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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2010.06.035 Byline: R. de Kok (a), P.G.J. Irwin (b), N.A. Teanby (b) Keywords: Titan; Atmospheres, Composition; Infrared observations Abstract: We use Cassini far-infrared limb and nadir spectra, together with recent Huygens results, to shed new light on the controversial far-infrared opacity sources in Titan's troposphere. Although a global cloud of large CH.sub.4 ice particles around an altitude of 30km, together with an increase in tropospheric haze opacity with respect to the stratosphere, can fit nadir and limb spectra well, this cloud does not seem consistent with shortwave measurements of Titan. Instead, the N.sub.2-CH.sub.4 collision-induced absorption coefficients are probably underestimated by at least 50% for low temperatures. Author Affiliation: (a) SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands (b) Atmospheric, Oceanic & Planetary Physics, Department of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, UK Article History: Received 18 February 2010; Revised 12 June 2010; Accepted 23 June 2010

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2010.03.017 Byline: P.G.J. Irwin (a), N.A. Teanby (a), G.R. Davis (b) Keywords: Spectroscopy; Uranus, Atmosphere; Neptune, Atmosphere; Infrared observations Abstract: Long-slit spectroscopy observations of Uranus by the United Kingdom InfraRed Telescope UIST instrument in 2006, 2007 and 2008 have been used to monitor the change in Uranus' vertical and latitudinal cloud structure through the planet's Northern Spring Equinox in December 2007. These spectra were analysed and presented by Irwin et al. (Irwin, P.G.J., Teanby, N.A., Davis, G.R. [2009]. Icarus 203, 287-302), but since publication, a new set of methane absorption data has become available (Karkoschka, E., Tomasko, M. [2010]. Methane absorption coefficients for the jovian planets from laboratory, Huygens, and HST data. Icarus 205, 674-694.), which appears to be more reliable at the cold temperatures and high pressures of Uranus' deep atmosphere. We have fitted k-coefficients to these new methane absorption data and we find that although the latitudinal variation and inter-annual changes reported by Irwin et al. (2009) stand, the new k-data place the main cloud deck at lower pressures (2-3bars) than derived previously in the H-band of [approximately equal to]3-4bars and [approximately equal to]3bars compared with [approximately equal to]6bars in the J-band. Indeed, we find that using the new k-data it is possible to reproduce satisfactorily the entire observed centre-of-disc Uranus spectrum from 1 to 1.75[mu]m with a single cloud at 2-3bars provided that we make the particles more back-scattering at wavelengths less than 1.2[mu]m by, for example, increasing the assumed single-scattering albedo from 0.75 (assumed in the J and H-bands) to near 1.0. In addition, we find that using a deep methane mole fraction of 4% in combination with the associated warm 'F' temperature profile of Lindal et al. (Lindal, G.F., Lyons, J.R., Sweetnam, D.N., Eshleman, V.R., Hinson, D.P. [1987]. J. Geophys. Res. 92, 14987-15001), the retrieved cloud deck using the new (Karkoschka and Tomasko, 2010) methane absorption data moves to between 1 and 2bars. The same methane absorption data and retrieval algorithm were applied to observations of Neptune made during the same programme and we find that we can again fit the entire 1-1.75[mu]m centre-of-disc spectrum with a single cloud model, providing that we make the stratospheric haze particles (of much greater opacity than for Uranus) conservatively scattering (i.e. I =1) and we also make the deeper cloud particles, again at around the 2bar level more reflective for wavelengths less than 1.2[mu]m. Hence, apart from the increased opacity of stratospheric hazes in Neptune's atmosphere, the deeper cloud structure and cloud composition of Uranus and Neptune would appear to be very similar. Author Affiliation: (a) Atmospheric, Oceanic, and Planetary Physics, Department of Physics, University of Oxford, Clarendon Laboratory, Parks Rd., Oxford OX1 3PU, United Kingdom (b) Joint Astronomy Centre, 660 N. A'ohoku Place, Hilo, HI 96720, United States Article History: Received 29 September 2009; Revised 12 March 2010; Accepted 15 March 2010

