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The addition of small amounts of H2 were investigated in a DC glow discharge in N2, at low pressure (~1 mbar) and low power (0.05 to 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 H2 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% H2 has a strong effect on the N2 plasma discharges. Hydrogen quenches the (higher) vibrational levels of N2 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: N2H+ and NH4+ being the major ones, and also H3+, NH+, NH2+, NH3+, N3H+ and N3H3+. The comparison to a radiofrequency capacitively coupled plasma (RF CCP) discharge in similar experimental conditions shows that both discharges led to similar observations. The study of N2-H2 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., Comment: Paper accepted in Plasma Sources Science and Technology in March 2023. The current version on arXiv is the submitted version

1 - Introduction : Titan, the largest satellite of Saturn, has a dense atmosphere composed mainly of N2 and CH4. The photochemistry of these two components leads to the formation of complex organic aerosols, from the upper atmosphere [1]. These solid particles are present throughout the atmosphere up to the surface, and have an important role in many processes taking place on Titan. Our aim is to constrain the news optical properties of these aerosols in the mid-Infrared spectral range (3 to 10 μm) determined by ellipsometry. We moreover considered two different samples representative of the expected evolution of the aerosols in Titan’s atmosphere. 2 – Aerosol Production : Analogs of Titan aerosols (tholins) are formed using the PAMPRE experiment [2]. PAMPRE is a reaction chamber, where a cold capacitively coupled radio frequency plasma is ignited at low pressure (~0.9 mbar). A gas mixture of 95% N2 and 5% CH4 is introduced into the reactor.In this study, two generations (productions) of tholins were performed, using the same gas mixture but injected at different flow rates. The variation of the flow rate allows to modify the residence time of the gas mixture in the reactor. Thus the chemistry evolves more or less slowly, and allows a more or less favored growth of powders. One of the two productions corresponds to tholins realized with the highest flow rate, and present a spherical nanometric morphology with average diameter of 400-500 nm [3]. The other production corresponds to tholins realized with a lower flow rate, presenting a spherical morphology but this time micrometric with an average diameter of 1-1.5 μm. 3 – Sample preparation for optical measurements : Compressed pellets For ellipsometry, the surface condition (roughness) of the sample will have an important impact on the measurements, theoretically the samples should be specular. In our case, tholins particles were collected and compressed into pellets. These pellets consist of a base of KBr compressed beforehand, then a layer of tholins deposited on the surface of this base and compressed afterwards. To realize the compression, the sample is placed between two mirrors, in order to obtain a smooth surface. At the time of the compression of the sample of tholins, the composition of this layer is not homogeneous, it is necessary to take into account a mixture of air and tholins (porosity). The air included into pellets takes a part in the optical constants determined from the measurements made. To get rid of the effect of porosity, the compression rate was varied for the different samples. 4 - Optical analysis : Reflectance measurements were performed on the surface of the pellets, using the ellipsometry technique, using an Infrared source. These measurements were performed in a spectral range from 3 to 10 μm (MIR), with a spectral resolution of 8 cm-1. In the ellipsometry analysis used in this study, the polarimetric angles (Δ and Ψ) are deduced by muller theory for an isotropic case. In order to fit the measured data as well as possible, we applied a one-layer model with a Lorentz oscillator, allowing to infer the refractive index n and the extinction coefficient k. 5 - Conclusion and Discussion : In this study, we propose new optical constants determined from Titan-like solid aerosols in the IR spectral ranges, to be compared with the previous determinations by [4] [5]. Morevover the measurements have been performed for two generations of analogues, with different physicochemical properties, which can simulate aerosols present at different atmospheric altitudes on Titan. [1] : Waite Jr. J.H et al., Science 316.5826 (2007) [2] : Szopa C. et al., Planetary and Space Science 54 (2006) [3] : Hadamcik E. et al., Planetary and Space Science 57 (2009) [4] Imanaka H. et al., Icarus, 218: 247-261 (2012). [5] Khare B.N. et al., Icarus, 60: 127-137 (1984).

1 - Introduction In the atmosphere of the satellite Titan, the photochemistry of its two main components N2 and CH4 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 N2 and CH4 such as HCN, HC3N, C2N2, C2H2, C2H6. We more specifically addressed the absorption processes of the gases on the particle (uptake coefficients). 2 - Experimental method 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. 3- Results 3.1 - Temporal evolution of the gas phase by mass spectrometry In a previous study [4], MID monitoring by mass spectrometry was performed for CH4 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”) : C2H2, C2H6, HC3N, C2N2. Figure 1 - Time evolution of the masses m/z 16 (CH4), 27 (HCN), obtained with a mass spectrometer [4]. 3.2 - Microphysical evolution by scanning electron microscopy 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). 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. 3.3 - Kinetic modeling of the gas-particle interaction 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. [1] : Israël G. et al., Nature 438 : 796-99 (2005). [2] : Lavvas P. et al., The Astrophysical Journal (2011). [3] : Szopa C. et al., Planetary and Space Science 54 (2006). [4] : Perrin et al. Processes, MDPI (2021) [5] : Pöschl U. et al., Atmospheric Chemistry and Physics 7 (2007)

