Titan's Upper Atmosphere Research Articles

Photoreactivity of condensed acetylene on Titan aerosols analogues

Volatile organic molecules formed by photochemistry in the upper atmosphere of Titan can undergo condensation as pure ices in the stratosphere and the troposphere as well as condense as ice layers onto the organic aerosols that are visible as the haze layers of Titan. As solar photons penetrate through Titan's atmosphere, shorter-wavelength photons are attenuated and longer-wavelength photons make it into the lower altitudes, where aerosols become abundant. We conducted an experimental study to evaluate the long wavelength (λ > 300 nm) photoreactivity of these ices accreted on the Titan aerosol-analogs (also known as tholins) made in the laboratory. We have focused on acetylene, the third most abundant hydrocarbon in Titan's atmosphere (after CH4 and C2H6). Further, acetylene is the most abundant unsaturated hydrocarbon in Titan's atmosphere. Our results indicate that the aerosols can act as activation centers to drive the photoreactivity of acetylene with the aerosols at the accretion interface at wavelengths where acetylene-ice alone does not show photoreactivity. We found that along with photochemistry, photodesorption plays an important role. We observed that about 15% of the initial acetylene is photodesorbed, with a photodesorption rate of (2.1 ± 0.2) × 10−6 molecules•photon−1 at 355 nm. This photodesorption is wavelength-dependent, confirming that it is mediated by the UV absorption of the aerosol analogues, similar to photochemistry. We conclude that the UV–Vis properties of aerosols would determine how they evolve further in Titan's atmosphere and on the surface through photochemical alterations involving longer-wavelength photons. Stronger extinction coefficients in the longer wavelength UV–Vis regions (> 300 nm) of the aerosol give higher efficiencies of photodesorption of accreted volatiles as well as photochemical incorporation of unsaturated condensates (such as acetylene) into the aerosols.

Enhanced Airglow Signature Observed at Titan in Response to its Fluctuating Magnetospheric Environment

AbstractOn rare occasions Titan has been observed in the magnetosheath, where the solar wind interferes with the regular magnetic field generated by Saturn. This particular orbital position allows for a deeper investigation of Titan's upper atmosphere response to its highly energetic magnetospheric environment. Although solar extreme ultraviolet radiation is known to be the main source of ionization in Titan's upper atmosphere, magnetospheric particle precipitation can also account for about 10% of the ionization process. We are reporting here on ultraviolet observations of Titan airglow enhancements occurring while the spacecraft and Titan are known to have been exposed to the magnetosheath environment. Using remote sensing Cassini Ultraviolet Imaging Spectrograph observations of Titan in correlation with in situ Cassini Plasma Spectrometer measurements, we present evidence of Titan's upper atmosphere response to an electron burst crossing the magnetosheath at the time of the T32 flyby, on 13 June 2007.

A Model of Titan-like Chemistry to Connect Experiments and Cassini Observations

Abstract A numerical model is presented for interpreting the chemical pathways that lead to the experimental mass spectra acquired in the Titan Haze Simulation (THS) laboratory experiments and for comparing the electron density and temperature of the THS plasma to observations made at Titan by the Cassini spacecraft. The THS plasma is a pulsed glow-discharge experiment designed to simulate the reaction of N2/CH4-dominated gas in Titan's upper atmosphere. The transient, one-dimensional model of THS chemistry tracks the evolution of more than 120 species in the direction of the plasma flow. As the minor species C2H2 and C2H4 are added to the N2/CH4-based mixture, the model correctly predicts the emergence of reaction products with up to five carbon atoms in relative abundances that agree well with measured mass spectra. Chemical growth in Titan's upper atmosphere transpires through ion–neutral and neutral–neutral chemistry, and the main reactions involving a series of known atmospheric species are retrieved from the calculation. The model indicates that the electron density and chemistry are steady during more than 99% of the 300 μs long discharge pulse. The model also suggests that the THS ionization fraction and electron temperature are comparable to those measured in Titan's upper atmosphere. These findings reaffirm that the THS plasma is a controlled analog environment for studying the first and intermediate steps of chemistry in Titan's upper atmosphere.

