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1.
David E. Woon  Jin-Young Park 《Icarus》2009,202(2):642-680
Barrierless reactions between unsaturated hydrocarbons and the ethynyl radical (C2H) can contribute to the growth of organic particulates in the haze-forming regions of Titan's atmosphere as well as in the gas giants and in the interstellar medium. We employed a combination of quantum chemistry and statistical rate theories to characterize reactions between ground state C2H and seven alkenes of the general structure R1R2C′C″R3R4 containing up to six carbons. The alkenes included ethene (C2H4); propene (C3H6); 1-butene, 2-butene, and isobutene (C4H8); trimethylethene (C5H10); and tetramethylethene (C6H12). Density functional theory calculations at the B3LYP/6-31 + G∗∗ level were used to characterize the adducts, isomers, products, and the intervening transition states for the addition-elimination reactions of all seven species. A multiple-well treatment was then employed to determine the outcome distributions for the range of temperatures and pressures relevant to Titan's atmosphere, the interstellar medium, and the outer atmospheres of the gas giants. Finally, trajectory calculations using an ROMP2 potential energy surface were used to calculate kinetic rates for the ethene + C2H reaction, where the agreement between the computed and measured values is very good. At low pressure and temperature, vinyl acetylene is a dominant product of several of the reactions, and all of the reactions yield at least one dominant product with both a double and a triple CC bond.  相似文献   

2.
We present new results of Cassini's T9 flyby with complementary observations from T18. Based on Cassini plasma spectrometer (CAPS) and Cassini magnetometer (MAG), compositional evidence shows the upstream flow for both T9 and T18 appears composed of light ions (H+ and H2+), with external pressures ∼30 times lower than that for the earlier TA flyby where heavy ions dominated the magnetospheric plasma. When describing the plasma heating and sputtering of Titan's atmosphere, T9 and T18 can be considered interactions of low magnetospheric energy input. On the other hand, T5, when heavy ion fluxes are observed to be higher than typical (i.e., TA), represents the limiting case of high magnetospheric energy input to Titan's upper atmosphere. Anisotropy estimates of the upstream flow are 1<T⊥/T‖<3 and the flow is perpendicular to B, indicative of local picked up ions from Titan's H and H2 coronae extending to Titan's Hill sphere radius. Beyond this distance the corona forms a neutral torus that surrounds Saturn. The T9 flyby unexpectedly resulted in observation of two “wake” crossings referred to as Events 1 and 2. Event 2 was evidently caused by draped magnetosphere field lines, which are scavenging pickup ions from Titan's induced magnetopause boundary with outward flux ∼2×106 ions/cm2/s. The composition of this out flow is dominated by H2+ and H+ ions. Ionospheric flow away from Titan with ion flux ∼7×106 ion/cm2/s is observed for Event 1. In between Events 1 and 2 are high energy field aligned flows of magnetosphere protons that may have been accelerated by the convective electric field across Titan's topside ionosphere. T18 observations are much closer to Titan than T9, allowing one to probe this type of interaction down to altitudes ∼950 km. Comparisons with previously reported hybrid simulations are made.  相似文献   

3.
Our model [Dimitrov, V., Bar-Nun, A., 1999. A model of energy dependent agglomeration of hydrocarbon aerosol particles and implication to Titan's aerosol. J. Aerosol. Sci. 30(1), 35-49] describes the experimentally found polymerization of C2H2 and HCN to form aerosol embryos, their growth and adherence to form various aerosols objects [Bar-Nun, A., Kleinfeld, I., Ganor, E., 1988. Shape and optical properties of aerosols formed by photolysis of C2H2, C2H4 and HCN. J. Geophys. Res. 93, 8383-8387]. These loose fractal objects describe well the findings of DISR on the Huygens probe [Tomasko, M.G., Bézard, B., Doose, L., Engel, S., Karkoschka, E., 2008. Measurements of methane absorption by the descent imager/spectral radiometer (DISR) during its descent through Titan's atmosphere. Planet. Space Sci., this issue, doi:10.1016/j.pss.2007]. These include (1) various regular objects of R=(0.035-0.064)×10−6 m, as compared with DISR's 0.05×10−6 m; (2) diverse low and high fractal structures composed of random combinations of various regular and irregular objects; (3) the number density of fractal particles is 6.9×106 m−3 at Z=100 km, as compared with DISR's finding of 5.0×106 m−3 at Z=80 km; (4) the number of structural units per higher fractals in the atmosphere at Z∼100 km is (2400-2700), as compared with DISR's 3000, and their size being of R=(5.4-6.4)×10−6 m will satisfy this value and (5) condensation of CH4 on the highly fractal structures could begin at the altitude where thin methane clouds were observed, filling somewhat the new open fractal structures.  相似文献   

