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Studying Titan’s atmosphere, a "frozen early Earth",
to improve our understanding of the beginnings of life
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Titan, the largest satellite of Saturn, is the site of intense chemical activity, particularly in its upper atmosphere where gases are irradiated by the sun’s UV rays . Titan could, through its similarities to Earth, help to understand by what (photo) chemical processes the first molecules of life appeared. This has been the work of one the LATMOS groups(1), users of the SMIS and DISCO beamlines.
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| Discovered by Huygens in 1655, approached by the Voyager 1 and 2 probes in 1980-1981 and truly revealed by the Cassini-Huygens mission since 2004-2005, Titan is a fascinating object in the solar system. Its atmosphere, composed of 97% N2, 2% methane CH4 and 1% of various hydrocarbons (acetylene C2H2, ethylene C2H4 , etc.) is fairly close to that of Earth, but Earth as it probably was more than 3 billion years ago, before oxygen O2 was produced by the first life forms. Another thing in common is water, which is also present on the surface of Titan, but in the form of ice, its surface temperature being -180°C, because of its distance from the sun. Therefore, Titan is often compared to a frozen early Earth and it is understandable that its study is likely to provide valuable information about the early chemistry that led to the synthesis of prebiotic molecules.
Figure1: Huygens separating from Cassini. It is going to enter the very high atmosphere of Titan. Artist view. Copyright: ESA | 
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 Reproducing Titan’s atmosphere in the laboratory
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During the Cassini –Huygens mission, the Huygens probe landed on the surface of Titan in January 2005, providing scientific information on the composition of its atmosphere. However, these data were incomplete because their transmission was only possible for a few hours, hardly more than the time it took for the probe to descend to the surface of Titan. The Cassini orbiter is however still in orbit around Saturn. It regularly provides results on the atmospheric composition of Titan each time it flies over it. Nevertheless, the instrumental limitations of its equipment do not allow it to identify precisely the reactions responsible for the complexification of organic molecules. To complement these in situ data, a great deal of work is therefore needed on modeling chemical reactions occurring in Titan's atmosphere, as well as laboratory experiments. This is precisely what Nathalie Carrasco and colleagues are doing in their LATMOS laboratory: they simulate the reactivity of Titan’s atmosphere by applying electric discharges in a N2-CH4 gaseous mixture reproducing this atmosphere. During these experiments, powders of organic particles are formed, called "tholins”, similar to those constituting the orange “haze” surrounding Titan to a thickness of several hundreds of kilometers and thus masking its surface. Analytical tools used in situ in the experiment (mass spectrometry, infrared (IR) absorption spectroscopy and optical emission spectroscopy) enable the chemical composition of the gas to be tracked when and where the reactions occur. The organic powders are collected for various ex-situ chemical and morphological analyses (mass spectrometry, IR spectroscopy and scanning electron microscopy, etc.). The processes involved in producing these powders are then examined to try to understand the formation of more and more complex molecules in this reaction mixture simulating Titan’s atmosphere.
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From the laboratory to SOLEIL
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The group began in December 2008 by using mass spectrometry on the DISCO beamline to analyze the soluble fraction of tholins, in collaboration with the CNRS ICSN laboratory. This fruitful collaboration highlighted the polymeric nature of these compounds, the nitrogen-rich content and in particular alternating carbon and nitrogen atoms in a systematic NC-N--CN fragment(2). The LATMOS group then turned to Paul Dumas, head of the SMIS IR micro-spectroscopy beamline, to analyze by IR and far-infrared spectroscopy the signatures of tholins prepared under various experimental conditions for comparison with spectra of Titan aerosols measured by Cassini in situ. The experimental protocol for the synthesis of tholins is comparable to that described above, and the IR beam is then used to analyze these molecules. The results obtained on SMIS have, on the one hand, shown a great variability in IR absorption in relation to the experimental conditions of synthesis, and on the other, allowed the first tholins spectrum in the far-infrared to be measured.
But the plasma setup used at LATMOS does not exactly reproduce the conditions present in Titan's atmosphere: the energy that initiates the chain of chemical reactions is not in fact electrical, as is the case in the studies conducted so far, but electromagnetic, since it comes from photons from the Sun. These are mainly photons in the ultra-violet (UV) range. Never mind: Nathalie Carrasco and her colleagues were going to use UV!  | Figure 2:
From left to right, Pr Yves Bénilan, LISA, Créteil ; Pr Guy Cernogora, LATMOS, Guyancourt ; Nathalie Carrasco, Senior Lecturer, LATMOS, Guyancourt, standing close to the experimental setup used for their experiments on DISCO. |
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An original setup on DISCO
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While working on tholins using mass spectrometry described above, Nathalie Carrasco and Alex Giuliani (scientist on DISCO), began to develop a new project based on global simulation of the atmospheric chemistry of Titan on the DISCO beamline. The DISCO beam includes the visible and UV down to 60 nm (1-20 eV), which permits the simulation of the "active" solar spectrum during reactions involving the atmosphere of Titan. A reaction cell has been specially designed for experiments on the beamline thanks to funding from PRES UniverParisSud and CNES, and involving several laboratories: LATMOS, the DISCO beamline, LCP, LGPM and LISA.
A first experiment studying the photochemistry of Titan was conducted at ALS recently. Nevertheless, this study took place under low pressure conditions and at only two different wavelengths. The DISCO beamline has a branch called APEX where irradiation at atmospheric pressure in the VUV is possible. Using a unique differential pumping system, it is possible to deliver UV photons at pressures up to atmospheric pressure. Thus, the reaction medium is subjected to UV radiation over a wide range of wavelengths covering the region previously studied by the U.S. team.
Monitoring of the syntheses taking place in the cell is achieved by mass spectrometry of the products obtained.
Results of the first series of measurements made by LATMOS researchers are very encouraging. Although the solid particles obtained during the experiments have not yet been detected, new chemical species were indeed produced after the dissociation of the molecules of N2 and CH4: these were CxHyNz compounds, rich in nitrogen. This result was unexpected since it was thought until now that the solid particles came from the agglomeration of benzene-type molecules, i.e. aromatic carbon cycles - without nitrogen atoms.
The full potential of this experiment is far from having been exploited, the time allocated to this first visit to the DISCO beamline being insufficient for Nathalie Carrasco and colleagues to do all the necessary tests. However, other sessions are planned. Will there soon be new results on the fundamental "building bricks" of life? (1) LATMOS : Laboratoire Atmosphères, Milieux, Observations Spatiales, based in Guyancourt. Its parent bodies: CNRS, Université de Versailles Saint-Quentin en Yvelines (UVSQ) and Université Pierre et Marie Curie (UPMC)
http://www.latmos.ipsl.fr
Reference : (2) N. Carrasco, I. Schmitz-Afonso, J-Y. Bonnet, E. Quirico, R. Thissen, O. Dutuit, A. Bagag, O. Laprévote, A. Buch, A. Giulani, G. Adandé, F. Ouni, E. Hadamcik, C. Szopa & G. Cernogora, J. Phys. Chem. A (2009), 113, 11195–11203
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