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1.
The formation of methylamine (CH3NH2) in the upper troposphere and lower stratosphere of Jupiter is investigated. Translationally hot hydrogen atoms are produced in the photolysis of ammonia, phosphine, and acetylene which react with methane to produce methyl (CH3) radicals; the latter recombine with NH2 to form CH3NH2. Also, methane is catalytically dissociated to CH3 + H by the species C2 and C2H produced in the photolysis of acetylene. It is shown that the combined production of CH3NH2 and subsequent photolysis to HCN is unlikely to account for the HCN observed near Jupiter's tropopause. Recombination of NH2 and C2H5N followed by photolysis to HCN is the preferred path. Production of C2H6 by these two processes is negligible in comparison to the downward flux of C2H6 from the Lyman α photolysis region of CH4. An upper limit column density on CH3PH2 is estimated to be ～1013 cm?2 as compared to 1015 cm?2 for CH3NH2. Hot H atoms account for a negligible fraction of the total ortho-para conversion by the reaction H + H2  相似文献   

2.
We report the detection of HCN on Jupiter. Three R-branch lines of the ν2 fundamental of HCN near 13.5 μm were observed in absorption, from which the HCN column density is inferred to be 5 × 10?3 cm-am with an uncertainty of a factor of 2. If emission from the stratosphere exists, then the derived column density is only a lowe limit. We suggest that the Jovian HCN most likely originates from the photolysis of CH4 and NH3 in the lower stratosphere and upper troposphere. In addition, an upper limit of 2.5 × 10?2 cm-am was established for the column density of HCN on Saturn.  相似文献   

3.
Robert E. Samuelson 《Icarus》1983,53(2):364-387
A simple global radiative equilibrium model is developed for Titan. It is restricted to the two-stream approximation, is vertically homogeneous in its scattering properties, and is spectrally divided into one thermal and two solar channels. A partially absorbing “violet” channel is responsible for heating in the stratosphere, while a conservatively scattering “red” channel permits heating at the surface. The optical thickness of the atmosphere in the red is 1 < τ1r < 3. Between 13 and 33% of the total incident solar radiation is absorbed at the planetary surface. The ratio of violet to thermal infrared absorption cross sections is between 30 and 60 in the stratosphere, leading to the large temperature inversion observed there. The observed and theoretically computed tropopause temperatures are 72 and 69°K, respectively, while their corresponding thermal optical depths are, respectively, ～0.1 and ～0.07. The spectrally integrated mass absorption coefficient at thermal wavelengths is approximately constant throughout the stratosphere and roughly linear with pressure in the troposphere. This in turn implies the presence of a uniformly mixed aerosol in the stratosphere, and suggests pressure-induced absorption by gaseous N2CH4H2 in the troposphere. In addition there appear to be two regions of enhanced opacity near 30 and 500 mbar which may be due to C2H2C2H6C3H8 and CH4 condensation clouds, respectively.  相似文献   

4.
High sensitivity observations were performed at 1.2- and 3-mm wavelengths with the IRAM 30-m telescope (Spain) between April 1996 and December 1999 to investigate the nitrile composition of Titan's stratosphere. A part of our dataset consists of high resolution spectra of HC14N taken at 88.6 GHz as well as spectra of HC15N recorded at 258.16 GHz. From a thorough analysis of both lines and with the help of appropriate radiative transfer calculations we show that the isotopic ratio 15N/14N is strongly enhanced compared to the terrestrial value. We propose that the range 3.9-4.5 should be considered as a basis for the enrichment factor. Five individual lines of HC3N were measured at 39-kHz resolution using a frequency-switched technique. Several CH3CN features were recorded at 78-kHz resolution in two transitions around 147.6 and 220.7 GHz. The high spectral resolution and the good signal-to-noise ratio affecting the spectra permit us to retrieve disk-averaged vertical profiles for HCN up to 450 km and for HC3N and CH3CN up to 500 km. Comparison of our inferred vertical profiles with relevant results of presently published photochemical models is presented. We show that the profiles of HCN and HC3N predicted by various authors below 450-km altitude appear inconsistent with our new observations. We find that the three distributions present very different gradients of abundance below 200-km altitude down to the condensation levels around 80 km. In the upper stratosphere HC3N and CH3CN have approximately the same mixing ratio of about 4×10−8 at 450 km, at least one order of magnitude lower than that of HCN. In the same time, another nitrile HC5N has been searched for by observing four transitions located between 109 and 221 GHz. As no spectral features could be detected after several hours of integration time, we propose an upper limit for the mixing ratio equal to 4×10−10 assuming a uniform distribution of this compound in the lower stratosphere.  相似文献   

