P11B-1259
Saturn's Magnetic Field Periodicity: Spectral Sidebands, their Origin and Significance
Magnetic field measurements during the first 15 Cassini orbits inside 10 Saturn radii yield spectra containing a central peak, Saturn's periodicity, and sidebands with difference frequencies corresponding to the orbital period. The sidebands are interpreted as a phase modulation of the periodicity caused by the periodic changes in longitude and distance from Saturn. The sideband amplitudes are used to determine a single parameter characterizing the modulation. This interpretation is consistent with outward propagation of a wave from an inner source rotating differentially with respect to the magnetosphere as proposed in the 'Cam' model. The modulation parameter coincides with a wave speed of ≈30 km/sec. This speed agrees with that derived by Cowley et al. (2006) who used the Cam model to reproduce the observed phase shift during a single orbit. Future studies of the periodicity and the wave properties should benefit from analyses based on this phase modulation.
P11B-1260
Periodic Tilting of Saturn's Plasma Sheet
From the vantage of the dawn sector, the INCA instrument on Cassini imaged neutral hydrogen atoms (20-50 keV) emitted from the center of the Saturn's plasma sheet at a time resolution of one hour for five days during late 2004. Points along the center of the plasma sheet were found from contoured images projected onto the noon-midnight plane; points within 20 RS of Saturn were fitted to straight lines, and the slopes of these lines were examined as a function of time. In the Sun-Saturn-orbit frame, these slopes vary between 17 deg and 25 deg with a well-defined period of 10.80 hours, the same period as that of Saturn kilometric radiation (SKR). This periodic tilting of the plasma sheet is in phase with SKR radiation in the sense that the maximum tilt angle occurs when the maximum in the SKR variation occurs. When fitted to a cosine in SLS3 longitude, the tilt angle periodicity has a phase angle of 47 deg. The periodic tilting of the plasma sheet agrees qualitatively with predictions of the "asymmetric-lift" model of Saturn's magnetosphere and offers direct evidence of a mechanism exciting waves that travel down the magnetotail.
P11B-1261
Jupiter's Rotation Period Derived From its Winds
Anderson and Schubert (2007, Science, 317, 1384) proposed that Saturn's rotation period might be ascertained by minimizing the dynamic heights of the 100 mbar isosurface with respect to the geoid; they derived a rotation period of 10h 32m 35s. We investigate the same approach for Jupiter to see if the Jovian rotation period is predicted by minimizing the dynamical heights of its isobaric (1 bar pressure level) surface. The shape of the Jovian pressure isosurface is derived from zonal wind data (García-Melendo and Sanchez-Lavega, 2001, Icarus, 316) and the Jovian geoid. Then, by regarding the Jovian geoid as undetermined in terms of Jupiter's rotation rate, we vary the rotation rate and the geoid and search for the geoid that minimizes the dynamical heights of the pressure isosurface in an rms sense. A rotation period of 9h 54m 29.7s is found to minimize the dynamical heights of the pressure isosurface. This rotation period is only one minute shorter than the measured period of Jupiter. The successful application of the method to Jupiter lends support to its relevance for Saturn. Application of the approach to Neptune and Uranus will also be discussed.
P11B-1262
Clues to the Origin of Saturn Kilometric Radiation: Why Morning? Why Responsive to the Solar Wind?
