P11C-1283
Saturn's South Polar Vortex Compared to Other Large Vortices in the Solar System
Observations made by the Imaging Science Subsystem (ISS), Visible and In- frared Mapping Spectrometer (VIMS) and the long-wavelength Composite Infrared Spectrometer (CIRS) aboard the Cassini spacecraft reveal that the large, long-lived cyclonic vortex at Saturn's south pole has a 4200-km-diameter cloud-free nearly circular region. This region has a 4 K warm core extending from the troposphere into the stratosphere, concentric cloud walls extending 20-70 km above the internal clouds, and numerous external clouds whose an- ticyclonic vorticity suggests a convective origin. The rotation speeds of the vortex reach 150-190 m/s, and may strengthen with depth. The Saturn polar vortex has features in common with terrestrial hurricanes and with the Venus polar vortex. Neptune and other giant planets may also have strong polar vortices.
P11C-1284
The Suppression of the Rhines Effect and the Location of Vortices on Saturn
Saturn, like other gas giant planets, has an atmosphere containing numerous vortices which tend to favor formation at latitudes where the zonal jet profile is barotropiclly unstable. Vortices which do form are often destroyed as they interact with Rossby waves in a process called the "Rhines effect" which acts to create alternating zonal flows. Interestingly, however, observations suggest that at some latitudes, vortices are more likely to persist, suggesting that at certain latitudes, Rossby waves may be prevented from interacting with vortices such that the Rhines effect is suppressed. Following the method applied to Jupiter as outlined by Theiss (2006), the two-dimensional Rhines scale is generalized to include depth dependent flow with a finite deformation radius in an effort to compare locations of known vortices with latitudes where the Rhines effect may be suppressed on Saturn. Several data sets of Saturn's zonal flow, including a recent profile as observed by Vasavada et al. (2006), are analyzed to determine at which latitudes the Rhines effect may be suppressed. These critical latitudes are then compared with locations where vortices are known to exist. A good correlation is found between the location of vortices and where the Rhines effect is shown to be suppressed on Saturn, suggesting that the criterion established by Theiss (2006) is not restricted to Jupiter.
P11C-1285
Saturn's North Polar Hexagon Numerical Modeling Results
In 1980, Voyager images revealed the presence of a circumpolar wave at 78 degrees planetographic latitude in the northern hemisphere of Saturn. It was notable for having a dominant planetary wavenumber-six zonal mode, and for being stationary with respect to Saturn's Kilometric Radiation rotation rate measured by Voyager. The center of this hexagonal feature was coincident with the center of a sharp eastward jet with a peak speed of 100 ms-1 and it had a meridional width of about 4 degrees. This hexagonal feature was confirmed in 1991 through ground-based observations, and it was observed again in 2006 with the Cassini VIMS instrument. The latest observations highlight the longevity of the hexagon and suggest that it extends at least several bars deep into the atmosphere. We use the Explicit Planetary Isentropic Code (EPIC) to perform high-resolution numerical simulations of this unique feature. We show that a wavenumber six instability mode arises naturally from initially barotropic jets when seeded with weak random turbulence. We also discuss the properties of the wave activity on the background vertical stability, zonal wind, planetary rotation rate and adjacent vortices. Computational resources were provided by the New Mexico Computing Applications Center and New Mexico Institute of Mining and Technology and the Comparative Planetology Laboratory at the University of Louisville.
P11C-1286
New Measurements of the Evolving Flow in Jupiter's Oval BA
We investigate the dynamics of Jupiter's Oval BA (Little Red Spot), a new anticyclonic vortex in Jupiter's southern hemisphere that is the result of successive mergers of the three White Oval anticyclones over a period of three years in the late 20th century. Oval BA has gained attention since late 2005 because it has attained a reddish hue, mimicking the Great Red Spot. We have processed high-resolution observations of Oval BA from Cassini and New Horizons flybys. We apply our automated cloud feature tracking technique to create a detailed wind flow field map of the vortex before and after the color change. We also use the flow fields to create maps of other dynamical quantities, such as vorticity. We discuss the implications of the evolving dynamics of Oval BA on the mechanisms proposed for the vortex's new appearance. We have also used the images of Oval BA to assess the dynamics of cyclonic regions surrounding the vortex. We note the possible appearance of active convection and thunderstorms in these regions, and examine the connection between the typical flow found in cyclones and their visible appearance. We discuss the implications of our measurements of these often overlooked and active regions of Jupiter's atmosphere and their possible connection with Oval BA.
