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1.
We report observation and analysis of a high-resolution 2.87-3.54 μm spectrum of the southern temperate region of Saturn obtained with NIRSPEC at Keck II. The spectrum reveals absorption and emission lines of five molecular species as well as spectral features of haze particles. The ν2+ν3 band of CH3D is detected in absorption between 2.87 and 2.92 μm; and we derived from it a mixing ratio approximately consistent with the Infrared Space Observatory result. The ν3 band of C2H2 also is detected in absorption between 2.95 and 3.05 μm; analysis indicates a sudden drop in the C2H2 mixing ratio at 15 mbar (130 km above the 1 bar level), probably due to condensation in the low stratosphere. The presence of the ν3+ν9+ν11 band of C2H6 near 3.07 μm, first reported by Bjoraker et al. [Bjoraker, G.L., Larson, H.P., Fink, U., 1981. Astrophys. J. 248, 856-862], is confirmed, and a C2H6 condensation altitude of 10 mbar (140 km) in the low stratosphere is determined. We assign weak emission lines within the 3.3 μm band of CH4 to the ν7 band of C2H6, and derive a mixing ratio of 9±4×10−6 for this species. Most of the C2H6 3.3 μm line emission arises in the altitude range 460-620 km (at ∼μbar pressure levels), much higher than the 160-370 km range where the 12 μm thermal molecular line emission of this species arises. At 2.87-2.90 μm the major absorber is tropospheric PH3. The cloud level determined here and at 3.22-3.54 is 390-460 mbar (∼30 km), somewhat higher than found by Kim and Geballe [Kim, S.J., Geballe, T.R., 2005. Icarus 179, 449-458] from analysis of a low resolution spectrum. A broad absorption feature at 2.96 μm, which might be due to NH3 ice particles in saturnian clouds, is also present. The effect of a haze layer at about 125 km (∼12 mbar level) on the 3.20-3.54 μm spectrum, which was not apparent in the low resolution spectrum, is clearly evident in the high resolution data, and the spectral properties of the haze particles suggest that they are composed of hydrocarbons. 相似文献
2.
Using synthetic spectra derived from an updated model atmosphere together with a continuum model that includes contributions from haze, cloud and ground, we have re-analyzed the recently published (Geballe et al., 2003, Astrophys. J. 583, L39-L42) high-resolution 3 μm spectrum of Titan which contains newly-detected bands of HCN (in emission) and C2H2 and CH3D (in absorption), in addition to previously detected bands of CH4. In the 3.10-3.54 μm interval the analysis yields strong evidence for the existence of a cloud deck or optically thick haze layer at about the 10 mbar (∼ 100 km) level. The haze must extend well above this altitude in order to mask the strong CH4 lines at 3.20-3.50 μm. These cloud and haze components must be transparent at 2.87-2.92 μm, where analysis of the CH3D spectrum demonstrates that Titan's surface is glimpsed through a second cloud deck at about the 100 mbar (∼ 50 km) level. Through a combination of areal distribution and optical depth this cloud deck has an effective transmittance of ∼ 20%. The spectral shape of Titan's continuum indicates that the higher altitude cloud and haze particles responsible for suppressing the CH4 absorptions have a largely organic make-up. The rotational temperature of the HCN ranges from 140 to 180 K, indicating that the HCN emission occurs over a wide range of altitudes. This emission, remodeled using an improved collisional deactivation rate, implies mesospheric mixing ratio curves that are consistent with previously predictions. The stratospheric and mesospheric C2H2 mixing ratios are ∼10−5, considerably less than previous model predictions (Yung et al., 1984), but approximately consistent with recent observational results. Upper limits to mixing ratios of HC3N and C4H2 are derived from non-detections of those species near 3.0 μm. 相似文献
3.
