P31A-0183
The Source of Heavy Element Enrichments on Saturn and the Other Giant Planets
We have developed a simple, top-down model for the initial composition of giant planets that explains the observed enrichment of heavy elements on Jupiter. It offers a new application of the pioneering work of Mizuno on giant planet structure. This model correctly predicted the enhancement of Carbon in Saturn's methane subsequently observed by CIRS on Cassini. It makes verifiable predictions for the other heavy elements on this planet that would provide a useful constraint on models for Saturn's interior. Predictions for Uranus and Neptune are consistent with the highly uncertain enrichments of C/H on these planets; they agree well with the better- determined values of D/H. The model implies the existence of a large mass of primitive icy planetesimals in the plane of the early solar nebula.
P31A-0184
Saturn's Deep Cloud Structure Derived From 5-Micron Spectra
The CSHELL and SpeX spectrometers on NASA's Infrared Telescope Facility were used to observe Saturn between 4.5 and 5.4 microns on several occasions from 2004-2007 at the same time as Cassini/VIMS and CIRS were mapping the planet. At these wavelengths thermal radiation originates from the deep atmosphere (5 bars) and it is attenuated by two cloud layers considered in equilibrium models to be composed of NH4SH and condensed NH3. In addition, there is a component of sunlight reflected from the upper (NH3) cloud that varies spatially on Saturn. CSHELL can spectrally resolve profiles of absorption lines of ammonia (NH3) and phosphine (PH3) on Saturn at selected wavelengths. These lines are very broad due to collisions with 3 to 5 bars of hydrogen. The Saturn spectrum exhibits numerous strong NH3 and PH3 lines, as well as Fraunhofer lines due to CO in the Sun. SpeX observations cover the entire 5-micron window sampling both thermal emission and reflected sunlight. Image cubes were obtained by stepping the slit across the planet. The best contrast in reconstructed images occurs at 5.05 microns, which coincides with the wavelength where VIMS sees spectacular structure on Saturn. The spatial variation of Saturn's 5-micron spectrum is dominated by the variable opacity of its deep cloud structure. Superimposed on this are smaller variations in the mixing ratios of NH3 and PH3. The abundances of these gases can be retrieved reliably in relatively cloud-free regions between 50 South and 65 South, which are analogous to Jupiter's belts and 5-micron hot spots. Elsewhere, it is more difficult to separate changes in cloud opacity from gas abundances. We use near-simultaneous CIRS observations which sound the ~500-mbar level to provide an upper boundary condition to PH3. The 5-micron spectrum of Saturn's Equatorial Zone (10 South) is significantly different from a region near 60 South. The NH3 and PH3 lines are weaker and narrower in the EQZ, while the Fraunhofer lines are stronger. We will present synthetic spectra calculated from models which fit both regions and which explore the tradeoffs between gas mixing ratios and cloud opacity. This will constrain the pressure level of the deep clouds and extend our knowledge of gas abundances to deeper levels than is possible using Cassini data alone. This work was supported by NASA's Planetary Astronomy program.
P31A-0185
Mapping the temperature and composition of Saturn's stratosphere from Cassini/CIRS observations
Using the limb observations provided by the Composite Infrared Spectrometer aboard the Cassini spacecraft, we obtained an unprecedended map of the temperature between 20~hPa to 0.003~hPa, and of the composition (ethane and acetylene) between 10~hPa and 0.01~hPa, with a vertical resolution of 1.5 times a scale height. The statospheric temperature structure and the derived thermal wind structure show the presence of an equatorial oscillation similar to the terrestrial QBO and jovian QQO. At extratropical latitudes, the temperature structure exhibits a smoother increase with altitude. However, in the upper stratosphere, the radiative cooling rates at mid-latitudes in northern and southern hemispheres does not balance with the local solar heating rates. This desiquilibrium can be explained by a meridional circulation from the southern to the northern hemisphere. The composition of the stratosphere, with enhancement of hydrocarbon abundance in the northern hemisphere at high altitudes constitutes another evidence for a meridional circulation.
P31A-0186
Results from the 2007 Radio Occultations of Cassini by Saturn
Six radio occultations of Cassini by Saturn have occurred or will occur in 2007, on May 10, June 11, June 26, October 24, December 3, and (after this meeting) on December 19. Unlike the 2005 series of occultations that were diametric and within 10 degrees of the equator, this series of occultations covered a wide range of latitudes from ~ 66 S to ~ 70 N. We present the temperature - pressure profiles obtained from the first 5 occultations of 2007, and compare them with the earlier Cassini and Voyager occultations.