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.10.021 Byline: R. de Kok (a)(b), P.G.J. Irwin (b), N.A. Teanby (b), S. Vinatier (c)(d), F. Tosi (e), A. Negrao (e)(f), S. Osprey (b), A. Adriani (e), M.L. Moriconi (g), A. Coradini (e) Keywords: Titan; Atmospheres, Composition; Infrared observations Abstract: Inspection of near-infrared images from Cassini's Imaging Science Subsystem and Visual and Infrared Mapping Spectrometer have revealed a new feature in Titan's haze structure: a narrow band of increased scattering by haze south of the equator. The band seems to indicate a region of very limited mixing in the lower stratosphere, which causes haze particles to be trapped there. This could explain the sharp separation between the two hemispheres, known as the north-south asymmetry, seen in images. The separation of the two hemispheres can also be seen in the stratosphere above 150km using infrared spectra measured by Cassini's Composite Infrared Spectrometer. Titan's behaviour in the lower tropical stratosphere is remarkably similar to that of the Earth's tropical stratosphere, which hints at possible common dynamical processes. Author Affiliation: (a) SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands (b) Atmospheric, Oceanic and Planetary Physics, Department of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, UK (c) Observatoire de Paris, LESIA, Meudon F-92195, France (d) Universite Pierre et Marie Curie-Paris 6, UMR 8109, Paris F-75005, France (e) INAF-IFSI, Via Fosso del Cavaliere 100, I-00133 Rome, Italy (f) Escola Superior de Tecnologia e Gestao do Instituto Politecnico de Leiria, Leiria, Portugal (g) CNR-ISAC, Via Fosso del Cavaliere 100, I-00133 Rome, Italy Article History: Received 26 August 2009; Revised 15 October 2009; Accepted 19 October 2009

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.07.027 Byline: N.A. Teanby (a), R. de Kok (a)(b), P.G.J. Irwin (a) Keywords: Titan; Atmospheres; Composition; Atmospheres; Dynamics Abstract: Fine scale layering of haze and composition in Titan's stratosphere and mesosphere was investigated using visible/UV images from Cassini's Imaging Science Sub-system (ISS) and IR spectra from Cassini's Composite Infra-Red Spectrometer (CIRS). Both ISS and CIRS independently show fine layered structures in haze and composition, respectively, in the 150-450km altitude range with a preferred vertical wavelength of around 50km. Layers are most pronounced around the north polar winter vortex, although some weaker layers do exist at more southerly latitudes. The amplitude of composition layers in each trace gas profile is proportional to the relative enrichment of that species in the winter polar vortex compared to equatorial latitudes. As enrichment is caused by polar subsidence, this suggests a dynamical origin. We propose that the polar layers are caused by cross-latitude advection across the vortex boundary. This is analogous to processes that lead to ozone laminae formation around Earth's polar vortices. Author Affiliation: (a) Atmospheric, Oceanic and Planetary Physics, Department of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, UK (b) SRON, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands Article History: Received 4 June 2009; Revised 21 July 2009; Accepted 23 July 2009

