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

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: II. Modeling results: The relevance of plasma-surface interaction

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%. 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 NH x 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 Portuguese Foundation sponsored this research for Science and Technology (FCT) in the framework of the Strategic Funding UID/FIS/04650/2019

The impact of lake shape and size on lake breezes and air-lake exchanges on Titan

Titan, the largest moon of Saturn, has many lakes on its surface, formed mainly of liquid methane. Like water lakes on Earth, these methane lakes on Titan likely profoundly affect the local climate. Previous studies (Rafkin and Soto 2020, Chatain et al 2022) showed that Titan's lakes create lake breeze circulations with characteristic dimensions similar to the ones observed on Earth. However, such studies used a model in two dimensions; this work investigates the consequences of the addition of a third dimension to the model. Our results show that 2D simulations tend to overestimate the extension of the lake breeze over the land, and underestimate the strength of the subsidence over the lake, due to divergence/convergence geometrical effects in the mass conservation equations. In addition, 3D simulations including a large scale background wind show the formation of a pocket of accelerated wind behind the lake, which did not form in 2D simulations. An investigation of the effect of shoreline concavity on the resulting air circulation shows the formation of wind currents over peninsulas. Simulations with several lakes can either result in the formation of several individual lake breeze cells (during the day), or the emergence of a large merged cell with internal wind currents between lakes (during the night). Simulations of several real-shaped lakes located at a latitude of 74{\deg}N on Titan at the spring equinox show that larger lakes trigger stronger winds, and that some sections of lakes might accumulate enough methane vapor to form a thin fog. The addition of a third dimension, along with adjustments in the parametrizations of turbulence and subsurface land temperature, results in a reduction in the magnitude of the average lake evaporate rate, namely to ~6 cm/Earth year.Comment: Submitted to Icarus on 2023-07-21. Dataset available at the DOI: 10.5281/zenodo.817227

Humid Evolution of Haze in the Atmosphere of Super-Earths in the Habitable Zone

Photochemical hazes are expected to form and significantly contribute to the chemical and radiative balance of exoplanets with relatively moderate temperatures, possibly in the habitable zone of their host star. In the presence of humidity, haze particles might thus serve as cloud condensation nuclei and trigger the formation of water droplets. In the present work, we are interested in the chemical impact of such a close interaction between photochemical hazes and humidity on the organic content composing the hazes and on the capacity to generate organic molecules with high prebiotic potential. For this purpose, we explore experimentally the sweet spot by combining N-dominated super-Earth exoplanets in agreement with Titan's rich organic photochemistry and humid conditions expected for exoplanets in habitable zones. A logarithmic increase with time is observed for the relative abundance of oxygenated species, with O-containing molecules dominating after 1 month only. The rapidity of the process suggests that the humid evolution of N-rich organic haze provides an efficient source of molecules with high prebiotic potential

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

Interaction aérosols - plasma dans l'ionosphère de Titan

The climatic system of Saturn’s moon Titan is governed by the intense production of organic aerosols in its upper atmosphere. This phenomenon also certainly happened on Earth at the beginning of life. These two points strongly motivate research on the formation and evolution processes of the aerosols in the atmosphere of Titan. The aerosols form and stay several weeks in the ionosphere, between ~900-1200 km of altitude. This atmospheric layer is ionized by UV solar rays and energetic particles coming from Saturn’s magnetosphere, forming a plasma with very reactive species: radicals, excited species, ions and electrons. In such an environment, the main question I tackle is how the organic aerosols interact with the plasma species.The phenomenon is simulated in the laboratory with a plasma setup developed on purpose: analogues of Titan aerosols are exposed to a N2-H2 plasma discharge. Both an evolution of the solid and the gas phase are observed. H and N atoms chemically interact with the aerosols. Then, hydrogen cyanide (HCN) and other organic molecules are ejected in the gas phase by ion sputtering. These results highlight an important contribution of heterogeneous processes in Titan’s upper atmosphere.My re-analysis of the Cassini Langmuir probe data revealed the presence of an unexpected electron population in the ionosphere, below 1200 km and on the day-side, where heavy ions are also detected. These electrons could be emitted by the aerosols, after collision with a photon, and/or heating by the active ion chemistry.Le système climatique de la lune de Saturne Titan est gouverné par la production intense d’aérosols organiques dans sa haute atmosphère. Ce phénomène s’est aussi certainement produit sur Terre au moment de l’apparition de la vie. Ces deux points motivent fortement les recherches sur les processus de formation et d’évolution des aérosols dans l’atmosphère de Titan. Les aérosols se forment et restent plusieurs semaines dans l’ionosphère, étendue d’environ 900 à 1200 km d’altitude. Cette région de l’atmosphère est ionisée par le rayonnement solaire UV et des particules énergétiques provenant de la magnétosphère de Saturne. Des espèces plasma très réactives sont ainsi présentes : des radicaux, des espèces excitées, des ions et des électrons. Dans un tel environnement, je me suis intéressée à l’interaction entre les aérosols organiques et le plasma.Ce phénomène est simulé en laboratoire avec une expérience développée à cet effet : des analogues des aérosols de Titan sont exposés à une décharge plasma en N2-H2. J’observe qu’à la fois les grains et la phase gaz évoluent. Les atomes H et N interagissent chimiquement avec les aérosols. Puis, du cyanure d’hydrogène (HCN) ainsi que d’autres molécules organiques sont éjectées en phase gaz par le bombardement ionique. Ces résultats mettent en évidence une contribution importante des processus hétérogènes dans l’ionosphère de Titan.Ma ré-analyse des données de la sonde de Langmuir de la mission Cassini a d’autre part révélé la présence d’une population d’électrons inattendue dans l’ionosphère, sous 1200 km d’altitude et côté jour, zone dans laquelle des ions lourds ont également été détectés. Ces électrons pourraient être émis par les aérosols, après collision avec un photon et/ou après chauffage par la chimie ionique très active dans cette région