5.
Observations of Jupiter by Cassini/CIRS, acquired during the December 2000 flyby, provide the latitudinal distribution of HCN and CO2 in Jupiter's stratosphere with unprecedented spatial resolution and coverage. Following up on a preliminary study by Kunde et al. [Kunde, V.G., and 41 colleagues, 2004. Science 305, 1582-1587], the analysis of these observations leads to two unexpected results (i) the total HCN mass in Jupiter's stratosphere in 2000 was (6.0±1.5)×1013 g, i.e., at least three times larger than measured immediately after the Shoemaker-Levy 9 (SL9) impacts in July 1994 and (ii) the latitudinal distributions of HCN and CO2 are strikingly different: while HCN exhibits a maximum at 45° S and a sharp decrease towards high Southern latitudes, the CO2 column densities peak over the South Pole. The total CO2 mass is (2.9±1.2)×1013 g. A possible cause for the HCN mass increase is its production from the photolysis of NH3, although a problem remains because, while millimeter-wave observations clearly indicate that HCN is currently restricted to submillibar (∼0.3 mbar) levels, immediate post-impact infrared observations have suggested that most of the ammonia was present in the lower stratosphere near 20 mbar. HCN appears to be a good atmospheric tracer, with negligible chemical losses. Based on 1-dimensional (latitude) transport models, the HCN distribution is best interpreted as resulting from the combination of a sharp decrease (over an order of magnitude in Kyy) of wave-induced eddy mixing poleward of 40° and an equatorward transport with velocity. The CO2 distribution was investigated by coupling the transport model with an elementary chemical model, in which CO2 is produced from the conversion of water originating either from SL9 or from auroral input. The auroral source does not appear adequate to reproduce the CO2 peak over the South Pole, as required fluxes are unrealistically high and the shape of the CO2 bulge is not properly matched. In contrast, the CO2 distribution can be fit by invoking poleward transport with a velocity and vigorous eddy mixing (). While the vertical distribution of CO2 is not measured, the combined HCN and CO2 results imply that the two species reside at different stratospheric levels. Comparing with the circulation regimes predicted by earlier radiative-dynamical models of Jupiter's stratosphere, and with inferences from the ethane and acetylene stratospheric latitudinal distribution, we suggest that CO2 lies in the middle stratosphere near or below the 5-mbar level.  相似文献   

