Introduction Titan is a unique environment in the Solar System. By the complex organic chemistry occurring in its very dense atmosphere and on its surface, this moon is considered as a natural laboratory for astrobiology studies. In addition to the identifications of organic compounds that have been suggested [1-3], Titan houses also a subsurface liquid water ocean (with some liquid water bodies remaining at the surface about a thousand years).Solar UV photons and Saturn’ magnetosphere electrons lead to the photo-dissociation and the ionization of the two main gaseous compounds of the Titan’ atmosphere, nitrogen N2 and methane CH4 (top, Fig 1). Simple organics are produced then reaction chains lead to more complex compounds to obtain solid organic aerosols. These aerosols form the haze of Titan and are organized in layers at different altitudes in the atmosphere. They are finally deposited at the surface (bottom, Fig 1).INMS and CAPS are two instruments of the Cassini-Huygens space mission that have extensively studied the upper atmosphere of Titan. They have for instance detected positive ions and neutrals up to 100 u (limit mass range of the INMS instrument) and heavy positive and negative ions up to thousands of mass units [1]. For most detections (mainly done by the CAPS instrument) the mass resolving power did not allow to decipher the chemical composition or the growth pathways involved in the aerosols’ formation. On the Huygens lander, the Gas Chromatograph Mass Spectrometer has also recorded thousands of mass spectra during its descent in the atmosphere and at the surface of Titan [4].Analogs of Titan’ aerosolsIn addition to the large amount of in situ data obtained thanks to the Cassini-Huygens space mission and to go further in our understanding of this moon, many laboratory experiments produce analogs of Titan’ aerosols called tholins. One of them, the PAMPRE experiment [5] uses a radio frequency reactive low-pressure plasma to produce solid particles and solid films onto metallic surfaces placed inside the PAMPRE reactor, mimicking the coupled ion-neutral chemistry occurring in Titan ionosphere [1,6]. In this study, tholins are produced with a gas mixture of 5% of methane (CH4) and 95% of nitrogen (N2). We consider this matter as an analog of the ionospheric aerosols of Titan (top, Fig 1). This kind of analog helps to understand the formation mechanisms of these aerosols in the ionosphere and the nature of the chemical species produced. Other laboratory experiments like CHARTS and VAHIA chambers can simulate the photochemistry of Titan’ clouds using gas mixtures of benzene and hydrogen cyanide [7] deposited at 130K. Cooling at 70K, irradiation (around 230 nm and during various times) and room temperature heating are among the different steps to obtain an organic residue to analyze. This other kind of analog is representative of the stratospheric matter on Titan (middle part Fig 1).The laser CosmOrbitrap-based mass spectrometer This work aims at studying these ionospheric and stratospheric analogs by the identification and the quantification of the main chemical species. As the tholins produced in PAMPRE (ionospheric analogs) can be placed in the CHARTS chamber mimicking the stratospheric clouds, the other goal is to monitor the chemical evolution of these tholins from ionospheric to stratospheric conditions. A better understanding of the growth pathways is also expected. Analyses are done with High Resolution Mass Spectrometry (HRMS). The technique used couples a laser ionization at 266 nm with the high-resolution mass analyzer CosmOrbitrap developed for future space applications [8]. This development led by LPC2E (Orléans, France) involves six laboratories and is funded by the CNES. The CORALS spaceflight prototype instrument that recently achieved Technical Readiness Level (TRL) of 5+ [9] is a laser CosmOrbitrap-based instrument developed in partnership with the University of Maryland and the Goddard Space Flight Center (GSFC). This instrumentation has demonstrated unprecedented analytical performances in the laboratory on a large organic samples range [10-13] including PAMPRE analogs [14] (blue, Fig 2).Using the same instrument, another on-going study is investigating hydrolyzed PAMPRE solid tholins [15] to understand the evolution of the surface organic matter in contact with liquid water in a crater melt pool (bottom, Fig 1). In this project, stratospheric analogs are analyzed with the laser CosmOrbitrap-based instrument for the first time. Changes in the nature of the compounds and evidences of a higher chemical complexity is looked for.Preparing the future exploration of Titan and other ocean worldsSeveral mission concepts and instruments are currently in development or in preparation for the future exploration of ocean worlds. Among them and dedicated to Titan, the Dragonfly space mission is planned to be launched in 2028. As future astrobiology space mission payloads should include HRMS instruments to provide unequivocal identifications of biosignatures and prebiotic molecules, this work participates to demonstrate the potential of laser CosmOrbitrap-based instruments for the future exploration of ocean worlds.Acknowledgments We gratefully acknowledge the CosmOrbitrap consortium (LPC2E, LATMOS, LISA, IPAG, IJC Lab, J. Heyrovsky institute of Physical Chemistry), Alexander Makarov (Thermo Fisher Scientific) and the CNES for its technical and financial support.We acknowledge the CORALS and AROMA teams, the NASA GSFC and the University of Maryland for a very fruitful collaboration.LS also acknowledges the University of Orleans for the funding of this study (BQR project “MOT”).References[1] Waite et al. 2007, Science, 316[2] Waite et al. 2009, Nature, 460[3] Postberg et al. 2018, Nature, 558[4] Niemann et al. 2005, Nature, 438[5] Szopa et al. 2006, Planetary and Space Science 54[6] Dubois et al. 2020, Icarus 338[7] Mouzay et al. 2021, The Planetary Science Journal, 2[8] Briois et al. 2016, Planetary and Space Science, 131[9] Willhite et al. 2021, IEEE Aerospace conference[10] Arevalo et al. 2018, Rapid Communications in Mass Spectrometry, 32[11] Selliez et al. 2019, Planetary and Space Science, 170[12] Selliez et al. 2023, Planetary and Space Science, 225[13] Ni et al., 2023, Astrobiology, 23[14] Selliez et al. 2020, Rapid Communications in Mass Spectrometry, 34[15] Maratrat et al. 2023, Titan Through Time workshop