1. Introduction Hydrogen emisssion by degassing magma is likely to have a significant impact on the H2 mixing ratio in early atmospheres of terrestrial planets depending on the surface pressure, volcanic flux and oxygen fugacity of the magma [1]. This process can account for the presence of a few percent of hydrogen in the early atmospheres of terrestrial exoplanets. The formation of water using a N2-CO2-H2 mixture was identified in a previous experimental study focusing on the early Earth with nitrogen as a dominant gas and an initial H2 mixing ratio fixed at 4% [2]. We now explore various amounts of hydrogen in the mixture to cover the unknown parameters regarding surface emission and to highlight the role played by hydrogen in the chemistry of these exoplanetary atmospheres. 2. Experimental method The PAMPRE (french for “aerosol production in microgravity using a reactive plasma”) experimental setup [3] is a plasma reactor used to simulate the photochemistry at low pressure (around 1 hPa) in planetary atmospheres. The CO2-N2-H2 mixture is injected in the reactor with various N2 to H2 abundance ratio and a fixed abundance of 70% for carbon dioxide. The molecular hydrogen mixing ratio is varied from 5% down to 0.5% in volume. The gas phase in the PAMPRE reactor is analyzed using a quadrupole mass spectrometer (Hiden Analyticals) to identify the species formed once the plasma is turned on. 3. Experimental results Figure 1. Production of water and oxygen with 0.5% H2 Figure 2. Production of water and oxygen with 5% H2 The significant oxygen production associated to a hydrogen ratio of 0.5% is reduced drastically when increasing the hydrogen mixing ratio by a factor of 10. The formation of oxygen via the HOx cycle well known in the case of Mars becomes secondary when the hydrogen mixing ratio increases. As a result, the production rate of the OH radical formed by (1) increases. O(1D) + H2 --> OH + H (1) The radical OH then reacts by (2) and (3) which explains the significant formation of water with an initial hydrogen mixing ratio of 5%. H2 + OH --> H2O + H (2) 2 OH --> H2O + O (3) These new experiments put forward the presence of a hydrogen mixing ratio threshold over which the formation of water dominates the formation of oxygen. Two regimes can therefore be identified : O2-dominant production regime at low amounts of H2 and H2O-dominant production regime at higher amounts of H2. 4. Conclusions and perspectives For terrestrial exoplanets known to be in the habitable zone of their host star, the present study shows that molecular hydrogen in the atmosphere is critical to characterize their habitability as this particular molecule leads to an important formation of water in oxydized conditions. It is also shown that based on the hydrogen abundance in the atmosphere, two regimes can be identified and this particular process suggests that high amounts of hydrogen could lead to a significant abundance of water in these atmospheres even without liquid water at the surface. Quantification of relative concentrations using quadrupole mass spectrometry by a recently developped tool [4] is in progress and would allow to quantify the H2 mixing ratio which seperates the two regimes in our experimental conditions. Based on these experimental results, the impact of this process is now being studied using the chemical kinetics model VULCAN [5] taking into account vertical mixing and using a known star spectrum. The predictions obtained from the model will be used to predict future observations of these specific environments in the frame of JWST and ARIEL space missions. Acknowledgements NC acknowledges the financial support of the European Research Council (ERC Starting Grant PRIMCHEM, Grant agreement no. 636829). References [1] Liggins, P., Shorttle, O., Rimmer, P., 2020. Can volcanism build hydrogen-rich early atmospheres ? Earth and Planetary Science Letters 550 [2] Fleury, B., Carrasco, N., Marcq, E., Vettier, L., Määttänen, A., 2015. The Astrophysical Journal Letters 807 [3] Szopa, C., Cernogora, G., Boufendi, L., Correia, J., Coll, P., 2006. Planetary and Space Science 54 [4] Gautier, T., Serigano, J., Bourgalais, J., Hörst, S.M., Trainer, M.G., 2020. Decomposition of electron ionization mass spectra for space application using a Monte-Carlo approach. Rapid Communications in Mass Spectrometry [5] Tsai, S-M., Lyons, J.R., Grosheintz, L, Rimmer, P.B., Kitzmann, D., Heng, K.,2017. VULCAN : An Open-Source, Validated Chemical Kinetics Python Code for Exoplanetary Atmospheres. The Astrophysical Journal Supplement Series 228