4.
We report the Balmer broad absorption lines (BALs) in the quasar SDSS J2220 + 0109 discovered from the SDSS data, and present a detailed analysis of the peculiar absorption line spectrum, including the He I* multiplet at λλ3189, 3889 arising from the metastable 23s-state helium and the Balmer Hα and Hβ lines from the excited hydrogen H I of n = 2 level, which are rarely seen in quasar spectra, as well as many absorption lines arising from the excited Fe II* of the levels 7 955 cm−1, 13 474 cm−1 and 13 673 cm−1 in the wavelength range 3100∼3300 Å. Ca II H, K absorption line doublets also clearly appear in the SDSS spectrum. All absorption lines show a similar blueshifted velocity structure of Δv ≈ − 1500 ∼ 0 km·s−1 relative to the quasar's systematic redshift determined from the emission lines. Detailed analysis suggests that the Balmer absorption lines should arise from the partially ionized region with a column density of NHI ≈ 1021 cm−2 for an electron density of ne ∼ 106 cm−3; and that the hydrogen n = 2 level may be populated via collisional excitation with Lyα pumping.  相似文献   

5.
A study has been made using a variable temperature flowing afterglow Langmuir probe technique (VT-FALP) to determine the equilibrium temperature dependencies of the dissociative electron-ion recombination of the protonated cyanide ions (RCNH+, where R=H, CH3 and C2H5) and their symmetrical proton-bound dimers (RCNH+NCR). The power law temperature dependencies of the recombination coefficients, αe, over the temperature range 180 to 600 K for the protonated ions are αe(T)(cm3 s−1)=3.5±0.5×10−7 (300/T)1.38 for HCNH+, αe(T)=3.4±0.5×10−7 (300/T)1.03 for CH3CNH+, and αe(T)=4.6±0.7×10−7 (300/T)0.81 for CH3CH2CNH+. The equivalent values for the proton-bound dimers are αe(T)(cm3 s−1)=2.4±0.4×10−6(300/T)0.5 for (HCN)2H+ to αe(T)=2.8±0.4×10−6(300/T)0.5 for (CH3CN)2H+, and αe(T)=2.3±0.3×10−6(300/T)0.5 for (CH3CH2CN)2H+. The relevance of these data to molecular synthesis in the interstellar medium and the Titan ionosphere are discussed.  相似文献   

6.
S. Carles  J.-C. Guillemin 《Icarus》2011,211(1):901-905
Rate coefficient of the cyanide anion (CN−) with cyanoacetylene (HC3N) reaction, has been studied in gas phase at room temperature using a Flowing Afterglow Langmuir Probe - Mass Spectrometer (FALP-MS) apparatus. The rate constant for the CN− + HC3N reaction is k = 4.8 × 10−9 cm3/s with an uncertainty of 30%.  相似文献   

7.
Laboratory spectra of methane-nitrogen mixtures have been recorded in the near-infrared range (1.0-1.65 μm) in conditions similar to Titan's near surface, to facilitate the interpretation of the DISR/DLIS (DISR—Descent Imager/Spectral Radiometer) spectra taken during the last phase of the descent of the Huygens Probe, when the surface was illuminated by a surface-science lamp. We used a 0.03 cm−1 spectral resolution, adequate to resolve the lines at high pressure (pN2∼1.5 bar). By comparing the laboratory spectra with synthetic calculations in the well-studied ν2+2ν3 band (7515-7620 cm−1), we determine a methane absorption column density of 178±20 cm atm and a temperature of 118±10 K in our experiment. From this, we derive the methane absorption coefficients over 1.0-1.65 μm with a 0.03 cm−1 sampling, allowing for the extrapolation of the results to any other methane column density under the relevant pressure and temperature conditions. We then revisit the calibration and analysis of the Titan “lamp-on” DLIS spectra. We infer a 5.1±0.8% methane-mixing ratio in the first 25 m of Titan's atmosphere. The CH4 mixing ratio measured 90 s after landing from a distance of 45 cm is found to be 0.92±0.25 times this value, thus showing no post-landing outgassing of methane in excess of ∼20%. Finally, we determine the surface reflectivity as seen between 25 m and 45 cm and find that the 1500 nm absorption band is deeper in the post-landing spectrum as compared to pre-landing.  相似文献   