P31A-0187
Auroral Movies and Spectroscopy from Cassini UVIS
Cassini's Ultraviolet Imaging Spectrograph (UVIS) has completed three years of study of Saturn's atmosphere and auroras. 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 construct spectral images of Saturn. Auroral emissions are seen from electron-excited molecular and atomic hydrogen. In 2007 UVIS obtained data with the spacecraft well out of Saturn's ring plane, permitting us to create images, spectra, and at times movies. We will present an auroral movie from 2007-145 that has been processed to remove flat-fielding artifacts and deconvolved to remove scattering along the slit. The movie shows near co- rotation of N polar auroral features with the planet's rotation. An auroral oval is present. The oval appears doubled on the midnight side. Other images from this year show emissions inside the auroral oval. We will discuss these images and their spectra. Additional images and movies are planned in coming months.
P31A-0188
Aurora at the North Pole of Saturn as Seen by Cassini ISS
Cassini ISS saw the visible aurora at Saturn for the first time. Brighter parts of the auroral circle are seen at latitude ~75 N. We will compare the brightness and structure of the aurora at several wavelengths, including the broadband clear filter, H-alpha (656 nm),Near-IR (825-925 nm), and UV (300-370 nm). We will discuss temporal variation of the aurora between several Cassini observations separated by months.
P31A-0189
Possible Resonances with Saturn's Rotation in the Rings
Images from the Cassini spacecraft reveal several unexpected structures in Saturn's faint rings that could be caused by irregularities in the planet's interior and/or magnetosphere. Images of the D ring from 2006 and early 2007 show structures between 71,000 and 73,000 km from Saturn's center that each have mode-2 rotational symmetry around the planet (i.e. the brightness variations with radius are the same on opposite sides of the planet). Similarly, images of the region between the A and F rings taken in late 2006 contain azimuthally periodic patterns around 137,500 km. These sorts of periodic structures are probably generated at resonances, where the orbital period of the particles in the ring is close to a whole number ratio times the period of some perturbing force. Indeed, similar brightness variations are found in the G ring near the 8:7 Inner Lindblad Resonance with Mimas. The locations and symmetry properties of the observed patterns, along with the speed at which they circle Saturn, can be used to identify not only the types of resonances that produce and sustain these features, but also the frequencies of the perturbing forces. Patterns in the D ring are consistent with 2:1 Inner Lindblad resonances, while those outside the A ring appear to be caused by 3:4 Outer Lindblad Resonances. In both locations, multiple perturbing forces with periods ranging between 10.5 and 10.9 hours appear to be affecting the ring material. As this overlaps the span of periods associated with Saturn's winds and magnetosphere, the observed ring structures may be created by anomalies in Saturn's gravitational and/or magnetic field. The relative strength and persistence of these patterns may therefore provide information about the the structure of the planet's interior or the dynamics of the inner magnetosphere.
P31A-0190
First Steps Toward an Accurate Quantification of the Saturnian Tidal Dissipation
Tidal dissipation inside gaseous planets is crucial for the study of their long term evolution. For the exo-planets tidal dissipation has often been estimated from the Solar system giant planets. The tidal dissipation inside Saturn and Jupiter can be determined from theoretical assumptions on the orbital evolution of their satellites. This provides a lower bound for their dissipation parameter Q. On the other hand, past studies to seek tidal accelerations in the astrometrical residuals of the Galilean and Saturnian satellites resulted in large differences. We have started to investigate the possibility of quantifying more accurately the Saturnian dissipation from its influence on the satellite dynamics. The key point of our method stands in the introduction of the tidal dissipation directly during the fitting procedure to the astrometrical observations, (jointly with the initial orbital elements of each satellite). An extensive set of observations covering the years [1886-2007] was considered. Preliminary results are discussed.
P31A-0191
Saturn periodicities as revealed by energetic neutral atoms
The ion and neutral camera (INCA) is part of the MIMI instrument on the Cassini spacecraft. It images high-speed neutrals that are created when an energetic ion obtains an electron from a neutral. In this way, energetic neutral atom (ENA) images are diagnostic of neutral distributions and their dynamics. Previously, we have explained ENA periodicities in terms of two components: a symmetric background distribution, and an asymmetric corotating component. This latter is likely an injection of ions rotating through the magnetosphere emitting ENAs as it interacts with the neutral gas. We now extend our understanding of ENA periodicities by examining multiple case studies involving various projections of the ENA images. Periodicities can be quantified and compared directly to SKR periodicities. A simple model using the standard neutral background and injection is used to investigate the relationship between ENA and SKR periods. We also look into the perturbation to the system due to the asymmetric ring current (corotating ion distribution) that dominates our images.