Experimental and Computational Studies of the Reactions of N and O Atoms with Small Heterocyclic Anions.

The existence of heterocyclic aromatic anions in extraterrestrial environments, such as the upper atmosphere of Titan, has been recently confirmed by data from the Cassini spacecraft. Nitrogen and oxygen atoms are also common species in the ionospheres of planets and moons and in the interstellar medium. In the current work, we extend previous studies to explore the reactivity of five-membered ring aromatic anions that contain nitrogen, oxygen, or sulfur (deprotonated pyrrole, furan, and thiophene) with N and O atoms both experimentally and computationally. Furanide and thiophenide anions react with the N atom by associative electron detachment (AED). All three anions react with the O atom both by AED and by processes that form ionic products. The reaction of pyrrolide anion with the O atom generates only one ionic product C4H3NO-, corresponding to an O addition and H loss process. The corresponding process is observed as the major channel for the reaction of furanide anion with the O atom while other ionic products HCOO- and C2H- are also formed. The reaction of thiophenide with the O atom is more complex, and four ionic products are generated, of which three are sulfur-containing ions. The reaction mechanisms are studied theoretically by employing density functional theory calculations, and spin conversion is found to be critical for understanding some product distributions. This work provides insight into the rich gas-phase chemistry of aromatic ion-atom reactions, which are relevant to ionospheric and interstellar chemistry.

Negative Ions in Space.

Until a decade ago, the only anion observed to play a prominent role in astrophysics was H-. The bound-free transitions in H- dominate the visible opacity in stars with photospheric temperatures less than 7000 K, including the Sun. The H- anion is also believed to have been critical to the formation of molecular hydrogen in the very early evolution of the Universe. Once H2 formed, about 500 000 years after the Big Bang, the expanding gas was able to lose internal gravitational energy and collapse to form stellar objects and "protogalaxies", allowing the creation of heavier elements such as C, N, and O through nucleosynthesis. Although astronomers had considered some processes through which anions might form in interstellar clouds and circumstellar envelopes, including the important role that polycyclic aromatic hydrocarbons might play in this, it was the detection in 2006 of rotational line emission from C6H- that galvanized a systematic study of the abundance, distribution, and chemistry of anions in the interstellar medium. In 2007, the Cassini mission reported the unexpected detection of anions with mass-to-charge ratios of up to ∼10 000 in the upper atmosphere of Titan; this observation likewise instigated the study of fundamental chemical processes involving negative ions among planetary scientists. In this article, we review the observations of anions in interstellar clouds, circumstellar envelopes, Titan, and cometary comae. We then discuss a number of processes by which anions can be created and destroyed in these environments. The derivation of accurate rate coefficients for these processes is an essential input for the chemical kinetic modeling that is necessary to fully extract physics from the observational data. We discuss such models, along with their successes and failings, and finish with an outlook on the future.

Modeling survey of ices in Titan's stratosphere

Processes in Titan's upper atmosphere, such as photochemical destruction of methane along with the destruction of nitrogen molecules from energetic electrons, result in the production of a number of hydrocarbon and nitrile compounds which are capable of condensing in the colder temperatures of Titan's mid to lower stratosphere. Stratospheric ices can contribute to the opacity of Titan's atmosphere as well as affect the chemistry of the more optically thick clouds seen in the troposphere, should they serve as condensation nuclei. We model the microphysics of a dozen trace species in Titan's atmosphere and show the resulting cloud properties. Clouds form and settle into layers between 50 and 80km. Condensation timescales can be slow, with half the species only growing to a radius ≲1μm. Ethane cloud particles grow the largest with radii up to 20µm. Factors such as the vapor pressure equation, nucleation rate, gas abundance, and temperature profile can have a significant effect on the appearance of the cloud particles. Though the data on optical constants is sparse for many of these ices, estimates show opacities of 10−5−10−3 for visible wavelengths.

Reactions of substituted benzene anions with N and O atoms: Chemistry in Titan's upper atmosphere and the interstellar medium.

