The exploration of planetary atmosphere is being advanced by the exciting results of the Cassin-Huygens mission to Titan. The complex chemistry revealed in such atmospheres leading to the synthesis of bigger molecules is providing new insights into our understanding of how life on Earth developed. In our experiments Titan's atmosphere is simulated in a glow discharge formed from a mixture of N2:CH4:CO2 gas. Samples of the discharge gas were analysed by GC-MS and FTIR. The major products identified in spectra were: hydrogen cyanide, acetylene and acetonitrile. The same compounds were detected in the FTIR: hydrogen cyanide, acetylene and ammonia. Whilst many of these compounds have been predicted and/or observed in the Titan atmosphere, the present plasma experiments provide evidence of both the chemical complexity of Titan atmospheric processes and the mechanisms by which larger species grow prior to form the dust that should cover much of the Titan's surface

Krcma, F.

Mason, N. J.

Matejcik, S.

Mazankova, V.

Morgan, G.

Torokova, L.

English

The exploration of planetary atmosphere is being advanced by the exciting results of the Cassin-Huygens mission to Titan. The complex chemistry revealed in such atmospheres leading to the synthesis of bigger molecules is providing new insights into our understanding of how life on Earth developed. In our experiments Titan\u27s atmosphere is simulated in a glow discharge formed from a mixture of N2:CH4:CO2 gas. Samples of the discharge gas were analysed by GC-MS and FTIR. The major products identified in spectra were: hydrogen cyanide, acetylene and acetonitrile. The same compounds were detected in the FTIR: hydrogen cyanide, acetylene and ammonia. Whilst many of these compounds have been predicted and/or observed in the Titan atmosphere, the present plasma experiments provide evidence of both the chemical complexity of Titan atmospheric processes and the mechanisms by which larger species grow prior to form the dust that should cover much of the Titan\u27s surface

CTU Open Journal Systems (Czech Technical University, Prague / České vysoké učení technické v Praze)