6.
Thirteen lines of the CO band near 4.7 μm have been observed on a jovian hot spot at a resolution of 0.045 cm−1. The measured line profiles indicate that the CO mole fraction is 1.0±0.2 ppb around the 6-bar level and is larger in the upper troposphere and/or stratosphere. An external source of CO providing an abundance of 4+3−2×1016 molecules cm−2 is implied by the observations in addition to the amount deposited at high altitude by the Shoemaker-Levy 9 collision. From a simple diffusion model, we estimate that the CO production rate is (1.5-10)×106 molecules cm−2 s−1 assuming an eddy diffusion coefficient around the tropopause between 300 and 1500 cm2 s−1. Precipitation of oxygen atoms from the jovian magnetosphere or photochemistry of water vapor from meteoroidal material can only provide a negligible contribution to this amount. A significant fraction of the CO in Jupiter's upper atmosphere may be formed by shock chemistry due to the infall of kilometer- to subkilometer-size Jupiter family comets. Using the impact rate from Levison et al. (2000, Icarus143, 415-420) rescaled by Bottke et al. (2002, Icarus156, 399-433), this source can provide the observed stratospheric CO only if the eddy diffusion coefficient around the tropopause is 100-300 cm2 s−1. Higher values, ∼700 cm2 s−1, would require an impact rate larger by a factor of 5-10, which cannot be excluded considering uncertainties in the distribution of Jupiter family comets. Such a large rate is indeed consistent with the observed cratering record of the Galilean satellites (Zahnle et al. 1998, Icarus136, 202-222). On the other hand, the ∼1 ppb concentration in the lower troposphere requires an internal source. Revisiting the disequilibrium chemistry of CO in Jupiter, we conclude that rapid vertical mixing can provide the required amount of CO at ∼6 bar for a global oxygen abundance of 0.2-9 times the solar value considering the uncertainties in the convective mixing rate and in the chemical constants.  相似文献   

7.
Limb spectra recorded by the Composite InfraRed Spectrometer (CIRS) on Cassini provide information on abundance vertical profiles of C2H2, C2H4, C2H6, CH3C2H, C3H8, C4H2, C6H6 and HCN, along with the temperature profiles in Titan's atmosphere. We analyzed two sets of spectra, one at 15° S (Tb flyby) and the other one at 80° N (T3 flyby). The spectral range 600-1400 cm−1, recorded at a resolution of 0.5 cm−1, was used to determine molecular abundances and temperatures in the stratosphere in the altitude range 100-460 km for Tb and 170-495 km for T3. Both temperature profiles show a well defined stratopause, at around 310 km (0.07 mbar) and 183 K at 13° S, and 380 km (0.01 mbar) with 207 K at 80° N. Near the north pole, stratospheric temperatures are colder and mesospheric temperatures are warmer than near the equator. C2H2, C2H6, C3H8 and HCN display vertical mixing ratio profiles that increase with height at 15° S and 80° N, consistent with their formation in the upper atmosphere, diffusion downwards and condensation in the lower stratosphere, as expected from photochemical models. The CH3C2H and C4H2 mixing ratios also increase with height at 15° S. But near the north pole, their profiles present an unexpected minimum around 300 km, observed for the first time thanks to the high vertical resolution of the CIRS limb data. C2H4 is the only molecule having a vertical abundance profile that decreases with height at 15° S. At 80° N, it also displays a minimum of its mixing ratio around the 0.1-mbar level. For C6H6, an upper limit of 1.1 ppb (in the 0.3-10 mbar range) is derived at 15° S, whereas a constant mixing ratio profile of is inferred near the north pole. At 15° S, the vertical profile of HCN exhibits a steeper gradient than other molecules, which suggests that a sink for this molecule exists in the stratosphere, possibly due to haze formation. All molecules display a more or less pronounced enrichment towards the north pole, probably due, in part, to subsidence of air at the north (winter) pole that brings air enriched in photochemical compounds from the upper atmosphere to lower levels.  相似文献   