Explanations of the periodic modulation of Io-associated decametric radiation from Jupiter invoke the rotating tilted dipole moment of Jupiter as a clock. Saturn, with an axially symmetric dipole moment, lacks such a clock, yet the corresponding kilometric radiation (SKR) is also periodically modulated (with a slowly drifting period). It remains unclear what controls the varying intensity of the emissions although it is widely accepted that the SKR power is generated in regions of intense field-aligned currents principally in the morning sector. Here we propose a model based on examination of the magnetospheric magnetic field that can account for the observed properties of SKR. Large scale perturbations of the azimuthal component of the magnetic field, corresponding to upward field- aligned currents, have been detected at invariant latitudes above 70 degrees on the high inclination orbits of the Cassini spacecraft in late 2006. The currents peak in intensity in the outer morning-side magnetosphere in a region where flux tubes extended in the antisolar direction drape over lower latitude flux tubes that remain roughly in meridian planes. The current strength is modulated by the rotating cam magnetic field, the periodic field structure observed deeper inside the magnetosphere, with intensification occurring when the peak upward cam currents rotate into the morning sector. Noting a strong morning-afternoon asymmetry of the magnetic structure of the outer magnetosphere, we argue that the spatial localization of the most intense SKR emissions can be understood if the power intensifies where plasma rotates into the morning sector, a region where the distance to the magnetopause diminishes steadily and the rotating cam structure encounters a strong field shear. The field shear to which we appeal is imposed by interaction with the solar wind, whose link to SKR properties has been noted in the past. Although there is evidence that SKR sources are found in both northern and southern regions at magnetically conjugate locations, it is not clear whether the northern hemisphere source is as strong as that in the south. Depending on whether the dominant emissions occur in one or two hemispheres, we require slightly different but closely related scenarios for ways in which the rotating cam structure produces peak SKR power.
P11B-1263
Modeling of a Giant Planet Magnetodisc
We present preliminary calculations of magnetic flux functions associated with the perturbation of a planetary dipole field by a rotating, equatorially-situated disk of plasma. Such structures are central to the dynamics of the rapidly rotating magnetospheres of Jupiter and Saturn. They are 'fed' internally by sources of plasma from moons such as Io (Jupiter) and Enceladus (Saturn). For these preliminary models, we use a scaled form of Caudal's Euler potentials for the Jovian magnetodisc [G. Caudal, J. Geophys. Res., 1986]. In this formalism, the field is assumed to be azimuthally symmetric about the planet's axis of rotation, and plasma temperature is constant along a field line. We perturb the dipole potential ('homogeneous' solution) by using simplified, corotating, planar disc structures with uniform plasma beta parameter. Our results quantify the degree of radial 'stretching' exerted on the dipolar field lines through the plasma's rotational motion and pressure. We comment on the degree of equatorial confinement as represented by the scale height associated with disk ions of varying mass and temperature. We also make preliminary comparisons with Cassini magnetic field measurements at Saturn.
P11B-1264
On the Bimodal Distribution of the Jovian and Kronian Magnetopause and Bow Shock Locations
A comprehensive statistical analysis of observed Jovian magnetopause and bow shock crossings revealed a bimodal distribution in the standoff distances of these boundaries [Joy et al., JGR, 2002], which was attributed to bimodality in the solar wind dynamic pressure. Recently a similar bimodality was identified also in the location of the Kronian magnetopause based on Cassini observations [Achilleos et al., Saturn Book Symposium, 2008]. It is still controversial whether this bimodality at Saturn is a result of a corresponding bimodal distribution in the solar wind or comes from internal magnetospheric processes characteristic to rotationally driven magnetospheres with significant internal mass loading rates. We investigate this question in two different ways. First, we analyze propagated upstream solar wind data at Jupiter and Saturn to test the presence of bimodality in the solar wind driving. For this purpose we use 13 years of upstream data obtained by the Michigan Solar Wind Model (MSWiM) that is a 1.5-D MHD model to propagate solar wind conditions from 1 AU to the outer planets. Second, we use our 3-D global MHD model of Saturn's magnetosphere to test whether the model can produce a bimodal distribution of standoff distances under conditions of low solar wind dynamic pressure and relatively high mass loading rate. Our model includes two magnetospheric plasma sources, a major disk-like source from Enceladus and the rings and a secondary toroidal plasma source from Titan. We fit a 9-parameter analytical magnetopause model and a 4-parameter bow shock model for Saturn to the simulated MHD boundaries at different times of the simulation to obtain a simulated distribution of the standoff distances. We expect that the internal magnetospheric dynamics is more likely to cause the observed bimodal distribution in the Kronian magnetopause locations rather than a bimodal distribution in the upstream solar wind.
