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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.04.002 Byline: Sandrine Guerlet (a), Thierry Fouchet (a), Bruno Bezard (a), Amy A. Simon-Miller (b), F. Michael Flasar (b) Keywords: Saturn; Infrared observations; Atmospheres; Composition Abstract: Measuring the spatial distribution of chemical compounds in Saturn's stratosphere is critical to better understand the planet's photochemistry and dynamics. Here we present an analysis of infrared spectra in the range 600-1400cm.sup.-1 acquired in limb geometry by the Cassini spacecraft between March 2005 and January 2008. We first determine the vertical temperature profiles from 3 to 0.01hPa, at latitudes ranging from 70[degrees]N to 80[degrees]S. We infer a similar meridional temperature gradient at 1-2hPa as in recent previous studies [Fletcher, L.N., Irwin, P.G.J., Teanby, N.A., Orton, G.S., Parrish, P.D., de Kok, R., Howett, C., Calcutt, S.B., Bowles, N., Taylor, F.W., 2007. Icarus 189, 457-478; Howett, C.J.A., Irwin, P.G.J., Teanby, N.A., Simon-Miller, A., Calcutt, S.B., Fletcher, L.N., de Kok, R., 2007. Icarus 190, 556-572]. We then retrieve the vertical profiles of C.sub.2H.sub.6 and C.sub.2H.sub.2 from 3 to 0.01hPa and of C.sub.3H.sub.8 around 1hPa. At 1hPa, the meridional variation of C.sub.2H.sub.2 is found to follow the yearly averaged solar insolation, except for a strong equatorial mole fraction of 8x10.sup.-7, nearly two times higher than expected. This enhancement in abundance can be explained by the descent of hydrocarbon-rich air, with a vertical wind speed at the equator of 0.25[+ or -]0.1 mm/s at 1hPa and 0.4[+ or -]0.15 mm/s at 0.1hPa. The ethane distribution is relatively uniform at 1hPa, with only a moderate 25% increase from 35[degrees]S to 80[degrees]S. Propane is found to increase from north to south by a factor of 1.9, suggesting that its lifetime may be shorter than Saturn's year at 1hPa. At high altitudes (1Pa), C.sub.2H.sub.2 and C.sub.2H.sub.6 abundances depart significantly from the photochemical model predictions of Moses and Greathouse [Moses, J.I., Greathouse, T.K., 2005. J. Geophys. Res. 110, 9007], except at high southern latitudes (62, 70 and 80[degrees]S) and near the equator. The observed abundances are found strongly depleted in the 20-40[degrees]S region and enhanced in the 20-30[degrees]N region, the latter coinciding with the ring's shadow. We favor a dynamical explanation for these anomalies. Author Affiliation: (a) LESIA - Observatoire de Paris, 92195 Meudon, France (b) NASA/Goddard Space Flight Center, Code 693, Greenbelt, MD 20771, USA Article History: Received 18 December 2008; Revised 3 April 2009; Accepted 13 April 2009

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2008.05.014 Byline: Richard K. Achterberg (a), Barney J. Conrath (b), Peter J. Gierasch (b), F. Michael Flasar (c), Conor A. Nixon (a) Keywords: Titan; Atmospheres; structure; Atmospheres; dynamics; Infrared observations Abstract: Maps of isotherms on surfaces of constant pressure in Titan's middle atmosphere encircle the poles but show an offset, implying that the mean zonal flow has an axis of symmetry that is tilted relative to the spin axis of the solid body. The effect is seen in both hemispheres around a consistent axis. Periodogram analysis of the temperature field shows that wavenumber one, the signal corresponding to the spin tilt, is the strongest wave component. We conjecture that the tilt of the atmospheric spin is due to a feedback between the flow and the solar heating. The spin adjusts itself to align the spin equator with the direction toward the Sun, and thereby maximizes the efficiency with which the meridional circulation pumps angular momentum upward to generate superrotation. Author Affiliation: (a) University of Maryland, Department of Astronomy, College Park, MD 20742, USA (b) Department of Astronomy, Cornell University, Ithaca, NY 14853, USA (c) NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA Article History: Received 11 December 2007; Revised 11 April 2008

