P21B-01 INVITED
Formation of Gas Giant Satellites
The Jovian and Saturnian regular satellites are believed to have accreted within circumplanetary disks of gas and solids. Such disks probably existed during the final stages of gas planet accretion, because gas inflowing from solar orbit that contained too much angular momentum to fall directly onto the planet would have instead flowed into circumplanetary orbit. As a circumplanetary gas disk viscously diffuses, most of its mass is accreted by the planet while most of its angular momentum (contained in a small fraction of its mass) is transferred outward and escapes. The gas planet disks would have been continually replenished by any ongoing inflow of nebular gas, allowing the disks to persist until the nebula itself dissipated. Here I will discuss a model [1,2,3] in which gas giant satellites form within inflow-supplied circumplanetary disks. Small solids carried into the disk by the inflowing gas provide the raw material for accreting satellites. A satellite grows until it reaches a critical mass for which the timescale for its further growth is comparable to the timescale for its orbit to decay via gravitational interactions with the gas disk. Once a satellite reaches this critical mass (comparable to that of Titan and the Galilean satellites), its orbit spirals inward and if inflow to the disk continues, the satellite will be lost to collision with the planet while a new one grows in its place. The current Jovian and Saturnian satellites represent the last surviving generation of satellites that formed as gas inflow to the circumplanetary disks ended. As the rate of inflow wanes, the disk cools, allowing the final satellites to incorporate substantial ices. For reasonable conditions, this model can account for the general structure (i.e., masses, numbers, and orbital spacings of the large satellites) and overall compositional trends in both the Jovian and Saturnian satellite systems. It implies that the current large satellites accreted slowly (in > 5 × 105 yr), within low-density, 'gas-starved' disks. The model also produces satellite systems that resemble that of Uranus, although its applicability to Uranus depends on how the planet acquired its obliquity. [1] Canup, R.M. & W.R. Ward, Astron. J. 124, 3404 (2002); [2] Canup, R.M. & W.R. Ward, Nature 441, 834 (2006). [3] Ward, W.R. & R.M. Canup, submitted (2008). Support from NASA's OPR and PGG programs is gratefully acknowledged.
P21B-02 INVITED
Comparative Formation of the Gas Giant Planets and their Atmospheres
The Galileo entry probe yielded measurements critical to constraining the models of the origin and evolution of Jupiter and its atmosphere. Supersolar abundances (relative to H) were found for the heavy noble gases, Ar, Kr and Xe, and for C, N and S (from CH4, NH3 and H2S), but O/H (from H2O) could not be determined as the probe entered a meteorologically anomalous (five-micron) "hot spot". These findings have led to two competing hypotheses, both requiring cold planetesimals. For Saturn, C/H has been determined, but abundances of the other heavy elements fall beyond the remote sensing capabilities of either Cassini or the Voyager Missions. While the 2011 Juno Mission is expected to measure the deep atmospheric abundance of water, hence O/H, at Jupiter, the missing suite of heavy elements, helium, and critical isotopes of hydrogen, nitrogen, carbon and helium in Saturn require an entry probe. A comparison of such data for the two gas giant planets is essential to fully comprehend the formation of the gas giant planets and their atmospheres. This talk will present the current status.
