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1.
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   

2.
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   

3.
Negative ions (anions) were identified in the coma of comet 1P/Halley during in situ Electron Electrostatic Analyzer measurements performed by the Giotto spacecraft in 1986. These anions were detected with masses in the range 7–110 amu, but with insufficient mass resolution to permit unambiguous identification. We present details of a new chemical‐hydrodynamic model for the coma of comet Halley that includes—for the first time—atomic and molecular anions, in addition to a comprehensive hydrocarbon chemistry. Anion number densities are calculated as a function of radius in the coma, and compared with the Giotto results. Important anion production mechanisms are found to include radiative electron attachment, polar photodissociation, dissociative electron attachment, and proton transfer. The polyyne anions C4H? and C6H? are found to be likely candidates to explain the Giotto anion mass spectrum in the range 49–73 amu. The CN? anion probably makes a significant contribution to the mass spectrum at 26 amu. Larger carbon‐chain anions such as C8H? can explain the peak near 100 amu provided there is a source of large carbon‐chain‐bearing molecules from the cometary nucleus.  相似文献   

4.
Simultaneous measurements of NO and NO2 in the stratosphere leading to an NOx determination have been performed by means of i.r. absorption spectrometry using the Sun as a source in the 5·2 μm band of NO and in the 6·2 μm band of NO2. The observed abundance of NOP peaks at 26 km where it is equal to (4·2 ± 1) × 109 cm?3. The volume mixing ratio of NOp was observed to vary from 1·3 × 10?9 at 20 km to 1·3 × 10?8 at 34 km.  相似文献   

5.
Altitude distributions of electronically excited atoms and molecules of oxygen and nitrogen in the aurora have been obtained by means of rocket-borne wavelength scanning interference filter photometers launched from Fort Churchill, Manitoba (58.4°N, 94.1°W) on January 23, 1974. Atomic oxygen densities derived from mass spectrometer measurements obtained during the flight are used in conjunction with the volume emission rate ratio of the N2(C3Πu?B3Πg) (0-0) second positive and N2(A3Σu+, v = 1?X1Σg+) Vegard-Kaplan bands to derive a rate constant for quenching of the N2(A3Σu+, v = 1) level with O(3P) of 1.7(±0.8) × 10?11 cm3 s?1 These data, together with O den derived from the O2(b1Σg+) state nightglow emission observed during the rocket ascent, suggest that quenching of the N2(A3Σu+, v = 1) level by O2 has a significant positive temperature dependence. The processes involved in the production and loss of the N2(A3Σu+) state are considered and energy transfer from the N2(A3Σu+) state to O(3P) is found to be a significant source of the OI 5577 Å green line in this aurora at altitudes below 130 km. Emission from the NO(A2Σ+?X2Π) gamma bands was not detected, an observation which is consistent with the mass spectrometer data obtained during the flight indicating that the NO density was <108 cm3 at 110 km. On the basis of previous rocket and satellite measurements of the NO gamma bands, energy transfer from the N2(A3Σu+) state to NO(X2Π) is shown to be an insignificant source of the gamma bands in aurora. Altitude profiles of the N2(a1Πg?X1Σg+) Lyman-Birge-Hopfield band system are presented.  相似文献   

6.
We have analyzed Titan observations performed by the Infrared Space Observatory (ISO) in the range 7-30 μm. The spectra obtained by three of the instruments on board the mission (the short wavelength spectrometer, the photometer, and the camera) were combined to provide new and more precise thermal and compositional knowledge of Titan’s stratosphere. With the high spectral resolution achieved by the SWS (much higher than that of the Voyager 1 IRIS spectrometer), we were able to detect and separate the contributions of most of the atmospheric gases present on Titan and to determine disk-averaged mole fractions. We have also tested existing vertical distributions for C2H2, HCN, C2H6, and CO2 and inferred some information on the abundance of the first species as a function of altitude. From the CH3D band at 1161 cm−1 and for a CH4 mole fraction assumed to be 1.9% in Titan’s stratosphere, we have obtained the monodeuterated methane-averaged abundance and retrieved a D/H isotopic ratio of 8.7−1.9+3.2 × 10−5. We discuss the implications of this value with respect to current evolutionary scenarios for Titan. The ν5 band of HC3N at 663 cm−1 was observed for the first time in a disk-averaged spectrum. We have also obtained a first tentative detection of benzene at 674 cm−1, where the fit of the ISO/SWS spectrum at R = 1980 is significantly improved when a constant mean mole fraction of 4 × 10−10 of C6H6 is incorporated into the atmospheric model. This corresponds to a column density of ∼ 2 × 1015 molecules cm−2 above the 30-mbar level. We have also tested available vertical profiles for HC3N and C6H6 and adjusted them to fit the data. Finally, we have inferred upper limits of a few 10−10 for a number of molecules proposed as likely candidates on Titan (such as allene, acetonitrile, propionitrile, and other more complex gases).  相似文献   