P11C-1287
Thunderstorm-Related Clouds of Saturn Observed by Cassini/VIMS
We present the first visible-to-near-infrared analysis of two new distinct types of bright and dark clouds in Saturn's thunderstorm region near 35 degrees S. latitude (planetocentric) between 290 and 330 degrees W. longitude as observed by the Visual-Infrared Mapping Spectrometer (VIMS) onboard the Cassini-Huygens orbiter on February 9, 2008. Specifically, we have analyzed the 0.35 - 3.1-micron spectra of these clouds to constrain their composition and structure. Our initial analysis indicates that the spectroscopic character of these spectrally bright clouds are consistent with the presence of ammonia ice absorption, supporting the possibility of ammonia ice particles in Saturnian clouds suggested previously by high-resolution spectroscopic ground-based observations (Kim et al., 2006, Icarus 185, 476-486). Specifically, these clouds are depleted in 2.73-micron reflectivity, similar to Jupiter's spectroscopically- identifiable clouds (Baines et al, 2002, Icarus 159, 74-94). The much darker clouds located downstream of the lightning site have unusually low albedo at all near-continuum wavelengths. The location of the dark clouds downstream in the zonal winds indicates that these are older features, suggesting that they are delayed convective upwellings of thermo-chemically generated material from deeper depths We present the characteristics as a function of wavelength on these both types of thunderstorm related clouds - single scattering albedos, cloud top altitudes, mean particle sizes, refractive indices, and constraints on the composition of both cloud types. We also present the effective pressure (depth) of sunlight penetration into the atmosphere of Saturn as a function of wavelength from 0.3 microns to 4.5 microns, calculated with the recent low-temperature methane absorption coefficients of Irwin et al (Icarus 181, 309-319, 2006). These show that most of the hazes and clouds of Saturn probed by reflected sunlight in the visible and near-infrared are limited to above the 1-bar level.
P11C-1288
A new Model of Saturn's Magnetic Field
Using data from the Cassini spacecraft, we have created a new model of Saturn's magnetic field model that includes non-axisymmetric components. Previous solutions have not unambiguously resolved such components, instead being limited to (spin) axisymmetric components. We analyse the model using resolution analysis, error analysis and spectral analysis. We also examine Slepian basis functions in addition to the canonical spherical harmonics basis functions in an effort to negate the effects of the mostly equatorial coverage by the Cassini orbiter. Such poor coverage can lead to errors when using spherical harmonics as global support is required to achieve orthogonality over the whole sphere whereas Slepian functions, which are orthogonal over both the whole sphere and the region of data coverage, and which have their energy optimally concentrated in the spatial and spectral domain, may be able to obtain much better results. Our model was derived using a regularized weighted linear least-squares estimation of model coefficients in which we estimated internal and external coefficients simultaneously as opposed to assuming and subtracting out an external field of specific shape and size. While small, we investigate whether the non-axisymmetric signature is sufficient to use in an autocorrelation timeseries analysis of the magnetic field measurements to infer a rotation rate directly from the internally generated magnetic field.
P11C-1289
Study of the drift energies to localise the ring current: a comparison between the terrestrial and the kronian magnetosphere.
In the Earth magnetosphere, a ring current can be observed in the dipolar region, between 2 and 9 RE. In the inner magnetosphere, the low energy particles (ions and electrons) corotate. Due to the gradient and the curvature of the field lines, the dipolar magnetic field influences the movement of the particles. Gradient and curvature are responsible of the existence of drift velocities perpendicular to the magnetic field lines. Above limit energy, the total drift velocity will be higher than the corotation velocity. Due to the orientation of the terrestrial magnetic field, the ions drift velocity is directed in an opposite direction than the corotation direction, and the electron drift velocity is directed in the same direction than the corotation direction. This explain the existence of the ring current. The same phenomenon can exist in the kronian magnetosphere. To localise this ring current it is necessary to determine the limit energy in the different part of the magnetosphere. In this paper, we will map the limit energy in the kronian magnetosphere and study the contribution of the different drift movements to this energy. Looking at the similarities and the differences between the Earth and the kronian magnetosphere, we'll present a comparative study. This work will give important information to study the kronian ring current. It also will be a start of a ring current model for the kronian magnetosphere.