P11B-1265
Modeling the Size and Shape of Saturn's Magnetopause Using Dynamic Pressure Balance
The shape and location of a planetary magnetopause can be determined by balancing the solar wind dynamic pressure with the magnetic and thermal pressures found inside the magnetopause. Previous studies on the kronian magnetopause have argued that the boundary is Earth-like (Slavin et al. 1985) and Jupiter- like (Arridge et al. 2006) in terms of its dynamics. In this poster we hope to find a solution by presenting a new pressure-dependent model of the magnetopause. We build upon previous findings by including estimated values for the solar wind thermal pressure and including low energy particle pressures from the Cassini plasma spectrometer (CAPS) and high energy particle pressures from the Cassini magnetospheric imaging instrument (MIMI). The results are compared to previous models to see whether the size and the shape of the boundary vary with these additional parameters. Directions for future studies are also outlined.
P11B-1266
Transient Ionization of Shock Compressed Water Near Planetary Isentropes
We report herein first principles simulations of water under shock loading near the isentropes of Neptune and Uranus. Accurate description of the chemical mechanism for the ionic conductivity at high pressures and temperatures is of particular importance to models of the planetary dynamo mechanism in these planets. Using a novel simulation technique for shock compression, we are able to make excellent comparison to the experimental results for the Hugoniot pressure, temperature and density final states. Our simulations resolve controversy by showing that a unimolecular mechanism for electric conduction dominates at high pressures along the shock Hugoniot. Near the approximate intersection of the Hugoniot and the planetary isentrope we observe high concentrations of negatively charged species that contribute electronic states near the band gap. Our results provide a microscopic picture of the chemistry at planetary depths of ca. 6000 km and greater. * This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
P11B-1267
Discovery of a transient radiation belt at Saturn
Radiation belts have been detected in situ at five planets. Only at Earth however has any variability in their intensity been heretofore observed, in indirect response to solar eruptions and high altitude nuclear explosions. The Cassini spacecraft's MIMI/LEMMS instrument has now detected systematic radiation belt variability elsewhere. We report three sudden increases in energetic ion intensity around Saturn, in the vicinity of the moons Dione and Tethys, each lasting for several weeks, in response to interplanetary events caused by solar eruptions in the year 2005. However, the intensifications, which could create temporary satellite atmospheres at the aforementioned moons, were sharply restricted outside the orbit of Tethys. Unlike Earth, Saturn has almost unchanging inner ion radiation belts: due to Saturn's near-symmetrical magnetic field, Tethys and Dione inhibit inward radial transport of energetic ions, shielding the planet's main, inner radiation belt from solar wind influences.
P11B-1268
Origin of Electron Plasma Populations Transport in Saturn's Magnetosphere
Because the dynamics of Saturn's magnetosphere fall between the large scale and rotationally driven dynamics of Jupiter's magnetosphere and the solar wind driven dynamics of the Earth's magnetosphere, the origins of the main processes governing the radial transport of charged particles in the Kronian magnetosphere are difficult to identify. As for other magnetospheric systems, we have used the Dll = Do.Ln parametric form in our diffusion theory model for simulating the diffusive motion of trapped particles within Saturn's system. Our first computations of plasma distributions suggest that the low-energy electrons (cold plasma) need to be numerically transported outward to improve the comparisons with the CAPS-ELS data. Although Dll is a term of diffusion in the transport equation, the quantity of particles drifting away from the planet is not sufficient to be considered as important. The primary result of the radial diffusion term has been to transport the particles towards the planet. To match plasma observations inside 12 Rs better, we are re-examining the expression of the inward radial transport coefficient. The incorporation of outward transport into our diffusion theory model is also carried out. In this paper, we discuss the effects of Coriolis and Centrifugal forces on the transport of plasma. Both forces are expected to make trapped particles drift away from the planet by changing the characteristics of their pitch-angle and energy. Our simulation results will provide the opportunity to discuss the origin of outward plasma flows, as well as how they balance the inward transport. This work will further explain the distribution and circulation of electron plasma populations in Saturn's inner magnetosphere.