Titan, Saturn’s biggest moon, is an ocean world, covered by organic materials and therefore one of the most promising astrobiological target in the Solar System, likely holding clues on the origin of life on Earth. That is why NASA has selected the Dragonfly mission to send in 2027 a rotorcraft lander to Titan and investigate its prebiotic chemistry and habitability. Making multiple flights (up to 24 in 2.5 years, starting in 2034), Dragonfly will explore a variety of locations, from the Shangri-La dune field to the rim of the young impact crater Selk (Lorenz et al., 2018), and therefore sample materials and determine surface properties in different geologic settings. The two permittivity probes - called DIEL- on board the Geophysical and Meteorological package (DraGMet) will be especially useful to characterize Dragonfly landing site environment. DIEL consist of 2 electrodes acting as self-impedance permittivity probes. They will be mounted on each skid of the Dragonfly rotorcraft and operate independently for sake of redundancy and safety. Their objective is to measure the complex ground permittivity at several low frequencies ( During this presentation, we will describe the tests that have been performed on prototypes of the DIEL electrode plates in order to estimate their sounding depth and sensitivity to composition variations. Tests were performed e.g., with the electrode lying on reference plastic slabs and natural materials (air, sand, soil, liquid water, etc.) of well-known complex permittivity, at ambient and Titan temperatures. The effect of porosity and of a bad ground-electrode contact was also investigated leading to suggestions to optimize DIEL electrode design, accommodation and performance. Indeed, the design of the DIEL experiments (size, shape, accommodation of the electrodes, modes of operation ect.) is not frozen yet and we are also conducting modelling simulations with COMSOL Multiphysics © to explore possible better designs and confirm the results obtained in laboratory. Lastly, in order to relate DIEL measurements to the ground composition, it is crucial to know the electrical properties of materials relevant to Titan’s surface. This is the purpose of the PAP (Permittivité d’Analogues Planétaires) measurement bench that have been developed at LATMOS. This bench includes a cryostat to perform measurements at Titan’s temperature (90 K). It was successfully used to investigate the complex permittivity of analogs of Titan’s organic aerosols called “tholins” (Lethuillier et al., 2018). Future measurements will focus on “eroded” tholins, that is tholins that have been modified during their descent to the surface by processes analogous to those at play in Titan’s atmosphere: UV radiations (Carrasco et al., 2018), interaction with ionosphere’s charged particles (Chatain et al., 2020), deposition of ice in the low stratosphere (Fleury et al., 2019; Dubois et al., 2020), wetting by droplets of liquid methane in the troposphere etc. References Beghin et al., Icarus 218 (2012) Carrasco et al., Nature Astronomy, Nature Publishing Group (2018) Chatain et al., Icarus 345 (2020) Dubois et al., Icarus 338 (2016) Hamelin et al., Icarus 270 (2016) Grard et al., Planet. Space Sciences 54 (2006) Fleury et al., Icarus 321 (2019) Lethuillier et al., Astronomy & Astrophysics 519 (2018) Lorenz et al., Johns Hopkins APL Technical Digest 34 (2018)

