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When lightning strikes the ground, it heats, melts, and fuses the sand in soils to form glass tubes known as fulgurites. We report here the composition of CO 2 , CO, and NO contained within the glassy bubbles of a fulgurite from the Libyan Desert. The results show that the fulgurite formed when the ground contained 0.1 wt% organic carbon with a C/N ratio of 10–15 and a δ 13 C of –13.96‰, compositions similar to those found in the present-day semiarid region of the Sahel, where the vegetation is dominated by C 4 plants. Thermoluminescence dating indicates that this fulgurite formed ~15 k.y. ago. These results imply that the semiarid Sahel (at 17°N) reached at least to 24°N at this time, and demonstrate that fulgurite gases and luminescence geochronology can be used in quantitative paleoecology.

UV spectroscopy is a powerful diagnostic tool in the study of organic composition and chemical processes in planetary atmospheres such as that of Titan. The Spectroscopie UV d'Interet Prebiologique dans l'Atmosphere de Titan (SIPAT) laboratory experiment has been developed in order to systematically determine at high resolution the absorption coefficients of expected molecular absorbers in Titan's atmosphere, with the goal of interpreting mid-UV albedo spectra. We have studied the mid-UV absorption coefficient of propynenitrile (cyanoacetylene), which is a key compound in prebiotic chemistry. In spite of all the precautions taken during the synthesis of this compound, the samples studied contain impurities. Using complementary techniques, we have been able to characterize the impurities (mainly acetone and acetonitrile) in the propynenitrile gas sample. Thus, we present new absorption coefficients of propynenitrile determined at 298 K and at 0.02 nm resolution between 185 and 200 nm (the wavelength range where acetone presents strong absorption features). A precise determination of impurity mixing ratios affecting previously published spectra leads to a reevaluation and the establishment of associated uncertainty values for HC3N absorption coefficients in the 185- to 230-nm range. Finally, these data have been corrected for an erroneous wavelength calibration implying a 3 A shift.

The interpretation of albedo spectra in the mid-UV range is critically dependent on the knowledge of the absolute absorption coefficient, at specific temperatures and with high resolution for molecular compounds likely to absorb the scattered solar radiation in this wavelength range. Propynenitrile, a key molecule in prebiotic synthesis, already detected with IR and microwave sounding in Titan's atmosphere, exhibits intense absorption bands in the 185 to 230 nm interval. This paper deals with the absolute absorption coefficient k of propynenitrile at low temperature and high resolution and compares it to the room temperature values. The consequences of the observed k variations on the interpretation of Titan's atmosphere spectra, expected in the near future, are considered here. As an additional result, the pressure regulation system of the experiment allows us to assess the determination of the temperature dependence of the propynenitrile vapor pressure.

Possible approaches to the study of organic chemistry in planetary atmospheres are threefold: they comprise theoretical modeling, simulation experiments, and observational programs. Because of their respective merits and limitations, these approaches are quite complementary, and their simultaneous improvement is the way to progress further in the field. All three ask for laboratory work, and the lack, or limited accuracy, of laboratory data is the main restriction to future improvement. Together with the development of theoretical modeling (based on chemical kinetics and depending on laboratory studies of reaction pathways and rate constants) laboratory simulation remains a powerful technique. Despite the inaccurate reproduction of all planetary conditions, this experimental approach yields precious information on the nature of middle and higher order molecular weight organics that can be expected in an atmosphere of a given overall composition; and there is, in general, good agreement between the data obtained from simulations and those derived from observations. Indeed, several of the organic species highlighted in such experiments, and their relative abundances, are compatible with those detected in related planetary atmospheres. This is shown in the particular case of Titan. Thus experimental results furnish information on the nature of organics to be searched for in planetary atmospheres, while, in turn, the detection of such candidates and possible indications of their concentration profiles, or the setting of upper limits to their abundancies, constrain the kinetic approach. Given the lists of candidates from simulation experiments, experimental programs for a systematic determination of spectroscopic characteristics, including frequencies and band or line intensities, of the likely organics in planetary atmospheres, have to be developed. As an example, experimental requirements and current results, both in the IR and the UV range, are presented concerning Titan's atmosphere in view of the Cassini-Huygens mission.