The likely existence of aromatic anions in many important extraterrestrial environments, from the atmosphere of Titan to the interstellar medium (ISM), is attracting increasing attention. Nitrogen and oxygen atoms are also widely observed in the ISM and in the ionospheres of planets and moons. In the current work, we extend previous studies to explore the reactivity of prototypical aromatic anions (deprotonated toluene, aniline, and phenol) with N and O atoms both experimentally and computationally. The benzyl and anilinide anions both exhibit slow associative electron detachment (AED) processes with N atom, and moderate reactivity with O atom in which AED dominates but ionic products are also formed. The reactivity of phenoxide is dramatically different; there is no measurable reaction with N atom, and the moderate reactivity with O atom produces almost exclusively ionic products. The reaction mechanisms are studied theoretically by employing density functional theory calculations, and spin conversion is found to be critical for understanding some product distributions. This work provides insight into the rich gas-phase chemistry of aromatic ion-atom reactions and their relevance to ionospheric and interstellar chemistry.

Characterization of aromaticity in analogues of titan's atmospheric aerosols with two-step laser desorption ionization mass spectrometry

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 3.28µm 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 analogs, 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 analogs. This analytical technique is based on the ionization of molecules by Resonance Enhanced Multi-Photon Ionization (REMPI) using a λ=248nm 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.

Examining the exobase approximation: DSMC models of Titan's upper atmosphere

Chamberlain ([1963] Planet. Space Sci., 11, 901–960) described the use of the exobase layer to determine escape from planetary atmospheres, below which it is assumed that molecular collisions maintain thermal equilibrium and above which collisions are deemed negligible. De La Haye et al. ([2007] Icarus., 191, 236–250) used this approximation to extract the energy deposition and non-thermal escape rates for Titan's atmosphere by fitting the Cassini Ion Neutral Mass Spectrometer (INMS) density data. De La Haye et al. assumed the gas distributions were composed of an enhanced population of super-thermal molecules (E >> kT) that could be described by a kappa energy distribution function (EDF), and they fit the data using the Liouville theorem. Here we fitted the data again, but we used the conventional form of the kappa EDF. The extracted kappa EDFs were then used with the Direct Simulation Monte Carlo (DSMC) technique (Bird [1994] Molecular Gas Dynamics and the Direct Simulation of Gas Flows) to evaluate the effect of collisions on the exospheric profiles. The INMS density data can be fit reasonably well with thermal and various non-thermal EDFs. However, the extracted energy deposition and escape rates are shown to depend significantly on the assumed exobase altitude, and the usefulness of such fits without directly modeling the collisions is unclear. Our DSMC results indicate that the kappa EDFs used in the Chamberlain approximation can lead to errors in determining the atmospheric temperature profiles and escape rates. Gas kinetic simulations are needed to accurately model measured exospheric density profiles, and to determine the altitude ranges where the Liouville method might be applicable.

Elusive anion growth in Titan’s atmosphere: Low temperature kinetics of the C3N[formula omitted] + HC3N reaction

Ion chemistry appears to be deeply involved in the formation of heavy molecules in the upper atmosphere of Titan. These large species form the seeds of the organic aerosols responsible for the opaque haze surrounding the biggest satellite of Saturn. The chemical pathways involving individual anions remain however mostly unknown. The determination of the rates of the elementary reactions with ions and the identification of the products are essential to the progress in our understanding of Titan’s upper atmosphere. We have taken steps in that direction through the investigation of the low temperature reactivity of C3N−, which was tentatively identified in the spectra measured by the CAPS-ELS instrument of the Cassini spacecraft during its high altitude flybys. The reaction of this anion with HC3N, one of the most abundant trace organics in the atmosphere, has been studied over the 49–294 K temperature range in uniform supersonic flows using the CRESU technique. The proton transfer is found to be the main exit channel (>91%) of the C315N− + HC3N reaction. It remains however indistinguishable with the non-isotopically labeled C314N− reactant. The T−1/2 temperature dependence of this proton transfer reaction and its global rate are reasonably well reproduced theoretically using an average dipole orientation model. A minor exit channel, reactive detachment (< 9%), has also been uncovered, although the nature of the neutral products has not been determined. It is concluded that the C314N− + HC3N reaction cannot contribute to the growth of molecular anions in the upper atmosphere of Titan. Due to the low branching into the neutral exit channel, it cannot contribute either to the growth of neutrals even assuming a complete mass transfer.