Interaction aérosols - plasma dans l'ionosphère de Titan

The climatic system of Saturn’s moon Titan is governed by the intense production of organic aerosols in its upper atmosphere. This phenomenon also certainly happened on Earth at the beginning of life. These two points strongly motivate research on the formation and evolution processes of the aerosols in the atmosphere of Titan. The aerosols form and stay several weeks in the ionosphere, between ~900-1200 km of altitude. This atmospheric layer is ionized by UV solar rays and energetic particles coming from Saturn’s magnetosphere, forming a plasma with very reactive species: radicals, excited species, ions and electrons. In such an environment, the main question I tackle is how the organic aerosols interact with the plasma species.The phenomenon is simulated in the laboratory with a plasma setup developed on purpose: analogues of Titan aerosols are exposed to a N2-H2 plasma discharge. Both an evolution of the solid and the gas phase are observed. H and N atoms chemically interact with the aerosols. Then, hydrogen cyanide (HCN) and other organic molecules are ejected in the gas phase by ion sputtering. These results highlight an important contribution of heterogeneous processes in Titan’s upper atmosphere.My re-analysis of the Cassini Langmuir probe data revealed the presence of an unexpected electron population in the ionosphere, below 1200 km and on the day-side, where heavy ions are also detected. These electrons could be emitted by the aerosols, after collision with a photon, and/or heating by the active ion chemistry.Le système climatique de la lune de Saturne Titan est gouverné par la production intense d’aérosols organiques dans sa haute atmosphère. Ce phénomène s’est aussi certainement produit sur Terre au moment de l’apparition de la vie. Ces deux points motivent fortement les recherches sur les processus de formation et d’évolution des aérosols dans l’atmosphère de Titan. Les aérosols se forment et restent plusieurs semaines dans l’ionosphère, étendue d’environ 900 à 1200 km d’altitude. Cette région de l’atmosphère est ionisée par le rayonnement solaire UV et des particules énergétiques provenant de la magnétosphère de Saturne. Des espèces plasma très réactives sont ainsi présentes : des radicaux, des espèces excitées, des ions et des électrons. Dans un tel environnement, je me suis intéressée à l’interaction entre les aérosols organiques et le plasma.Ce phénomène est simulé en laboratoire avec une expérience développée à cet effet : des analogues des aérosols de Titan sont exposés à une décharge plasma en N2-H2. J’observe qu’à la fois les grains et la phase gaz évoluent. Les atomes H et N interagissent chimiquement avec les aérosols. Puis, du cyanure d’hydrogène (HCN) ainsi que d’autres molécules organiques sont éjectées en phase gaz par le bombardement ionique. Ces résultats mettent en évidence une contribution importante des processus hétérogènes dans l’ionosphère de Titan.Ma ré-analyse des données de la sonde de Langmuir de la mission Cassini a d’autre part révélé la présence d’une population d’électrons inattendue dans l’ionosphère, sous 1200 km d’altitude et côté jour, zone dans laquelle des ions lourds ont également été détectés. Ces électrons pourraient être émis par les aérosols, après collision avec un photon et/ou après chauffage par la chimie ionique très active dans cette région

Experimental simulation of aerosols evolution in Titan’s ionosphere

International audienceMany recent studies on Titan are concerned with aerosols. In particular, questions are asked on how these complex organic molecules are formed and evolve in Titan’s atmosphere. Here for the first time we experimentally study how harsh plasma environment simulating Titan ionosphere can affect these aerosols. Titan tholins are placed in a N2-H2 plasma reactor and sample signatures are measured by infrared transmission spectroscopy. First results show an evolution of the absorption bands. Therefore, plasma conditions seem to change tholin chemical structure