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1.
Vladimir A. Krasnopolsky 《Icarus》2008,197(2):377-385
Venus was observed at 2.4 and 3.7 μm with a resolving power of 4×104 using the long-slit high-resolution spectrograph CSHELL at NASA IRTF. The observations were made along a chord that covered a latitude range of ± 60° at a local time near 8:00. The continuous reflectivity and limb brightening at 2.4 μm are fitted by the clouds with a single scattering albedo 1−a=0.01 and a pure absorbing layer with τ=0.09 above the clouds. The value of 1−a agrees with the refractive index of H2SO4 (85%) and the particle radius of 1 μm. The absorbing layer is similar to that observed by the UV spectrometer at the Pioneer Venus orbiter. However, its nature is puzzling. CO2 was measured using its R32 and R34 lines. The retrieved product of the CO2 abundance and airmass is constant at 1.9 km-atm along the instrument slit in the latitude range of ± 60°. The CO mixing ratio (measured using the P21 line) is rather constant at 70 ppm, and its variations of ∼10% may be caused by atmospheric dynamics. The observed value is higher than the 50 ppm retrieved previously from a spectrum of the full disk, possibly, because of some downward extension of the mesospheric morningside bulge of CO. The observations of the HF R3 line reveal a constant HF mixing ratio of 3.5±0.5 ppb within ± 60° of latitude, which is within the scatter in the previous measurements of HF. OCS has been detected for the first time at the cloud tops by summing 17 lines of the P-branch. The previous detections of OCS refer to the lower atmosphere at 30-35 km. The retrieved OCS mixing ratio varies with a scale height of 1 to 3 km. The mean OCS mixing ratio is ∼2 ppb at 70 km and ∼14 ppb at 64 km. Vertical motions in the atmosphere may change the OCS abundance. The detected OCS should significantly affect Venus' photochemistry. A sensitive search for H2S using its line at 2688.93 cm−1 results in a 3 sigma upper limit of 23 ppb, which is more restrictive than the previous limit of 100 ppb. 相似文献
2.
Rapid temporal variability of SO2 and SO in the Venus 85–100 km mesosphere (Sandor, B.J., Clancy, R.T., Moriarty-Schieven G.H. [2007]. Bull. Am. Astron. Soc. 39, 503; Sandor, B.J., Clancy, R.T., Moriarty-Schieven, G.H., Mills, F.P. [2010]. Icarus 208, 49–60) requires in situ sources and sinks for these molecules. While many loss mechanisms are recognized, no process for in situ production is known. Observational investigations to find, or constrain other potential sulfur reservoirs offer one method toward understanding the applicable photochemistry. Here, we report upper limits for gas-phase H2SO4 (sulfuric acid) abundances in Venus’ 85–100 km upper mesosphere, derived from 16 ground-based sub-mm spectroscopic observations in the period 2004–2008. Unlike the ubiquitous sulfuric acid solid/liquid aerosol, the gas phase would be photochemically active, potentially both source and sink for SO and SO2. H2SO4 is retrieved from sub-mm lines located in the same bandpass as the SO2 and SO lines described by Sandor et al. (Sandor, B.J., Clancy, R.T., Moriarty-Schieven, G.H., Mills, F.P. [2010]. Icarus 208, 49–60). H2SO4 upper limits reported here are thus simultaneous and spatially coincident with measurements of SO2 and SO, providing for analysis of the three sulfur species collectively. The average H2SO4 abundance over 16 observations is 1 ± 2 ppb (i.e. <3 ppb). Upper limits for individual observations range from 3 to 44 ppb, where quality of the observing weather is the dominant constraint on measurement precision. The sum of H2SO4, SO2 and SO varies widely. In one comparison, the sum [H2SO4 + SO2 + SO] measured on one date differs by 10-σ from the sum measured 2 months later. We conclude that upper mesospheric sulfur atoms are not conserved among the three molecules, that H2SO4 is not a significant sulfur reservoir for balancing the observed variations of [SO2 + SO], and is not relevant to the (still unknown) photochemistry responsible for observed behavior of SO2 and SO. Having ruled out H2SO4, we infer that elemental sulfur is the most probable candidate for the needed third reservoir. 相似文献
3.