Uncertainty for calculating transport on Titan: A probabilistic description of bimolecular diffusion parameters

Bimolecular diffusion coefficients are important parameters used by atmospheric models to calculate altitude profiles of minor constituents in an atmosphere. Unfortunately, laboratory measurements of these coefficients were never conducted at temperature conditions relevant to the atmosphere of Titan. Here we conduct a detailed uncertainty analysis of the bimolecular diffusion coefficient parameters as applied to Titan's upper atmosphere to provide a better understanding of the impact of uncertainty for this parameter on models. Because temperature and pressure conditions are much lower than the laboratory conditions in which bimolecular diffusion parameters were measured, we apply a Bayesian framework, a problem-agnostic framework, to determine parameter estimates and associated uncertainties. We solve the Bayesian calibration problem using the open-source QUESO library which also performs a propagation of uncertainties in the calibrated parameters to temperature and pressure conditions observed in Titan's upper atmosphere. Our results show that, after propagating uncertainty through the Massman model, the uncertainty in molecular diffusion is highly correlated to temperature and we observe no noticeable correlation with pressure. We propagate the calibrated molecular diffusion estimate and associated uncertainty to obtain an estimate with uncertainty due to bimolecular diffusion for the methane molar fraction as a function of altitude. Results show that the uncertainty in methane abundance due to molecular diffusion is in general small compared to eddy diffusion and the chemical kinetics description. However, methane abundance is most sensitive to uncertainty in molecular diffusion above 1200km where the errors are nontrivial and could have important implications for scientific research based on diffusion models in this altitude range.

Influence of local ionization on ionospheric densities in Titan's upper atmosphere

AbstractTitan has the most chemically complex ionosphere of the solar system. The main sources of ions on the dayside are ionization by EUV solar radiation and on the nightside include ionization by precipitated electrons from Saturn's magnetosphere and transport of ions from the dayside, but many questions remain open. How well do models predict local ionization rates? How strongly do the ionization processes drive the ionospheric densities locally? To address these questions, we have carried out an analysis of ion densities from the Ion and Neutral Mass Spectrometer (INMS) from 16 close flybys of Titan's upper atmosphere. Using a simple chemical model applied to the INMS data set, we have calculated the ion production rates and local ionization frequencies associated with primary ions and . We find that on the dayside the solar energy deposition model overestimates the INMS‐derived production rates by a factor of 2. On the nightside, however, the model driven by suprathermal electron intensities from the Cassini Plasma Spectrometer Electron Spectrometer sometimes agrees and other times underestimates the INMS‐derived production rates by a factor of up to 2–3. We find that below 1200 km, all ion number densities correlate with the local ionization frequency, although the correlation is significantly stronger for short‐lived ions than long‐lived ions. Furthermore, we find that, for a given N2 local ionization frequency, has higher densities on the dayside than on the nightside. We explain that this is due to CH4 being more efficiently ionized by solar photons than by magnetospheric electrons for a given amount of N2 ionization.

A combined crossed molecular beam and quasiclassical trajectory study of the Titan-relevant N(2D) + D2O reaction

Following a recent investigation on the N(2D) + H2O reaction [Homayoon et al., J. Phys. Chem. Lett. 5, 3508 (2014)], we report on an experimental and theoretical study of the isotopologue N(2D) + D2O reaction. Crossed molecular beam (CMB) experiments were conducted at a collision energy of 10.3 kcal mol–1. Quasiclassical trajectory calculations were performed on a recent potential energy surface to derive the centre-of-mass functions necessary to simulate the CMB laboratory distributions. Excellent agreement was found. The importance of the channel leading to HON/DON was confirmed. The inclusion of this channel, in addition to that leading to the isomer HNO/DNO, can affect the models considering the coupling between nitrogen and oxygen chemistry in the upper atmosphere of Titan.