Available laboratory UV spectroscopic data do not usually fulfill all the conditions which are required for reducing observational data of planetary atmospheres: absolute wavelength calibration, wide wavelength range including the mid-UV, spectral resolution, absolute determination of the absorption coefficients, data at various temperatures. Faced with this fact in our research program on the structure and composition of Titan atmosphere, we have developed a suitable laboratory facility. We describe this experimental facility: it gives absolute absorption coefficients at 0.1 A resolution in the range 185–350 nm with sample temperatures adjustable between −50°C and +50°C. The future program to be implemented with this facility includes studies on nitriles and polyacetylenes.

A conference on Life in the Universe, organized by the Council of Europe, and held in Paris, November 19 to 21, 1979, brought together more than 100 scientists from different fields. The aim of this conference was to promote scientific cooperation in Europe, in conjunction with other countries, in fields related to the problem of Life in the Universe. Our knowledge concerning this problem was reviewed in three different sessions. The first session was devoted to the question of planetary evolution; the second session was devoted to the question of the origin and evolution of life, and the third session was devoted to the problem of evolution of intelligence and the search for extraterrestrial intelligence. The Drake formula was the keynote of the conference. Each session tried to estimate some of the parameters of this formula, in order to examine the question: how many extraterrestrial civilizations with a level of technology at least equal to ours may be present in the universe? Even if no definite answer is given to this question, this meeting pointed out the need for more research.

Previous kinetic studies on the addition of aliphatic thiols to activated nitriles suggest that the formation of iminothioesters (possible prebiotic precursors of thioesters) occurs through the nucleophilic addition of thiolate to the C=N group of the activated nitrile in its acidic form: $$R S^ {\ominus} + A - CH{_2} - C \equiv N {\overset {{H^ + }} \leftrightarrows} A - CH{_2} - \begin{array}{*{20}c} C \\ | \\ {SR} \\ \end{array} = NH$$

Organic photochemical syntheses in the Jovian atmosphere was simulated by irradiating, at 147 nm, gaseous mixtures of methane and ammonia with varying amounts of hydrogen. Some results relevant to the photochemistry of the Jupiter atmosphere at several tens of kilometers above the clouds were obtained: (1) a favorable effect of the pressure of high amounts of H2 on the yield of hydrocarbon synthesis when NH3 is mixed with CH4; (2) a very low yield of synthesis of unsaturated hydrocarbons in such conditions; and (3) the possibility of formation of detectable amounts of HCN and CH3CN.

The recent Voyager mission and the simulation experiments in the laboratory suggest that a complex nitrogen-organic chemistry is occuring at the periphery of Titan. Thus, this satel lite of Saturn appears as a privileged place in the solar system for the study of extraterrestrial organic chemistry which can be considered as part of Exobiology. Projects of space mission relating to Titan are already under investigation, in particular with the “CASSINI” proposal. The CASSINI project is a combination of a Saturn orbiter and a Titan probe mission. Such a mission would allow the first study “in situ” of a complex extraterrestrial organic chemistry in atmospheric phase.

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 Saturn's atmosphere are presented.

On Titan, most of the organics present in the atmosphere must condense in the lower stratosphere and be solid near the surface, except methane, ethane, propane, propene and 1-butene which must be liquid and could form oceans containing large fractions of dissolved N2. The solid organics, depending on their density relatively to this liquid, can accumulate at the surface or at the bottom of the oceans. In addition, depending on their solubility in this liquid and on their atmospheric flux down to the surface, they can dissolve partly or totally in the oceans. From this stage, Chemical Evolution on Titan must have followed a way very different from the terrestrial one, involving physical chemical processes in a cryogenic apolar solvent mainly composed of CH4-C2H6-N2, in place of organic chemistry in water. Systematic study of the volumic mass and solubility of organics in such a cryogenic mixture of various compositions, at 94 K, is presented, using thermodynamic modelling. The results suggest that the oceans of Titan could be free of any “icebergs” of organic compounds. These oceans could be very rich in dissolved organics, with relatively high concentrations, in the range 1–10 −6 M. In addition, the concentration of several of the organic solutes should be constant, buffered by a bottom layer of the corresponding compound in the solid phase.

