David Dubois

Nathalie Carrasco

Marie Petrucciani

Ludovic Vettier

Sarah Tigrine

Pascal Pernot

In situ Investigation of Neutrals Involved in the Formation of Titan Tholins

Keywords: Titan, Saturn, Atmospheres, chemistry, Plasma discharge, Experimental techniques

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 N 2 -CH 4 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 NH 3 , HCN, C 2 H 2 and C 2 H 4 . 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% N 2 -CH 4 , covering the range of methane concentrations encountered in Titan's ionosphere. We also detect in situ heavy molecules (C 7 ). In particular, we show the role of ethylene and other volatiles as key solid-phase precursors.

I) Introduction

Titan is Saturn's largest moon and second largest in the Solar System after Ganymede.

It has a mean radius of 2,575 km. One major interest in Titan is its uniquely dense (a 1.5-bar ground pressure) and extensive (~1,500 km) atmosphere, primarily made of N 2 and CH 4 . In fact, the extent of the atmosphere alone accounts for about 58% in size of Titan's radius. Titan's first flyby by Pioneer 11 (1979) revealed the moon as being a faint brownish dot orbiting Saturn. The subsequent Voyager 1 (1980), Voyager 2 (1981), Cassini (2004Cassini ( -2017) ) and Huygens (2005) spacecraft have uncovered physicochemical processes unique in the solar system occurring both in the atmosphere and surface. Early seminal studies [START_REF] Khare | Red clouds in reducing atmospheres[END_REF][START_REF] Sagan | The greenhouse of Titan[END_REF][START_REF] Hanel | Infrared Observations of the Saturnian System from Voyager 1[END_REF] showed that Titan's reducing atmosphere is also composed of complex and heavy organic molecules and aerosols, their laboratory analogous counterparts coined tholins. This chemistry is initiated at high altitudes and participates in the formation of solid organic particles [START_REF] Khare | Organic solids produced by electrical discharge in reducing atmospheres: Tholin molecular analysis[END_REF][START_REF] Waite | The Process of Tholin Formation in Titan's Upper Atmosphere[END_REF][START_REF] Hörst | Titan's Atmosphere and Climate[END_REF] for a more recent and holistic review of Titan's atmospheric chemistry). At altitudes higher than 800 km, the atmosphere is under the influence of energy deposition such as solar harsh ultraviolet (VUV) radiation, solar X-rays, Saturn's magnetospheric energetic electrons and solar wind [START_REF] Krasnopolsky | A photochemical model of Titan's atmosphere and ionosphere[END_REF][START_REF] Sittler | Heavy ion formation in Titan's ionosphere: Magnetospheric introduction of free oxygen and a source of Titan's aerosols?[END_REF]. Cosmic dust input is also thought to chemically interact with volatiles in the middle atmosphere [START_REF] English | Ablation and chemistry of meteoric materials in the atmosphere of Titan[END_REF][START_REF] Frankland | Uptake of acetylene on cosmic dust and production of benzene in Titan's atmosphere[END_REF]. To wit, Titan is presumably one of the most chemically complex bodies in the solar system. Some of the techniques used to better understand the processes occurring in Titan's atmosphere are detailed hereafter.

Earth-based observations of Titan have been one way to determine the atmospheric composition and structure, as well as surface and subsurface features. For example, direct monitoring of seasonal tropospheric clouds from Earth-based observations helped us infer their transient locations [START_REF] Brown | Direct detection of variable tropospheric clouds near Titan's south pole[END_REF]. Ground-based submillimeter observations were used to infer the stratospheric composition with for example, the determination of hydrogen cyanide (HCN) and acetonitrile (CH 3 CN) mixing ratios [START_REF] Tanguy | Stratospheric Profile of HCN on Titan from Millimeter Observations 57[END_REF][START_REF] Bezard | Detection of acetonitrile on Titan[END_REF][START_REF] Hidayat | Millimeter and Submillimeter Heterodyne Observations of Titan : Retrieval of the Vertical Profile of HCN and the 12 C / 13[END_REF], and with the rise of ground-based observation techniques such as the Atacama Large Millimeter Array (ALMA) (Cordiner et al., 2014a(Cordiner et al., , 2014b;;[START_REF] Molter | ALMA Observations of HCN and its Isotopologues on Titan 1-14[END_REF][START_REF] Serigano | Isotopic Ratios of Carbon and Oxygen in Titan's CO using ALMA[END_REF][START_REF] Palmer | ALMA detection and astrobiological potential of vinyl cyanide on Titan 1-7[END_REF]. The use of other space-based telescopes such as the Infrared Space Observatory (ISO) and the Herschel Space Observatory has additionally provided information on the stratospheric composition and neutral abundances [START_REF] Coustenis | Titan's atmosphere from ISO mid-infrared spectroscopy[END_REF][START_REF] Courtin | First results of Herschel -SPIRE observations of Titan[END_REF][START_REF] Rengel | Herschel/PACS spectroscopy of trace gases of the stratosphere of Titan[END_REF]. Early on, atmospheric studies brought by the Voyager 1 mission were able to identify several organic molecules [START_REF] Hanel | Infrared Observations of the Saturnian System from Voyager 1[END_REF]Kunde et al., 1981;[START_REF] Maguire | C3H8 and C3H4 in Titan's atmosphere[END_REF]. Later, the Cassini-Huygens (NASA/ESA) mission provided us with a more precise atmospheric characterization of the composition [START_REF] Teanby | Active upper-atmosphere chemistry and dynamics from polar circulation reversal on Titan[END_REF][START_REF] Vinatier | Seasonal variations in Titan's middle atmosphere during the northern spring derived from Cassini/CIRS observations[END_REF][START_REF] Coustenis | Titan's temporal evolution in stratospheric trace gases near the poles[END_REF], wind [START_REF] Flasar | Titan's Atmospheric Temperatures, Winds, and Composition[END_REF][START_REF] Achterberg | Temporal variations of Titan's middle-atmospheric temperatures from 2004 to 2009 observed by Cassini/CIRS[END_REF] and upper atmosphere thermal profiles (e.g. [START_REF] Snowden | The thermal structure of Titan's upper atmosphere, I: Temperature profiles from Cassini INMS observations[END_REF], while the Huygens lander (ESA) was able to provide insight on the in situ composition [START_REF] Israel | Complex organic matter in Titan's atmospheric aerosols from in situ pyrolysis and analysis[END_REF] and optical properties of the aerosols [START_REF] Doose | Vertical structure and optical properties of Titan's aerosols from radiance measurements made inside and outside the atmosphere[END_REF]). Cassini's Composite Infrared Spectrometer (CIRS) limb spectral imaging [START_REF] Vinatier | Analysis of Cassini/CIRS limb spectra of Titan acquired during the nominal mission I: Hydrocarbons, nitriles and CO2 vertical mixing ratio profiles[END_REF] examined the role of volatiles in Titan's chemistry and atmospheric dynamics. Many hydrocarbons and nitriles were shown to have wide latitudinal variations of vertical mixing ratios (sometimes contradicting Global Circulation Models, e.g. de [START_REF] De Kok | HCN ice in Titan's high-altitude southern polar cloud[END_REF], emphasizing the importance of understanding the complex coupling between gas phase chemistry and global dynamics. Both polar enrichments of nitriles (e.g. [START_REF] Teanby | Dynamical implications of seasonal and spatial variations in Titan's stratospheric composition[END_REF] as well as the presence of heavy ions [START_REF] Crary | Heavy ions, temperatures and winds in Titan's ionosphere: Combined Cassini CAPS and INMS observations[END_REF][START_REF] Wellbrock | Cassini CAPS-ELS observations of negative ions in Titan's ionosphere: Trends of density with altitude[END_REF][START_REF] Desai | Carbon chain anions and the growth of complex organic molecules in Titan's ionosphere[END_REF] have also been suggested to participate in Titan's aerosol production. Thus, ground-and space-based observations have and will be especially important in the post-Cassini era [START_REF] Nixon | Titan Science with the James Webb Space Telescope[END_REF].

Photochemical models have been complementarily used to predict the thermal, compositional and structural evolution of Titan's atmospheric column (e.g. [START_REF] Atreya | Evolution of a nitrogen atmosphere on Titan[END_REF][START_REF] Carrasco | Influence of neutral transport on ion chemistry uncertainties in Titan ionosphere[END_REF][START_REF] Carrasco | Sensitivity of a Titan ionospheric model to the ion-molecule reaction parameters[END_REF]. Furthermore, these models offer us testable hypotheses for experiments and predictions for chemical pathways leading to the formation of aerosols (e.g. [START_REF] Waite | The Process of Tholin Formation in Titan's Upper Atmosphere[END_REF][START_REF] Yelle | Formation of NH3 and CH2NH in Titan's upper atmosphere[END_REF].