C.A. Nixon R.K. Achterberg P.G.J. Irwin T. Fouchet P.N. Romani A. LeClair A.A. Simon-Miller F.M. Flasar 《Icarus》2007,188(1):47-71
Hydrocarbons such as acetylene (C2H2) and ethane (C2H6) are important tracers in Jupiter's atmosphere, constraining our models of the chemical and dynamical processes. However, our knowledge of the vertical and meridional variations of their abundances has remained sparse. During the flyby of the Cassini spacecraft in December 2000, the Composite Infrared Spectrometer (CIRS) instrument was used to map the spatial variation of emissions from 10 to 1400 cm−1 (1000-7 μm). In this paper we analyze a zonally averaged set of CIRS spectra taken at the highest (0.48 cm−1) resolution, firstly to infer atmospheric temperatures in the stratosphere at 0.5-20 mbar via the ν4 band of CH4, and in the troposphere at 150-400 mbar, via the H2 absorption at 600-800 cm−1. Stratospheric temperatures at 5 mbar are generally warmer in the north than the south by 7-8 K, while tropospheric temperatures show no such asymmetry. Both latitudinal temperature profiles however do show a pattern of maxima and minima which are largely anti-correlated between the two levels. We then use the derived temperature profiles to infer the vertical abundances of C2H2 and C2H6 by modeling tropospheric absorption (∼200 mbar) and stratospheric emission (∼5 mbar) in the C2H2ν5 and C2H6ν9 bands, and also emission of the acetylene (ν4+ν5)−ν4 hotband (∼0.1 mbar). Acetylene shows a distinct north-south asymmetry in the stratosphere, with 5 mbar abundances greatest close to 20° N and decreasing from there towards both poles by a factor of ∼4. At 200 mbar in contrast, acetylene is nearly flat at a level of ∼3×10−9. Additionally, the abundance gradient of C2H2 between 10 and 0.1 mbar is derived, based on interpolated temperatures at 0.1 mbar, and is found to be positive and uniform with latitude to within errors. Ethane at both 5 and 200 mbar shows increasing VMR towards polar regions of ∼1.75 towards 70° N and ∼2.0 towards 70° S. An explanation for the meridional trends is proposed in terms of a combination of photochemistry and dynamics. Poleward, the decreasing UV flux is predicted to decrease the abundances of C2H2 and C2H6 by factors of 2.7 and 3.5, respectively, at latitude 70°. However, the lifetime of C2H6 in the stratosphere (3×1010 s at 5 mbar) is much longer than the dynamical timescale for meridional mixing inferred from Comet SL-9 debris (5-50×108 s), and therefore the rising abundance towards high latitudes likely indicates that meridional mixing dominates over photochemical effects. For C2H2, the opposite occurs, with the relatively short photochemical lifetime (3×107 s), compared to meridional mixing times, ensuring that the expected photochemical trends are visible. 相似文献
4.
Jupiter exhibits bright H+3 auroral arcs at 3-4 microns that cool the hot (>1000 K) ionosphere above the ∼10−7 bar level through the infrared bands of this trace constituent. Below the 10−7 bar level significant cooling proceeds through infrared active bands of CH4, C2H2, and C2H6. We report the discovery of 3-micron line emission from these hydrocarbon species in spectra of the jovian south polar region obtained on April 18 and 20, 2006 (UT) with CGS4 on the United Kingdom Infrared Telescope. Estimated cooling rates through these molecules are 7.5×10−3, 1.4×10−3, and , respectively, for a total nearly half that of H+3. We derive a temperature of 450 ± 50 K in the 10−7-10−5 bar region from the C2H2 lines. 相似文献
5.
The infrared AOTF spectrometer is a part of the SPICAM experiment onboard the Mars-Express ESA mission. The instrument has a capability of solar occultations and operates in the spectral range of 1-1.7 μm with a spectral resolution of ∼3.5 cm−1. We report results from 24 orbits obtained during MY28 at Ls 130°-160°, and the latitude range of 40°-55° N. For these orbits the atmospheric density from 1.43 μm CO2 band, water vapor mixing ratio based on 1.38 μm absorption, and aerosol opacities were retrieved simultaneously. The vertical resolution of measurements is better than 3.5 km. Aerosol vertical extinction profiles were obtained at 10 wavelengths in the altitude range from 10 to 60 km. The interpretation using Mie scattering theory with adopted refraction indices of dust and H2O ice allows to retrieve particle size (reff∼0.5-1 μm) and number density (∼1 cm−3 at 15-30 km) profiles. The haze top is generally below 40 km, except the longitude range of 320°-50° E, where high-altitude clouds at 50-60 km were detected. Optical properties of these clouds are compatible with ice particles (effective radius reff=0.1-0.3 μm, number density N∼10 cm−3) distributed with variance νeff=0.1-0.2 μm. The vertical optical depth of the clouds is below 0.001 at 1 μm. The atmospheric density profiles are retrieved from CO2 band in the altitude range of 10-90 km, and H2O mixing ratio is determined at 15-50 km. Unless a supersaturation of the water vapor occurs in the martian atmosphere, the H2O mixing ratio indicates ∼5 K warmer atmosphere at 25-45 km than predicted by models. 相似文献
6.