8.
The reactivity of C2(X1Σ+g) with simple saturated (CH4, C2H6 and C3H8) and unsaturated (C2H2 and C2H4) hydrocarbons has been studied in the gas phase over the temperature range 24-300 K using the CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme or Reaction Kinetics in a Uniform Supersonic Flow) technique. All reactions have been found to be very rapid in this temperature range and the rate coefficients are typically ?10−10 cm3 molecule−1 s−1 with the exception of methane for which the rate coefficient is one order of magnitude lower: ∼10−11 cm3 molecule−1 s−1. These results have been analyzed in terms of potential destruction sources of C2(X1Σ+g) in the atmospheres of Titan and the Giant Planets. It appears that the rate coefficient of the reaction 1C2 + CH4 should be updated with our new data and that reactions with C2H2, C2H4 and C2H6 should also be included in the existing photochemical models.  相似文献   

9.
The Electron Spectrometer (ELS), one of the sensors making up the Cassini Plasma Spectrometer (CAPS) revealed the existence of numerous negative ions in Titan's upper atmosphere. The observations at closest approach (∼1000 km) show evidence for negatively charged ions up to ∼10,000 amu/q, as well as two distinct peaks at 22±4 and 44±8 amu/q, and maybe a third one at 82±14 amu/q. We present the first ionospheric model of Titan including negative ion chemistry. We find that dissociative electron attachment to neutral molecules (mostly HCN) initiates the formation of negative ions. The negative charge is then transferred to more acidic molecules such as HC3N, HC5N or C4H2. Loss occurs through associative detachment with radicals (H and CH3). We attribute the three low mass peaks observed by ELS to CN−, C3N−/C4H− and C5N−. These species are the first intermediates in the formation of the even larger negative ions observed by ELS, which are most likely the precursors to the aerosols observed at lower altitudes.  相似文献   

10.
The early evolution of Titan's atmosphere is expected to produce enrichment in the heavy isotopomers of CO, 13CO and C18O, relative to 12C16O. However, the original isotopic signatures may be altered by photochemical reactions. This paper explains why there is no isotopic enrichment in C in Titan's atmosphere, despite significant enrichment of heavy H, N, and O isotopes. We show that there is a rapid exchange of C atoms between the CH4 and CO reservoirs, mediated by the reaction 1CH2+*CO→1*CH2+CO, where *C is 13C. Based on recent laboratory measurements, we estimate the rate coefficient for this reaction to be 3.2×10−12 cm3 s−1 at the temperature appropriate for the upper atmosphere of Titan. We investigate the isotopic dilution of CO using the Caltech/JPL one-dimensional photochemical model of Titan. Our model suggests that the time constant for isotopic exchange through the above reaction is about 800 Myr, which is significantly shorter than the age of Titan, and therefore any original isotopic enhancement of 13C in CO may have been diluted by the exchange process. In addition, a plausible model for the evolution history of CO on Titan after the initial escape is proposed.  相似文献   

11.
Discovery by Cassini's plasma instrument of heavy positive and negative ions within Titan's upper atmosphere and ionosphere has advanced our understanding of ion neutral chemistry within Titan's upper atmosphere, primarily composed of molecular nitrogen, with ~2.5% methane. The external energy flux transforms Titan's upper atmosphere and ionosphere into a medium rich in complex hydrocarbons, nitriles and haze particles extending from the surface to 1200 km altitudes. The energy sources are solar UV, solar X-rays, Saturn's magnetospheric ions and electrons, solar wind and shocked magnetosheath ions and electrons, galactic cosmic rays (GCR) and the ablation of incident meteoritic dust from Enceladus’ E-ring and interplanetary medium. Here it is proposed that the heavy atmospheric ions detected in situ by Cassini for heights >950 km, are the likely seed particles for aerosols detected by the Huygens probe for altitudes <100 km. These seed particles may be in the form of polycyclic aromatic hydrocarbons (PAH) containing both carbon and hydrogen atoms CnHx. There could also be hollow shells of carbon atoms, such as C60, called fullerenes which contain no hydrogen. The fullerenes may compose a significant fraction of the seed particles with PAHs contributing the rest. As shown by Cassini, the upper atmosphere is bombarded by magnetospheric plasma composed of protons, H2+ and water group ions. The latter provide keV oxygen, hydroxyl and water ions to Titan's upper atmosphere and can become trapped within the fullerene molecules and ions. Pickup keV N2+, N+ and CH4+ can also be implanted inside of fullerenes. Attachment of oxygen ions to PAH molecules is uncertain, but following thermalization O+ can interact with abundant CH4 contributing to the CO and CO2 observed in Titan's atmosphere. If an exogenic keV O+ ion is implanted into the haze particles, it could become free oxygen within those aerosols that eventually fall onto Titan's surface. The process of freeing oxygen within aerosols could be driven by cosmic ray interactions with aerosols at all heights. This process could drive pre-biotic chemistry within the descending aerosols. Cosmic ray interactions with grains at the surface, including water frost depositing on grains from cryovolcanism, would further add to abundance of trapped free oxygen. Pre-biotic chemistry could arise within surface microcosms of the composite organic-ice grains, in part driven by free oxygen in the presence of organics and any heat sources, thereby raising the astrobiological potential for microscopic equivalents of Darwin's “warm ponds” on Titan.  相似文献   

