P13F-01 INVITED
MHD Simulations of the Interaction between Ganymede and Jupiter's Magnetosphere
Ganymede is unique among planetary moons because it has its own magnetic field strong enough to form a magnetosphere within Jupiter's magnetospheric environment. We have performed a series of global MHD simulations to understand the interaction between Ganymede and Jupiter's magnetosphere using several field and particle data sets from Galileo as boundary conditions. Our simulations show that, in addition to the familiar structures such as the magnetopause and equatorial current sheet, Ganymede's magnetosphere extends into an Alfv\acute{e}n wing that mediates the interaction of Ganymede with the plasma and ionosphere of Jupiter. We show that the magnetospheric current system at Ganymede is analogous to the interaction of Io with the Jovian plasma as well as to the interactions between the solar wind and planetary magnetospheres. In the simulation, magnetic reconnection, which is the primary process for plasma and energy transport in the magnetosphere, is found to take place over a large longitude range around the moon. The simulations reproduce quite closely the magnetic field structure measured by the Galileo magnetometer for all six close encounters. However, our simulated magnetosphere is slightly smaller than the observed one. We will discuss several factors that can possibly cause the discrepancies in the current model, such as mass-loading, Ganymede's intrinsic magnetic field and grid resolution in the simulation.
P13F-02
The Global Expansion and Resurfacing of Ganymede
Ganymede's surface is dominated by relatively young, extensional tectonic deformation. While it is generally accepted that this deformation formed during global expansion of the satellite, the cause of the expansion remains unclear. Here we investigate the feasibility of a scenario in which global expansion was caused by extensive melting of Ganymede's ice shell during the Galilean satellites' passage through a Laplace-like resonance. The current Laplace resonance does not pump Ganymede's eccentricity. However, Malhotra (1991) and Showman and Malhotra (1997) showed that the Galilean satellites may have passed through Laplace-like resonances that did force Ganymede's eccentricity, leading to internal heating of the satellite. Showman et al. (1997) explored the effects of such tidal heating on Ganymede and found that it can lead to thermal runaway, melting, and global expansion, helping to explain the extensive resurfacing of the satellite. While the likelihood of thermal runaway appeared small, improved understanding of the internal structure of Ganymede and the nature of stagnant lid convection warrants a new study. Here we present simulations of Ganymede's coupled thermal and orbital evolution. The orbital model allows a dynamical investigation of the orbital histories of the Galilean satellites near the observed 2:1 mean motion resonance. The thermal model simultaneously solves the energy balance in Ganymede's ice shell, silicate mantle, and Fe/FeS core. Stagnant lid convection, radiogenic heating, ocean formation, and inner core growth are included in the model. Additionally, we investigate the effect partial melting has on the thermal evolution of the ice shell. Coupling between the orbital and thermal models occurs via tidal dissipation, which is calculated with the model of Tobie et al. 2005 to determine how tidal heating is distributed throughout the satellite interior. These simulations reveal that passage through the Laplace-like resonance can lead to melting under a much broader range of initial conditions than suggested by Showman et al. (1997). The degree to which melting occurs is sensitive to the grain size of the ice and the assumed convective stress. The generation of melt would not only lead to satellite expansion and extensional stress, but would also provide a source of near surface melt, permitting cryovolcanic activity. It therefore appears plausible that resonance passage may have significantly contributed to the resurfacing of Ganymede.
P13F-03
Thermal Evolution of Charon and the Major Satellites of Uranus: Constraints on Early Differentiation
A thermal history model developed for medium-sized icy satellites containing silicate rock at low volume fractions is applied to Charon and the satellites of Uranus Ariel, Umbriel, Titania, Oberon and Miranda. The model assumes homogeneously accreted satellites. To calculate the initial temperature profile we assume that infalling planetesimals deposit a fraction h of their kinetic energy as heat at the instantaneous surface of the growing satellites. The parameter h is varied between models. The model continuously checks for convectively unstable shells in the interior by updating the temperature profile and calculating the Rayleigh number and the temperature-dependent viscosity. The viscosity parameter values are taken as those of ice I although the satellites under consideration likely contain admixtures of lighter constituents. Their effects and those of rock on the viscosity are discussed. Convective heat transport is calculated assuming the stagnant lid model for strongly temperature dependent viscosity. In convectively stable regions heat transfer is by conduction with a temperature dependent thermal conductivity. Thermal evolution calculations considering radiogenic heating by the long-lived radiogenic isotopes of U, Th, and K suggest that Ariel, Umbriel, Titania, Oberon and Charon may have started to differentiate after a few hundred million years of evolution. With short-lived isotopes -- if present in sizeable concentrations -- this time will move earlier. Results for Miranda -- the smallest satellite of Uranus -– indicate that it never convected or differentiated if heated by the said long-lived isotopes only. Miranda's interior temperature was found to be not even close to the melting temperatures of reasonable mixtures of water and ammonia. This finding is in contrast to its heavily modified surface and supports theories that propose alternative heating mechanisms such as the decay of short-lived isotopes or early tidal heating.
P13F-04
Tidal and librational feedback in icy satellite shells
Tides and librations are both dynamic responses of satellites to gravitational perturbations from their primaries, and are usually considered separately. We examine the case in which resonantly enhanced libration of a decoupled ice shell becomes large enough to influence the amount and pattern of tidal heating. This can control the thickness and moments of inertia of the shell, which thereby influences the libration. The dynamics, stability, and characteristic time scales of this feedback loop are not yet well characterized, but "anomalously active" bodies like Enceladus suggest that the conventional view of tidal heating needs to be expanded somewhat. A synchronously rotating satellite in an eccentric orbit has a tidal bulge which changes in amplitude and position, as the distance and apparent direction to the primary changes over the course of each orbital circuit. The optical librations, or oscillations in angular position of the tidal bulge, have amplitude γ = 2e, where e is the orbital eccentricity. When averaged over an orbital period, the tidal heating rate can be expressed as\[ \frac{dE}{dt}=-\frac{3}{2}\frac{k}{Q}\frac{n5R5}{G}\left( 3e2+γ 2\right) \] where k is a tidal Love number, Q is a tidal quality factor, R is the satellite mean radius, and G is the gravitational constant. The first term in parentheses reflects the contribution from radial tides, or change in amplitude of the tidal bulge, while the second term is due to change in its position. Forced librations in longitude are periodic deviations from uniform rotation which a satellite may experience due to gravitational torques in an eccentric orbit. The free libration rate ω is given by \[ ω 2=3n2(B-A)/C, \] where n is the orbital angular rate, and A < B < C are the principal moments of inertia. In most cases, the equatorial moment difference B-A for a satellite is a small fraction of the polar moment C, and the free libration period is much longer than the orbital period. In that case, the libration amplitude is small. However, if an ice shell is decoupled from the interior by a fluid layer, the libration response is controlled by the moments of inertia of the shell itself, and resonant enhancement is possible. If the amplitude of the forced libration is comparable to the optical libration, the amount and spatial pattern of tidal heating will change.