Measured ion-molecule reaction rates for modelling Titan's atmosphere

Rate coefficients for several two- and three-body ion-molecule reactions involving hydrocarbons have been determined at thermal energies and above using drift tube-mass spectrometer techniques. The measured rates for clustering and breakup reactions involving CH₅⁺ and C₂H₅⁺ ions in methane are found to be strongly temperature dependent in the range from 80 to 240 K. The equilibrium constants determined for these reactions differ somewhat from those of Hiraoka and Kebarle. Rate coefficients for two-body reactions of CH₅⁺, C₂H₅⁺, N⁺, H⁺ and D⁺ ions with methane and/or ethane have been measured. The results indicate that the product yields of several of the fast ion-molecule reactions depend strongly on ion energy (temperature), and therefore previous room-temperature results may be of limited value for model calculations of Titan's atmosphere.     

Radial distribution of production rates, loss rates and densities corresponding to ion masses ?40 amu in the inner coma of Comet Halley: Composition and chemistry

In this paper we have studied the chemistry of C, H, N, O, and S compounds corresponding to ions of masses ?40 amu in the inner coma of the Comet 1P/Halley. The production rates, loss rates, and ion mass densities are calculated using the Analytical Yield Spectrum approach and solving coupled continuity equation controlled by the steady state photochemical equilibrium condition. The primary ionization sources in the model are solar EUV photons, photoelectrons, and auroral electrons of the solar wind origin. The chemical model couples ion-neutral, electron-neutral, photon-neutral and electron-ion reactions among ions, neutrals, electrons, and photons through over 600 chemical reactions. Of the 46 ions considered in the model the chemistry of 24 important ions (viz., CH₃OH⁺₂, H₃CO⁺, NH⁺₄, H₃S⁺, H₂CN⁺, H₂O⁺, NH⁺₃, CO⁺, C₃H⁺₃, OH⁺, H₃O⁺, CH₃OH⁺, C₃H⁺₄, C₂H⁺₂, C₂H⁺, HCO⁺, S⁺, CH⁺₃, H₂S⁺, O⁺, C⁺, CH⁺₄, C⁺₂, and O⁺₂) are discussed in this paper. At radial distances <1000 km, the electron density is mainly controlled by 6 ions, viz., NH⁺₄, H₃O⁺, CH₃OH⁺₂, H₃S⁺, H₂CN⁺, and H₂O⁺, in the decreasing order of their relative contribution. However, at distances >1000 km, the 6 major ions are H₃O⁺, CH₃OH⁺₂, H₂O⁺, H₃CO⁺, C₂H⁺₂, and NH⁺₄; along with ions CO⁺, OH⁺, and HCO⁺, whose importance increases with further increase in the radial distance. It is found that at radial distances greater than ∼1000 km (±500 km) the major chemical processes that govern the production and loss of several of the important ions in the inner coma are different from those that dominate at distances below this value. The importance of photoelectron impact ionization, and the relative contributions of solar EUV, and auroral and photoelectron ionization sources in the inner coma are clearly revealed by the present study. The calculated ion mass densities are compared with the Giotto Ion Mass Spectrometer (IMS) and Neutral Mass Spectrometer (NMS) data at radial distances 1500, 3500, and 6000 km. There is a reasonable agreement between the model calculation and the Giotto measurements. The nine major peaks in the IMS spectra between masses 10 and 40 amu are reproduced fairly well by the model within a factor of two inside the ionopause. We have presented simple formulae for calculating densities of the nine major ions, which contribute to the nine major peaks in the IMS spectra, throughout the inner coma that will be useful in estimating their densities without running the complex chemical models.     

The lower ionosphere of Titan

Ionization of the atmosphere of Titan by galactic cosmic rays is a very significant process throughout the altitude range of 100 to 400 km. An approximate form of the Boltzmann equation for cosmic ray transport has been used to obtain local ionization rates. Models of both ion and neutral chemistry have been employed to compute electron and ion density profiles for three different values of the H₂/CH₄ abundance ratio. The peak electron density is of the order 10³ cm^?3. The most abundant positive ions are C₂H₉⁺ and C₃H₉⁺, while the predicted densities of the negative ions H^? and CH₃^? are very small (<10^?4 that of the positive ions). It is suggested that inclusion of the ion chemistry is important in the computation of the H and CH₃ density profiles in the lower ionosphere.     