12.
In Titan's atmosphere consisting of N2 and CH4, large amounts of atomic hydrogen are produced by photochemical reactions during the formation of complex organics. This atomic hydrogen may undergo heterogeneous reactions with organic aerosol in the stratosphere and mesosphere of Titan. In order to investigate both the mechanisms and kinetics of the heterogeneous reactions, atomic deuterium is irradiated onto Titan tholin formed from N2 and CH4 gas mixtures at various surface-temperatures of the tholin ranging from 160 to 310 K. The combined analyses of the gas species and the exposed tholin indicate that the interaction mechanisms of atomic deuterium with the tholin are composed of three reactions; (a) abstraction of hydrogen from tholin resulting in gaseous HD formation (HD recombination), (b) addition of D atom into tholin (hydrogenation), and (c) removal of carbon and/or nitrogen (chemical erosion). The reaction probabilities of HD recombination and hydrogenation are obtained as ηabst=1.9(±0.6)×10−3×exp(−300/T) and ηhydro=2.08(±0.64)×exp(−1000/T), respectively. The chemical erosion process is very inefficient under the conditions of temperature range of Titan's stratosphere and mesosphere. Under Titan conditions, the rates of hydrogenation > HD recombination ? chemical erosion. Our measured HD recombination rate is about 10 times (with an uncertainty of a factor of 3-5) the prediction of previous theoretical model. These results imply that organic aerosol can remove atomic hydrogen efficiently from Titan's atmosphere through the heterogeneous reactions and that the presence of aerosol may affect the subsequent organic chemistry.  相似文献   

13.
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 H2 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 C12H10 for neutrals and C10H+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 C6H + C4H2, C3N + C4H2, 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.  相似文献   

14.
Far-IR (25-50 μm, 200-400 cm−1) nadir and limb spectra measured during Cassini's four year prime mission by the Composite InfraRed Spectrometer (CIRS) instrument have been used to determine the abundances of cyanogen (C2N2), methylacetylene (C3H4), and diacetylene (C4H2) in Titan's stratosphere as a function of latitude. All three gases are enriched at northern latitudes, consistent with north polar subsidence. C4H2 abundances agree with those derived previously from mid-IR data, but C3H4 abundances are about 2 times lower, suggesting a vertical gradient or incorrect band intensities in the C3H4 spectroscopic data. For the first time C2N2 was detected at southern and equatorial latitudes with an average volume mixing ratio of 5.5±1.4×10−11 derived from limb data (>3-σ significance). This limb result is also corroborated by nadir data, which give a C2N2 volume mixing ratio of 6±3×10−11 (2-σ significance) or alternatively a 3-σ upper limit of 17×10−11. Comparing these figures with photochemical models suggests that galactic cosmic rays may be an important source of N2 dissociation in Titan's stratosphere. Like other nitriles (HCN, HC3N), C2N2 displays greater north polar relative enrichment than hydrocarbons with similar photochemical lifetimes, suggesting an additional loss mechanism for all three of Titan's main nitrile species. Previous studies have suggested that HCN requires an additional sink process such as incorporation into hazes. This study suggests that such a sink may also be required for Titan's other nitrile species.  相似文献   

