Marie-Claire Gazeau

Cometary organic chemistry: a review from observations, numerical and experimental simulations

Abstract This paper is a review dealing with the organic chemistry of comets. It describes how the chemical composition of comets can provide information about the chemistry of the interstellar medium, and the formation of the solar system. We discuss to what extent they could have brought to Earth the ingredients essential to the emergence of life: water and prebiotic compounds. We review all molecules which have been detected or tentatively detected in comets by remote sensing or in-situ observations, inputs of theoretical models, and all other organic species expected to be present from the results of experimental simulations. This compilation yields a list of more than a hundred molecules which can be used as a reference for the preparation of experiments developed for the Rosetta and Deep Space 4 cometary missions. We point out that further experiments are necessary to investigate the connections between the solid and gaseous phases of comets, especially studying the photodegradation of high molecular weight compounds which could be present in the nuclei.

Organic chemistry in Titan's atmosphere: New data from laboratory simulations at low temperature

Many experiments have already been carried out to simulate organic chemistry on Titan, the largest satellite of Saturn. They can provide fruitful information on the nature of minor organic constituents likely to be present in Titans atmosphere, both in gas and aerosol phases. Indeed, all the organic compounds but one already detected in Titans atmosphere have been identified in simulation experiments. The exception, C4N2, as well as other compounds expected in Titan from theoretical modeling, such as other N-organics, and polyynes, first of all C6H2, have never been detected in experimental simulation thus far. All these compounds are thermally unstable, and the temperature conditions used during the simulation experiments were not appropriate. We have recently started a new program of simulation experiments with temperature conditions close to that of Titans environment. It also uses dedicated analytical techniques and procedures compatible with the analysis of organics only stable at low temperatures, as well solid products of low stability in the presence of O2 and H2O. Spark discharge of N2-CH4 gas mixtures was carried out at low temperature in the range 100-150 K. Products were analysed by FTIR, GC and GC-MS techniques. GC-peaks were identified by their mass spectrum, and, in most cases, by comparison of the retention time and mass spectrum with standard ones. We report here the first detection in Titan simulation experiments of C6H2 and HC5N. Their abundance is a few percent relative to C4H2 and HC3N, respectively. Preliminary data on the solid products indicate an elemental composition corresponding to (H11C11N)n. These results open new prospects in the modeling of Titans haze making.

Experimental simulation of Titan's organic chemistry at low temperature

A wide range of experiments has already been carried out to simulate the chemical evolution of Titan. Such experiments can provide useful information on the possible nature of minor constituents, mostly organic, likely to be present in Titans atmosphere. Indeed, all but one of the organic compounds already detected in Titans atmosphere have been identified in simulation experiments. The exception, C4N2, as well as other compounds expected in Titan from theoretical modeling, such as other N-organics, mainly CH2N2, and polyynes, namely C6H2, have never been detected in experimental simulation. It turned out that these compounds were thermally unstable, and the temperature conditions used during the simulation experiments (including conditions used for chemical analysis) were not appropriate. We have recently started a new program of simulation experiments using temperature conditions close to those of Titans environment, more compatible with the build-up and detection of organics only stable at low temperature. Spark discharge of N2-CH4 gas mixtures was carried out at low temperature in the range of 100-150 K. The analysis of the obtained products was performed through FTIR, GC and GC-MS techniques. GC-peak identification was done owing to its mass spectrum and, in most cases, by comparison of the retention time and of the mass spectrum with standards. We report here the first detection in Titans simulation experiments of C6H2. Its abundance is a few 10(-2) relative to C4H2. We also report a tentative identification of HC5N (to be confirmed by use of standard) with an abundance of a few 10(-2) relative to HC3N. The possible presence of HC5N suggested by our work provides the occurrence of very novel pathways in the formation of Titans organic aerosols, involving not only C and H but also N atoms.

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 Titans 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.

Polyoxymethylene as Parent Molecule for the Formaldehyde Extended Source in Comet Halley

Among unsolved questions raised by observations of comets is the origin of extended sources, i.e., the distribution of molecules in the coma which cannot be explained by a direct sublimation from the nucleus. Polyoxymethylene [formaldehyde polymer: also called POM] is sometimes ([CH 2 -O[) n , invoked as a potential parent compound, the degradation of which could produce the required amount of across the coma, but no quantitative study has ever been undertaken with relevant parameters. H 2 CO From new experimental data, we are now able to consider multiphase chemistry: POM in the solid state on cometary grains slowly degrades by solar photons and heat and produces in the gaseous H 2 CO phase. This is a new approach to cometary organic chemistry. We show, by considering simple assumptions about the cometary environment, that the hypothesis of POM on grains leads to a very good agreement with Giotto observations if we assume that the cometary grains are D7% POM by mass at a temperature of 330 K.