Products of meteoric metal ion chemistry within planetary atmospheres. 1. Mg at Titan

We report results of quantum chemical calculations of Mg⁺/ligand bond dissociation energies involving ligands identified as major constituents of Titan's upper atmosphere. Trends identified in these results allow elucidation of the important bimolecular and termolecular reactions of Mg⁺, and of simple molecular ions containing Mg⁺, arising from meteoric infall into Titan's atmosphere. Our study highlights, and includes calculated rate coefficients for, crucial ligand-switching and ligand-stripping reactions which ensure that a dynamic equilibrium exists between atomic and molecular ions of Mg⁺. Neutralization of ionized meteoric Mg is expected to produce the radical MgNC in high yield. The highly polar MgNC radical should provide an excellent nucleation site for condensation of polar (e.g., HCN, CH₃CN, and HC₃N) and highly unsaturated (e.g., C₂H₂, C₄H₂, and C₂N₂) neutrals at comparatively high altitude, leading to precipitation of Mg-doped tholin-like material. The implications for Titan's prebiotic chemical evolution, of the surface deposition of such material (which may feasibly contain magnesium porphyrins, or other bioactive Mg-containing complexes) remain to be assessed.     

Ion chemistry in the cometary atmosphere

Dirty ice of a second kind (major components, H₂O, CO, and N₂; minor components less than several percents, NH₃, CH₄, and other organic substances such as HCN, CH₃CN etc.) is assumed for the composition of volatiles in the cometary nucleus. The consistency with the observations of molecular ions and daughter molecules in the cometary atmosphere is argued by taking into account various ion-molecular reactions and dissociative recombinations. There is a satisfactory agreement for the second kind of dirty-ice model, but the presence of large amounts of CH₄ and NH₃ is found to be rather in contradiction with observational evidence. A velocity of 8 km s^?1 for the hydrogen atoms, derived from analysis of the hydrogen Lyman-alpha corona around comets, is found from the dissociative recombination of H₃O⁺, the dominant constituent of cometary ionosphere, in accordance with H₃O⁺+e ^?→OH+H+H.     

The role of hydrocarbons in the lonospheres of the outer planets

The role of hydrocarbons as a possible sink for H⁺ and H₃⁺ ions in the lower ionosphere of the outer planets is examined. Calculations indicate that H⁺ and H₃⁺ are efficiently converted to hydrocarbon ions on reaction with methane. The terminal ions, CH₅⁺ and C₂H₅⁺ are rapidly neutralized in dissociative recombination with electrons. Extreme ultraviolet photolysis of hydrocarbons as a potential additional source of lower elevation ions in investigated.     

Ion composition and chemistry in the coma of Comet 1P/Halley—A comparison between Giotto's Ion Mass Spectrometer and our ion-chemical network

In order to understand the cometary plasma environment it is important to track the closely linked chemical reactions that dominate ion evolution. We used a coupled MHD ion-chemistry model to analyze previously unpublished Giotto High Intensity Ion Mass Spectrometer (HIS-IMS) data. In this way we study the major species, but we also try to match some minor species like the CH_x and the NH_x groups. Crucial for this match is the model used for the electrons since they are important for ion-electron recombination. To further improve our results we included an enhanced density of supersonic electrons in the ion pile-up region which increases the local electron impact ionization. In this paper we discuss the results for the following important ions: C⁺, CH⁺, CH⁺₂, CH⁺₃, N⁺, NH⁺, NH⁺₂, NH⁺₃, NH⁺₄, O⁺, OH⁺, H₂O⁺, H₃O⁺, CO⁺, HCO⁺, H₃CO⁺, and CH₃OH⁺₂. We also address the inner shock which is very distinctive in our MHD model as well as in the IMS data. It is located just inside the contact surface at approximately 4550 km. Comparisons of the ion bulk flow directions and velocities from our MHD model with the data measured by the HIS-IMS give indication for a solar wind magnetic field direction different from the standard Parker angle at Halley's position. Our ion-chemical network model results are in a good agreement with the experimental data. In order to achieve the presented results we included an additional short lived inner source for the C⁺, CH⁺, and CH⁺₂ ions. Furthermore we performed our simulations with two different production rates to better match the measurements which is an indication for a change and/or an asymmetric pattern (e.g. jets) in the production rate during Giotto's fly-by at Halley's comet.     