Vladimir A. Krasnopolsky 《Icarus》2010,209(2):314-322
While CO, HCl, and HF, that were considered in the first part of this work, have distinct absorption lines in high-resolution spectra and were detected four decades ago, the lines of HDO, OCS, and SO2 are either very weak or blended by the telluric lines and have not been observed previously by ground-based infrared spectroscopy at the Venus cloud tops. The H2O abundance above the Venus clouds is typically below the detection limit of ground-based IR spectroscopy. However, the large D/H ratio on Venus facilitates observations of HDO. Converted to H2O with D/H ≈ 200, our observations at 2722 cm−1 in the Venus afternoon show a H2O mixing ratio of ∼1.2 ppm at latitudes between ±40° increasing to ±60° by a factor of 2. The observations in the early morning reveal the H2O mixing ratio that is almost constant at 2.9 ppm within latitudes of ±75°. The measured H2O mixing ratios refer to 74 km. The observed increase in H2O is explained by the lack of photochemical production of sulfuric acid in the night time. The recent observations at the P-branch of OCS at 4094 cm−1 confirm our detection of OCS. Four distributions of OCS along the disk of Venus at various latitudes and local times have been retrieved. Both regular and irregular components are present in the variations of OCS. The observed OCS mixing ratio at 65 km varies from ∼0.3 to 9 ppb with the mean value of ∼3 ppb. The OCS scale height is retrieved from the observed limb darkening and varies from 1 to 4 km with a mean value of half the atmospheric scale height. SO2 at the cloud tops has been detected for the first time by means of ground-based infrared spectroscopy. The SO2 lines look irregular in the observed spectra at 2476 cm−1. The SO2 abundances are retrieved by fitting by synthetic spectra, and two methods have been applied to determine uncertainties and detection limits in this fitting. The retrieved mean SO2 mixing ratio of 350 ± 50 ppb at 72 km favors a significant increase in SO2 above the clouds since the period of 1980-1995 that was observed by the SOIR occultations at Venus Express. Scale heights of OCS and SO2 may be similar, and the SO2/OCS ratio is ∼500 and may be rather stable at 65-70 km under varying conditions on Venus. 相似文献
4.
The condensation and vaporization behavior of ices containing SO2, H2S, and CO2: implications for Io
In an extension of previously reported work on ices containing CO, CO2, H2O, CH3OH, NH3, and H2, measurements of the physical and infrared spectral properties of ices containing molecules relevant to Jupiter's moon Io are presented. These include studies on ice systems containing SO2, H2S, and CO2. The condensation and sublimation behaviors of each ice system and surface binding energies of their components are discussed. The surface binding energies can be used to calculate the residence times of the molecules on a surface as a function of temperature and thus represent important parameters for any calculation that attempts to model the transport of these molecules on Io's surface. The derived values indicate that SO2 frosts on Io are likely to anneal rapidly, resulting in less fluffy, "glassy" ices and that H2S can be trapped in the SO2 ices of Io during night-time hours provided that SO2 deposition rates are on the order of 5 micrometers/hr or larger. 相似文献
5.
The high-latitude cloud (HLC) MBM 7 has been observed in the 21 cm H I line and the 12CO(1-0) and 13CO(1-0) lines with similar spatial resolutions. The data reveal a total mass approximately 30 M solar for MBM 7 and a complex morphology. The cloud consists of a cold dense core of 5 M solar surrounded by atomic and molecular gas with about 25 M solar, which is embedded in hotter and more diffuse H I gas. We derive a total column density N(H I + 2H2) of 1 x 10(21) cm-2 toward the center and 1 x 10(20) cm-3 toward the envelope of MBM 7. The CO line indicates the existence of dense cores [n(H2) > or = 2000 cm-3] of size (FWHM) approximately 0.5 pc. The morphology suggests shock compression from the southwest direction, which can form molecular cores along the direction perpendicular to the H I distribution. The H I cloud extends to the northeast, and the velocity gradient appears to be about 2.8 km s-1 pc-1 in this direction, which indicates a systematic outward motion which will disrupt the cloud in approximately 10(6) yr. The observed large line widths of approximately 2 km s-1 for CO suggest that turbulent motions exist in the cloud, and hydrodynamical turbulence may dominate the line broadening. Considering the energy and pressure of MBM 7, the dense cores appear not to be bound by gravity, and the whole cloud including the dense cores seem to be expanding. The distance to HLCs suggest that they belong to the galactic plane, since the scale height of the cloud is < or approximately equal to 100 pc. Compared to the more familiar dense dark clouds, HLCs may differ only in their small mass and low density, with their proximity reducing the filling factor and enhancing the contrast of the core and envelope structure. 相似文献
6.