7.
R.K. Khanna 《Icarus》2005,178(1):165-170
Infrared spectra of crystalline HC3N and C2H2 were investigated at several temperatures between 15 and 150 K. The characteristics of the 505 and 753 cm−1 bands of HC3N are in complete agreement with the emission spectral data on Titan obtained by the Voyager IRIS instrument, thus confirming the identification of crystalline HC3N on Titan. A composite spectrum in the 720-800 cm−1 region, with contributions from HC3N and C2H2 in crystalline phases, reproduces the Voyager emission data in that region, thus providing a suggestion for the identification of crystalline C2H2 on Titan.  相似文献   

8.
We have obtained the infrared spectra and the corresponding absolute band intensities for two HC3N isotopomers: DC3N and HC315N. Our results for DC3N are in good agreement with previous measurements except for the ν2 and ν3 stretching modes. For HC315N, this study is the first including intensity measurements.We have also studied the possible detection of these isotopomers in Titan's atmosphere using the CIRS spectrograph onboard the Cassini spacecraft. Our simulation of the expected spectra shows that for a signal-to-noise ratio better than 100, the 15N isotopomer of HC3N could be detected. But, further study of HC3N hot bands are needed since some of them overlap the HC315N Q-branch.  相似文献   

9.
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   

10.
The two basic components of the neutral hydrogen, cool dense clouds merged in a hotter tenuous medium, are studied using 21 cm absorption data of the Parkes Survey. The mean parameters obtained for the typical clouds next to the galactic plane are τp = 1.7, velocity half-width=3.3 km s?1. Their temperatures areT sc ≥40 K with a meanT sc =63±12 K and the obtained hot gas density isn HH=(0.15±0.05) atom cm?3. Theoretical analysis following Giovanelli and Brown (1973) reveals that the pressure equilibrium condition (n HH+2n e )·T SH≈n HC·T sc is compatible with the quoted values if it is assumed that the cosmic abundances in the interstellar medium are below the adopted normal solar abundance. This lack of heavy elements suggests accretion to grains which is consistent with the observed narrow concentration of the dark matter on the galactic layer (≤100 pc halfwidth). The same pressure condition leads to a mean cool cloud density ofn HC～30 atom cm?3 and a hot gas temperature ofT SH～10 500 K. Comparison with data from Hii regions suggests that the cool clouds are somewhat denser and less extensive than such regions. An explanation for it is the expansion that the Hii regions went through in their origin. Comparison with 21 cm emission data shows that the cloud galactic layer is only about a quarter as thick as the hot gas layer. All the present results suggest that only such clouds can be spatially related with the typical I population associated with the spiral structure.  相似文献   

11.
We present a systematic investigation of the parametric evolution of both retrograde and direct families of periodic motions as well as their stability in the inner region of the peripheral primaries of the planar N-body regular polygonal configuration (ring model). In particular, we study the change of the bifurcation points as well as the change of the size and dynamical structure of the rings of stability for different values of the parameters ν = N?1 (number of peripheral primaries) and β (mass ratio). We find some types of bifurcations of families of periodic motions, namely period doubling pitchfork bifurcations, as well as bifurcations of symmetric and non-symmetric periodic orbits of the same period. For a given value of N ? 1, the intervals Δx and ΔC of the rings of stability (where the periodic orbits are stable) of both retrograde and direct families increase with β increasing, while for a given value of β, the interval ΔC decreases with increasing N ? 1. In general, it seems that the dynamical properties of the system depend on the ratio (N ? 1)/β. The size of each ring of stability tends to zero as the ratio (N ? 1)/β → ∞, that is, if N ? 1→∞ or β → 0, the size of each ring of stability tends to zero (Δx → 0 and ΔC → 0) and, in general, the retrograde and direct families tend to disappear. This study gives us interesting information about the evolution of these two families and the changes of the bifurcation patterns since, for example, in some cases the stability index A oscillates between ?1 ≤ Α ≤ + 1. Each time the family becomes critically stable a new dynamical structure appears. The ratios of the Jacobian constant C between the successive critical points, C i /C i+1, tend to 1. All the above depend on the parameters N ? 1, β and show changes in the topology of the phase space and in the dynamical properties of the system.  相似文献   