Structure and time variations of the Jovian ionosphere

The problem of the ionospheric formation in the Jovian upper atmosphere is examined. By adopting two plausible atmospheric models, we solve coupled time-dependent continuity equations for ions H₂⁺, H₅⁺, H⁺, H₃⁺ and H_eH⁺ simultaneously. It is shown that both radiative and three body association of H⁺ to H₂ are important for the determination of the structure of the Jovian ionosphere. The maximum electron density in the daytime is found to be about 10⁵ cm^?3. It is also shown that diurnal variation with large-amplitude can exist in the Jovian ionosphere.     

Initial interpretation of Titan plasma interaction as observed by the Cassini plasma spectrometer: Comparisons with Voyager 1

The Cassini plasma spectrometer (CAPS) instrument made measurements of Titan's plasma environment when the Cassini Orbiter flew through the moon's plasma wake October 26, 2004 (flyby TA). Initial CAPS ion and electron measurements from this encounter will be compared with measurements made by the Voyager 1 plasma science instrument (PLS). The comparisons will be used to evaluate previous interpretations and predictions of the Titan plasma environment that have been made using PLS measurements. The plasma wake trajectories of flyby TA and Voyager 1 are similar because they occurred when Titan was near Saturn's local noon. These similarities make possible direct, meaningful comparisons between the various plasma wake measurements. They lead to the following: (A) The light and heavy ions, H⁺and N⁺/O⁺, were observed by PLS in Saturn's magnetosphere in the vicinity of Titan while the higher mass resolution of CAPS yielded H⁺ and H₂⁺as the light constituents and O⁺/CH₄⁺ as the heavy ions. (B) Finite gyroradius effects were apparent in PLS and CAPS measurements of ambient O⁺ ions as a result of their absorption by Titan's extended atmosphere. (C) The principal pickup ions inferred from both PLS and CAPS measurements are H⁺, H₂⁺, N⁺, CH₄⁺ and N₂⁺. (D) The inference that heavy pickup ions, observed by PLS, were in narrow beam distributions was empirically established by the CAPS measurements. (E) Slowing down of the ambient plasma due to pickup ion mass loading was observed by both instruments on the anti-Saturn side of Titan. (F) Strong mass loading just outside the ionotail by a heavy ion such as N₂⁺ is apparent in PLS and CAPS measurements. (G) Except for the expected differences due to the differing trajectories, the magnitudes and structures of the electron densities and temperatures observed by both instruments are similar. The high-energy electron bite-out observed by PLS in the magnetotail is consistent with that observed by CAPS.     

Cosmic ray synthesis of organic molecules in Titan's atmosphere

We have studied the possible synthesis of organic molecules by the absorption of galactic cosmic rays in an N₂CH₄H₂ Titan model atmosphere. The cosmic-ray-induced ionization results in peak electron densities of 2 × 10³ cm^?3, with NH₄⁺, C₃H₉⁺, and C₄H₉⁺ being among the important positive ions. Details of the ion and neutral chemistry relevant to the production of organic molecules are discussed. The potential importance of $\text{N}\text{(}{}^2\text{D)}$ reactions with CH₄ and H₂ is also demonstrated. Although the integrated production rate of organic matter due to the absorption of the cosmic ray cascade is much less than that by solar ultraviolet radiation, the production of nitrogen-bearing organic molecules by cosmic rays may be greater.     

Ion-ion mutual neutralization and ion-neutral switching reactions of some stratospheric ions

Following the recent mass spectrometric observations of the ambient stratospheric positive and negative ions we have carried out co-ordinated laboratory experiments using a selected ion flow tube apparatus and a flowing afterglow apparatus for the following purposes: (i) to consider whether CH₃CN is a viable candidate molecule for the species X in the observed stratospheric ion series H⁺ (H₂O_n (X)_m and (ii) to determine the binary mutual neutralization rate coefficients α_i for the reactions ofH⁺ (H₂O₄ and H⁺(H₂O)(CH₃CN)₃ with several of the negative ion species observed in the stratosphere. We conclude from (i) that CH₃CN is indeed a viable candidate for X and from (ii) that the α_i for stratospheric ions are within the limited range (5–6) × 10^?8 cm³ s^?1.     

