of amino acids when volatilized in the presence of silica gel and pulverized basaltic lava

To evaluate the types of amino acid thermal transformations caused by silicate materials, we studied the volatilization products of Aib, L-Ala, L-Val and L-Leu under temperatures of up to 270 degrees C in the presence of silica gel as a model catalyst and pulverized basaltic lava samples. It was found that silica gel catalyzes nearly quantitative condensation of amino acids, where piperazinediones are the major products, whereas lava samples have much lower catalytic efficiency. In addition bicyclic and tricyclic amidines and several products of their subsequent thermal decomposition have been identified using the coupled technique of GC-FTIR-MS and HPLC-PB-MS, with auxiliary computer simulation of IR spectra and NMR spectroscopy. The decomposition is due to dehydrogenation, elimination of the alkyl substituents and dehydration as well as cleavage of the bicyclic ring system. The imidazole ring appears to be more resistant to thermal decomposition as compared to the pyperazine moiety, giving rise to the formation of different substituted imidazolones. The amidines were found to hydrolyze under treatment with concentrated HCl, releasing the starting amino acids and thus behaving as amino acid anhydrides. The thermal transformations cause significant racemization of amino acid residues. Based on our observations, the formation of amidine-type products is suggested to be rather common in the high-temperature experiments on amino acid condensation.

Power measurements of spark discharge experiments

An accurate and precise knowledge of the amount of energy introduced into prebiotic discharge experiments is important to understand the relative roles of different energy sources in the synthesis of organic compounds in the primitive Earth's atmosphere and other planetary atmospheres. Two methods widely used to determine the power of spark discharges were evaluated, namely calorimetric and oscilloscopic, using a chemically inert gas. The power dissipated by the spark in argon at 500 Torr was determined to be 2.4 (+12%/-17%) J s-1 by calorimetry and 5.3 (+/- 15%) J s-1 by the oscilloscope. The difference between the two methods was attributed to (1) an incomplete conversion of the electric energy into heat, and (2) heat loss from the spark channel to the connecting cables through the electrodes. The latter contribution leads to an unwanted effect in the spark channel by lowering the spark product yields as the spark channel cools by mixing with surrounding air and by losing heat to the electrodes. Once the concentrations of the spark products have frozen at the freeze-out temperature, any additional loss of heat from the spark channel to the electrodes has no consequence in product yields. Therefore, neither methods accurately determines the net energy transferred to the system. With a lack of a quantitative knowledge of the amount of heat loss from the spark channel during the interval from ignition of the spark to when the freeze-out temperature is reached, it is recommended to derive the energy yields of the spark products from the mean value of the two methods with the uncertainty being their standard deviation. For the case of argon at 500 Torr, this would be 3.8 (+/-50%) J s-1.

Corona discharge of Titan's troposphere

The atmosphere of Titan is constantly bombarded by galactic cosmic rays and Saturnian magnetospheric electrons causing the formation of free electrons and primary ions, which are then stabilized by ion cluster formation and charging of aerosols. These charged particles accumulate in drops in cloud regions of the troposphere. Their abundance can substantially increase by friction, fragmentation or collisions during convective activity. Charge separation occurs with help of convection and gravitational settling leading to development of electric fields within the cloud and between the cloud and the ground. Neutralization of these charge particles leads to corona discharges which are characterized by low current densities. These electric discharges could induce a number of chemical reactions in the troposphere and hence it is of interest to explore such effects. We have therefore, experimentally studied the corona discharge of a simulated Titan's atmosphere (10% methane and 2% argon in nitrogen) at 500 Torr and 298 K by GC-FTIR-MS techniques. The main products have been identified as hydrocarbons (ethane, ethyne, ethene, propane, propene + propyne, cyclopropane, butane, 2-methylpropane, 2-methylpropene, n-butene, 2-butene, 2,2-dimethylpropane, 2-methylbutane, 2-methylbutene, n-pentane, 2,2-dimethylbutane, 2-methylpentane, 3-methylpentane, n-hexane, 2,2-dimethylhexane, 2,2-dimethylpentane, 2,2,3-trimethylbutane, 2,3-dimethylpentane and n-heptane), nitriles (hydrogen cyanide, cyanogen, ethanenitrile, propanenitrile, 2-methylpropanenitrile and butanenitrile) and an uncharacterized film deposit. We present their trends of formation as a function of discharge time in an ample interval and have derived their initial yields of formation. These results clearly demonstrate that a complex organic chemistry can be initiated by corona processes in the lower atmosphere. Although photochemistry and charged particle chemistry occurring in the stratosphere can account for many of the observed hydrocarbon species in Titan, the predicted abundance of ethene is to low by a factor of 10 to 40. While some ethene will be produced by charged-particle chemistry, its production by corona processes and subsequent diffusion into the stratosphere appears to be an adequate source. Because little UV penetrates to the lower atmosphere to destroy the molecules formed there, the corona-produced species may be long-lived and contribute significantly to the composition of the lower atmosphere and surface.