P21B-03
Origin of the Regular Satellites of Jupiter and Saturn: Mass and Composition Constraints
It has often been noted that the compositional gradient of the Galilean satellites may provide a link to the environment in which they formed (e.g., Estrada et al. 2008). The similarities in the bulk properties of the regular satellites of Jupiter and Saturn favor a unified framework for their origin; yet, the inner, icy satellites of Saturn exhibit no such trend. We set-aside for now the inner satellites, and focus instead on the large, outer regular satellites of each satellite system. We seek to account for the masses and compositions of Ganymede and Callisto in the case of Jupiter, and Titan and Iapetus, for Saturn. For objects the size of Iapetus or larger, the porosity is likely to be small not only because the internal pressure is large enough to close pore spaces, but also because the presence of short-lived radioactive nuclides heats the interior causing ice to flow. For such satellites, densities can be interpreted in terms of rock/ice fractions. Iapetus' low density, and correspondingly low rock/ice fraction, presents a puzzle when compared to the other three satellites, each of which is roughly 50% ice and rock. In turn, the rock/ice fractions for Ganymede, Titan and Callisto are comparable to that of (captured) Saturnian irregular satellite Phoebe. Progress in understanding these observations requires tying the properties of solar nebula planetesimals to subnebula satellitesimals. We argue that planetesimal break-up following giant planet formation, in tandem with delivery via ablation of planetesimal fragments crossing the subdisk can provide a framework for understanding the mass budget and compositions of regular satellites. In particular, ablation can result in fractionation, and account for the observed density of Iapetus provided this satellite formed in situ (Mosqueira and Estrada, 2005). For this to work (solar nebula) planetesimals of size 10 km or larger may need to be at least partially differentiated, which argues that the first generation of planetesimals in the Jupiter-Saturn region (and possibly beyond) incorporated significant quantities of 26Al. Acknowledgements: This work is supported by PG&G and OPR NASA grants.
P21B-04
Deuterium in the Outer Planets: New Constraints and New Questions from Infrared Spectroscopy
We discuss how new observations of far-infrared rotational lines of HD and mid-infrared vibrational features of CH3D are challenging the accepted measurements for the deuterium abundance in the outer solar system. New derivations of D/H will be presented from the Cassini Composite Infrared Spectrometer (CIRS) for Saturn, the Spitzer Infrared Spectrometer (IRS) for Uranus and Neptune and the grism mode of the AKARI Infrared camera (IRC) for Neptune. Many thousands of spatially resolved Cassini/CIRS spectra at an unapodized spectral resolution of 0.25 cm-1 covering a variety of latitudes on Saturn have been acquired during Cassini's prime mission, and are coadded to give ten independent estimates of the HD mole fraction and hundreds of estimates of the CH3D mole fraction. Spitzer/IRS acquired disc-averaged spectra of Uranus during Cycle 1 and more recently with Director Discretionary time in December 2007. Neptune disc-averaged spectra were acquired during Cycle 2 (November 2005). ISAS/JAXA's AKARI satellite recorded disc-integrated spectra of Neptune in May 2007 with a resolving power of 50 in the 5.5-13 micron range. These spectra have been analysed using two separated radiative transfer and retrieval models to check for consistency of results. On Saturn, we retrieve lower estimates of D/H from HD and CH3D than were obtained from ISO/SWS by Lellouch et al. (2001). Preliminary analysis of Uranus spectra suggest that the CH3D/CH4 ratio is significantly smaller than that predicted by the HD abundance determined from ISO/SWS by Feuchtgruber et al. (1999), suggesting a Uranian ratio more like that of Saturn, or a substantially different fractionation factor from that in the current literature. Furthermore, although constraints on CH3D from mid-IR Neptune spectroscopy are weaker, preliminary findings are that the CH3D/CH4 ratio is lower than that obtained by Orton et al. (1992) and inferred from HD measurements from ISO/SWS (Feuchtgruber et al., 1999). Fletcher is supported by an appointment to the NASA Postdoctoral Program at the Caltech/Jet Propulsion Laboratory, administered by Oak Ride Associated Universities through a contract with NASA.
P21B-05 INVITED
Saturn and Jupiter: Surprising Similarities and Stark Differences in Dynamics and Chemistry in the Gas Giants as Revealed by Galileo, Cassini, and New Horizons
Imagery and spectra obtained by a variety of spacecraft over the past decade have revealed much about the atmospheres of the two gas giants. A partial listing of salient phenomena documented by these spacecraft on both planets include aurorae, lightning, the 3-D nature of zonal winds, thunderstorm-related clouds, spectrally-identifiable ammonia clouds, wave features, and long-lived discrete features at mid-latitudes and near the poles. Temporal variability in regional cloud structures are observed – seasonally on Saturn, more episodically on Jupiter. Molecular abundances of disequilibrium gases also vary spatially on both planets. These and other relevant phenomena will be discussed in this talk comparing the dynamics and chemistry of the two gas giants of our solar system.