Measurements of positive ion conversion and removal reactions relating to the Jovian ionosphere

Measured rates are presented for the reaction of He⁺ ions with H₂ (and D₂) molecules to form H⁺, H₂⁺, and HeH⁺ ions, as well as for the subsequent reactions of H⁺ and HeH⁺ ions with H₂ to form H₃⁺. The neutralization of H₃⁺ (and H₅⁺) ions by dissociative recombination with electrons is shown to be fast. The reaction He⁺ + H₂ is slow (k = 1.1 × 10^?13 cm³/sec at300°K) and produces principally H⁺ by the dissociative charge transfer branch. It is concluded that there may be a serious bottleneck in the conversion of two of the primary ions of the upper Jovian ionosphere, H⁺ and He⁺ (which recombine slowly), to the rapidly recombining H₃⁺ ion (α[H₃⁺]?3.4 × 10^?7 cm³/sec at 150°K).     

The role of ion-molecule reactions in the conversion of N2O5 to HNO3 in the stratosphere

We have investigated the role of several ion-molecule reactions in the conversion of N₂O₅ to HNO₃. In the proposed conversion, an N₂O₅ molecule would react with an H₂O molecule clustered to an inert ion to produce two HNO₃ molecules. Subsequent clustering of an H₂O molecule to the inert ion would make the reaction catalytic. If such an ion-catalysed conversion of N₂O₅ to HNO₃ occurs, it would probably play a role in the stratospheric chemistry at high latitudes in winter. In this paper we present reaction rate constant measurements made in a flowing afterglow apparatus for hydrated H₃O⁺, H⁺(CH₃CN)_m (m = 1, 2, 3), and several negative ions reacting with N₂O₅. Slow rate constants were found for these ions for hydration levels that are predominant in the stratosphere. With the known stratospheric ion density, these slow rate constants preclude significant N₂O₅ conversion by ion-molecule reactions.     

Representative abundances of a few positive ions in the innermost coma of comet Halley

The production rate of H₂O molecules at a heliocentric distance of 1 AU for comet Halley and the abundance ratio with respect to water (H₂O) of parent molecules at the cometary nucleus from the paper of Yamamoto (1987) have been used to compute the number densities of positive ions viz. H₃O⁺, H₃S⁺, H₂CN⁺, H₃CO⁺, CH₃OH ₂ ⁺ and NH ₄ ⁺ at various cometocentric distances within 600 kms from the nucleus.The role of proton transfer reactions in producing major ionic species is discussed. A major finding of the present investigation is that NH ₄ ⁺ ion which may be produced through proton transfer reactions is the most abundant ion near the nucleus of a comet unless the abundance of NH₃ as a parent is abnormally low. Using the quoted value of Q(NH₃)/Q(H₂O) for comet Halley and the life times of NH₃ and H₂O molecules, the abundance ratio N(NH₃)/N(H₂O) is found to be one-third of that used in the present paper. The consequent proportionate decrease in the NH ₄ ⁺ ions does not, however, affect its superiority in number density over other ions near the nucleus.The number density of the next most abundant ion viz. H₃O⁺ is found to be 4 × 10⁴ cm^-3 at the nucleus of comet Halley and decreases by a factor of two only upto a distance of 600 K ms from the nucleus. The ionic mass peak recorded by VEGA and GIOTTO spacecrafts atm/q = 18 is most probably composite of the minor ionic species H₂O⁺, as its number density = 10² cm^-3 remains virtually constant in the inner coma and of NH ₄ ⁺ , the number density of which at large cometocentric distances may add to the recorded peak atmlq = 18. The number densities of other major ions produced through proton transfer from H₃O⁺ are also discussed in the region within 600 K ms from the nucleus of comet Halley.     