Lastly, experimental techniques have also been used to simulate Titan's atmospheric conditions [START_REF] Khare | Optical constants of organic tholins produced in a simulated Titanian atmosphere: From soft x-ray to microwave frequencies[END_REF][START_REF] Sagan | Production and condensation of organic gases in the atmosphere of Titan[END_REF][START_REF] Pintassilgo | Methane decomposition and active nitrogen in a N2-CH4 glow discharge at low pressures[END_REF][START_REF] Cable | Titan tholins: Simulating Titan organic chemistry in the Cassini-Huygens era[END_REF][START_REF] Carrasco | VUV photochemistry simulation of planetary upper atmosphere using synchrotron radiation[END_REF][START_REF] Sciamma-O'brien | The THS experiment: Simulating Titan's atmospheric chemistry at low temperature (200 K)[END_REF][START_REF] Tigrine | A microwave plasma source for VUV atmospheric photochemistry[END_REF][START_REF] Hörst | Laboratory investigations of Titan haze formation: In situ measurement of gas and particle composition[END_REF] and determine chemical mechanisms that are otherwise out of Cassini's reach. A wide range of laboratory simulations have been used to complementarily investigate Titan's atmospheric chemistry and different energy inputs (see [START_REF] Cable | Titan tholins: Simulating Titan organic chemistry in the Cassini-Huygens era[END_REF] for a thorough review of these experiments). Typically in our lab, plasma discharges [START_REF] Szopa | PAMPRE: A dusty plasma experiment for Titan's tholins production and study[END_REF][START_REF] Sciamma-O'brien | Titan's atmosphere: An optimal gas mixture for aerosol production?[END_REF][START_REF] Carrasco | Volatile products controlling Titan's tholins production[END_REF] or UV irradiation [START_REF] Peng | Titan's atmosphere simulation experiment using continuum UV-VUV synchrotron radiation[END_REF] have been used to achieve Titan-like ionospheric conditions. In this context, it is deemed necessary to apply and exploit such a technique to better understand the chemical reactivity occurring in Titan-like upper atmospheric conditions. Hence, the gas phase which we know is key to the formation of aerosols on Titan [START_REF] Waite | The Process of Tholin Formation in Titan's Upper Atmosphere[END_REF] is emphasized. Indeed, chemical precursors that are present in the gas phase [START_REF] Carrasco | Volatile products controlling Titan's tholins production[END_REF] follow certain chemical pathways that still need to be investigated. For instance, Sciamma-O'Brien et al., 2010 showed how the production of atomic hydrogen coming from CH 4 was anti-correlated with aerosol formation, by changing the initial methane amount in the discharge. They argued that the presence of molecular and atomic hydrogen in too large amounts in the gas mixture may have an inhibiting effect on the polymerization reaction that produce the tholins.

To simulate the methane mixing ratios and energy conditions present in Titan's ionosphere, we used the PAMPRE cold dusty plasma experiment [START_REF] Szopa | PAMPRE: A dusty plasma experiment for Titan's tholins production and study[END_REF] with an N 2 -CH 4 gas mixture under controlled gas influx and pressure. Cold plasma discharges have been shown to faithfully simulate the electron energy distribution of magnetospheric electrons and protons, and UV irradiation deposited at Titan's atmosphere within a range of ~ 10 eV -5 keV [START_REF] Sittler | Survey of low-energy plasma electrons in Saturn's magnetosphere: Voyagers 1 and 2[END_REF][START_REF] Chang | Corona Discharge Processes[END_REF][START_REF] Maurice | Comprehensive analysis of electron observations at Saturn: Voyager 1 and 2[END_REF][START_REF] Brown | Titan from Cassini-Huygens[END_REF][START_REF] Johnson | Evolution of an early Titan atmosphere[END_REF], making them good laboratory analogs to simulate the ionospheric chemistry.

A first study [START_REF] Gautier | Nitrile gas chemistry in Titan's atmosphere[END_REF] dealt with the identification of gas products in the PAMPRE reactor and used an external cold trap to study the gas phase. Analyses were made by gas-chromatography coupled to mass spectrometry. More than 30 reaction products were detected and a semi-quantitative study was performed on nitriles, the most abundant molecules collected. However, one of the limitations to this technique is how the products are transported ex situ, and may allow for loss or bias in their chemical reactivity and measurements. In situ detections were then made by mass spectrometry with no cryogenic trapping [START_REF] Carrasco | Volatile products controlling Titan's tholins production[END_REF]. Unfortunately, the only use of mass spectrometry prevented any univocal identification of large organic molecules likely involved as precursors for aerosol formation. Since then, setup improvements such as an internal cryogenic trap and infrared spectroscopy have been made in order to study the gas phase in situ using these different complementary techniques. The purpose of the cryotrap is to act as a cold finger enabling the volatiles to condensate and preventing them from being evacuated from the chamber. This has allowed us to avoid any volatile loss and to take on a quantitative approach on volatile products. Thus, to better characterize the initial volatile products precursors to aerosols, we used a coupled analysis of mass and infrared spectrometry. The presence of an internal cold trap made the condensation of these volatiles possible, allowing for in situ measurements.

We will first describe our experimental chamber, which was designed to be relevant to simulate Titan's ionosphere conditions. This was achieved by choosing methane concentrations pertaining to those found in the upper atmosphere. Mass spectrometry results of volatiles at two methane concentrations will be then presented, as well as their infrared spectroscopic analyses with a quantitative approach taken on four major neutral volatiles (NH 3 , C 2 H 2 , C 2 H 4 and HCN) acting as precursors to the formation of aerosols. Finally, we will discuss the discrepancies between both methane conditions, as well as ethylene, ammonia and hydrogen cyanide chemistry.

II) Experimental Setup

The PAMPRE experiment and in situ cryogenic analysis

The PAMPRE cold plasma reactor used in this study aims, by design, at reproducing exogenic energy sources, for example Saturn's magnetosphere particles, UV radiation, cosmic rays and coronal processes impacting Titan's atmosphere [START_REF] Szopa | PAMPRE: A dusty plasma experiment for Titan's tholins production and study[END_REF][START_REF] Alcouffe | Capacitively coupled plasma used to simulate Titan's atmospheric chemistry[END_REF]. The reactor delivers a capacitively coupled plasma discharge, produced by a 13.56 MHz radiofrequency generator through a tuning match box. The gas influx is held constant and monitored with mass flow controllers. The latter can tune an N 2 -CH 4 mixing ratio from 0% to 10% of CH 4 from highly pure N 2 (>99.999%) and 90-10% N 2 -CH 4 (>99.999%) bottles. An in situ mass spectrometer (MS) and Fourier-Transform Infrared (FT-IR) spectrometer are also integrated to the system.

To simulate Titan's atmospheric conditions, 90-10% and 99-1% N 2 -CH 4 ratios were considered.

Both correspond to a 55 sccm flow in standard conditions. Pressure gauges are set at 0.9 mbar of constant pressure in the reactor while the plasma power is constant at 30W.

In order to detect, analyze and preserve all of the products during our experiments, we need a trapping mechanism to capture these compounds, and thus prevent them from being evacuated. [START_REF] Gautier | Nitrile gas chemistry in Titan's atmosphere[END_REF] used an external cryogenic trap system, shaped as a cylindrical coil, immersed in liquid nitrogen. Then, they isolated the coiled trap and analyzed the gas phase ex situ. Here, the setup is different, and we believe more efficient as the trapping and subsequent analyses are performed in situ and avoid any transfer line bias. Moreover, this cryo-trap mainly traps the gas-phase chemistry in the preceding stages to the formation of solid organic aerosols, preventing their production. In the absence of such a cryo-trap, the plasma is dusty and several mg/h of solid particles are produced. As a result, this enables us to effectively study in detail these gas-phase precursors of tholins, trapped and accumulated during a 2h plasma discharge. Fig. 1 shows the 3D schematics of our cryogenic trap. An external liquid nitrogen tank extends as a copper circuit into the chamber for cooling. This cooling circuit goes through the upper lid of the chamber, and spirals down onto and lays flat on the polarized electrode. This cooling system is semi-closed, in that the remaining liquid nitrogen is stored in another container outside the reactor. The electrode temperature is controlled with a temperature regulator (Cryo-Diffusion Pt100 probe) which controls the liquid nitrogen circulation in the cooling circuit with a solenoid valve. The initial temperature is adjusted to -180 ±2ºC prior to the experiment.

Chamber lid Liquid N 2

Cold copper coil Polarized electrode the chamber, which is then in contact with the polarized electrode. The electrode is thus cooled to low-controlled temperatures and the liquid nitrogen goes out through an exit system. This cryogenic trap enables the in situ condensation of the produced volatiles within the plasma.

Using the cryogenic trap set at T = -180 ±2 ºC, the protocol used was the following: The plasma discharge and cooling system are turned on and the electrodes are kept for 2h at T = -180ºC.

The duration of the plasma discharge was chosen to trap and accumulate the gas phase products, and improve their detection. The discharge reaches a steady state after a few minutes, as shown in [START_REF] Alcouffe | Capacitively coupled plasma used to simulate Titan's atmospheric chemistry[END_REF]. A similar 2h trapping was previously performed in [START_REF] Gautier | Nitrile gas chemistry in Titan's atmosphere[END_REF] with an external cold trap, which was then analyzed by GC-MS. For consistency, we have chosen the same discharge duration. During this time, the products formed in the N 2 -CH 4 plasma are accumulated and adsorbed onto the chamber walls still at lowcontrolled temperatures thanks to the cryogenic system. After 2h, the plasma is switched off, the chamber is pumped down from 0.9 mbar to 10 -4 mbar, and the cold trap is stopped. The volatile products are gradually released under vacuum. We then proceed to do in situ gas phase measurements as the chamber goes back to room temperature (Fig. 3). During this temperature increase, it is possible that some additional neutral-neutral chemistry could occur. Our analytical capacity enables us to address the most abundant and therefore stable molecules, which are not expected to vary strongly apart from the sublimation process.

