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PAMPRE: A dusty plasma experiment for Titan's tholins production and study
Abstract
Organic aerosols play a significant role in the properties and evolution of Titan's atmosphere. But our knowledge of them and their physico-chemical mechanisms of formation and evolution are currently limited to a few data obtained by Titan observations from the Earth or from space probes. For this reason, laboratory experiments are developed to simulate the atmospheric chemistry and produce analogues of these aerosols in order to understand better their properties and how they are formed. The plasma discharges are the most efficient devices for the production of such analogues. However, the existing plasmas simulations introduce experimental biases compared with the conditions of aerosols production in Titan's atmosphere: chemistry is induced by electrons instead of photons; the solid analogues are produced and deposited on solid surfaces; direct analysis of the particles inside the reactive chamber is not easy. In order to avoid some of these experimental problems, we have developed another method of production of Titan's aerosols analogues. It is based on a capacitively coupled radio-frequency (RF) cold plasma system at low pressure in a N2–CH4 gaseous mixture. In this plasma, solid particles produced from the gas phase are in levitation, thus preventing any wall effect on their production, and allowing the study of the formation and growth of the particles directly in the plasma. Moreover, the electron energy distribution of this plasma can be compared with the solar spectrum. This article describes the RF plasma experiment and presents the first results obtained with an initial N2–CH4 (90:10) gaseous mixture which produced our first studied analogues of Titan's aerosols.
... One powerful methodology is to synthesize and study the physical chemistry of analogues of Titan's aerosols (so called tholins) in the laboratory. In the past, photochemical reactors or plasma discharge experiments have been used for this purpose (Khare et al., 1984;Coll et al., 1999;Tran et al., 2003;Imanaka et al., 2004;Szopa et al., 2006;Vuitton et al., 2009;Hasenkopf et al., 2010, Carrasco et al., 2013. ...
... The aim of the present study is to identify univocally aromatic molecules in tholins with a specific method described in detail below. For tholins synthesis we use the reactor described by Szopa et al. (2006) which is a plasma RF discharge applied to a N2/CH4 gas mixture. The chemical and physical properties of tholins produced by this set--up have been studied previously by a variety of techniques namely infrared spectroscopy (Derenne et al., 2012) and ellipsometry (Sciamma--O'Brien et al., 2012;Mahjoub et al., 2012;Mahjoub et al., 2014). ...
... The experimental setup dedicated to the production of Titan's aerosol analogues is based on a Capacitively Coupled Plasma Radio Frequency (CCP RF) operating at 13.56 MHz (Szopa et al., 2006;Alcouffe et al., 2010). The reactor is a stainless steel cylinder 30 cm in diameter and 40 cm high. ...
The role of polycyclic aromatic hydrocarbons (PAH) and Nitrogen containing PAH (PANH) as intermediates of aerosol production in the atmosphere of Titan has been a subject of controversy for a long time. An analysis of the atmospheric emission band observed by the Visible and Infrared Mapping Spectrometer (VIMS) at 3.28 micrometer suggests the presence of neutral polycyclic aromatic species in the upper atmosphere of Titan. These molecules are seen as the counter part of negative and positive aromatics ions suspected by the Plasma Spectrometer onboard the Cassini spacecraft, but the low resolution of the instrument hinders any molecular speciation. In this work we investigate the specific aromatic content of Titan's atmospheric aerosols through laboratory simulations. We report here the selective detection of aromatic compounds in tholins, Titan's aerosol analogues, produced with a capacitively coupled plasma in a N2:CH4 95:5 gas mixture. For this purpose, Two-Step Laser Desorption Ionization Time-of-Flight Mass Spectrometry (L2DI-TOF-MS) technique is used to analyze the so produced analogues. This analytical technique is based on the ionization of molecules by Resonance Enhanced Multi-Photon Ionization (REMPI) using a {\lambda}=248 nm wavelength laser which is selective for aromatic species. This allows for the selective identification of compounds having at least one aromatic ring. Our experiments show that tholins contain a trace amount of small PAHs with one to three aromatic rings. Nitrogen containing PAHs (PANHs) are also detected as constituents of tholins. Molecules relevant to astrobiology are detected as is the case of the substituted DNA base adenine.
... The formation of various types of dust grains has been studied within laboratory plasmas, which simulate the freefloating conditions and ion chemistry present in many astrophysical settings, as grains are formed within the plasma bulk and are electrostatically levitated (Bouchoule 1999). Previous studies have focused on chemical precipitation within weakly ionized plasmas of solid aerosols, such as analogs of Titan's tholins (Szopa et al. 2006) and polycyclic aromatic hydrocarbons that are common in the interstellar medium (Ricketts et al. 2011). One common configuration, used by Szopa et al. (2006), is the steady-state radio-frequency (RF) capacitively coupled plasma, in which a sustained plasma discharge is produced between parallel-plate electrodes. ...
... Previous studies have focused on chemical precipitation within weakly ionized plasmas of solid aerosols, such as analogs of Titan's tholins (Szopa et al. 2006) and polycyclic aromatic hydrocarbons that are common in the interstellar medium (Ricketts et al. 2011). One common configuration, used by Szopa et al. (2006), is the steady-state radio-frequency (RF) capacitively coupled plasma, in which a sustained plasma discharge is produced between parallel-plate electrodes. The plasma develops a positive electric potential, while the grains acquire a negative charge (Shukla & Mamun 2002), causing the grains to be electrostatically suspended within the plasma while they accrete new material and interact with their surroundings both physically and chemically. ...
Grains of ice are formed spontaneously when water vapor is injected into a weakly ionized laboratory plasma in which the background gas has been cooled to cryogenic temperatures comparable to those of deep space. These ice grains are levitated indefinitely within the plasma so that their time evolution can be observed under free-floating conditions. Using microscope imaging, ice grains are shown to have a spindle-like fractal structure and grow over time. Both crystalline and amorphous phases of ice are observed using Fourier transform infrared spectroscopy. A mix of crystalline and amorphous grains coexists under certain thermal conditions, and a linear mixing model is used on the ice absorption band surrounding 3.2 μ m to examine the ice phase composition and its temporal stability. The extinction spectrum is also affected by inelastic scattering as grains grow, and characteristic grain radii are obtained from Mie scattering theory and compared to size measurements from direct imaging. Observations are used to compare possible ice nucleation mechanisms, and it is concluded that nucleation is likely catalyzed by ions, as ice does not nucleate in the absence of plasma and impurities are not detected. Ice grain properties and infrared extinction spectra show similarity to observations of some astrophysical ices observed in protoplanetary disks, implying that the fractal morphology of the ice and observed processes of homogeneous ice nucleation could occur as well in such astrophysical environments with weakly ionized conditions.
... Haze analogs are produced using the PAMPRE (French acronym for "production of aerosols in micro-gravity by a reactive plasma") experimental setup described in detail in Szopa et al. (2006). This plasma reactor triggers disequilibrium chemical reactions from electron impact at energies equivalent to vacuumultraviolet photons. ...
... This plasma reactor triggers disequilibrium chemical reactions from electron impact at energies equivalent to vacuumultraviolet photons. The relative electron energy distribution is similar to a solar spectrum, with an increased high-frequency tail enhancing the dissociation and ionization of the gas molecules (Szopa et al. 2006;Alves et al. 2012). The plasma is confined in a stainless-steel cage with the base acting as the grounded electrode onto which we place optical substrates. ...
We report new optical constants (refractive index, n, and extinction coefficient, k) for exoplanet haze analogs from 0.3 to 30 microns. The samples were produced in a simulated N_ -dominated atmosphere with two different abundance ratios of CO_ and CH_ , using the PAMPRE plasma reactor at LATMOS. We find that our haze analogs present a significantly lower extinction coefficient in the optical and near-infrared (NIR) range compared to the seminal data obtained on Titan haze analogs. We confirm the stronger IR absorption expected for hazes produced in a gas mixture with higher CO abundances. Given the strong impact of the atmospheric composition on the absorbing power of hazes, these new data should be used to characterize early-Earth and CO-rich exoplanet atmospheres. The data presented in this paper can be found in the Optical Constants Database. Using ellipsometry or spectrophotometry, the retrieved optical constants are affected by the sensitivity of the measurement and the accuracy of the calculations. A comparative study of both techniques was performed to identify limitations and better understand the discrepancies present in the previous data. For the refractive index n, errors of 1-3 are observed with both optical techniques and the different models, caused by the correlation with the film thickness. We find that UV-visible reflection ellipsometry provides similar n values, regardless of the model used; whereas the Swanepoel method on transmission is more subjected to errors in the UV. In the UV and mid-infrared (MIR), the different calculations lead to rather small errors on k. Larger errors of k arise in the region of weak absorption, where calculations are more sensitive to errors on the refractive index n.
... Results with the DC discharge in N 2 -H 2 are compared to measurements done on a RF CCP discharge in N 2 -H 2 in similar experimental conditions and with the same MS, previously published in Chatain et al [13], abbreviated 'C20' below. The motivation of this work is that both discharges are used to simulate Titan's ionosphere [8,31]. Therefore, investigating the similarities and differences in the formed species in both discharges is fundamental to draw conclusions on Titan's case. ...
... Therefore, on Titan NH 4 + is often used as an indicator of the NH 3 density [49]. [28][29][30][31]. In all the experiments except in pure N 2 plasma, N 2 H + is the main ion, at 70-80%. ...
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.
... The aerosols are likely to have an aromatic component, with a substantial fraction of polycyclic aromatic hydrocarbons (PAHs) depending on the trace gas compositions (Trainer et al., 2004(Trainer et al., , 2013Gautier et al., 2017). Further, Gautier et al. (2014) and Schulz et al. (2021) reported the presence of a large variety nitrogen-bearing molecules in tholins, especially aromatic cycles including nitrogen atom and amines, which are probably the source of nitriles and unsaturated hydrocarbons detected when pyrolyzing these same tholins (Szopa et al., 2006). ...
... These tholins were produced within the PAMPRE experiment at LATMOS laboratory and prepared following the same procedure detailed in previous publications (Szopa et al., 2006;Gautier et al., 2014). The initial gas mixture was N 2 :CH 4 = 95:5. ...
Titan is a key planetary body for astrobiology, with the presence of a subsurface ocean and a dense atmosphere, in which complex chemistry is known to occur. Approximately 1-Titan-year after the Cassini-Huygens mission arrived in the saturnian system, Dragonfly rotorcraft will land on Titan's surface by 2034 for an exhaustive geophysical and chemical investigation of the Shangri-La organic sand sea region. Among the four instruments onboard Dragonfly, the Dragonfly Mass Spectrometer (DraMS) is dedicated to analyze the chemical composition of surface samples and noble gases in the atmosphere. One of the DraMS analysis modes, the Gas Chromatograph-Mass Spectrometer (GC-MS), is devoted to the detection and identification of organic molecules that could be involved in the development of a prebiotic chemistry or even representative of traces of past or present life. Therefore, DraMS-GC subsystem should be optimized to detect and identify relevant organic compounds to meet this objective. This work is focused on the experimental methods employed to select the chromatographic column to be integrated in DraMS-GC, to assess the analytical performances of the column selected, and also to assess the performances of the second DraMS-GC column, which is devoted to the separation of organic enantiomers. Four different stationary phases have been tested to select the most relevant one for the separation of the targeted chemical species. The results show that the stationary phase composed of polymethyl (95%) diphenyl (5%) siloxane is the best compromise in terms of efficiency, robustness, and retention times of the molecules. The combination of the general and the chiral columns in DraMS is perfectly suited to in situ chemical analysis on Titan and for the detection of expected diverse and complex organic compounds.
... Tholins, complex organic compounds rich in carbon and nitrogen used as analogs to astrophysical solid matter (Sagan & Khare 1979), were produced using the PAMPRE setup, a low pressure (0.95 mbar) radiofrequency (RF) plasma reactor located at LATMOS (Guyancourt, France) (Szopa et al. 2006). A 13.56 MHz RF power source tuned at 30 W generates a capacitively coupled plasma (CCP) fed by a gas mixture of N 2 :CH 4 = 95:5, conditions chosen for optimal tholin production (Sciamma-O'Brien et al. 2012). ...
