M.-C. Gazeau

Experimental laboratory simulation of Titan's atmosphere: aerosols and gas phase

The discovery that Titan, the largest satellite of Saturn, has an atmosphere and that methane is a significant constituent of it, was the starting point for a systematic study of Titan’s atmospheric organic chemistry. Since then, the results from numerous ground-based observations and two flybys of Titan, by Voyager I and II, have led to experimental laboratory simulation studies and photochemical and physical modeling. All these works have provided a more detailed picture of Titan. We report here a continuation of such a study performing an experimental laboratory simulation of Titan’s atmospheric chemistry, and considering the two physical phases involved: gases and aerosols. Concerning the gaseous phase, we report the first detection of C4N2 and we propose possible atmospheric abundances for 70 organic compounds on Titan’s upper atmosphere. Concerning the solid phase, we have characterized aerosol analogues synthesized in conditions close to those of Titan’s environment, using elemental analysis, pyrolysis, solubility studies and infrared spectroscopy. # 1999 Elsevier Science Ltd. All rights reserved.

Characterization of an N2 flowing microwave post-discharge by OES spectroscopy and determination of absolute ground-state nitrogen atom densities by TALIF

A flowing microwave post-discharge source sustained at 2.45 GHz in pure nitrogen has been investigated by optical emission spectroscopy (OES) and two-photon absorption laser-induced fluorescence (TALIF) spectroscopy. Variations of the optical emission along the post-discharge (near, pink and late afterglow) have been studied and the gas temperature has been determined. TALIF spectroscopy has been used in the late afterglow to determine the absolute ground-state nitrogen atomic densities using krypton as a reference gas. Measurements show that the microwave flowing post-discharge is an efficient source of N ( 4 S) atoms in late afterglow. In our experimental conditions, the maximum N ( 4 S) density is about 2.2 × 10 15 cm −3 for a pressure of 22 Torr, at 300 K. The decay of N ( 4 S) density as a function of the time spent in the quartz tube has been modelled and a wall recombination probability γ of (2.1 ± 0.3) × 10 −4 is obtained. (Some figures in this article are in colour only in the electronic version)

Report and implications of the first observation of C4N2 in laboratory simulations of Titan’s atmosphere

Abstract C4N2 is an exceptional organic compound in Titan; it is the only one to have been detected in condensed phase but not in gas phase. After the description of the delicate C4N2 laboratory synthesis and of the determination of C4N2 mass spectrum, we report here the first identification of gaseous C4N2 in laboratory simulations of Titan’s atmosphere, using our last experimental system, based on a N2–CH4 cold plasma at low temperature and low pressure. Finally, we discuss the implications of this identification in the frame of remote sensing observations of gaseous C4N2 in Titan’s atmosphere.

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 Titans 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 Titans 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.

In Situ Exploration of Titan and Cometary Nucleus: Implication for Exobiology

Titan, the largest satellite of Saturn, is in many aspects similar to the Earth. In spite of much lower temperatures and the absence of liquid water, because its environment is very rich in organics, and many couplings are involved in the various parts of its “geofluid”, Titan is a reference for studying prebiotic chemistry on a planetary scale. The NASA-ESA Cassini-Huygens mission, which will send Cassini into orbit around Saturn and the Huygens probe into Titan’s atmosphere, will systematically study organic chemistry in Titan’s “geofluid”. 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. Comets are also of prime importance. With their nuclei rich in C,H,N & O-containing compounds, cometary impacts may have largely contributed to the build up of organic matter on early Earth. The ESA Rosetta mission will include a spacecraft, which will approach and follow a comet for several years. It will carry landers which will be delivered to the surface of the comet, and will perform in situ studies of the cometary nucleus, including chemical (elemental and molecular) analysis. With the Horizon 2000 ESA program, these two very ambitious missions should provide many information of crucial importance for the field of exobiology.

