Observations of C2H6 and C2H2 in the stratosphere of Saturn

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).     

Meridional variations of temperature, C2H2 and C2H6 abundances in Saturn's stratosphere at southern summer solstice

Measurements of the vertical and latitudinal variations of temperature and C₂H₂ and C₂H₆ abundances in the stratosphere of Saturn can be used as stringent constraints on seasonal climate models, photochemical models, and dynamics. The summertime photochemical loss timescale for C₂H₆ in Saturn's middle and lower stratosphere (∼40-10,000 years, depending on altitude and latitude) is much greater than the atmospheric transport timescale; ethane observations may therefore be used to trace stratospheric dynamics. The shorter chemical lifetime for C₂H₂ (∼1-7 years depending on altitude and latitude) makes the acetylene abundance less sensitive to transport effects and more sensitive to insolation and seasonal effects. To obtain information on the temperature and hydrocarbon abundance distributions in Saturn's stratosphere, high-resolution spectral observations were obtained on September 13-14, 2002 UT at NASA's IRTF using the mid-infrared TEXES grating spectrograph. At the time of the observations, Saturn was at a LS≈270°, corresponding to Saturn's southern summer solstice. The observed spectra exhibit a strong increase in the strength of methane emission at 1230 cm⁻¹ with increasing southern latitude. Line-by-line radiative transfer calculations indicate that a temperature increase in the stratosphere of ≈10 K from the equator to the south pole between 10 and 0.01 mbar is implied. Similar observations of acetylene and ethane were also recorded. We find the 1.16 mbar mixing ratio of C₂H₂ at −1° and −83° planetocentric latitude to be and , respectively. The C₂H₂ mixing ratio at 0.12 mbar is found to be at −1° planetocentric latitude and at −83° planetocentric latitude. The 2.3 mbar mixing ratio of C₂H₆ inferred from the data is and at −1° and −83° planetocentric latitude, respectively. Further observations, creating a time baseline, will be required to completely resolve the question of how much the latitudinal variations of C₂H₂ and C₂H₆ are affected by seasonal forcing and/or stratospheric circulation.     

Meridional distribution of CH3C2H and C4H2 in Saturn’s stratosphere from CIRS/Cassini limb and nadir observations

Limb and nadir spectra acquired by Cassini/CIRS (Composite InfraRed Spectrometer) are analyzed in order to derive, for the first time, the meridional variations of diacetylene (C₄H₂) and methylacetylene (CH₃C₂H) mixing ratios in Saturn’s stratosphere, from 5 hPa up to 0.05 hPa and 80°S to 45°N. We find that the C₄H₂ and CH₃C₂H meridional distributions mimic that of acetylene (C₂H₂), exhibiting small-scale variations that are not present in photochemical model predictions. The most striking feature of the meridional distribution of both molecules is an asymmetry between mid-southern and mid-northern latitudes. The mid-southern latitudes are found depleted in hydrocarbons relative to their northern counterparts. In contrast, photochemical models predict similar abundances at north and south mid-latitudes. We favor a dynamical explanation for this asymmetry, with upwelling in the south and downwelling in the north, the latter coinciding with the region undergoing ring shadowing. The depletion in hydrocarbons at mid-southern latitudes could also result from chemical reactions with oxygen-bearing molecules.Poleward of 60°S, at 0.1 and 0.05 hPa, we find that the CH₃C₂H and C₄H₂ abundances increase dramatically. This behavior is in sharp contradiction with photochemical model predictions, which exhibit a strong decrease towards the south pole. Several processes could explain our observations, such as subsidence, a large vertical eddy diffusion coefficient at high altitudes, auroral chemistry that enhances CH₃C₂H and C₄H₂ production, or shielding from photolysis by aerosols or molecules produced from auroral chemistry. However, problems remain with all these hypotheses, including the lack of similar behavior at lower altitudes.Our derived mean mixing ratios at 0.5 hPa of (2.4 ± 0.3) × 10⁻¹⁰ for C₄H₂ and of (1.1 ± 0.3) × 10⁻⁹ for CH₃C₂H are compatible with the analysis of global-average ISO observations performed by Moses et al. (Moses, J.I., Bézard, B., Lellouch, E., Gladstone, G.R., Feuchtgruber, H., Allen, M. [2000a]. Icarus 143, 244-298). Finally, we provide values for the ratios [CH₃C₂H]/[C₂H₂] and [C₄H₂]/[C₂H₂] that can constrain the coupled chemistry of these hydrocarbons.     