Plasma Physics and Technology 3(3):163–167, 2016 © Department of Physics, FEE CTU in Prague, 2016THE INFLUENCE OF CO2 ADMIXTURES ON PROCESS IN TITAN’SATMOSPHERIC CHEMISTRYTorokova L.a,∗, Mazankova V.a, Mason N. J.b, Krcma F.a, Morgan G.b,Matejcik S.ca Faculty of Chemistry, Brno University of Technology, Purkynova 118, 612 00 Brno, Czech Republicb Department of Physical Sciences, Open University, Milton Keynes MK7 6AA, United Kingdomc Faculty of Mathematics, Physics and Informatics, Comenius University, 842 48 Bratislava, Slovakia∗ torokova@fch.vutbr.czAbstract. The exploration of planetary atmosphere is being advanced by the exciting results of theCassin-Huygens mission to Titan. The complex chemistry revealed in such atmospheres leading to thesynthesis of bigger molecules is providing new insights into our understanding of how life on Earthdeveloped. In our experiments Titan’s atmosphere is simulated in a glow discharge formed from amixture of N2:CH4:CO2 gas. Samples of the discharge gas were analysed by GC-MS and FTIR. Themajor products identified in spectra were: hydrogen cyanide, acetylene and acetonitrile. The samecompounds were detected in the FTIR: hydrogen cyanide, acetylene and ammonia. Whilst many ofthese compounds have been predicted and/or observed in the Titan atmosphere, the present plasmaexperiments provide evidence of both the chemical complexity of Titan atmospheric processes and themechanisms by which larger species grow prior to form the dust that should cover much of the Titan’ssurface.Keywords: atmosphere of Titan, glow discharge, GC-MS analysis and FTIR spectroscopy.1. IntroductionSimple organic molecules play an important role inthe formation of complex organics in planetary at-mospheres. On the last decade, several lower hy-drocarbons and nitriles have been confirmed in theatmosphere of Titan, the largest satellite of the Saturn.The first detailed information was brought by Voyagerspace mission in 1980. More important complex infor-mation about Titan atmosphere was obtained fromCassini-Huygens mission in 2005. Its atmosphere com-position is principally nitrogen with 2-6 % methaneand some trace gases as nitriles (HCN, HC3N, HC5N,and C2N2) and lower molecule hydrocarbons (C2H2,C2H4, C2H6, C3H8, C3H4) as well as hydrogen [1, 2].These compounds can be produced by low power dis-charges in the methane clouds [3, 4] or by dissociationof nitrogen and methane either by solar induced pho-tolysis, photodissociation or by electron impact. Thesame processes generate also charged particles that aretransported through the atmospheric clouds. Neutral-ization of these charged particles leads to high powerdischarges like lightning within the clouds which caninduce other chemical reactions in the troposphere[5]. Organic radicals produced by electron impactcould also supply a significant fraction of Titan’s hazeand surface material [6]. Lightning activity is oneof the most probable initiators of organic moleculesformation on Titan lower atmosphere and has alsobeen suggested as a mechanism for triggering the pre-biotic chemistry on Earth [7], for which the loweratmosphere of Titan may be a good mimic [8–11].The possibility of lightning on Titan is supported bytheoretical models and observations of Titan’s tro-posphere during the last decade, which indicate thatmethane droplets may suddenly condense and undergovertical motions in the Titan’s atmosphere [12]. Mostof the experiments reproduce a gas mixture close tothe Titan’s atmosphere, or at least to a part of itsatmosphere, and then supply energy into this gas mix-ture to induce chemical reactions. Different plasmadischarges have been shown to be good mimics toa planetary atmosphere providing insights into bothphysical and chemical processes of such atmosphere.Various studies of electron-molecule and ion-moleculereactions in the planetary atmosphere were presentedrecently. Namely results obtained by corona dischargewere presented in [9–13], by DBD discharge in [14],by glow discharge in [15–17], by microwave dischargein [18], and by RF discharges in [10, 19, 20]. However,the mechanisms of such organic chemistry are stillunclear.2. ExperimentThe experimental setup schematic drawing is shownin Figure 1. The special high vacuum stainless steel re-actor was constructed for our experiments to preventany oxygen contamination during the experiments.Nitrogen and methane flows were automatically con-trolled by Bronkhorst controllers. The measurementswere carried out at total gas flows 100 sccm at