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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2010.06.020 Byline: Amy A. Simon-Miller (a), Peter J. Gierasch (b) Keywords: Jupiter, Atmosphere; Atmospheres, Evolution; Atmospheres, Dynamics Abstract: Hubble Space Telescope Wide Field Planetary Camera 2 imaging data of Jupiter were combined with wind profiles from Voyager and Cassini data to study long-term variability in Jupiter's winds and cloud brightness. Searches for evidence of wind velocity periodicity yielded a few latitudes with potential variability; the most significant periods were found nearly symmetrically about the equator at 0[degrees], 10-12[degrees]N, and 14-18[degrees]S planetographic latitude. The low to mid-latitude signals have components consistent with the measured stratospheric temperature Quasi-Quadrennial Oscillation (QQO) period of 4-5years, while the equatorial signal is approximately seasonal and could be tied to mesoscale wave formation. Robustness tests indicate that a constant or continuously varying periodic signal near 4.5years would appear with high significance in the data periodograms as long as uncertainties or noise in the data are not of greater magnitude. However, the lack of a consistent signal over many latitudes makes it difficult to interpret as a QQO-related change. In addition, further analyses of calibrated 410-nm and 953-nm brightness scans found few corresponding changes in troposphere haze and cloud structure on QQO timescales. However, stratospheric haze reflectance at 255-nm did appear to vary on seasonal timescales, though the data do not have enough temporal coverage or photometric accuracy to be conclusive. Sufficient temporal coverage and spacing, as well as data quality, are critical to this type of search. Author Affiliation: (a) Code 693, NASA Goddard Space Flight Center, Greenbelt, MD 20771, United States (b) 318 Space Sciences Department, Cornell University, Ithaca, NY 14853, United States Article History: Received 19 March 2010; Revised 16 June 2010; Accepted 16 June 2010

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2010.01.027 Byline: Anthony D. Toigo (a), Peter J. Gierasch (b), Bruno Sicardy (c), Emmanuel Lellouch (c) Keywords: Pluto, Atmosphere; Tides, Atmospheric; Occultations; Pluto; Atmospheres, Structure Abstract: Stellar occultations have shown that vertical profiles of density fluctuations in the atmosphere of Pluto typically show wave-like structure with an amplitude of a few percent and vertical wavelengths of a few kilometers. Here we calculate the tidal response of Pluto's atmosphere to solar-induced sublimation 'breathing' from N.sub.2 frost patches. Solutions show global-scale wave-like density structure capable of explaining the observations. The atmospheric response is a combination of eastward and westward migrating tides, together with a zonally symmetric mode. Calculated vertical wavelengths and amplitudes are similar to observations. Author Affiliation: (a) Center for Radiophysics and Space Research, Cornell University, Ithaca, NY 14853-6801, United States (b) Astronomy Department, Cornell University, Ithaca, NY 14853-6801, United States (c) LESIA, Observatoire de Paris, 5 place Jules Janssen, 92195 Meudon, France Article History: Received 31 July 2009; Revised 19 January 2010; Accepted 25 January 2010

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

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2006.01.019 Byline: Adam P. Showman (a), Peter J. Gierasch (b), Yuan Lian (a) Keywords: Jupiter; atmosphere; Atmospheres; dynamics Abstract: A simple model shows that acceleration of Jupiter and Saturn's multiple jets at altitudes confined near the top of the adiabatic region (e.g., at a few bars pressure) can produce jets that penetrate deeply into the molecular envelope. This result disproves the common assertion that jet acceleration near the outer margin can only produce zonal winds that are confined to these outer layers. Author Affiliation: (a) Department of Planetary Sciences, Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA (b) Department of Astronomy, Cornell University, Ithaca, NY 14853, USA Article History: Received 7 July 2005; Revised 10 December 2005