Jupiter's hydrocarbon polar brightening: Discovery of 3-micron line emission from south polar CH4, C2H2, and C2H6

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.     

The 2.9-4.2 micron spectrum of Saturn: Clouds and CH4, PH3, and NH3

We have used synthetic spectra to analyze a medium resolution 2.9-4.2 μm spectrum of Saturn's temperate region observed at UKIRT using CGS4. The synthetic spectra include CH₄, PH₃, and NH₃ lines, for which mixing ratios were adopted from recent Cassini results. The observed absorption features in the spectrum are well accounted for by lines of these molecular species formed 22 +/− 8 km above the 1 bar pressure level at ∼610 mbar. The influence of optically thin haze particles at higher altitudes on the spectrum is not pronounced, with higher spectral resolution probably required to constrain the effects of haze in this wavelength region. Fluorescent line emission by CH₄ in its ν₃ and ν₃+ν₄−ν₄ bands, detected in the 3.2-3.5 μm region, originates between 400 km (∼0.06 mbar) and 800 km (∼0.01 μbar) above the 1 bar level, with peak contributions from the two major contributing bands at 550 km (∼3 μbar) and 700 km (∼0.1 μbar), respectively.     

Meridional variations of C2H2 and C2H6 in Jupiter's atmosphere from Cassini CIRS infrared spectra

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.     

Clouds, haze, and CH4, CH3D, HCN, and C2H2 in the atmosphere of Titan probed via 3 μm spectroscopy

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.     

Saturn's latitudinal C2H2 and C2H6 abundance profiles from Cassini/CIRS and ground-based observations

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.     

Speckle imaging of Titan at 2 microns: surface albedo, haze optical depth, and tropospheric clouds 1996-1998

We present results from 14 nights of observations of Titan in 1996-1998 using near-infrared (centered at 2.1 microns) speckle imaging at the 10-meter W.M. Keck Telescope. The observations have a spatial resolution of 0.06 arcseconds. We detect bright clouds on three days in October 1998, with a brightness about 0.5% of the brightness of Titan. Using a 16-stream radiative transfer model (DISORT) to model the central equatorial longitude of each image, we construct a suite of surface albedo models parameterized by the optical depth of Titan's hydrocarbon haze layer. From this we conclude that Titan's equatorial surface albedo has plausible values in the range of 0-0.20. Titan's minimum haze optical depth cannot be constrained from this modeling, but an upper limit of 0.3 at this wavelength range is found. More accurate determination of Titan's surface albedo and haze optical depth, especially at higher latitudes, will require a model that fully considers the 3-dimensional nature of Titan's atmosphere.     

Titan at 3 microns: Newly identified spectral features and an improved analysis of haze opacity

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.     

Saturn's auroral/polar H3 infrared emission: II. A comparison with plasma flow models

We present a detailed analysis of the H⁺₃ intensity and velocity profiles crossing Saturn's auroral/polar region, as described by Stallard et al. [Stallard, T., Miller, S., Melin, H., Lystrup, M., Dougherty, M., Achilleos, N., 2007. Icarus 189, 1-13], with a view to understanding the magnetospheric processes with which they are connected. The data are not consistent with the theory that Saturn's main auroral oval is associated with corotation enforcement currents in the middle magnetosphere. This implies that the main auroral oval can be associated with the open-closed field line boundary [Cowley, S.W.H., Bunce, E.J., O'Rourke, J.M., 2004. J. Geophys. Res. 109. A05212]; a third model, by Sittler et al. [Sittler, E.C., Blanc, M.F., Richardson, J.D., 2006. J. Geophys. Res. 111. A06208] associates the main oval with centrifugal instabilities in the outer magnetosphere, but does not make predictions about ionospheric plasma flows with which we can compare our data. We do, however, tentatively identify emission at latitudes lower than the main auroral oval which may be associated with the corotation enforcement currents in the middle magnetosphere. We also find that at latitudes higher than the main auroral oval there is often a region of the ionosphere that is in rigid corotation with the planet. We suggest that this region corresponds to field lines embedded in the centre of the magnetotail which are shielded from the solar wind such that their rotation is controlled only by the neutral atmosphere.     