Abstract Titan’s aerosols start forming in the ionosphere, in a reactive environment hosting electrons, ions and radicals. In this work we study the interaction of the aerosols with the ‘carbon free’ plasma species. In this objective, analogues of Titan’s aerosols (tholins) are exposed to a N2-H2 plasma in the laboratory. A previous work observed modifications on the solid aerosols [1]. Complementarily, this study investigates a possible feedback of the tholins erosion on the gas phase composition. The decrease of ammonia and the formation of carbon-bearing (and especially nitrile-bearing) species is observed by neutral and ion mass spectrometry. We suggest surface processes combining reactions with radicals and ion sputtering to explain these observations. 1- Introduction Saturn’s biggest moon, Titan, has a thick atmosphere of N2 and CH4 (2-5%), covered by a haze of orange organic aerosols. The mission Cassini discovered that these aerosols start forming around 900 – 1200 km, in the ionosphere [2]. The ionosphere is the upper part of the atmosphere, ionized by UV solar photons and energetic particles from Saturn’s magnetosphere. It therefore hosts reactive plasma species (electrons, ions, radicals, excited species) that are likely to interact with the newly formed aerosols. Carbon-bearing species help the carbon growth of the aerosols, but what is the result of the interaction of the aerosols with the other plasma species? Laboratory analogues of Titan’s aerosols can be formed in plasma discharges (e.g. in the experiment PAMPRE, Szopa et al., 2006); they are called ‘tholins’. A few studies have previously studied the effect of extreme UV photons on tholins [4], [5], and the recombination of atomic hydrogen at the surface of tholins [6]. Nevertheless, the interaction of Titan’s aerosols with the other ‘carbon-free’ plasma species has never been studied before. We built a dedicated experimental setup to expose tholins to a N2-H2 plasma (named ‘THETIS’ for THolins Evolution in Titan’s Ionosphere Simulation). The evolution of the tholins have previously been analysed by IR absorption spectroscopy and reported in Chatain et al. (2020a). We observed a physical erosion of the grains (with holes of ~20 nm) and chemical changes (disappearance of isonitriles and unsaturated structures, and formation of a new nitrile band). In this context, the aerosols erosion is likely to have a feedback effect on the composition of the gas phase in Titan’s ionosphere. In this work, we therefore investigate the modifications of the gas phase during the exposure of tholins to a N2-H2 plasma. 2- The experimental setup The ionosphere of Titan ‘free of carbon species’ is simulated by a DC glow plasma discharge in N2-H2 (with up to 5% H2). The pressure is varied from 0.5 to 2 mbar, and the current from 10 to 40 mA. Tholins formed in PAMPRE are spread on a thin metallic grid, which is exposed to the plasma during the experiment. The gas phase composition (neutrals and positive ions) is measured by a mass spectrometer (EQP series from Hiden), whose collecting head is positioned at ~5 cm from the plasma glow, next to the grounded electrode (see Figure 1). The mass spectrometer transmittance has previously been finely determined [7] to enable a quantitative comparison between all the mass intensities. 3- Evolution of neutral species Figure 2 shows mass spectra acquired close to the N2-H2 plasma discharge, without (blue) and with (yellow) the tholins sample. Ammonia (NH3), which is formed in the N2-H2 plasma, decreases with the insertion of tholins in the plasma. It suggests that ammonia or its precursors are consumed by tholins. On the opposite, we observed the production of HCN and other carbon-containing molecules. Most of them contains nitriles (-CN), like acetonitrile (CH3-CN) and cyanogen (C2N2). 4- Evolution of positive ions Figure 3 similarly presents mass spectra without (blue) and with (yellow) tholins exposed to the N2-H2 plasma. Observations are consistent with the evolution of neutral species: ammonia related ions (NHx+) are formed in the N2-H2 plasma and decrease with the addition of tholins; HCN related ions (CN+, HCN+, HCNH+) increase strongly; and new carbon-containing ions are formed. Among those we observe nitrile-bearing ions (related to acetonitrile and cyanogen), and highly unsaturated hydrocarbons (C+, CH+, C2+, C2H+, C3+, C3H+…). 5- Conclusions on the production of new volatiles by heterogeneous chemistry From the evolution of the gas phase species observed in this work, the modifications of the tholins chemical functions presented in Chatain et al. (2020a) and previous heterogeneous chemistry modelling work in microelectronics [8], we suggest in Figure 4 some surface processes. Radicals are likely to chemically react with the tholins. In particular, we suggest that an interaction of tholins carbon atoms with the radicals N, NH and H leads to the formation of HCN(s) (which stays adsorbed at the surface). In parallel, ion sputtering ejects fragments of tholins into the gas phase. It is especially the case for HCN which is already a stable molecule. This could also explain the nitrile-containing species and the highly unsaturated hydrocarbon ions observed in the previous sections. In addition, NH is fundamental in the production of NH3. The fact that NH is used at the surface of tholins to form HCN(s) is a reasonable explanation to the decrease of ammonia production. In conclusion, in parallel to their formation in the ionosphere of Titan, aerosols are likely to undergo heterogeneous erosive processes and be a source of new volatiles. It would be interesting to implement such processes in the ionospheric chemical models. Acknowledgements A.C. acknowledges ENS Paris-Saclay Doctoral Program. N.C. acknowledges the financial support of the European Research Council (ERC Starting Grant PRIMCHEM, Grant agreement no. 636829). References [1] A. Chatain et al. Icarus 345 (2020) [2] J. H. Waite et al. Science 316 (2007) [3] C. Szopa et al. Planet. Space Sci. 54 (2006) [4] N. Carrasco et al. Nat. Astron. 2 (2018) [5] S. Tigrine et al. Astrophys. J. 867 (2018) [6] Y. Sekine et al. Icarus 194 (2008) [7] A. Chatain et al. Plasma Sources Sci. Technol. (2020) [8] K. Van Laer et al. Plasma Sources Sci. Technol. 22 (2013)

