Photochemical Models Of Titan Research Articles

Low‐Pressure EUV Photochemical Experiments: Insight on the Ion Chemistry Occurring in Titan's Atmosphere

AbstractThanks to the Cassini spacecraft onboard instruments, it has been known that Titan's ionospheric chemistry is complex and the molecular growth is initiated through the photolysis of the most abundant species directly in the upper atmosphere. Among the pool of chemical compounds formed by the photolysis, N‐bearing species are involved in the haze formation, but the chemical incorporation pathways need to be better constrained. In this work, we performed low‐pressure extreme ultraviolet (EUV) photochemical laboratory experiments. The APSIS reactor was filled with a N2/CH4 (90/10%) gas mixture relevant to the upper atmosphere of Titan. The cell was irradiated by using a EUV photon source at 73.6 nm, which has been difficult to produce in the laboratory for previous studies. The photoproducts (both neutral and ionic species) were monitored in situ with a quadrupole mass spectrometer. The chemical pathways are explained by confronting experimental observations and numerical predictions of the photoproducts. The most interesting result in this work is that methanimine was the only stable N‐bearing neutral molecule detected during the experiments, and it relies on N production. This experimental result is in agreement with the relatively high abundance predicted by 1‐D photochemical models of Titan's atmosphere and comforts methanimine as an intermediate toward the formation of complex N‐bearing organic molecules. This experiment is only testing one part of the overall chemical scheme for Titan's upper atmosphere due to the selective wavelength but demonstrates the capability to probe the chemical pathways occurring in Titan's atmosphere by minimizing bias coming from wall surface reactions.

Detection of Propadiene on Titan

Abstract The atmosphere of Titan, the largest moon of Saturn, is rich in organic molecules, and it has been suggested that the moon may serve as an analog for the pre-biotic Earth due to its highly reducing chemistry and existence of global hazes. Photochemical models of Titan have predicted the presence of propadiene (historically referred to as allene), CH2CCH2, an isomer of the well-measured propyne (also called methylacetylene) CH3CCH, but its detection has remained elusive due to insufficient spectroscopic knowledge of the molecule. This has recently been remedied with an updated spectral line list. Here we present the first unambiguous detection of the molecule in any astronomical object, observed with the Texas Echelle Cross Echelle Spectrograph on the NASA Infrared Telescope Facility in 2017 July. We model its emission line near 12 μm and measure a volume mixing ratio of (6.9 ± 0.8) × 10−10 at 175 km, assuming a vertically increasing abundance profile as predicted in photochemical models. Cassini measurements of propyne made during 2017 April indicate that the abundance ratio of propyne to propadiene is 8.2 ± 1.1 at the same altitude. This initial measurement of the molecule in Titan’s stratosphere paves the way toward constraining the amount of atomic hydrogen available on Titan, as well as future mapping of propadiene on Titan from 8 m and larger ground-based observatories, and future detection on other planetary bodies.

FUV Photoionization of Titan Atmospheric Aerosols

Abstract Thanks to the Cassini–Huygens mission, it is now established that the first aerosols in Titan’s upper atmosphere are found from an altitude of ∼1200 km. Once they are formed and through their descent toward the surface, these nanoparticles are submitted to persistent far-ultraviolet (FUV) radiation that can reach lower atmospheric layers. Such an interaction has an impact, especially on the chemistry and charge budget of the atmospheric compounds. Models are useful to understand this photoprocessing, but they lack important input data such as the photoemission threshold or the absolute photoabsorption/emission cross sections of the aerosols. In order to quantify the photoemission processes, analogs of Titan’s aerosols have been studied with the DESIRS FUV beamline at the synchrotron SOLEIL as isolated substrate-free nanoparticles. We present here the corresponding angle-resolved photoelectron spectroscopy data recorded at different FUV photon energies. The results show a very low photoionization threshold (6.0 ± 0.1 eV ∼ 207 nm) and very high absolute ionization cross sections (∼106 Mb), indicating that FUV photoemission from aerosols is an intense source of slow electrons that has to be taken into account in photochemical models of Titan’s atmosphere.