The deuterium enrichment of organics in the interstellar medium, protoplanetary disks and meteorites has been proposed to be the result of ionizing radiation. The goal of this study is to quantify the effects of soft X-rays (0.1 - 2 keV), a component of stellar radiation fields illuminating protoplanetary disks, on the refractory organics present in the disks. We prepared tholins, nitrogen-rich complex organics, via plasma deposition and used synchrotron radiation to simulate X-ray fluences in protoplanetary disks. Controlled irradiation experiments at 0.5 and 1.3 keV were performed at the SEXTANTS beam line of the SOLEIL synchrotron, and were followed by ex-situ infrared, Raman and isotopic diagnostics. Infrared spectroscopy revealed the loss of singly-bonded groups (N-H, C-H and R-NC) and the formation of sp carbon defects. Raman analysis revealed the introduction of defects and structural amorphization. Finally, tholins were measured via secondary ion mass spectrometry (SIMS), revealing that significant D-enrichment is induced by X-ray irradiation. Our results are compared to previous experimental studies involving the thermal degradation and electron irradiation of organics. The penetration depth of soft X-rays in m-sized tholins leads to volume rather than surface modifications: lower energy X-rays (0.5 keV) induce a larger D-enrichment than 1.3 keV X-rays, reaching a plateau for doses larger than 5 10 eV cm. Our work provides experimental evidence of a new non-thermal pathway to deuterium fractionation of organic matter.
... Examples include comet tails, 1 Saturn's rings, 2 interstellar dust clouds, 3,4 noctilucent clouds in the Earth's mesosphere, 5,6 and aerosols in the atmospheres of Titan and Pluto. 7,8 In astrophysical and atmospheric plasmas, ionization of a background gas is typically caused by stellar radiation, x-rays, or cosmic rays. Electrons and ions from the background plasma impact the dust grains and stick, causing them to accumulate large amounts of charge. ...
A model for a weakly ionized dusty plasma is proposed in which UV or x-ray radiation continuously creates free electrons at high energy, which then cool through collisions with a cold neutral gas before recombining. The transition of a free electron from high energy at birth to low energy at demise implies that the electron energy distribution is not the simple Maxwellian of an isolated system in thermal equilibrium, but instead has a high-energy tail that depends on the recombination time. This tail can have a major effect on dust grain charging because the flux of tail electrons can be substantial even if the density of tail electrons is small. Detailed analytic and numerical calculations of dust grain charging show that situations exist in which a small high-energy tail dominates charge behavior. This implies that dust grain charge in terrestrial and space dusty plasmas may be significantly underestimated if a Maxwellian distribution is assumed and the non-thermal dynamics are neglected.
... They compared the optical constants of tholin samples produced with plasma discharge to tholin samples produced with UV irradiation, and found that the latter had much lower k values in the near-infrared (NIR) spectral range. Plasma and UV energy sources are known to lead to variations in the chemistry, as UV sources emitting above 80 nm are unable to directly dissociate nitrogen (small amounts of nitrogen have been found to be incorporated in the chemistry induced by UV sources through indirect reactions, though; e.g., Trainer et al. 2012) while plasma discharges are able to directly dissociate nitrogen (e.g., Szopa et al. 2006). ...
We have measured the complex refractive indices, from 0.4 to 1.6 μ m, of five laboratory-generated organic refractory materials (tholins) produced at low temperature (150 K) using plasma chemistry in the stream of a supersonic expansion in NASA Ames’ COsmic SImulation Chamber (COSmIC) facility. Three samples were produced from N 2 :CH 4 gas precursors (with different voltages inducing different degrees of ionization in the plasma), one sample was produced from N 2 :CH 4 :C 2 H 2 , and one sample was produced from Ar:CH 4 in order to produce a purely carbonaceous sample. The optical constants, n and k , of the samples were determined using spectral reflectance measurements. We observe that both n and k appear to be correlated with the nitrogen content in the solid sample, with samples containing more nitrogen having higher n and k . Comparisons to previous laboratory studies and Titan aerosol optical constants derived from observations show that the COSmIC tholins with a higher nitrogen content (higher n and k ) are closer analogs of Titan aerosols. We also present a new analysis of Cassini Visible Infrared Mapping Spectrometer observations of Titan’s atmosphere in the visible to near infrared using the COSmIC tholin optical constants in a radiative transfer model. The COSmIC tholin sample produced from N 2 :CH 4 with the lowest energy level has a spectral behavior that appears well suited to reproduce the observed Titan aerosol properties. This study has therefore demonstrated that this COSmIC tholin sample has valuable and promising optical properties for the analysis of Cassini’s Titan atmospheric observations.
... Results with the DC discharge in N2-H2 are compared to measurements done on a radiofrequency capacitively coupled plasma (RF CCP) discharge in N2-H2 in similar experimental conditions and with the same mass spectrometer, previously published in Chatain et al. [13], abbreviated 'C20' below. The motivation of this work is that both discharges are used to simulate Titan's ionosphere [8,28]. Therefore, investigating the similarities and differences in the formed species in both discharges is fundamental to draw conclusions on Titan's case. ...
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.
... The experimental basis is the same as described in Chatain et al. (2020a), which should be referred to for more experimental details. First, analogues of Titan's aerosols ('tholins') are produced using the PAMPRE experiment (Szopa et al., 2006), in its standard working conditions (Sciamma-O'Brien et al., 2010). Round grains of 200-300 nm are formed. ...
... The experimental basis is the same as described in Chatain et al. (2020a). First, analogues of Titan's aerosols ('tholins') are produced using the PAMPRE experiment (Szopa et al., 2006), in its standard working conditions (Sciamma-O'Brien et al., 2010). After production, tholins are temporarily exposed to ambient air during their collection and the making of pellets, which consists in the spreading of 1 mg of tholins on stainless steel grids of 1.3 cm in diameter, with 25 μm-large threads and 38 μm-wide meshes. ...
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.
... Generally, experiments that form tholins with CO or CO 2 agree on the overall impact of increasing oxygen: the real optical index n increases toward shorter wavelengths, while k makes them more absorbing in the UV-Vis (Hasenkopf et al. 2010;Gavilan et al. 2017Gavilan et al. , 2018Ugelow et al. 2018;Jovanović et al. 2021). Gavilan et al. (2017, hereafter G17) and Gavilan et al. (2018, hereafter G18) investigated the role of atmospheric CO 2 on the optical properties of tholins prepared using the PAMPRE chamber (Szopa et al. 2006) located at LATMOS (U. Paris-Saclay, France). ...
Photochemical hazes are suspected to obscure molecular features, such as water, from detection in the transmission spectra of exoplanets with atmospheric temperatures <800 K. The opacities of laboratory produced organic compounds (tholins) from Khare et al. have become a standard for modeling haze in exoplanet atmospheres. However, these tholins were grown in an oxygen-free, Titan-like environment that is very different from typical assumptions for exoplanets, where C/O ∼ 0.5. This work presents the 0.13–10 μ m complex refractive indices derived from laboratory transmission measurements of tholins grown in environments with different oxygen abundances. With the increasing uptake of oxygen, absorption increases across the entire wavelength range, and a scattering feature around 6 μ m shifts toward shorter wavelengths and becomes more peaked around 5.8 μ m, due to a C = O stretch resonance. Using GJ 1214 b as a test case, we examine the transmission spectra of a sub-Neptune planet with C/O ratios of solar, 1, and 1000 to evaluate the effective differences between our opacities and those of Khare. For an atmosphere with solar hydrogen and helium abundances, we find a difference of 200–1500 ppm, but for high-metallicity ( Z = 1000) environments, the difference may only be 20 ppm. The 1–2 μ m transmission data for GJ 1214 b rule out the Titan-like haze model, and are more consistent with C/O = 1 and C/O = solar haze models. This work demonstrates that using haze opacities that are more consistent with underlying assumptions about bulk atmospheric composition are important for building self-consistent models that appropriately constrain the atmospheric C/O ratio, even when molecular features are obscured.
... Generally, experiments that form tholins with CO or CO 2 agree on the overall impact of increasing Oxygen: the real optical index n increases towards shorter wavelengths, while k makes them more absorbing in the UV-Vis (Hasenkopf et al. 2010;Ugelow et al. 2018;Gavilan et al. 2017Gavilan et al. , 2018Jovanović et al. 2021). Gavilan et al. (2017, G17) and Gavilan et al. (2018, G18 hereafter) investigated the role of atmospheric CO 2 on the optical properties of tholins prepared using the PAMPRE chamber (Szopa et al. 2006) located at LATMOS (U. Paris-Saclay, France). ...
Photochemical hazes are suspected to obscure molecular features, such as water, from detection in the transmission spectra of exoplanets with atmospheric temperatures < 800 K. The opacities of laboratory produced organic compounds (tholins) from Khare et al. (1984) have become a standard for modeling haze in exoplanet atmospheres. However, these tholins were grown in an oxygen-free, Titan-like environment that is very different from typical assumptions for exoplanets, where C/O~0.5. This work presents the 0.13-10 micron complex refractive indices derived from laboratory transmission measurements of tholins grown in environments with different oxygen abundances. With the increasing uptake of oxygen, absorption increases across the entire wavelength range, and a scattering feature around 6 micron shifts towards shorter wavelengths and becomes more peaked around 5.8 micron, due to a C=O stretch resonance. Using GJ 1214b as a test-case, we examine the transmission spectra of a sub-Neptune planet with C/O ratios of solar, 1, and 1000 to evaluate the effective differences between our opacities and those of Khare. For an atmosphere with solar hydrogen and helium abundances, we find a difference of 200-1500 ppm, but for high-metallicity (Z=1000) environments, the difference may only be 20 ppm. The 1-2 micron transmission data for GJ 1214b rule out the Titan-like haze model, and are more consistent with C/O=1 and C/O=solar haze models. This work demonstrates that using haze opacities that are more consistent with underlying assumptions about bulk atmospheric composition are important for building self-consistent models that appropriately constrain the atmospheric C/O ratio, even when molecular features are obscured.
... Titan tholins were produced with the Production d'Aérosols en Microgravité par Plasma REactifs (PAMPRE) experiment at Laboratoire Atmosphère, Milieux, Observations Spatiales (LATMOS), which simulates Titan's atmosphere by radio-frequency plasma discharge in a gas mixture composed of nitrogen (95%) and methane (5%). The experiment is described in detail in Szopa et al. (2006). In the present work, tholins have quasi-stoechiometric H, C, and N ratios (Sciamma-O'Brien et al. 2011); they present strong mid-infrared absorption bands due to CH and NH functions (Gautier et al. 2012); and they are made of spheres of a few hundred nanometres in diameter (Hadamcik et al. 2009;Perrin et al. 2021). ...
Context. The Martian Moon eXploration mission (MMX) of the Japanese space agency (JAXA) is scheduled to take off in September 2024 to explore Phobos and Deimos – the two martian moons – by in situ observations, but also by a sampling and returning regolith samples to Earth. The origins of Phobos and Deimos are still unknown and their understanding is one of the main goals of the MMX mission. In one scenario, Phobos could be a captured asteroid, as the Phobos spectrum is similar to dark D-type asteroids.
Aims. For the present work, we considered the hypothesis of Phobos being a captured D-type asteroid, and we investigated the detectability of organics on Phobos using laboratory spectral analogues.
Methods. We synthesised a near-infrared spectral analogue of Phobos composed of olivine (77 vol.%, 50–125 µm), hyperfine anthracite (20 vol.%, <1 µm), and organic tholins (3 vol.%, ~400 nm) by measuring the reflectance spectrum from 0.4 to 4.75 µm with the SHADOWS spectrogonio-radiometre developped at IPAG. The best spectral match for a Phobos regolith analogue was chosen based on its reflectance level and spectral slope similarities to Phobos’ observed spectrum. Several samples were then prepared by adding a different volume content of organic matter (Titan tholins). We monitored the 3 µm band attributed to N - H bands stretching modes absorption due to the amine function in the tholins, so as to assess the detectability of the NH-rich organics on Phobos.
Results. We have demonstrated that the organic compounds become detectable for more than 5 vol.% in the mixture. We further studied the observation geometry effects on the absorption band depth and found no significant effect except at large phase angles (>80º). These results will be useful to interpret the data of the MMX Infrared Spectrometer (MIRS) onboard the MMX spacecraft, which will measure the spectral reflectance of Phobos from 0.9 to 3.6 µm.
... As already mentioned, HCN also plays/played a major role in the formation of H/C/N compounds on earth as well as on other celestial bodies, e.g. Titan (Carrasco et al., 2009;Coll et al., 1998;Hörst et al., 2012;Israël et al., 2005;Kaiser and Mebel, 2012;Maillard et al., 2021;Thompson et al., 1994;Rüger et al., 2019;Selliez et al., 2020;Somogyi et al., 2016;Szopa et al., 2006;Vuitton et al., 2010). Titan, the second largest moon in our Solar System, has a dense atmosphere consisting mainly of nitrogen and a few percent methane. ...