Mass Spectrometry

QMS quadrupole mass spectrometers were used (Pfeiffer and Hiden Analytics) for in situ measurements inside the chamber. Once the neutral gas is ionized within the MS chamber (SEM voltage of 1000V), each mass is collected by the detector. The MS samples near the reactive plasma with a 0.8 mm-diameter capillary tube, long enough to keep a high-pressure gradient between the MS and plasma chambers. Thus, an effective 10 -5 -10 -6 mbar resident pressure in the MS chamber is maintained throughout the experiment. This vacuum background was thoroughly checked before each simulation. The experiments were done using a 1u resolution, covering a 1-100 mass range, at a speed of 2s/mass with a channeltron-type detection. Finally, a baseline noise filter was applied to each spectrum.

Infrared Spectroscopy

A ThermoFisher Nicolet 6700 Fourier Transform Infrared (FT-IR) spectrometer was used for in situ analysis of the gas phase in transmission mode. The spectrometer uses a Mercury Cadmium Telluride (MCT/A) high sensitivity detector with a KBr beamsplitter. During each measurement, 500 scans were taken at a resolution of 1 cm -1 for 650-4000 cm -1 corresponding to a short-wavelength mid-IR (15.4-2.5 µm) range. The optical path length is 30 cm.

III) Results

We will now present results from two sets of experiments done at two initial methane concentrations, 1% and 10% (three and two replicates, respectively), representative of the methane concentration range found in Titan's ionosphere [START_REF] Waite | Ion Neutral Mass Spectrometer Results from the First Flyby of Titan[END_REF]. The effect of the cold trap on the plasma can be appreciated by comparing the mass spectra in the same conditions with and without the cold trap (Fig. 2). Under usual conditions, without cold-trap, several mg/h of solid particles are produced in the plasma volume in steady state conditions after the very first minutes of the discharge (Sciamma-O' Brien et al., 2010). Now, with the cold-trap, solid particles are no longer produced even after a 2h plasma duration: the aerosol formation is prevented. So, we must investigate how far the gas-phase chemistry goes in the presence of the cold trap. The plasma is out of thermodynamical equilibrium and hosts fast ion-neutral reactions. If the entrapment is faster than the chemistry, then the absence of aerosols might be explained by the fact that any product larger than CH 4 would be removed from the reaction space. But, if the chemical kinetics of the gas phase chemistry is competitive with the entrapment, then complex molecules are produced before their entrapment on the cold reactor walls. In this case, the absence of aerosols is caused by the low amount of gas-phase precursors in the discharge, thus preventing the gas-to-solid conversion. The analysis of the mass spectra obtained in situ during the plasma discharge with and without cold-trap (Fig. 2) informs us on this effect of the cold-trap on the gasphase chemistry.

The methane consumption can be estimated with m/z 15 and m/z 16 on Fig. 2 corresponding to the CH 3 + and CH 4 + fragments. Following the method described in Sciamma-O'Brien et al., 2010, we quantify the methane consumption with the CH 3 + (m/z 15) fragment at a pressure of 0.9 mbar. With the current cold trap setup, we found this consumption to be of 52% and 60% at 10% and 1% [CH 4 ] 0 , respectively. Without the current cryotrap system, Sciamma-O'Brien et al., 2010 found methane consumption efficiencies of 45% and 82% at 10% and 1% [CH 4 ] 0 . The behavior of methane consumption obtained with and without cold trap are comparable, even if an effect of the different electrode temperature cannot be discarded.

In the following sections, we define C n families as molecular classes using a C x H y N z nomenclature for N-bearing and hydrocarbon species, where x + z = n. As such, C n blocks detected using mass spectrometry can be traced, where n is related to the presence of heavy aliphatic and N-bearing compounds.

In the C 1 block, in addition to the methane ion peaks, we can notice the signature of the residual water of the mass spectrometer at m/z 17, 18. Other compounds due to the residual air in the mass spectrometer can be seen at m/z 32 (O 2 ) and m/z 44 (CO 2 ).

The role of the cryotrap has a substantive effect on trapping organic volatile products. Heavy compounds such as the C 4 block (Fig. 2a) vanish with the cold trap (Fig. 2b), despite similar methane consumptions.

This shows the role of the cryo-trap, which is to efficiently condense and accumulate in situ the organic products on the cold walls of the reactor instead of being evacuated out as volatiles during the discharge.

Methane is consumed in similar quantities in the plasma discharge with and without the cryotrap. Some large and complex molecules are still detected during the discharge by mass spectrometry (Fig. 2b), up to the upper mass limits of our instrument (m/z 100) with the cold trap, showing that the gas phase chemistry ensuring their formation is still occurring. Their concentration in the gas phase is drastically reduced, by two orders of magnitude (Fig. 2b), suggesting that they are efficiently trapped on the plasma walls while they are forming. The cold trap does not prevent the formation of complex gas-phase molecules, but mainly blocks the gasto-solid conversion step. The cryogenic trap stops the gas-phase chemistry in the stage preceding the formation of solid organic aerosols. It condenses the gas-phase precursors while they are forming, and no aerosols are observed during the 2h plasma discharge.

Release of volatiles back to room temperature

We leave the cold trap continuously on during a 2h plasma discharge. After 2h, the plasma and cryo-trap are stopped, the chamber is pumped down to 10 -4 mbar, isolated, and left to go back to room temperature. Hence, all the species condensed on the cold walls are progressively desorbed according to their vaporization temperatures and analyzed in situ. We define MS1, This desorption is recorded with a pressure gauge within the isolated chamber as the temperature increases. Figure 3 (top) shows the pressure of the gas products being released and the temperature of the electrode (bottom) according to time, both at [CH 4 ] 0 = 10% and [CH 4 ] 0 = 1%, in blue and red, respectively. Both pressures increase, owing to the volatiles sublimating though in different behaviors. Most of the volatiles have been released at ~6h of release (Figure 3, top) and the curve starts to reach an asymptotic limit in the case of 10% methane, (i) the release starts immediately with the temperature increase until 0.74 mbar at -79ºC where it reaches a small plateau, followed by (ii) a second volatile release starting at -75ºC with a sharp decrease in heating rate at 300 min until it (iii) reaches a second plateau at 1.84 mbar. On the other hand, products formed in [CH 4 ] 0 = 1% conditions have a different release pattern. Notwithstanding the discrepancy in final pressure obtained (0.35 mbar, 5x less), the products take a much longer time to be desorbed, the release starting at about 90 min with a much smoother slope. Unlike at 10% methane, there is no plateau; the release of the volatiles is continuous and in a rather limited pressure range.

These qualitative differences suggest, as a first approximation, different products in the two methane conditions. In particular, all the products released at temperatures below -100ºC at 10% methane are absent in the case of 1% methane. This means that these specific compounds are not produced in significant amounts in the plasma discharge at 1% CH 4 .

Products released at temperatures below -100°C in the case of [CH

4 ] 0 = 10%,
We are first interested in the molecules released below -100°C in the case of [CH 4 ] 0 = 10%, which are not produced (or at least not in detectable quantities) in [CH 4 ] 0 = 1%.

As a first qualitative guess, we use the Antoine Law [START_REF] Mahjoub | Effect of the synthesis temperature on the optical indices of organic materials produced by N2-CH4 RF plasma[END_REF] Eq. ( 1) to calculate the vapor pressure of C 2 H 6, which, at -180ºC, is slightly above the triple point of -183.15ºC [START_REF] Fray | Sublimation of ices of astrophysical interest: A bibliographic review[END_REF]. C 2 H 6 is indeed one of four volatiles expected to be present our final gas mixture [START_REF] Gautier | Nitrile gas chemistry in Titan's atmosphere[END_REF], along with C 2 H 2 , C 2 H 4 , and HCN. We use the A, B, C parameters from the NIST database [START_REF] Carruth | Vapor Pressure of Normal Paraffins Ethane Through n-Decane from Their Triple Points to About 10Mm Hg[END_REF]. The low temperature used in the present study is at the edge of the validity range for C 2 H 6 . Nonetheless, we extend this law's validity domain and assume it to be applicable to these low-temperature conditions and thus must consider this calculation as a first approximation only. The vapor pressure is calculated at -180°C.

log 10 (𝑃 𝑠 ) = 𝐴 -( 𝐵 𝑇 + 𝐶 ) (1)
For C 2 H 6 , we find P s = 5.7 x 10 2 mbar. This vapor pressure combined with the fact that we are slightly above the triple point of C 2 H 6 near the liquid/vapor equilibrium, shows that this species will hardly condense.

HCN, C 2 H 2 and C 2 H 4 all have their triple points [START_REF] Fray | Sublimation of ices of astrophysical interest: A bibliographic review[END_REF] (2)

ln (𝑃 𝑠𝑢𝑏 ) = 𝐴 0 + ∑ 𝑛 𝑖 = 1 𝐴 𝑖 𝑇 𝑖 T p (ºC) A 0 A 1 A 2 A 3 A 4 P Sub (-180ºC)
in mbar HCN -14.15 1.39 x 10 1 -3.62 x 10 3 -1.33 x 10 5 6.31 x 10 6 -1.13 x 10 8 1.69 x 10 -12

C 2 H 2 -81.15 1.34 x 10 1 -2.54 x 10 3 0 0 0 9.48 x 10 -4

C 2 H 4 -169.15 1.54 x 10 1 -2.21 x 10 3 -1.22 x 10 4 2.84 x 10 5 -2.20 x 10 6 1.3 x 10 3 The larger values (Table 1) for the two C 2 hydrocarbons in comparison with HCN agree with a first substantial release of the light C 2 hydrocarbons at temperatures below -100°C. HCN contributes to the gas composition mainly in the second stage, for temperatures larger than -80°C, because of its low vapor pressure at colder temperatures.

