P44A-01 INVITED 16:00h
Cassini Ultraviolet Imaging Spectrograph Initial Observations of Saturn
Cassini's Ultraviolet Imaging Spectrograph (UVIS) has begun making detailed studies of Saturn. Two long slit spectral channels are used to obtain EUV data from 56.3-118.2 nm and FUV data from 111.5-191.3 nm. 64 spatial pixels along each slit are combined with slit motion to build up spectral images of Saturn, with sufficient spatial resolution to reveal Saturn's auroral oval. Observed emissions include H Lyman-alpha and H2 bands from Saturn's auroras and dayglow. The auroral spectrum is remarkably similar to that of Jupiter, showing short-wavelength FUV absorption due to methane, CH4. Saturn's aurora is observed to vary in brightness by at least a factor of four. The brightest auroral emissions seen so far occurred after 2004 day 207 19:30 when Cassini CAPS recorded passage of a solar wind shock. The enhanced auroral brightness persisted for days, and is seen at both poles of Saturn. Saturn's auroral and dayglow spectrum show striking differences. At the longest wavelengths, Saturn's reflected sunlight spectrum is strongly modulated by absorption bands of acetylene, C2H2. Maps of the distribution of C2H2 will provide clues to Saturn's upper atmospheric circulation.
P44A-02 16:15h
Cassini UVIS Observations of Saturn H$_{2}$ Dayglow Emission
An analysis of the Cassini UVIS Saturn H$_{2}$ Lyman and Werner band emission is presented. The spectrum was obtained during the Cassini pre-SOI period by binning multiple Saturn observations from April 2, 2004 to May 12, 2004. The H$_{2}$ dayglow emission is concentrated near the sub-solar point at a latitude of approximately -21 degrees. The H$_{2}$ emission is examined for both the EUV and FUV instruments, covering a total spectral range of $\sim$(561 to 1900) $\AA$. The H$_{2}$ emission is modeled using fine-structure, hydrogen physical chemistry. The relative contributions of solar and low energy electron excitation in the observed emission are analyzed with the fine-structure chemistry model. H$_{2}$ solar resonance scattering is more prevalent in the Saturn dayglow spectrum than the Jupiter dayglow spectrum obtained by Cassini UVIS in December of 2000. Details of non-LTE H$_{2}$ ($v$ : $J$) partitioning and altitude range of excitation predicted from the hydrogen chemistry model are discussed. The source processes and altitude inferred from the spectrum have important implications for the examination of the Saturn thermospheric heating mechanism.
P44A-03 INVITED 16:30h
Saturn Temperatures, Winds, and Composition from Cassini CIRS
We summarize CIRS observations of Saturn's southern hemisphere during the approach phase and early part of the Cassini tour. Stratospheric temperatures near 1 mbar increase from the equator to the south pole by 20 K. It is approximately three saturnian weeks after southern summer solstice, suggesting an efficient radiative response to insolation. At the tropopause (100 mbar), the equator-pole temperature contrast is more subdued, consistent with a larger radiative relaxation time. Zonal winds have been derived from the observed temperature field. The limited latitude resolution the early CIRS observations (8 degrees) precludes resolving the mid-latitude jets, but at low latitudes there is a large decrease (~140 m/s) of the zonal winds above the cloud tops to the upper stratosphere. The CIRS spectral range (7 micrometers - 1 mm) encompasses the signatures of a panoply of compounds, including hydrocarbons, oxygen compounds, phosphine, ammonia, and isotopes of carbon, nitrogen, and hydrogen. The extension of coverage to submillimeter wavelengths offers a unique capability not fully realized before. A brief preview of CIRS observations in the next few months of the Cassini tour will be presented.
P44A-04 16:45h
Spectral Imaging of Saturn by Cassini/VIMS: Early Science Results
We report results obtained from spectral images of Saturn acquired during the first two Saturn encounters by the Visual and Infrared Mapping Spectrometer (VIMS) on board the Cassini spacecraft. The broad spectral coverage of the VIMS instrument - from 0.3 to 5.1 microns - enables VIMS to observe a wide variety of atmospheric phenomena and processes. These include: (1) the vertical and spatial distribution and microphysical properties (size, shape, and composition) of stratospheric hazes and tropospheric clouds, (2) the distributions of condensable vapors (e.g., water and ammonia) and disequilibrium species (e.g., phosphine), diagnostic of meteorology and global circulation, (3) the distribution of species generated by auroral processes (e.g., H$_3$+), (4) methane fluorescence near 10-microbar level, and (5) lightning. Preliminary results will be presented, including images and spectra of the cloudy equatorial region, the southern temperate latitudes, and the south polar aurorae.