12.
Excitation functions for collision-induced dissociation reactions of CO 3? and NO3? to give O? and the corresponding neutral species have been studied using an in-line tandem mass spectrometer. When these ions were prepared from certain gaseous mixtures, larger cross-sections and lower thresholds were observed for the dissociation processes than those found for the same ions in their apparent ground states. These observations suggest the existence of long-lived excited states of CO3?1 and NO3?1. The heats of formation of these excited ionic states were determined to be ?4.8 ± 0.1 and ?0.3 ± 0.2 eV for CO3?1 and NO3?1, respectively. Possible implications of these findings with respect to the D -region negative ion reaction scheme are discussed.  相似文献   

13.
Having analyzed the 1999 scanning observations of the Galactic-center region with the PCA spectrometer onboard the RXTE observatory, we obtained upper limits on the flux from the microlensing black hole OGLE-1999-BUL-32 in 1999–2000. We show that the X-ray luminosity of this black hole did not exceed L x ? 3 × 1033(d/1kpc)2 erg s?1. Near the maximum amplification of the background star (on June 6, 1999), the upper limit was L x ? 7 × 1033(d/1kpc)2 erg s?1.  相似文献   

14.
Vertical distributions and spectral characteristics of Titan’s photochemical aerosol and stratospheric ices are determined between 20 and 560 cm?1 (500–18 μm) from the Cassini Composite Infrared Spectrometer (CIRS). Results are obtained for latitudes of 15°N, 15°S, and 58°S, where accurate temperature profiles can be independently determined.In addition, estimates of aerosol and ice abundances at 62°N relative to those at 15°S are derived. Aerosol abundances are comparable at the two latitudes, but stratospheric ices are ～3 times more abundant at 62°N than at 15°S. Generally, nitrile ice clouds (probably HCN and HC3N), as inferred from a composite emission feature at ～160 cm?1, appear to be located over a narrow altitude range in the stratosphere centered at ～90 km. Although most abundant at high northern latitudes, these nitrile ice clouds extend down through low latitudes and into mid southern latitudes, at least as far as 58°S.There is some evidence of a second ice cloud layer at ～60 km altitude at 58°S associated with an emission feature at ～80 cm?1. We speculate that the identify of this cloud may be due to C2H6 ice, which in the vapor phase is the most abundant hydrocarbon (next to CH4) in the stratosphere of Titan.Unlike the highly restricted range of altitudes (50–100 km) associated with organic condensate clouds, Titan’s photochemical aerosol appears to be well-mixed from the surface to the top of the stratosphere near an altitude of 300 km, and the spectral shape does not appear to change between 15°N and 58°S latitude. The ratio of aerosol-to-gas scale heights range from 1.3–2.4 at about 160 km to 1.1–1.4 at 300 km, although there is considerable variability with latitude. The aerosol exhibits a very broad emission feature peaking at ～140 cm?1. Due to its extreme breadth and low wavenumber, we speculate that this feature may be caused by low-energy vibrations of two-dimensional lattice structures of large molecules. Examples of such molecules include polycyclic aromatic hydrocarbons (PAHs) and nitrogenated aromatics.Finally, volume extinction coefficients NχE derived from 15°S CIRS data at a wavelength of λ = 62.5 μm are compared with those derived from the 10°S Huygens Descent Imager/Spectral Radiometer (DISR) data at 1.583 μm. This comparison yields volume extinction coefficient ratios NχE(1.583 μm)/NχE(62.5 μm) of roughly 70 and 20, respectively, for Titan’s aerosol and stratospheric ices. The inferred particle cross-section ratios χE(1.583 μm)/χE(62.5 μm) appear to be consistent with sub-micron size aerosol particles, and effective radii of only a few microns for stratospheric ice cloud particles.  相似文献   

15.
For the first time, height profiles of the stratospheric negative ion composition are presented. The results are from two nights of balloon borne mass spectrometers and cover an altitude range from 23.8 to 38.9 km. Below approx. 30km, NO?3 · mHNO3 ions are dominant. These are replaced by HSO4? · nH2SO4 · oHNO3 ions above this height. There are indications that the most abundant ions above 32 km have masses greater than 280 atomic mass units (amu), the instruments' mass range. The fractional ion count rates as a function of altitude are presented and their significance for neutral trace gas analysis and ion sampling is discussed.  相似文献   

16.
Three organic compounds (HC3N, C6H2, and C4N2) relevant of Titan's atmosphere have been studied within the framework of the SIPAT (Spectroscopie UV d'Intérêt Prébiologique dans l'Atmosphère de Titan) program. Since this facility is still unable to reach the very low temperatures (170 K) of Titan's high atmosphere, spectra have to be obtained at several absorption-cell temperatures, and the data extrapolated towards lower temperatures. Previously published HC3N and C6H2 absorption coefficient data are reviewed, while new spectroscopic data are presented on C4N2. Integrated intensity calculations over the vibrational bands are performed apart from the background continuum. Thus, only the band contrast is considered here. While, the temperature dependence of the hot-band integrated intensity follows a Boltzmann distribution, we have enhanced the fit through an empirical parametrisation to account for the observed temperature dependence of the C4N2 and HC3N absorption coefficients, and to extrapolate those data to the low temperature conditions of Titan's high atmosphere. Finally, we discuss the implications of the results to possible detection by remote sensing observations of these minor compounds in Titan's atmosphere.  相似文献   