RAPID ASSOCIATION REACTIONS AT LOW PRESSURE: IMPACT ON THE FORMATION OF HYDROCARBONS ON TITAN

Photochemical models of Titan's atmosphere predict that three-body association reactions are the main production route for several major hydrocarbons. The kinetic rate constants of these reactions strongly depend on density and are therefore only important in Titan's lower atmosphere. However, radiative association reactions do not depend on pressure. The possible existence of large rates at low density suggests that association reactions could significantly affect the chemistry of Titan's upper atmosphere and better constraints for them are required. The kinetic parameters of these reactions are extremely difficult to constrain by experimental measurements as the low pressure of Titan's upper atmosphere cannot be reproduced in the laboratory. However, in the recent years, theoretical calculations of kinetics parameters have become more and more reliable. We therefore calculated several radical-radical and radical-molecule association reaction rates using transition state theory. The calculations indicate that association reactions are fast even at low pressure for adducts having as few as four C atoms. These drastic changes have however only moderate consequences for Titan's composition. Locally, mole fractions can vary by as much as one order of magnitude but the column-integrated production and condensation rates of hydrocarbons change only by a factor of a few. We discuss the impact of these results for the organic chemistry. It would be very interesting to check the impact of these new rate constants on other environments, such as giant and extrasolar planets as well as the interstellar medium.

The photochemical model of Titan’s atmosphere and ionosphere: A version without hydrodynamic escape

There are observational and theoretical evidences both in favor of and against hydrodynamic escape (HDE) on Titan, and the problem remains unsolved. A test presented here for a static thermosphere does not support HDE on Titan and Triton but favors HDE on Pluto. Cooling of the atmosphere by the HCN rotational lines is limited by rotational relaxation above 1100 km and self-absorption below 900 km on Titan. HDE can affect the structure and composition of the atmosphere and its evolution. Hydrocarbon, nitrile, and ion chemistries are strongly coupled on Titan, and attempts to calculate them separately may result in significant errors. Here we apply our photochemical model of Titan’s atmosphere and ionosphere to the case of no hydrodynamic escape. Our model is still the only after-Cassini self-consistent model of coupled neutral and ion chemistry. The lack of HDE is a distinct possibility, and comparing models with and without HDE is of practical interest. The mean difference between the models and the neutral and ion compositions observed by INMS are somewhat better for the model with HDE. A reaction of NH 2 with H 2CN suggested by Yelle et al. (2009) reduces but does not remove a significant difference between the ammonia abundances in the models and INMS observations. Losses of methane and nitrogen and production and deposition to the surface of hydrocarbons and nitriles are evaluated in the model, along with lifetimes and evolutionary aspects.

Isomer specific spectroscopy of C10Hn, n = 8–12: Exploring pathways to naphthalene in Titan's atmosphere

Laboratory investigations of the isomer-specific spectroscopy of several C10Hn isomers with n = 8-12 are described, focusing on structures of relevance to the formation or subsequent reaction of naphthalene. The photochemical models of Titan's atmosphere have now progressed to the point that further development of the large-molecule end of the model must recognize and explicitly incorporate the unique spectroscopy, photochemistry, and reactivity of structural isomers. Mass-resolved, resonant two-photon ionization (R2PI) was used to record ultraviolet spectra of specific C10Hn composition, while hole-burning methods were used to resolve the spectra of different structural and conformational isomers under jet-cooled conditions. The R2PI spectrum of a new C10H8 isomer, 1-phenyl-1-butyne-3-ene, is described and contrasted with other C10H8 isomers. The anticipated role for resonance-stabilized radicals is illustrated by studies of the visible spectroscopy of two hydronaphthyl radical isomers, 1-C10H9 and 2-C10H9, and the trihydronaphthyl radical 1,2,3-C10H11. Conformation-specific spectra of an anticipated C10H12 recombination product of benzyl and allyl radicals is also reported. A reaction scheme that fleshes out the experimental data surrounding naphthalene and its hydrogenated radicals and ions is proposed as a basis for future modeling under Titan's conditions.