P11B-1269
Global Neutral gas Distribution at Saturn and Jupiter Derived From ENA Images
Neutral gas distributions around giant planets are important indicators of the source, loss and transport
processes that redistribute material from the planet, its moons and rings, through interaction with the ambient
plasma of the magnetosphere. Energetic Neutral Atoms (ENAs) are produced by charge exchange between
energetic ions and neutral gas and can be imaged by the INCA camera on board Cassini providing a marker
for plasma-neutral processes. As Cassini flew by Jupiter Cassini-INCA data was used to reveal a trans-
Europa gas torus [Mauk et al., Nature, 2003]. We demonstrate a technique to retrieve the global neutral gas
distribution in Saturn's magnetosphere using ENA images obtained from the INCA imager on board Cassini.
The neutral gas distribution at Saturn is retrieved by simulating INCA images using ion distributions of
combined CHEMS, LEMMS and INCA in-situ ion measurements that cover several passes from the SOI
(183/2004) to day 100/2007, at various local times over the dipole L range 5 P11B-1270 Models of Jupiter's Growth Incorporating Thermal and Hydrodynamics Constraints
We have modeled the growth of Jupiter incorporating both thermal and hydrodynamical constraints on its
accretion of gas from the circumsolar disk. We have used a planetary formation code, based on a Henyey-
type stellar evolution code, to compute the planet's internal structure and a three-dimensional hydrodynamics
code to calculate the planet's interactions with the protoplanetary disk. Our principal results are: (1) Three
dimensional hydrodynamics calculations show that the flow of gas in the circumsolar disk limits the region
occupied by the planet's tenuous gaseous envelope to within about 0.25 Rh (Hill sphere radii) of the
planet's center, which is much smaller than the value of ~ 1 Rh that was assumed in previous
studies. (2) This smaller size of the planet's envelope increases the planet's accretion time, but only by 5--
10%. In general, in agreement with previous results of Hubickyj et al. [Hubickyj, O., Bodenheimer, P.,
Lissauer, J.J., 2005. Icarus, 179, 415-431], Jupiter formation times are in the range 2.5--3 Myr,
assuming a protoplanetary disk with solid surface density of 10 g/cm2 and dust opacity in the
protoplanet's envelope equal to 2% that of interstellar material. Thermal pressure limits the rate at which a
planet less than a few dozen times as massive as Earth can accumulate gas from the protoplanetary disk,
whereas hydrodynamics regulates the growth rate for more massive planets. (3) In a protoplanetary disk
whose alpha-viscosity parameter is ~ 0.004, giant planets will grow to several times the mass of Jupiter
unless the disk has a small local surface density when the planet begins to accrete gas hydrodynamically, or
the disk is dispersed very soon thereafter. The large number of planets known with masses near Jupiter's
compared with the smaller number of substantially more massive planets is more naturally explained by
planetary growth within circumstellar disks whose alpha-viscosity parameter is ~ 0.0004.
(4) Capture of Jupiter's irregular satellites within the planet's diffuse and extended thermally-supported
envelope is not consistent with the Jupiter formation models presented in this study.
P11B-1271 Latitudinal Variations In Vertical Cloud Structure Of Jupiter As Determined By Ground- based Observation With Multispectral Imaging
We conducted ground-based observation of Jupiter with the liquid crystal tunable filter (LCTF) and EM-CCD
camera in two methane absorption bands (700-757nm, 872-950nm at 3 nm step: total of 47 wavelengths) to
derive detailed Jupiter's vertical cloud structure. The 2-meter reflector telescope at Nishi-Harima astronomical
observatory in Japan was used for our observation on 26-30 May, 2008. After a series of image processing
(composition of high quality images in each wavelength and geometry calibration), we converted observed
intensity to absolute reflectivity at each pixel using standard star. As a result, we acquired Jupiter's data
cubes with high-spatial resolution (about 1") and narrow band imaging (typically 7nm) in each methane
absorption band by superimposing 30 Jupiter's images obtained in short exposure time (50 ms per one
image). These data sets enable us to probe different altitudes of Jupiter from 100 mbar down to 1bar level
with higher vertical resolution than using convectional interference filters. To interpret observed center-limb
profiles, we developed radiative transfer code based on layer adding doubling algorithm to treat multiple
scattering of solar light theoretically and extracted information on aerosol altitudes and optical properties
using two-cloud model. First, we fit 5 different profiles simultaneously in continuum data (745-757 nm) to
retrieve information on optical thickness of haze and single scattering albedo of cloud. Second, we fit 15
different profiles around 727nm methane absorption band and 13 different profiles around 890 nm methane
absorption band to retrieve information on the aerosol altitude location and optical thickness of cloud. In this
presentation, we present the results of these modeling simulations and discuss the latitudinal variations of
Jupiter's vertical cloud structure.