The three-dimensional structure of Saturn's intense equatorial jet from latitudes 8° N to 20° S is revealed from detailed measurements of the motions and spectral reflectivity of clouds at visible wavelengths on high resolution images obtained by the Cassini Imaging Science Subsystem (ISS) in 2004 and early 2005. Cloud speeds at two altitude levels are measured in the near infrared filters CB2 and CB3 matching the continuum (effective wavelengths 750 and 939 nm) and in the MT2 and MT3 filters matching two methane absorption bands (effective wavelengths 727 and 889 nm). Radiative transfer models in selective filters covering an ample spectral range (250-950 nm) require the existence of two detached aerosol layers in the equator: an uppermost thin stratospheric haze extending between the pressure levels ∼20 and 40 mbar (tropopause level) and below it, a dense tropospheric haze-cloud layer extending between 50 mbar and the base of the ammonia cloud (between ∼1 and 1.4 bar). Individual cloud elements are detected and tracked in the tropospheric dense haze at 50 and 700 mbar (altitude levels separated by 142 km). Between latitudes 5° N and 12° S the winds increase their velocity with depth from 265 m s−1 at the 50 mbar pressure level to 365 m s−1 at 700 mbar. These values are below the high wind speeds of 475 m s−1 measured at these latitudes during the Voyager era in 1980-1981, indicating that the equatorial jet has suffered a significant intensity change between that period and 1996-2005 or that the tracers of the flow used in the Voyager images were rooted at deeper levels than those in Cassini images. 相似文献
7.
We present 1.25-19 μm infrared spectra of pure solid CH4 and H2O/CH4=87, 20, and 3 solid mixtures at temperatures from 15 to 150 K. We compare and contrast the absorptions of CH4 in solid H2O with those of pure CH4. Changes in selected peak positions, profiles, and relative strength with temperature are presented, and absolute strengths for absorptions of CH4 in solid H2O are estimated. Using the two largest (ν3+ν4) and (ν1+ν4) near-IR absorptions of CH4 at 2.324 and 2.377 μm (4303 and 4207 cm−1), respectively, as examples, we show that peaks of CH4 in solid H2O are at slightly shorter wavelength (higher frequency) and broader than those of pure solid CH4. With increasing temperature, these peaks shift to higher frequency and become increasingly broad, but this trend is reversible on re-cooling, even though the phase transitions of H2O are irreversible. It is to be hoped that these observations of changes in the positions, profiles, and relative intensities of CH4 absorptions with concentration and temperature will be of use in understanding spectra of icy outer Solar System bodies. 相似文献
8.