Chemical dynamics of triacetylene formation and implications to the synthesis of polyynes in Titan's atmosphere

For the last four decades, the role of polyynes such as diacetylene (HCCCCH) and triacetylene (HCCCCCCH) in the chemical evolution of the atmosphere of Saturn's moon Titan has been a subject of vigorous research. These polyacetylenes are thought to serve as an UV radiation shield in planetary environments; thus, acting as prebiotic ozone, and are considered as important constituents of the visible haze layers on Titan. However, the underlying chemical processes that initiate the formation and control the growth of polyynes have been the least understood to date. Here, we present a combined experimental, theoretical, and modeling study on the synthesis of the polyyne triacetylene (HCCCCCCH) via the bimolecular gas phase reaction of the ethynyl radical (CCH) with diacetylene (HCCCCH). This elementary reaction is rapid, has no entrance barrier, and yields the triacetylene molecule via indirect scattering dynamics through complex formation in a single collision event. Photochemical models of Titan's atmosphere imply that triacetylene may serve as a building block to synthesize even more complex polyynes such as tetraacetylene (HCCCCCCCCH).

A photochemical model of Titan's atmosphere and ionosphere

A global-mean model of coupled neutral and ion chemistry on Titan has been developed. Unlike the previous coupled models, the model involves ambipolar diffusion and escape of ions, hydrodynamic escape of light species, and calculates the H 2 and CO densities near the surface that were assigned in some previous models. We tried to reduce the numbers of species and reactions in the model and remove all species and reactions that weakly affect the observed species. Hydrocarbon chemistry is extended to C 12H 10 for neutrals and C 10H + 11 for ions but does not include PAHs. The model involves 415 reactions of 83 neutrals and 33 ions, effects of magnetospheric electrons, protons, and cosmic rays. UV absorption by Titan's haze was calculated using the Huygens observations and a code for the aggregate particles. Hydrocarbon, nitrile, and ion chemistries are strongly coupled on Titan, and attempt to calculate them separately (e.g., in models of ionospheric composition) may result in significant error. The model densities of various species are typically in good agreement with the observations except vertical profiles in the stratosphere that are steeper than the CIRS limb data. (A model with eddy diffusion that facilitates fitting to the CIRS limb data is considered as well.) The CO densities are supported by the O + flux from Saturn's magnetosphere. The ionosphere includes a peak at 80 km formed by the cosmic rays, steplike layers at 500–700 and 700–900 km and a peak at 1060 km (SZA = 60°). Nighttime densities of major ions agree with the INMS data. Ion chemistry dominates in the production of bicyclic aromatic hydrocarbons above 600 km. The model estimates of heavy positive and negative ions are in reasonable agreement with the Cassini results. The major haze production is in the reactions C 6H + C 4H 2, C 3N + C 4H 2, and condensation of hydrocarbons below 100 km. Overall, precipitation rate of the photochemical products is equal to 4–7 kg cm −2 Byr −1 (50–90 m Byr −1 while the global-mean depth of the organic sediments is ∼3 m). Escape rates of methane and hydrogen are 2.9 and 1.4 kg cm −2 Byr −1, respectively. The model does not support the low C/N ratio observed by the Huygens ACP in Titan's haze.

Epistemic bimodality and kinetic hypersensitivity in photochemical models of Titan's atmosphere

We show that photochemical models of Titan's atmosphere can give rise to bimodal distributions in the abundances of some major compounds, like C 2H 2 and C 2H 4. Sensitivity analysis enabled us to identify the causes and conditions of this bimodality. We propose several methods to control this behavior in photochemical models. In particular, we point out the importance of two key reactions and the needs for a critical evaluation of the kinetic data. We also show that the abundances of some compounds are hypersensitive to the ratio [CH 4]/[H], suggesting that a time-dependent variation of this ratio might lead to a real bistability in the high atmosphere of Titan.

Experimental designs for the determination of key reactions in photochemical models: Application to the photochemistry of hydrocarbons in the atmosphere of Titan

Experimental design is known to be a powerful tool that permits to identify the most active factors in an experiment. Many works have shown that this technique can also be applied to the domain of numerical simulation. In the present paper, we show that experimental design-based methods are suitable for sensitivity analysis studies in photochemical models. Such a method is presented and applied to a simple photochemical model of Titan's atmosphere. The key reactions that control the concentration of the main hydrocarbons of the model are identified. Comparison with a global sensitivity method is discussed. We show that experimental design-based methods permit to significantly reduce the number of simulations to identify the key reactions in complex kinetic models.