Infrared imaging spectroscopy of Mars: H2O mapping and determination of CO2 isotopic ratios

High-resolution infrared imaging spectroscopy of Mars has been achieved at the NASA Infrared Telescope Facility (IRTF) on June 19-21, 2003, using the Texas Echelon Cross Echelle Spectrograph (TEXES). The areocentric longitude was 206°. Following the detection and mapping of hydrogen peroxide H₂O₂ [Encrenaz et al., 2004. Icarus 170, 424-429], we have derived, using the same data set, a map of the water vapor abundance. The results appear in good overall agreement with the TES results and with the predictions of the Global Circulation Model (GCM) developed at the Laboratory of Dynamical Meteorology (LMD), with a maximum abundance of water vapor of 3±1.5×¹⁰⁻⁴(17±9 pr-μm). We have searched for CH₄ over the martian disk, but were unable to detect it. Our upper limits are consistent with earlier reports on the methane abundance on Mars. Finally, we have obtained new measurements of CO₂ isotopic ratios in Mars. As compared to the terrestrial values, these values are: (¹⁸O/¹⁷O)[M/E] = 1.03 ± 0.09; (¹³C/¹²C)[M/E] = 1.00 ± 0.11. In conclusion, in contrast with the analysis of Krasnopolsky et al. [1996. Icarus 124, 553-568], we conclude that the derived martian isotopic ratios do not show evidence for a departure from their terrestrial values.     

Variability in the H3 emission of Saturn: Consequences for ionisation rates and temperature

We present an analysis of observations of the auroral/polar regions of Saturn, carried out in 1999, 2004 and 2005, making use of the facility spectrometer, CGS4, on the United Kingdom Infrared Telescope (UKIRT), Mauna Kea, Hawaii. We obtain temperatures of 380(±70) K in 1999 and 420(±70) K in 2004. (The 2005 data has insufficient spectral resolution for a temperature determination to be made.) Our most probable interpretation is that the temperature of Saturn's auroral/polar H⁺₃ layer should be taken as 400(±50) K. This is lower than the value obtained by Miller et al. [Miller, S., and 10 colleagues, 2000. Philos. Trans. R. Soc. 358, 2485-2502], which is shown to be in error. Our analysis reveals clearly that the line emission due to H⁺₃ varies considerably, showing nearly an order of magnitude increase when one compares the data obtained in 1999 with those of 2004. Our conclusion is that this variability is (mainly) due to the changing H⁺₃ column density. By analogy with modelling results obtained for Jupiter, we estimate that the particle (keV electron) precipitation experienced by Saturn must be ∼20 times greater in 2004 than in 1999, to produce this additional ionisation. The H⁺₃ emission increases, but this is insufficient to offset most of the heating due to the extra particle precipitation, indicating that this ion does not act as a “thermostat” on Saturn, in the same way that it does on Jupiter.     

Simultaneous mapping of H2O and H2O2 on Mars from infrared high-resolution imaging spectroscopy

New maps of martian water vapor and hydrogen peroxide have been obtained in November-December 2005, using the Texas Echelon Cross Echelle Spectrograph (TEXES) at the NASA Infra Red Telescope facility (IRTF) at Mauna Kea Observatory. The solar longitude Ls was 332° (end of southern summer). Data have been obtained at 1235-1243 cm⁻¹, with a spectral resolution of 0.016 cm⁻¹ (R=8×¹⁰⁴). The mean water vapor mixing ratio in the region [0°-55° S; 345°-45° W], at the evening limb, is 150±50 ppm (corresponding to a column density of 8.3±2.8 pr-μm). The mean water vapor abundance derived from our measurements is in global overall agreement with the TES and Mars Express results, as well as the GCM models, however its spatial distribution looks different from the GCM predictions, with evidence for an enhancement at low latitudes toward the evening side. The inferred mean H₂O₂ abundance is 15±10 ppb, which is significantly lower than the June 2003 result [Encrenaz, T., Bézard, B., Greathouse, T.K., Richter, M.J., Lacy, J.H., Atreya, S.K., Wong, A.S., Lebonnois, S., Lefèvre, F., Forget, F., 2004. Icarus 170, 424-429] and lower than expected from the photochemical models, taking in account the change in season. Its spatial distribution shows some similarities with the map predicted by the GCM but the discrepancy in the H₂O₂ abundance remains to be understood and modeled.     