A radiative–conductive model for the vertical thermal structure of Pluto's atmosphere is developed with a non-LTE treatment of solar heating in the CH43.3 μm and 2.3 μm bands, non-LTE radiative exchange and cooling in the CH47.6 μm band, and LTE cooling by CO rotational line emission. The model includes the effects of opacity and vibrational energy transfer in the CH4molecule. Partial thermalization of absorbed solar radiation in the CH43.3 and 2.3 μm bands by rapid vibrational energy transfer from the stretch modes to the bending modes generates high altitude heating at sub-microbar pressures. Heating in the 2.3 μm bands exceeds heating in 3.3 μm bands by approximately a factor of 6 and occurs predominantly at microbar pressures to generate steep temperature gradients ∼10–20 K km−1forp> 2 μbar when the surface or tropopause pressure is ∼3 μbar and the CH4mixing ratio is a constant 3%. This calculated structure may account for the “knee” in the stellar occultation lightcurve. The vertical temperature structure in the first 100 km above the surface is similar for atmospheres with Ar, CO, and N2individually as the major constituent. If a steep temperature gradient ∼20 K km−1is required near the surface or above the tropopause, then the preferred major constituent is Ar with 3% CH4mixing ratio to attain a calculated ratio ofT/(= 3.5 K amu−1) in agreement with inferred values from stellar occultation data. However, pure Ar and N2ices at the same temperature yield an Ar vapor pressure of only ∼0.04 times the N2vapor pressure. Alternative scenarios are discussed that may yield acceptable fits with N2as the dominant constituent. One possibility is a 3 μbar N2atmosphere with 0.3% CH4that has 106 K isothermal region (T/= 3.8 K amu−1) and ∼8 K km−1surface/tropopause temperature gradient. Another possibility would be a higher surface pressure ∼10 μbar with a scattering haze forp> 2 μbar. Our model with appropriate adjustments in the CH4density profile to Triton's inferred profile yields a temperature profile consistent with the UVS solar occultation data (Krasnopolsky, V. A., B. R. Sandel, and F. Herbert 1992.J. Geophys. Res.98, 3065–3078.) and ground-based stellar occultation data (Elliot, J. L., E. W. Dunham, and C. B. Olkin 1993.Bull. Am. Astron. Soc.25, 1106.). 相似文献
9.
We have reanalyzed the high-resolution spectrum of Titan between 2.87 and 3.12 μm observed with NIRSPEC/Keck II on 2001 Nov. 21 in southern summer, using updated CH3D and C2H6 line-by-line models. From new synthetic spectra, we identify all but a few of the previously unidentified significant absorption spectral features in this wavelength range as due to these two species, both of which had been previously detected by Voyager and ground-based observations at other wavelengths. We also derive opacities and reflectivities of haze particles as functions of altitude for the 2.87-2.92 μm wavelength range, where Titan's atmosphere is partially transparent down to the surface. The extinction per unit altitude is observed to increase from 100 km (∼8 mbar) toward lower altitude. The derived total optical depth is approximately 1.1 for the 2.87-2.92 μm range. At wavelengths increasing beyond 2.92 μm the haze layers become much more optically thick, and the surface is rapidly hidden from view. These conclusions apply to equatorial and southern-temperate regions on Titan, excluding polar regions. We also find it unlikely that there is a large enhancement of the tropospheric CH4 mole fraction over the value reported from analysis of the Huygens/GCMS observations. 相似文献
10.
We present high signal precision optical reflectance spectra of 2005 FY9 taken with the Red Channel Spectrograph and the 6.5-m MMT telescope on 2006 March 4 UT (5000-9500 Å; 6.33 Å pixel−1) and 2007 February 12 UT (6600-8500 Å; 1.93 Å pixel−1). From cross-correlation experiments between the 2006 March 4 spectrum and a pure CH4-ice Hapke model, we find the CH4-ice bands in the MMT spectrum are blueshifted by 3 ± 4 Å relative to bands in the pure CH4-ice Hapke spectrum. The higher resolution MMT spectrum of 2007 February 12 UT enabled us to measure shifts of individual CH4-ice bands. We find the 7296, 7862, and 7993 Å CH4-ice bands are blueshifted by 4 ± 2, 4 ± 4, and 6 ± 5 Å. From four measurements we report here and one of our previously published measurements, we find the CH4-ice bands are shifted by 4 ± 1 Å. This small shift is important because it suggest the presence of another ice component on the surface of 2005 FY9. Laboratory experiments show that CH4-ice bands in spectra of CH4 mixed with other ices are blueshifted relative to bands in spectra of pure CH4-ice. A likely candidate for the other component is N2-ice because its weak 2.15 μm band and blueshifted CH4 bands are seen in spectra of Triton and Pluto. Assuming the shift is due to the presence of N2, spectra taken on two consecutive nights show no difference in CH4/N2. In addition, we find no measurable difference in CH4/N2 at different depths into the surface of 2005 FY9. 相似文献
11.