Introduction Titan, Saturn's largest moon, has a thick nitrogen-based atmosphere, where during its flyby, the IRIS infrared spectrometer aboard the Voyager 1 spacecraft detected nitriles, such as HCN, formed by EUV photochemistry based on methane CH4 and molecular nitrogen N2. Titan is also surrounded by an organic photochemical haze, according to the observations of the Cassini-Huygens and Voyager 1 missions.In-situ measurements by the Huygens spacecraft [1], and laboratory experiments synthesizing Titan analog aerosols (called tholins), have revealed that HCN is one of the major chemical signatures extracted from the aerosols, and their laboratory analogues.Laboratory experiments were conducted using a powder plasma reactor mimicking Titan's ionosphere, replicating the formation of Titan-like organic aerosols, and the associated chemistry. In this work, we simultaneously study the temporal evolution of HCN present in the gas phase, and the formation and growth of Titan tholins. 1 - Aerosol production Analogues of Titan aerosols are formed using the experiment PAMPRE [4]. PAMPRE is a reaction chamber, where a cold radiofrequency plasma capacitively coupled is ignited at low pressure (~0.9 mbar). A gas mixture of 95% N2 and 5% CH4 is introduced into the reactor. Subsequently, a 12W discharge is generated between two electrodes, which ionizes the N2 and CH4 present.In this plasma, the radicals and products formed by ionization of methane and nitrogen, will combine by several reaction pathways, to form more complex organic particles and nitriles such as HCN, in the same way as in the ionosphere of Titan.In this study, the gas flow rate was optimized, to increase the residence time of the gas mixture in the reactor as much as possible. The chemical growth of the solid particles is thus favored, allowing to follow simultaneously the formation and the evolution of the particles, as well as the co-evolution of the gas mixture composition. 2 - Morphology analysis of tholins by scanning electron microscopy The formed samples were observed by scanning electron microscopy (SEM field emission gun).Figure 1 - SEM picture of Titan tholins obtained after 160 s in the dischargeThe images show primary nanometer monomers coagulated and formed aggregates (Figure 1). These aggregates continue to evolve to form single spherical particles of ~1.5 µm in diameter (Figure 1). These experimental results are in agreement with the evolution predicted by the model [2] on aerosols residing in the atmosphere of Titan.3 - Chemical analysis of tholins by infrared spectroscopy Absorbance measurement is performed using an FTIR, on pellets composed of ~ 99.6% KBr and ~ 0.4% tholins. Three absorption bands stand out on the mid-infrared absorption spectrum. In particular, the intense bands at 1560 cm-1 and 1630 cm-1 (Figure 2) which correspond to different functional groups, such as aromatic or aliphatic -NH2, C=N, C=C, aromatic or heteroaromatic groups, difficult to distinguish from each other. The presence of these nitrogenous aromatic compounds can be promoted by HCN [3] [5].Figure 2 - Normalized IR spectra showing the aromatic or aliphatic groups bands 4 - Conclusions and Perspectives The growth of the particles could be observed by the SEM analysis. Moreover, the IR analysis shows the presence of nitrogenous chemical bonds, corresponding to aromatic compounds or nitrile. This growth may be due to the incorporation of HCN chemistry on the surface of the tholins. Indeed, MS analysis of the gas phase shows a progressive consumption of HCN during the formation and growth of Titan tholins. Our experimental results are consistent with numerical models based on Cassini-Huygens observations: A formation of atmospheric HCN on Titan, and its efficient incorporation in Titan’s haze to produce large organic polymers.References[1] Israël G. et al., Nature 438 : 796-99 (2005).[2] Lavvas P. et al., The Astrophysical Journal (2011).[3] Imanaka H. et al., Icarus, 168: 344-66 (2004).[4] Szopa C. et al., Planetary and Space Science 54 (2006).[5] Gautier T. et al., Icarus, 221, 320-327 (2012).