Abstract. We have constructed a chemical reaction system in a contracting interstellar cloud. In paper (I) we have presented the details of the physical and chemical scheme and the method of solution. The results of our chemical model produce fractional abundances of H2CO, CO, OH, H2O, SO and OCS which are in good agreement with the results of observations. On the other hand, the results of chlorine-bearing species are not in agreement with those of the observations. The calculated abundances of H2CO, CO, OH, H2O, SO, OCS and Cl+ are in agreement with the results of previous theoretical studies. 相似文献
7.
Analysis of high-spatial-resolution (approximately 0.8 arcsec) methane band and continuum imagery of Neptune's relatively homogeneous Equatorial Region yields significant constraints on (1) the stratospheric gaseous methane mixing ratio (fCH4,s), (2) the column abundances and optical properties of stratospheric and tropospheric hydrocarbon hazes, and (3) the wavelength-dependent single-scattering albedo of the 3-bar opaque cloud. From the center-to-limb behavior of the 7270-angstroms and 8900-angstrom sCH4 bands, the stratospheric methane mixing ratio is limited to fCH4,s < 1.7 x 10(-3), with a nominal value of fCH4,s = 3.5 x 10(-4), one to two orders of magnitude less than pre-Voyager estimates, but in agreement with a number of recent ultraviolet and thermal infrared measurements, and largely in agreement with the tropopause mixing ratio implied by Voyager temperature measurements. Upper limits to the stratospheric haze mass column abundance and 6190-angstroms and 8900-angstroms haze opacities are 0.61 microgram cm-2 and 0.075 and 0.042, respectively, with nominal values of 0.20 microgram cm-2 and 0.025 and 0.014 for the 0.2-micrometer radius particles preferred by the recent Voyager PPS analysis of Pryor et al. (1992, Icarus 99, 302-316). The tropospheric CH4 haze opacities are comparable to that found in the stratosphere, upper limits of 0.104 and 0.065 at 6190 angstroms and 8900 angstroms, respectively, with nominal values of 0.085 and 0.058. This indicates a column abundance less than 11.0 micrograms cm-2, corresponding to the methane gas content within a well-mixed 3% methane tropospheric layer only 0.1 cm thick near the 1.5-bar CH4 condensation level. Constraints on the single-scattering albedos of these hazes include (1) for the stratospheric component, 6190-angstroms and 8900-angstroms imaginary indices of refraction less than 0.047 and 0.099, respectively, with 0.000 (conservative scattering) being the nominal value at both wavelengths, and (2) CH4 haze single-scattering albedos greater than 0.85 and 0.50 at these two wavelengths, with conservative scattering again begin the preferred value. However, conservative scattering is ruled out for the opaque cloud near 3 bars marking the bottom of the visible atmosphere. Specifically, we find cloud single-scattering albedos of 0.915 +/- 0.006 at 6340 angstroms, 0.775 +/- 0.012 at 7490 angstroms, and 0.803 +/- 0.010 at 8260 angstrom. Global models utilizing a complete global spectrum confirm the red-absorbing character of the 3-bar cloud. The global-mean model has approximately 7.7 times greater stratospheric aerosol content then the Equatorial Region. An analysis of stratospheric haze precipitation rates indicates a steady-state haze production rate of 0.185-1.5 x 10(-14) g cm-2 sec-1, in agreement with recent theoretical photochemical estimates. Finally, reanalysis of the Voyager PPS 7500-angstroms phase angle data utilizing the fCH4,s value derived here confirms the Pryor et al. result of a tropospheric CH4 haze opacity of a few tenths in the 22-30 degrees S latitude region, several times that of the Equatorial Region or of the globe. The factor-of-10 reduction in fCH4,s below that assumed by Pryor et al. implies decreased gas absorption and consequently a decrease in the forward-scattering component of tropospheric aerosols. 相似文献
8.