17.
Interference filter photometry was taken of Comet Encke on June 14, 1974 (1.07 AU heliocentric distance, postperihelion) at the CTIO (Cerro Tololo Interamerican Observatory) 150-cm reflector. Production rates were calculated of 4.1 × 1023 mol sec?1 of CN, 5.3 × 1023 mol sec?1 of C3, and 4.3 × 1024 mol sec?1 of C2. These are about three times smaller than at comparable heliocentric distance preperihelion, assuming a value of 100 for the ratio H2O/ (C2 + C3 + CN). An upper limit was placed on the production of nonvolatiles at about one-third that of volatiles in mass by assuming a bulk density of 1 g cm?3, a particle geometric albedo of 0.1, and a phase function of 0.2.  相似文献   

18.
The origin of CN radicals in comets is not completely understood so far. We present a study of CN and HCN production rates and CN Haser scale lengths showing that: (1) at heliocentric distances larger than 3 AU, CN radicals could be entirely produced by HCN photolysis; (2) closer to the Sun, for a fraction of comets CN production rates are higher than HCN ones whereas (3) in the others, CN distribution cannot be explained by the HCN photolysis although CN and HCN production rates seem to be similar. Thus, when the comets are closer than 3 AU to the Sun, an additional process to the HCN photolysis seems to be required to explain the CN density in some comets.The photolysis of HC3N or C2N2 could explain the CN origin. But the HC3N production rate is probably too low to reproduce CN density profile, even if uncertainties on its photolysis leave the place for all possible conclusions. The presence of C2N2 in comets is a reliable hypothesis to explain the CN origin; thus, its detection is a challenging issue. Since C2N2 is very difficult to detect from ground-based observations, only in situ measurements or space observations could determine the contribution of this compound in the CN origin.Another hypothesis is a direct production of CN radicals by the photo- or thermal degradation of complex refractory organic compounds present on cometary grains. This process could explain the spatial profile of CN inside jets and the discrepancy noted in the isotopic ratio 14N/15N between CN and HCN. Laboratory studies of the thermal and UV-induced degradation of solid nitrogenated compounds are required to model and validate this hypothesis.  相似文献   

19.
G. Paubert  D. Gautier  R. Courtin 《Icarus》1984,60(3):599-612
The flux emitted by Titan's disk in millimeter lines of HCN, HC3N, CH3CN, and CO is calculated by means of a radiative transfer formulation which takes into account the sphericity of the atmosphere. It is demonstrated that the plane-parallel approximation for radiative transfer is no longer valid, especially in the core of emission lines, when Titan is not spatially resolved. The antenna temperatures which would be measured by large radiotelescopes observing Titan at frequencies of (1?0) and (2?1) transitions of CO, of (1?0), (2?1), and (3?2) transitions of HCN, and of selected transitions of HC3N and CH3CN in the range 80–300 GHz are calculated. The observability of these transitions is investigated. It is concluded that there is the possibility of inferring the vertical stratospheric distribution of these species from line shape measurements to be achieved with existing or forthcoming radioastronomical instrumentation. The determination of the CO abundance by D. O. Muhleman, G. L. Berge, and R. T. Clancy (1984, (Science (Washington, D.C.), 223, 393–396) from measurements at 115.3 GHz in two 200 MHz bands, is reinterpreted by means of this radiative transfer formulation. A CO mixing ratio between 3 × 10?5 and 18 × 10?5, with a most plausible value of 7.5 × 10?5, is found.  相似文献   

20.
We report here on unique post-perihelion (2.3 AU) measurements of Comet Hale-Bopp in the FUV-range (950–1250 Å) by means of the UVSTAR spectrometer from the space shuttle with the main purpose of searching for argon and other FUV emitters. New methods for separating the strong airglow emission at shuttle altitudes are here discussed in detail. Due to our low resolution (15 Å) and S/N ratio the possible rocket-borne detection of argon near perihelion (0.9 AU) could not be confirmed. New species as N2 are suspected but difficult to separate from the strong airglow emission at shuttle altitudes. From the Lyα brightness (1.30± 0.08 kRy) a water production rate Q = 5.9 ± 0.4 × 1029 molecules s?1 could be derived and compared with other post-perihelion observations.  相似文献   

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