The absence of C 2 hydrocarbons in the case of [CH 4 ] 0 = 1% is in agreement with the exsitu analysis made in [START_REF] Gautier | Nitrile gas chemistry in Titan's atmosphere[END_REF].

Detections by mass spectrometry

To go further in the analysis of the gas release, we monitored the gas phase composition by mass spectrometry at three threshold moments MS1, MS2, MS3 for 10% methane and only in the final gas mixture for 1%.

In the case of 10% methane (Fig. 3 dashed blue line), the assigned MS1, MS2 and MS3 spectra are defined as the T, P conditions of -130ºC and 0.38 mbar, -79ºC and 0.74 mbar and +22ºC and 1.84 mbar of products formed, respectively. MS1 is representative of the first release stage occurring at temperatures lower than -100°C, MS2 is on the pressure plateau beginning at -100°C, and MS3 corresponds to the final gas mixture obtained. For 1% methane conditions, we will only focus on the last point, where a substantial volatile pressure was measured.

A first iteration at [CH 4 ] 0 = 10% was done. Mass spectra of the volatiles are shown Fig. 4. at different stages in their release after the plasma discharge, namely MS1, MS2 and MS3.

-MS1: 7min after stopping the cryogenic trap (blue line), C 1 , C 2 and C 3 block species have started to appear, with C 1 and C 2 (light volatiles of m/z < 30 amu) dominating the spectrum. Masses 15 and 28 stand out for the C 1 and C 2 blocks, respectively.

Mass 28 cannot be attributed to N 2 as it was not trapped at -180°C during the experiment. It can be attributed to ethylene C 2 H 4 or ethane C 2 H 6 as both molecules are expected to be released at temperatures lower than -100°C and both have their major fragment at m/z 28 according to the NIST database. However, the previous ethane vapor pressure calculations and its triple point [START_REF] Fray | Sublimation of ices of astrophysical interest: A bibliographic review[END_REF] show that it could not condense. In these plasma conditions, it is risky to attribute m/z 29 to methanimine (CH 2 =NH) as the contribution of propane (C 3 H 8 ) becomes non negligible [START_REF] Carrasco | Volatile products controlling Titan's tholins production[END_REF]. m/z 29 corresponds indeed to the main peak in the fragmentation pattern of C 3 H 8 . A few heavier volatiles in the C 3 and C 4 block are also beginning to be detected. At m/z 44 we could also suspect the contribution of CO 2 . A blank mass spectrum of the mass spectrometer only has previously been measured [START_REF] Carrasco | Volatile products controlling Titan's tholins production[END_REF], showing the same intensity of 2 x 10 -12 A at m/z 44 as the one seen in Fig. 4 (blue plot), at low temperature. It corresponds to the weak air signature within the mass spectrometer at a vacuum limit of about 3 x 10 -8 mbar.

Consequently, the contribution of CO 2 observed in the mass spectra of If we now compare the final stages obtained in the case of [CH 4 ] 0 = 1% and 10% on Fig. 3, the total pressure of volatiles obtained (0.35 mbar, blue line) is much less than in the case of [CH 4 ] 0 = 10% (red). Fig. 5 shows a comparison of the final states at [CH 4 ] 0 = 1% (brown) and [CH 4 ] 0 = 10% (blue), with the initial state (black), taken just before the volatile release at low-controlled temperature T = -180ºC. As expected from the lower final pressure, the mass spectrum is much less intense in the case of [CH 4 ] 0 = 1% than 10%. No signature is detected beyond the C 5 block for 1% whereas large intensities are found until the C 7 block in the case of 10%. However, the major peaks are the same in both cases. The C 1 block is dominated by m/z 17, accountable for ammonia. This important production, discussed in section 4.2, is a new result compared to the work by [START_REF] Gautier | Nitrile gas chemistry in Titan's atmosphere[END_REF] and [START_REF] Carrasco | Volatile products controlling Titan's tholins production[END_REF]. Indeed, as detailed in [START_REF] Carrasco | Volatile products controlling Titan's tholins production[END_REF], ammonia was hardly detectable with the analytical methods used in the two latter studies. The C 2 block has high peak intensities at m/z 26, 27, 28, 30 for [CH 4 ] 0 = 10% but the one at 28 stands out in both methane conditions, compatible with C 2 H 4 or C 2 H 6 . The C 3 block is dominated by m/z 41, accountable for acetonitrile CH 3 CN; m/z 54 dominates C 4 , which could correspond to butadiene C 4 H 6 or C 2 N 2 H 2 (HCN dimer). For [CH 4 ] 0 = 10%, the C 5 , C 6 and C 7 blocks are dominated by masses 69, 83 and 97, respectively. The odd masses predominantly detected suggest a strong contribution of N-bearing molecules in both conditions.

The intensity evolution of m/z 17, 26, 27, 28 and 41 over time are shown Figure 11

Monitoring and quantification using Mid-Infrared spectroscopy

Alongside the neutral mass analysis, infrared spectra are simultaneously taken throughout the release of the volatiles inside the chamber to provide quantification of these volatiles.

The measurement range is 650 cm -1 to 4000 cm -1 (2.5 µm to 15.4 µm) at a resolution of 1 cm -1 during the release of the gas phase products analyzed in situ. Fig. 6 shows the spectra taken in both initial methane conditions. The top plot was taken for a [CH 4 ] 0 = 10%, the bottom at [CH 4 ] 0 = 1%.

There is a clear discrepancy in the volatile products formed between the two conditions. [CH 4 ] 0 = 10% produces -CH and -CH 2 compounds visible in the stretching mode region 3,000-3,500 cm -1 that are absent in the [CH 4 ] 0 = 1% case. Likewise, there is a relatively far greater absorption of molecules bearing C=N and C=C bonds (~1,500 cm -1 ) with [CH 4 ] 0 = 10%. Fig. 6 (bottom) also has a large, broad absorption spread out over 2,000-2,500 cm -1 absent in the top figure. These signatures could correspond to a scattering feature of solid grains in suspension which could be the result of the released precursors reacting post-discharge. Furthermore, in Sciamma-O'Brien et al. ( 2010), it had been shown that the gas to solid conversion yield was more important in the case of the 1% experiment than in the 10% experiment, in agreement with a higher amount of N-bearing gas phase products compared to hydrocarbons in the 1% experiment. The present results are consistent with this previous study.

Fig. 6 FT-IR spectra of the volatiles taken after the 90-10% (top) and 99-1% (bottom) N 2 -CH 4 plasma conditions, plotted with an arbitrary absorbance against a 650-4000 cm -1 wavenumber range. The volatile density produced during the plasma discharge is being incrementally released and analyzed through IR spectroscopy. Top: Total gas pressures of 0.38 mbar (-130ºC), 0.93 mbar (-66ºC), 1.28 mbar (-44ºC) and 1.84 mbar (+22ºC) are shown in black, blue, cyan and red, respectively. Bottom: Total measured gas pressures of 0 mbar (-73ºC), 0.12 mbar (-41ºC), 0.20 mbar (-8ºC) and 0.34 mbar (+22ºC) with the same color code. One clear difference is in the absence of any substantial aliphatic compounds (2800-3100 cm -1 ) at [CH 4 ] 0 = 1% (bottom) that stand out at [CH 4 ] 0 = 10% (top).

Among all the signatures observed, we identified and calculated the concentrations of four volatiles as they are released in the chamber: NH 3 , C 2 H 2 , C 2 H 4 , and HCN. These species are listed in Table 2. Ammonia (NH 3 ), acetylene (C 2 H 2 ), hydrogen cyanide (HCN), ethylene (C 2 H 4 ), are all species present at ionospheric and/or stratospheric altitudes on Titan, considered to be key in the formation of Titan's aerosols [START_REF] Hanel | Infrared Observations of the Saturnian System from Voyager 1[END_REF][START_REF] Kunde | C4H2, HC3N and C2N2 in Titan's atmosphere[END_REF][START_REF] Maguire | C3H8 and C3H4 in Titan's atmosphere[END_REF]Wilson andAtreya, 2003, 2004) and formed in the upper atmosphere [START_REF] Hörst | Titan's Atmosphere and Climate[END_REF]. To track the production and evolution with time/temperature of these four compounds, we used in situ infrared quantification. By using Beer-Lambert's Law (Eq. 3), we can calculate the concentration of any given gas phase product.