at-mospheric pressure and laboratory temperature. Thedischarge electrode system had the standard config-163Torokova L., Mazankova V., Mason N.J. et al. Plasma Physics and Technologyuration of the gliding arc discharge. The dischargewas formed in the stable abnormal glow regime, andplasma was occurred between the electrodes at theirshortest distance of 2 mm in the form of a plasmachannel of 1 mm in its diameter. The reactor chambervolume was 0.5 l. The discharge was supplied by a DCstabilized HV source. Discharge breakdown voltagewas 5500 V, a stable plasma channel was operatingat 400 V at current of 30 mA during all presentedexperiments. The measurements were performed fordifferent N2:CH4 ratios in the range from 1 % to 3% of methane in nitrogen (both gases having quotedpurities of 99.995 %) and admixture 1, 2 and 3 % ofcarbon dioxide (CO2). The exhaust gas was sampledusing the cold trap technique. The liquid nitrogenstainless steel trap (diameter of 15 mm, length of 165mm, the total volume of 116 cm3) was mounted atthe outlet of the reactor, as it is shown in Figure1, as a side removable arm. The sampling time was5 min and all gas products were subsequently anal-ysed by GC-MS. The sampling efficiency closed to100 % was confirmed by in situ FTIR done behindthe sampling point (see description of experimentaldevice). Sampling time was adjusted based on pre-liminary experiments to avoid saturation at GC-MSmeasurements.3. ResultsThe chromatogram given in Figure 2 correspondingto experiment at nitrogen, methane and carbon diox-ide are shown as typical examples. All the peakshave been identified using their retention time in thesequence and their mass spectra using the programMSD Chemistry with the NIST MS library [21]. Thepeaks corresponding to nitrogen, methane and carbondioxide as the original gas mixture were recorded atretention times under 2 minutes and thus they are notdepicted in Figure 2.The other peaks of the highest intensity in thesechromatograms can be attributed to lower hydrocar-bons and nitrile compounds. The dominant peakcorresponds to hydrogen cyanide (HCN), the seconddominant peak is acetylene (C2H2) and the third mainproduct is acetonitrile (CH3CN). Other abundantdetermined products were ethane (C2H6), ethylene(C2H4), cyanogen (C2N2), propenenitrile (C2H3CN),and propanenitrile (C2H5CN). A lot of nitriles, manyhydrocarbons, and even aromatic compound, havebeen detected from the analysis of all obtained chro-matogram. No oxygen containing compounds weredetected because of our special high vacuum reactorpreserved any oxygen contamination. The results forselected C2-hydrocarbons (ethane, ethylene and acety-lene) and nitriles (hydrogen cyanide, acetonitrile andcyanogen) are shown in 1. The dominant hydrocarbonwas acetylene and its amount increased nearly directlyproportionally to the carbon dioxide concentration.Ethane and ethylene show the similar trend of relativeintensity. Relative intensities of hydrogen cyanide,acetonitrile and cyanogen as the main detected com-pounds are depicted on carbon dioxide concentrationin the gas mixture again at both total gas flows. Thesecompounds present various chemical structures withdifferent number of carbon and nitrogen atoms.A typical FTIR spectrum showing the productsformed in the nitrogen discharge fed by 1% of CH4and 1% of CO2 is shown in Figure 3. Similar spectrawere observed for other N2:CH4:CO2 ratios. Hydro-gen cyanide (HCN) was found to be the most abun-dant product at wavenumbers of 1430 cm−1 and of720 cm−1. It is in agreement with [17]. Ammonia(NH3) was identified at 966 cm−1 which was surpris-ing because ammonia was not detected in previousexperimental studies. The other major products wereacetylene (C2H2) as well as carbon monoxide (CO)and water (H2O). These products were recognized inall N2:CH4:CO2 gas mixtures. The products concen-trations are strongly dependent on the compositionof the gas mixtures.Relative quantifications of selected hydrocarbonswere done in dependence on the methane concentra-tion and gas mixture flow rate. The relative intensitywas calculated as an area under the recorded peaks.Figures 4 and 5 shows dependences of the calculatedrelative hydrocarbon intensity on carbon dioxide ad-ditions and discharge current. Figure 4 shows thequantitative analysis of HCN (1430 cm−1) and Figure5 NH3 (966 cm−1) formed under different experimen-tal conditions. The increase of the initial dischargecurrent from 15 mA to 40 mA leads to increase in theproduct yield of this compound. The