V.A. Krasnopolsky 《Planetary and Space Science》1985,33(1):109-117
From estimates of drying effect in the cloud layer, data of the Venera 14 X-ray fluorescent spectroscopy, and evaluation of photochemical production of sulfuric acid, it follows that sulfuric acid and/or products of its further conversion should constitute not only the Mode 2 particles but most of the Mode 3 particles as well. The eddy mixing coefficient equals 2 × 104 cm2 s?1 in the cloud layer. The presence of ferric chloride in the cloud layer is indicated by the Venus u.v. absorption spectrum in the range of 3200–5000 Å, by the Venera 12 X-ray fluorescent spectrum, by the coincidence of the calculated FeCl3 condensate density profile and that of the Mode 1 in the middle and lower cloud layer, as well as by the upward flux of FeCl3 from the middle cloud layer which provides the necessary concentration of FeCl3 in H2SO4 solution. FeCl3 as the second absorber explains the localization of absorption in the upper cloud layer due to the FeCl3 conversion to ferric sulfate near the boundary between the upper and middle cloud layers. Other possible absorbers such as sulfur, ammonium pyrosulfite, nitrosylsulfuric acid, etc. are discussed. 相似文献
9.
《Planetary and Space Science》2006,54(13-14):1389-1397
We review the progress in our understanding of the composition of the Venus atmosphere since the publication of the COSPAR Venus International Reference Atmosphere volume in 1985. Results presented there were derived from data compiled in 1982–1983. More recent progress has resulted in large part from Earth-based studies of the near-infrared radiation from the nightside of the planet. These observations allow us to probe the atmosphere between the cloud tops and the surface. Additional insight has been gained through: (i) the analysis of ultraviolet radiation by satellites and rockets; (ii) data collected by the Vega 1 and 2 landers; (iii) complementary analyses of Venera 15 and 16 data; (iv) ground-based and Magellan radio occultation measurements, and (v) re-analyses of some spacecraft measurements made before 1983, in particular the Pioneer Venus and Venera 11, 13 and 14 data. These new data, and re-interpretations of older data, provide a much better knowledge of the vertical profile of water vapor, and more information on sulfur species above and below the clouds, including firm detections of OCS and SO. In addition, some spatial and/or temporal variations have been observed for CO, H2O, H2SO4, SO2, and OCS. New values of the D/H ratio have also been obtained. 相似文献
10.
Infrared spectra of Io in the region 2.5-5.0 micrometers, including new observational data, are analyzed using detailed laboratory studies of plausible surface ices. Besides the absorption bands attributable to sulfur dioxide frosts, four infrared spectral features of Io are shown to be unidentified. These unidentified features show spatial and temporal band strength variations. One pair is centered around 3.9 micrometers (3.85 and 3.91 micrometers) and the second pair is centered around 3.0 micrometers (2.97 and 3.15 micrometers). These absorptions fall close to the fundamental stretching modes in H2S and H2O, respectively. The infrared absorption spectra of an extensive set of laboratory ices ranging from pure materials, to binary mixtures of H2S and H2O (either mixed at different concentrations or layered), to H2O:H2S:SO2 mixtures are discussed. The effects of ultraviolet irradiation (120 and 160 nm) and temperature variation (from 9 to 130 K) on the infrared spectra of the ices are examined. This comparative study of Io reflectance spectra with the laboratory mixed ice transmission data shows the following: (1) Io's surface most likely contains H2S and H2O mixed with SO2. The 3.85- and 3.91-micrometers bands in the Io spectra can be accounted for by the absorption of the S-H stretching vibration (nu 1) in H2S clusters and isolated molecules in an SO2-dominated ice. The weak 2.97- and 3.15-micrometers bands which vary spatially and temporally in the Io spectra coincide with the nu 3 and nu 1 O-H stretching vibrations of clusters of H2O molecules complexed, through hydrogen bonding and charge transfer interactions, with SO2. (2) The observations are well matched qualitatively by the transmission spectra of SO2 ices containing about 3% H2S and 0.1% H2O which have been formed by the condensation of a mixture of the gases onto a 100 K surface. (3) No new features are produced in the region 2.5 to 5.0 micrometers in the spectrum of these ices under prolonged ultraviolet irradiation or temperature variation up to 120 K. (4) Comparison of the Io spectra to transmission spectra of both mixed molecular ices and layered ices indicates that only the former can explain the shifts and splitting of the absorption bands seen in the Io spectrum and additionally can account for the fact that solid H2S is observed in the surface material of Io at temperature and pressure conditions above the sublimation point of pure H2S. 相似文献
11.