𝐼 𝑡 = 𝐼 0 × 𝑒 -𝑙.𝜎.𝑁 (3)
, are, respectively, the transmitted, incident intensities, optical path (cm) and the 𝐼 𝑡 𝐼 0 𝑎𝑛𝑑 𝑙 𝜎 absorption cross-section (cm 2 .molecule -1 ) given by the Pacific Northwest National Lab (PNNL), University of Washington [START_REF] Sharpe | Gas Phase Database for Quantitative Infrared Spectroscopy[END_REF], GEISA [START_REF] Armante | The 2015 edition of the GEISA spectroscopic database[END_REF] and ExoMol [START_REF] Harris | Improved HCN/HNC linelist, model atmospheres and synthetic spectra for WZ Cas[END_REF][START_REF] Hill | Temperature-dependent molecular absorption cross sections for exoplanets and other atmospheres[END_REF][START_REF] Tennyson | The ExoMol database: molecular line lists for exoplanet and other hot atmospheres[END_REF] databases. In addition, N is the molecular density (cm -3 ). The absorption A is as follows:

𝐴 (𝜆) = 𝑙 × 𝜎 (𝜆) × 𝑁 (4)
Using the integrated experimental absorption A with the integrated database absorption cross-section between the two wavelengths λ 1 and λ 2 , (4) becomes:

𝜎 ∫ 𝜆 2 𝜆 1 𝐴𝑑𝜆 = 𝑙 × 𝑁 × ∫ 𝜆 2 𝜆 1 𝜎𝑑𝜆 (5)
So, we obtain the molecular number density N (molecules.cm -3 ):

𝑁 = 1 𝑙 × ∫ 𝜆 2 𝜆 1 𝐴𝑑𝜆 ∫ 𝜆 2 𝜆 1 𝜎𝑑𝜆 (6)
Number densities are derived over a wavenumber range (see Table 3 of the Supplementary Material), covering the absorption band considered for each species.

The main absorption band of each species was taken for all cases. The density results for both conditions are shown Fig. 7 and Fig. 8. For the detailed number density calculation results and data points used with the specific integration bands, the reader is invited to peruse the Supplementary Material of the online version of this article.

The 962 cm -1 (10.4 µm) NH 3 assigned band refers to the υ 2 N-H symmetric deformation. It is part of the NH 3 doublet at 930 and 960 cm -1 , caused by the motion of nitrogen through the plane of the three protons leading to an energy barrier. In the case of C 2 H 2 , we used acetylene's 729.25 cm -1 frequency of oscillation (13.7 µm). This band corresponds to the symmetric CH bending mode υ 5 of every other C and H atom to its respective neighboring atom. For HCN, we used the fundamental υ 2 bend frequency at 712 cm -1 (14.0 µm); C 2 H 4 the υ 7 CH 2 wag deformation at 949 cm -1 (10.5 µm). C 2 H 4 centered at 950 cm -1 is surrounded by the NH 3 doublet at 930 and 960 cm -1 . This absorption represents the fundamental absorption of ammonia's υ 2 normal vibrational mode. The motion of nitrogen through the plane of the three protons causes this vibration with an energy barrier. For a summary on each band and frequency used, the reader is referred to Table 3 of the Supplementary Material.

Molecular number densities at N 2 /CH 4 : 99/1% are listed in Table T-4. The measurements taken at t 1-6 correspond approximately to 30 min, 60 min, 120 min, 180 min, 1200 min and 1440 min, respectively. Three datasets were acquired after three experiments in the same initial conditions. At this point, it is noteworthy to remember that t 0 represents the time when the plasma conditions and cryo-trap cooling are ended (Fig. 3), and thus when the condensed volatiles can effectively desorb from the chamber still under vacuum walls into the gas phase.

Overall, ammonia is the most dominant C 1 neutral (~ 1.1 x 10 15 cm -3 ) produced at 1% CH 4 , confirming its high intensity at m/z 17 detected in mass spectrometry (Fig. 4). C 2 H 2 and C 2 H 4 also reached relatively high abundance ~ 3.2 x 10 12 cm -3 and ~ 9.9 x 10 12 cm -3 , respectively.

These number densities correspond to the final measurements. Fig. 8 shows the same kinetic profiles from a 10% methane initial gas mixture, with the molecular densities listed in Table T-4. The C 2 species all reach concentrations at an order of magnitude higher than those at 1%, correlating with the higher methane concentration initially injected, which promotes the production of hydrocarbon species. C 2 species are thus strongly favorably produced with increasing methane concentrations. This is especially notable with the amount of HCN produced (~ 1.2 x 10 15 cm -3 ) in the 10% [CH 4 ] 0 condition. 4 and 5 of the online version of this article).

Methane consumption and hydrocarbon yield

With a 10% CH 4 mixing ratio, the condition with the most products, we consider the CH 4 consumption to provide yield estimates on the carbon conversion to hydrocarbon gas products.

We note D as the total gas flow rate (N 2 -CH 4 ) into the chamber in standard conditions. We first calculate the amount of CH 4 consumed in standard conditions (mg.h -1 ), with D = 55 sccm. If n tot is the total number of gas phase moles entering the reactor per second, the ideal gas law becomes:

𝑑 𝑛𝑡𝑜𝑡 𝑑𝑡 = 𝑃 0 𝑅 × 𝑇 0 × 𝐷 (7)
With P 0 the gas pressure in Pa, R the ideal gas constant, T the absolute temperature (300 K), molecules.s -1 . As seen previously, with a mixing ratio of [CH 4 ] 0 = 10%, we 𝑑 𝑛𝑡𝑜𝑡 𝑑𝑡 = 2.22 × 10 19 find a CH 4 consumption of 52%. So,

𝑑𝑛 𝐶𝐻 4𝑐𝑜𝑛𝑠 𝑑𝑡 = 𝐷 𝐶𝐻 4 × 𝐶𝐻 4𝑐𝑜𝑛𝑠 × 𝑑 𝑛𝑡𝑜𝑡 𝑑𝑡 (8)
And the methane consumption rate is molecules.s -1 . As the plasma 𝑑𝑛 𝐶𝐻 4𝑐𝑜𝑛𝑠 𝑑𝑡 = 1.15 × 10 18 runs for 2h, the total methane consumption is molecules. The 𝑛 𝐶𝐻 4𝑐𝑜𝑛𝑠 (2ℎ) 𝑑𝑡 = 8.28 × 10 21 total volume of the PAMPRE reactor is ~ 28×10 3 cm 3 . Consequently, there is a CH 4 total consumption of about molecules.cm -3 over the 2h of the plasma discharge. With a 2.9 × 10 17 calculated HCN total number of molecules of ~ 1.2 x 10 15 cm -3 , HCN accounts for ~ 0.4% of the carbon conversion from the consumed CH 4 , while it is ~ 0.3 ‰ and 0.2 ‰ for C 2 H 2 and C 2 H 4 , respectively.

IV) Discussion

Volatile discrepancies at [CH 4 ] 0 = 1% and [CH 4 ] 0 = 10%

There are evident differences in the volatile products formed in both methane concentrations used in this study (Fig. 3, Fig. 6, Tables T-3 and T-4 of the online version of this article). Despite obtaining ~ 1.84 mbar solely in volatile products with 10% methane and ~ 0.35 mbar at 1% methane, both spectra on Fig. 5 appear to qualitatively show the same nitrogen-bearing odd mass compounds, albeit at different intensities. One major discrepancy though, lies in the overwhelming amount of hydrocarbon species produced at 10% methane. The C 2 block is dominated by the m/z 28 amu signal (Fig. 5), dominating the rest of the spectra, which according to the NIST database, corresponds to ethylene and ethane's main fragment (this mass cannot be linked to a release of N 2 , as the cryotrap temperatures were not cold enough to trap any nitrogen during the experiment). However, the vapor pressure indicates that ethane is difficult to trap, and so m/z 28 can be mainly attributed to C 2 H 4 . Furthermore, the I 30 /I 28 ratio of the absolute intensities in the mass spectra at m/z 30 and m/z 28 amu correspond to the ones given by NIST, ~ 30-35%. On Titan, short-lived species such as C 2 H 4 remain tantalizing to study, as they experience stratospheric dynamical processes with changing mixing ratios. By preventing the condensation of N 2 , this study shows how m/z 28 may be attributed to C 2 H 4 as one of the most abundant hydrocarbons at 10% CH 4 .