The addition of small amounts of H 2 were investigated in a DC glow discharge in N 2 , at low pressure (∼1 mbar) and low power (0.05–0.2 W cm −3 ). We quantified the electric field, the electron density, the ammonia production and the formation of positive ions for amounts of H 2 varying between 0 and 5%, pressure values between 0.5 and 4 mbar, and currents between 10 and 40 mA. The addition of less than 1% H 2 has a strong effect on the N 2 plasma discharges. Hydrogen quenches the (higher) vibrational levels of N 2 and some of its highly energetic metastable states. This leads to the increase of the discharge electric field and consequently of the average electron energy. As a result, higher quantities of radical and excited species are suspected to be produced. The addition of hydrogen also leads to the formation of new species. In particular, ammonia and hydrogen-bearing ions have been observed: N 2 H + and NH 4 + being the major ones, and also H 3 + , NH + , NH 2 + , NH 3 + , N 3 H + and N 3 H 3 + . The comparison to a radiofrequency capacitively coupled plasma discharge in similar experimental conditions shows that both discharges led to similar observations. The study of N 2 –H 2 discharges in the laboratory in the adequate ionization conditions then gives some insights on which plasma species made of nitrogen and hydrogen could be present in the ionosphere of Titan. Here, we identified some protonated ions, which are reactive species that could participate to the erosion of organic aerosols on Titan.