In the atmosphere of Titan, Saturn's main satellite, molecular growth is initiated by a chemistry involving charged and free-radical species. However, the respective contribution of these species to the complexification of matter is far from being known. This work presents a chemical analysis by mass spectrometry at relatively low pressure to characterize the formation pathways of the first aromatics, notably benzene, by irradiating a mixture N2/CH4 with EUV photons. Introduction It is now accepted that photochemical processes in the atmosphere of Titan, the largest of Saturn's 62 moons, directly influence the chemical composition of the liquid lakes and the surface hydrological activity. Thus, a detailed understanding of the chemistry of Titan's atmosphere is the way forward to a whole Titan modeling. In the highest atmospheric layers between 700 and 1100 km in altitude, N2 and CH4 are ionized and dissociated mainly by energetic EUV photons from solar radiation, leading to the formation of reactive species like ions and radicals among a diverse range of species hydrocarbons, amines, and imines. In the lower layers of the atmosphere, below 500km, hydrocarbons such as benzene and other aromatics by interaction with lower energy UV photons (>155 nm) are key compounds in the formation of larger and more complex molecules like Polycyclic Aromatic Hydrocarbons (PAHs) and Polycyclic Aromatic Nitrogen Heterocycles (PANHs). These intermediate species between small organic precursor molecules and larger carbonaceous materials, contribute to the production of aerosols which form the opaque photochemical haze layers that obscure the surface of Titan. Thus the identification of the pathways of formation of these species is essential for understanding the chemistry of Titan's atmosphere, but is yet to be fully understood even for the simplest PAH, namely benzene (C6H6). In situ and direct observations are insufficient to constrain photochemical models and understand the molecular complexification pathways of Titan's atmosphere. Thus the way forward is to reproduce the chemistry in the laboratory by irradiating a mixture of gases representative of this environment from a photon-producing energy source. Experimental Method In this study a nonlinear optics approach has been used to generate intense and stable light in the Extreme Ultraviolet (EUV) because energetic photons are absorbed in the upper atmospheric layers and initiate the formation of reactive species in the satellite ionosphere. The EUV light was produced by the high-order harmonic generation of the femtosecond BLASBEAT laser at CELIA in Bordeaux and injected into the closed and removable SURFACAT photoreactor to irradiate for several hours, at a pressure of 0.1mbar, a gas mixture composed of nitrogen and methane (95/5%) at a wavelength of 85.58 nm. This wavelength falls within the energy range of interest between the CH4 dissociation threshold (98 nm) and the N2 ionization threshold (79.4 nm), which allows the production of nitrogen atoms in its ground and excited state that are suspected of playing a fundamental role in the molecular growth of complex but as yet unknown nitrogen species. Results The results presented in Figure 1, show overall a good agreement between the products formed in the photochemical reactor and the data obtained in situ in Titan's atmosphere. Especially, there is good agreement for the peak at mass 78 which has been identified as benzene in Titan's atmosphere. Because of the importance of benzene in Titan's atmosphere, this study is dedicated to analyze its formation pathways in the photochemical reactor and the implications for our understanding of the chemistry of Titan's atmosphere. At the wavelength used in this experiment, photolysis of methane leads mainly to the formation of the CH4+ ion with a branching ratio of 47% and methyl radical ion (CH3+) and methylidyne radical (CH) with branching ratios of 19% and 18% respectively. While the formation of CH4+ leads to a closed cycle of methane formation, CH3+ is a source of C2H5+ ions by reaction with methane. These ions recombine to lead mainly to the formation of highly reactive ethylenyl radicals (C2H3). By reacting with hydrocarbon radicals such as butadienyl (C4H3), they are at the origin of phenyl radicals (C6H5), precursor of benzene by addition of a hydrogen atom. At the wavelength used in this experiment, the other dominant photoproduct of methane photolysis, CH, leads mainly to the formation of C2H4, whose photolysis by energetic photons leads mainly to the formation of hydrocarbon ions C2H2+ and C2H3+. These ions then lead to the formation of propargyl radicals (C3H3) which by recombining give rise to the formation of benzene. Implications for Titan's atmosphere The literature points out that the production of benzene in the upper atmospheric layers of Titan is due to radical-radical reactions but mainly to the chemistry of the ions via the dissociative recombination of aromatic ions such as C6H6+ and C6H7+. In less energetic conditions, such as photons from solar radiation arriving on Titan, the photolysis of methane leads mainly to the formation of hydrocarbon radicals (CH2 and CH3) which will contribute to many small neutral hydrocarbons (C2H2, C2H4) that will form heavy ions and prevent electronic recombinations. However, the troubling overlap between the benzene signal in this EUV-dominated work and that of INMS shows that it is crucial to have a good assessment of the branching ratios of radical-radical reactions at low pressures and low temperatures representative of Titan's atmosphere. Especially, association reactions can be rapid even at low pressures for adducts of significant size. Indeed, the most energetic photons are absorbed in the upper atmospheric layers to initiate molecular growth while the less energetic photons are absorbed lower in the atmosphere. Additionally, the detection of a signal at mass 92 in Figure 1, could be assigned to toluene (C7H8) and signal at mass 91 to its main fragment due to electronic impact by the ionization of the mass spectrometer used in this work. It confirms the importance of a rigorous analysis of neutral reactions, since toluene can be formed by the reaction between phenyl and methyl radicals, which, although poorly understood, involves one of the precursors of benzene whose formation would be favored in an environment with energetic photons.

Organic aerosols accumulated in Titan's orange haze start forming in its ionosphere. This upper part of the atmosphere is highly reactive and complex ion chemistry takes place at altitudes from 1200 to 900 km. The ionosphere is a nitrogen plasma with a few percent of methane and hydrogen. Carbon from methane enables the formation of macromolecules with long organic chains, finally leading to the organic aerosols. On the other hand, we suspect that hydrogen and the protonated ions have a different erosive effect on the aerosols. Here we experimentally studied the effect of hydrogen and protonated species on organic aerosols. Analogues of Titan's aerosols were formed in a CCP RF plasma discharge in 95% N2 and 5% CH4. Thereafter, the aerosols were exposed to a DC plasma in 99% N2 and 1% H2. Samples were analysed by scanning electron microscopy and in situ infrared transmission spectroscopy. Two pellet techniques - KBr pressed pellets and thin metallic grids - were compared to confirm that modifications seen are not due to the material used to make the pellet. We observed that the spherical aerosols of ~500 nm in diameter were eroded under N2-H2 plasma exposure, with the formation of holes of ~10 nm at their surface. Aerosols were globally removed from the pellet by the plasma. IR spectra showed a faster disappearance of isonitriles and/or carbo-diimides compared to the global band of nitriles. The opposite effect was seen with beta-unsaturated nitriles and/or cyanamides. Double bonds as C=C and C=N were more affected than amines and C-H bonds. N-H and C-H absorption bands kept a similar ratio in intensity and their shape did not vary. Therefore, it seems that carbon and hydrogen play opposite roles in Titan's ionosphere: the carbon from methane lead to organic growth while hydrogen and protonated species erode the aerosols and react preferentially with unsaturated chemical functions., Comment: 30 pages, 16 figures