Ammonia

Moreover, NH 3 stands out as being the most dominant N-bearing molecule in both [CH 4 ] 0 = 1% and [CH 4 ] 0 = 10% conditions, with final calculated concentrations of ~ 1.1 x 10 15 and ~ 1.4 x 10 15 molecules.cm -3 , respectively. Ammonia production was previously shown to be positively correlated with an increasing methane concentration [START_REF] Carrasco | Volatile products controlling Titan's tholins production[END_REF]. However, the latter study did not consider ammonia detection due to lack of a quantification approach as well as the m/z 17 H 2 O fragment contamination. NH 3 , as well as HCN, are nitrogen-bearing volatiles of utmost importance related to prebiotic chemistry in reducing atmospheric environments. So, studying NH 3 in Titan's upper atmosphere is crucial to understanding the chemical pathways leading to the formation of aerosols and how it is incorporated in them. Cassini/INMS analyses of early Titan flybys [START_REF] Vuitton | Ion chemistry and N-containing molecules in Titan's upper atmosphere[END_REF] first reported the presence of ammonia in the ionosphere, which was later more robustly confirmed by Cui et al. (2009). [START_REF] Yelle | Formation of NH3 and CH2NH in Titan's upper atmosphere[END_REF], [START_REF] Carrasco | Volatile products controlling Titan's tholins production[END_REF], [START_REF] Loison | The neutral photochemistry of nitriles, amines and imines in the atmosphere of Titan[END_REF] discussed NH 3 chemical pathways. As such, the formation of ammonia in Titan's atmosphere is still unclear, though it is thought to most likely involve methanimine CH 2 NH which itself acts as an important source for NH 2 radicals in plasma conditions, to eventually forming NH 3 . An ionic characterization of pathways and reactions pertaining to ammonia production is out of the scope of this paper. [START_REF] Yelle | Formation of NH3 and CH2NH in Titan's upper atmosphere[END_REF] proposed an explanation for the production of NH 3 . Indeed, NH 3 is being photochemically formed in the upper atmosphere and fed down into the stratosphere. While photochemical models [START_REF] Lara | Vertical distribution of Titan's atmospheric neutral constituents[END_REF][START_REF] Krasnopolsky | A photochemical model of Titan's atmosphere and ionosphere[END_REF] did not initially predict the formation of NH 3 in the upper atmosphere, its production can be explained through ion reactions [START_REF] Yelle | Formation of NH3 and CH2NH in Titan's upper atmosphere[END_REF], by measuring the protonated ion NH 4 + . Based on the long chain reaction forming NH 3 detailed by [START_REF] Yelle | Formation of NH3 and CH2NH in Titan's upper atmosphere[END_REF] and its importance in our experimental results (major nitrogen-bearing molecule), the contribution of nitrogen-bearing functional groups (e.g. amines, nitriles or imine) or their contribution to eventually be incorporated into the organic aerosols seems pertinent. This appears to be the case for NH 3 , being the main N-bearing compound. In more advanced environments whether on Titan [START_REF] Neish | Low temperature hydrolysis of laboratory tholins in ammonia-water solutions: Implications for prebiotic chemistry on Titan[END_REF] or in the interstellar medium (e.g. [START_REF] Largo | The reaction between NH + and CH 3 COOH : a possible process for the formation of glycine precursors in the interstellar medium[END_REF], NH 3 remains an open and current topic to study prebiotic chemistry.

C 2 H 4 pathways to tholin formation

The presence of ethylene in the ionosphere [START_REF] Waite | The Process of Tholin Formation in Titan's Upper Atmosphere[END_REF] and stratosphere (e.g. [START_REF] Coustenis | The composition of Titan's stratosphere from Cassini/CIRS mid-infrared spectra[END_REF] is well established. However, similar to the other volatiles chosen in this study, its influence and participation in tholin formation needs to be further investigated. The relative concentration of C 2 H 4 in Titan's atmospheric column is subject to distinct features. It was first noted by [START_REF] Coustenis | The composition of Titan's stratosphere from Cassini/CIRS mid-infrared spectra[END_REF] and [START_REF] Vinatier | Vertical abundance profiles of hydrocarbons in Titan's atmosphere at 15 o S and 80 o N retrieved from Cassini/CIRS spectra[END_REF] how the retrieved stratospheric vertical abundances of C 2 H 4 from Cassini/CIRS data showed both a unique decrease in the mixing ratios towards high northern latitudes as well as being the only species whose mixing ratio decreases with altitude at 15ºS near the equator. Compared to other C 2 molecules (e.g. C 2 H 2 , C 2 H 6 , HCN), C 2 H 4 is unique in that its mixing ratio decreases with altitude at equatorial and northern polar latitudes. One hypothesis given by [START_REF] Vinatier | Vertical abundance profiles of hydrocarbons in Titan's atmosphere at 15 o S and 80 o N retrieved from Cassini/CIRS spectra[END_REF] was that C 2 H 4 (along with C 4 H 2 and CH 3 C 2 H), is subject to a photodissociative sink along with equatorward transport from the winter polar vortex in the lower stratosphere and troposphere [START_REF] Crespin | Seasonal variations of Titan's stratospheric composition in the LMD/SA Global ClimateModel[END_REF], essentially removing it away from the gaseous atmosphere. This sink could eventually be the multiple haze layer.

As discussed previously and shown Fig. 5 and Table 5 (Supplementary Material), C 2 H 4 is an important C 2 compound in the 10% [CH 4 ] 0 mixture. Similar experiments [START_REF] Gautier | Nitrogen incorporation in titan's tholins inferred by high resolution orbitrap mass spectrometry and gas chromatography-mass spectrometry[END_REF] argued that (although in the absence of a cooling plasma system) tholins are formed with CH 2 patterns, coherent with C 2 H 4 putatively acting as a strong gaseous precursor. This study comes in agreement, at least on a tholin formation scale that C 2 H 4 participates in their formation. This experimental result also confirms that, if the stratospheric multiple haze layer is indeed the sink to this molecule [START_REF] Vinatier | Vertical abundance profiles of hydrocarbons in Titan's atmosphere at 15 o S and 80 o N retrieved from Cassini/CIRS spectra[END_REF], the latter can easily take chemical pathways leading to the formation of aerosols.

Pathways involving C 2 H 4 (alongside C 2 H 2 ) loss have been suggested by [START_REF] Yelle | Formation of NH3 and CH2NH in Titan's upper atmosphere[END_REF].

They proposed that C 2 H 4 (and C 2 H 2 ) reacts with NH radicals (Reaction 9) to produce heavier nitrile molecules such as CH 3 CN (Reaction 10) or the cyanomethylene radical HC 2 N.

N + + CH 4 → CH 3 + + NH (9) NH + C 2 H 4 → CH 3 CN + H 2 (10)
The formation of NH radicals, reaction (9), occurs in ion chemistry conditions through proton exchange, while these same radicals can then react with ethylene, reaction (10) in neutral conditions. Given the large abundance of produced NH 3 , it is possible that NH radicals were formed and reacted with C 2 H 4 during our 2h plasma, which incidentally is favorably produced with increasing CH 4 concentration (Reaction 9). Interestingly, C 2 H 4 is implicated in NH 3 formation. According to [START_REF] Yelle | Formation of NH3 and CH2NH in Titan's upper atmosphere[END_REF], NH 2 + H 2 CN → NH 3 + HCN (11) Prior to this, the formation of the amino radical NH 2 involves ion chemistry, with the following ion-neutral reaction:

N + + C 2 H 4 → NH 2 + C 2 H 2 + (12)
Thus, C 2 H 4 in our plasma may participate in the production of heavier nitrile species or ammonia during the plasma discharge.

HCN production

Hydrogen cyanide in Titan's atmosphere is well documented and known to be the most abundant nitrile trace volatile (e.g. [START_REF] Kim | Clouds, haze, and CH4, CH3D, HCN, and C2 H2 in the atmosphere of Titan probed via 3 ??m spectroscopy[END_REF][START_REF] Vinatier | Vertical abundance profiles of hydrocarbons in Titan's atmosphere at 15 o S and 80 o N retrieved from Cassini/CIRS spectra[END_REF], with high mixing ratios at stratospheric altitudes in the northern polar regions.

HCN is also of prime importance in the search for prebiotic conditions in the solar system and beyond [START_REF] Oró | Comets and the Formation of Biochemical Compounds on the Primitive Earth[END_REF] as well as being one of the precursors to amino acids and peptides [START_REF] Oró | Comets and the Formation of Biochemical Compounds on the Primitive Earth[END_REF][START_REF] Hörst | Formation of Amino Acids and Nucleotide Bases in a Titan Atmosphere Simulation Experiment[END_REF][START_REF] He | Identification of nitrogenous organic species in Titan aerosols analogs: Implication for prebiotic chemistry on Titan and early Earth[END_REF][START_REF] Rahm | Polymorphism and electronic structure of polyimine and its potential significance for prebiotic chemistry on[END_REF]. In addition, the chemistry of HCN mainly relies on its relatively high polarity and quite strong C≡N triple bond, with a bond dissociation energy D(H-CN) of ~ 5.2 eV calculated by photodissociation and photoionization models [START_REF] Berkowitz | Heat of formation of the CN radical[END_REF][START_REF] Davis | Determination of Bond Dissociation Energies in Hydrogen Cyanide. Cyanogen and Cyanogen Halides by the Photodissociation Method[END_REF][START_REF] Cicerone | The atmospheric chemistry of hydrogen cyanide (HCN)[END_REF].

Thus, making it a relatively stable molecule.

As shown Figs. 7 and8, HCN is one of the most abundant products formed in both [CH 4 ] 0 = 1% and [CH 4 ] 0 = 10% conditions, with ~ 1.5 x 10 14 cm -3 and 1.2 x 10 15 cm -3 on average, respectively. Carbon yield calculations stresses the carbon conversion to HCN (~ 0.4%), while other C 2 species all are favorably produced by an order of magnitude at 10% CH 4 than at 1% CH 4 . HCN and HCN-based copolymers are also known to participate in the chemical growth patterns of tholin formation [START_REF] Pernot | Tholinomics-Chemical Analysis of Nitrogen-Rich Polymers[END_REF][START_REF] Gautier | Nitrogen incorporation in titan's tholins inferred by high resolution orbitrap mass spectrometry and gas chromatography-mass spectrometry[END_REF] along with the CH 2 monomer, which is in agreement with the idea that haze forming acts as an HCN sink in the stratosphere [START_REF] Mckay | Elemental composition, solubility, and optical properties of Titan's organic haze[END_REF][START_REF] Vinatier | Vertical abundance profiles of hydrocarbons in Titan's atmosphere at 15 o S and 80 o N retrieved from Cassini/CIRS spectra[END_REF]. This study seems to confirm the prevalent role of HCN in the formation of tholins.