<p><strong>1 - Introduction</strong></p> <p>In the atmosphere of the satellite Titan, the photochemistry of its two main components N<sub>2 </sub>and CH<sub>4 </sub>leads to the formation of complex organic molecules, up to the production of solid aerosols, in the form of an orange haze. Observations from the Cassini-Huygens mission [1], as well as models [2] and laboratory experiments [3], strongly suspect that once formed in the ionosphere, the haze will reside for some time in Titan's atmosphere until settling on the surface. Our aim is to investigate experimentally the interaction of the haze particles with their atmospheric chemical environment, focusing on possible reactive molecules produced by gas phase photochemistry of N<sub>2</sub> and CH<sub>4</sub> such as HCN, HC3N, C<sub>2</sub>N<sub>2</sub>, C<sub>2</sub>H<sub>2</sub>, C<sub>2</sub>H<sub>6</sub>. We more specifically addressed the absorption processes of the gases on the particle (uptake coefficients).</p> <p> </p> <p><strong>2 - Experimental method</strong></p> <p>In this experimental study, a dusty plasma reactor is used to simulate the atmospheric chemistry of Titan [3], as well as the synthesis of Titan’s aerosols analogues (tholins). The gaseous precursors formed by electronic dissociation were monitored in-situ by mass spectrometry, simultaneously with the formation and growth of the haze particles. The properties of the tholins are analyzed by scanning electron microscopy (morphology and size) and high resolution mass spectrometry, LDI-FTICR (chemical composition). In this study, the injection gas flow rate was optimized in order to increase as much as possible the residence time of the gas mixture in the reactor. The chemical growth of the solid particles is thus favored, allowing to follow simultaneously the formation and the evolution of the particles, as well as the co-evolution of the composition of the gas mixture until reaching a stationary gas chemistry, which will not change any more during the whole experiment.</p> <p><strong>3- Results</strong></p> <p><strong>3.1 - Temporal evolution of the gas phase by mass spectrometry</strong></p> <p>In a previous study [4], MID monitoring by mass spectrometry was performed for CH<sub>4</sub> and HCN (Figure 1).  From these results, we distinguish two kinetic regimes of gas-particle interaction: a transient regime corresponding to the production and consumption of gases and correlated to the evolution of tholins solid particles, and a stationary regime where the gas mixture ratio is stabilized. In this study, the MID monitoring is carried out for gas-phase molecules suspected to participate to the tholins chemical growth (so called “precursors”) : C<sub>2</sub>H<sub>2</sub>, C<sub>2</sub>H<sub>6</sub>, HC3N, C<sub>2</sub>N<sub>2</sub>.</p> <p><strong><img src="" alt="" width="822" height="478" /></strong></p> <p><em>Figure 1 - Time evolution of the masses m/z 16 (CH<sub>4</sub>), 27 (HCN), obtained with a mass spectrometer [4].</em></p> <p> </p> <p><strong>3.2 - Microphysical evolution by scanning electron microscopy </strong></p> <p>The samples were observed by scanning electron microscopy. The images show two growth phases, each corresponding to a gas-particle kinetic regime distinguished by the MID monitoring. Tholins during the transient regime exhibit nanoscale spherical monomers, not exceeding ~200 nm in diameter (Figure 2.A). Tholins formed in the stationary regime show an evolution of spherical monomers up to diameters of a few µm, and the formation of aggregates (Figure 2.B et 2.C).</p> <p><img src="" alt="" /></p> <p><em>Figure 2- Morphologies of Titan's tholins obtained with SEM. Figure 2.A : Tholins formed during the transient regime have an average diameter of 200 nm. Figure 2.B : Evolution of spherical nanometric to micrometric particles. Figure 2.C : Tholins formed during the strationnary regime, have an average diameter of a few µm.</em></p> <p> </p> <p><strong>3.3 - Kinetic modeling of the gas-particle interaction </strong></p> <p><strong> </strong>Based on a kinetic model performed by Pöschl et al. in 2007 [5], the two kinetic regimes observed in the experiment are fitted. From it, the absorption coefficient γ (uptake coefficient) of Titan tholins was deduced for each monitored precursor.. For each regime, an absorption coefficient γ is calculated taking into account the different interactions between gas-surface of the particles, as well as between surface-bulk of the particles, i.e. adsorption, desorption and diffusion effects.</p> <p> </p> <p> </p> <p>[1] : Israël G. et al., Nature 438 : 796-99 (2005).</p> <p>[2] : Lavvas P. et al., The Astrophysical Journal (2011).</p> <p>[3] : Szopa C. et al., Planetary and Space Science 54 (2006).</p> <p>[4] : Perrin et al.  Processes, MDPI (2021)</p> <p>[5] : Pöschl U. et al., Atmospheric Chemistry and Physics 7 (2007)</p> <p> </p> <div> <div> <div> </div> </div> </div>

Titan’s haze is strongly suspected to be an HCN-derived polymer, but despite the first in situ measurements by the ESA-Huygens space probe, its chemical composition and formation process remain largely unknown. To investigate this question, we simulated the atmospheric haze formation process, experimentally. We synthesized analogues of Titan’s haze, named Titan tholins, in an irradiated N2–CH4 gas mixture, mimicking Titan’s upper atmosphere chemistry. HCN was monitored in situ in the gas phase simultaneously with the formation and evolution of the haze particles. We show that HCN is produced as long as the particles are absent, and is then progressively consumed when the particles appear and grow. This work highlights HCN as an effective precursor of Titan’s haze and confirms the HCN-derived polymer nature of the haze.

In this work, we present the results of simulations carried out for N2-H2 capacitively coupled radio-frequency discharges, running at low pressure (0.3-0.9 mbar), low power (5-20 W), and for amounts of H2 up to 5 pct. Simulations are performed using a hybrid code that couples a two-dimensional time-dependent fluid module, describing the dynamics of the charged particles in the discharge, to a zero-dimensional kinetic module, that solves the Boltzmann equation and describes the production and destruction of neutral species. The model accounts for the production of several vibrationally and electronic excited states, and contains a detailed surface chemistry that includes recombination processes and the production of NHx molecules. The results obtained highlight the relevance of the interactions between plasma and surface, given the role of the secondary electron emission in the electrical parameters of the discharge and the critical importance of the surface production of ammonia to the neutral and ionic chemistry of the discharge.