8.
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   

9.
The dynamics of Titan's stratosphere is discussed in this study, based on a comparison between observations by the CIRS instrument on board the Cassini spacecraft, and results of the 2-dimensional circulation model developed at the Institute Pierre-Simon Laplace, available at http://www.lmd.jussieu.fr/titanDbase [Rannou, P., Lebonnois, S., Hourdin, F., Luz, D., 2005. Adv. Space Res. 36, 2194-2198]. The comparison aims at both evaluating the model's capabilities and interpreting the observations concerning: (1) dynamical and thermal structure using temperature retrievals from Cassini/CIRS and the vertical profile of zonal wind at the Huygens landing site obtained by Huygens/DWE; and (2) vertical and latitudinal profiles of stratospheric gases deduced from Cassini/CIRS data. The modeled thermal structure is similar to that inferred from observations (Cassini/CIRS and Earth-based observations). However, the upper stratosphere (above 0.05 mbar) is systematically too hot in the 2D-CM, and therefore the stratopause region is not well represented. This bias may be related to the haze structure and to misrepresented radiative effects in this region, such as the cooling effect of hydrogen cyanide (HCN). The 2D-CM produces a strong atmospheric superrotation, with zonal winds reaching 200 m s−1 at high winter latitudes between 200 and 300 km altitude (0.1-1 mbar). The modeled zonal winds are in good agreement with retrieved wind fields from occultation observations, Cassini/CIRS and Huygens/DWE. Changes to the thermal structure are coupled to changes in the meridional circulation and polar vortex extension, and therefore affect chemical distributions, especially in winter polar regions. When a higher altitude haze production source is used, the resulting modeled meridional circulation is weaker and the vertical and horizontal mixing due to the polar vortex is less extended in latitude. There is an overall good agreement between modeled chemical distributions and observations in equatorial regions. The difference in observed vertical gradients of C2H2 and HCN may be an indicator of the relative strength of circulation and chemical loss of HCN. The negative vertical gradient of ethylene in the low stratosphere at 15° S, cannot be modeled with simple 1-dimensional models, where a strong photochemical sink in the middle stratosphere would be necessary. It is explained here by dynamical advection from the winter pole towards the equator in the low stratosphere and by the fact that ethylene does not condense. Near the winter pole (80° N), some compounds (C4H2, C3H4) exhibit an (interior) minimum in the observed abundance vertical profiles, whereas 2D-CM profiles are well mixed all along the atmospheric column. This minimum can be a diagnostic of the strength of the meridional circulation, and of the spatial extension of the winter polar vortex where strong descending motions are present. In the summer hemisphere, observed stratospheric abundances are uniform in latitude, whereas the model maintains a residual enrichment over the summer pole from the spring cell due to a secondary meridional overturning between 1 and 50 mbar, at latitudes south of 40-50° S. The strength, as well as spatial and temporal extensions of this structure are a difficulty, that may be linked to possible misrepresentation of horizontally mixing processes, due to the restricted 2-dimensional nature of the model. This restriction should also be kept in mind as a possible source of other discrepancies.  相似文献   

10.
A model is presented for the photochemistry of PH3 in the upper troposphere and lower stratosphere of Saturn that includes the effects of coupling with NH3 and hydrocarbon photochemistry, specifically the C2H2 catalyzed photodissociation of CH4. PH3 is rapidly depleted with altitude (scale height ～35 km) in the upper troposphere when K～104cm2sec?1; an upper limit for K at the tropopause is estimated at ～105cm2sec?1. If there is no gas phase P2H4 because of sublimation, P2 and P4 formation is unlikely unless the rate of the spin-forbidden recombination reaction PH + H2 + M → PH3 + M is exceedingly slow. An upper limit P4 column density of ～2×1015cm?2 is estimated in the limit of no recombination. If sublimation does not remove all gas phase P2H4, P2 and P4 may be produced in potentially larger quantities, although they would be restricted almost entirely to the lowest levels of our model, where T?100°K. Potentially observable amounts of the organophosphorus compounds CH3P2H2 and HCP are predicted, with column densities of >1017 cm?2 and production rates of ～2×108cm?2sec?1. The possible importance of electronically excited states of PHx and additional PH3/hydrocarbon photochemical coupling paths are also considered.  相似文献   