Vladimir A. Krasnopolsky 《Icarus》2012,218(1):230-246
The model is intended to respond to the recent findings in the Venus atmosphere from the Venus Express and ground-based submillimeter and infrared observations. It extends down to 47 km for comparison with the kinetic model for the lower atmosphere (Krasnopolsky, V.A. [2007]. Icarus 191, 25–37) and to use its results as the boundary conditions. The model numerical accuracy is significantly improved by reduction of the altitude step from 2 km in the previous models to 0.5 km. Effects of the NUV absorber are approximated using the detailed photometric observations at 365 nm from Venera 14. The H2O profile is not fixed but calculated in the model. The model involves odd nitrogen and OCS chemistries based on the detected NO and OCS abundances. The number of the reactions is significantly reduced by removing of unimportant processes. Column rates for all reactions are given, and balances of production and loss may be analyzed in detail for each species.The calculated vertical profiles of CO, H2O, HCl, SO2, SO, OCS and of the O2 dayglow at 1.27 μm generally agree with the existing observational data; some differences are briefly discussed. The OH dayglow is ～30 kR, brighter than the OH nightglow by a factor of 4. The H + O3 process dominates in the nightglow excitation and O + HO2 in the dayglow, because of the reduction of ozone by photolysis. A key feature of Venus’ photochemistry is the formation of sulfuric acid in a narrow layer near the cloud tops that greatly reduces abundances of SO2 and H2O above the clouds. Delivery of SO2 and H2O through this bottleneck determines the chemistry and its variations above the clouds. Small variations of eddy diffusion near 60 km result in variations of SO2, SO, and OCS at and above 70 km within a factor of ～30. Variations of the SO2/H2O ratio at the lower boundary have similar but weaker effect: the variations within a factor of ～4 are induced by changes of SO2/H2O by ±5%. Therefore the observed variations of the mesospheric composition originate from minor variations of the atmospheric dynamics near the cloud layer and do not require volcanism. NO cycles are responsible for production of a quarter of O2, SO2, and Cl2 in the atmosphere. A net effect of photochemistry in the middle atmosphere is the consumption of CO2, SO2, and HCl from and return of CO, H2SO4, and SO2Cl2 to the lower atmosphere. These processes may be balanced by thermochemistry in the lower atmosphere even without outgassing from the interior, though the latter is not ruled out by our models. Some differences between the model and observations and the previous models are briefly discussed. 相似文献
12.
We have detected a new interstellar molecule, H2CN (methylene amidogen), in the cold, dark molecular cloud TMC-l. The column density of H2CN is estimated to be approximately 1.5 x 10(11) cm-2 by assuming an excitation temperature of 5 K. This column density corresponds to a fractional abundance relative to H2 of approximately 1.5 x 10(-11). This value is more than three orders of magnitude less than the abundance of the related molecule HCN in TMC-1. We also report a tentative detection of H2CN in Sgr B2(N). The formation mechanism of H2CN is discussed. Our detection of the H2CN molecule may suggest the existence of a new series of carbon-chain molecules, CH2CnN (n = 0, 1, 2,...). 相似文献
13.