The mixing of N2 with H2 leads to very different plasmas from pure N2 and H2 plasma discharges. Numerous issues are therefore raised involving the processes leading to ammonia (NH3) formation. The aim of this work is to better characterize capacitively-coupled radiofrequency plasma discharges in N2 with few percents of H2 (up to 5%), at low pressure (0.3–1 mbar) and low coupled power (3–13 W). Both experimental measurements and numerical simulations are performed. For clarity, we separated the results in two complementary parts. The actual one (first part), presents the details on the experimental measurements, while the second focuses on the simulation, a hybrid model combining a 2D fluid module and a 0D kinetic module. Electron density is measured by a resonant cavity method. It varies from 0.4 to 5 × 109 cm−3, corresponding to ionization degrees from 2 × 10−8 to 4 × 10−7. Ammonia density is quantified by combining IR absorption and mass spectrometry. It increases linearly with the amount of H2 (up to 3 × 1013 cm−3 at 5% H2). On the contrary, it is constant with pressure, which suggests the dominance of surface processes on the formation of ammonia. Positive ions are measured by mass spectrometry. Nitrogen-bearing ions are hydrogenated by the injection of H2, N2H+ being the major ion as soon as the amount of H2 is >1%. The increase of pressure leads to an increase of secondary ions formed by ion/radical–neutral collisions (ex: N2H+, NH4+, H3+), while an increase of the coupled power favours ions formed by direct ionization (ex: N2+, NH3+, H2+).

Ammonia is an interesting molecule suspected to be formed in Titan ionosphere and leading to further complex chemistry. Here we experimentally study one of the two ways of formation of ammonia in the ionosphere: the catalysis on surfaces in a N2-H2 plasma. We vary plasma conditions in a CCP RF discharge and follow the ammonia formation thanks to IR and mass spectrometries. We show the strong effect of pressure, H2 percentage, plasma power and metallic surfaces.

The main carrier of primordial heavy noble gases in chondrites is thought to be an organic phase, known as phase Q, whose precise characterization has resisted decades of investigation. The Q noble gas component shows elemental and isotopic fractionation relative to the Solar, in favor of heavy elements and isotopes. These noble gas characteristics were experimentally simulated using a plasma device called the “Nebulotron”. In this study, we synthesized thirteen solid organic samples by electron-dissociation of CO, in which a noble gas mixture was added. The analysis of their heavy noble gas (Ar, Kr and Xe) contents and isotopic compositions reveals enrichment in the heavy noble gas isotopes and elements relative to the light ones. The isotope fractionation is mass-dependent and is consistent with a mn-type law, where n≥1. Based on a plasma model, we propose that the ambipolar diffusion of ions in the ionized CO gas medium is at the origin of the noble gas isotopic fractionation. In addition, the elemental fractionation of experimental and chondritic samples can be accounted for by the Saha law of plasma equilibrium, which does not depend on the respective noble gas masses but rather on their ionization potentials. Our results suggest that the Q noble gases were trapped into growing organic particles starting from solar gases that were fractionated in an ionized medium by ambipolar diffusion and Saha processes. This would imply that both the formation of chondritic organic matter and the trapping of noble gases took place simultaneously in the ionized areas of the protoplanetary disk.

Two sorts of solid organic samples can be produced in laboratory experiments simulating Titan's atmospheric reactivity: grains in the volume and thin films on the reactor walls. We expect that grains are more representative of Titan's atmospheric aerosols, but films are used to provide optical indices for radiative models of Titan's atmosphere. The aim of the present study is to address if these two sorts of analogues are chemically equivalent or not, when produced in the same N2–CH4 plasma discharge. The chemical compositions of both these materials are measured by using elemental analysis, XPS analysis and Secondary Ion Mass Spectrometry. The main parameter probed is the CH4/N2 ratio to explore various possible chemical regimes. We find that films are homogeneous but significantly less rich in nitrogen and hydrogen than grains produced in the same experimental conditions. This surprising difference in their chemical compositions could be explained by the efficient etching occurring on the films, which stay in the discharge during the whole plasma duration, whereas the grains are ejected after a few minutes. The higher nitrogen content in the grains possibly involves a higher optical absorption than the one measured on the films, with a possible impact on Titan's radiative models.