Homoleptic cyanide compounds exist of almost all main group elements. While the alkali metals and alkaline earth metals form cyanide salts, the cyanides of the lighter main group elements occur mainly as covalent compounds. This review gives an overview of the status quo of main group element cyanides and cyanido complexes. Information about syntheses are included as well as applications, special substance properties, bond lengths, spectroscopic characteristics and computations. Cyanide chemistry is presented mainly from the field of inorganic chemistry, but aspects of chemical biology and astrophysics are also discussed in relation to cyano compounds.
... We also show the production rates (L. Jovanović, personal communication) of tholin experiments for Pluto gas composition from the PAMPRE experimental set-up (Szopa et al., 2006;Alcouffe et al., 2009) at LATMOS, which used 500 ppm CO in N 2 with either 1% CH 4 to simulate Pluto's atmosphere at altitude of 400 km or 5% CH 4 for an altitude of 600 km, as well as a CO-free experiment with 99% N 2 and 1% CH 4 (Jovanović et al., 2020), all at room temperature. A description of the experimental conditions for both sets of experiments can also be found in Section 3.3.2 in Table 3. ...
... UV lamps (e.g., Clarke et al. 2000;Trainer et al. 2006;Sebree et al. 2018) and synchrotron sources (e.g., Imanaka & Smith 2007;Thissen et al. 2009;Peng et al. 2013) can simulate the solar UV radiation of the mid-to upper atmosphere, while gamma-rays/ soft X-rays mimic protons from Saturn's magnetosphere and cosmic rays in the lower atmosphere (e.g., Ramírez et al. 2001;Pilling et al. 2012). Plasma discharges can simulate the electron bombardment of Titan's upper atmosphere from Saturn's magnetosphere and have a broad energy spectrum resembling the solar spectrum, with a high-energy tail that allows for the dissociation of nitrogen (e.g., Coll et al. 1999;Szopa et al. 2006;He et al. 2012He et al. , 2017Sciamma-O'Brien et al 2017). The choice of energy source, as well as other experimental parameters like the molecular mixing ratio, pressure, or temperature used in the laboratory, can have an impact on chemical and material properties of the tholin samples (e.g., Sciamma-O'Brien et al. 2010Imanaka et al. 2004;Hörst et al. 2018;Mahjoub et al. 2012Mahjoub et al. , 2014. ...
In Titan’s nitrogen-methane atmosphere, photochemistry leads to the production of complex organic particles, forming Titan’s thick haze layers. Laboratory-produced aerosol analogs, or “tholins,” are produced in a number of laboratories; however, most previous studies have investigated analogs produced by only one laboratory rather than a systematic, comparative analysis. In this study, we performed a comparative study of an important material property, the surface energy, of seven tholin samples produced in three independent laboratories under a broad range of experimental conditions, and we explored their commonalities and differences. All seven tholin samples are found to have high surface energies and are therefore highly cohesive. Thus, if the surface sediments on Titan are similar to tholins, future missions such as Dragonfly will likely encounter sticky sediments. We also identified a commonality between all the tholin samples: a high dispersive (nonpolar) surface energy component of at least 30 mJ m ⁻² . This common property could be shared by the actual haze particles on Titan as well. Given that the most abundant species interacting with the haze on Titan (methane, ethane, and nitrogen) are nonpolar in nature, the dispersive surface energy component of the haze particles could be a determinant factor in condensate−haze and haze−lake liquid interactions on Titan. With this common trait of tholin samples, we confirmed the findings of a previous study by Yu et al. that haze particles are likely good cloud condensation nuclei for methane and ethane clouds and would likely be completely wetted by the hydrocarbon lakes on Titan.
... We also show the production rates (L. Jovanović, personal communication) of tholin experiments for Pluto gas composition from the PAMPRE experimental set-up (Alcouffe et al., 2009;Szopa et al., 2006) at LATMOS, which used 500 ppm CO in N 2 with either 1% CH 4 to simulate Pluto's atmosphere at altitude of 400 km or 5% CH 4 for an altitude of 600 km, as well as a CO-free experiment with 99% N 2 and 1% CH 4 (Jovanović et al., 2020), all at room temperature. A description of the experimental conditions for both sets of experiments can also be found in Section 3.3.2 in Table 3. ...
Triton is the largest moon of the Neptune system and possesses a thin nitrogen atmosphere with trace amounts of carbon monoxide and methane, making it of similar composition to that of the dwarf planet Pluto. Like Pluto and Saturn's moon Titan, Triton has a haze layer thought to be composed of organics formed through photochemistry. Here, we perform atmospheric chamber experiments of 0.5% CO and 0.2% CH4 in N2 at 90 K and 1 mbar to generate Triton haze analogs. We then characterize the physical and chemical properties of these particles. We measure their production rate, their bulk composition with combustion analysis, their molecular composition with very high resolution mass spectrometry, and their transmission and reflectance from the optical to the near‐infrared with Fourier Transform Infrared (FTIR) Spectroscopy. We compare these properties to existing measurements of Triton's tenuous atmosphere and surface, as well as contextualize these results in view of all the small, hazy, nitrogen‐rich worlds of our solar system. We find that carbon monoxide present at greater mixing ratios than methane in the atmosphere can lead to significantly oxygen‐ and nitrogen‐rich haze materials. These Triton haze analogs have clear observable signatures in their near‐infrared spectra, which may help us differentiate the mechanisms behind haze formation processes across diverse solar system bodies.
... We also show the production rates (L. Jovanović, personal communication) of tholin experiments for Pluto gas composition from the PAMPRE experimental set-up (Szopa et al., 2006;Alcouffe et al., 2009) at LATMOS, which used 500 ppm CO in N 2 with either 1% CH 4 to simulate Pluto's atmosphere at altitude of 400 km or 5% CH 4 for an altitude of 600 km, as well as a CO-free experiment with 99% N 2 and 1% CH 4 (Jovanović et al., 2020), all at room temperature. A description of the experimental conditions for both sets of experiments can also be found in Section 3.3.2 in Table 3. ...
Triton is the largest moon of the Neptune system and possesses a thin nitrogen atmosphere with trace amounts of carbon monoxide and methane, making it of similar composition to that of the dwarf planet Pluto. Like Pluto and Saturn's moon Titan, Triton has a haze layer thought to be composed of organics formed through photochemistry. Here, we perform atmospheric chamber experiments of 0.5% carbon monoxide and 0.2% methane in molecular nitrogen at 90 K and 1 mbar to generate Triton haze analogues. We then characterize the physical and chemical properties of these particles. We measure their production rate, their bulk composition with combustion analysis, their molecular composition with very high resolution mass spectrometry, and their transmission and reflectance from the optical to the near-infrared (0.4 to 5 microns) with Fourier Transform Infrared (FTIR) spectroscopy. We compare these properties to existing measurements of Triton's tenuous atmosphere and its surface, as well as contextualize these results in view of all the small, hazy nitrogen-rich worlds of our solar system. We find that carbon monoxide present at greater mixing ratios than methane in the atmosphere can lead to significantly oxygen- and nitrogen-rich haze materials. These Triton haze analogues have clear observable signatures in their near-infrared spectra, which may help us differentiate the mechanisms behind haze formation processes across diverse solar system bodies.
... From a general standpoint, it is difficult to conduct a rigorous comparison of the C/N ratios determined for the tholins of our study with other Titan tholins produced in previous, independent studies reported in the literature (e.g., McDonald et al., 1994;Coll et al., 1995Coll et al., , 1997Coll et al., , 1999Coll et al., , 2001McKay, 1996;Sarker et al., 2003;Tran et al., 2003Tran et al., , 2008Imanaka et al., 2004;Curtis et al., 2005;Ferris et al., 2005;Somogyi et al., 2005;Bernard et al., 2006;Szopa et al., 2006;Neish et al., 2008;Quirico et al., 2008;Sekine et al., 2008;Carrasco et al., 2009Carrasco et al., , 2016Imanaka and Smith, 2010;Sciamma-O'Brien et al., 2010;Cable et al., 2012), as these tholins were produced using a wide range of experimental set-ups with many varying parameters (gas mixture composition, energy source, pressure, temperature, etc.) that could all have an impact on the final measured C/N ratios. ...
We have used the Titan Haze Simulation (THS) experimental set-up at NASA Ames' COSmIC facility to produce four laboratory analogs of Titan's aerosols, or tholins. These tholin samples were produced from four gas mixtures, representative of Titan's atmosphere, of initial compositions N2:CH4 (90:10 and 95:5) and N2:CH4:C2H2 (89.5:10:0.5 and 94.5:5:0.5) at 150 K, using a plasma discharge in the stream of a jet-cooled gas expansion. Here we present an ex-situ X-ray absorption near-edge structure (XANES) spectroscopy analysis of these four tholin samples. The C- and N-XANES spectra show the presence of various functional groups (aromatic carbon, imines, nitriles, etc.) whose abundances are correlated with (i) the relative proportions between N2 and CH4, and (ii) the presence or absence of C2H2 in the initial mixtures. In particular, mixtures containing C2H2 result in the formation of tholins consisting of more aromatic structures and displaying larger relative amounts of imines and nitriles, but lower overall nitrogen content. XANES spectroscopy also allowed for the determination of the elemental C/N ratios for the four tholins, which were all found to be low (< 2.5) and consistent with those measured for tholins produced in other, independent experimental set-ups. The C/N ratios of tholins produced from gas mixtures that contained C2H2 (2.2–2.4) were found to be about twice as large as those of tholins produced from gas mixtures that did not contain C2H2 (0.9–1.3). These new experimental results from the COSmIC/THS experiment demonstrate the impact initial precursors have on the nitrogen chemistry during the formation of tholins, which may help in the interpretation of observations and the development of models of Titan's atmosphere.
... UV lamps (e.g., Clarke et al. 2000;Trainer et al. 2006;Sebree et al. 2018) and synchrotron sources (e.g., Imanaka & Smith 2007;Thissen et al. 2009;Peng et al. 2013) can simulate the solar UV radiation of the mid-to upper atmosphere, while gamma rays/soft x-rays mimic protons from Saturn's magnetosphere and cosmic rays in the lower atmosphere (e.g., Ramírez et al. 2001;Pilling et al. 2012). Plasma discharges can simulate the electron bombardment of Titan's upper atmosphere from Saturn's magnetosphere and have a broad energy spectrum resembling the solar spectrum, with a high energy tail that allows for the dissociation of nitrogen (e.g., Coll et al. 1999;Szopa et al. 2006;He et al. 2012;Sciamma-O'Brien et al. 2017;He et al. 2017). The choice of energy source as well as other experimental parameters like the molecular mixing ratio, pressure, or temperature used in the laboratory can have an impact on chemical and material properties of the tholin samples (e.g., Sciamma-O'Brien et al. 2010Imanaka et al. 2004;Hörst et al. 2018;Mahjoub et al. 2012Mahjoub et al. , 2014. ...
In Titan's nitrogen-methane atmosphere, photochemistry leads to the production of complex organic particles, forming Titan's thick haze layers. Laboratory-produced aerosol analogs, or "tholins", are produced in a number of laboratories; however, most previous studies have investigated analogs produced by only one laboratory rather than a systematic, comparative analysis. In this study, we performed a comparative study of an important material property, the surface energy, of seven tholin samples produced in three independent laboratories under a broad range of experimental conditions, and explored their commonalities and differences. All seven tholin samples are found to have high surface energies, and are therefore highly cohesive. Thus, if the surface sediments on Titan are similar to tholins, future missions such as Dragonfly will likely encounter sticky sediments. We also identified a commonality between all the tholin samples: a high dispersive (non-polar) surface energy component of at least 30 mJ/m2. This common property could be shared by the actual haze particles on Titan as well. Given that the most abundant species interacting with the haze on Titan (methane, ethane, and nitrogen) are non-polar in nature, the dispersive surface energy component of the haze particles could be a determinant factor in condensate-haze and haze-lake liquids interactions on Titan. With this common trait of tholin samples, we confirmed the findings of a previous study by Yu et al. (2020) that haze particles are likely good cloud condensation nuclei (CCN) for methane and ethane clouds and would likely be completely wetted by the hydrocarbon lakes on Titan.