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2005.12.020 Byline: Richard K. Achterberg (a), Barney J. Conrath (b), Peter J. Gierasch (b) Keywords: Jupiter; atmosphere; Atmospheres; structure; Atmospheres; dynamics Abstract: The zonal mean ammonia abundance on Jupiter between the 400- and 500-mbar pressure levels is inferred as a function of latitude from Cassini Composite Infrared Spectrometer data. Near the Great Red Spot, the ammonia abundance is mapped as a function of latitude and longitude. The Equatorial Zone is rich in ammonia, with a relative humidity near unity. The North and South Equatorial Belts are depleted relative to the Equatorial Zone by an order of magnitude. The Great Red Spot shows a local maximum in the ammonia abundance. Ammonia abundance is highly correlated with temperature perturbations at the same altitude. Under the assumption that anomalies in ammonia and temperature are both perturbed from equilibrium by vertical motion, we find that the adjustment time constant for ammonia equilibration is about one third of the radiative time constant. Author Affiliation: (a) Science Systems and Applications, Inc., 10210 Greenbelt Road, Suite 600, Lanham, MD 20706, USA (b) Astronomy Department, Cornell University, Ithaca, NY 14853, USA Article History: Received 10 December 2004; Revised 8 December 2005

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 compositions of the atmospheres of the outer planets are dominated by molecular hydrogen. The hydrogen ortho and para forms (proton spins parallel and antiparallel) are observed to have ratios that are not in thermodynamic equilibrium, with spatial variations, probably due to vertical motions that transport fluid from a different temperature regime. Conversion between the two forms produces significant 'latent heat' release, but conversion is thought to be so slow that this heating is extremely small. Because the two forms of hydrogen have different specific heats and their abundance ratio is spatially variable, Ertel's potential vorticity is not conserved, even in the adiabatic and frictionless limit. In this paper the degree of nonconservation is assessed by scale analysis, for typical observed ortho-para inhomogeneity. A numerical example similar to Jupiter's Great Red Spot is presented. Analysis is restricted to large-scale motions in the stable upper tropospheres of the planets, where the quasigeostrophic approximation applies. A major result is that a generalization of quasigeostrophic potential vorticity is still conserved, and that the para fraction is merely an inert tracer in this regime. The Ertel isentropic potential vorticity is not conserved, even to leading order, except in special regions where the ortho-para ratio is exceptionally homogeneous.

The linear stability of a zonal jet that decays with depth is investigated under the assumption that the thermal stratification is very small. A westerly cosine jet is found to be more stable than it is in a thin fluid shell with two-dimensional flow. This is in agreement with observations of Jupiter and Saturn, where jet curvature exceeds the barotropic stability criterion. This result constitutes an alternative hypothesis to that of Ingersoll and Pollard [Icarus 52 (1982) 62], who showed that deep jets extending through the interior are also more stable than thin shell jets. The flow regime assumed in the present work requires that a small stratification can exist and persist even in the presence of horizontal temperature gradients. Further work will be needed to test whether this is realistic. Keywords: Jupiter; Saturn; Atmospheres; Outer planet atmospheres; Atmospheric dynamics

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.

Local radiative-convective thermal structure is calculated on Jupiter and Saturn under the assumption that a low opacity zone exists near the 2000-K depth, as suggested by T. Guillot et al. (1994a, Icarus 112, 337-353). The opacity is uncertain, and a simple but qualitatively accurate model is used to explore a range of opacities. Under the assumption of local radiative-convective equilibrium, insolation gradients lead to latitudinal temperature gradients in the shell outside the stably stratified radiative zone near 2000 K, and to a latitudinal variation in the emission to space. Measurements of the net heat balance (solar and planetary radiation) do not yet extend to high enough latitudes to test these predictions. On Jupiter and especially Saturn, the preference for westerly surface motion may be due to equator-to-pole temperature gradients in this shell. It is speculated that the dynamical response to radiative forcing due to low opacity near the 2000-K level may limit latitudinal temperature gradients and may be an important drive for meteorology on the outer planets.