P11C-1290
Future observations in the IR of Jupiter Auroras during the Juno mission
The Juno mission to Jupiter has been planned for reaching Jupiter in October 2016. Juno's orbits have been designed for being polar. This will be a great chance for the Italian Jupiter InfraRed Auroral Mapper (JIRAM) to observe and map Jupiter auroras on the both north and south pole. JIRAM is an image spectrometer sensitive in the IR range 2-5 micron and it is provided with two focal planes: one for taking images on the L- band where the aurora IR light is emitted and the second one for making spectra with a resolution of 9 nm of part of the image. JIRAM will operate in IR wavelength and will be able to observe the H3+ emission at 3.4- 3.6 micron on the dark underlying atmosphere due to the strong absorption of methane at those wavelengths. Other mission has observed Jupiter auroras but never at such small distance and with this spectral resolution. NIMS on Galileo was the first imaging spectrometer able to observe auroras, but its spectral resolution was about three times less than JIRAM and the instrument was not able to get a large number of quality observations. Another imager spectrometer which observed Jupiter was VIMS on Cassini during its Jupiter fly-by when the spacecraft was flying at a distance of about 1 million of kilometers from the planet for a short observation at very low spatial and lower spectral resolutions. In the future, Juno will permit JIRAM to observe auroras from an unique point of view thanks to its polar orbiting and for flying so close to the planet. Juno will fly over the poles at an altitude of 1 planet radius. IR observations from JIRAM will be complemented by another instrument (UVS) observing the ultraviolet emission of the auroras.
P11C-1291 [WITHDRAWN]
Structural and Symmetric Properties of Oxygen at 350 GPa
We have performed density functional electronic structure calculations for oxygen at 350 GPa, using the quantum espresso package with ultrasoft pseudopotentials and PBE exchange and correlation functional. The random search method was used to determine the enthalpies and lattice parameters of monatomic oxygen at 350 GPa. Other chalcogen elements (Po, Te, Se, S) all take a rhombohedral β-Po structure when compressed. Polonium forms its structure at zero pressure, Te at 11 GPa, Se at 60 GPa, and Sulfur at 153 GPa. As you move up the table, higher pressures are needed to make the element take a β-Po form. My hypothesis was that at pressures near 350 GPa, oxygen would have a β-Po structure as well. Within random search, several starting configurations are randomly chosen and relaxed, until the final structures, with the lowest enthalpy, are found several times. First, the crystal lattice is generated by randomly selecting cell-vector lengths between 0.5 and 1.5 (in arbitrary units) and three cell angles between 40° and 140°. The cell vectors are then scaled to match a new volume, which is also chosen randomly between 0.5 and 1.5 of some physically sensible volume. Then, atomic positions are obtained by generating three random numbers between 0 and 1 for each atom, which represents the positions of the atoms in terms of the crystal vectors. First-principles methods are then used to relax the cell towards the closest minimum in enthalpy. We performed random search using one oxygen atom per unit cell. Therefore, only the six cell degrees of freedom were necessary. 150 random starting systems were selected and then converged towards equilibrium at 350 GPa. 1000 iterations or more were necessary for each structure to reach hydrostaticity of the stress tensor. The systems' enthalpies were then calculated and the structures with the lowest enthalpies were analyzed. Eight different values for enthalpy local minima were obtained. The global minimum was obtained with 14 out of 150 samples. It had an enthalpy that was 50 meV lower than that of the closest local minima and had a C2/m space group. The results show that oxygen forms a monoclinic arrangement of 1-D chains at 350 GPa and zero temperature. For the global minima, the spacings between oxygens along the chains is 1.44 Å, while the distance from one chain to the closest other is 1.98 Å. Keywords: DFT, high pressure, oxygen, structural search
P11C-1292
Optical Constants of the Ammonium Ion: A Potential Ice Product in the Outer Solar System
Optical constants, the real and imaginary indices of refraction, are the fundamental values used in a wide
variety of radiative transfer models that are relevant to many environments including circumstellar disks,
planetary atmospheres, and planetary surfaces. Cook et al. (2007) observed a feature near 2.2 micrometers
(um) in the reflectance spectrum of Pluto's moon, Charon. They suggested evidence for both ammonia di-
hydrate and ammonia hemi-hydrate on opposite hemispheres of Charon. Moore et al. (2003) discuss the
irradiation of N2-CH4-CO ices; mixtures relevant for Pluto, Triton, and a plausible constituent for some Kuiper
Belt Objects and Centaurs. One stable product observed after irradiation of the ice mixture is the ammonium
ion, NH4+. NH4+ has strong spectral features in the 3-4 um region and weaker overtones near 2.21 and
2.24 um (Moore et al. 2003). The 2.21 um feature is the strongest of the overtone bands and is near the
wavelengths observed for Charon. Using the measurements of Moore et al. (2003) the optical constants of
NH4+ are estimated and will be available to investigate NH4+ as an alternative candidate for the feature
observed on Charon.
Moore, M. et al. 2003, Earth Moon Planets, 92, 291-306.
Cook, J. et al. 2007, Ap J, 663, 1406-1419.