Organic molecules are ubiquitous in primitive solar system bodies such as comets and asteroids. These primordial organic compounds may have formed in the interstellar medium and in protoplanetary disks (PPDs) before being accreted and further transformed in the parent bodies of meteorites, icy moons, and dwarf planets. The present study describes the composition of primordial organics analogs produced in a laboratory simulator of the PPD (the Nebulotron experiment at the CRPG laboratory) with nitrogen contents varying from N/C < 0.01 to N/C = 0.63. We present the first Fourier transform ion cyclotron resonance mass spectrometry analysis of these analogs. Several thousands of molecules with masses between m/z 100 and 500 are characterized. The mass spectra show a Gaussian shape with maxima around m/z 250. Highly condensed polyaromatic hydrocarbons (PAH) are the most common compounds identified in the samples with lower nitrogen contents. As the amount of nitrogen increases, a dramatic increase of the chemical diversity is observed. Nitrogen-bearing compounds are also dominated by polyaromatic hydrocarbons (PANH) made of 5- and 6-membered rings containing up to four nitrogen atoms, including triazine and pyrazole rings. Such N-rich aromatic species are expected to decompose easily in the presence of water at higher temperatures. Pure carbon molecules are also observed for samples with relatively small fractions of nitrogen. MS peaks compatible with the presence of amino acids and nucleobases, or their isomers, are detected. When comparing these Nebulotron samples with the insoluble fraction of the Paris meteorite organic matter, we observe that the samples with intermediate N/C ratios bracketing that of the Paris insoluble organic matter (IOM) display relative proportions of the CH, CHO, CHN, and CHNO chemical families also bracketing those of the Paris IOM. Our results support that Nebulotron samples are relevant laboratory analogs of primitive chondritic organic matter.
Experimental studies are key to investigating the physical and chemical processes that drive cloud and haze formation from gas and solid phase molecular precursors in (exo)planetary environments, and validating the theoretical calculations used in models of (exo)planetary atmospheres. They allow characterizing the physical, optical, and chemical properties of laboratory-generated analogs, hence providing critical input parameters to models for observational data analysis. In this paper, we present examples of (1) experiments performed with different facilities to produce analogs of Titan and exoplanet atmospheric aerosols from gas phase molecular precursors, and (2) the characterization of these analogs to provide information on their composition, morphology, and optical constants to the scientific community. We also introduce the recently launched NASA Center for Optical Constants (NCOC), which will provide this critical data to the scientific community for (exo)planetary-relevant ices and organic refractory materials produced in the laboratory from the irradiation of gas and ice precursors.
Complex organic aerosols in the upper atmosphere of Saturn's moon Titan reach the troposphere and surface, where a methane (CH4)‐based hydrological cycle occur. Previous studies have assumed no interactions between organic aerosols and liquid CH4, although the dissolution of low‐molecular‐weight photochemical products in liquid CH4 has been considered. Here we report experimental results of soaking a laboratory analog (so‐called tholin) of Titan's organic aerosols in liquid CH4 at 93–98 K for several hours and then evaporating the liquid, simulating wet–dry cycling on Titan. After wet–dry cycling, residual tholin particles form aggregates through cementation. Solid evaporitic deposits formed by evaporation of interacted liquid contain nitrogen‐bearing aromatics, suggesting selective dissolution of aromatics. Our results suggest that organic aerosols or high‐molecular‐weight compounds adsorbed on them partly dissolve in liquid CH4 on Titan, even during short‐term wetting events, promoting the growth of aerosols to dune particles via aggregation and providing aromatics to evaporites.
Titan has a diverse range of materials in its atmosphere and on its surface: the simple organics that reside in various phases (gas, liquid, and ice) and the solid complex refractory organics that form Titan’s haze layers. These materials all actively participate in various physical processes on Titan, and many material properties are found to be important in shaping these processes. Future in situ explorations on Titan would likely encounter a range of materials, and a comprehensive database to archive the material properties of all possible material candidates will be needed. Here, we summarize several important material properties of the organic liquids, ices, and the refractory hazes on Titan that are available in the literature and/or that we have computed. These properties include thermodynamic properties (phase-change points, sublimation and vaporization saturation vapor pressure, and latent heat), and physical properties (organic liquid densities and organic ice and haze densities). We have developed a new database to provide a repository for these data and make them available to the science community. These data can be used as inputs for various theoretical models to interpret current and future remote sensing and in situ atmospheric and surface measurements on Titan. The material properties of the simple organics may also be applicable to giant planets and icy bodies in the outer solar system, interstellar medium, protoplanetary disks, and exoplanets.
Abstract 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.
Microwave-enhanced methane and nitrogen plasma is known to produce dense and highly active plasma species at atmospheric pressure. The main challenge in simultaneous production of ethylene and ammonia under plasma...
Titan hazes, and their laboratory analogs, named tholins, have been extensively studied in the laboratory. Previously, cold plasmas, such as radio frequency discharge, were the dominant plasma sources to produce energetic electrons and ions to simulate photoelectrons. In this work, the electron cyclotron resonance (ECR) discharge plasma was extended to study the formation of hazes encountered in Titan's upper atmosphere. ECR plasma has many advantages such as the electrons being cyclotron accelerated with a broad-energy-spectrum that can dissociate nitrogen, and with the temperature of the ions and neutrals remaining at room temperature. In addition, ECR plasma is free of radio-frequency interference, which ensures that the background parameters and processes can be precisely measured. The N2/CH4 gas mixtures were discharged under different pressures and CH4 mixing ratios, and the basic plasma parameters, emission spectra and gas composition, as well as the parameters related to the molecular weight functional groups of the reaction products were detected simultaneously during the reaction. The optical emission spectra show that the increasing production of CN species was observed with the addition of CH4. In addition, the solid products were collected and analyzed using infrared spectroscopy and mass spectrometry, and the results are generally consistent with previous works. Therefore, the ECR plasma has unique advantages and can provide an alternative energy source to study the generation of hazes encountered in Titan's upper atmosphere.
Titan, the largest of Saturn's moons, shares remarkable similarities with Earth. Its thick atmosphere is composed primarily of nitrogen; it features the most complex organic chemistry known outside of Earth and, uniquely, hosts an analog to Earth's hydrological cycle, with methane forming clouds, rain and seas. Using the latest data from the ongoing Cassini–Huygens missions, laboratory measurements and numerical simulations, this comprehensive reference examines the physical processes that shape Titan's fascinating atmospheric structure and chemistry, weather, climate, circulation and surface geology. The text also surveys leading theories about Titan's origin and evolution, and assesses their implications for understanding the formation of other complex planetary bodies. Written by an international team of specialists, chapters offer detailed, comparative treatments of Titan's known properties and discuss the latest frontiers in the Cassini–Huygens mission, offering students and researchers of planetary science, geology, astronomy and space physics an insightful reference and guide.
Titan, the largest of Saturn's moons, shares remarkable similarities with Earth. Its thick atmosphere is composed primarily of nitrogen; it features the most complex organic chemistry known outside of Earth and, uniquely, hosts an analog to Earth's hydrological cycle, with methane forming clouds, rain and seas. Using the latest data from the ongoing Cassini–Huygens missions, laboratory measurements and numerical simulations, this comprehensive reference examines the physical processes that shape Titan's fascinating atmospheric structure and chemistry, weather, climate, circulation and surface geology. The text also surveys leading theories about Titan's origin and evolution, and assesses their implications for understanding the formation of other complex planetary bodies. Written by an international team of specialists, chapters offer detailed, comparative treatments of Titan's known properties and discuss the latest frontiers in the Cassini–Huygens mission, offering students and researchers of planetary science, geology, astronomy and space physics an insightful reference and guide.
Titan, the largest of Saturn's moons, shares remarkable similarities with Earth. Its thick atmosphere is composed primarily of nitrogen; it features the most complex organic chemistry known outside of Earth and, uniquely, hosts an analog to Earth's hydrological cycle, with methane forming clouds, rain and seas. Using the latest data from the ongoing Cassini–Huygens missions, laboratory measurements and numerical simulations, this comprehensive reference examines the physical processes that shape Titan's fascinating atmospheric structure and chemistry, weather, climate, circulation and surface geology. The text also surveys leading theories about Titan's origin and evolution, and assesses their implications for understanding the formation of other complex planetary bodies. Written by an international team of specialists, chapters offer detailed, comparative treatments of Titan's known properties and discuss the latest frontiers in the Cassini–Huygens mission, offering students and researchers of planetary science, geology, astronomy and space physics an insightful reference and guide.
Titan, the largest of Saturn's moons, shares remarkable similarities with Earth. Its thick atmosphere is composed primarily of nitrogen; it features the most complex organic chemistry known outside of Earth and, uniquely, hosts an analog to Earth's hydrological cycle, with methane forming clouds, rain and seas. Using the latest data from the ongoing Cassini–Huygens missions, laboratory measurements and numerical simulations, this comprehensive reference examines the physical processes that shape Titan's fascinating atmospheric structure and chemistry, weather, climate, circulation and surface geology. The text also surveys leading theories about Titan's origin and evolution, and assesses their implications for understanding the formation of other complex planetary bodies. Written by an international team of specialists, chapters offer detailed, comparative treatments of Titan's known properties and discuss the latest frontiers in the Cassini–Huygens mission, offering students and researchers of planetary science, geology, astronomy and space physics an insightful reference and guide.
Titan, the largest of Saturn's moons, shares remarkable similarities with Earth. Its thick atmosphere is composed primarily of nitrogen; it features the most complex organic chemistry known outside of Earth and, uniquely, hosts an analog to Earth's hydrological cycle, with methane forming clouds, rain and seas. Using the latest data from the ongoing Cassini–Huygens missions, laboratory measurements and numerical simulations, this comprehensive reference examines the physical processes that shape Titan's fascinating atmospheric structure and chemistry, weather, climate, circulation and surface geology. The text also surveys leading theories about Titan's origin and evolution, and assesses their implications for understanding the formation of other complex planetary bodies. Written by an international team of specialists, chapters offer detailed, comparative treatments of Titan's known properties and discuss the latest frontiers in the Cassini–Huygens mission, offering students and researchers of planetary science, geology, astronomy and space physics an insightful reference and guide.
Titan, the largest of Saturn's moons, shares remarkable similarities with Earth. Its thick atmosphere is composed primarily of nitrogen; it features the most complex organic chemistry known outside of Earth and, uniquely, hosts an analog to Earth's hydrological cycle, with methane forming clouds, rain and seas. Using the latest data from the ongoing Cassini–Huygens missions, laboratory measurements and numerical simulations, this comprehensive reference examines the physical processes that shape Titan's fascinating atmospheric structure and chemistry, weather, climate, circulation and surface geology. The text also surveys leading theories about Titan's origin and evolution, and assesses their implications for understanding the formation of other complex planetary bodies. Written by an international team of specialists, chapters offer detailed, comparative treatments of Titan's known properties and discuss the latest frontiers in the Cassini–Huygens mission, offering students and researchers of planetary science, geology, astronomy and space physics an insightful reference and guide.
Titan, the largest of Saturn's moons, shares remarkable similarities with Earth. Its thick atmosphere is composed primarily of nitrogen; it features the most complex organic chemistry known outside of Earth and, uniquely, hosts an analog to Earth's hydrological cycle, with methane forming clouds, rain and seas. Using the latest data from the ongoing Cassini–Huygens missions, laboratory measurements and numerical simulations, this comprehensive reference examines the physical processes that shape Titan's fascinating atmospheric structure and chemistry, weather, climate, circulation and surface geology. The text also surveys leading theories about Titan's origin and evolution, and assesses their implications for understanding the formation of other complex planetary bodies. Written by an international team of specialists, chapters offer detailed, comparative treatments of Titan's known properties and discuss the latest frontiers in the Cassini–Huygens mission, offering students and researchers of planetary science, geology, astronomy and space physics an insightful reference and guide.
Titan, the largest of Saturn's moons, shares remarkable similarities with Earth. Its thick atmosphere is composed primarily of nitrogen; it features the most complex organic chemistry known outside of Earth and, uniquely, hosts an analog to Earth's hydrological cycle, with methane forming clouds, rain and seas. Using the latest data from the ongoing Cassini–Huygens missions, laboratory measurements and numerical simulations, this comprehensive reference examines the physical processes that shape Titan's fascinating atmospheric structure and chemistry, weather, climate, circulation and surface geology. The text also surveys leading theories about Titan's origin and evolution, and assesses their implications for understanding the formation of other complex planetary bodies. Written by an international team of specialists, chapters offer detailed, comparative treatments of Titan's known properties and discuss the latest frontiers in the Cassini–Huygens mission, offering students and researchers of planetary science, geology, astronomy and space physics an insightful reference and guide.
Titan, the largest of Saturn's moons, shares remarkable similarities with Earth. Its thick atmosphere is composed primarily of nitrogen; it features the most complex organic chemistry known outside of Earth and, uniquely, hosts an analog to Earth's hydrological cycle, with methane forming clouds, rain and seas. Using the latest data from the ongoing Cassini–Huygens missions, laboratory measurements and numerical simulations, this comprehensive reference examines the physical processes that shape Titan's fascinating atmospheric structure and chemistry, weather, climate, circulation and surface geology. The text also surveys leading theories about Titan's origin and evolution, and assesses their implications for understanding the formation of other complex planetary bodies. Written by an international team of specialists, chapters offer detailed, comparative treatments of Titan's known properties and discuss the latest frontiers in the Cassini–Huygens mission, offering students and researchers of planetary science, geology, astronomy and space physics an insightful reference and guide.