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.05.003 Byline: P.G.J. Irwin (a), N.A. Teanby (a), G.R. Davis (b) Keywords: Spectroscopy; Uranus; Atmosphere; Infrared observations Abstract: Long-slit spectroscopy observations of Uranus by the United Kingdom Infrared Telescope UIST instrument in 2006, 2007 and 2008 have been used to monitor the change in Uranus' vertical and latitudinal cloud structure through the planet's northern spring equinox in December 2007. The observed reflectance spectra in the Long J (1.17-1.31[mu]m) and H (1.45-1.65[mu]m) bands, obtained with the slit aligned along Uranus' central meridian, have been fitted with an optimal estimation retrieval model to determine the vertical cloud profile from 0.1 to 6-8bar over a wide range of latitudes. Context images in a number of spectral bands were used to discriminate general zonal cloud structural changes from passing discrete clouds. From 2006 to 2007 reflection from deep clouds at pressures between 2 and 6-8bar increased at all latitudes, although there is some systematic uncertainty in the absolute pressure levels resulting from extrapolating the methane coefficients of Irwin et al. (Irwin, P.G.J., Sromovsky, L.A., Strong, E.K., Sihra, K., Teanby, N.A., Bowles, N., Calcutt, S.B., Remedios, J.J. [2006] Icarus, 181, 309-319) at pressures greater than 1bar, as noted by Tomasko et al. and Karkoschka and Tomasko (Tomasko, M.G., Bezard, B., Doose, L., Engel, S., Karkoschka, E. [2008] Planet. Space Sci., 56, 624-647; Karkoschka, E., Tomasko, M. [2009] Icarus). However, from 2007 to 2008 reflection from these clouds throughout the southern hemisphere and from both northern and southern mid-latitudes (30[degrees] N,S) diminished. As a result, the southern polar collar at 45[degrees]S has diminished in brightness relative to mid-latitudes, a similar collar at 45[degrees]N has become more prominent (e.g. Rages, K.A., Hammel, H.B., Sromovsky, L. [2007] Bull. Am. Astron. Soc., 39, 425; Sromovsky, L.A., Fry, P.M., Ahue, W.M., Hammel, H.B., de Pater, I., Rages, K.A., Showalter, M.R., van Dam, M.A. [2008] vol. 40 of AAS/Division for Planetary Sciences Meeting Abstracts, pp. 488-489; Sromovsky, L.A., Ahue, W.K.M., Fry, P.M., Hammel, H.B., de Pater, I., Rages, K.A., Showalter, M.R. [2009] Icarus), and the lowering reflectivity from mid-latitudes has left a noticeable brighter cloud zone at the equator (e.g. Sromovsky, L.A., Fry, P.M. [2007] Icarus, 192, 527-557;Karkoschka, E., Tomasko, M. [2009] Icarus). For such substantial cloud changes to have occurred in just two years suggests that the circulation of Uranus' atmosphere is much more vigorous and/or efficient than is commonly thought. The composition of the main observed cloud decks between 2 and 6-8bar is unclear, but the absence of the expected methane cloud at 1.2-1.3bar (Lindal, G.F., Lyons, J.R., Sweetnam, D.N., Eshleman, V.R., Hinson, D.P. [1987] J. Geophys. Res., 92, 14987-15001) is striking (as previously noted by, among others, Sromovsky, L.A., Irwin, P.G.J., Fry, P.M. [2006] Icarus, 182, 577-593; Sromovsky, L.A., Fry, P.M. [2007] Icarus, 192, 527-557; Sromovsky, L.A., Fry, P.M. [2008] Icarus, 193, 252-266; Karkoschka, E., Tomasko, M. [2009] Icarus) and suggests that cloud particles may be considerably different from pure condensates and may be linked with stratospheric haze particles drizzling down from above, or that tropospheric hazes are generated near the methane condensation level and then drizzle down to deep pressures as suggested by Karkoschka and Tomasko (Karkoschka, E., Tomasko, M. [2009] Icarus). The retrieved cloud structures were also tested for different assumptions of the deep methane mole fraction, which Karkoschka and Tomasko (Karkoschka, E., Tomasko, M. [2009] Icarus) find may vary from [approximately equal to]1-2% in polar regions to perhaps as much as 4% equatorwards of 45[degrees]N,S. We found that such variations did not significantly affect our conclusions. Author Affiliation: (a) Atmospheric, Oceanic, and Planetary Physics, Department of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, United Kingdom (b) Joint Astronomy Centre, 660 N. A'ohoku Place, Hilo, HI 96720, USA Article History: Received 21 January 2009; Revised 27 April 2009; Accepted 3 May 2009