P31A-0192
Saturn's Drifting Period: Evidence From Analysis of Magnetic Perturbations.
It is well known that the SKR period has changed over time, increasing by ~0.5 percent per year during the Cassini epoch. This impacts many aspects of magnetospheric science but also poses a problem both for the elucidation of the internal field of Saturn and the rotation rate of the interior. Long term monitoring of the continuous radio signals has allowed derivation of a formula governing the evolution of the phase and thereby the change of period during the Cassini epoch up to August 2006 (Kurth et al., 2007). The radio data have the virtue of being nearly continuous but are not well suited for determining an instantaneous frequency and phase. We show evidence from the magnetometer data measured in situ by the Cassini spacecraft while the spacecraft is within the cam region (L shells less than 11 - 17) of fairly stable phase in the oscillations and thereby refine the estimates of the time varying period. We propose a mechanism whereby the magnetometer signal and the radio signal are synchronized based on the proposed phase model.
P31A-0193
Solar Wind modulation of Saturn's radio clock
The internal rotation rates of the giant planets can be estimated by cloud motions, but that is not very precise because absolute wind speeds are not known a priori and depend on latitude. Periodicities in the radio emissions, thought to be tied to the internal planetary magnetic field, are instead used. At Saturn, in spite of an apparently axisymmetric magnetic field, kilometer-wavelength emission arising from auroral sources is modulated at a period initially identified as 10h 39m 24s ±7s, and this has been adopted as Saturn's rotation period. Subsequent observations, however, revealed that this period varies by ±6 min on a timescale of several months to years. Here we report measurements of the periodicity of the kilometric radiation that reveal that the period varies systematically by ±1 percent with a characteristic timescale of 20-30 days. We demonstrate that these fluctuations are correlated with those of the solar wind speed at Saturn. This provides evidence that Saturn's radio clock is controlled, at least in part, by conditions external to the planet's magnetosphere. No correlation is found with the solar wind density, dynamic pressure or magnetic field, therefore the solar wind speed plays a special role. We also demonstrate that the long term fluctuations are simply an average of the short term ones, and therefore the long term variations are probably also driven by changes in the solar wind.
P31A-0194
Periodic crossings of Saturn's magnetospheric current/plasma sheet observed by Cassini CAPS/ELS and MAG
Cassini observations in Saturn's magnetotail during 2006 show evidence of periodic encounters with the plasma and current sheet. In this talk we discuss these encounters as observed by the Cassini magnetometer (MAG) and electron spectrometer (ELS). We emphasise that the tail observations can simply be interpreted in terms of periodic vertical oscillations of the sheet, obviating the need for a sub-corotating plume structure as advocated by some authors. Furthermore, many encounters with the sheet are "double-peaked" which can also be simply explained by the vertical oscillation paradigm. We explore these vertical oscillations, consider simple structural models to fit the observations, and compare our results with data from other instruments (e.g., Carbary et al. 2007).
P31A-0195
Updated Model of Saturn's Magnetic Field Based on All Available Data
Models of Saturn's magnetic field based on the initial orbits of the Cassini spacecraft [Dougherty et al, Science, 2005] revealed an axisymmetric magnetic field very much like that observed by the Pioneer and Voyager spacecraft decades earlier. Later a reference model based on the first fifteen orbits was derived that allowed for non-axial terms. These terms were small, but nonzero [Giamperi et al, 2006]. A distinct periodicity in the magnetic field at or close to the presumed rotation period of the planet was identified in the data as well, a signature inconsistent with an axisymmetric planetary field and inconsistent with a tilted dipole, the simplest model that would produce a periodic field. Since orbit insertion in July, 2004 Cassini has executed more than forty-five orbits in Saturn's magnetosphere and the magnetometer onboard has provided nearly continuous measurements during this time. With periapses at a variety of radial distances, latitudes, and planetary longitudes the global spatial coverage is now sufficient to reassess the planetary magnetic field. We use all available data obtained during the first three years of the Cassini prime mission and standard inversion techniques to derive an updated model of Saturn's planetary magnetic field.