P11B-1272 Polar Phenomena in Outer Planet Atmospheres
Infrared observations of the polar regions of the outer planets have revealed similarities to the Earth's
atmosphere and some new phenomena. The most dominant force which is apparent in time-dependent
studies of the poles is seasonal radiative forcing, which was detected in Saturn's stratosphere as early as
1973. For Saturn, Uranus and Neptune, planets with substantial obliquities, the seasonally dependent
changes are predictable and can be used to constrain abundances of optically active gases and the rate of
restoration by stratospheric circulation. In the case of Neptune, recent evidence shows that the heating is
sufficient to allow a "leak" from the reservoir of methane in the deep atmosphere into the polar stratosphere.
New thermal images of Uranus show that the winter pole of Uranus which has only recently emerged fully from
darkness is colder than when it was in the middle of winter when Voyager 2 visited, confirming the substantial
seasonal phase delay associated with radiative heating and cooling models. Even Jupiter with its 3-degree
obliquity shows clear evidence for seasonal forcing of temperatures in the upper troposphere and
stratosphere. The second most prominent characteristic
of the resolvable polar temperature fields in Jupiter and Saturn is the formation of polar vortices. Jupiter's
polar vortices are cold, similar to those detected in the terrestrial planets; they have sharp equatorward
boundaries which are characterized by Rossby waves which rotate at the speed of the local zonal wind flow
and are coincident with the similarly irregular boundaries of a polar haze, also known as "polar hoods". The
cold vortex at Saturn's northern winter pole is muted, but Saturn also has a unique "warm polar vortex" in the
south (late summer) pole which shows no apparent wave structure. Saturn's warm polar vortex has no
counterpart in the Earth's atmosphere, where summer radiative warming simply dissipates the cold winter
vortex. Saturn also possesses dynamically driven hot regions within 2 degrees of its poles where dynamics is
driving relatively dry air downwards, causing adiabatic warming and clearing the atmosphere; this
phenomenon also has no terrestrial counterpart. Jupiter's upper polar stratosphere is warmed in discrete
local regions by Joule heating from energetic particles cascading into the neutral atmosphere. The northern
auroral-related polar "hot spot" has a very predictable geometry, but an amplitude that is variable over time
scales of months. On the other hand, the stratosphere 25-30 degrees from Neptune's pole shows signs of
ephemeral hot spots which are more likely to related to dynamics. These phenomena provide a rich basis of
constraints for global climate models which must, at least for Jupiter, be coupled with models of auroral
energy transport.
P11B-1273 Photolytical Generation of Carbon Dioxide
Carbon dioxide has been found by Cassini VIMS throughout the Saturnian system in locations such as
Iapetus' equator where the temperature is too high for it to remain as free ice for more than a few hundred
years. We suggest that the 4.26 micron absorption feature found on Iapetus and Hyperion (that has been
attributed to complexed CO2) is the result of either UV photolysis or ion bombardment driving chemistry
between the carbon rich layer and the water ice regolith.
We conducted experiments to simulate the generation of CO2
by UV radiation under conditions similar to those on the
surface of Iapetus. A simulated icy regolith was created in
an argon atmosphere using flash-frozen, degassed water
crushed into sub-millimeter sized particles. Isotopically
labeled amorphous carbon (13C), which was ground into a
fine dust, was mixed into the regolith allowing for
extensive grain contact. This sample was placed in a vacuum
chamber and cooled to temperatures as low at 60K. The
sample was irradiated with UV light, and the products were
measured using both a mass spectrometer to identify free
molecules and an IR spectrometer for molecules that remained
trapped on and in the simulated regolith.