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.03.023 Byline: L.N. Fletcher (a), G.S. Orton (a), N.A. Teanby (b), P.G.J. Irwin (b) Keywords: Saturn; Jupiter; Atmospheres; composition; Atmospheres; structure Abstract: The global distribution of phosphine (PH.sub.3) on Jupiter and Saturn is derived using 2.5 cm.sup.-1 spectral resolution Cassini/CIRS observations. We extend the preliminary PH.sub.3 analyses on the gas giants [Irwin, P.G.J., and 6 colleagues, 2004. Icarus 172, 37-49; Fletcher, L.N., and 9 colleagues, 2007a. Icarus 188, 72-88] by (a) incorporating a wider range of Cassini/CIRS datasets and by considering a broader spectral range; (b) direct incorporation of thermal infrared opacities due to tropospheric aerosols and (c) using a common retrieval algorithm and spectroscopic line database to allow direct comparison between these two gas giants. The results suggest striking similarities between the tropospheric dynamics in the 100-1000 mbar regions of the giant planets: both demonstrate enhanced PH.sub.3 at the equator, depletion over neighbouring equatorial belts and mid-latitude belt/zone structures. Saturn's polar PH.sub.3 shows depletion within the hot cyclonic polar vortices. Jovian aerosol distributions are consistent with previous independent studies, and on Saturn we demonstrate that CIRS spectra are most consistent with a haze in the 100-400 mbar range with a mean optical depth of 0.1 at 10 [mu]m. Unlike Jupiter, Saturn's tropospheric haze shows a hemispherical asymmetry, being more opaque in the southern summer hemisphere than in the north. Thermal-IR haze opacity is not enhanced at Saturn's equator as it is on Jupiter. Small-scale perturbations to the mean PH.sub.3 abundance are discussed both in terms of a model of meridional overturning and parameterisation as eddy mixing. The large-scale structure of the PH.sub.3 distributions is likely to be related to changes in the photochemical lifetimes and the shielding due to aerosol opacities. On Saturn, the enhanced summer opacity results in shielding and extended photochemical lifetimes for PH.sub.3, permitting elevated PH.sub.3 levels over Saturn's summer hemisphere. Author Affiliation: (a) Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA (b) Atmospheric, Oceanic & Planetary Physics, Department of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, UK Article History: Received 12 December 2008; Revised 16 March 2009; Accepted 18 March 2009

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.03.022 Byline: N.A. Teanby (a), P.G.J. Irwin (a), R. de Kok (a), A. Jolly (b), B. Bezard (c), C.A. Nixon (d), S.B. Calcutt (a) Keywords: Titan; Atmospheres; composition Abstract: Far-IR (25-50 [mu]m, 200-400 cm.sup.-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 (C.sub.2N.sub.2), methylacetylene (C.sub.3H.sub.4), and diacetylene (C.sub.4H.sub.2) in Titan's stratosphere as a function of latitude. All three gases are enriched at northern latitudes, consistent with north polar subsidence. C.sub.4H.sub.2 abundances agree with those derived previously from mid-IR data, but C.sub.3H.sub.4 abundances are about 2 times lower, suggesting a vertical gradient or incorrect band intensities in the C.sub.3H.sub.4 spectroscopic data. For the first time C.sub.2N.sub.2 was detected at southern and equatorial latitudes with an average volume mixing ratio of 5.5[+ or -]1.4x10.sup.-11 derived from limb data (3-I significance). This limb result is also corroborated by nadir data, which give a C.sub.2N.sub.2 volume mixing ratio of 6[+ or -]3x10.sup.-11 (2-I significance) or alternatively a 3-I upper limit of 17x10.sup.-11. Comparing these figures with photochemical models suggests that galactic cosmic rays may be an important source of N.sub.2 dissociation in Titan's stratosphere. Like other nitriles (HCN, HC.sub.3N), C.sub.2N.sub.2 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. Author Affiliation: (a) Atmospheric, Oceanic & Planetary Physics, Department of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, UK (b) LISA, Universite Paris 12, 61 Av. General de Gaulle, F-94010 Creteil Cedex, France (c) Observatoire de Paris, LESIA, Meudon, F-92195, France (d) Department of Astronomy, University of Maryland, College Park, MD 20742, USA Article History: Received 1 December 2008; Revised 13 March 2009; Accepted 18 March 2009