A spectrum of Jupiter between 6000 and 12 000 cm? at high resolution (0.05 cm?) was recorded with a Michelson interferometer at Palomar Mountain in October 1974. An analysis of the R branch of the 3ν3CH4 band with the reflecting-layer model, taking into account the H2 absorption which occurs in the same spectral range, leads to a Lorentzian half-width of 0.09 ± 0.02 cm?1, a rotational temperature of 175 ± 10° K, and a CH4 abundance of order 52m atm. Five lines of the 13CH43ν3 band have been identified; a comparison with new laboratory spectra indicates that the 13CH4/12CH4 ratio in the Jupiter atmosphere is close to the terrestrial ratio. 相似文献
12.
D. Jacquemart E. Lellouch C. de Bergh N. Lacome M. Tomasko 《Planetary and Space Science》2008,56(5):613-623
Laboratory spectra of methane-nitrogen mixtures have been recorded in the near-infrared range (1.0-1.65 μm) in conditions similar to Titan's near surface, to facilitate the interpretation of the DISR/DLIS (DISR—Descent Imager/Spectral Radiometer) spectra taken during the last phase of the descent of the Huygens Probe, when the surface was illuminated by a surface-science lamp. We used a 0.03 cm−1 spectral resolution, adequate to resolve the lines at high pressure (pN2∼1.5 bar). By comparing the laboratory spectra with synthetic calculations in the well-studied ν2+2ν3 band (7515-7620 cm−1), we determine a methane absorption column density of 178±20 cm atm and a temperature of 118±10 K in our experiment. From this, we derive the methane absorption coefficients over 1.0-1.65 μm with a 0.03 cm−1 sampling, allowing for the extrapolation of the results to any other methane column density under the relevant pressure and temperature conditions. We then revisit the calibration and analysis of the Titan “lamp-on” DLIS spectra. We infer a 5.1±0.8% methane-mixing ratio in the first 25 m of Titan's atmosphere. The CH4 mixing ratio measured 90 s after landing from a distance of 45 cm is found to be 0.92±0.25 times this value, thus showing no post-landing outgassing of methane in excess of ∼20%. Finally, we determine the surface reflectivity as seen between 25 m and 45 cm and find that the 1500 nm absorption band is deeper in the post-landing spectrum as compared to pre-landing. 相似文献
13.
The reactivity of C2(X1Σ+g) with simple saturated (CH4, C2H6 and C3H8) and unsaturated (C2H2 and C2H4) hydrocarbons has been studied in the gas phase over the temperature range 24-300 K using the CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme or Reaction Kinetics in a Uniform Supersonic Flow) technique. All reactions have been found to be very rapid in this temperature range and the rate coefficients are typically ?10−10 cm3 molecule−1 s−1 with the exception of methane for which the rate coefficient is one order of magnitude lower: ∼10−11 cm3 molecule−1 s−1. These results have been analyzed in terms of potential destruction sources of C2(X1Σ+g) in the atmospheres of Titan and the Giant Planets. It appears that the rate coefficient of the reaction 1C2 + CH4 should be updated with our new data and that reactions with C2H2, C2H4 and C2H6 should also be included in the existing photochemical models. 相似文献
14.
We present near-IR spectra of solid CO2 in H2O and CH3OH, and find they are significantly different from that of pure solid CO2. Peaks not present in either pure H2O or pure CO2 spectra become evident when the two are mixed. First, the putative theoretically forbidden CO2 (2ν3) overtone near 2.134 μm (4685 cm−1), that is absent from our spectrum of pure solid CO2, is prominent in the spectra of H2O/CO2=5 and 25 mixtures. Second, a 2.74-μm (3650 cm−1) dangling OH feature of H2O (and a potentially related peak at 1.89 μm) appear in the spectra of CO2-H2O ice mixtures, but are probably not diagnostic of the presence of CO2. Other CO2 peaks display shifts in position and increased width because of intermolecular interactions with H2O. Warming causes some peak positions and profiles in the spectrum of a H2O/CO2=5 mixture to take on the appearance of pure CO2. Absolute strengths for absorptions of CO2 in solid H2O are estimated. Similar results are observed for CO2 in solid CH3OH. Since the CO2 (2ν3) overtone near 2.134 μm (4685 cm−1) is not present in pure CO2 but prominent in mixtures, it may be a good observational (spectral) indicator of whether solid CO2 is a pure material or intimately mixed with other molecules. These observations may be applicable to Mars polar caps as well as outer Solar System bodies. 相似文献
15.