V) Conclusions

The present study was aimed at expanding previous work focused on the volatile production and their participation in tholin production using a cold dusty plasma experiment [START_REF] Gautier | Nitrile gas chemistry in Titan's atmosphere[END_REF][START_REF] Carrasco | Volatile products controlling Titan's tholins production[END_REF]), but with one major twofold novelty (i) the ability to trap these volatiles in situ within our plasma reactor with a new experimental setup at lowcontrolled temperatures, and (ii) quantitatively analyze their formation using IR spectroscopy. It is important to be cautious in interpreting these quantitative results, as three (T, P and t) parameters change simultaneously during the sublimation phase entailing possible unsuspected ice-volatile chemistry. The approach we took was solely focused on the gas phase. Indeed, these volatiles, still poorly understood and which give Titan its unique nature of being one of the most chemically complex bodies in the Solar System, act as substantial gas phase precursors to the formation of Titan's organic aerosols populating its haze layers. Hence, simulating ionosphere conditions is crucial in constraining the volatile population, its reactivity and potential chemical pathways.

Major discrepancies exist between the two initial methane concentrations we used, 1% and 10%, with many more hydrocarbons formed in the latter case (detection of C 7 species), but with similar amounts of N-bearing species, only at different concentrations. Our results confirm the important role of C 2 species in the tholin precursor volatile family (m/z 28 e.g.), as well as that of NH 3 . One of them, ethylene C 2 H 4 , being a relatively major C 2 hydrocarbons, might even reveal an important "hub" role at least at 10% [CH 4 ] 0 through which specific chemical pathways involving C 2 H 4 are favored (see Section 4.3). The importance of C 2 H 4 might also be relevant at 1% [CH 4 ] 0 although this is difficult to assess in this study due to the cryotrapping efficiency which is not optimum at -180ºC to quantify it, as suggested by the thermodynamical calculations and mass spectra during the discharge. Eventually, tholins themselves may be benefitting from the incorporation of C 2 H 4 , which itself stresses the value of considering a yet unaccomplished fully-coupled ion and neutral chemistry study. This is out of the scope of this paper, as the cryotrap system prevents the formation of tholins. These experiments have also shown a strong incorporation of carbon into HCN, along with NH 3 being a competitive product to HCN, especially with a [CH 4 ] 0 = 1% (an order of magnitude more in final concentration).

Future compelling work to expand this study might be to consider other molecular families participating in the neutral reactivity, therefore increasing our knowledge of volatile chemistry precursor to tholin production. In addition, adding selected volatiles such as HCN, an important nitrile, in trace amounts into our gaseous mixture would improve the understanding of its influence on the chemical reactivity in plasma conditions in future work. Moreover, predicting ion densities and characterizing ion-neutral reactions in these conditions would be key to understanding their incorporation into the solid phase and validating chemical pathways which are still largely unknown in Titan ionosphere simulations. Perhaps most importantly, a fullycoupled ion-neutral characterization would fundamentally bring credence to understanding the complex organic realm encompassing Titan's gas phase chemistry. [CH 4 ] 0 = 10%. Main absorption bands of (A) NH 3 (930 cm -1 and 960 cm -1 doublet), (B) C 2 H 2 (729.25 cm -1 ), (C) HCN (713 cm -1 ), (D) C 2 H 4 (949.55 cm -1 ). The color code is the same as in Figure 7. The vertical dashed gray lines correspond to the integration band used for the density calculations on either side of the absorption peaks. 3. IR peak locations, fundamental frequencies and integrated wavenumber ranges, absorption cross-sections (cm 2 .molecule -1 ) for each selected volatile, as analyzed in both methane conditions. For the calculation results (derived from Equ. 6), see the following Tables 45 correspond to measurements done at approximately 7 min (-130ºC), 125 min (-66ºC), 190 min (-44ºC) and 21h (+22ºC), respectively. Each column corresponds to one experiment. Number density calculations are performed with the integrated absorption cross-section of the ExoMol or Hitran databases at the resolution of our experimental spectra (see Table 3). Blank boxes indicate the absence of the species where no number density was derived.

Fig. 1 .

 1 Fig. 1. 3D schematics of the cryogenic trap. The cold trap goes through the top lid of the reactor and coils down into

Fig. 2 .

 2 Fig. 2. (a) Upper panel: No cryogenic trap. Mass spectra of a 90-10% N 2 -CH 4 mixture with plasma off (red) and

  MS2 and MS3 (Figures3 and 4) as three mass spectra for 10%[CH 4 ] 0 taken at three different pressures (~ 0.38, 0.74 and 1.84 mbar) and temperatures (-130ºC, -79ºC and +22ºC) during volatile release, respectively. The temperature heat-up rate initially increases relatively fast (~ 7 ºC/min between t = 0 min and the acquisition of the MS1 spectrum), then decreases to ~ 0.3 ºC/min near MS2 and finally reaches a temperature decrease rate of ~ 0.1 ºC/min above -50ºC (MS3).

Fig. 3 .

 3 Fig. 3. (Top) Pressure evolution of the gas products according to degassing time in a 90-10% (blue) and 99-1% N 2 -CH 4 (red) gas mixture. Labeled at each data point are the times and vessel pressures, respectively. The MS1 (-130ºC, 0.38 mbar), MS2 (-79ºC, 0.74 mbar), MS3 (+22ºC, 1.84 mbar) color-coded labels correspond to the three spectra of Fig. 4 taken at their corresponding temperatures and pressures, underlying significant detections at 10% methane. (Bottom) Temperature evolution during degassing of the volatiles with the same color code for both mixing ratios.

Fig. 4 .

 4 Fig. 4. Detection and evolution of several C n blocks for [CH 4 ] 0 = 10%. Plots marked in blue, red and black represent intermittent spectra taken 7 min, 76 min and 21h after commencing volatile release back to room temperature (MS1, MS2 and MS3, respectively). The indicated temperatures correspond to those taken at the start of a mass scan. Note that during a scan acquisition (~200s long), the temperature may change over 0.3 ºC to 23 ºC depending on the heating rates. The color code used here is the same as MS1, MS2 and MS3 of Fig. 3.

Fig. 5 .

 5 Fig. 5. Three superimposed spectra at different [CH 4 ] 0 concentrations. In black, the initial mass spectrum taken before release of the volatiles, still at low-controlled temperature (and representative of the blank of our mass spectrometer). In blue and brown, the final state of volatiles at [CH 4 ] 0 = 10% after 21h and [CH 4 ] 0 = 1% after 21h of release, respectively.

  main infrared absorption bands which were used for density calculations are also given. References are 1: Vinatier et al. (2007), 2: Cui et al. (2009), 3: Coustenis et al. (2007), 4: Nelson et al. (2009), 5: Paubert et al. (1984), 6: Teanby et al. (2007), 7: Moreno et al. (2015), 8: Molter et al. (2016). For more details on the bands and absorption cross-sections used for the molecular density calculations, the reader is referred to Table3and Figures 9 and 10 of the Supplementary Material.

Fig. 7 .

 7 Fig. 7. 1% methane conditions. Top: Molecular densities and average fitted kinetic profiles of C 2 H 2 and C 2 H 4 . Bottom: NH 3 and HCN. Approximate temperatures are also labeled over each data point. The error bars represent the dispersion of the data points for all three experiments (Table 4 and 5 of the online version of this article).

Fig. 8 .

 8 Fig. 8. 10% methane conditions. Top: Molecular densities and average fitted kinetic profiles of C 2 H 2 and C 2 H 4 .

Fig. 9 .

 9 Fig.9.[CH 4 ] 0 = 1%. Main absorption bands of (A) NH 3 (930 cm -1 and 960 cm -1 doublet), (B) C 2 H 2 (729.25 cm -1 ), (C) HCN (713 cm -1 ), (D) C 2 H 4 (949.55 cm -1 ). The color code is the same as in Figure6. The vertical dashed gray lines correspond to the integration band used for the density calculations on either side of the absorption peaks.

Fig

  Fig.10.[CH 4 ] 0 = 10%. Main absorption bands of (A) NH 3 (930 cm -1 and 960 cm -1 doublet), (B) C 2 H 2 (729.25 cm -1 ), (C)

Fig. 11 .

 11 Fig. 11. Evolution of the intensities (same time scale as Figure 3) of selected species, NH 3 (m/z 17), C 2 H 2 (m/z 26), HCN (m/z 27), C 2 H 4 (m/z 28) and CH 3 CN (m/z 41) over time.

  .1 [START_REF] Sharpe | Gas Phase Database for Quantitative Infrared Spectroscopy[END_REF] http://vpl.astro.washington.edu/spectra/allmoleculeslist.htm Calculated molecular densities from IR absorption at [CH 4 ] 0 = 1%, for three different data sets. T 1-6 correspond to measurements done at approximately 30 min (-130ºC), 60 min (-85ºC), 120 min (-70ºC), 180 min (-41ºC), 1200 min (+22ºC) and 1440 min (+22ºC), respectively. Each column corresponds to one experiment. Number density calculations are performed with the integrated absorption cross-section of the ExoMol or Hitran databases at the resolution of our experimental spectra (see

Table 1 .