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2008.09.019 Byline: L.N. Fletcher (a), G.S. Orton (a), N.A. Teanby (b), P.G.J. Irwin (b), G.L. Bjoraker (c) Keywords: Saturn; Atmospheres; composition; Atmospheres; structure Abstract: High spectral resolution observations from the Cassini Composite Infrared Spectrometer [Flasar, F.M., and 44 colleagues, 2004. Space Sci. Rev. 115, 169-297] are analysed to derive new estimates for the mole fractions of CH.sub.4, CH.sub.3D and.sup.13CH.sub.4 of (4.7[+ or -]0.2)x10.sup.-3, (3.0[+ or -]0.2)x10.sup.-7 and (5.1[+ or -]0.2)x10.sup.-5 respectively. The mole fractions show no hemispherical asymmetries or latitudinal variability. The analysis combines data from the far-IR methane rotational lines and the mid-IR features of methane and its isotopologues, using both the correlated-k retrieval algorithm of Irwin et al. [Irwin, P., and 9 colleagues, 2008. J. Quant. Spectrosc. Radiat. Trans. 109, 1136-1150] and a line-by-line approach to evaluate the reliability of the retrieved quantities. C/H was found to be enhanced by 10.9[+ or -]0.5 times the solar composition of Grevesse et al. [Grevesse, N., Asplund, M., Sauval, A., 2007. Space Sci. Rev. 130 (1), 105-114], 2.25[+ or -]0.55 times larger than the enrichment on Jupiter, and supporting the increasing fractional core mass with distance from the Sun predicted by the core accretion model of planetary formation. A comparison of the jovian and saturnian C/N, C/S and C/P ratios suggests different reservoirs of the trapped volatiles in a primordial solar nebula whose composition varies with distance from the Sun. This is supported by our derived D/H ratio in methane of (1.6[+ or -]0.2)x10.sup.-5, which appears to be smaller than the jovian value of Lellouch et al. [Lellouch, E., Bezard, B., Fouchet, T., Feuchtgruber, H., Encrenaz, T., de Graauw, T., 2001. Astron. Astrophys. 370, 610-622]. Mid-IR emission features provided an estimate of C12/C13= 91.8.sub.-7.8.sup.+8.4, which is consistent with both the terrestrial ratio and jovian ratio, suggesting that carbon was accreted from a shared reservoir for all of the planets. Author Affiliation: (a) Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA (b) Atmospheric, Oceanic & Planetary Physics, Department of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, UK (c) NASA/Goddard Spaceflight Center, Greenbelt, MD 20771, USA Article History: Received 19 June 2008; Revised 23 September 2008; Accepted 28 September 2008

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2008.05.024 Byline: R. de Kok, P.G.J. Irwin, N.A. Teanby Keywords: Titan; Atmospheres; composition; Infrared observations Abstract: 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 C.sub.4N.sub.2 gas was determined to be 9x10.sup.-9, which is less than required to make the C.sub.4N.sub.2 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.sup.-1 and is located around an altitude of 140 km. The upper limit for propionitrile (C.sub.2H.sub.5CN) gas shows that the feature around 220 cm.sup.-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.sup.-1 is not consistent with that of pure HCN ice at 172 cm.sup.-1 and mixing of HCN ice with other ices, or chemical alteration of HCN ice might mask the HCN ice signature. Author Affiliation: Atmospheric, Oceanic and Planetary Physics, Department of Physics, Clarendon Laboratory, University of Oxford, Parks Road, Oxford, OX1 3PU, UK Article History: Received 26 February 2008; Revised 12 May 2008

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2008.05.010 Byline: A. Crespin (a), S. Lebonnois (a), S. Vinatier (b), B. Bezard (b), A. Coustenis (b), N.A. Teanby (c), R.K. Achterberg (d), P. Rannou (e), F. Hourdin (a) Keywords: Titan; Atmospheres; dynamics; Satellites; composition Abstract: 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 mas.sup.-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 C.sub.2H.sub.2 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[degrees] 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[degrees] N), some compounds (C.sub.4H.sub.2, C.sub.3H.sub.4) 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[degrees] 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. Author Affiliation: (a) Laboratoire de Meteorologie Dynamique, IPSL, CNRS/UPMC, Box 99, F-75252 Paris Cedex 05, France (b) LESIA, Observatoire de Paris-Meudon, 5 place Jules Janssen, 92195 Meudon Cedex, France (c) Clarendon Laboratory, AOPP, Department of Physics, University of Oxford, Parks Road, Oxford OX1 3PU, UK (d) Department of Astronomy, University of Maryland, College Park, MD 20742, USA (e) Service d'Aeronomie, IPSL, CNRS/UPMC/UVSQ, BP3, F-91371 Verrieres-le-Buisson Cedex, France Article History: Received 13 September 2007; Revised 30 April 2008