Titan, the largest of Saturn's moons, shares remarkable similarities with Earth. Its thick atmosphere is composed primarily of nitrogen; it features the most complex organic chemistry known outside of Earth and, uniquely, hosts an analog to Earth's hydrological cycle, with methane forming clouds, rain and seas. Using the latest data from the ongoing Cassini–Huygens missions, laboratory measurements and numerical simulations, this comprehensive reference examines the physical processes that shape Titan's fascinating atmospheric structure and chemistry, weather, climate, circulation and surface geology. The text also surveys leading theories about Titan's origin and evolution, and assesses their implications for understanding the formation of other complex planetary bodies. Written by an international team of specialists, chapters offer detailed, comparative treatments of Titan's known properties and discuss the latest frontiers in the Cassini–Huygens mission, offering students and researchers of planetary science, geology, astronomy and space physics an insightful reference and guide.
Titan, the largest of Saturn's moons, shares remarkable similarities with Earth. Its thick atmosphere is composed primarily of nitrogen; it features the most complex organic chemistry known outside of Earth and, uniquely, hosts an analog to Earth's hydrological cycle, with methane forming clouds, rain and seas. Using the latest data from the ongoing Cassini–Huygens missions, laboratory measurements and numerical simulations, this comprehensive reference examines the physical processes that shape Titan's fascinating atmospheric structure and chemistry, weather, climate, circulation and surface geology. The text also surveys leading theories about Titan's origin and evolution, and assesses their implications for understanding the formation of other complex planetary bodies. Written by an international team of specialists, chapters offer detailed, comparative treatments of Titan's known properties and discuss the latest frontiers in the Cassini–Huygens mission, offering students and researchers of planetary science, geology, astronomy and space physics an insightful reference and guide.
Titan, the largest of Saturn's moons, shares remarkable similarities with Earth. Its thick atmosphere is composed primarily of nitrogen; it features the most complex organic chemistry known outside of Earth and, uniquely, hosts an analog to Earth's hydrological cycle, with methane forming clouds, rain and seas. Using the latest data from the ongoing Cassini–Huygens missions, laboratory measurements and numerical simulations, this comprehensive reference examines the physical processes that shape Titan's fascinating atmospheric structure and chemistry, weather, climate, circulation and surface geology. The text also surveys leading theories about Titan's origin and evolution, and assesses their implications for understanding the formation of other complex planetary bodies. Written by an international team of specialists, chapters offer detailed, comparative treatments of Titan's known properties and discuss the latest frontiers in the Cassini–Huygens mission, offering students and researchers of planetary science, geology, astronomy and space physics an insightful reference and guide.
Titan has a diverse range of materials in its atmosphere and on its surface: the simple organics that reside in various phases (gas, liquid, ice) and the solid complex refractory organics that form Titan's haze layers. These materials all actively participate in various physical processes on Titan, and many material properties are found to be important in shaping these processes. Future in-situ exploration on Titan would likely encounter a range of materials, and a comprehensive database to archive the material properties of all possible material candidates will be needed. Here we summarize several important material properties of the organic liquids, ices, and the refractory hazes on Titan that are available in the literature and/or that we have computed. These properties include thermodynamic properties (phase change points, sublimation and vaporization saturation vapor pressure, and latent heat), physical property (density), and surface properties (liquid surface tensions and solid surface energies). We have developed a new database to provide a repository for these data and make them available to the science community. These data can be used as inputs for various theoretical models to interpret current and future remote sensing and in-situ atmospheric and surface measurements on Titan. The material properties of the simple organics may also be applicable for giant planets and icy bodies in the outer solar system, interstellar medium, and protoplanetary disks.
On July 14th, 2015, NASA’s New Horizons spacecraft flew by Pluto, revealing a complex atmosphere and surface seen nowhere else in the Solar System. Pluto’s surface ices are composed of molecular nitrogen N2, methane CH4, and carbon monoxide CO. During Pluto’s elliptical orbit, these ices undergo a sublimation/condensation cycle resulting in a tenuous atmosphere (~11 µbar at the surface). This atmosphere is mostly composed of N2 and CH4, with ~500 ppm of CO. Subjected to extreme ultraviolet radiation and Lyman-α photons, it is the place of photochemical aerosol production, aerosols being solid particles in suspension in the atmosphere. The exact processes of formation of these aerosols are however not well constrained yet. These solid particles, whose chemical composition and optical properties are unknown, are observed up to more than 350 km of altitude in the atmosphere of Pluto. Numerical models have shown that the presence of these aerosols in the atmosphere could have an impact on the atmospheric chemistry and climate of Pluto. Moreover, it has been suggested that these aerosols sediment and constitute a source of organic matter on the surface of Pluto.During my Ph.D., I used an experimental approach to study the aerosols of Pluto, from their formation in the upper atmosphere to their evolution on the surface, through their interactions with the atmosphere. The formation of Pluto’s aerosols by photochemistry and their chemical composition are the subjects of the first and second part of this Ph.D. thesis (Chapter III and Chapter IV). The interaction of Pluto’s aerosols with solar radiation and the contribution of photochemical aerosols as a coloring agent on the surface of Pluto are the subjects of the third and fourth part of this Ph.D. thesis (Chapter V and Chapter VI).The experimental setup PAMPRE (Production of Aerosols in Microgravity by REactive Plasma), located at LATMOS, has been used to simulate the atmospheric chemistry of Pluto and to synthesize analogues of photochemical aerosols, usually called "tholins". Experiments have also been performed at GANIL, using the IGLIAS (Irradiation of astrophysical ices) experimental setup. By irradiating tholins with heavy ions, the objective was to simulate the ageing of organic matter on the surface of Pluto due to the charged particles constituting the galactic cosmic rays.Thanks to the physicochemical analyses carried out to characterize the chemical composition of Pluto-simulated atmosphere as well as that of the synthesized aerosol analogues, I was able to conclude to the importance of N2 and CO reactivity in the atmospheric chemistry of Pluto. The nitrogen constituting the molecules produced in the gas phase and ultimately incorporated in the solid particles is included not only in the form of terminal functional groups (amine, nitrile, isocyanide), but also in the form of nitrogen heterocycles (triazine, pyrazole, pyrazine, pyrrole). Regarding oxygen, only terminal oxygenated chemical functions (alcohol, carboxylic acid, carbonyl) were detected. These nitrogenous and/or oxygenated organic molecules are responsible for a strong absorption in the ultraviolet spectral range by Pluto aerosol analogues and a more moderate absorption in the visible and near-infrared. These results are consistent with spectral observations of Pluto’s surface and atmosphere by instruments onboard New Horizons. Finally, thanks to the experiments of irradiation of Pluto aerosol analogues by heavy ions, I was able to conclude that the surface of Pluto is processed by galactic cosmic ray irradiation, probably explaining the characteristic featureless spectra of the Cthulhu region.
L'évolution des réserves de pétrole implique l'utilisation en raffinerie de pétroles bruts non conventionnels, bien souvent plus lourds et donc difficiles à caractériser. Les produits pétroliers sont en effet des mélanges chimiques extrêmement complexes. La partie légère et volatile peut être analysée par chromatographie en phase gazeuse couplée à la spectrométrie de masse (GC/MS), permettant l'identification des composés par l'utilisation de mesures de masses précises et de modèles de fragmentation. Cependant ces techniques sont inadaptées à l'analyse des fractions lourdes. Dans la pratique, la caractérisation des mélanges les plus complexes implique l'utilisation de spectromètres de masse à ultra-haute résolution généralement par analyse directe sans séparation chromatographique. La technique de référence est aujourd’hui la spectrométrie de masse à transformée de Fourier par résonance cyclotronique des ions (FTICR). Grâce à une résolution supérieure à 106 et à une précision de mesure de masse inférieure à 0,1 ppm, cet instrument permet de séparer toutes les espèces présentes dans un produit pétrolier et d'attribuer à chaque valeur de m/z une composition élémentaire unique. Ceci permet d'obtenir très facilement des cartes moléculaires qui peuvent être présentées graphiquement en utilisant le diagramme de Kendrick, le diagramme de van Krevelen ou le nombre d'insaturations (DBE) en fonction du nombre de carbones. Ce travail de thèse a permis grâce à la caractérisation moléculaire de produits pétroliers (Vacuum Gas Oil, Pétroles Bruts, Matériel Interfacial, Asphaltènes et Bio-Oil…) d'aborder la complexité de leur traitement dans l’outil de raffinage. Des protocoles d'analyses des échantillons ont été développés, à l'aide de différentes sources d'ionisation à pression atmosphérique (ESI, APCI et APPI) ainsi que par désorption/ionisation laser (LDI) sur le spectromètre de masse FTICR 12T. Les informations sur le contenu isomérique des produits pétroliers ont ensuite été déterminées grâce à l'apport de la spectrométrie de mobilité ionique (IMS).
Pluto’s fly-by by the New Horizons spacecraft in July 2015 has revealed a dark reddish equatorial region, named Cthulhu, covered by a dark, non-icy material whose origin and composition have yet to be determined. It has been suggested that this material could form from the sedimentation of photochemical aerosols, originating from dissociation and ionisation processes in Pluto’s high atmosphere (similarly to aerosols forming Titan’s haze). This hypothesis is here further investigated by comparing New Horizons spectra collected both in the visible and the near-infrared to laboratory reflectance measurements of analogues of Pluto’s aerosols (Pluto tholins). These aerosols were synthesised in conditions mimicking Pluto’s atmosphere, and their optical and reflectance properties were determined, before being used in Hapke models. In particular, the single scattering albedo and phase function of Pluto tholins were retrieved through Hapke model inversion, performed from laboratory reflectance spectra collected under various geometries. From reconstructed reflectance spectra and direct comparison with New Horizons data, some of these tholins are shown to reproduce the photometric level (i.e. reflectance continuum) reasonably well in the near-infrared. Nevertheless, a misfit of the red visible slope still remains and tholins absorption bands present in the modelled spectra are absent in those collected by the New Horizons instruments. Several hypotheses are considered to explain the absence of these absorption features in LEISA data, namely high porosity effects or GCR irradiation. The formation of highly porous structures, which is currently our preferred scenario, could be promoted by either sublimation of ices initially mixed with the aerosols, or gentle deposition under Pluto’s weak gravity.
One of the most widespread features of stone weathering by gaseous and microparticulate atmospheric pollution consist of a general sulphatation under two major forms: the first is the sulphur deposition on stone subsurface; the second is the growth of gypseous black crusts on part of buildings sheltered from direct or running rain.
The study of numerous modern black crusts reveals microparticles, anthropogenic in origin, cemented by gypsum. On the contrary, ancient crusts are scarcely gypseous and contain mainly unburnt wood debris. The formation of modern black crusts results of sulphur gaseous inputs by atmospheric SO2, and of solid deposition. Microparticles, especially those emitted by heavy fuel oil combustion (fly-ash and soots) are carbon and sulphur composition. They also contain metals, able to induce the oxydation of SO2 in sulphates. The chemical composition and constant presence of the microparticles lead to think that they play a role in black crust formation.
In order to verify this hypothesis and to observe the incipient stage of stone weathering, an atmospheric simulation chamber was performed. Experimental conditions have been choosen, close to real urban atmospheric pollution. Two types of calcareous stones were selected: a limestone and a molassic sandstone. Each test samples were exposed, naked or covered with fly-ash or soots.
In any case, the transformation of calcite to gypsum is observed, either in surface or in subsurface. This sulphatation occurs also in the absence of microparticles. The limestone, covered or not, shows, all over the experiment, a degree of sulphatation higher than the sandstone. These differences depend on chemical, mineralogical and petrophysical properties of both stones.
It has been also demonstrated that the fly-ash emitted by heavy fuel oil combustion are very reactives. They are able to enucleate numerous crystallized species, especially gypsum, when they are deposited on the stones surface. They also induce the development of gypsum on the nearby parts of material. In any case, test samples covered with fly-ash are the most sulphated. Soots do not induce such important transformations. In the experimental conditions, they reduce the sulphatation playing a screen role protecting the stone surface.
The measurements and observations show the incipient stages of sulphated black crusts formation. Substrat sulphatation and sulphated black crusts growth are two independant phenomenons progressing one hand towards the depth, and the other hand from the material-polluted atmosphere interface.