During the first six orbits of the Galileo spacecraft's prime mission, the Solid State Imaging (SSI) system acquired multispectral image mosaics of Jupiter's Great Red Spot, an equatorial belt/zone boundary, a '5-[[micro]meter] hot spot' similar to the Galileo Probe entry site, and two of the classic White Ovals. We present mosaics of each region, approximating their appearance at visible wavelengths and showing cloud height and opacity variations. The local wind field is derived by tracking cloud motions between multiple observations of each region with time separations of roughly 1 and 10 hr. Vertical cloud structure is derived in a companion paper by Banfield et al. (Icarus 135, 230-250). Galileo's brief, high-resolution observations complement Earth-based and Voyager studies and offer local meteorological context for the Galileo Probe results. Our results show that the dynamics of the zonal jets and large vortices have changed little since Voyager, with a few exceptions. We detect a cyclonic current within the center of the predominantly anticyclonic Great Red Spot. The zonal velocity difference between 0 [degrees] S and 6 [degrees] S has increased by 20 m [sec.sup.-1]. We measure a strong northeast flow approaching the hot spot. This flow indicates either massive horizontal convergence or the presence of a large anticyclonic vortex southeast of the hot spot. The current compact arrangement of two White Ovals and a cyclonic structure greatly perturbs the zonal jets in that region. Key Words: Jupiter atmosphere; atmospheric dynamics; image processing; spacecraft.

Galileo Solid State Imaging (SSI) data taken during the E6 orbit on February 19, 1997 offer a brief, high-resolution glimpse of Jupiter's White Ovals. These data can be used to infer relative cloud heights and systematic rotations over a small area of latitude and longitude (approximately 20 [degrees] by 30 [degrees]) around the White Ovals. To fully understand the behavior of these cloud systems, however, a longer time span of global data is necessary. In this paper, we utilize ground-based Voyager and Hubble Space Telescope data to interpret the Galileo images. We find that the White Ovals' translation rates have slowed from 0.39 deg [day.sup.-1] during the Voyager era to 0.13 deg [day.sup.-1] in 1996 and they, along with three smaller anticyclonic systems, have coalesced into a system of alternating cyclonic and anticyclonic systems that has not been observed before. The largest of the ovals, BC, is now impeding the eastward motion of the system, causing more rapidly translating anticyclonic cells to catch the system and increase the westward extension of the equally spaced features. Rotational velocities for BC and a small anticyclonic system at 42 [degrees] S have also been measured and peak at 120 m [sec.sup.-1] for both systems, comparable to the Mitchell et al. (J. Geophys. Res. 86, 8751-8757) measurements of BC and the Great Red Spot (GRS) from Voyager data. The motion of individual features and of the whole system in the ambient wind flow is discussed. Key Words: Jupiter; Galileo; atmospheres.

Photometrically calibrated grism spectra of Jupiter in the H (1.45-1.8 [[micro]meter]) and K (1.95-2.5 [[micro]meter]) bands with a resolution of about 100 were taken with the 5-m Hale telescope at Palomar in August of 1995. The spectra were obtained as meridional cuts at six different longitudes on the planet, with one cut crossing the Great Red Spot. The technique outlined in D. Banfield et al. (1996, Icarus 121, 389-410) for the retrieval of scatterer density with altitude from near-infrared spectra is used and refined. It is expected that this retrieval technique will find use in the interpretation of many atmospheric near-infrared reflection spectra, especially those from Galileo NIMS and Cassini VIMS. For the wavelengths and spectral resolution used in this study, the sensitivity of the inversions extends from pressure levels near 400 mbar up to about 20 mbar. Employing this inversion technique on the spectra yields well-resolved jovian cloud densities for [Tau] [approximately less than] 0.1, as a function of latitude and altitude. The density of scatterers is minimum at a height where the pressure is about 100 mbar and increases both upward and downward from this level. The minimum near 100 mbar can be explained by coagulation of settling particles, leading to an increase in fall speed. The results indicate that stratospheric haze particles are generated at heights where p [approximately less than] 20 mbar. Key Words: atmospheres, abundances and dynamics; jovian atmosphere; radiative transfer; spectrophotometry.