We report on the production and reaction rates of CO2 and
CO, as well as the generation of free hydrogen and oxygen as
detected by a SRS-100 mass spectrometer. We also identify
residual products that either freeze on the surface or
become entrained by or adsorbed onto the ice grains. We
attempt to match the CO2 absorption feature found on
Iapetus with that seen in our simulation, perhaps
identifying a possible source of CO2 in the Saturnian
system. Finally, we estimate the time required for these
reactions to occur on Iapetus to see if UV photolysis would
be effective.
P11B-1274 General circulation of giant planet atmospheres
The atmospheres of the giant planets are driven by differential solar
heating and intrinsic heat fluxes emanating from the deep interior. We
show that if both processes are taken into account in an energetic
consistent manner, the observed large-scale features of the general
circulations of all giant planet atmospheres can be reproduced.
We use energetically consistent general circulation models to simulate
the outer atmospheres of Jupiter, Saturn, Uranus, and Neptune. In the
models, the solar radiative fluxes are deposited in the upper
atmosphere by absorption and scattering, and temporally constant and
spatially homogeneous heat fluxes consistent with the observed
intrinsic heat fluxes are imposed at the bottom boundary. Convection
transports heat from the bottom boundary into the upper atmosphere
when the intrinsic heat fluxes are sufficiently strong to generate
statically unstable conditions. For Jupiter and Saturn, the intrinsic
heat fluxes are strong enough to lead to convection, which generates
Rossby waves in the equatorial upper atmosphere. Momentum transport
associated with these Rossby waves leads to the generation of
equatorial superrotation on Jupiter and Saturn. For Uranus and
Neptune, the intrinsic heat fluxes are not strong enough to lead to
convection penetrating into the upper atmosphere; as a consequence,
the equatorial flow is retrograde.
Differences in the optical properties of the atmospheres and in
planetary parameters such as the gravitational acceleration and
rotation rate can account for the differences in the general
circulations of the giant planets, such as the different jet widths
and strengths.
P11B-1275 Detailed Measurements of Ion Anisotropies by the Cassini INCA Experiment and Calculated Convection Velocities in Saturn's Magnetosphere
The Ion and Neutral Camera (INCA), part of a cluster of instruments on the Cassini spacecraft, measures
intensities of hydrogen and oxygen ions and neutral atoms in the Saturnian magnetosphere. We use the
measured intensity spectrum and anisotropy of hot hydrogen and oxygen ions to calculate the plasma bulk
velocity. We find that, throughout a wide region of this magnetosphere, the bulk plasma is capable of nearly
rigid co-rotation within Titan's orbit, beyond which the bulk flow falls increasingly further behind the rigid rate.
The anisotropies are sometimes characterized well by convecting isotropic populations, but at other times are
not. In some cases time aliasing due to a dynamic or inhomogeneous ion population convecting past the
spacecraft prevents such a calculation. However, we have performed calculations in a wide region of the
magnetosphere, where some ion populations are nearly isotropic in their convection frame. We find that, in
the dawn and dusk regions, convection approaching rigid co-rotation speeds can be maintained out to the
vicinity of the Titan orbit. We have examined equatorial orbits as well as data at low to moderate latitudes in
the dusk side. Thus far, we find no evidence for a dusk side depression similar to that found within Jupiter's
magnetosphere. We have performed calculations at low to moderate latitudes within orbits highly inclined to
Saturn's equatorial plane on the night side and find evidence of convection at substantial fractions of the
rigid co-rotation as determined by the given L-shell equatorial distance, consistent with the pattern found
within equatorial orbits. We find evidence of similar behavior in the day side region.