Possible role of volcanic ash-gas clouds in the Earth's prebiotic chemistry

Volcanic ash-gas clouds represent versatile local atmospheric environments appropriate for abiotic synthesis of rather complex organic molecules due to the simultaneous presence of various gaseous reagents, catalytically active inorganic particles, electric discharges, pressure and temperature gradients. They are relatively readily attainable for the scientists, contrary to objects or events of space origin (interstellar and planetary dust, meteoritic/cometary impacts, etc.), providing excellent opportunities for in situ studies and grounded simulating experiments. This paper reviews the available data on this environment, its most important chemical and physical parameters. Based on this analysis, it is suggested in brief experimental conditions for the simulation.

Dust in the Universe: implications for terrestrial prebiotic chemistry

In the present review we analyze the available literature on the distribution of dust in the Universe, methods of its observation and determination of the chemical composition, and the roles for terrestrial prebiotic chemistry. The most plausible natural sources of dust on the Earth in the prebiotic era are sedimentation of interplanetary dust, meteoritic and cometary impacts, volcanic eruptions, and soil microparticulates; the interplanetary medium being among the most powerful supplier of the dust matter. Two fundamental roles of dust particles for the origins of life are considered: (1) catalytic formation of prebiotic compounds; and (2) delivery of organic matter to the Earth by space dust particles. Due to the fact that there is only approximate information on the chemical composition and properties of interstellar, circumstellar, and major part of interplanetary dust, even the simulating experiments are difficult to perform. Until these gaps are filled, it seems reasonable to focus efforts of the scientists dealing with dust-driven catalytic formation of prebiotically important compounds on the volcanic and meteoritic/cometary impact environments.

Radiolysis of aqueous formaldehyde relevant to cometary environments

The radiation chemistry of aqueous solutions of formaldehyde was studied in order to obtain an insight into the possible role of ionizing radiation on cometary environments. Aqueous solutions of 1.0 mol dm-3 formaldehyde were exposed to gamma-radiation in the dose range from 0.01 to 1200 kGy at 298 K. The radiation chemical yield of decomposition of formaldehyde was determined to be: G(-CH2(OH)2)-26.3 +/- 1.2. The high radiation chemical yield of decomposition was explained by a chain reaction initiated by the radical CH(OH)2 with formaldehyde. Computer fitting of the experimental data gives k(CH(OH)2 + CH2(OH)2)- 8.0xl0(1) dm3 mol-1 s-1. In the computer treatment of experimental findings we used 54 equations to consider the radiolysis of water and 11 reactions for the radiolysis of aqueous formaldehyde. Based on previous estimates of the total dose of ionizing radiation that comets have accumulated over 4.6 billion years, we predict a radiation damage-depth curve of formaldehyde in comet nuclei.

Radiation-induced syntheses in cometary simulated models

The behavior of an aqueous-dominant multicomponent cometary model is examined at high doses of ionizing radiation. The system is composed of a water mixture of HCN (0.2 mol dm-3), CH3CN (0.04 mol dm-3), C2H5CN (0.02 mol dm-3), CH3OH (0.12 mol dm-3) and HCO2H (0.01 mol dm-3. It was exposed to gamma rays at doses up to 18.5 MGy. The chemical kinetic database used in the computer treatment of experimental data consists of 79 reactions. A complex mixture of products has been synthesized: gases, amino acids, carboxylic acids and polymeric material. The results suggest that the pristine material in cometary nuclei may have been chemically altered by the action of cosmic rays and embedded radionuclides.