15.
This paper reports 13CO, C18O, HCO+ (J = 1−0) spectral observations toward IRAS 23133+6050 with the 13.7 m millimeter-wave telescope at Qinghai Station of PMO. Corresponding to the 13CO, C18O, HCO+ line emissions, the size of the observed molecular cloud core is 4.0 pc, 2.1 pc and 2.3 pc, the virial mass is 2.7 × 103 M⊙, 0.9 × 103 M⊙ and 2.3 × 103 M⊙, and the volume density of H2 is 2.7 × 103 cm−3, 5.1 × 103 cm−3 and 4.6 × 103 cm−3, respectively. Using the power-law function n(r) ∼r−p, the spatial density distribution of the cloud core was analyzed, the obtained exponent p is respectively 1.75, 1.56 and 1.48 for the 13CO, C18O and HCO+ cores, and it is found that the density distribution becomes gradually flatter from the outer region to the inner region of the core. The HCO+ abundance is 4.6 × 10−10, one order of magnitude less than the value for dark clouds, and slightly less than that for giant molecular clouds. The 13CO/C18O relative abundance ratio is 12.2, comparable with the value 11.8 for dark clouds, and the value 9.0 ∼ 15.6 for giant molecular clouds. A 13CO bipolar outflow is found in this region. The IRAS far-infrared luminosity and the virial masses give the luminosity-mass ratios 18.1, 51.1 and 21.2 from the three lines.  相似文献   

16.
After molecular nitrogen, methane is the most abundant species in Titan’s atmosphere and plays a major role in its energy budget and its chemistry. Methane has strong bands at 3.3 μm emitting mainly at daytime after absorption of solar radiation. This emission is strongly affected by non-local thermodynamic equilibrium (non-LTE) in Titan’s upper atmosphere and, hence, an accurate modeling of the non-LTE populations of the emitting vibrational levels is necessary for its analysis. We present a sophisticated and extensive non-LTE model which considers 22 CH4 levels and takes into account all known excitation mechanisms in which they take part. Solar absorption is the major excitation process controlling the population of the v3-quanta levels above 1000 km whereas the distribution of the vibrational energy within levels of similar energy through collisions with N2 is also of importance below that altitude. CH4-CH4 vibrational exchange of v4-quanta affects their population below 500 km. We found that the ν3 → ground band dominates Titan’s 3.3 μm daytime limb radiance above 750 km whereas the ν3 + ν4 → ν4 band does below that altitude and down to 300 km. The ν3 + ν2 → ν2, the 2ν3 → ν3, and the 13CH4ν3 → ground bands each contribute from 5% to 8% at regions below 800 km. The ν3 + 2ν4 → 2ν4and ν2 + ν3 + ν4 → ν2 + ν4 bands each contribute from 2% to 5% below 650 km. Contributions from other CH4 bands are negligible. We have used the non-LTE model to retrieve the CH4 abundance from 500 to 1100 km in the southern hemisphere from Cassini-VIMS daytime measurements near 3.3 μm. Our retrievals show good agreement with previous measurements and model results, supporting a weak deviation from well mixed values from the lower atmosphere up to 1000 km.  相似文献   

17.
The vertical profile of H2SO4 vapor is calculated using current atmospheric and thermodynamic data. The atmospheric data include the H2O profiles observed at 70-112 km by the SOIR solar occultations, the SPICAV-UV profiles of the haze extinction at 220 nm, the VeRa temperature profiles, and a typical profile of eddy diffusion. The thermodynamic data are the saturated vapor pressures of H2O and H2SO4 and chemical potentials of these species in sulfuric acid solutions. The calculated concentration of sulfuric acid in the cloud droplets varies from 85% at 70 km to a minimum of 70% at 90 km and then gradually increasing to 90-100% at 110 km. The H2SO4 vapor mixing ratio is ∼10−12 at 70 and 110 km with a deep minimum of 3 × 10−18 at 88 km. The H2O-H2SO4 system matches the local thermodynamic equilibrium conditions up to 87 km. The column photolysis rate of H2SO4 is 1.6 × 105 cm−2 s−1 at 70 km and 23 cm−2 s−1 at 90 km. The calculated abundance of H2SO4 vapor at 90-110 km and its photolysis rate are smaller than those presented in the recent model by Zhang et al. (Zhang, X., Liang, M.C., Montmessin, F., Bertaux, J.L., Parkinson, C., Yung, Y.L. [2010]. Nat. Geosci. 3, 834-837) by factors of 106 and 109, respectively. Assumptions of 100% sulfuric acid, local thermodynamic equilibrium, too warm atmosphere, supersaturation of H2SO4 (impossible for a source of SOX), and cross sections for H2SO4·H2O (impossible above the pure H2SO4) are the main reasons of this huge difference. Significant differences and contradictions between the SPICAV-UV, SOIR, and ground-based submillimeter observations of SOX at 70-110 km are briefly discussed and some weaknesses are outlined. The possible source of high altitude SOX on Venus remains unclear and probably does not exist.  相似文献   