P11B-1276 Numerical models of the transition from zonal flow to dynamo action in Jupiter and Saturn
The surface winds and magnetic fields of Jupiter and Saturn are broadly comparable. Each planet has a
strong and prograde equatorial jet and weaker, alternating jets at higher latitudes. Furthermore, both planets
exhibit relatively strong, dipolar magnetic fields. Saturn's magnetic field is weaker and more axisymmetric
than that of Jupiter. In addition, Saturn's equatorial jet is broader and stronger than that of Jupiter. We have
performed a set of numerical simulations of rotating convection and dynamo action in spherical shells. The
model fluid is Boussinesq with radially varying electrical conductivity. The electrical conductivity, which is
nearly constant in the deeper parts of the shell, exponentially decreases outward, starting at a chosen radius
parameter. In some of the simulations a strong, mainly dipolar dynamo develops in the deeper region of high
electrical conductivity. In contrast, strong zonal flow with an equatorial jet develops near the low-conductivity,
free slip outer surface, and penetrates to a depth associated with the conductivity profile. The strong zonal
flow is attenuated by Lorentz forces at depth and does not persist in the dynamo region. The relationship
between the structure of equatorial jets and the magnetic fields generated in our models imply that major
differences between the surface zonal flow and magnetic fields of Jupiter and Saturn arise from the different
depths of transition from their low-conductivity molecular envelopes to their liquid metal interiors.
P11B-1277 Convective Heat Transfer and the Pattern of Thermal Emission on the Gas Giants
Jupiter and Saturn emit nearly twice the thermal energy they receive from the Sun. Although insolation
decreases toward the poles, the large-scale outward heat flux is nearly uniform, with smaller-scale latitudinal
undulations that correlate with the zonal jet streams. Here we present numerical models of rapidly rotating,
turbulent three-dimensional convection in geometrically thin, uniformly-forced layers of Boussinesq fluid that
approximate the deep convection zones of Jupiter and Saturn. In previous studies we have demonstrated
that such models generate zonal flows comparable to those observed on the gas giants. By analyzing the
simulated patterns of convective heat transfer, we show that deep convection in the gas giants can explain
the anomalously uniform large-scale thermal emissions as well as the jet-scale variations. In particular, we
find that convective heat transfer by quasi-geostrophic thermal plumes in relatively thin spherical shell
geometries generates an outward heat flow pattern with a broad equatorial minimum and peaks at the poles.
The results suggest an alternative to the longstanding hypothesis that insolation controls the large-scale
patterns of heat flux and zonal flow on the gas giants. Instead, we propose that the large-scale thermal and
zonal flow fields can originate deep within the planets' molecular envelopes.
P11B-1278 Saturn's Thermal Emission at 2-cm Wavelength and Implications for Atmospheric Composition and Dynamics
The thermal emission from Saturn's atmosphere has been mapped using the passive radiometer of the
Cassini RADAR instrument. The radiometer operates at a frequency of 13.78 GHz, or 2.18-cm wavelength,
and uses the spacecraft's main communication antenna to form a beam of 0.36º width at half power. Images
of absolute brightness at normal incidence were constructed using data obtained from raster scans during
close periapsis passes in September, 2005, and February, 2008. In 2005 most of the equatorial region was
mapped within latitudes +/- 60 degrees, while the polar regions were observed in 2008. The sensitivity was
about 0.1 K and the best spatial resolution in the images is about 500 km, providing more than an order of
magnitude improvement in both parameters over all previous microwave observations of Saturn. At 2-cm
wavelength the radiometric weighting function lies at altitudes almost entirely within the ammonia saturation
region, and the brightness temperature is therefore primarily sensitive to the cloud-level ammonia
concentration. A variety of heretofore-unseen structure attributable to cloud-level ammonia variations was
seen and implications for atmospheric circulations will be presented.
P11B-1279 Interior Models of Jupiter and Saturn with Density Discontinuities
Interior models of Jupiter and Saturn, with the density profile represented as a 6th degree polynomial,
provide a good fit to gravitational and atmospheric data (Anderson & Schubert, Science 317, 1384,
2007; Helled et al., submitted to Icarus, 2008). However, the representation of the density profile by a
polynomial function of radius is inadequate to account for a density discontinuity at the surface of a heavy
element core.