The role of polycyclic aromatic hydrocarbons (PAH) and Nitrogen containing PAH (PANH) as intermediates of aerosol production in the atmosphere of Titan has been a subject of controversy for a long time. An analysis of the atmospheric emission band observed by the Visible and Infrared Mapping Spectrometer (VIMS) at 3.28 mu m suggests the presence of neutral polycyclic aromatic species in the upper atmosphere of Titan. These molecules are seen as the counter part of negative and positive aromatics ions suspected by the Plasma Spectrometer onboard the Cassini spacecraft, but the low resolution of the instrument hinders any molecular speciation.In this work we investigate the specific aromatic content of Titan's atmospheric aerosols through laboratory simulations. We report here the selective detection of aromatic compounds in tholins, Titan's aerosol analogs, produced with a capacitively coupled plasma in a N-2:CH4 95:5 gas mixture. For this purpose, Two-Step Laser Desorption Ionization Time-of-Flight Mass Spectrometry (L2DI-TOF-MS) technique is used to analyze the so produced analogs. This analytical technique is based on the ionization of molecules by Resonance Enhanced Multi-Photon Ionization (REMPI) using a lambda=248 nm wavelength laser which is selective for aromatic species. This allows for the selective identification of compounds having at least one aromatic ring. Our experiments show that tholins contain a trace amount of small PAHs with one to three aromatic rings. Nitrogen containing PAHs (PANHs) are also detected as constituents of tholins. Molecules relevant to astrobiology are detected as is the case of the substituted DNA base adenine. (C) 2016 Published by Elsevier Ltd.

The Cassini mission around Titan revealed that the interaction between the N2 and CH4 molecules and the solar VUV radiation leads to a complex chemistry above an altitude of 800km with the detection of heavy organic molecules like benzene (C6H6). This is consistent with an initiation of the aerosols in Titan’s upper atmosphere. The presence of those molecules makes Titan a natural laboratory to witness and understand prebiotic-like chemistry but despite all the data collected, all the possible photochemical processes in such a hydrocarbon-nitrogen-rich environment are not precisely understood. This is why Titan’s atmospheric chemistry experiments are of high interest, especially those focusing on the photochemistry as most of the Titan-like experiments are based on N2-CH4 plasma techniques. In order to reproduce this VUV photochemistry of N2 and CH4, we designed a photochemical reactor named APSIS which is to be coupled window-less with a VUV photon source as N2 needs wavelengths shorter than 100 nm in order to be dissociated. Those wavelengths are available at synchrotron beamlines but are challenging to obtain with common laboratory discharge lamps. At LATMOS, we developed a table-top VUV window-less source using noble gases for the micro-wave discharge. We started with Neon, as it has two resonance lines at 73.6 and 74.3 nm which allow us to dissociate and/or ionize both CH4 and N2. We will present here our first experimental results obtained with APSIS coupled with this VUV source and then discuss them regarding the Cassini data and other previous laboratory photochemical studies.

Since the Cassini-CAPS measurements, organic aerosols are known to be present and formed at high altitudes in the diluted and partially ionized medium that is Titan's ionosphere [1]. This unexpected chemistry can be further investigated in the laboratory with plasma experiments simulating the complex ion-neutral chemistry starting from N 2 -CH 4 [2]. Two sorts of solid organic samples can be produced in laboratory experiments simulating Titan's atmospheric reactivity: grains in the volume and thin films on the reactor walls. We expect that grains are more representative of Titan's atmospheric aerosols, but films are used to provide optical indices for radiative models of Titan's atmosphere.The aim of the present study is to address if these two sorts of analogues are chemically equivalent or not, when produced in the same N 2 -CH 4 plasma discharge. The chemical compositions of both these materials are measured by using elemental analysis, XPS analysis and Secondary Ion Mass Spectrometry. We find that films are homogeneous but significantly less rich in nitrogen and hydrogen than grains produced in the same experimental conditions. This surprising difference in their chemical compositions is explained by the efficient etching occurring on the films, which stay in the discharge during the whole plasma duration, whereas the grains are ejected after a few minutes [3]. The impact for our understanding of Titan's aerosols chemical composition is important. Our study shows that chemical growth and etching process are simultaneously at stake in Titan's ionosphere. The more the aerosols stay in the ionosphere, the more graphitized they get through etching process. In order to infer Titan's aerosols composition, our work highlights a need for constraints on the residence time of aerosols in Titan's ionosphere. [1] Waite et al. (2009) Science , 316, p. 870[2] Szopa et al. (2006) PSS, 54, p. 394[3] Carrasco et al. (2016) PSS, 128, p. 52