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2007.09.029 Byline: Richard K. Achterberg (a), Barney J. Conrath (b), Peter J. Gierasch (b), F. Michael Flasar (c), Conor A. Nixon (a) Keywords: Titan; Atmospheres; structure; Atmospheres; dynamics; Infrared observations Abstract: The Composite Infrared Radiometer-Spectrometer (CIRS) instrument, on the NASA Cassini Saturn orbiter, has been acquiring thermal emission spectra from the atmosphere of Titan since orbit insertion in 2004. Observation sequences for measuring stratospheric temperatures have been obtained using both a nadir mapping mode and a limb viewing mode. The limb observations give better vertical resolution, and give information from higher altitudes, while the nadir observations provide more complete longitude coverage. Because the scale height of Titan's atmosphere is large enough so that emission from a grazing ray is influenced by horizontal temperature variations in the atmosphere, we have developed a two-dimensional temperature retrieval algorithm for reducing the limb spectra, which solves simultaneously for meridional and vertical temperature variations. The analyzed nadir mapping data have sampled nearly all longitudes at latitudes from about 90[degrees] S to 60[degrees] N, providing temperatures between pressure levels of about 5 to 0.2 mbar. The limb data covers latitudes between about 75[degrees] S and 85[degrees] N, and yields temperatures between about 1 and 0.005 mbar, at a small number of longitudes. The retrieved temperatures are consistent with early results from nadir observations [Flasar, F.M., and 44 colleagues, 2005. Science 308, 975-978] between 0.5 and 5 mbar where both results are valid, with the warmest temperatures at the equator, and much stronger meridional temperature gradients in the northern (winter) hemisphere than in the southern. At higher altitudes not probed by nadir viewing, the limb data reveal that the stratopause is nearly 20 K warmer in the northern polar regions than at the equator and southern hemisphere, and that the altitude of the stratopause shifts from [approximately equal to]0.1 mbar (300 km) near the equator to 0.01 mbar (400 km) poleward of about 40[degrees] N. When the gradient wind equation is used to construct a zonal mean wind, the reversal in sign of the temperature leads to capping of the winter westerly flow. The core of the resulting jet is about 190 mas.sup.-1 in magnitude, spans between 30[degrees] N and 60[degrees] N, and peaks near 0.1 mbar. Estimates of the radiative heating associated with the radiative disequilibrium lead to a meridional overturning timescale of about three Earth years. Author Affiliation: (a) University of Maryland, Department of Astronomy, College Park, MD 20742, USA (b) Department of Astronomy, Cornell University, Ithaca, NY 14853, USA (c) NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA Article History: Received 31 May 2007; Revised 13 September 2007

The Cassini Imaging Science Subsystem (ISS) and Composite Infrared Spectrometer (CIRS) reported a North Equatorial Belt (NEB) wave in Jupiter's atmosphere from optical images [Porco, C.C., and 23 colleagues, 2003. Science 299, 1541-1547] and thermal maps [Flasar, EM., and 39 colleagues, 2004. Nature 427, 132-135], respectively. The connection between the two waves remained uncertain because the two observations were not simultaneous. Here we report on simultaneous ISS images and CIRS thermal maps that confirm that the NEB wave shown in the ISS ultraviolet (UV1) and strong methane band (MT3) images is correlated with the thermal wave in the CIRS temperature maps, with low temperatures in the CIRS maps (upwelling) corresponding to dark regions in the UV1 images (UV-absorbing particles) and bright regions in the MT3 images (high clouds and haze). The long period of the NEB wave suggests that it is a planetary (Rossby) wave. The combined observations from the ISS and CIRS are utilized to discuss the vertical and meridional propagation of the NEB wave, which offers a possible explanation for why the NEB wave is confined to specific latitudes and altitudes. Further, the ISS UV1 images reveal a circumpolar wave centered at 48.5[degrees] S (planetocentric) and probably located in the stratosphere, as suggested by the ISS and CIRS observations. The simultaneous comparison between the ISS and CIRS also implies that the large dark oval in the polar stratosphere of Jupiter discovered in the ISS UV1 images [Porco, C.C., and 23 colleagues, 2003. Science 299, 1541-1547] is the same feature as the warm regions at high northern latitudes in the CIRS 1-mbar temperature maps [Flasar, EM., and 39 colleagues, 2004. Nature 427, 132-135]. This comparison supports a previous suggestion that the dark oval in the ISS UV1 images is linked to auroral precipitation and heating [Porco, C.C., and 23 colleagues, 2003. Science 299, 1541-1547]. Keywords: Atmospheres, dynamics; Jupiter, atmosphere; Planetary dynamics