 1 Vapor pressures P sub (mbar) calculated at 93K (-180ºC) for HCN, C 2 H 2 and C 2 H 4 . The T p and A i columns correspond to the triple points and coefficients of the polynomials of extrapolations, respectively, as given by[START_REF] Fray | Sublimation of ices of astrophysical interest: A bibliographic review[END_REF]

Table 2 .

 2 Four major volatile compounds detected and analyzed by infrared spectroscopy at [CH 4 ] 0 = 1% and [CH 4

Table

  Table 3). Blank boxes indicate the absence of the species where no number density was derived.

	Time/Species	NH 3 (cm -3 )	C 2 H 2 (cm -3 )	HCN (cm -3 )	C 2 H 4 (cm -3 )
				2.0 x	2.2 x			8.3 x	5.5 x
	T 1	-	-	10 13	10 13	-	-	10 12	10 12
	mean	-		2.1 x 10 13	-		6.9 x 10 12
		3.5 x		2.9 x	3.0 x	6.5 x	4.5 x	1.4 x	3.3 x
	T 2	10 14	-	10 13	10 13	10 14	10 15	10 13	10 12
	mean	3.5 x 10 14	3.0 x 10 13	2.6 x 10 15	8.7 x 10 12
		8.6 x	1.6 x	3.5 x	4.9 x	1.1 x	2.4 x	1.1 x	5.9 x
	T 3	10 14	10 15	10 13	10 13	10 15	10 15	10 13	10 13
	mean	1.2 x 10 15	4.2 x 10 13	1.8 x 10 15	3.5 x 10 13
		2.3 x	5.0 x	4.1 x	3.2 x	8.5 x	1.5 x	2.2 x	4.3 x
	T 4	10 15	10 14	10 13	10 13	10 14	10 15	10 13	10 13
	mean	1.4 x 10 15	3.7 x 10 13	1.2 x 10 15	3.3 x 10 13

Table 5 .

 5 Calculated molecular densities from IR absorption at [CH 4 ] 0 = 10%, for two different data sets. T 1-4

in intensity in the bottom figure. This shows the efficient role of the cryotrap, which is to trap (albeit not completely

here) the gas products formed in the plasma.

Acknowledgements

We are thankful to the European Research Council Starting Grant PRIMCHEM, grant agreement no. 636829 for funding this work. We thank C. Szopa, A. Mahjoub, A. Oza and gratefully acknowledge discussions with T. Gautier. We are grateful to two anonymous reviewers for their insightful and valuable comments which improved the content of this paper.

Supplementary

t=2h30 (0 mbar and -73ºC) t=3h30 (0.12 mbar and -41ºC) t=6h10 (0.20 mbar and -8ºC) t=20h20 (0.34 mbar and +22ºC) 10.9 10.7 10.5 10.3


Raffaele Petrone

Nadia Yousfi Steiner

Samir Jemei

Fabien Harel

Daniel Hissel

Marie-Cécile Pera

Model-based strategy oriented to PEMFC system prognostic for Bus

come   L'archive ouverte pluridisciplinaire

Model-based strategy oriented to PEMFC system prognostic for Bus transportation applications based on EMR formalism.

R. PETRONE 1,2 , N. YOUSFI STEINER 1,2 3 , S. JEMEI 1,2 , F. HAREL 2,4 , D. HISSEL 1,2 , M-C. PERA Proton exchange membrane fuel cells (PEMFCs) appear nowadays to be a promising solution to face energy transition challenges in automotive applications. In this scenario, PEMFCs lifespan and their remaining Useful Life (RUL) under dynamic operations is currently object of main interest. This work introduces the fundamentals to design a generic strategy to support PEMFC durability enhancement in case of bus transportation applications. To this purpose, the energetic macroscopic formalism (EMR) is used to represent and model both the stack and the balance of plant (BoP) interactions. Based on power exchanges, each element will be related to another according to the action-reaction principle. The expected model will be able to simulate the system' response under a given load mission profile. Ageing behavior will be also considered in the model development. The model is expected to predict the voltage degradation profile with respect to the operating time and/or the produced energy. The voltage trend will be subsequently exploited by prognostic techniques to evaluate the PEMFC and BOP RUL and support their maintenance. This kind of approach is useful to integrate the existing diagnostic algorithms and support PEMFC makers in control decisions and maintenance scheduling as illustrated below. The research leading to these results has received funding from the European Union's Horizon2020 Programme (H2020-JTI-FCH-2015-1) for the Fuel Cells and Hydrogen Joint Undertaking, under grant agreement n° 700101 -Project : GIANTLEAP (Giantleap Improves Automation of Non-polluting Transportation with Lifetime Extension of Automotive PEM fuel cells). The work is performed in collaboration with the Labex ACTION Program (contract N° ANR-11-LABX-0001-01).

Figure :

 : Figure: Generic strategy fundamentals to support PEMFC durability based on EMR


Shrey Joshi

Jean-Pierre Franc

Giovanni Ghigliotti

Marc Fivel
email: marc.fivel@simap.grenoble-inp.fr

Smoothed Particle Hydrodynamics simulations of cavitation bubble collapse induced plasticity

Keywords: SPH, cavitation erosion, bubble collapse, plasticity, damage

Cavitation erosion is a major concern in many industrial applications such as water pump, marine propellers, hydroelectric turbines, fuel injectors, or even in health science like for hearth valves. Cavitation erosion is the consequence of mass loss of a solid material subjected to multiple impacts of collapsing bubbles in the fluid. The bubbles are generally nucleated in the flow at locations where the local pressure decreases below vapour pressure, i.e. in the regions of high velocity. They will later collapse when the velocity will decrease. In the case the collapsing bubble is located close to a solid interface, the dynamics of the collapse is not symmetric and a liquid microjet is formed toward the solid interface as depicted in Figure 1b. When the microjet hits the opposite side of the bubble (Figure 1c), a shock wave is generated in the fluid. The liquid microjet will then reach the solid surface (Figure 1d) at a velocity of several meters per second leading to high strain rates in the material. With time, the solid/liquid interface will experience a large number of impacts that will cumulate plasticity in the material. This will finally lead to the nucleation of a crack at the origin of mass loss. The objective of this paper is to simulate the plasticity induced in the solid material by bubble collapses. Very few attempts have been conducted so far. Typical simulations consists of using a Finite Element (FE) solver where the load corresponding to the bubble collapse is introduced via a pressure distribution at the solid boundary. However, it is extremely difficult to measure experimentally such a load distribution. Indeed, measurements based on PVDF only give an integration of multiple bubble collapses on the entire sensor area with a poor temporal resolution. An alternative solution consists of realizing Computational Fluid Dynamics (CFD) simulations but a key point is to accurately account for the deformation of the solid interface. This could be done within a classical fluid/structure interaction framework, for example using an Arbitrary Lagrangian Eulerian description of the fluid domain and a strong coupling between the CFD code and the FE solver [START_REF] Fivel | Towards numerical prediction of cavitation erosion[END_REF]. In the present paper, we chose to use a meshless method in order to solve both the fluid and the solid domain in the same code. Smoothed Particle Hydrodynamics (SPH) method is a Lagrangian meshless method for which the dynamics of particles representative of an infinitesimal part of the continuum domain is computed based on a set of constitutive equations [START_REF] Monaghan | Smoothed Particle Hydrodynamics[END_REF]. Although originally developed to numerical studies of planetary collisions [START_REF] Benz | The origin of the moon and the single-impact hypothesis I[END_REF], SPH is nowadays mostly dedicated to fluid mechanics for which the code can easily handle compressibility and subsequent wave propagations.

Starting from the 2D open-source software SPHysics dedicated to fluid mechanics and jointly developed at Johns Hopkins University and the University of Rome, La Sapienza [4], we have modified the code so that it could account for elasto-visco-plastic media within axisymmetric conditions. The solid solver has been validated by comparisons with FE simulations in simple situations such as the case of indentation. The fluid solver has been verified by computing the velocity field during a spherical bubble collapse in an infinite fluid domain and comparing the result of the analytical Rayleigh-Plesset solution. The code has then been applied to the case of a spherical bubble collapsing near an Al-2205 aluminum alloy sample for which the constitutive parameters have been identified experimentally. Different standoff positions of the bubble have been tested. Namely, we have investigated the case of a detached cavity for which the standoff ratio, SR, defined as the distance to the wall divided by the bubble radius is greater than 1 and the case of an attached cavity for which SR<1. It is observed that the attached cavities induce a much more localized plastic zone precisely located along the symmetry axis (Figure 2a). The detached cavity behaves differently because of the shock wave generated during the bubble collapse which cumulates plasticity outside the symmetry axis (Figure 2b) although it is not the location of the highest pressure induced by the fluid. The plasticity expansion process is attributed to inertial effects induced by the elastic wave traveling in the solid (Figure 2c). It is concluded that numerical investigations of cavitation erosion should not be limited to pressure distribution at the interface.

Figure 1 :

 1 Figure 1: Snapshots of the dynamics a cavitation bubble collapse near a solid interface.

Figure 2 :

 2 Figure 2: SPH results of plasticity induced by an attached cavity (a) compared to a detached cavity (b). Illustration of the Von Mises stress induced by the wave propagation in the case of a detached cavity (c).

Acknowledgement

This project has received funding from the European Union's Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No 642536.