A linear [Sigma]-coordinate model is used in conjunction with a Mars GCM northern hemisphere winter zonal mean circulation to study the orographically forced stationary disturbances on Mars. The disturbances are compared to their counterparts on Earth and to the bright streak data from the Mariner 9 and Viking images. Diabatic heating on Mars is predominantly radiative, and inversion near the surface causes mountains to cool and depressions to heat the atmosphere. Due to the short radiative timescales and large meridional surface pressure gradient, the atmosphere responds to radiative forcing by changing its temperature and pressure gradient. The strong meridional flow near the equator and short radiative timescales combine to make responses to mechanical and thermal forcing of the same order in the tropics and in parts of the northern midlatitudes. In the subtropics, small zonal and meridional flows allow the response to radiative forcing to dominate. All these factors contribute to making the Martian atmospheric responses to orographic forcing different from the responses found on Earth. The model near-surface winds forced by realistic topography agree well with the Mars GCM stationary wave winds and with the orientation of the bright streaks. The addition of stationary eddies to the zonal mean winds improves all measures of alignment, as does the inclusion of radiative forcing in the linear model. Using the dusty basic-state fields improves wind-streak alignment through a better match between the zonal mean winds and streaks. Even near Olympus Mons, linear stationary waves improve the wind-streak alignment when added to the zonal mean winds.

Atmospheric jets and vortices on Jupiter persist for decades in the presence of vigorous turbulence which might be expected to destroy organized flows. The penetration depth of the jets into the planet has been a major question, and new data from the NASA Galileo entry probe suggest that they are not shallow atmospheric phenomena. Numerical simulations and Galileo orbiter data promise further progress in the near future.

The orientations of the small cloud streaks that are always found at mid-latitudes in ultraviolet images of the Venus cloud-top provide a means for characterizing the global-scale circulation, including the wind field on the night side of Venus. Assuming that small clouds act as passive tracers of the cloud-top wind field, the orientation of cloud streaks is determined by the wind shear that the cloud has recently experienced. The typical deformation time for small clouds is about 50 hr, or one-half of a cloud-top rotation, so clouds viewed on the sunlit half of Venus show the influence of daytime and nighttime winds. By recording the orientations of cloud streaks and their variation with local solar time, we for the first time use cloud orientation information to place a constraint on the amplitudes of the Hadley circulation and solar thermal tides at the cloud-top level. A large solar thermal tide with a cloud-top meridional wind amplitude at 45 [degrees] latitude of between 5.5-10.5 m [sec.sup.-1] is found to be necessary to account for the observed variation with local solar time of cloud streak orientations. Maximum poleward tidal winds occur in the early afternoon. The cloud-top Hadley circulation is estimated to have an amplitude at 45 [degrees] latitude of between 3.5 and 8.5 m [s.sup.-1]. This means the Hadley circulation may not be the dominant contributor to cloud-top meridional velocities, and that equatorward meridional velocities may exist over a portion of the night side at the Venus cloud-top.