We present interior models of Jupiter and Saturn with density profiles accounting for the existence of a core.
The density profile of the planet is represented by a piecewise function,
which includes a constant density region (core), and a polynomial for the planetary envelope. The core
density and radius, and the polynomial coefficients are a priori unknown and are found by iterating until the
gravitational harmonics of the interior models converge to the measured ones.
The density profiles, together with an integration of the hydrostatic equation, provide a pressure-density
relation, referred to as an empirical equation of state (EOS). The empirical EOS makes no assumption about
the planet's composition or how different elements are distributed with depth. It is also independent of any
theoretical models of the behavior of hydrogen, helium, and heavier element mixtures at high temperature
and pressure.
The interior models reveal information on the planets' internal structure and whether interiors with cores are
consistent with the gravitational data. The models also improve our understanding of the effect of a central
core on the measured gravitational moments.
P11B-1280 Convection in Planets With Oblate Geometry
Current simulations of the giant planets assume a spherical geometry, however centrifugal forces on these
rapidly rotating gaseous bodies result in non-spherical geometries. Saturn, for example, is visibly non-
spherical, with an ellipticity, or flattening of 10%, where the flattening is the ratio of the equatorial-polar
length difference to the equatorial length. While the ellipticity of a giant planet is likely a function of the semi-
major axis, as the density and the distortion caused by rotation vary with depth in the planet, these initial
simulations assume a constant ellipticity with semi-major axis for simplicity. Simulations with varying ellipticity
are compared at multiple Rayleigh numbers to see how the convective patterns change as the gravitational
field becomes increasingly non-radial. The number of density scale heights is also varied to ascertain the
influence of density stratification on the convective patterns.
P11B-1281 Visible Phase Curves and Internal Heat Flow for Uranus
Images of Uranus acquired by Voyager 1 during the 1980s were re-analyzed using improved calibrations.
These images provide disk-averaged reflectivities in six bands, at phase angles from 28 to 100 degrees.
These, along with measurements made by the Voyager 2 spacecraft during 1986, are being used to
constrain an atmospheric model of Uranus to generate a new bolometric Bond albedo for the planet in the
1980s epoch. Improved phase curves, atmospheric models, albedos, and a recomputed heat flow will be
presented.
P11B-1282 Uranian Equinoctial Observations
In December of 2007, the planet Uranus passed through its northern hemisphere spring equinox. The
northernmost latitudes of the planet and regular satellites were exposed to sunlight for the first time in 42
years. Unique circumstances during the equinoctial event also concerning the viewing geometries of the rings
and satellites provided rare opportunities to determine the physical nature of these elements of the Uranian
system and to study the short-term and evolutionary effects of seasonal insolation in the outer solar system.
Furthermore, the approaching perspective afforded opportunities to characterize details of surfaces that had
not been viewable, even by Voyager 2, since the advent of modern instrumentation.
We present preliminary results from these observations made over several nights during the 2006, 2007, and
2008 observing semesters. Our imaging using the Palomar adaptive optics system on the observatory's 200-
inch telescope has been used to obtain high-resolution images. These observations have provided
constraints on the planet's atmospheric dynamics and structure by monitoring the increasing storm activity
and changing large-scale features in the atmosphere, such as the shifting polar collar, and sampling the
vertical structure from multiple planetary occultations. These same images also provide unique photometric
information regarding the ring-system particles by viewing the system from its dark side, accessible only
during the 2007 season. Spectral and spectro-photometric observations of the newly exposed surfaces of the
major satellites have also been obtained from the IRTF, the SOAR telescope, and Palomar 200-inch,
including observations of some mutual event phenomena.
Acknowledgements: These results are based in part on observations obtained at the Hale Telescope,
Palomar Observatory, as part of a collaborative agreement between Caltech, JPL and Cornell University.
Some observations were also obtained at the Infrared Telescope Facility, which is operated by the University
of Hawaii, using the SpeX near-IR spectrometer and at the SOAR telescope, Cerro Pachon, Chile using the
OSIRIS camera.