admixture of 1%CO2 had little influence on HCN and NH3 production.However, there is visible effect of CO2 addition tonitrogen methane mixtures with 1% methane with theyield of HCN increasing with increasing CO2 concen-trations. This is in contrast to earlier study [22] thatsuggested that the kinetics of HCN formation slowsdown in presence of CO, showing an inhibiting role ofCO on HCN formation.4. ConclusionThe gaseous phase products formed in the at-mospheric glow discharge fed by different mixtures ofnitrogen plus methane (1 % and 3 %) and admixtureof carbon dioxide (1, 2 and 3 %) were determinedby in situ FTIR and GC-MS analysis. The dischargewas operated in the flowing regime at different dis-charge currents at laboratory temperature. An in-situFTIR technique for the exhaust gas phase samplingwas successfully used for chemical analysis to deducethe gas composition in the N2:CH4:CO2 reactive gasmixture mimics of Titan’s atmosphere. Various nitrilecompounds and hydrocarbons were observed in allexperiments. HCN was identified as the major gasphase product in all of measurements. Others minorproducts detected were C2H2, NH3, CO2, CO andalso some nitrile oxides. These results are consistentwith the Titan’s atmospheric composition because the164vol. 3 no. 3/2016 The influence of CO2 ... on Titan’s atmospheric chemistryFigure 1. Experimental setup. 1- storage bottle of nitrogen, 2- storage bottle of methane, 3- storage bottle of carbondioxide, 4- Bronkhorst controllers, 5- reactor body, 6- electrode system, 7- oscilloscope, 8- IR gas cell, 9- FTIRspectrometer, 10- cold trap.Figure 2. An example of exhaust gas chromatogram in 98 % N2:1 % CH4 :1 % CO2 gas mixture.165Torokova L., Mazankova V., Mason N.J. et al. Plasma Physics and Technology1% of CH4 concentration 3% of CH4 concentrationconcentration of CO2 (%) 0 1 2 0 1 2acetylene 1.45 × 106 1.72 × 106 2.12 × 106 1.79 × 106 2.77 × 106 3.33 × 106ethene 1.94 × 105 2.63 × 105 3.02 × 105 2.69 × 105 3.19 × 105 3.24 × 105ethane 4.35 × 104 5.39 × 104 7.04 × 104 5.36 × 105 6.19 × 105 7.44 × 105hydrogen cyanide 8.87 × 106 1.82 × 107 2.26 × 107 2.89 × 107 3.43 × 107 5.08 × 107acetonitrile 9.28 × 105 1.86 × 106 3.43 × 106 2.15 × 106 2.70 × 106 3.60 × 106cyanogen 2.81 × 105 4.23 × 105 8.06 × 105 4.15 × 105 6.16 × 105 8.26 × 105Table 1. The list of selected compounds from GC-MS analysis. The relative intensity was calculated as an area underthe recorded peaks (arbitrary unit).Figure 3. An example of IR spectrum in 98 % N2:1 %CH4 :1% CO2 gas mixture.Figure 4. The dependence of hydrogen cyanide con-centration on discharge current for different initialconcentration of methane and carbon dioxide.same compounds were detected during the Cassini-Huygens space mission. CO could have an effect onthe atmospheric reactivity of Titan. The formationof organic molecules incorporating oxygen in gasescould occur in the upper atmosphere of Titan wherethe dissociations of N2 and CO2 by VUV photonsand magnetospheric electrons are possible. This factclearly demonstrates that laboratory experiments canFigure 5. The dependence of ammonia concentrationon discharge current for different initial concentrationof methane and carbon dioxide.be used for prediction of both the presence and pos-sible concentrations of compounds which have notbeen detected, yet. These simple organics should betracers of the chemical groups constituting the dustyproducts.AcknowledgementsThe research was supported by Czech Ministry of Educa-tion, Youth and Sports, projects No. COST CZ LD15011,within the collaboration of the COST Action TD 1308,CEEPUS network AT-0063.References[1] C.A. Nixon. Detection of propene in Titan’sstratosphere. Astrophys J, 776(1), 2013.[2] V.A. Krasnopolsky. Chemical composition of Titan’satmosphere and ionosphere: Observations and thephotochemical model. Icarus, 236:83–91, 2014.[3] K.L. Aplin. Atmospheric electrification in the solarsystem. Surveys in Geophysics, 27(1):63–108, 2006.[4] S. Vinatier. Vertical abundance profiles ofhydrocarbons in titan’s atmosphere at 15 degrees s and80 degrees n retrieved from CASSINI/CIRS spectra.Icarus, 188(1):120–138, 2007.[5] R. Navarro Gonzalez. Corona discharge of titan’stroposphere. 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The Influence of CO2 Admixtures on Process in Titan\u27s Atmospheric Chemistry

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The Influence of CO2 Admixtures on Process in Titan's Atmospheric Chemistry

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