P44A-05 INVITED 17:00h
The Saturn Atmosphere: An Early Cassini ISS Perspective
During the approach and early part of the Cassini tour of the Saturn system the Imaging Science Subsystem (ISS) obtained a large number of images of Saturn. At this early phase of the mission we report quantitative results for some of the ISS atmospheric science objectives. Positional measurements of small spots show the same zonal wind profile reported from Voyager investigations (data from 1980-81) at latitudes outside of +- 18 degrees. In the latitude range within +- 18 degrees most of the Cassini measurements cluster around 270 m/s (Eastward), whereas the Voyager results are higher (300-470 m/s). The lower ISS values agree with values reported by Sanches-LaVega et al. (Nature 2003, 423, 623-625) measured from Hubble Space Telescope images between 1994 and 2002. A few of the Cassini measurements within this latitude band give speeds in the range 350-390 m/s which overlap (in latitude) the other points with lower values. Latitudinal temperature gradients point to a decrease in wind with height so some or all of the differences might be interpreted as differences in pressure levels of the cloud features used to track winds, with the Voyager measurements sensing the deepest. Two large storms in 1990 and 1995 put clouds at higher altitudes in the HST and Cassini epochs. Cloud height models from images in methane bands will be used to assess this hypothesis. We also observed spot interactions. The westward-moving jet at approximately 36 degrees south (the one closest to the equator in the southern hemisphere) was particularly active. It had many counter-clockwise rotating spots, which merged and divided during a few months of observation. A combination of polarization images from the violet to the near-infrared, methane-band images and ultraviolet images show a complex latitudinal behavior for cloud and haze structure and haze properties at high southern latitudes.
P44A-06 17:15h
Cassini RPWS Observations of Saturn Lightning
The Radio and Plasma Wave (RPWS) instrument on Cassini began observing Saturn Electrostatic Discharges (SED) on a routine basis on 13 July, shortly after closest approach to the planet. SED, first discovered by the Planetary Radio Astronomy instrument on Voyager, are widely believed to be the radio signature of lightning discharges in the atmosphere of Saturn. The radio signatures appear very similar to the dayside signatures observed over 20 years ago by Voyager in that the frequency range extends from about 1 MHz to the highest receiver frequency (~16 MHz), and the individual bursts are of short (10's of msec) duration. However, the Cassini observations differ in important ways: (1) the lightning is generally weaker in intensity, (2) the period of reoccurrence of the storms (~10.24 - 10.75 hr) is decidedly longer than that observed by Voyager (~10.1 hr), and (3) the lightning storms, lasting from minutes to hours, now occur episodically, rather than repeatedly on every rotation. This last point is underscored by the fact that a possible SED storm of great intensity was observed over a year before closest approach in July, 2003. As of this writing, RPWS continues to detect SED. Since 13 July, 40 well-defined episodes, or storms, have been observed, corresponding to a storm occurrence rate of about 40 percent. Careful tracking of the storm phase relative to the planet's rotation has revealed that two probably separate and distinct storm systems have developed since 13 July. The first, lasting from 13 July (or before) until 27 July, reappeared with a period of 10 hr 45 min with highly predictable start times, apparently corresponding to the emergence of the storm over the nightside limb of the planet. The storm waxed and waned in intensity, sometimes disappearing for many rotations, but always reappearing in phase. The second system appeared on 30 July, and has been observed on and off to date. This second system has reoccurred with a period of about 10hr 15min. Presumably the storm periodicity can be related to a particular latitude on Saturn where the measured wind speed in the atmosphere gives the appropriate period of rotation. Attempts to associate SED storms with particular atmospheric features are continuing.
P44A-07 INVITED 17:30h
Cassini Magnetic Field Measurements and Saturn's Interior
Cassini measurements at Saturn Orbit Insertion enable a search for temporal changes in the planetary field, reexamination of a possible departure from axial symmetry and high degree moments all of which could have important implications for the planet's interior. Discovery of a secular variation would be important in testing the hypothesis that the field is decaying away and could provide a means of estimating the depth to the fluid electrically-conducting core in which the field is being generated. The presence of a measurable tilt angle or non- axisymmetric components would test the hypothesis that the field is being shielded by an electrically-conducting shell and provide a determination of the rotation rate of the core. Detecting higher degree source terms than the dipole, quadrupole and octupole moments would enable another approach to estimating the size of the fluid core based on an Earth-like dynamo. These issues are addressed by recent analyses of the Cassini magnetic field observations including subsequent improvements to the spacecraft/ magnetic sensor orientations.
P44A-08 17:45h
Modeling Zonal Flows on the Giant Planets
Large-scale zonal flows, as observed on the giant planets, can result from deeply driven thermal convection in a rapidly rotating spherical shell. Using 3D numerical models, we study the affects of velocity boundary conditions and spherical shell geometry on zonal flow generation. The radius ratio, $\chi = r_i/r_o$, where $r_i$ is the inner shell boundary and $r_o$ is the outer shell boundary, is varied over the range $0.60 \le \chi \le 0.923$. The resulting surface zonal flow Rossby numbers for the models are comparable to those measured on the giant planets. In all our calculations a prograde barotropic jet forms in the equatorial region. This jet is flanked by large-scale alternating baroclinic jets at higher latitudes. Near the poles, an upwelling along the rotation axis is associated with a strong vortex structure. Superimposed on this are small-scale zonal flows driven by local 3D vortical convection that occurs only in this polar region. Scaling behavior for the number of alternating jets and their strength will be presented. The relevance of our numerical models to the characteristics and differences between zonal flows on Jupiter and Saturn will be discussed.