Keywords: sulphated black crusts, limestone, sandstone, atmospheric pollution, sulphur dioxyde, fly-ash, soot, atmospheric simulation chamber, sulphatation, gypsum, scanning electron microscopy
The fluorescence of N{2}(B3Pi_g) and N{2}(C3Pi_u) states is observed in a time afterglow. This fluorescence is due to repopulation via pooling reaction of N{2}(A3Sigma+_u) state. From a kinetic model, it is shown that the N{2}(C3Pi_u) characteristic decay time depends on the atomic nitrogen density. Atomic density is deduced from the observed fluorescence of N{2}(C3Pi_u) for density higher than 5×10^{13} cm^{-3}. This density increases with pulse duration and current intensity. La fluorescence des états N{2}(B3Pi_g) et N{2}(C3Pi_u) est observée dans une post-décharge temporelle. Ces fluorescences sont dues à un repeuplement par la réation de pooling entre états N{2}(A3Sigma+_u). A partir d'un modèle cinétique, il est montré que le temps caractéristique de décroissance de l'état N{2}(C3Pi_u) dépend de la densité d'azote atomique. La densité d'atomes est déduite de la fluorescence observée pour des densités supérieures à 5×10^{13} cm^{-3}. La densité atomique augmente avec la durée et l'intensité de l'impulsion.
PAHs are important components of the interstellar medium and
carbonaceous chondrites, but have never been identified in the reducing
atmospheres of the outer solar system. Incompletely characterized
complex organic solids (tholins) produced by irradiating simulated Titan
atmospheres reproduce well the observed UV/visible/IR optical constants
of the Titan stratospheric haze. Titan tholin and a tholin generated in
a crude simulation of the atmosphere of Jupiter are examined by two-step
laser desorption/multiphoton ionization mass spectrometry. A range of
two- to four-ring PAHs, some with one to four alkylation sites, are
identified, with a net abundance of about 0.0001 g/g (grams per gram) of
tholins produced. Synchronous fluorescence techniques confirm this
detection. Titan tholins have proportionately more one- and two-ring
PAHs than do Jupiter tholins, which in turn have more four-ring and
larger PAHs. The four-ringed PAH chrysene, prominent in some discussions
of interstellar grains, is found in Jupiter tholins.
Time-resolved emission spectroscopy is used to investigate the relaxation of N2(B 3Πg), N2(C 3Πu) and CN(B 2Σ) states in the time afterglow of a low-pressure N2-CH4 pulsed discharge, with time duration of 1 ms and in the range [CH4]/[N2] = 0-2%. The decays in the relative measured concentrations in the afterglow are interpreted by modelling the relaxation of a set of time-varying kinetic master equations for the various species produced in the discharge, with conditions at the beginning of the afterglow calculated from a time-dependent kinetic model for the pulsed discharge. It is observed that the N2(B 3Πg) state is populated in the afterglow mainly via the reaction N2(A 3Σu+) + N2(X 1Σg+, 5≤v≤14)→N2(B 3Πg) + N2(X 1Σg+, v = 0), since the pulse duration is large enough to populate the N2(X 1Σg+, v) levels at its end and, to a smaller extent, also by pooling of N2(A 3Σu+). The N2(C 3Πu) state is populated by pooling of N2(A 3Σu+) only, whereas the CN(B 2Σ) state is created through reactions involving either N2(A 3Σu+) states or N2(X 1Σg+, v) levels in collisions with CN(X 2Σ+) molecules. The agreement between measured and calculated concentrations of N2(B 3Πg) and N2(C 3Πu) states is very good in pure N2 and it may be considered satisfactory in the case of N2-CH4 mixtures, and for the CN(B 2Σ) state the agreement between theory and experiment is also reasonably good.
Mass spectrometry and optical emission spectroscopy are used in a N2-xCH4 glow discharge with x = 0.5-2%, at low pressures (1-2 Torr) and small flow rates (6 sccm), in order to determine the CH4 and H2 absolute concentrations and the N2(B 3g) and N2(C 3u) relative concentrations. A kinetic model is developed based on the steady-state solutions to the homogeneous electron Boltzmann equation coupled to a system of rate balance equations for the most populated neutral and ionic species produced, either from active nitrogen and CH4 dissociation or as a result of reactions between radicals from N2 and CH4. It is observed that CH4 is very efficiently decomposed through a sequence of reactions in which at the end HCN and H2 appear as the most abundant products in the discharge. A brown deposition on the tube walls has been detected which is attributed to HCN, in agreement with other investigations of Titan's atmosphere, since this species is poorly destroyed in volume. The accordance between theory and experiment is very satisfactory allowing an insight to be obtained into the basic elementary mechanisms in these discharges.
Titan, the largest satellite of Saturn, has an atmosphere chiefly made up of N2 and CH4, and including many organics. This atmosphere also partly consists of hazes and aerosol particles which shroud the surface of this satellite, giving it a reddish appearance. The aerosols observed in Titan''s atmosphere are thought to be synthesized at high altitudes (>300 km) and fall to the surface. Varying with temperature profiles, condensation phenomena take place in the lower atmosphere, about 100 km below. These solid particles, often called tholins, have been currently investigated for many years by laboratory scientists and physics modellers. This paper assesses past research and results in different fields (elemental composition, optical constants, pyrolysis, particle size), highlighting interests and questions aroused by these studies. It also presents the latest results and advances, and concludes with existing problems and future pathways.
The SOLar SPECtrum (SOLSPEC) and the SOlar SPectrum (SOSP) spectrometers are two twin instruments built to carry out solar spectral irradiance measurements. They are made of three spectrometers dedicated to observations in the ultraviolet, visible and infrared domains. SOLSPEC flew with the ATmospheric Laboratory for Applications and Science (ATLAS) while SOSP flew on the EUropean Retrieval CArrier (EURECA) missions. ATLAS1 and 2 data being already published, this paper is mostly dedicated to the ATLAS3 and EURECA data in the IR domain. Comparisons between the ATLAS data sets and the Upper Atmosphere Research Satellite (UARS) results are made. EURECA IR data are shown and compared with previous results. Our best UV, visible and IR spectra are finally merged into a single absolute solar irradiance spectrum covering the 200 to 2400nm domain.
ACP’s main objective is the chemical analysis of the aerosols in Titan’s atmosphere. For this purpose, it will sample the aerosols during descent and prepare the collected matter (by evaporation, pyrolysis and gas products transfer) for analysis by the Huygens Gas Chromatograph Mass Spectrometer (GCMS). A sampling system is required for sampling the aerosols in the 135–32 km and 22–17 km altitude regions of Titan’s atmosphere. A pump unit is used to force the gas flow through a filter. In its sampling position, the filter front face extends a few mm beyond the inlet tube. The oven is a pyrolysis furnace where a heating element can heat the filter and hence the sampled aerosols to 250 °C or 600 °C. The oven contains the filter, which has a thimble-like shape (height 28 mm). For transferring effluent gas and pyrolysis products to GCMS, the carrier gas is a labeled nitrogen 15N2, to avoid unwanted secondary reactions with Titan’s atmospheric nitrogen.
Aeraulic tests under cold temperature conditions were conducted by using a cold gas test system developed by ONERA. The objective of the test was to demonstrate the functional ability of the instrument during the descent of the probe and to understand its thermal behavior, that is to test the performance of all its components, pump unit and mechanisms.
In order to validate ACP’s scientific performance, pyrolysis tests were conducted at LISA on solid phase material synthesized from experimental simulation. The chromatogram obtained by GCMS analysis shows many organic compounds. Some GC peaks appear clearly from the total mass spectra, with specific ions well identified thanks to the very high sensitivity of the mass spectrometer. The program selected for calibrating the flight model is directly linked to the GCMS calibration plan. In order not to pollute the two flight models with products of solid samples such as tholins, we excluded any direct pyrolysis tests through the ACP oven during the first phase of the calibration. Post probe descent simulation of flight results are planned, using the much representative GCMS and ACP spare models.
The Gas Chromatograph Mass Spectrometer (GCMS) on the Huygens Probe will measure the chemical composition of Titan's atmosphere from 170 km altitude (∼1 hPa) to the surface (∼1500 hPa) and determine the isotope ratios of the major gaseous constituents. The GCMS will also analyze gas samples from the Aerosol Collector Pyrolyser (ACP) and may be able to investigate the composition (including isotope ratios) of several candidate surface materials.
The GCMS is a quadrupole mass filter with a secondary electron multiplier detection system and a gas sampling system providing continuous direct atmospheric composition measurements and batch sampling through three gas chromatographic (GC) columns. The mass spectrometer employs five ion sources sequentially feeding the mass analyzer. Three ion sources serve as detectors for the GC columns and two are dedicated to direct atmosphere sampling and ACP gas sampling respectively. The instrument is also equipped with a chemical scrubber cell for noble gas analysis and a sample enrichment cell for selective measurement of high boiling point carbon containing constituents. The mass range is 2 to 141 Dalton and the nominal detection threshold is at a mixing ratio of 10− 8. The data rate available from the Probe system is 885 bit/s. The weight of the instrument is 17.3 kg and the energy required for warm up and 150 minutes of operation is 110 Watt-hours.
A ‘‘reference cell’’ for generating radio‐frequency (rf) glow discharges in gases at a frequency of 13.56 MHz is described. The reference cell provides an experimental platform for comparing plasma measurements carried out in a common reactor geometry by different experimental groups, thereby enhancing the transfer of knowledge and insight gained in rf discharge studies. The results of performing ostensibly identical measurements on six of these cells in five different laboratories are analyzed and discussed. Measurements were made of plasma voltage and current characteristics for discharges in pure argon at specified values of applied voltages, gas pressures, and gas flow rates. Data are presented on relevant electrical quantities derived from Fourier analysis of the voltage and current wave forms. Amplitudes, phase shifts, self‐bias voltages, and power dissipation were measured. Each of the cells was characterized in terms of its measured internal reactive components. Comparing results from different cells provides an indication of the degree of precision needed to define the electrical configuration and operating parameters in order to achieve identical performance at various laboratories. The results show, for example, that the external circuit, including the reactive components of the rf power source, can significantly influence the discharge. Results obtained in reference cells with identical rf power sources demonstrate that considerable progress has been made in developing a phenomenological understanding of the conditions needed to obtain reproducible discharge conditions in independent reference cells.
Titan, the largest satellite of Saturn, has a thick nitrogen/methane atmosphere with a thick global organic haze. A laboratory analogue of Titan's haze, called tholin, was formed in an inductively coupled plasma from nitrogen/methane=90/10 gas mixture at various pressures ranging from 13 to 2300 Pa. Chemical and optical properties of the resulting tholin depend on the deposition pressure in cold plasma. Structural analyses by IR and UV/VIS spectroscopy, microprobe laser desorption/ionization mass spectrometry, and Raman spectroscopy suggest that larger amounts of aromatic ring structures with larger cluster size are formed at lower pressures (13 and 26 Pa) than at higher pressures (160 and 2300 Pa). Nitrogen is more likely to incorporate into carbon networks in tholins formed at lower pressures, while nitrogen is bonded as terminal groups at higher pressures. Elemental analysis reveals that the carbon/nitrogen ratio in tholins increases from 1.5–2 at lower pressures to 3 at 2300 Pa. The increase in the aromatic compounds and the decrease in C/N ratio in tholin formed at low pressures indicate the presence of the nitrogen-containing polycyclic aromatic compounds in tholin formed at low pressures. Tholin formed at high pressure (2300 Pa) consists of a polymer-like branched chain structure terminated with CH3, NH2, and CN with few aromatic compounds. Reddish-brown tholin films formed at low pressures (13–26 Pa) shows stronger absorptions (almost 10 times larger k-value) in the UV/VIS range than the yellowish tholin films formed at high pressures (160 and 2300 Pa). The tholins formed at low pressures may be better representations of Titan's haze than those formed at high pressures, because the optical properties of tholin formed at low pressures agree well with that of Khare et al. (1984a, Icarus 60, 127–137), which have been shown to account for Titan's observed geometric albedo. Thus, the nitrogen-containing polycyclic aromatic compounds we find in tholin formed at low pressure may be present in Titan's haze. These aromatic compounds may have a significant influence on the thermal structure and complex organic chemistry in Titan's atmosphere, because they are efficient absorbers of UV radiation and efficient charge exchange intermediaries. Our results also indicate that the haze layers at various altitudes might have different chemical and optical properties.
Table of Persistent Band Heads.- Individual Band Systems.- Spectra of Deuterides.- Practical Procedure and Precautions.- On the identification of bands.- Sources.- Collimation.- Comparison spectra.- Measurement.- Spurious bands.- Literature.- Description of Plates.- Author Index.