Titan is the biggest satellite of Saturn whose atmosphere is mainly composed of molecular nitrogen (N2) and methane (CH4) with an average ratio of 98/2 %. The Cassini/Huygens mission revealed that the interaction between those neutral molecules and the UV solar light leads to a complex photochemistry that produces heavy organic molecules. When those molecules condense, they will then become the solid aerosols which are responsible for the brownish haze surrounding Titan. Between 1000 and 600km, the VUV solar radiations are still significant and will continue to modify the physical, chemical and optical properties of those grains. A change in these parameters can impact the radiative budget of Titan’s atmosphere.

Nitrogen-rich refractory organics are scarce phases recovered as a fraction of stratospheric IDPs and constitute the bulk of the organic matter of some ultracarbonaceous Antarctic micrometeorites. They are likely formed under very specific conditions within a nitrogen-rich environment and may provide valuable clues on the origin of the population of interplanetary dusts accreted by Earth. In this study, we produced relevant analogs of such refractory organics characterized in three ultracarbonaceous Antarctic micrometeorites, starting from the carbonization of an HCN polymer and a tholin. Indeed, carbonization is a process that can increase the polyaromatic character toward a structure similar to that observed in these cosmomaterials. Both these precursors were degraded in an Ar atmosphere at 300, 500, 700 and 1000 degrees C over similar to 1h and characterized by elemental analysis, micro-FTIR and Raman micro-spectroscopy (at 244 and 514 nm excitation wavelengths). Our results show that the precursors evolve along distinct chemical and structural pathways during carbonization and that the influence of the precursor structure is still very strong at 1000 degrees C. Interestingly, these different carbonization routes appear in the spectral characteristics of the G and D bands of their Raman spectra. Several of the residues present chemical and structural similarities with three recently studied ultracarbonaceous micrometeorites (Dobrica et al. [2011]. Meteorit. Planet. Sci. 46, 1363; Dartois et al. [2013]. Icarus 224, 243) and with N-rich inclusions in stratospheric IDPs. However, the residues do not simultaneously account for the carbon structure (Raman) and the chemical composition (IR, N/C ratio). This indicates that the precursors and/or heating conditions in our experiments are not fully relevant. Despite this lack of full relevancy, the formation of a polyaromatic structure fairly similar to that of UCAMMs and IDPs suggests that the origin of N-rich refractory organics lies in a thermal process in the proto-solar disk, however radiolysis cannot be excluded. Crown Copyright (C) 2014 Published by Elsevier Inc. All rights reserved.

The carbon conversion in a N2CH4 radiofrequency (RF) discharge is investigated by analyzing both the gas phase and the dust produced. HCN is identified as the main reactive product and both HCN and NH3 are quantified. This quantification allows determining the carbon conversion yield from CH4 to HCN, which appears to be surprisingly low. The comparison of the infrared absorbance spectra between dust produced in two similar RF plasma setups emphasizes small differences on the CH3/CH2 bonds ratio in the samples. This difference is explained by the aging processes of dust within the plasma.

The carbon conversion in a N2CH4 radiofrequency (RF) discharge is investigated by analyzing both the gas phase and the dust produced. HCN is identified as the main reactive product and both HCN and NH3 are quantified. This quantification allows determining the carbon conversion yield from CH4 to HCN, which appears to be surprisingly low. The comparison of the infrared absorbance spectra between dust produced in two similar RF plasma setups emphasizes small differences on the CH3/CH2 bonds ratio in the samples. This difference is explained by the aging processes of dust within the plasma.

A RF CCP plasma discharge in N2/CH4 mixture is used to produce organic films. Here is presented the influence of a cryogenic cooling during the film synthesis on the optical refractive indices. The optical indices in the visible range are determined by spectroscopic ellipsometry. It is shown that the optical absorption is lower for a material produced at low temperature (105 K) than at room temperature. The absorption decrease is assigned to nitrogen containing products depletion confirmed both by IR absorption spectroscopy of the film on the one hand and by mass spectrometry of the reactive gas phase on the other hand.