H2S has been often invoked as the initial source of sulfur in prebiotic evolution, and several sulfur-containing compounds have been proposed as intermediates in the primordial synthesis of biologically relevant sulfur-containing chemicals. The possibilities of synthesis of the principal key intermediates by glow discharges in CH4-N2-H2S mixtures is studied. It is shown that synthesis of important intermediates such as HCN, (CN)2, CHCCN and CH3SH is possible from such mixtures if the amount of H2S is not more than 10%. For higher amounts of H2S, the syntheses are strongly inhibited.

In the coming decade, UV spectroscopy of Titan's atmosphere should appear as a powerful diagnostic tool which is likely to complement IR and microwave spectroscopy and future in situ experiments in the study of organic chemical processes which, on the Earth, could have led to the emergence of life. However, the interpretation of albedo spectra in the mid-UV range is critically dependent on the knowledge of the absorption coefficient of molecular compounds likely to absorb the scattered solar radiation at these wavelengths. Among them, propynenitrile is a key molecule in prebiotic synthesis and 2-butynenitrile is another simple acetylenic molecule: both present intense absorption in the UV. This first paper in a series aimed at interpreting observational data presents spectra of these two acetylenic nitriles obtained in the laboratory at room temperature between 185 and 250 nm and at resolutions from a few tenths of a nanometer up to 0.02 nm. Vibrational analysis of the 2-butynenitrile spectrum is presented for the first time. Also in the case of propynenitrile, some assignments are new relative to the previous work of V.A. Job and G.W. King (1966b, J. Mol. Spectrosc. 19, 178–184). From the analysis it is concluded that the excited state of the corresponding band system is a 1 Δ , rather than a 1 Σ − , a possibility considered as equally possible in the paper of Job and King.

Voyager1 radio-occultation refractivity profiles of Titan in the 0–200 km altitude range are reanalyzed in order to derive limiting profiles for the equatorial thermal structure, taking into account uncertainties on the mean molecular weight m. The major atmospheric constituents are assumed to be N2 and CH4 (already detected) and Ar which is a plausible additional constituent when m is greater than 28. The stratospheric abundance of CH4 is assumed to be limited by saturation at the tropopause. Spectra calculated from the infrared thermal profiles in the CH4 ν4 band at 7.7 μm are compared with the Voyager IRIS observations. This allows us to constrain both the abundances of major atmospheric components and the stratospheric temperatures. The methane mole fraction is between 0.5 and 3.4% in the stratosphere but may be as high as 21% at the surface, whereas the atmospheric Ar/N2 ratio can be anything between 0 and 0.27. The temperature is between 92.5 and 101°K at the ground level, and between 70.5 and 74.5°K at the tropopause. The maximum uncertainty (±4°K) occurs between 150 and 200 km altitude. A “nominal” temperature and composition profile of Titan's atmosphere between the ground and 200 km is given with tabulated values. The implications of composition and surface temperature uncertainties on the abundances of the expected major oceanic constituents are discussed, as well as the consequences for the solubility of noncodenssable atmospheric constituents (H2 and CO) and organic compounds raining out from the atmosphere.

A list of volatile nitriles, not yet detected in the atmosphere of Titan, but likely to be present in this environment, has been selected: acetonitrile, propionitrile, acrylonitrile, crotononitrile, allyl cyanide, methacrylonitrile, and cyanopropyne. The spectra of these compounds in the gas phase have been systematically studied, in the mid- and far-infrared ranges. For each selected nitrile, the most intense vibration bands have been determined. Their strengths have been characterized by estimating the monochromatic absorption coefficient at the maximum(s) of the bands, and the integrated absorbance over the entire band. Then, in order to estimate the detectability of the selected compounds by infrared spectroscopy in the atmosphere of Titan, the data obtained have been extrapolated to the case of Titan.