11.
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   

12.
Mid-infrared limb spectra in the range 600-1400 cm−1 taken with the Composite InfraRed Spectrometer (CIRS) on-board the Cassini spacecraft were used to determine vertical profiles of HCN, HC3N, C2H2, and temperature in Titan's atmosphere. Both high (0.5 cm−1) and low (13.5 cm−1) spectral resolution data were used. The 0.5 cm−1 data gave profiles at four latitudes and the 13.5 cm−1 data gave almost complete latitudinal coverage of the atmosphere. Both datasets were found to be consistent with each other. High temperatures in the upper stratosphere and mesosphere were observed at Titan's northern winter pole and were attributed to adiabatic heating in the subsiding branch of a meridional circulation cell. On the other hand, the lower stratosphere was much colder in the north than at the equator, which can be explained by the lack of solar radiation and increased IR emission from volatile enriched air. HC3N had a vertical profile consistent with previous ground based observations at southern and equatorial latitudes, but was massively enriched near the north pole. This can also be explained in terms of subsidence at the winter pole. A boundary observed at 60° N between enriched and un-enriched air is consistent with a confining polar vortex at 60° N and HC3N's short lifetime. In the far north, layers were observed in the HC3N profile that were reminiscent of haze layers observed by Cassini's imaging cameras. HCN was also enriched over the north pole, which gives further evidence for subsidence. However, the atmospheric cross section obtained from 13.5 cm−1 data indicated a HCN enriched layer at 200-250 km, extending into the southern hemisphere. This could be interpreted as advection of polar enriched air towards the south by a meridional circulation cell. This is observed for HCN but not for HC3N due to HCN's longer photochemical lifetime. C2H2 appears to have a uniform abundance with altitude and is not significantly enriched in the north. This is consistent with observations from previous CIRS analysis that show increased abundances of nitriles and hydrocarbons but not C2H2 towards the north pole.  相似文献   

13.
A. Bar-Nun  M. Podolak 《Icarus》1985,64(1):112-124
The lightning energy dissipation rate on Jupiter from Voyager's observation is used, together with shock-tube experimental results and reasonable eddy diffusion coefficients for the various atmospheric layers, to compute the column abundances of lightning-produced CO, C2H2, and HCN. Shock-tube experiments on the hydrogenation of CO clearly rule out chemical “freezing” of CO at the 1064°K and 400-bar level and its subsequent upwelling to the upper atmosphere. Also, lightning in the water cloud cannot produce enough CO to meet its observed abundance. Hence, the CO is formed from an external source of oxygen or water. The production of acetylene both by lightning above the water cloud and by startospheric methane photolysis is required to maintain its observed abundance against destruction processes. This explains the decrease in the C2H2/C2H6 ratio from the equator to the pole, as observed in the IR. HCN production by lightning above the water cloud is sufficient to account for its observed abundance and meets the observational requirement of a tropospheric HCN source.  相似文献   

14.
Dale W. Smith 《Icarus》1980,44(1):116-133
The Galilean satellite eclipse technique for measuring the aerosol distribution in the Jovian lower stratosphere and upper troposphere is described and applied using 30 color observations of 12 natural satellite eclipses obtained with the 200-in Hale telescope. These events probe the North and South Polar Regions, the North Temperate Belt, the South Equatorial Belt, the South Tropical Zone, the South Temperate Zone, and the Great Red Spot. Aerosol is found above the visible cloud tops in all locations. It is very tenuous and varies with altitude, increasing rapidly with downward passage through the tropopause. The aerosol extinction coefficient at 1.05 μm is 1.0 ± 0.05 × 10?8 cm?1 at the tropopause and the mass density is a few times 10?13 g cm?3. The observations require some aerosol above the tropopause but do not clearly determine its structure. The present analysis emphasizes an extended haze distribution, but the alternate possibility that the stratospheric aerosol resides in a thin layer is not excluded. The vertical aerosol optical depth above the tropopause at 1.05 μm exceeds 0.04 in the NPR, SPR, NTB, SEB, and StrZ, is ～0.006 ± 0.003 in the STZ, and is ～ 0.003 ± 0.001 above the GRS. The aerosol extinction increases with decreasing wavelength in the STZ and NTB and indicates a particle radius of 0.2–0.5 μm; a radius of ～0.9 μm is indicated in the STrZ.  相似文献   