We present the 2320-2050 cm-1 (4.31-4.88 micrometers) infrared spectra of 16 solid-state nitriles, isonitriles, and related compounds in order to facilitate the assignment of absorption features in a spectral region now becoming accessible to astronomers for the first time through the Infrared Space Observatory (ISO). This frequency range spans the positions of the strong C triple bond N stretching vibration of these compounds and is inaccessible from the ground due to absorption by CO2 in the terrestrial atmosphere. Band positions, profiles, and intrinsic strengths (A values) were measured for compounds frozen in Ar and H2O matrices at 12 K. The molecular species examined included acetonitrile, benzonitrile (phenylcyanide), 9-anthracenecarbonitrile, dimethylcyanamide, isopropylnitrile (isobutyronitrile), methylacrylonitrile, crotononitrile, acrylonitrile (vinyl cyanide), 3-aminocrotononitrile, pyruvonitrile, dicyandiamide, cyanamide, n-butylisocyanide, methylisocyanoacetate, diisopropylcarbodiimide, and hydrogen cyanide. The C triple bond N stretching bands of the majority of nitriles fall in the 2300-2200 cm-1 (4.35-4.55 micrometers) range and have similar positions in both Ar and H2O matrices, although the bands are generally considerably broader in the H2O matrices. In contrast, the isonitriles and a few exceptional nitriles and related species produce bands at lower frequencies spanning the 2200-2080 cm-1 (4.55-4.81 micrometers) range. These features also have similar positions in both Ar and H2O matrices, and the bands are broader in the H2O matrices. Three of the compounds (pyruvonitrile, dicyandiamide, and cyanamide) show unusually large shifts of their C triple bond N stretching frequencies when changing from Ar to H2O matrices. We attribute these shifts to the formation of H2O:nitrile complexes with these compounds. The implications of these results for the identification of the 2165 cm-1 (4.62 micrometers) "XCN" interstellar feature and the 4550 cm-1 (2.2 micrometers) feature of various objects in the solar system are discussed. 相似文献
14.
Vladimir A. Krasnopolsky 《Icarus》2010,208(2):539-547
Variations of the upper cloud boundary and the CO, HF, and HCl mixing ratios were observed using the CSHELL spectrograph at NASA IRTF. The observations were made in three sessions (October 2007, January 2009, and June 2009) at early morning and late afternoon on Venus in the latitude range of ±60°. CO2 lines at 2.25 μm reveal variations of the cloud aerosol density (∼25%) and scale height near 65 km. The measured reflectivity of Venus at low latitudes is 0.7 at 2.25 μm and 0.028 at 3.66 μm, and the effective CO2 column density is smaller at 3.66 μm than those at 2.25 μm by a factor of 4. This agrees with the almost conservative multiple scattering at 2.25 μm and single scattering in the almost black aerosol at 3.66 μm. The expected difference is just a factor of (1 − g)−1 = 4, where g = 0.75 is the scattering asymmetry factor for Venus’ clouds. The observed CO mixing ratio is 52 ± 4 ppm near 08:00 and 40 ± 4 ppm near 16:30 at 68 km, and the higher ratio in the morning may be caused by extension of the CO morningside bulge to the cloud tops. The observed weak limb brightening in CO indicates an increase of the CO mixing ratio with altitude. HF is constant at 3.5 ± 0.2 ppb at 68 km in both morningside and afternoon observations and in the latitude range ±60°. Therefore the observations do not favor a bulge of HF, though HF is lighter than CO. Probably a source in the upper atmosphere facilitates the bulge formation. The recent measurements of HCl near 70 km are controversial (0.1 and 0.74 ppm) and require either a strong sink or a strong source of HCl in the clouds. The HCl lines of the (2-0) band are blended by the solar and telluric lines. Therefore we observed the P8 lines of the (1-0) band at 3.44 μm. These lines are spectrally clean and result in the HCl mixing ratio of 0.40 ± 0.03 ppm at 74 km. HCl does not vary with latitude within ±60°. Our observations support a uniformly mixed HCl throughout the Venus atmosphere. 相似文献
15.
Ricard A Cernogora G Fitaire M Hochard L Kouassi N Speller C Vacher JR 《Planetary and Space Science》1995,43(1-2):41-46
The kinetic reactions in N2-xCH4(C2H2) gas discharges with x less than 1% have been studied by emission spectroscopy in the afterglow of D.C. discharges and by mass spectroscopy from radiolysis ionization using alpha particles. The pressure range is from several Torr to 100 Torr. At the end of N2 D.C. discharges at room temperature, for a residence time of about 10(-2) s, the dominant active species are the N atoms with density of 10(14)-10(15) cm-3 for N2 density of about 10(17) cm-3 (3 Torr), the N2(X,V) vibrational molecules with for example [N2(X,V = 10)] approximately 10(14) cm-3 and the electronic metastable molecules N2(A 3 sigma u +) with a density of 10(12) cm-3. In such conditions, the following kinetic reactions have been studied: N2(A) + N2(A) --> N2(C,B,V') + N2(X), N2(A) + N2(X,V>5) --> N2(X) + N2(B,V') in pure N2 post-discharges and N2(A) + CH4 --> products, C + N + M2 --> CN(B,V') + M2, N2(X,V>4) + CN --> N2(X) + CN(B,A,V'), in N2-1% CH4 post-discharges. The clustering reactions of N2-(1-5%)CH4(C2H2) gas mixtures after radiolysis ionization have been studied for the H2CN+ nN2 ions and the equilibrium constants have been determined in the temperature range T = 140-300 K. 相似文献
16.