The evolution of infalling sulfur species in Titan’s atmosphere

Aims. We studied the hypothesis that micrometeorites and Enceladus’ plume activity could carry sulfur-bearing species into the upper atmosphere of Titan, in a manner similar to oxygen-bearing species. Methods. We have developed a detailed photochemical model of sulfur compounds in the atmosphere of Titan that couples hydrocarbon, nitrogen, oxygen, and sulfur chemistries. Results. Photochemical processes produce mainly CS and H2CS in the upper atmosphere of Titan and C3S, H2S and CH3SH in the lower atmosphere. Mole fractions of these compounds depend significantly on the source of sulfur species. Conclusions. A possible future detection of CS (or the determination of a low upper limit) could be used to distinguish the two scenarios for the origin of sulfur species, which then could help to di erentiate the various scenarios for the origin of H2O, CO, and CO2 in the stratosphere of Titan.

Developing a self‐consistent description of Titan's upper atmosphere without hydrodynamic escape

AbstractIn this study, we develop a best fit description of Titan's upper atmosphere between 500 km and 1500 km, using a one‐dimensional (1‐D) version of the three‐dimensional (3‐D) Titan Global Ionosphere‐Thermosphere Model. For this modeling, we use constraints from several lower atmospheric Cassini‐Huygens investigations and validate our simulation results against in situ Cassini Ion‐Neutral Mass Spectrometer (INMS) measurements of N2, CH4, H2, 40Ar, HCN, and the major stable isotopic ratios of 14N/15N in N2. We focus our investigation on aspects of Titan's upper atmosphere that determine the amount of atmospheric escape required to match the INMS measurements: the amount of turbulence, the inclusion of chemistry, and the effects of including a self‐consistent thermal balance. We systematically examine both hydrodynamic escape scenarios for methane and scenarios with significantly reduced atmospheric escape. Our results show that the optimum configuration of Titan's upper atmosphere is one with a methane homopause near 1000 km and atmospheric escape rates of 1.41–1.47 ×1011 CH4 m−2 s−1 and 1.08 ×1014 H2 m−2 s−1 (scaled relative to the surface). We also demonstrate that simulations consistent with hydrodynamic escape of methane systematically produce inferior fits to the multiple validation points presented here.

THE HIGH-RESOLUTION EXTREME-ULTRAVIOLET SPECTRUM OF N 2 BY ELECTRON IMPACT

We have analyzed high-resolution (FWHM = 0.2 A) extreme-ultraviolet (EUV, 800-1350 A) laboratory emission spectra of molecular nitrogen excited by an electron impact at 20 and 100 eV under (mostly) optically thin, single-scattering experimental conditions. A total of 491 emission features were observed from N2 electronic-vibrational transitions and atomic N I and N II multiplets and their emission cross sections were measured. Molecular emission was observed at vibrationally excited ground-state levels as high as v'' = 17, from the a 1Π g , b 1Π u , and b'1Σ u + excited valence states and the Rydberg series c'n +1 1Σ u +, cn 1Π u , and on 1Π u for n between 3 and 9. The frequently blended molecular emission bands were disentangled with the aid of a sophisticated and predictive quantum-mechanical model of excited states that includes the strong coupling between valence and Rydberg electronic states and the effects of predissociation. Improved model parameters describing electronic transition moments were obtained from the experiment and allowed for a reliable prediction of the vibrationally summed electronic emission cross section, including an extrapolation to unobserved emission bands and those that are optically thick in the experimental spectra. Vibrationally dependent electronic excitation functions were inferred from a comparison of emission features following 20 and 100 eV electron-impact collisional excitation. The electron-impact-induced fluorescence measurements are compared with Cassini Ultraviolet Imaging Spectrograph observations of emissions from Titan's upper atmosphere.

Density waves in Titan's upper atmosphere

Analysis of the Cassini Ion Neutral Mass Spectrometer data reveals the omnipresence of density waves in various constituents of Titan's upper atmosphere, with quasi‐periodical structures visible for N2, CH4,29N2, and some of the minor constituents. The N2 amplitude lies in the range of ≈4%–16%with a mean of ≈8%. Compositional variation is clearly seen as a sequence of decreasing amplitude with increasing scale height. The observed vertical variation of amplitude implies significant wave dissipation in different constituents, possibly contributed by molecular viscosity for N2but by both molecular viscosity and binary diffusion for the others. A wave train with near horizontally propagating wave energy and characterized by a wavelength of ≈730 km and a wave period of ≈10 h is found to best reproduce various aspects of the observations in a globally averaged sense. Some horizontal and seasonal trends in wave activity are identified, suggesting a connection between the mechanism driving the overall variability in the background atmosphere and the mechanism driving the waves. No clear association of wave activity with magnetospheric particle precipitation can be identified from the data.