The ultimate aim of the various diagnostic methods applied in fusion research is to obtain information, as detailed as possible, on the local thermodynamic state and of the transport properties of the plasma investigated. Very different diagnostic methods are applied, very often making use of rather different physical effects. There exist authoritive articles and books in which specific or various methods are treated, see e.g. Refs.1–3. In this article some specific diagnostic problems and results will be discussed which are based on the spectroscopy of photons emitted by atoms (ions). First, however, we make some general remarks concerning plasma spectroscopic methods.
Since several years, GPCOS team (Groupe de Physico-Chimie Organique Spatiale) of LISA has developed an experimental program, of which the goal is to simulate Titan's atmospheric chemistry. Analytical techniques has been developed in order to detect and to quantify compounds produced during these experimental simulations : gaseous compounds by IR spectrometry and GC-MS, solid compounds (deposited on the walls of the reactor) by elemental analysis and pyrolysis coupled with GC-MS. However, due to the complexity of the studied chemistry, production mechanisms (especially of Titan's aerosols analogues) were not correctly described. In addition the representativity of the energy in this kind of glow discharge (electrons in place of UV photons), in order to simulate the Titan's stratospheric conditions was seriously contested. Using a kinetic model of the glow discharge for analysing experimental results, we managed to explain mechanisms taking place inside the reactor. We have determined the energy deposited in the discharge by the electrons; detected in situ short time species (radicals, ions, excited species) by UV-Vis emission spectrometry; and compared the evolution of relative abundances of species with the modeling results Even if we detected CH radical, it is not predicted by photochemical models of Titan's atmosphere. Thus, if it is present in Titan's atmosphere, it may participate to the C2H2 formation, which present an underestimated abundance with about 30% by the photochemical models. We also have detected ammonia (NH3) in the major products. Its possible presence in condensed phase on Titan could explain Titan's albedo at about 5 µm. Effectively NH3 ices have a strong absorption feature around 5.25 µm. If its presence is confirmed by CIRS, the IR spectrometer onboard Cassini-Huygens mission, this exo/astrobiological compound could participate to the grow of Titan's aerosols. Notably, it could react with hydrogen cyanide (HCN) in order to form NH4CN which can product, in presence of water, purine bases like adenine and diaminopurine. Coming from cometary and meteoritic impacts, oxygenated compounds (CO, CO2 and H2O) are present in Titan's atmosphere. Thus, we have performed the first experimental simulation with an initial mixture made of N2/CH4/CO (98/1,99/0,01) in order to verify the impact of carbon monoxide (CO), major O-containing compound in Titan's atmosphere, on the produced gaseous phase. We have identified by two analysis techniques (IR spectrometry and CG-MS) oxirane (or ethylene oxide, an O-cyclic molecule), the major O-containing organic product. This compound has been detected in the Interstellar Medium and its possible presence on Titan will be confirmed by CIRS (with a strong feature at 11.4 µm). The evolution as a function of the experimental parameters of : 1. species abundances in the discharge 2. atomic composition of tholins has allowed to propose a mechanism concerning the gaseous organic compounds : an exchange of an hydrogen fixed on a carbon by a C≠N radical. This hypothesis is in agreement with reactions proposed by the atmospheric chemical models : HCN + CN C2N2 + H k = 6,31.10-17 Tg1,57 exp (-50/Tg) cm3.s-1 C2H2 + CN HC3N + H k = 5,67.10-9 Tg-0,55 exp(-4/Tg) cm3 s-1 C2H4 + CN CH2CHCN + H k = 1,25.10-10 (Tg/300)0,7 exp(-30/Tg) cm3 s-1 This kind of reactions could be possible with polyynes in order to product cyanopolyynes : C4H2 + CN HC5N + H k = 2.10-10 cm3 s-1 C6H2 + CN HC7N + H k = 2.10-10 cm3 s-1 C8H2 + CN HC9N + H k = 2.10-10 cm3 s-1 Concerning tholins, the same kind of mechanism could take place on a chemical structure made of conjugated systems. For the first time, the comparison between the energy deposited in the reactor and the one arriving in Titan's atmosphere has been realised, allowing to discuss the energetic representativity of the glow discharge. The power provided by the electrons in the plasma is 108 times bigger than the one brought by photons responsible of CH4 and N2 dissociations (> 10eV ; < 150 nm). A comparison with the production rates of solid compounds shows that the experimental simulation has a production rate about 104 times smaller than the one of Titan (rate relative to the deposited power in the two cases).
Rotational and vibrational temperatures are measured in arc jets by optical emission spectroscopy. The arc jets are produced in nitrogen, air, and in nitrogen methane mixture. In nitrogen jets, the rotational and vibrational temperatures are deduced from the (Delta = 0) transition of the first negative spectra of nitrogen by comparison with calculated spectra. When rotational temperature increases from 2500 K to 5500 K, vibrational temperature increases from 4000 K to 7000 K. In the nitrogen methane mixture, rotational and vibrational temperatures are also deduced from comparison between experimental and calculated spectra of CN. CN spectra is observed far away from the arc generator with a good signal over noise ratio. Downstream the rotational temperature decreases from 5500 K to 400 K and the vibrational temperature from 7000 K to 6000 K.
The shapes and sizes of photochemically produced aerosol particles of polyacetylene, polyethylene, and polyhydrogen cyanide were studied experimentally. All of the single particles were found to be perfectly spherical and semiliquid. However, they aggregate readily, with a sticking coefficient near unity, to form nonspherical particles, which could give rise to the observed polarization from Titan's and Jupiter's upper haze layers. The absorbance of polyacetylene was remeasured and corrected, and it is now much closer to that of polyethylene. The measured real and imaginary indices of refraction of the two materials make them both suitable material for Titan's and Jupiter's upper haze layers. However, the larger abundance and higher rate of polymerization of acetylene would make it the dominant aerosol-forming material in both atmospheres.
C4N2 is an exceptional organic compound in Titan; it is the only one to have been detected in condensed phase but not in gas phase. After the description of the delicate C4N2 laboratory synthesis and of the determination of C4N2 mass spectrum, we report here the first identification of gaseous C4N2 in laboratory simulations of Titan’s atmosphere, using our last experimental system, based on a N2–CH4 cold plasma at low temperature and low pressure. Finally, we discuss the implications of this identification in the frame of remote sensing observations of gaseous C4N2 in Titan’s atmosphere.
This is a critical review and compilation of the observed and predicted spectroscopic data on the molecule N2 and its ions N2 −, N2 +, N2 2+, and the molecule N3. Each electronic band system is discussed in detail, and tables of band origins and heads are given. In addition to the gas phase electronic, electron and Raman spectra, there are also examined the spectra of condensed molecular nitrogen as well as the pressure- and field-induced infrared and microwave absorption. Dissociation energy of N2, predissociations, and perturbations are discussed. Potential energy curves are given, as well as radiative lifetimes, f-values, and Franck-Condon integrals. Molecular constants are listed for the known electronic states. Electronic structure and theoretical calculations are reviewed.
Results are reported from laboratory continuous-flow plasma-discharge experiments designed to simulate the formation of hydrocarbons and nitriles from Nâ and CHâ in the atmosphere of Titan. Gas-chromatography and mass-spectrometry data were obtained in experiments lasting up to 100 h at temperature 295 K and pressure 17 or 0.24 mbar, modeling (1) cosmic-ray-induced processes in the Titan troposphere and (2) processes related to stratospheric aurorae excited by energetic electrons and ions from the Saturn magnetosphere, respectively. The results are presented in extensive tables and graphs, and the 0.24-mbar yields are incorporated into an eddy-mixing model to give stratospheric column abundances and mole fractions in good agreement with Voyager IRIS observations. 60 refs.
A low pressure arcjet is used to create high temperatures and high speed flows in the simulation of flow conditions around a space vehicle during its hypersonic atmospheric reentry. Here, a new optical emission spectroscopy method for measuring temperature from high resolution spectra in such low pressure arcs is presented. The possibility of tuning the arc temperature is demonstrated.
Electron energy distributions have been obtained for electrically excited N2, CO, CO2, and their mixtures by numerically solving the Boltzmann equation for conditions typical of electric discharges. Reported electron cross-section data have been used in the calculation. The calculated distribution functions were found to be markedly non-Maxwellian, having energy variations which reflect the important electron-molecule energy exchange processes. Solution of the electron energy conservation equation using these distribution functions revealed that vibrational and electronic excitation of N2, CO, and CO2 dominates electron-molecule energy exchange processes for average electron energy in the 1-3-eV range typical of electric discharges. Electron-molecule vibrational excitation rates were also evaluated for a variety of gas mixtures and discharge conditions. The importance of these results to molecular gas-discharge lasers is discussed.
Recent radar, microwave and infrared observations of Titan suggest that a significant fraction of the surface may be covered by ice, in conflict with previous photochemical models which suggested a global ocean, 700 m deep, of ethane. We present here results of a new photochemical model, including updated reaction coefficients, and improved treatments of transport and condensation processes, which predict a lower ethane production (<285 m equivalent). We additionally consider the likely existence of a deep porous icy regolith on Titan's surface, which could “hide” the liquid hydrocarbons from observation, while permitting communication with the atmosphere to maintain the observed methane abundance against photolysis. This “shallow, buried ocean” model is compatible with current observational constraints on Titan's surface.
We use an rf discharge to test a new method of using fluctuations in light-scattering signals to provide particle number densities without assuming particle optical properties or shapes. The plane parallel electrode geometry for an rf discharge of silane in argon provides particle sizes which are very dependent on spatial position (0.1 mm sensitivity) and which have densities ranging from 106 to 108 particles/cm3 for estimated radii of 16 nm at a 108 density. The discharge conditions and spatial positions were varied to probe low particle density and high particle density limits of the theory. Under favorable conditions of low particle number in the scattering volume, surprisingly narrow particle size variations of ∼5% were inferred from the unique distributions of light-scattering intensity.
The growth of particle size has been measured in a low-pressure argon-silane plasma using high-resolution transmission electronic microscopy. The results show that formation and growth of dust particles is an homogeneous process; the first generation size distribution is monodispersed; and the growth kinetics reveals a three-step process from molecular ions to large particles. Together with measurements of particle concentration obtained by laser light scattering, these measurements give a clear indication that the growth proceeds through three successive steps: (i) 'rapid' formation of crystalline clusters (as shown by dark-field high-resolution transmission electron microscopy) with concentrations of up to 1010 cm-3; (ii) formation of aggregates, of diameters up to 50 nm, by coagulation (during coagulation the particle concentration decreases dramatically); and (iii) growth of the particles with a constant concentration by surface deposition of SiHx radicals, whilst the numerical density remains constant. Laser-induced particle explosive evaporation has been performed using a XeCl (308 nm) laser. This experiment allowed detection of nanocrystallites and also the beginning of their coagulation and gave clear evidence of the temperature effect on particle formation.
The electron transport parameters and excitation rate coefficients for individual N2(X, v-Y, v') transitions (Y denoting an excited electronic state) were calculated for typical operating conditions of a stationary N2 discharge at low pressures (0.1<or=p<or=2 Torr) and moderate currents (0.5<or=l<or=50 mA) taking into account the coupling between the electron energy distribution function (EDF) and the vibrational distribution function (VDF) of N2(X) molecules. The coupled EDF and VDF were self-consistently calculated by solving simultaneously the Boltzmann equation and the system of rate balance equations for the vibrational levels including e-V, V-V and V-T energy exchange processes. The calculations are based on a complete set of electron cross sections for the transitions X v-Y, v' generated from a consistent set of cross sections for excitation of the entire states Y from X, v=0 using the Franck-Condon approximation.
A review is presented of the phenomena associated with particles in low pressure plasmas. Dust particles which are typically micrometers in diameter have been observed by laser light scattering in various low-pressure, radiofrequency-excited plasmas. Experiments have been designed so that the origin of the dust material is unambiguous and, to some extent, quantitative. The processes involved in the appearance of the mesoscopic dust particles are outlined and compared with our experimental observations. The source material and its required generation rate, nucleation, charging, growth mechanisms, growth rates, and saturation mechanisms are discussed. The mutual influences of dust and plasma, particularly the role of geometric and circuit boundary conditions in laboratory plasmas, are described.
Experimental simulations of organic chemistry taking place in the
atmosphere of Titan are presented in light of the recent atmospheric
composition data provided by the Voyager spacecraft. In the experiments,
a gas mixture of N2 and CH4 in proportions from about 100:1 to 100:4 was
irradiated by UV radiation, electric discharges, an electron beam, gamma
radiation and a proton beam to assess the possible contributions of the
various possible energy sources to atmospheric chemistry on Titan.