Several interstellar molecules have been detected toward the highly perturbed B and G clouds associated with the supernova remnant IC 443 via their 3 mm transitions, including N2H+, SiO, SO, CN, HNC, and H13CO+. The (J, K) = (1, 1) and (2, 2) inversion lines of metastable ammonia have also been observed, as well as the J = 3-2 transition of HCO+ at 1.2 mm. Analysis of the (1, 1) and (2, 2) inversion lines of NH3 indicates minimum gas kinetic temperatures of TK = 70 K toward cloud B, and TK = 33 K in cloud G. Modeling of the J = 1-0 and J = 3-2 transitions of HCO+ implies densities greater than 10(5) cm-3 toward both positions. These data clearly show that hot and dense material is present in IC 443, and they suggest the presence of shocks in both regions. A careful analysis of the HCO+ lines indicates that the HCO+ abundance is at most enhanced by factors of a few over that found in cold, quiescent gas. This conclusion contradicts past claims of HCO+ abundance enhancements of several orders of magnitude in the perturbed regions. The N2H+ abundance was also found to be similar to that in cold gas, suggesting that there is no increase in ionization in the clouds. The abundances of SO and CS, as well as CN and NH3, do not appear to differ significantly from those found in cold dark clouds, although chemistry models predict sulfur-containing species to undergo high-temperature enhancements. SiO, however, is found to have an abundance in the perturbed gas 100 times larger than the upper limits observed in the dark cloud TMC 1, a result in agreement with high temperature chemistry models. In addition, the HNC/HCN ratio in both IC 443 B and G was found to be approximately 0.1--far from the ratio of 1 predicted by low-temperature ion-molecule chemistry, but similar to the values observed in clouds where elevated temperatures are present. 相似文献
17.
Ohishi M Ishikawa SI Amano T Oka H Irvine WM Dickens JE Ziurys LM Apponi AJ 《The Astrophysical journal》1996,471(1):L61-L64
A new interstellar molecular ion, H2COH+ (protonated formaldehyde), has been detected toward Sgr B2, Orion KL, W51, and possibly in NGC 7538 and DR21(OH). Six transitions were detected in Sgr B2(M). The 1(1,0)-1(0,1) transition was detected in all sources listed above. Searches were also made toward the cold, dark clouds TMC-1 and L134N, Orion (3N, 1E), and a red giant, IRC + 10216, without success. The excitation temperatures of H2COH+ are calculated to be 60-110 K, and the column densities are on the order of 10(12)-10(14) cm-2 in Sgr B2, Orion KL, and W51. The fractional abundance of H2COH+ is on the order of 10(-11) to 10-(9), and the ratio of H2COH+ to H2CO is in the range 0.001-0.5 in these objects. The values in Orion KL seem to be consistent with the "early time" values of recent model calculations by Lee, Bettens, & Herbst, but they appear to be higher than the model values in Sgr B2 and W51 even if we take the large uncertainties of column densities of H2CO into account. We suggest production routes starting from CH3OH may play an important role in the formation of H2COH+. 相似文献
18.