Latitudinal variations of HCN, HC3N, and C2N2 in Titan's stratosphere derived from Cassini CIRS data
N.A. Teanby P.G.J. Irwin C.A. Nixon B. Bézard N.E. Bowles L. Fletcher F.W. Taylor 《Icarus》2006,181(1):243-255
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
16.
Uwe Fink Bradford A. Smith D. Chris Benner James R. Johnson H.J. Reitsema James A. Westphal 《Icarus》1980,44(1):62-71
Using a low-resolution spectrograph and a CCD array, a spectrum of Pluto from 0.58 to 1.06 μm was obtained. The spectrum had a resolution of and a signal-to-noise ratio of ～300. It showed CH4 absorption bands at 6200, 7200, 7900, 8400, 8600, 8900 and 10,000 Å. The strongest of these bands was at 8900 Å with an absorption depth of 0.23. This band was heavily saturated, compared to the weaker bands, providing proof for the gaseous origin of the observed absorptions. By applying CH4 band model parameters to our data, a total CH4 abundance of 80 ± 20 m-am was derived. This translates into a one-way abundance of 27 ± 7 m-am and a CH4 surface pressure of 1.5 × 10?4 atm. An upper limit to the total pressure of ～0.05 atm could be set. First-order calculations on atmospheric escape showed that this methane atmosphere would be stable if the mass of Pluto is increased 50% over its current value and its radius is 1400 km. Alternatively a heavier gas mixed with the CH4 atmosphere would aid its stability. The relatively large amount of gaseous CH4 observed implies that the absorption bands recently reported at 1.7 and 2.3 μm are likely due to atmospheric CH4 absorptions rather than surface frost as interpreted earlier. 相似文献
17.
Previous modeling by Banaszkiewicz et al. (2000a,b) showed that the CH4 thermospheric mixing ratio on Titan could vary as much as 35-40% due to ion-neutral chemical reactions. A new vertical methane profile has been computed by simultaneously modifying the stratospheric methane mixing ratio and the K(z) previously considered by Lara et al. (1996) and Banaszkiewicz et al. (2000a,b). A satisfactory fit of the methane thermospheric abundance and stratospheric mixing ratio of other minor constituents is achieved by placing the homopause at ∼1000 km and increasing the methane stratospheric mixing ratio (qCH4) up to 3.8%. The new proposed eddy diffusion coefficient steadily rises from 1×107 cm2 s−1 at 700 km to 1×1010 cm2 s−1 at 1500 km, whereas the stratospheric values are in the range (4-20)×103 cm2 s−1. Other likely ionization sources that can influence the methane distribution are (i) a metallic ion layer produced by micrometeoroid infall and (ii) frequent X-rays solar flares. Analysis of the effects of these ionization sources on the methane distribution indicates that, unlike previously assumed, CH4 can suffer considerable variations. These variations, although proved in this work, must be cautiously regarded since several assumptions have to be made on the rate of N2 and CH4 ionization by the processes previously mentioned. Hence, these results are only indicative of methane sensitivity to ionospheric chemistry. 相似文献
18.
We have performed high-resolution spectral observations at mid-infrared wavelengths of C2H6 (12.16 μm), and C2H2 (13.45 μm) on Saturn. These emission features probe the stratosphere of the planet and provide information on the hydrocarbon photochemical processes taking place in that region of the atmosphere. The observations were performed using our cryogenic echelle spectrometer Celeste, in conjunction with the McMath-Pierce 1.5-m solar telescope in November and December 1994. We used Voyager IRIS CH4 observations (7.67 μm) to derive a temperature profile on the saturnian atmosphere for the region of the stratosphere. This profile was then used in conjunction with height-dependent volume mixing ratios of each hydrocarbon to determine global abundances for ethane and acetylene. Our ground-based measurements indicate abundances of for C2H6 (1.0 mbar pressure level), and for C2H2 (1.6 mbar pressure level). We also derived new mixing ratios from the Voyager mid-latitude IRIS observations; 8.6±0.9×10−6 for C2H6 (0.1-3.0 mbar pressure level), and 1.6±0.2×10−7 for C2H2 (2.0 mbar pressure level). 相似文献
19.