The in situ exploration of Titan’s atmosphere requires the development of laboratory experiments to understand the molecular growth pathways initiated by photochemistry in the upper layers of the atmosphere. Key species and dominant reaction pathways are used to feed chemical network models that reproduce the chemical and physical processes of this complex environment. Energetic UV photons initiate highly efficient chemistry by forming reactive species in the ionospheres of the satellite. We present here a laboratory experiment based on a new closed and removable photoreactor coupled here to an Extreme Ultraviolet (EUV) irradiation beam produced by the high-order harmonic generation of a femtosecond laser. This type of EUV stable source allow long-term irradiation experiments in which a plethora of individual reactions can take place. In order to demonstrate the validity of our approach, we irradiated for 7 hours at 89.2 nm, a gas mixture based on N2/CH4 (5%). Using only one wavelength, products of the reaction reveal an efficient photochemistry with the formation of large hydrocarbons but especially organic compounds rich in nitrogen similar to Titan. Among these nitrogen compounds, new species had never before been identified in the mass spectra obtained in situ in Titan’s atmosphere. Their production in this experiment, on the opposite, corroborates previous experimental measurements in the literature on the chemical composition of aerosol analogues produced in the laboratory. Diazo-compounds such as dimethyldiazene (C2H6N2), have been observed and are consistent with the large nitrogen incorporation observed by the aerosols collector pyrolysis instrument of the Huygens probe. This work represents an important step forward in the use of a closed cell chamber irradiated by the innovative EUV source for the generation of photochemical analogues of Titan aerosols. This approach allows to better constrain and understand the growth pathways of nitrogen incorporation into organic aerosols in Titan’s atmosphere.

International audience; The detection of thin haze layers in Pluto's atmosphere highlighted the complex photochemistry occurring in it. In order to understand the processes leading to the formation of Pluto's aerosols, we produced in laboratory a plasma mimicking Pluto's atmospheric chemistry and we analyzed this plasma by in situ mass spectrometry.

International audience; Organic aerosols formed in Titan's upper atmosphere are surrounded by very reactive plasma species. The present work aims to study the potential interactions between aerosols and plasma species. The exposure of aerosols to plasma is simulated in a cold plasma reactor. IR transmission spectroscopy gives clues about the chemical modifications of the aerosols. Mass spectrometry simultaneously measures the neutral species and positive ions in the gas phase. We observe the formation of HCN and carbocations while ammonia density is decreased by the addition of organic aerosols in the N2-H2 plasma.

International audience; The discovery of haze in Pluto's atmosphere on July 14 th , 2015, has raised lots of questions. To help understand the data provided by the New Horizons spacecraft, Pluto's aerosols analogues were synthetized and their chemical composition was determined by high-resolution mass spectrometry (Orbitrap technique).

International audience; We will present laboratory investigation of Pluto's atmosphere and its aerosols formation to help understand the data provided by the New Horizons spacecraft.

The Cassini Plasma Spectrometer (CAPS) Electron Spectrometer (ELS) instrument onboard Cassini revealed an unexpected abundance of negative ions above 950 km in Titan's ionosphere. \textit{In situ} measurements indicated the presence of negatively charged particles with mass-over-charge ratios up to 13,800 \textit{u/q}. At present, only a handful of anions have been characterized by photochemical models, consisting mainly of C$_n$H$^-$ carbon chain and C$_{n-1}$N$^-$ cyano compounds ($n=2-6$); their formation occurring essentially through proton abstraction from their parent neutral molecules. However, numerous other species have yet to be detected and identified. Considering the efficient anion growth leading to compounds of thousands of \textit{u/q}, it is necessary to better characterize the first light species. Here, we present new negative ion measurements with masses up to 200 \textit{u/q} obtained in an \ce{N2}:\ce{CH4} dusty plasma discharge reproducing analogous conditions to Titan's ionosphere. We perform a comparison with high altitude CAPS-ELS measurements near the top of Titan's ionosphere from the T18 encounter. The main observed peaks are in agreement with the observations. However, a number of other species (\textit{e.g.} \ce{CNN-}, \ce{CHNN-}) previously not considered suggests an abundance of N-bearing compounds, containing two or three nitrogen atoms, consistent with certain adjacent doubly-bonded nitrogen atoms found in tholins. These results suggest that an N-rich incorporation into tholins may follow mechanisms including anion chemistry, further highlighting the important role of negative ions in Titan's aerosol growth., Comment: 8 pages, 2 figures, accepted for publication in the Astrophysical Journal Letters

