Interaction dust-plasma in Titan's ionosphere: feedbacks on the gas phase composition

Titan's organic aerosols are formed in the ionosphere, a layer ionized by solar VUV photons and energetic particles from the magnetosphere of Saturn, forming a natural N2-CH4-H2 plasma. Previous works showed some chemical evolution processes: VUV photons slightly alter the aerosols nitrile bands, hydrogen atoms tend to hydrogenate their surface and carbon-containing species participate to the growth of the aerosols. This work investigates the effect of the other plasma species, namely the N2-H2 derived ions, radicals and excited states. Industrial plasmas often use N2-H2 discharges to form ammonia-based fertilizers, for metal nitriding, and to erode organic surfaces. Consequently, these are likely to affect Titan's organic aerosols. We therefore developed the THETIS experiment to study the interactions between analogues of Titan's aerosols (tholins) and the erosive N2-H2 plasma species found in Titan's ionosphere. Following a first paper on the evolution of the solid phase by Scanning Electron Microscopy and IR transmission spectroscopy (Chatain et al., Icarus, 2020), this paper focuses on evolution of the gas phase composition, by neutral and ion mass spectrometry. Newly formed HCN, NH3-CN and C2N2 are extracted from the tholins as well as some other carbon-containing species and their derived ions. On the other hand, the production of ammonia strongly decreases, probably because the H, NH and N radicals are rather used for the production of HCN at the surface of tholins. Heterogeneous processes are suggested: chemical processes induced by radicals at the surface would modify and weaken the tholin structure, while ion sputtering would desorb small molecules and highly unsaturated ions. The effect of plasma erosion on aerosols in Titan's ionosphere could therefore lead to the formation of CN bonds in the aerosol structure and the production of HCN or R-CN species in the gas phase.Comment: This paper has been accepted in Icarus (February 2023). The current version in arXiv is the submitted versio

Simulating Titan's upper atmosphere and its photochemistry in the vacuum ultra-violet (VUV)

International audienceTitan, the largest moon of Saturn, has a dense atmosphere whose upper layers are mainly composed of methane (CH4) and molecular nitrogen (N2). The Cassini mission revealed that the interaction between those molecules and the solar VUV radiation, as well as the electrons from Saturn’s magnetosphere, leads to a complex chemistry above an altitude of 800km.Cassini instruments such as INMS or CAPS revealed that this naturally ionized environment contains heavy organic molecules like benzene (C6H6) even at altitudes higher than 900 k

Characterization of a DC glow discharge in N2-H2 with electrical measurements and neutral and ion mass spectrometry

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 versio

N2-H2 capacitively coupled radio-frequency discharges at low pressure. Part I. Experimental results: Effect of the H2 amount on electrons, positive ions and ammonia formation

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 +).N. Carrasco acknowledges the financial support of the European Research Council (ERC Starting Grant PRIMCHEM, Grant agreement no. 636829). A. Chatain acknowledges ENS Paris-Saclay Doctoral Program. A. Chatain is grateful to Gilles Cartry and Thomas Gautier for fruitful discussions on the MS calibration. L.L. Alves acknowledges the financial support of the Portuguese Foundation for Science and Technology (FCT) through the project UID/FIS/50010/2019. L. Marques and M. J. Redondo acknowledge the financial support of the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Funding UIDB/04650/2019

Experimental Approach to the Positive Ion Chemistry in Titan’s Upper Atmosphere

International audienceTitan is Saturn’s largest satellite. This object is unique in the solar system as it hosts a dense atmosphere [1] mainly made of molecular nitrogen N2 and methane CH4, with a surface pressure of 1.5 bar. The nitrogen-rich atmosphere and the presence of liquid areas at the surface make it one of the most interesting objects to understand the evolution of the primitive Earth before the emergence of life and to look for habitable worlds outside the solar system. The Cassini-Huygens Mission has been probing Titan since 2004. It has revealed an intense atmospheric photochemistry initiated by the photo-dissociation and ionization of N2 and CH4 [2]. Photochemistry on Titan leads to the formation of solid organic aerosols responsible for a smog permanently surrounding the moon [2,3]. These aerosols are produced in large amounts and have a significant interest for astrobiology because they are among the most complex organic materials ever detected in extra-terrestrial bodies. In the upper atmosphere, the plasma spectrometer onboard Cassini detected signatures compatible with the presence of heavily charged molecules which are precursors for the solid core of the aerosols [4,5,6]. These observations hint at the key role of ion chemistry for organic growth. Further, it is now known that aerosols are initiated in the ionosphere, where gas and solid aerosols coexist in a fully coupled ionic and neutral chemistry. However, the processes coupling ion chemistry and aerosol production are mostly still unknown. For the first time, we use an experimental approach, investigating the ion chemistry, responsible for the organic growth that we observe in Titan’s upper atmosphere. To do this, we use a plasma reactor simulating Titan’s heterogeneous ionosphere chemistry [7]. Positive ions are investigated by in situ ion mass spec- trometry, alongside neutral products complementarily studied through infrared absorption spectroscopy and mass spectrometry

Organic chemistry in the ionosphere of the early Earth

International audienceThe emergence of life on the Early Earth during the Archean has required a prior complex organic chemistry providing the prerequisite bricks of life. The origin of the organic matter and its evolution on the early Earth is far from being understood. Several hypotheses are investigated, possibly complementary, which can be divided in two main categories: the endogenous and the exogenous sources. In this work we have been interested in the contribution of a specific endogenous source: the organic chemistry occurring in the ionosphere of the early Earth. At these high altitudes, the VUV contribution of the young sun was important, involving an efficient production of reactive species. Here we address the issue whether this chemistry can lead to the production of larger molecules with a prebiotic interest in spite of the competitive lysing effect of the harsh irradiation at these altitudes

Interaction aérosols-plasma dans l'ionosphère de Titan

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Cation Chemistry in Titan’s Upper Atmosphere and its Influence on Tholin Formation

International audienceTitan is Saturn’s largest satellite. This object is unique in the solar system as it hosts a dense atmosphere mainly made of molecular nitrogen N and methane CH4, with a surface pressure of 1.5 bar. The nitrogen rich atmosphere and the presence of liquid areas on the surface make it one of the most interesting nearby objects to understand the evolution of the primitive Earth before the emergence of life and to look for habitable environments in the solar system. The Cassini-Huygens Mission has been probing Titan since 2004. It has revealed an intense atmospheric photochemistry initiated by the photo-dissociation and ionization of N2 and CH4. Photochemistry on Titan leads to the formation of solid organic aerosols responsible for asmog permanently surrounding the moon. In the upper atmosphere, Cassini detected signatures compatible with the presence of heavily charged molecules which are precursors for the solid core of the aerosols. These observations indicate that ion chemistry has an important role for organic growth. However, the processes coupling ion chemistry and aerosol production are mostly still unknown. In this study, we investigate the cation chemistry, responsible for the organic growth that we observe in Titan’s upper atmosphere, simulated using the PAMPRE plasma reactor. Positive ions are investigated by in situ ion mass spectrometry in a dusty cold plasma, alongside neutral products additionally studied through infrared absorption spectroscopy and mass spectrometry