Titan's stratospheric C2N2, C3H4, and C4H2 abundances from Cassini/CIRS far-infrared spectra

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.     

Seasonal variation of Titan's stratospheric ethylene (C2H4) observed

All previous observations of seasonal change on Titan have been of physical phenomena such as clouds and haze. We present here the first observational evidence of chemical change in Titan's atmosphere. Images taken during 1999-2002 (late southern spring on Titan) with the W.M. Keck I 10-meter telescope at 8-13 μm show a significant accumulation of ethylene (C₂H₄) in the south polar stratosphere as well as north-south stratospheric temperature variation (colder at poles). Our observations restrict this newly discovered south polar ethylene accumulation to latitudes south of 60° S. The only other observations of the spatial distribution of C₂H₄ were those of Voyager I, which found a significant north polar accumulation in early northern spring. We see no build-up in the north, although the highest northern latitudes are obstructed from view in the current season. Our observations constrain any unobserved north polar accumulation of C₂H₄ to north of 50° N latitude. Comparison of the Voyager I results with our new results show seasonal chemical change has occurred in Titan's atmosphere.     

Improved Determination of Ethane (C2H6) Abundance in Titan's Stratosphere

Ethane spectral lines were observed in emission from Titan in August 1993, October 1995, and September 1996, at a spectral resolution of λ/Δλ≈10⁶, at wavelength 11.7−11.9 μm using the Goddard Infrared Heterodyne Spectrometer at the NASA Infrared Telescope Facility on Mauna Kea, Hawaii. The ethane mole fraction is determined to be (8.8±2.2)×10⁻⁶ (68.3% confidence limits, “1σ”), averaging the retrievals from each observing run obtained using the “recommended” thermal profile of R. V. Yelle, D. Strobel, E. Lellouch, and D. Gautier (1997, in Huygens: Science, Payload, and Mission (J.-P. Lebreton, Ed.), pp. 243-256, European Space Agency SP-1177).     

Meridional variations in stratospheric acetylene and ethane in the southern hemisphere of the saturnian atmosphere as determined from Cassini/CIRS measurements

These are the first results from nadir studies of meridional variations in the abundance of stratospheric acetylene and ethane from Cassini/CIRS data in the southern hemisphere of Saturn. High resolution, 0.5 cm⁻¹, CIRS data was used from three data sets taken in June-November 2004 and binned into 2° wide latitudinal strips to increase the signal-to-noise ratio. Tropospheric and stratospheric temperatures were initially retrieved to determine the temperature profile for each latitude bin. The stratospheric temperature at 2 mbar increased by 14 K from 9° to 68° S, including a steep 4 K rise between 60° and 68° S. The tropospheric temperatures showed significantly more meridional variation than the stratospheric ones, the locations of which are strongly correlated to that of the zonal jets. Stratospheric acetylene abundance decreases steadily from 30 to 68° S, by a factor of 1.8 at 2.0 mbar. Between 18° and 30° S the acetylene abundance increases at 2.0 mbar. Global values for acetylene have been calculated as (1.9±0.19)×¹⁰⁻⁷ at 2.0 mbar, (2.6±0.27)×¹⁰⁻⁷ at 1.6 mbar and (3.1±0.32)×¹⁰⁻⁷ at 1.4 mbar. Global values for ethane are also determined and found to be (1.6±0.25)×¹⁰⁻⁵ at 0.5 mbar and (1.4±0.19)×¹⁰⁻⁵ at 1.0 mbar. Ethane abundance in the stratosphere increases towards the south pole by a factor of 2.5 at 2.0 mbar. The increase in stratospheric ethane is especially pronounced polewards of 60° S at 2.0 mbar. The increase of stratospheric ethane towards the south pole supports the presence of a meridional wind system in the stratosphere of Saturn.     

High-resolution spectroscopy of Venus: Detection of OCS, upper limit to H2S, and latitudinal variations of CO and HF in the upper cloud layer

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.