High-temperature measurements of VUV-absorption cross sections of CO2 and their application to exoplanets

Context. Ultraviolet (UV) absorption cross sections are an essential ingredient of photochemical atmosphere models. Exoplanet searches have unveiled a large population of short-period objects with hot atmospheres, very different from what we find in our solar system. Transiting exoplanets whose atmospheres can now be studied by transit spectroscopy receive extremely strong UV fluxes and have typical temperatures ranging from 400 to 2500 K. At these temperatures, UV photolysis cross section data are severely lacking. Aims. Our goal is to provide high-temperature absorption cross sections and their temperature dependency for important atmospheric compounds. This study is dedicated to CO2, which is observed and photodissociated in exoplanet atmospheres. We also investigate the influence of these new data on the photochemistry of some exoplanets. Methods. We performed these measurements with synchrotron radiation as a tunable VUV light source for the 115–200 nm range at 300, 410, 480, and 550 K. In the 195–230 nm range, we used a deuterium lamp and a 1.5 m Jobin-Yvon spectrometer and we worked at seven temperatures between 465 and 800 K. We implemented the measured cross section into a 1D photochemical model. Results. For λ> 170 nm, the wavelength dependence of ln(σCO2 (λ,T ) × 1 Qv(T ) ) can be parametrized with a linear law. Thus, we can interpolate σCO 2 (λ,T ) at any temperature between 300 and 800 K. Within the studied range of temperature, the CO2 cross section can vary by more than two orders of magnitude. This, in particular, makes the absorption of CO2 significant up to wavelengths as high as 230 nm, while it is negligible above 200 nm at 300 K. Conclusions. The absorption cross section of CO2 is very sensitive to temperature, especially above 160 nm. The model predicts that accounting for this temperature dependency of CO 2 cross section can affect the computed abundances of NH 3 ,C O 2 , and CO by one order of magnitude in the atmospheres of hot Jupiter and hot Neptune. This effect will be more important in hot CO 2 -dominated atmospheres.

An exobiological view of Titan and the Cassini-Huygens mission

Abstract The largest satellite of Saturn, Titan, is the only one in the solar system having a dense atmosphere. In many aspects it is similar to the Earth. Moreover, organic chemistry on Titan and prebiotic chemistry on Earth involve the same N-containing organic molecules: nitriles, including acetylenic nitriles and dinitriles, and their oligomers. Thus, in spite of much lower temperatures and the absence of liquid water, because of its environment very rich in organics, and the many couplings involved in the various parts of its “geofluid”, Titan is a reference laboratory for studying prebiotic chemistry on a planetary scale. In the frame of the NASA-ESA Cassini-Huygens mission, which includes an orbiter (Cassini) around Saturn and a probe (Huygens) in Titans atmosphere, organic chemistry in Titans “geofluid” will be studied in great detail. In situ measurements, in particular from Huygens GC-MS and ACP instruments, will provide detailed analysis of the organics present in the air, aerosols, and surface. The mission will be launched in october 1997, for an arrival in the Saturn System in 2004. Thus, at horizon 2000, we can expect many information of crucial importance for the field of exobiology.

A COMBINED EXPERIMENTAL AND THEORETICAL STUDY OF THE CATALYTIC DISSOCIATION OF METHANE BY THE PHOTOLYSIS OF ACETYLENE AT 185 NM

Abstract Experimental irradiations of CH4–C2H2 (1000 : 1) gas mixtures, at 185 nm, have been carried out in order to test the catalytic scheme of dissociation of methane, via the photolysis of acetylene, that is supposed to occur in Titans stratosphere. The theoretical evolution of such a system has been investigated using three chemical models, and the predictions compared with our experimental data. Our experiments confirm that dissociation of methane can be catalysed by the photolysis of acetylene at 185 nm. The quantitative comparison with theoretical results reveals an experimental effect, which we explain as a heterogeneous process occurring on the walls of the reactor. We propose a mechanism for this wall effect that is in qualitative agreement with our observations. We are currently developing an approach that will allow quantification of this effect, with the aim of extrapolating laboratory results to an open system. We will therefore be able to perform a critical comparison with existing gas-phase schemes to improve the chemical modelling of Titans atmosphere.

Semi-empirical calculation of electronic absorption wavelengths of polyynes, monocyano- and dicyanopolyynes. Predictions for long chain compounds and carbon allotrope carbyne

Abstract Absorption wavelengths and oscillator strengths have been calculated using the ZINDO method for the allowed low-energy electronic transition 1 Σ + ← 1 Σ + of monocyanopolyynes ( HC y N , y=1–13 ), 1 Σ u + ← 1 Σ g + of polyynes and dicyanopolyynes (HCyH and NC y N , y=1–40 ). For y>18, the geometries were extrapolated from DFT optimization of the shortest members. Extrapolation formulas have been drawn up for longer chains, the asymptotic values of those yield an estimation of the absorption wavelength (ca. 400 nm) of the hypothetical carbon allotrope carbyne.

Thermally unstable polyynes and N-organics of planetological interest: New laboratory data and implications for their detection by in situ and remote sensing techniques

In the frame of a laboratory program on organic compounds of planetological interest only stable at low temperatures, we are currently studying the gas chromatographic (GC) and mass spectrometric (MS) behaviour of gaseous polyynes and N-organics, together with their UV and IR spectra. We report here new experimental data on triacetylene (C6H2) expected to be present in Titans atmosphere and dicyanoacetylene (C4N2), already detected in this environment. The GC-MS behaviour of these compounds has been studied using fused silica wall coated open capillary column and Ion trap mass spectrometer. Quantitative GC-MS analysis is possible if the chromatographic conditions use room temperature, fast separation (less than about 20 minutes) and low partial pressure of these unstable compounds. The limit of detectability of C4N2 by GC-MS analysis is 500 pg, about 50 times higher than that of C6H2. The UV and IR spectra of these compounds, which were only partially available in the literature, have been studied, including at low temperature. The strength of the main signatures (UV absorption coefficients and IR integrated intensities) have been systematically determined. In particular, we report the first quantitative spectroscopic data for C6H2. Such combined experimental studies avoid the risk of wrong diagnostics due to possible chemical contamination. The application of these techniques for searching for these organics in planetary atmospheres is also discussed.