Thanks to the \textit{Cassini} spacecraft onboard instruments, it has been known that Titan's ionospheric chemistry is complex and the molecular growth is initiated through the photolysis of the most abundant species directly in the upper atmosphere. Among the pool of chemical compounds formed by the photolysis, N-bearing species are involved in the haze formation but the chemical incorporation pathways need to be better constrained.}\\ In this work, we performed low-pressure EUV photochemical laboratory experiments. The APSIS reactor was filled with a N$_2$/CH$_4$ (90/10\%) gas mixture relevant to the upper atmosphere of Titan. The cell was irradiated by using a EUV photon source at 73.6 nm {which has been difficult to produce in the laboratory for previous studies.}. The photoproducts (both neutral and ionic species) were monitored \textit{in situ} with a quadrupole mass spectrometer. The chemical pathways are explained by confronting experimental observations and numerical predictions of the photoproducts. \\ The most interesting result in this work is that methanimine was the only stable N-bearing neutral molecule detected during the experiments and it relies on N$_2^+$ production. This experimental result is in agreement with the relatively high abundance predicted by 1D-photochemical models of Titan's atmosphere and comforts methanimine as an intermediate towards the formation of complex N-bearing organic molecules.\\ {This experiment is only testing one part of the overall chemical scheme for Titan's upper atmosphere due to the selective wavelength but demonstrates the capability to probe the chemical pathways occurring in Titan's atmosphere by minimizing bias coming from wall surface reactions.}

International audience; In Titan's upper atmosphere, Cassini detected mass signatures compatible with the presence of heavily charged molecules which are volatile precursors to the solid core of the aerosols. INMS measurements revealed molecular groupings of masses below 100 amu, while CAPS-IBS saw positive ions up to ~350 amu, well beyond the INMS detection limit. Photochemical processes are also influenced by dayside vs. nightside conditions, with photoionization dominating the dayside. From a dynamical and transport viewpoint, the ion densities were also found to be sensitive to modeled turbulent neutral transport. A previous laboratory study has also explored the influence of trace species on the first and intermediate steps of the neutral and positive ion chemistry, in hydrocarbon-enriched mixtures. Furthermore, models have indicated that primary production processes and ion-molecule reactions mainly control the production of the lighter and subsequent heavier positive ions. These reactions rely on the large abundancies of hydrocarbon building blocks (e.g. C2H2 and C2H4). These observations indicate that ion chemistry has an important role for organic growth in the upper atmosphere. However, the processes coupling ion chemistry and the formation of larger tholins have yet to be further explored. In this study, we investigate the positive ion chemistry in the laboratory, responsible for the efficient organic growth producing tholins, simulated in the PAMPRE plasma reactor. We use a RF-generated plasma at 1 mbar pressure, coupled with an ion mass spectrometer , probing the gas phase volatiles in situ. We study N2:CH4 mixtures with 1-10% CH4 mixing ratios, and investigate the impact of light ions on the larger ones. We further compare our results to Cassini Ion and Neutral Mass Spectrometer (INMS) measurements taken during the T40 flyby.

International audience; The emergence of life on the Earth has required a prior organic chemistry leading to the formation of prebiotic molecules. The origin and the evolution of the organic matter on the early Earth is not yet firmly understood. Several hypothesis, possibly complementary, are considered. They can be divided in two categories: endogenous and exogenous sources. In this work we investigate the contribution of a specific endogenous source: the organic chemistry occurring in the ionosphere of the early Earth where the significant VUV contribution of the young Sun involved an efficient formation of reactive species. We address the issue whether this chemistry can lead to the formation of complex organic compounds with CO2 as only source of carbon in an early atmosphere made of N2, CO2 and H2, by mimicking experimentally this type of chemistry using a low pressure plasma reactor. By analyzing the gaseous phase composition, we strictly identified the formation of H2O, NH3, N2O and C2N2. The formation of a solid organic phase is also observed, confirming the possibility to trigger organic chemistry in the upper atmosphere of the early Earth. The identification of Nitrogen-bearing chemical functions in the solid highlights the possibility for an efficient ionospheric chemistry to provide prebiotic material on the early Earth.

International audience; The goal of this experimental study is to investigate how the presence of atmospheric CO2 affects the optical properties of organic photochemical aerosols. To this end, we add CO2 to a N2:CH4 gas mixture used in a plasma typically used for Titan studies. We produce organic thin films (tholins) in plasmas where the CO2 / CH4 ratio is increased from 0 to 4. We measure these films via spectrometric ellipsometry in the ultraviolet, visible and near-infrared (270 nm to 2 µm). The ellipsometry parameters are fitted with a Tauc-Lorentz model used for optically transparent materials, to obtain the thickness of the thin film, its optical band gap, and the refractive indices. According to this optical mode, oxidized organic aerosols are transparent in the visible up to the near ultraviolet (k = 0 from 900-400 nm), while the fully reduced organic aerosol absorbs in the visible as well as in the near UV (k = 0 from 900-500 nm). As the CO2 /CH4 ratio is quadrupled, the position of the UV absorption resonance is shifted from ∼177 nm to 264 nm, and its strength is also quadrupled: oxidized tholins absorb more efficiently in the middle UV with respect to the far UV for reduced tholins. Our laboratory wavelength-tabulated refractive indices provide further constraints to atmospheric models of the early Earth and Earth-like exoplanets including photochemical hazes formed under increasingly oxidizing conditions.