18.
Extreme ultraviolet (EUV) resonance fluorescence of the (0,v″) bands of the c′41Σu+→X1Σg+ and the (1,v″) bands of the b′ 1Σu+→X1Σg+ transitions of N2 has been observed by photon excitation of N2 in the vicinity of 95.8 nm. The integrated fluorescence intensities of the c′4→X (0,v″) emission become saturated at N2 pressures higher than ∼0.16 mTorr. The emission features in the spectral region between 105 and 130 nm become progressively significant as the N2 pressure is increased. The (1,v″) progression for v″ up to 11 of the b′→X transition and two progressions of the Lyman-Birge-Hopfield (LBH) system have been identified. The multiple scattering processes apparently cause significant reduction in the c′4→X (0,0) emission rates. The present results may be useful in the explanation of the weak c′4→X (0,0) fluorescence as well as the significant c′4→X (0,v″) features in the dayglow of the Earth observed by the Far Ultraviolet Spectroscopic Explorer.  相似文献   

19.
Jacques Gustin  Ian Stewart 《Icarus》2010,210(1):270-283
This study reports the analysis of far ultraviolet (FUV) limb spectra of the airglow of Saturn in the 1150-1850 Å spectral window, obtained with the Ultraviolet Imaging Spectrograph (UVIS) onboard Cassini, spanning altitudes from −1200 to 4000 km. The FUV limb emission consists of three main contributions: (1) H Ly-α peaking at 1100 km with a brightness of 0.8 kilo-Rayleighs (kR), (2) reflected sunlight longward of 1550 Å which maximizes at −950 km with 16.5 kR and (3) H2 bands in the 1150-1650 Å bandwidth, peaking at 1050 km reaching a maximum of 3.9 kR.A vertical profile of the local H2 volume emission rate has been derived using the hydrocarbon density profiles from a model of the Saturn equatorial atmosphere. It is well matched by a Chapman function, characterized by a maximum value of 3.5 photons cm−3 s−1 in the 800-1650 Å UV bandwidth, peaking at 1020 km.Comparisons between the observed spectra and a first-order synthetic airglow H2 model in the 1150-1650 Å bandwidth show that the spectral shape of the H2 bands is accounted for by solar fluorescence and photoelectron excitation. The best fits are obtained with a combination of H2 fluorescence lines and 20 eV electron impact spectra, the latter contributing ∼68% of the total H2 airglow emission.  相似文献   

20.
Mid- and far-infrared spectra from the Composite InfraRed Spectrometer (CIRS) have been used to determine volume mixing ratios of nitriles in Titan's atmosphere. HCN, HC3N, C2H2, and temperature were derived from 2.5 cm−1 spectral resolution mid-IR mapping sequences taken during three flybys, which provide almost complete global coverage of Titan for latitudes south of 60° N. Three 0.5 cm−1 spectral resolution far-IR observations were used to retrieve C2N2 and act as a check on the mid-IR results for HCN. Contribution functions peak at around 0.5-5 mbar for temperature and 0.1-10 mbar for the chemical species, well into the stratosphere. The retrieved mixing ratios of HCN, HC3N, and C2N2 show a marked increase in abundance towards the north, whereas C2H2 remains relatively constant. Variations with longitude were much smaller and are consistent with high zonal wind speeds. For 90°-20° S the retrieved HCN abundance is fairly constant with a volume mixing ratio of around 1 × 10−7 at 3 mbar. More northerly latitudes indicate a steady increase, reaching around 4 × 10−7 at 60° N, where the data coverage stops. This variation is consistent with previous measurements and suggests subsidence over the northern (winter) pole at approximately 2 × 10−4 m s−1. HC3N displays a very sharp increase towards the north pole, where it has a mixing ratio of around 4 × 10−8 at 60° N at the 0.1-mbar level. The difference in gradient for the HCN and HC3N latitude variations can be explained by HC3N's much shorter photochemical lifetime, which prevents it from mixing with air at lower latitude. It is also consistent with a polar vortex which inhibits mixing of volatile rich air inside the vortex with that at lower latitudes. Only one observation was far enough north to detect significant amounts of C2N2, giving a value of around 9 × 10−10 at 50° N at the 3-mbar level.  相似文献   

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