Magnesium ion chemistry in the stratosphere

Laboratory measurements of reaction rate constants of magnesium ions and magnesium containing ions with O₃, NO, HNO₃, and H₂O₂ have been carried out in a flowing afterglow experiment. Mg⁺ ions react with O₃ to produce MgO⁺ ions, which in turn react with O₃ to produce Mg⁺ ions. Mg⁺ ions react with HNO₃ and H₂O₂ to produce MgOH⁺ ions. MgOH⁺ ions react rapidly with HNO₃ to produce NO⁺₂ ions and Mg(HO)₂. One can therefore conclude that Mg⁺, MgO⁺, or MgOH⁺ ions could not have significant concentrations in the stratosphere if gas phase magnesium compounds were present. The failure to observe these ions therefore cannot be used as evidence that the stratospheric magnesium, resulting from meteor ablation at higher altitudes, is in condensed phases. This is in contrast to the case for sodium where the ion chemistry is such that the failure to observe hydrated Na⁺ ions proves that gas phase sodium compounds are not present in the stratosphere.     

Reflectance and transmittance spectra (2.2-2.4 μm) of ion irradiated frozen methanol

We present near-IR (2.2-2.4 μm) reflectance and transmittance spectra of frozen (16 and 77 K) methanol (CH₃OH) and water-methanol (1:1) mixtures before and after irradiation with 30 keV He⁺ and 200 keV H⁺ ions. Spectra of other simple hydrocarbons (CH₄, C₂H₂, C₂H₄, C₂H₆) and CO have also been obtained both to help in the identification of the new molecules formed after ion irradiation of methanol-rich ices, and to get insight into the question of the presence of simple frozen hydrocarbons on the surface of some objects in the outer Solar System. The results confirm what obtained by studies performed in different spectral ranges, namely the ion-induced formation of CO and CH₄, and, for the first time, evidence a strong decrease of the intensity of the methanol band at about 2.34 μm in comparison with that at 2.27 μm. The results are discussed in view of their relevance for icy objects in the Solar System (namely comets, Centaurs, and Kuiper belt objects) where CH₃OH has been observed or suggested to be present.     

Jovian ionospheric models

This paper reports results obtained on ionosphere formation in the Jovian upper atmosphere with special reference to some of the recently available reaction rates, and to recent models of the Jovian neutral atmosphere based on the possibility of a warmer mesopause. We find that the role of the hypothetical radiative association of H⁺ to H₂ to form H₃⁺, as brought to light in our earlier study, is still important, even with a reaction rate as low as 10^?15 cm³sec^?1. In the lower regions of the ionosphere three-body processes leading to the formation of H₃⁺ and H₅⁺ ions, which have very fast dissociative recombination rates, produce a dramatic reduction in the electron density. When no radiative association takes place, and the H⁺ ions are lost by radiative recombination alone, we confirm that the photochemical equilibrium profile is also the diffusive equilibrium profile. However, with collisional-radiative recombination, whose rate becomes altitude-dependent, diffusion tends to bring about some redistribution of the ionization. Inclusion of radiative association enhances the role of diffusion. In this case, diffusion brings about all the expected changes. In particular, the differences in the electron density profile, originated in the lower-middle ionosphere by radiative association, are propagated up to all higher altitudes by diffusion. The rate constant of radiative association is, however, unknown. It is hoped that the critical importance of this reaction for the Jovian ionosphere will be an incentive towards a careful laboratory determination of its rate coefficient. In the older models of the Jovian ionosphere the major ions were H⁺ which were lost only by pure radiative recombination. This led to high electron densities and practically no diurnal change. In contrast, our new models have relatively much smaller electron densities, especially in lower regions, and may be susceptible to significant diurnal variation.     

Predictions of the electrical conductivity and charging of the aerosols in Titan's atmosphere