Photochemical kinetics uncertainties in modeling Titan's atmosphere: First consequences

Uncertainties carried by the different kinetic parameters included in photochemical models of planetary atmospheres have rarely been considered even if they are supposed to be contributing mostly to the inconsistencies between observations and computed predictions. In this paper, we report the first detailed analysis of the propagation of uncertainties carried by the reaction rate coefficients included in an up-to-date photochemical model of Titan's atmosphere. Monte Carlo calculations performed on these reaction rate coefficients have been used to introduce their uncertainties and to investigate their significance on the photochemical modeling of Titan's atmosphere. Crude approximations in the implemented physical processes have been adopted to limit the number of free parameters. This allows us to pinpoint specifically the importance of chemical processes uncertainties in Titan's photochemical models and to evaluate their chemical robustness. First implications of this preliminary study related to purely chemical rate coefficient uncertainties are discussed. They are important enough to question indeed any comparisons between theoretical models with observations as well as any potential conclusions subsequently inferred. Since the latest missions, such as Cassini–Huygens, are likely to induce an ever-increasing interest for such kind of comparing studies, our conclusions show that it is crucial to reform the way we think of, and use, current photochemical models to understand the processes occurring in the atmospheres of the outer Solar System.

Methane photochemistry: A brief review in the frame of a new experimental program of Titan’s atmosphere simulations

Abstract As methane is the main precursor of Titan’s chemistry, its photolytic behaviour is of great importance for the understanding of Titan’s atmospheric system. Thus, the main purpose of this paper is to present a critical review about available laboratory methane photolysis results. The way these results are further exploited in photochemical models of Titan’s atmosphere is also discussed, as those models may lead to different and sometimes contradictory conclusions. Following this literature discussions, pertinent aspects of a new experimental project for a re-examination of methane photochemistry in the frame of new Titan’s atmospheric simulations are also described.

Current state of modeling the photochemistry of Titan's mutually dependent atmosphere and ionosphere

In the context of recent observations, microphysical models, and laboratory data, a photochemical model of Titan's atmosphere, including updated chemistry focusing on rate coefficients and cross sections measured under appropriate conditions, has been developed to increase understanding of these processes and improve upon previous Titan photochemical models. The model employs a two‐stream discrete ordinates method to characterize the transfer of solar radiation, and the effects of electron‐impact, cosmic‐ray deposition, and aerosol opacities from fractal and Mie particles are analyzed. Sensitivity studies demonstrate that an eddy diffusion profile with a homopause level of 850 km and a methane stratospheric mole fraction of 2.2% provides the best fit of stratospheric and upper atmosphere observations and an improved fit over previous Titan photochemical models. Lack of fits for C3H8, HC3N, and possibly C2H3CN can be resolved with adjustments in aerosol opacity. The model presents a benzene profile consistent with its detection in Titan's stratosphere [Coustenis et al., 2003], which may play an important role in the formation of Titan hazes. An electron peak concentration of 4200 cm−3 is calculated, which exceeds observations by 20%, considerably lower than previous ionosphere models. With adjustments in aerosol opacities and surface fluxes the model illustrates that reasonable fits to existing observations are possible with a single eddy diffusion profile, contrary to the conclusions of previous Titan models. These results will aid in the receipt and interpretation of data from Cassini‐Huygens, which will arrive at Titan in 2004 and deploy a probe into Titan's atmosphere in January 2005.

Actinic fluxes in Titan's atmosphere, from one to three dimensions: Application to high‐latitude composition

We present a study on diurnally and annually averaged values of the actinic fluxes used in one‐dimensional (1‐D) photochemical models, as well as a 3‐D radiative transfer model, based on Monte Carlo calculations with application to the atmosphere of Titan. This study shows that the commonly used value <θ> = 30° for the mean incident angle at the equator in photochemical models of Titan is not the best choice, though changing the value has no dramatic effects on photochemistry. The results of the 3‐D code give direct access to the photolysis rates at any point in the atmosphere. The necessity of 3‐D values in a deep atmosphere such as Titan's is demonstrated particularly for high‐latitude winter conditions. These 3‐D photolysis rates are used to model the latitudinal variations of the chemical composition of Titan's atmosphere in a 1‐D photochemical model adapted to different latitudes. This study shows that these kinds of simple photochemical models cannot reproduce the observed latitudinal behavior and that we need to develop real 2‐D photochemical models of Titan's atmosphere.