Low temperature reaction kinetics of CN− + HC3N and implications for the growth of anions in Titan’s atmosphere

The Cassini–Huygens probe has uncovered the existence of a profusion of negatively charged molecular species in the upper atmosphere of Titan (∼950km). The presence of large amounts of anions was unexpected and the chemical pathways leading to their formation mostly unknown. The investigation of the negative ion chemistry appears therefore to be a key factor for modeling Titan’s upper atmosphere. We present here the first low temperature experimental kinetic study involving CN−, proposed by Vuitton et al. (2009) to be one of the negative ions detected by the CAPS-ELS instrument onboard the Cassini spacecraft. The temperature dependence of the rate coefficient of the reaction CN−+HC3N, was explored over the 49–294K temperature range in uniform supersonic flows using the CRESU technique. We find that the kinetics of this reaction is fast (k≳4×10−9cm3molecule−1s−1) and presents a weak negative temperature dependence which, considering the experimental error bars, agrees with long-range based capture theory. We also observe that C3N−+HCN represents the main exit channel demonstrating that the studied reaction participates efficiently to the chemical growth of negative ions in the atmosphere of Titan.

Cyclopropenyl cation – the simplest Huckel's aromatic molecule – and its cyclic methyl derivatives in Titan's upper atmosphere

The recent measurements by Cassini Ion Neutral Mass Spectrometer (INMS) showed the presence of numerous carbocations and shed light on their composition in Titan's upper atmosphere. The present research identifies an important class of ion-molecule reactions proceeding via carbocation collision complexes, and its implications in the chemistry of Titan's thermosphere and ionosphere. An analysis (based on the kinetics and dynamics of the elementary chemical processes identified) of the Cassini measurements reveals the mechanism of formation of the three-membered Huckel aromatic rings-Cyclopropenyl cation and its cyclic methyl derivatives. For carbocations, a nonclassical three-carbon-center two-electron-bond structure is no longer a controversial topic in chemistry literature. Emphasis has been placed on a future coordinated effort of state-of-the-art laboratory experiments, quantum-chemical calculations, and astronomical ALMA and JWST observations including planetary in situ measurements at millimeter and submillimeter wavelengths to elucidate the structure, energetics and dynamics of the compositions of carbocations detected by Cassini cationic mass spectrometry. The carbocation chemistry in Titan's upper atmosphere has a possible bearing on the organic chemistry and aromaticity in the atmosphere of primitive earth.

LARGE ABUNDANCES OF POLYCYCLIC AROMATIC HYDROCARBONS IN TITAN'S UPPER ATMOSPHERE

In this paper, we analyze the strong unidentified emission near 3.28 μm in Titan's upper daytime atmosphere recently discovered by Dinelli et al. We have studied it by using the NASA Ames PAH IR Spectroscopic Database. The polycyclic aromatic hydrocarbons (PAHs), after absorbing UV solar radiation, are able to emit strongly near 3.3 μm. By using current models for the redistribution of the absorbed UV energy, we have explained the observed spectral feature and have derived the vertical distribution of PAH abundances in Titan's upper atmosphere. PAHs have been found to be present in large concentrations, about (2-3) × 104 particles cm–3. The identified PAHs have 9-96 carbons, with a concentration-weighted average of 34 carbons. The mean mass is ~430 u; the mean area is about 0.53 nm2; they are formed by 10-11 rings on average, and about one-third of them contain nitrogen atoms. Recently, benzene together with light aromatic species as well as small concentrations of heavy positive and negative ions have been detected in Titan's upper atmosphere. We suggest that the large concentrations of PAHs found here are the neutral counterpart of those positive and negative ions, which hence supports the theory that the origin of Titan main haze layer is located in the upper atmosphere.