Retrievals performed on Cassini Composite Infrared Spectrometer data obtained during the distant Jupiter flyby in 2000/2001 have been used to generate global temperature maps of the planet in the troposphere and stratosphere, but to higher latitudes than were shown previously by Flasar et al. [Flasar, F.M., 39 colleagues, 2004a. Nature 427, 132-135, Flasar, F.M., 44 colleagues, 2004b. Space Sci. Rev. 115, 169-297]. Similar retrievals were performed on Voyager 1 IRIS data to provide the first detailed IRIS map of the stratosphere, and high latitudes in the troposphere. Thermal winds were calculated for each data set and show strong vertical shears in the zonal winds at low latitudes, and meridional temperature gradients indicate the presence of circumpolar jets, as well. The temperatures retrieved from the two spacecraft were also compared with yearly ground-based data obtained over the intervening two decades. Tropospheric temperatures reveal gradual changes at low latitudes, with little obvious seasonal or short-term variation [Orton et al., 1994. Science 265, 625-631]. Stratospheric temperatures show much more complicated behavior over short timescales, consistent with quasi-quadrennial oscillations at low latitudes, as suggested in prior analyses of shorter intervals of ground-based data [Orton et al., 1991. Science 252, 537-542; Friedson, A.J., 1999. Icarus 137, 34-55]. A scaling analysis indicates that meridional motions, mechanically forced by wave or eddy convergence, play an important role in modulating the temperatures and winds in the upper troposphere and stratosphere on seasonal and shorter timescales. At latitudes away from the equator, the mechanical forcing can be derived simply from a temporal record of temperature and its vertical derivative. Ground-based observations with improved vertical resolution and/or long-term monitoring from spacecraft are required for this purpose, though the Voyager and Cassini data given indications that the magnitude of the forcing is very small. Keywords: Jupiter, atmosphere; Atmospheres, dynamics; Atmospheres, temperature

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2005.06.020 Byline: Katia I. Matcheva (a), Barney J. Conrath (a), Peter J. Gierasch (a), F. Michael Flasar (b) Keywords: Jupiter; Jupiter; atmosphere; Atmospheres; structure Abstract: We analyze the thermal infrared spectra of Jupiter obtained by the Cassini-CIRS instrument during the 2000 flyby to infer temperature and cloud density in the jovian stratosphere and upper troposphere. We use an inversion technique to derive zonal mean vertical profiles of cloud absorption coefficient and optical thickness from a narrow spectral window centered at 1392 cm.sup.-1 (7.18 [mu]m). At this wavenumber atmospheric absorption due to ammonia gas is very weak and uncertainties in the ammonia abundance do not impact the cloud retrieval results. For cloud-free conditions the atmospheric transmission is limited by the absorption of molecular hydrogen and methane. The gaseous optical depth of the atmosphere is of order unity at about 1200 mbar. This allows us to probe the structure of the atmosphere through a layer where ammonia cloud formation is expected. The results are presented as height vs latitude cross-sections of the zonal mean cloud optical depth and cloud absorption coefficient. The cloud optical depth and the cloud base pressure exhibit a significant variability with latitude. In regions with thin cloud cover (cloud optical depth less than 2), the cloud absorption coefficient peaks at 1.1[+ or -]0.05bar, whereas in regions with thick clouds the peak cloud absorption coefficient occurs in the vicinity of 900[+ or -]50 mbar. If the cloud optical depth is too large the location of the cloud peak cannot be identified. Based on theoretical expectations for the ammonia condensation pressure we conclude that the detected clouds are probably a system of two different cloud layers: a top ammonia ice layer at about 900 mbar covering only limited latitudes and a second, deeper layer at 1100 mbar, possibly made of ammonium hydrosulfide. Author Affiliation: (a) Department of Astronomy, Cornell University, Ithaca, NY 14853, USA (b) NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA Article History: Received 25 October 2004; Revised 24 April 2005

The changing size and aspect ratio of the Great Red Spot from 1880 to 2000 are reviewed, indicating that the length of the system has decreased significantly over the last 100 years and continues to decrease at present at a rate of 0.19 degrees per year. Voyager IRIS maps of para hydrogen fraction and potential temperature over the system are presented, showing the internal structure and surrounding areas at pressures from 100 to 500 mbars. In these maps, the GRS appears as a cold temperature anomaly and a low para fraction anomaly. Vertical structure analyses and wind velocity measurements are presented from data acquired by the Galileo spacecraft in 1996 and 2000, respectively. The vertical structure data suggest a very thick, white, cloud deck with a top near 800 [+ or -] 100 mbars and an extended, blue-absorbing, optically thick haze up to about 200 mbars with a thin stratospheric haze above it. The wind field shows a substantial increase in the maximum tangential velocity from 150 to 190 m [s.sup.-1] over the Galileo time period. The vertical structure analyses from Galileo data and the wind and temperature fields from Voyager data all show evidence of a tilt to the Great Red Spot at the cloud deck from north to south and more subtly from east to west. The data support the hypothesis of the GRS as a tilted 'pancake' with temporal variability to the tilt. Key Words: Jupiter; atmosphere; atmospheres, dynamics; atmospheres, evolution; atmospheres, structure.