Modeling of methane photolysis in the reducing atmospheres of the outer solar system

It is important that methane photolysis is accurately treated in models of planetary atmospheres of the outer solar system and of the interstellar medium. Recent measurements of the H atom and H2 yields in the photolysis of methane have been considered and appear to be consistent with quantum yields, at Lyman α (121.6 nm), of (R1) CH3 + H = 0.41, (R2) 1CH2 + H2 = 0.53, (R3) 1CH2 + H + H = 0.0, (R4) 3CH2 + H + H = 0.0, and (R5) CH + H2 + H = 0.06. At shorter wavelengths it is likely that the production of CH will increase due to the decomposition of excited CH3 molecules, although further data are required to be able to accurately model the wavelength dependence of methane photolysis. At the low temperatures of Titan's atmosphere, for example, the possible temperature dependence of CH4 absorption cross sections may need to be considered. By reference to other hydrocarbons, the absorption of methane may increase by 30% or more at 200 K with respect to room temperature values. These dependences, not previously considered in photochemical models of Titan, could significantly change the predicted composition of Titan's atmosphere and the chemical pathways implied.

Rate coefficients for reactions of ethynyl radical (C2H) with HCN and CH3CN: Implications for the formation of complex nitriles on Titan

Rate coefficients for the reactions of C2H+HCN→products and C2H+CH3CN→products have been measured over the temperature range 262–360 K. These experiments represent an ongoing effort to accurately measure reaction rate coefficients of the ethynyl radical, C2H, relevant to planetary atmospheres such as those of Jupiter and Saturn and its satellite Titan. Laser photolysis of C2H2 is used to produce C2H, and transient infrared laser absorption is employed to measure the decay of C2H to obtain the subsequent reaction rates in a transverse flow cell. Rate constants for the reaction C2H+HCN→products are found to increase significantly with increasing temperature and are measured to be (3.9–6.2)×10−13 cm3 molecules−1 s−1 over the temperature range of 297–360 K. The rate constants for the reaction C2H+CH3CN→products are also found to increase substantially with increasing temperature and are measured to be (1.0–2.1)×10−12 cm3 molecules−1 s−1 over the temperature range of 262–360 K. For the reaction C2H+HCN→products, ab initio calculations of transition state structures are used to infer that the major products form via an addition/elimination pathway. The measured rate constants for the reaction of C2H+HCN→products are significantly smaller than values currently employed in photochemical models of Titan, which will affect the HC3N distribution.

Photochemical Modeling of Titan's Atmosphere

We have developed a new photochemical model of Titan's atmosphere which includes all the important compounds and reactions in spherical geometry from the surface to 1240 km. Compared to the previous model of Yung et al. (1984, Astrophys. J. Suppl. 55, 465-506), the most significant recent change in the reactions used is the updated methane photodissociation scheme (Mordaunt et al. 1993, J. Chem. Phys. 98(3), 2054-2065). Moreover, the transfer of the solar radiation in the atmosphere and the photolysis rates have been calculated by using a Monte Carlo code. Finally, the eddy diffusion coefficient profile is adjusted in order to fit the mean vertical distribution of HCN retrieved from millimeter groundbased observations of Tanguy et al. (1990, Icarus, 85, 43-57) using new values for the boundary flux of atomic nitrogen (Strobel et al. 1992, Icarus 100, 512-526). We have run the model in both steady-state and diurnal modes, with 62 species involved in 249 reactions. There is little difference between diurnal and steady-state results. Overall our results are in a closer agreement with the abundances inferred from the Voyager infrared measurements at the equator than the Yung et al. results. We find that the catalytic scheme for H recombination invoked by Yung et al. only slightly improves the model results and we conclude that this scheme is not essential to fit observations.