Laboratory experiments as support to the built up of Titan’s theoretical models and interpretation of Cassini-Huygens data

To interpret the concentrations of the products measured in Titan’s atmosphere and to better understand the associated chemistry, many theoretical models have been developed so far. Unfortunately, large discrepancies are still found between theoretical and observational data. A critical examination of the chemical scheme included in these models points out some problems regarding the reliability of the description of critical reaction pathways as well as the accuracy of kinetic parameters. Laboratory experiments can be used to reduce these two sources of uncertainty. It can be: i) experimental simulations: in our laboratory (LISA), representative Titan’s simulation experiments are planned to be carried out in a reactor where the initial gas mixture will be exposed, for the first time, to both electrons and photons. Thus, the chemistry between N atoms and CH3 , CH2 , CH fragments, issued from electron dissociation of N2 and photo-dissociation of CH4 respectively, will be initiated. Thank to a time resolved technique, we will be able to analyse “in situ”, qualitatively and quantitatively, the stable species as well as the short life intermediates. Then, the implied chemistry will be determined precisely, and consequently, its description will be refined in theoretical models. The current status of this program will be given. ii) specific experiments: they are devoted, for example, to determine kinetic rate constants and low temperature VUV spectra that will be used to feed models and to interpret observational data. Such experiments performed in LISA and in Rennes’ laboratory concern polyynes and cyanopolyynes as these compounds could link the gaseous and the solid phase in planetary atmosphere. Results concerning C4H hydrocarbons kinetic rate constants and VUV cross section of HC3N and HC5N will be detailed.

The new Titan: an astrobiological perspective

Since the first Voyager data, Titan, the largest satellite of Saturn and only satellite in the solar system having a dense atmosphere, became one of the key planetary bodies for astrobiological studies, due to: i) its many analogies with planet Earth, in spite of much lower temperatures, ii) the already well observed presence of an active organic chemistry, involving several of the key compounds of prebiotic chemistry, in the gas phase but also assumed to occur in the solid phase through the haze particles. And the potential development of a prebiotic chemistry in liquid water, with a possible water ocean in its internal structure, and the possible episodic formation of small liquid water bodies for short but not negligible time duration at the surface (from the melting of surface water ice by impact), iii) the resulting possibility that life may have emerged on or in Titan and may have been able to adapt and to persist. These aspects are examined with some of the associated questions on the basis of the already available Cassini-Huygens data.

Titan : Exobiology and the Cassini-Huygens Mission

In the solar system, one can consider two classes of extraterrestrial bodies of prime interest for Exobiology. These are planetary bodies where (extinct or extent) life may be present, such as Mars and Europa, and bodies where a complex organic chemistry is taking place. Titan, Saturn’s largest satellite is probably, with the comets, one of the most exobiologically interesting bodies of this second kind.

Titan’s Chemistry : Exobiological Aspects and Expected Contribution from Cassini-Huygens

Because of the presence of a dense atmosphere, of an environment very rich in organics, and of many couplings involved in the various parts of its “geofluid”, Titan, the largest satellite of Saturn, is a reference for studying prebiotic chemistry on a planetary scale. By coupling the data obtained from simulation experiments carried out in the laboratory, theoretical modeling and observations, it is already possible to draw a quite precise figure of such organic chemistry, with all its complexity. In particular, new data have been obtained from experiments simulating the organic chemistry of Titan’s atmosphere (gas and aerosol phases), in an open reactor, within the correct range of temperature, pressure, carefully avoiding any chemical contamination. They show a very good agreement with the observational data, demonstrating for the first time the formation of all the organic species already detected in Titan’s atmosphere. In particular, C4N2, never detected before in this kind of laboratory experiment, although it has been detected in Titan’s atmosphere, is now clearly identified, together with many other species not yet detected in Titan. This strongly suggests the presence of more complex organics in Titan’s atmosphere and surface, including high molecular weight polyynes and cyanopolyynes. The NASA-ESA Cassini-Huygens mission was successfully launched in October 1997. The Cassini spacecraft will reach the Saturn system in 2004 and will become an orbiter around Saturn and fly-by Titan, while, simultaneously it will send the Huygens probe into Titan’s atmosphere. Both will systematically study organic chemistry in Titan’s “geofluid”. 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. This very ambitious mission should provide much information of crucial importance for our knowledge of the complexity of Titan’s chemistry, and, more generally for the field of exobiology.