Analysis of reaction products by GC/MS reveals UV light to produce
saturated hydrocarbons such as C2H6 and C3H8 but no appreciable amounts
of unsaturated hydrocarbons or nitrogen-containing compounds. Electric
discharges and gamma, beta and proton radiation, however, are found to
produce HCN and more unsaturated than saturated hydrocarbons. Acetylene
is believed to be produced from ethane or ethylene in methanephotolysis,
while HCN may be produced from CH2 radicals. The presence of HCN on
Titan is interpreted as implying that the chemical processes postulated
as involved in the formation of bases and amino acids on the primitive
earth may be common in the solar system.
High densities of submicron particles have been created in an Ar/SiH 4 parallel plate radio‐frequency (rf) discharge. The particles were collected and measured by electron microscopy and the mean particle diameter was found to be 230±60 nm. Laser scattering from the dense clouds of such particles showed that the concentration was 1×108 cm-3. A laser Doppler anemometer was used to measure the particle velocity distribution and hence the mean particle mass. This is consistent with the specific density of the hydrogenated amorphous silicon. The mean velocities of particles were measured at two different gas flows when the discharge was extinguished, so that the particles are neutral and do not interact, and the particles move with the gas velocity. However, during the discharge the particles have almost no mean axial velocity, even though the gas flow is as large as before. This is due to the strong interparticle interactions that keep the particle cloud, as a whole, stationary. The charge on the particles is estimated, leading to a value of the Coulomb coupling parameter of Γ=10. This large value suggests that the particle cloud can be viewed as a Coulomb liquid.
The rates and altitudes for the dissociation of atmospheric constituents of Titan are calculated for solar UV, solar wind protons, interplanetary electrons, Saturn magnetospheric particles, and cosmic rays. The resulting integrated synthesis rates of organic products range from 102–103 g cm−2 over 4.5 × 109 years for high-energy particle sources to 1.3 × 104 g cm−2 for UV at , and to 5.0 × 105 g cm−2 if (acting primarily on C2H2, C2H4, and C4H2) is included. The production rate curves show no localized maxima corresponding to observed altitudes of Titan's hazes and clouds. For simple to moderately complex organic gases in the Titanian atmosphere, condensation occurs below the top of the main cloud deck at 2825 km. Such condensates comprise the principal cloud mass, with molecules of greater complexity condensing at higher altitudes. The scattering optical depths of the condensates of molecules produced in the Titanian mesosphere are as great as ∼ 102/(particulate radius, μm) if column densities of condensed and gas phases are comparable. Visible condensation hazes of more complex organic compounds may occur at altitudes up to ∼ 3060 km provided only that the abundance of organic products declines with molecular mass no faster than laboratory experiments indicate. Typical organics condensing at 2900 km have molecular masses = 100–150 Da. At current rates of production the integrated depth of precipitated organic liquids, ices, and tholins produced over 4.5 × 109 years ranges from a minimum ∼ 100 m to kilometers if UV at is important. The organic nitrogen content of this layer is expected to be ∼ 10−1−10−3 by mass.
The preliminary measurements by Pioneer 11 of the limb darkening and polarization of Titan at red and blue wavelenghts (M. G. Tomasko, 1980,J. Geophys. Res., 85, 5937–5942) are refined and the measurements of the brightness of the integrated disk at phase angles from 22 to 96° are reduced. At 28° phase, Titan's reflectivity in blue light at southern latitudes is as much as 25% greater than that at northern latitudes, comparable to the values observed by Voyager 1 (L. A. Sromovsky et al., 1981,Nature (London), 292, 698–702). In red light the reflectivity is constant to within a few percent for latitudes between 40°S and 60°N. Titan's phase coefficient between 22 and 96° phase angle averages about 0.014 magnitudes/degree in both colors—a value considerably greater than that observed at smaller phase from the Earth. Comparisons of the data with vertically homogeneous multiple-scattering models indicate that the single-scattering phase functions of the aerosols in both colors are rather flat at scattering angles between 80 and 150° with a small peak at larger scattering (i.e., small phase) angles. The models indicate that the phase integral, q, for Titan in both red and blue light is about 1.66 ± 0.1. Together with Younkin's value for the bolometric geometric albedo scaled to a radius of 2825 km, this implies an effective temperature in equilibrium with sunlight of 84 ± 2°K, in agreement with recent thermal measurements. The single-scattering polarizations produced by the particles at 90° scattering angle are quite large, >85% in blue light and >95% in red. A vertically homogeneous model in which the particles are assumed to scatter as spheres cannot simultaneously match the polarization observations in both colors for any refractive index. However, the observed polarizations are most sensitive to the particle properties near optical depth in each color, and so models based on single scattering by spheres can be successful over a range of refractive indices if the size of the particles increases with depth and if the cross section of the particles increases sufficiently rapidly with decreasing wavelenght. For example, with nr = 1.70, the polarization (and the photometry) are reproduced reasonably well in both colors when the area-weighted average radous of the particles, α, is given by α = (0.117 μm)(τred/0.5)0.217. While this model does not reproduce the large increase in brightness from 129 to 160° phase observed by Voyager 1, the observed increase is determined by the properties of the particles in the top few hundredths of an optical depth. Thus the addition of a very thin layer of forward-scattering aerosols on top of the above model offers one way of satisfying both the Pioneer 11 and Voyager 1 observations. Of course, other models, using bimodal size distributions or scattering by nonspherical particles, may also be capable of reproducing these data.
The optical properties of polymers, produced photolytically from ethylene, which was detected in Titan's atmosphere and from acetylene or hydrogen cyanide which may be present there, were studied experimentally. It is shown that an aerosol consisting of polyethylene provides an excellent fit to the variation of Titan's albedo with wavelength, while polymers of acetylene or hydrogen cyanide do not. This fit seems to remove the requirement of nitrogen-bearing polymers, which was proposed earlier to account for Titan's red coloration. Therefore, Titan's coloration does not necessarily imply the presence of nitrogen in its atmosphere. It is also proposed that above the layer of larger aerosol particles, whose scattering determines the phase function, there are smaller particles of the same material, which act as an absorbing haze to darken and slightly redden the underlying aerosol. This high-altitude haze also causes the observed strong limb-darkening.
Calculations of the optical properties of aggregate particles are able to resolve a persistent problem in understanding the shape and size of haze aerosols in the atmospheres of Titan and Jupiter. Most of the photometric and polarimetric observations for Titan can be explained by the presence of aggregate particles whose mean projected area is equal to that of a sphere with radius 0.14 μm, containing monomers with mean radii near 0.06 μm. An additional mode of smaller particles is needed to fit ultraviolet data. Aggregate particles can also account for the observed optical properties of Jupiter's high altitude haze. Knowledge of the size and shape of the particles will allow for more precise estimates of the sedimentation rates and provide a key constraint on the coupled surface/atmosphere evolution of Titan.
As part of a continuing series of experiments on the production of dark reddish organic solids, called tholins, by irradiation of cosmically abundant reducing gases, the synthesis from a simulated Titanian atmosphere of a tholin with a visible reflection spectrum similar to that of the high altitude aerosols responsible for the albedo and reddish color of Titan has been reported and , Orig. Life. 12, 280) and [C. Sagan, B. N. Khare, and J. Lewis, in press. In Saturn (M. S. Matthews and T. Gehrels, Eds.), Univ. of Arizona Press, Tucson]. The determination of the real (n) and imaginary (k) parts of the complex refractive index of thin films of such tholin prepared by continuous D.C. discharge through a 0.9 N2/0.1 CH4 gas mixture at 0.2 mb are reported. For have been determined from a combination of transmittance, specular reflectance, interferometric, Brewster angle, and ellipsometric polarization measurements; experimental uncertainties in n are estimated to be ±0.5, and in k ± 30%. Values of n(≅1.65) and k (≅0.004 to 0.08) in the visible range are consistent with deductions made by ground-based and spacecraft observations of Titan. Maximum values of k (≅0.8) are near 1000 Å, and minimum values (≅4 × 10−4) are near 1.5 μm. Many infrared absorption features are present in k(γ), including the 4.6-μm nitrile band.
An analysis of Titan's solar phase variation as a function of wavelength together with the continuum geometric albedo makes it possible to set limits on the real part of the refractive index and on the average particle size of the aerosol component of Titan's atmosphere: . If nris known can be determined to within a few percent, and varies inversely with nr. Using this information in a two-layer model of a methane-aerosol atmosphere and comparing the result with Titan's visible and near-infrared methane spectrum leads to the conclusion that the top layer of Titan's atmosphere contains 0.01 km atm of methane and 2.5 extinction optical depths of aerosol, while the data are consistent with a bottom layer containing 2.2 km atm of methane and about 7.5 aerosol optical depths for .
The laboratory investigation of the atmospheric photochemistry of planets and satellites is mainly carried out in static systems. These studies are often poor models of chemical processes in atmospheres because: (1) much higher mixing ratios of minor constituents must be used to accurately determine the amount of reactant consumed and to obtain sufficient products for analysis, (2) secondary photolysis of the initial photoproducts often occurs, (3) wall reactions occur, and (4) most of the starting material is converted to products to obtain enough for spectroscopic analysis. The use of a photochemical flow reactor either circumvents or minimizes these problems by using gas mixtures and photolysis conditions more representative of a planetary atmosphere. A gas mixture, composed of a small amount of a reactant gas diluted in a much larger amount of carrier gas, is flowed past a UV lamp for an extended period of time. Unconsumed reactants and products are collected in traps downstream until amounts sufficient for spectral analysis are collected. FTIR and NMR analysis provides structural information and quantitative data on their rates of formation.
The photochemical flow reactor (D.W. Clarke et al., 2000, Icarus 147, 282–291) has been modified to minimize the incorporation of oxygen and other impurities in the photoproducts. A mixture of gases that approximate their mixing ratios on Titan (N2, CH4, H2, C2H2, C2H4, and HC3N) (0.98, 0.018, 0.002, 3.5 × 10−4, 3 × 10−4, 1.7 × 10−5, respectively) was irradiated in the flow photochemical reactor using a 185-nm source to give a Titan haze analog as a solid product. X-ray photoelectron spectroscopy (XPS) gave a composition of 93.3% C, 5.3% N, and 1.4% O. Of the 93.3% carbon, high-resolution XPS revealed that 81.2% was present as CH, CC, and CC groups, 12.1% may be CO, CN, CN, CN, and/or CN groups, 5.3% as a CN group. The peak for N was symmetrical and was assigned to the CN while that for oxygen was assigned to the CO and/or the CO group. Some of these assignments were confirmed by FTIR spectroscopy. The polymeric product had a C:N ratio of 17.6, which is significantly greater than that for Titan haze analogs prepared in discharge reactions. When the polymer was exposed to air for seven days the oxygen content increased by 6% along with an increase in the infrared absorption at 1710 cm−1 assigned to the CO group of a ketone. The oxidation is attributed to the reaction of oxygen with free radicals trapped in the polymer matrix. It is proposed that the photochemical initiation of Titan haze formation from compounds formed from starting materials formed high in Titan’s atmosphere is a more plausible model than haze formed in reactions initiated by solely by discharges. These data will be helpful in the interpretation of the data returned from the Huygens probe of the Cassini mission.
It is found that solid organic material produced by electrical discharge in a simulated Titan atmosphere (10% CH4, 90% N2) has an elemental composition corresponding to C11H11N2. Immersion in liquid ethane followed by filtration indicates that the organic solid is insoluble in ethane and, it is supposed, in methane. An upper limit is placed on the soluble fraction of 0.03% by mass. When the mixing ratio of CH4 is varied from 10 to 100% it is found that the organic solid produced becomes progressively darker in the ultraviolet and violet (0.2–0.4 μm) compared to the visible and near-infrared (0.4–0.75 μm). The optical properties are variable by more than a factor of 2 between experiments. Sedimentation of solid haze material on the surface of Titan represents a comparatively small loss for carbon. For nitrogen, however, the loss rate of 108 N atoms cm−2s−1 in haze material is greater than 10% of the total production of N in the upper atmosphere.
The discovery that Titan, the largest satellite of Saturn, has an atmosphere and that methane is a significant constituent of it, was the starting point for a systematic study of Titan’s atmospheric organic chemistry. Since then, the results from numerous ground-based observations and two flybys of Titan, by Voyager I and II, have led to experimental laboratory simulation studies and photochemical and physical modeling. All these works have provided a more detailed picture of Titan. We report here a continuation of such a study performing an experimental laboratory simulation of Titan’s atmospheric chemistry, and considering the two physical phases involved: gases and aerosols. Concerning the gaseous phase, we report the first detection of C4N2 and we propose possible atmospheric abundances for 70 organic compounds on Titan’s upper atmosphere. Concerning the solid phase, we have characterized aerosol analogues synthesized in conditions close to those of Titan’s environment, using elemental analysis, pyrolysis, solubility studies and infrared spectroscopy.