We have used temperatures retrieved from the Voyager IRIS (Infrared Interferometer Spectrometer) data at pressures of 130 and 270 mbar to search for waves in the upper troposphere of Saturn. The data are from three global mapping sequences taken by Voyagers 1 and 2, which together provide coverage between 60 [degrees] S and 80 [degrees] N latitudes. To analyze the temperatures, we grouped them within 10 [degrees] wide latitude bins and applied a periodogram analysis of their zonal variance. Since the data for each map were taken over periods of 12 to 18 hours, the phase velocity of any spectral component identified is ambiguous. We estimated the phase velocity of these components by varying the assumed phase velocity to maximize the amplitude of the periodogram response at integer zonal wavenumbers. The results are dominated by zonal wavenumber 2 at 130 mbar in northern midlatitudes. In the Voyager 1 data this component is statistically significant at the 99% confidence level over latitudes 20 [degrees] N to 40 [degrees] N; the zonal phase of the wave is roughly constant over this latitude range. Estimates of the phase velocity using the combined Voyager 1 inbound and outbound maps allow a solution for the midlatitude zonal wavenumber 2 wave that is quasi-stationary in the reference frame defined by the rotation period of Saturn's magnetic field. Raytracing models suggest that the midlatitude waves are quasi-stationary Rossby waves. The waves are confined meridionally by meridional variations of the zonal mean winds, and confined vertically by variations in the static stability of Saturn's atmosphere. The meridional structure of these features suggests that they may be excited by overreflection of waves incident on a critical surface at midlatitudes, and that the low phase velocities are a consequence of the gradient in potential vorticity vanishing where the zonal winds are small. The waves can leak into the upper stratosphere, saturating or 'breaking' at pressure levels [approximately] [10.sup.-2][-10.sup.-1] mbar. The viscous damping times are -10 years, comparable to the radiative damping time in the tropopause region and in the stratosphere.

We present the first detection of NH3 ice in the thermal infrared in Jupiter''s atmosphere using Cassini CIRS observations in the 10-μm region obtained on 31 December 2000 and 1 January 2001.We identify a brightness temperature difference α≡TB(1040 cm−1)−TB(1060 cm−1) as an indicator of spectrally identifiable NH3 ice, where 1040 cm−1 is an adjacent continuum region and 1060 cm−1 is the NH3 ice feature. Higher values of α imply a stronger NH3 ice signature in the spectrum. Using midlatitude zonally averaged CIRS spectra, we demonstrate systematic spatial variations in α, with the highest values at the equator and near 23°N.In one CIRS spectral average (covering 22–25°N and 140–240°W), our radiative transfer models are consistent with an optical depth of 0.75±0.25 for NH3 ice particles modeled as randomly oriented 4:1 prolate spheroids (volume equivalent radius=0.79 μm). Particles larger or smaller than 1 μm by about a factor of 2 would be unable to duplicate the observed NH3 ice feature at 1060 cm−1: absorption due to larger particles is excessively broadened, and absorption due to smaller particles is hidden by NH3 gas absorption at 1067 cm−1. We also modeled an average spectrum for a second region on Jupiter (14–17°N and 10–70°W), finding an upper limit of τ=0.2 for the same NH3 ice particle type. The choice of prolate spheroid particles is based on laboratory studies of NH3 ice aerosols, although 1-μm Mie-scattering spheres would also have detectable signatures at 1060 cm−1. We model the 1-μm NH3 ice cloud with a particle-to-gas scale height ratio Hp/Hg=1. For both CIRS spectra analyzed, the spectrum at frequencies greater than 1100 cm−1 also requires a second cloud with essentially grey absorption, which we modeled using 10-μm NH3 ice spheres distributed with Hp/Hg=1/8 and a cloud base at 790 mbar. [Copyright &y& Elsevier]