P21B-06
A General Radiative Seasonal Climate Model Applied to Saturn, Uranus, and Neptune.
With similar compositions, a range of planet-sun distances, different orbital periods, and a variety of axial tilts, the Giant Planets are a unique test set for seasonal climate variation studies. We have created a general radiative seasonal climate model in an attempt to reproduce observed and predict future stratospheric temperatures of the Giant Planets. We present here a description of the radiative heating and cooling algorithms used in calculating the change in temperature with time. We will discuss the methods used to decrease run time, the opacity tables used, and indicate where more detailed opacity information would prove useful. We will use Saturn seasonal models to show the impact variations of the key coolants, acetylene and ethane, and the dominant heaters, methane and aerosols, have on predicted stratospheric temperatures. We will also present the initial results from the application of our model to the atmospheres of Uranus and Neptune. The same planet independent heating/cooling code implemented in our radiative seasonal climate model is being incorporated into the global circulation model EPIC. This work was funded by NASA PATM grants NNX08AE64G and NNX08AL95G.
P21B-07 INVITED
Planetary Magnetosphere Comparisons at the End of the Cassini Prime Mission
Over the last four years, Cassini has comprehensively mapped the low and mid-latitude magnetosphere of Saturn over a wide range of local times. New insights are emerging on the roles of rotation and solar wind in shaping and driving this magnetosphere. New electric current systems and plasma convection systems have been reported and the mystery of rotational signals from its magnetosphere appears to be near conclusion. It is therefore an appropriate time to revisit our knowledge of other magnetospheric systems and compare and contrast Cassini findings with the knowledge gained from other systems. In this presentation we will review the magnetospheric scales and sizes, current sheet shapes and structures, field-aligned current systems, auroral phenomena and plasma convection systems of the outer planet magnetospheres (especially Saturn and Jupiter) to learn the roles of solar wind and internal rotation in shaping these magnetospheres and driving dynamics in them.
P21B-08
Magnetospheric Storms at Saturn and Jupiter and Their Relation to Rotational Periodicities
A magnetospheric storm is a well-known phenomenon in the terrestrial magnetosphere and leads to intensified plasma pressure, electrical currents and magnetic field perturbations. While terrestrial magnetospheric storms are externally driven predominantly by dayside reconnection with the interplanetary magnetic field (IMF), imaging of Saturn's magnetospheric energetic plasma distribution by Cassini/INCA reveals that storms occur during enhanced solar wind dynamic pressure. Galileo measurements in Jupiter's magnetosphere strongly suggest a similar scenario for Jovian magnetospheric storms. We discuss the possible mechanisms behind the stormy behaviors and the consequences for rotational periodicities that appear to become more pronounced during storms. One such periodic phenomenon is the periodic magnetic field perturbations seen in Saturn's magnetosphere. We show that these can be explained by the currents driven by the injected and energized plasma pressure distribution drifting around Saturn. We use a magnetic field model based on the Tsyganenko et al. [2000, 2002] formulation, with a magnetodisc like configuration, seasonal tilt, and a rotating partial ring current (PRC) distribution. The asymmetric PRC pressure is retrieved from Cassini/MIMI and CAPS observations of hot and cold plasma. Our preliminary findings show that the magnetic field perturbations are consistent with the hot plasma pressure distributions observed by the Cassini/MIMI/INCA. However, there appears to be a need for a component of cold plasma, not observed by MIMI, to fully match the magnetic field measurements. With a seasonal tilt of the plasmasheet, the PRC causes perturbations that expand the plasmasheet asymmetrically in the north-south direction, resulting in a periodic oscillation of the plasmasheet tilt southward in winter and northward in summer. This is consistent with the idea discussed by Khurana et al. [2008]. We seek direct evidence in the INCA images for this periodic oscillation.