Both laboratory measurements and theory indicate that CO2 should be a common component in interstellar ices. We show that the exact band position, width, and profile of the solid-state 12CO2 infrared bands near 3705, 3600, 2340, and 660 cm-1 (2.70, 2.78, 4.27, and 15.2 micrometers) and the 13CO2 band near 2280 cm-1 (4.39 micrometers) are dependent on the matrix in which the CO2 is frozen. Measurements of these bands in astronomical spectra can be used to determine column densities of solid-state CO2 and provide important information on the physical conditions present in the ice grains of which the CO2 is a part. Depending on the composition of the ice, the CO2 asymmetric stretching band was observed to vary from 2328.7 to 2346.0 cm-1 and have full widths at half-maxima (FWHMs) ranging from 4.7 to 29.9 cm-1. The other CO2 bands showed similar variations. Both position and width are also concentration dependent. Absorption coefficients were determined for the five CO2 bands. These were found to be temperature independent for CO2 in CO and CO2 matrices but varied slightly with temperature for CO2 in H2O-rich ices. For all five bands this variation was found to be less than 15% from 10 to 150 K, the temperature at which H2O ice sublimes. A number of parameters associated with the physical behavior of CO2 in CO2- and H2O-rich ices were also determined. The CO2-CO2 surface binding energy in pure CO2 ices is found to be (delta Hs/k) = 2690 +/- 50 K. CO2-H2O and CO-H2O surface binding energies were determined to be (delta Hs/k) = 2860 +/- 200 K and 1740 +/- 100 K, respectively. Under our experimental conditions, CO2 condenses in measurable quantities into H2O-rich ices at temperatures up to 100 K, only slightly higher than the temperature at which pure CO2 condenses. Once frozen into an H2O-rich ice, the subsequent loss of CO2 upon warming is highly dependent on concentration. For ices with H2O/CO2 > 20, the CO is physically trapped within the H2O lattice, and little CO2 is lost until the sublimation temperature of the H2O matrix is reached. In contrast, in ices having H2O/CO2 < 5, the CO2 remains only to temperatures of about 90 K. Above this point the CO2 readily diffuses out of the H2O matrix. These results suggest that two different forms of H2O lattice are produced. The implications of these data for cometary models and our understanding of cometary formation are considered. 相似文献
19.
We speculate on the origin and physical properties of haze in the upper atmosphere of Venus. It is argued that at least four distinct types of particles may be present. The densest and lowest haze, normally seen by spacecraft, probably consists of a submicron sulfuric acid aerosol which extends above the cloud tops (at ～70 km) up to ～80 km; this haze represents an extension of the upper cloud deck. Measurements of the temperature structure between 70 and 120 km indicate that two independent water ice layers may occasionally appear. The lower one can form between 80 and 100 km and is probably the detached haze layer seen in high-contrast limb photography. This ice layer is likely to be nucleated on sulfuric acid aerosols, and is analogous to the nacreous (stratospheric) clouds on Earth. At the Venus “mesopause” near 120 km, temperatures are frequently cold enough to allow ice nucleation on meteoric dust or ambient ions. The resulting haze (which is analogous to noctilucent clouds on Earth) is expected to be extremely tenous, and optically invisible. On both Earth and Venus, meteoric dust is present throughout the upper atmosphere and probably has similar properties. 相似文献
20.
First measurements of SO2 and SO in the Venus mesosphere (70-100 km) are reported. This altitude range is distinctly above the ∼60-70 km range to which nadir-sounding IR and UV investigations are sensitive. Since July 2004, use of ground-based sub-mm spectroscopy has yielded multiple discoveries. Abundance of each molecule varies strongly on many timescales over the entire sub-Earth Venus hemisphere. Diurnal behavior is evident, with more SO2, and less SO, at night than during the day. Non-diurnal variability is also present, with measured SO2 and SO abundances each changing by up to 2× or more between observations conducted on different dates, but at fixed phase, hence identical sub-Earth Venus local times. Change as large and rapid as a 5σ doubling of SO on a one-week timescale is seen. The sum of SO2 and SO abundances varies by an order of magnitude or more, indicating at least one additional sulfur reservoir must be present, and that it must function as both a sink and source for these molecules. The ratio SO2/SO varies by nearly two orders of magnitude, with both diurnal and non-diurnal components. In contrast to the strong time dependence of molecular abundances, their altitude distributions are temporally invariant, with far more SO2 and SO at 85-100 km than at 70-85 km. The observed increase of SO2 mixing ratio with altitude requires that the primary SO2 source be upper mesospheric photochemistry, contrary to atmospheric models which assert upward transport as the only source of above-cloud SO2. Abundance of upper mesospheric aerosol, with assumption that it is composed primarily of sulfuric acid, is at least sufficient to provide the maximum gas phase (SO + SO2) sulfur reported in this study. Sulfate aerosol is thus a plausible source of upper mesospheric SO2. 相似文献