The electrical conductivity and electrical charge on the aerosols in atmosphere of Titan are computed for altitudes between 0 and 400 km. Ionization of methane and nitrogen due to galactic cosmic rays (GCR) is important at night where these ions are converted to ion clusters such as CH⁺₅CH₄, C₇H⁺₇, C₄H⁺₇, and H₄C₇N⁺. The ubiquitous aerosols observed also play an important role in determining the charge distribution in the atmosphere. Because polycyclic aromatic hydrocarbons (PAHs) are expected in Titan's atmosphere and have been observed in the laboratory and found to be electrophilic, we consider the formation of negative ions. During the night, the very smallest molecular complexes accept free electrons to form negative ions. This results in a large reduction of the electron abundance both in the region between 150 and 350 km over that predicted when such aerosols are not considered. During the day time, ionization by photoemission from aerosols irradiated by solar ultraviolet (UV) radiation overwhelms the GCR-produced ionization. The presence of hydrocarbon and nitrile minor constituents substantially reduces the UV flux in the wavelength band from the cutoff of CH₄ at 155 to 200 nm. These aerosols have such a low ionization potential that the bulk of the solar radiation at longer wavelengths is energetic enough to produce a photoionization rate sufficient to create an ionosphere even without galactic cosmic ray (GCR) bombardment. At altitudes below 60 km, the electron and positive ion abundances are influenced by the three-body recombination of ions and electrons. The addition of this reaction significantly reduces the predicted electron abundance over that previously predicted. Our calculations for the dayside show that the peaks of the charge distributions move to larger values as the altitude increases. This variation is the result of the increased UV flux present at the highest altitudes. Clearly, the situation is quite different than that for the night where the peak of the distribution for a particular size is nearly constant with altitude when negative ions are not present. The presence of very small aerosol particles (embryos) may cause the peak of the distribution to decrease from about 8 negative charges to as little as one negative charge or even zero charge. This dependence on altitude will require models of the aerosol formation to change their algorithms to better represent the effect of charged aerosols as a function of altitude. In particular, the charge state will be much higher than previously predicted and it will not be constant with altitude during the day time. Charging of aerosol particles, whether on the dayside or nightside, has a major influence on both the electron abundance and electrical conductivity. The predicted conductivities are within the measurement range of the HASI PWA instrument over most but not all, of the altitude range sampled.     

The evolution of charged particles in a model of contracting cloud

The evolution of the charged particles are followed during contraction of a model of an interstellar cloud, with initial density number of n = 10 cm^–3. The contraction is followed up to density increase by five orders of magnitude. Special care is given to the details of the negative ions. In addition, we have tested the ambipolar diffusion according to the results of the ion density.The results predict the importance of atomic ions in the diffuse regions. H⁺ and C⁺ are distinctly enhanced in the beginning of contraction but decrease as contraction proceeds. Molecular ions enhance as contraction proceeds and becomes important in dense regions. The most enhanced molecular ions are HCO⁺, O₂ ⁺, C₂H₃ ⁺, H₃O⁺ and SO⁺, H₃ ⁺ is less abundant. The atomic ions (except metalic ions) decrease noticeably as density increases. In general the negative ions are of negligible fractional abundances. It has also been found that the time of ambipolar diffusion is shorter than the dynamical time, hence the magnetic field should be weakened in the central core as the central density increases to n = 10⁴ cm^–3.     

Photochemistry of minor constituents in the troposphere

Altitude profiles for the number densities of NO, NO₂, NO₃, N₂O₅, HNO₂, CH₃O, CH₃O₂, H₂CO, OH, and HO₂ are calculated as a function of time of day with a steady-state photochemical model in which the altitude profiles for the number densities of H₂O, CH₄, H₂, CO, O₃, and the sum of NO and NO₂ are fixed at values appropriate to a summer latitude of 34°. Average daily profiles are calculated for the long-lived species, HNO₃, H₂O₂, and CH₃O₂H.The major nitrogen compound HNO₃ may have a number density approaching 5 × 10¹¹ molecules cm^?3 at the surface, although an effective loss path due to collisions with particulates could greatly reduce this value.The number density of OH remains relatively unchanged in the first 6 km and reaches 1 × 10⁷ molecules cm^?3 at noon, while the number density of HO₂ decreases throughout the lower troposphere from its noontime value of 8 × 10⁸ molecules cm^?3 at the surface.H₂O₂ and H₂CO both have number densities in the ppb range in the lower troposphere.Owing to decreasing temperature and water concentration, the production of radicals and their steady-state number densities decrease with altitude, reaching a noontime minimum of 1 × 10⁸ molecules cm^?3 for OH and 3 × 10⁷ molecules cm^?3 for HO₂ at the tropopause. The related minor species show even sharper decreases with increasing altitude.The primary path for interconverting OH and HO₂ serves as the major sink for CO and leads to a tropospheric lifetime for CO of ~0.1 yr.Another reaction cycle, the oxidation of CH₄, is quite important in the lower troposphere and leads to the production of H₂CO along with the destruction of CH₄ for which a tropospheric lifetime of ~2 yr is estimated.The destruction of H₂CO that was produced in the CH₄ oxidation cycle provides the major source of CO and H₂ in the atmosphere.