15.
New thermal profiles of Jupiter are retrieved from recent far infrared spectral measurements and for H2 mixing ratios varying from 0.8 to 0.94. The effective temperature corresponding to the inferred thermal profile is 123.15 ± 0.35°K. Far-infrared brightness temperature spectra computed from these profiles are compared to experimental data including measurements made at high spectral resolution in the NH3ν2 band at 10 μm and in NH3 pure rotational bands between 40 and 110 μm. It is found that a strong depletion of NH3 does occur in the Jovian stratosphere and that ammonia seems to be undersaturated in the upper troposphere.  相似文献   

16.
R. de Kok  P.G.J. Irwin  N.A. Teanby 《Icarus》2008,197(2):572-578
In Titan's north polar region stratospheric clouds are expected to form due to a combination of low temperatures and downward motion of volatile-enriched air. Here we investigate possible sources of stratospheric clouds at Titan's pole using data from the Cassini Composite Infrared Spectrometer and a simple condensation model. An upper limit for C4N2 gas was determined to be 9×10−9, which is less than required to make the C4N2 cloud at the Voyager epoch. Hence, the presence of this cloud after equinox remains a mystery. The largest cloud seen in far-infrared spectra has a feature around 220 cm−1 and is located around an altitude of 140 km. The upper limit for propionitrile (C2H5CN) gas shows that the feature around 220 cm−1 is probably not due to pure propionitrile ice. Instead, our model calculations show that HCN should cause by far the largest cloud around 140 km. We therefore propose that HCN ice plays an important role in the formation of the massive polar cloud, because of the unavailability of sufficient condensable gas other than HCN to produce a strong enough condensate feature. However, the signature at 220 cm−1 is not consistent with that of pure HCN ice at 172 cm−1 and mixing of HCN ice with other ices, or chemical alteration of HCN ice might mask the HCN ice signature.  相似文献   

17.
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   

18.
A radiative seasonal model which incorporates a multilayer radiative transfer treatment at wave-lengths longward of 7 μm is presented and applied to Saturn's stratosphere. Opacities due to H2-He, CH4, C2H2, and C2H6 are included. Season-dependent insolation is shown to produce a strong hemispheric asymmetry decreasing with depth at the Voyager encounter times, and seasonal amplitudes of 30°K at the poles are predicted in the high stratosphere. The ring-modulated dependence of the insolation and the orbital eccentricity are shown to have a significant effect. Calculations agree closely with the Voyager 1 and 2 radio occultation ingress profiles recorded at 76°S and 36.5°S for CH4/H2 = 3.5 + 1.4/? 1.0 × 10?3;the estimated errors include modeling systematic errors and uncertainties in the occultations profiles. The possible role of aerosols in the stratospheric heating is analyzed. The Voyager 2 egress profile recorded at 31°S cannot be reproduced by calculations. Some constraints on the C2H2 and C2H6 abundances are derived. The upper portion of the occultation profiles (p < 3mbar) can be matched for C2H2/H2 = 1.0 + 1.3/?0.6 × 10?7, C2H6/H2 = 1.5 + 1.8/?0.9 × 10?6 at 76°S and C2H2/H2 = 4 + 6/?4 × 10?8, C2H6/H2 = 6 + 9/?6 × 10?7 at 36.5°N. At the northern occultation latitude, the discrepancy with the concentrations derived from analysis of IRIS spectra by R. Courtin, D. Gautier, A. Marten, B. Bézard, and R. Hanel (1984, Astrophys. J.287) can be explained by a sharp variation of the mixing ratios of these gases with altitude in the upper stratosphere. Other interpretations are discussed.  相似文献   

