Paul Bruston

UV spectroscopy of Titan's atmosphere, planetary organic chemistry, and prebiological synthesis: I. Absorption spectra of gaseous propynenitrile and 2-butynenitrile in the 185- to 250-nm region

Abstract In the coming decade, UV spectroscopy of Titans 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.

The low temperature organic chemistry of Titan's geofluid.

Organic chemistry on Titan and prebiotic chemistry on Earth involve the same N-containing organics: nitriles and their oligomers. Couplings of their chemistry in the three parts of Titans geofluid (atmosphere, aerosols and surface) seem to play a key role in the organic chemical evolution of the planet. If liquid water was present on Titan, then a prebiotic chemistry, involving eutectics, similar to that of the early Earth, may have occurred. However, liquid water is currently absent and a prebiotic chemistry based only on N-organics may be evolving now on Titan. The other consequence of the low temperatures of Titan is the possible formation of organics unstable at room temperature and very reactive. So far, these compounds have not been systematically searched for in experimental studies of Titans organic chemistry. C4N2 has already been detected on Titan. Powerful reactants in organic chemistry, CH2N2, and CH3N3, may be also present. They exhibit spectral signatures in the mid-IR strong enough to allow their detection at the 10-100 ppb level. They may be detectable on future IR spectra (ISO and Cassini) of Titan.

THE TEMPERATURE DEPENDENT ABSORPTION CROSS SECTIONS OF C4H2 AT MID ULTRAVIOLET WAVELENGTHS

Abstract We have measured the mid UV (195–265 nm) absorption coefficients of diacetylene, C4H2, over the temperature range 193–293 K. An impurity involved in the synthesis of C4H2, namely C4H3Cl, can strongly influence the measured absorption coefficients. This impurity has a mid UV absorption approximately 300 times stronger than C4H2 and we demonstrate that its presence, at low levels, in previously published spectra has strongly influenced the published coefficients and their temperature dependence. We have obtained an ultra-pure sample of C4H2 by repeated distillation and show here its clean spectrum and temperature dependence. We confirm previous band assignments, but our superior resolution (0.02 nm) has shown the presence of other, previously undetected bands. These new results have wide implications for the modelling of Titans atmosphere and the analysis of observational data.

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.

Infrared spectra of triacetylene in the 4000-220 cm−1 region: Absolute band intensity and implications for the atmosphere of Titan

Abstract The gas phase infrared spectra of triacetylene were studied at room temperature in the 4000-220 cm −1 wavenumber region. The absolute intensity of all the bands of noticeable strength were experimentally determined and the associated uncertainties were systematically estimated. The data obtained were then used for studying the detectability of C 6 H 2 in Titans atmosphere by means of infrared spectroscopy. The upper limit of mean stratospheric abundance derived from the already available Titan IR spectra (a fraction of ppb) is in agreement with the photochemical model estimates.

Mid‐UV spectroscopy of propynenitrile at low temperature: Consequences on expected results from observations of Titan's atmosphere

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 Titans 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 Titans 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.

Gas infrared spectra, assignments, and absolute IR band intensities of C4N2 in the 250–3500 cm−1 region: implications for Titan's stratosphere

Abstract 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 Titans atmosphere. In particular a better estimate of the upper limits of its abundance in Titans atmosphere is obtained when our results are used to reanalyse Voyager IRIS infrared spectra.

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.

Laboratory studies of organic chemistry in planetary atmopheres: From simulation experiments to spectroscopic determinations

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 Titans atmosphere in view of the Cassini-Huygens mission.

A laboratory facility for mid‐UV absorption spectroscopy of molecular compounds for planetary atmospheres

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.