Narrow-band 2.0-2.35 [[micro]meter] spectrophotometric observations of Jupiter at nine wavelengths were taken with the 5-m Hale Telescope at Palomar during the month following the impact of Comet P/Shoemaker-Levy 9. Spectra were obtained for the principal impact sites, and also for regions not affected by the comet impacts (the South Polar Region, North Equatorial Zone, South Equatorial Zone, and the Great Red Spot). A technique by which near-infrared reflection spectra can be inverted to yield vertical profiles of scatterer density is presented. For our wavelengths and bandwidths, the sensitivity of the inversions extends from pressure levels near 1 bar up to about 20 mbar. We find that all comet-induced aerosol clouds lie near or above the vertical limit of our sensitivity, i.e., 20 mbar. The lower limit of the clouds is around 50 mbar. The total scattering opacity of the clouds decreased by a factor of about 1.7 over the 35 days spanned by these observations, while the area covered by them increased by a factor of about 1.5. West et al. (West, R. A., E. Karkoschka, A. J. Friedson, M. Seymour, K. H. Baines, and H. B. Hammel 1995. Science 267, 1296-1301.) suggest particle coagulation during this time which fits with our observations, but particle fallout is also likely. All the aerosols comprise a volume the equivalent of a [similar to] 0.6-km radius sphere assuming a particle size of 0.25 [[micro]meter]. We also measured the drift speeds of these clouds using two different techniques. One involved scaling down Voyager-measured cloud-top winds (Limaye, S. S. 1986. Icarus 65,335-352.) and deforming early images to match later images, while the other involved directly cross-correlating longitudinally shifted images of spots from different times. Both techniques indicate wind speeds of order 5 m/sec, or [approximately]30% of the tropospheric wind speeds measured by Voyager in this region.

Two terrestrial methods, convective adjustment and diffusive methods, are applied to outer planet applications for developing and understanding an algorithm that can be incorporated in dynamical modeling of outer planetary flows. This model postulates that the atmosphere of the outer planets are cooled from the top, as by radiation to space and this cooling is regulated to some extent by ortho-para hydrogen conversion. After ortho- and para- hydrogen are mixed by a convective event, their ratio begins to relax toward thermodynamic equilibrium and this leads to atmospheric stability.

Galileo imaging system observations of the Venus cloud deck at 400 nanometers for 16 hours during Feb 1990 revealed the presence of solar tides and huge sub-global scale albedo features. Differential sunlight absorption triggers the local dynamics of Venus to general cell-like structures at low latitudes. Cell-like or streaky albedo features depend on horizontal shear flow.

The nature of global-scale waves that can exist in the atmosphere of Venus is examined. A linear three-dimensional model atmosphere with spherical geometry is used to study large-scale forced and free waves. Solutions are obtained numerically with grid points in the vertical and a spherical harmonic expansion in the horizontal. Observations have shown a global-scale traveling wave with phase speed near the cloud-top wind velocity. Global-scale wave modes are found to exist in the model at this velocity. When a radiative- dynamic cloud feedback is added to the model, the most unstable wave mode is found to have the same phase speed as the observed wave. The horizontal structure of this wave is consistent with the observed horizontal structure of the 'Y' feature seen in ultraviolet images of the Venus cloud top.

A two-layer rigid-lid model is used to examine the initial development of an anticyclonic body in a high speed jet flow for the incompressible fluid by the shallow water equations. Gravity waves propagate in all directions in the case of a large Rossby deformation radius. In small deformations, the gravity waves can propagate only in the direction of the upstream, which causes the dark spots in the North Temperate Belt of Jupiter.

Multispectral images of the lunar western limb and far side obtained from Galileo reveal the compositional nature of several prominent lunar features and provide new information on lunar evolution. The data reveal that the ejecta from the Orientale impact basin (900 kilometers in diameter) lying outside the Cordillera Mountains was excavated from the crust, not hte mantle, and covers pre-Orientale terrain that consisted of both highland materials and relatively large expenses of ancient mare basalts. The inside of the far side South Pole--Aitken basinc (>2000 kilometers in diameter) has low albedo, red color, and a relatively high abundance of iron- and magnesium-rich materials. These features suggest that the impact may have penetrated into the deep crust or lunar mantle or that the basin contains ancient mare basalts that were later covered by highlands ejecta., THE GALILEO SPACECRAFT ENCOUNTERED THE EARTH-moon system in December 1990 in the first of two flybys that are part of a sequence of planetary gravity assists that will deliver the [...]