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2007.03.009 Byline: C.J.A. Howett (a), P.G.J. Irwin (a), N.A. Teanby (a), A. Simon-Miller (b), S.B. Calcutt (a), L.N. Fletcher (a), R. de Kok (a) Keywords: Infrared observations; Saturn; atmosphere Abstract: 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.sup.-1, CIRS data was used from three data sets taken in June-November 2004 and binned into 2[degrees] 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[degrees] to 68[degrees] S, including a steep 4 K rise between 60[degrees] and 68[degrees] 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[degrees] S, by a factor of 1.8 at 2.0 mbar. Between 18[degrees] and 30[degrees] S the acetylene abundance increases at 2.0 mbar. Global values for acetylene have been calculated as (1.9[+ or -]0.19)x10.sup.-7 at 2.0 mbar, (2.6[+ or -]0.27)x10.sup.-7 at 1.6 mbar and (3.1[+ or -]0.32)x10.sup.-7 at 1.4 mbar. Global values for ethane are also determined and found to be (1.6[+ or -]0.25)x10.sup.-5 at 0.5 mbar and (1.4[+ or -]0.19)x10.sup.-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[degrees] 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. Author Affiliation: (a) Atmospheric, Oceanic & Planetary Physics, Department of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, UK (b) Goddard Space Flight Center, Mail Code 693, Greenbelt, MD 20771, USA Article History: Received 27 October 2006; Revised 5 March 2007

We have investigated the abundances of Titan's stratospheric oxygen compounds using 0.5 [cm.sup.-1] resolution spectra from the Composite Infrared Spectrometer on the Cassini orbiter. The CO abundance was derived for several observations of far-infrared nadir spectra, taken at a range of latitudes (75[degrees] S-35[degrees] N) and emission angles (0[degrees]-60[degrees]), using rotational lines that have not been analysed before the arrival of Cassini at Saturn. The derived volume mixing ratios for the different observations are mutually consistent regardless of latitude. The weighted mean CO volume mixing ratio is 47 [+ or -] 8 ppm if CO is assumed to be uniform with latitude. [H.sub.2]O could not be detected and an upper limit of 0.9 ppb was determined. C[O.sub.2] abundances derived from mid-infrared nadir spectra show no significant latitudinal variations, with typical values of 16 [+ or -] 2 ppb. Midinfrared limb spectra at 55[degrees] S were used to constrain the vertical profile of C[O.sub.2] for the first time. A vertical C[O.sub.2] profile that is constant above the condensation level at a volume mixing ratio of 15 ppb reproduces the limb spectra very well below 200 km. This is consistent with the long chemical lifetime of C[O.sub.2] in Titan's stratosphere. Above 200 km the C[O.sub.2] volume mixing ratio is not well constrained and an increase with altitude cannot be ruled out there. Keywords: Titan; Atmospheres, composition

Mid-infrared limb spectra in the range 600-1400 [cm.sup.-1] taken with the Composite InfraRed Spectrometer (CIRS) on-board the Cassini spacecraft were used to determine vertical profiles of HCN, H[C.sub.3] N, [C.sub.2][H.sub.2], and temperature in Titan's atmosphere. Both high (0.5 [cm.sup.-1]) and low (13.5 [cm.sup.-1]) spectral resolution data were used. The 0.5 [cm.sup.-1] data gave profiles at four latitudes and the 13.5 [cm.sup.-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. H[C.sub.3]N 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[degrees] N between enriched and un-enriched air is consistent with a confining polar vortex at 60[degrees] N and H[C.sub.3]N's short lifetime. In the far north, layers were observed in the H[C.sub.3]N 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.sup.-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 H[C.sub.3]N due to HCN's longer photochemical lifetime. [C.sub.2][H.sub.2] 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 [C.sub.2][H.sub.2] towards the north pole. Keywords: Titan; Atmospheres, composition