Brigette E. Hesman Donald E. Jennings Gordon L. Bjoraker Amy A. Simon-Miller Robert J. Boyle Leigh N. Fletcher 《Icarus》2009,202(1):249-259
Hydrocarbons in the upper atmosphere of Saturn are known, from Voyager, ground-based, and early Cassini results, to vary in emission intensity with latitude. Of particular interest is the marked increase in hydrocarbon line intensity near the south pole during southern summer, as the increased line intensity cannot be simply explained by the increased temperatures observed in that region since the variations between C2H2 and C2H6 emission in the south pole region are different. In order to measure the latitudinal variations of hydrocarbons in Saturn's southern hemisphere we have used 3 cm−1 resolution Cassini CIRS data from 2006 and combined this with measurements from the ground in October 2006 at NASA's IRTF using Celeste, an infrared high-resolution cryogenic grating spectrometer. These two data sets have been used to infer the molecular abundances of C2H2 and C2H6 across the southern hemisphere in the 1-10 mbar altitude region. We find that the latitudinal acetylene profile follows the yearly average mean daily insolation except at the southern pole where it peaks in abundance. Near the equator (5° S) the C2H2 abundance at the 1.2 mbar level is (1.6±0.19)×10−7 and it decreases by a factor of 2.7 from the equator toward the pole. However, at the pole (∼87° S) the C2H2 abundance jumps to (1.8±0.3)×10−7, approximately the equatorial value. The C2H6 abundance near the equator at the 2 mbar level is (0.7±0.1)×10−5 and stays approximately constant until mid-latitudes where it increases gradually toward the pole, attaining a value of (1.4±0.4)×10−5 there. The increase in ethane toward the pole with the corresponding decrease in acetylene is consistent with southern hemisphere meridional winds [Greathouse, T.K., Lacy, J.H., Bézard, B., Moses, J.I., Griffith, C.A., Richter, M.J., 2005. Icarus 177, 18-31]. The localized increase in acetylene at the pole provides evidence that there is dynamical transport of hydrocarbons from the equator to the southern pole. 相似文献
20.
Maya García-Comas Manuel López-Puertas Bianca M. Dinelli Alberto Adriani Angioletta Coradini 《Icarus》2011,214(2):571-583
After molecular nitrogen, methane is the most abundant species in Titan’s atmosphere and plays a major role in its energy budget and its chemistry. Methane has strong bands at 3.3 μm emitting mainly at daytime after absorption of solar radiation. This emission is strongly affected by non-local thermodynamic equilibrium (non-LTE) in Titan’s upper atmosphere and, hence, an accurate modeling of the non-LTE populations of the emitting vibrational levels is necessary for its analysis. We present a sophisticated and extensive non-LTE model which considers 22 CH4 levels and takes into account all known excitation mechanisms in which they take part. Solar absorption is the major excitation process controlling the population of the v3-quanta levels above 1000 km whereas the distribution of the vibrational energy within levels of similar energy through collisions with N2 is also of importance below that altitude. CH4-CH4 vibrational exchange of v4-quanta affects their population below 500 km. We found that the ν3 → ground band dominates Titan’s 3.3 μm daytime limb radiance above 750 km whereas the ν3 + ν4 → ν4 band does below that altitude and down to 300 km. The ν3 + ν2 → ν2, the 2ν3 → ν3, and the 13CH4ν3 → ground bands each contribute from 5% to 8% at regions below 800 km. The ν3 + 2ν4 → 2ν4and ν2 + ν3 + ν4 → ν2 + ν4 bands each contribute from 2% to 5% below 650 km. Contributions from other CH4 bands are negligible. We have used the non-LTE model to retrieve the CH4 abundance from 500 to 1100 km in the southern hemisphere from Cassini-VIMS daytime measurements near 3.3 μm. Our retrievals show good agreement with previous measurements and model results, supporting a weak deviation from well mixed values from the lower atmosphere up to 1000 km. 相似文献