19.
Analysis of the 250-560 cm−1 spectral continuum of Titan's north polar hood just after spring equinox reveals, in addition to the ubiquitous aerosol, a tenuous but relatively uniform cloud of small particles permeating the lower stratosphere at altitudes between 58 and 90 km. Voyager 1 IRIS data suggest the particles are highly scattering, almost certainly condensed organics, with radii between 1 and 5 μm. Mole fractions for the condensed material range between 4×10−8 and 4×10−6, depending upon particle size. Vapor pressure arguments imply condensed nitriles near 90 km, the most likely being HCN, with condensed hydrocarbons such as C2H6 restricted to regions considerably nearer the tropopause. No direct chemical identification is possible. Negligible methane supersaturation in the troposphere at 67.4° N latitude, when compared with degrees of supersaturation at other latitudes, hints at precipitation fluxes of north polar stratospheric condensates during the previous northern winter that were perhaps three orders of magnitude greater than those at low latitudes during that time. A scale height of 1.5 times the density scale height above 160 km is reaffirmed for the photochemical aerosol of the north polar hood. There appears to be a depletion of aerosol somewhere below 160 km. An aerosol mole fraction ∼8×10−8 at 160 km is inferred, about 33% greater than the value derived in a previous study. The Cassini CIRS instrument, with its expanded spectral range and higher spectral resolution, should be able to provide highly complementary information for the time period covering most of the northern winter season.  相似文献   

20.
Infrared emission lines of stratospheric ammonia (NH3) were observed following the collisions of the fragments of Comet Shoemaker-Levy 9 with Jupiter in July of 1994 at the impact sites of fragments G and K. Infrared heterodyne spectra near 10.7 μm were obtained by A. Betz et al. (in Abstracts for Special Sessions on Comet Shoemaker-Levy 9, The 26th Meeting of the Division for Planetary Sciences, Washington DC, 31 Oct.-4 Nov. 1994, p. 25) using one of the Infrared Spatial Interferometer telescope systems on Mount Wilson. Lineshapes of up to three different NH3 emission lines were measured at a resolving power of ∼107 at multiple times following the impacts. We present here our radiative transfer analysis of the fully resolved spectral lineshapes of the multiple rovibrational lines. This analysis provides information on temperature structure and NH3 abundance distributions and their temporal changes up to 18 days after impact. These results are compared to photochemical models to determine the role of photochemistry and other mechanisms in the destruction and dilution of NH3 in the jovian stratosphere after the SL9 impacts.One day following the G impact, the inferred temperature above 0.001 mbar altitude is 283±13 K, consistent with a recent plume splashback model. Cooling of the upper stratosphere to 204 K by the fourth day and to quiescence after a week is consistent with a simple gray atmosphere radiative flux calculation and mixing with cold jovian air. During the first 4 days after impact, NH3 was present primarily at altitudes above 1 mbar with a column density of (7.7±1.6)×1017 cm−2 after 1 day and (3.7±0.8)×1017 cm−2 after 4 days. (Errors represent precision.) We obtained >2.5 times more NH3 than can be supplied by nitrogen from a large cometary fragment, suggesting a primarily jovian source for the NH3. By 18 days postimpact, a return to quiescent upper stratospheric temperature is retrieved for the G region, with an NH3 column density of 7.3×1017 cm−2 or more in the lower stratosphere, possibly supplied by NH3 upwelling across an impact-heated and turbulent tropopause, which may have been masked by initial dust and haze. Above the 1-mbar level, the maximum retrieved column density decreased to 6.5×1016 cm−2. Comparison to photochemical models indicates that photolysis alone is not sufficient to account for the loss of NH3 above 1 mbar by that time, even when chemical reformation of NH3 is ignored. We speculate that the dispersion of plume material at high altitudes (above 1 mbar) is responsible for the change in the spectra observed a few days postimpact. Data on the K impact region provide qualitatively consistent results.  相似文献   

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