Surface temperature and atmospheric composition inferred from a recent reanalysis of Voyager 1 measurements (E. Lellouch et al. 1989, Icarus 79, 328–349) are used to constrain several properties of Titan's ocean. Thermodynamic equilibrium is assumed between atmosphere and ocean. The ocean is considered as a nonideal solution. Depending on atmospheric parameters, the main ocean features can vary drastically. Assuming a 92.5°K surface temperature, a 1.5-atm surface pressure, and an argon free atmosphere, a 1.55% methane mole fraction in the low troposphere implies an ocean composed of more than 90% ethane (and heavier alkanes), about 7.3% methane, and 1.8% nitrogen. At 101°K for the same surface pressure with an atmosphere containing 17% argon and 21.1% methane, the ocean is composed of 5% ethane and higher alkanes, 83.4% methane, 6% nitrogen, and 5.6% argon. Other components are quantitatively negligible. The solubilities and thicknesses of precipitate for several minor oceanic components are derived and presented (Table II). Thicknesses of solid deposits formed at the bottom of the ocean are not very dependent on surface temperature and atmospheric composition. The ocean can be an important CO reservoir, able to hold from 0.25 time (ethane rich ocean at 92.5°K) up to 11 times (methane rich ocean at 101°K) the atmospheric CO inventory.

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 H 2 escape. A study of mixtures of nitrogen and methane shows a very strong effect of H 2 escape on the formation of organic nitriles, the only nitrogen containing organics detected in the gas phase. These are HCN, CH  CCN, (CN) 2 , CH 2 CHCN, CH 3 CN and CH 3 CH 2 CN. 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 H 2 from the atmosphere of the primitive Earth may have played a crucial role in the formation of reactive organic molecules such as CH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.

Etude de syntheses prebiotiques en phase gazeuse avec precision de mecanismes de formation des precurseurs HCN, HC 3 N et HCHO, de l'influence de la composition du melange gazeux et de la source d'energie. Les previsions relatives aux composes organiques susceptibles d'etre formes, sont corroborees par les examens de composition atmospherique des planetes du systeme solaire

A PoraPLOT Q column was utilized for gas chromatographic analyses of various mixtures of low-molecular-weight saturated and unsaturated hydrocarbons and nitriles, which are plausible constituents of the atmosphere of Titan, the largest satellite of Saturn. This capillary column coated with styrene—divinylbenzene co-polymer has a chromatographic behaviour similar to that of a Porapak Q packed column when analyzing these compounds. Its chromatographic behaviour however, is drastically improved by its much higher resolving power. With more than 1000 theoretical plates per metre for nitriles, this column offers a rapid separation of these compounds from C1C8 hydrocarbons, even under isothermal conditions. It appears to be a viable choice for the gas chromatograph—mass spectrometer aboard the Huygens probe of the Cassini mission which is planned to explore Titan's atmosphere.

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 occurring 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.

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.

It is generally accepted that the amount of molecular hydrogen in the atmosphere of the Earth, must have always been small, on account of its escape. Now, in classical experiments simulating the chemical evolution of model reducing atmospheres, large quantities of hydrogen are generally formed and accumulate. The results of these experiments must be therefore reassessed if we want to apply them to the modelisation of the chemical evolution of terrestrial planets. For this reason, we have carried out sytematic experiments with and without hydrogen escape simulation. The influence of this parameter: On the evolution of the atmosphere from a CH4-H2O containing atmosphere to a CO2- H2O containing atmosphere. On the formation of atmospheric organic precursors such as aldehydes, is reported and discussed.