International audience; The water concentration and distribution in the early Earth's atmosphere are important parameters that contribute to the chemistry and the radiative budget of the atmosphere. If the atmosphere above the troposphere is generally considered as dry, photochemistry is known to be responsible for the production of numerous minor species. Here we used an experimental setup to study the production of water in conditions simulating the chemistry above the troposphere of the early Earth with an atmospheric composition based on three major molecules: N2, CO2, and H2. The formation of gaseous products was monitored using infrared spectroscopy. Water was found as the major product, with approximately 10% of the gas products detected. This important water formation is discussed in the context of the early Earth.

International audience; In the atmosphere of Titan, Saturn’s main satellite, molecular growth is initiated by 85.6 nm extreme ultraviolet (EUV) photons triggering a chemistry with charged and free-radical species. However, the respective contribution of these species to the complexification of matter is far from being known. This work presents a chemical analysis in order to contribute to a better understanding of aromatic formation pathways. A gas mixture of N2/CH4 (90/10%) within the closed SURFACAT reactor was irradiated at a relatively low pressure (0.1 mbar) and room temperature for 6 h by EUV photons (∼85.6 nm). The neutral molecules formed at the end of the irradiation were condensed in a cryogenic trap and analyzed by electron ionization mass spectrometry. An analysis of the dominant chemical pathways highlights the identification of benzene and toluene and underlies the importance of small ion and radical reactions. On the basis of the experimental results, a speculative mechanism based on sequential H-elimination/CH3-addition reactions is proposed for the growth of aromatics in Titan’s atmosphere. Elementary reactions to be studied are given to instill future updates of photochemical models of Titan’s atmosphere.

The Cassini Mission has greatly improved our understanding of the dynamics and chemical processes occurring in Titan's atmosphere. It has also provided us with more insight into the formation of the aerosols in the upper atmospheric layers. However, the chemical composition and mechanisms leading to their formation were out of reach for the instruments onboard Cassini. In this context, it is deemed necessary to apply and exploit laboratory simulations to better understand the chemical reactivity occurring in the gas phase of Titan-like conditions. In the present work, we report gas phase results obtained from a plasma discharge simulating the chemical processes in Titan's ionosphere. We use the PAMPRE cold dusty plasma experiment with an N2-CH4 gaseous mixture under controlled pressure and gas influx. An internal cryogenic trap has been developed to accumulate the gas products during their production and facilitate their detection. The cryogenic trap condenses the gas-phase precursors while they are forming, so that aerosols are no longer observed during the 2h plasma discharge. We focus mainly on neutral products NH3, HCN, C2H2 and C2H4. The latter are identified and quantified by in situ mass spectrometry and infrared spectroscopy. We present here results from this experiment with mixing ratios of 90-10% and 99-1% N2-CH4, covering the range of methane concentrations encountered in Titan's ionosphere. We also detect in situ heavy molecules (C7). In particular, we show the role of ethylene and other volatiles as key solid-phase precursors., Comment: 55 pages, 10 figures, 5 tables, Accepted for publication in Icarus

Photochemical aerosols were observed in Pluto atmosphere during the New Horizons flyby on July 14th, 2015, as several thin haze layers extending at >350 km of altitude. This flyby has raised numerous questions on the aerosols formation processes and their impact on Pluto radiative transfer and climate. In order to gain a better understanding, we synthesized Pluto aerosol analogues in a room-temperature dusty plasma experiment and inferred their chemical composition from infrared spectroscopy, elemental composition analysis and very high-resolution mass spectrometry (ESI+/Orbitrap device). Three types of samples were synthesized at 0.9 ± 0.1 mbar, called P400, P600 and PCO-free. The samples P400 and P600 were produced from gas mixtures mimicking Pluto atmosphere at around 400 and 600 km of altitude, respectively, in order to determine if CH4 mixing ratio has an influence on the chemical composition of the aerosols. The sample PCO-free was produced from a gas mixture similar to the one forming the sample P400, but without carbon monoxide, in order to identify the impact of CO. Our study shows that the molecules constituting samples P400 and P600 are very rich in nitrogen atoms (up to 45% in mass of N elements) and, compared to the molecules constituting the PCO-free sample, a significant incorporation of oxygen atoms was detected. Moreover, our results on the variation of CH4 mixing ratio demonstrate that different ratios lead to different reactivity between N2, CH4 and CO. In particular, more nitrogen and oxygen atoms are detected in the bulk composition of the analogues P400. Due to the presence of nitrogenated and oxygenated molecules in the analogues of Pluto aerosols, we suggest that these aerosols will have an impact on Pluto radiative transfer, and thus on climate, that will differ from predictions based on the optical constants of Titan aerosol analogues.