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2006.07.011 Byline: N.A. Teanby (a), L.N. Fletcher (a), P.G.J. Irwin (a), T. Fouchet (b)(c), G.S. Orton (d) Keywords: Saturn; Atmospheres; composition Abstract: Far infrared spectra (10-600 cm.sup.-1) from Cassini's Composite InfraRed Spectrometer (CIRS) were used to determine improved upper limits of hydrogen halides HF, HCl, HBr, and HI in Saturn's atmosphere. Three observations, comprising a total of 3088 spectra, gave 3I upper limits on HF, HCl, HBr, and HI volume mole fractions of 8.0x10.sup.-12, 6.7x10.sup.-11, 1.3x10.sup.-10, and 1.4x10.sup.-9, respectively, at the 500 mbar pressure level. These upper limits confirm sub-solar abundances of halide species for HF, HCl, and HBr in Saturn's upper atmosphere -- consistent with predictions from thermochemical models and influx of material from meteoroids. Our upper limit for HCl is 16 times lower than the tentative detection at 1.1x10.sup.-9 reported by Weisstein and Serabyn [Weisstein, E.W., Serabyn, E., 1996. Icarus 123, 23-36]. These observations are not sensitive to the deep halide abundance, which is expected to be enriched relative to the solar composition. Author Affiliation: (a) Atmospheric, Oceanic & Planetary Physics, Department of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, UK (b) Observatoire de Paris, LESIA, Meudon, F-92195, France (c) Universite Pierre et Marie Curie-Paris 6, UMR 8109, Paris, F-75005, France (d) CALTECH, Jet Propulsion Laboratory, MS 169-237, 4800 Oak Grove Dr., Pasadena, CA 91109, USA Article History: Received 29 March 2006; Revised 11 July 2006

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2005.11.003 Byline: P.G.J. Irwin (a), L.A. Sromovsky (b), E.K. Strong (c), K. Sihra (d), N.A. Teanby (a), N. Bowles (a), S.B. Calcutt (a), J.J. Remedios (e) Keywords: Spectroscopy; Uranus; Titan Abstract: The band model fits of Sihra [1998. Ph.D. Thesis. University of Oxford], subsequently reported by Irwin et al. [2005. Icarus 176, 255-271], to new measurements of low-temperature near-infrared self-broadened methane absorption spectra combined with earlier warmer, longer path measurements of both self- and hydrogen-broadened methane spectra measured by Strong et al. [1993. J. Quant. Spectrosc. Radiat. Transfer 50, 363-429], have been found to contain severe artefacts at wavelengths of very low methane absorption. Although spectra calculated from these new band data appear to be reliable for paths with low to medium absorption, transmissions calculated for long paths of high methane absorption, such as for Uranus, Neptune and Titan are severely compromised. The recorded laboratory transmission spectra of Sihra [1998. Ph.D. Thesis. University of Oxford] and Strong et al. [1993. J. Quant. Spectrosc. Radiat. Transfer 50, 363-429] have thus been refitted with a more robust model and new k-distribution data for both self- and hydrogen-broadened methane absorption derived. In addition, a new model of the temperature dependence of the absorption has been employed that improves the quality of the fit and should also provide more accurate extrapolations to low temperatures. Author Affiliation: (a) Atmospheric, Oceanic, and Planetary Physics, University of Oxford, Clarendon Laboratory, Parks Rd, Oxford, OX1 3PU, UK (b) Space Science and Engineering Center, University of Wisconsin-Madison, 1225 W. Dayton Street, Madison, WI 53706, USA (c) Department of Physics, University of Toronto, 60 St. George Street, Toronto, Ontario, Canada M5S 1A7 (d) National Radiological Protection Board, Chilton, Didcot, Oxon, OX11 0RQ, UK (e) EOS, Space Research Centre, Department of Physics and Astronomy, University of Leicester, LE11 7RH, UK Article History: Received 12 September 2005; Revised 3 November 2005