The Round Table on prebiotic photochemistry was mainly devoted to the problem of the origin of Life and the possible role of light. Most of the questions concerning the origin of living systems on the Earth, involving photochemistry, were reviewed during the pre sentations by the speakers and the discussions which followed. Those topics were mainly, by logical order : organic photochemical synthesis in planetary atmosphere, prebiotic photochemical synthesis in aqueous media, model systems of photosynthesis, origin of chirality, and living systems as open systems.

In order to evidence new reactions of prebiotic synthesis in aqueous solution between the S-containing compounds, such as alkanethiols, which can be obtained in gaseous phase during the simulation of the evolution of primitive atmosphere, and the malonic nitriles which are generally considered as important intermediates in the Chemical Evolution, a kinetic study on the eventual reactions of ethanethiol with cyanoacetaldehyde and malononitrile has been carried out.

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.

Sulphur may have played an important role, mainly as an energy converter, during the initial steps of Chemical Evolution.

The 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 Miller's 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.

Similarities in the early histories of Mars and Earth suggest the possibility that life may have arisen on Mars as it did on Earth. If this were the case, early deterioration of the environment on Mars (loss of surface water, decrease in temperature) may have inhibited further evolution of life. Thus, life on Mars would probably be similar to the simplest form of life on Earth, the prokaryotes. We present a hypothetical strategy to search for life on Mars consisting of (i) identifying a suitable landing site with good exobiological potential, and (ii) searching for morphological and biogeochemical signatures of extinct and extant life on the surface, in the regolith subsurface, and within rocks. The platform to be used in this theoretical exercise is an integrated, multi-user instrument package, distributed between a lander and rover, which will observe and analyse surface and subsurface samples to obtain the following information: 1. environmental data concerning the surface geology and mineralogy, UV radiation and oxidation processes; 2. macroscopic to microscopic morphological evidence of life; 3. biogeochemistry indicative of the presence of extinct or extant life; 4. niches for extant life.

The formation of prebiotic organics in outer space has been simulated on ground. In order to verify abiotic formation of such compounds in earth orbit, the concept of cosmobiology experiment was developed. Simulated interstellar ice over dust grains is exposed to vacuum ultraviolet light and other space environment to induce organic formation. A system configuration and its engineering specification were determined to meet the scientific requirements, which were defined by the ground based study. The system consists of cryogenics to keep volatile species solidified, optics to filter and amplify the ultraviolet portion of the solar light for the accelerated irradiation of sample, and analysis subsystem to evaluate formation of organics in-situ together with post flight analysis of involatile products in trace.

Three organic compounds (HC3N, C6H2, and C4N2) relevant of Titan's atmosphere have been studied within the framework of the SIPAT (Spectroscopie UV d'Interet Prebiologique dans l'Atmosphere de Titan) program. Since this facility is still unable to reach the very low temperatures (∼170 K) of Titan's high atmosphere, spectra have to be obtained at several absorption-cell temperatures, and the data extrapolated towards lower temperatures. Previously published HC3N and C6H2 absorption coefficient data are reviewed, while new spectroscopic data are presented on C4N2. Integrated intensity calculations over the vibrational bands are performed apart from the background continuum. Thus, only the band contrast is considered here. While, the temperature dependence of the hot-band integrated intensity follows a Boltzmann distribution, we have enhanced the fit through an empirical parametrisation to account for the observed temperature dependence of the C4N2 and HC3N absorption coefficients, and to extrapolate those data to the low temperature conditions of Titan's high atmosphere. Finally, we discuss the implications of the results to possible detection by remote sensing observations of these minor compounds in Titan's atmosphere.

The IR spectrum of gaseous dicyanoacetylene, NCCCCN, is studied in the 3500-220 cm−1 range, employing Fourier transform spectroscopy. A total of 17 bands are analyzed and assigned. The absolute intensity is determined and the associated uncertainty is estimated for each of the bands. The resulting data are used to study the detectability of gaseous C4N2 in Titan's atmosphere. In particular a better estimate of the upper limits of its abundance in Titan's atmosphere is obtained when our results are used to reanalyse Voyager IRIS infrared spectra.