P31C-01
On the Persistence of an Ocean on Europa
Data from the Galileo mission indicate that Europa possesses a liquid ocean beneath its icy shell. However, the thickness of the outer ice shell and the depth of the ocean remain uncertain. The past state of the outer H2O layer is even less constrained. We present results of a computational model of the thermal evolution of Europa's interior, that suggest an ocean could have existed beneath an ice shell throughout most of Europa's history, maintained in part by pore water convection in the silicate mantle. Our numerical simulator predicts a thermal history by solving the time-dependent governing equations of mass, momentum and energy conservation in spherical coordinates. The various processes included in the simulations are hydrothermal convection (through silicate mantle pores), thermal diffusion, salt transport, phase changes, radiogenic heating, parameterized convection in the ocean and ice layers, and tidal dissipative heating (tdh) in an ice shell. The vigor of hydrothermal convection is characterized by the Rayleigh number, which depends on the product, kH, where k is mantle permeability and H is the thickness of the convecting layer. Studies of terrestrial permeabilities are used to constrain the permeability of Europa's silicate mantle. Significant permeability has been found on Earth at depths up to 25 km. Accounting for differing gravity, this scales to about 200 km on Europa. There is considerable uncertainty as to when tidal dissipative heating may have commenced in Europa; we adopt Yoder's analysis in which the Laplace resonance among Io, Europa, and Ganymede formed about 500 Myr ago. Despite the 4 Gyr period in our model without tdh, we find that an ocean layer can be maintained by pore water convection in the mantle. This ocean layer, not as thick as when tdh is active, varies in depth with latitude, and thins slowly over time. Concurrently, parameterized convection in the ice shell occurs, and is non-uniform in space and time. Water heated in the silicate mantle is released into the ocean at sites of hydrothermal venting as warm, buoyant plumes that tend to rise through the ocean to the base of the ice shell. The presence of salt in mantle pores, but not initially in the ocean, will limit plume rise for a period of time, but eventually the bulk of the ocean becomes homogenized due to turbulent mixing. Further, a brine layer slowly forms at the base of the ice shell, resulting in an unstable concentration profile. The brine accumulation results from exclusion of water as cold brine cools further, and from the high viscosity of sub-zero brines. The heat delivered by the rising plumes can contribute to local melting and thinning of the ice shell. Without tdh, significant topography at the base of the ice shell develops due to coupling between fluid flow and thermodynamics of ice melt, but most of this topography is lost when tdh is activated. Some variation remains with ice shell thicknesses ranging from 22 km at the poles to 26 km at 11° latitude to 15 km at the equator. The thickest ice occurs near the latitude of Conamara, suggesting that convection in the ice shell could contribute to chaotic terrain.
P31C-02
Theoretical Modeling of Europan Cycloids: The Importance of Obliquity
Cycloids are thought to be cracks that form in response to tidal stress on Europa. Obliquity and non- synchronous rotation (NSR) alter the pattern of tidal stress and can thus be constrained through cycloid modeling. Several authors have published fits of theoretical models to three cycloids in Europa's southern hemisphere. The fits in Hoppa et al. (2001) captured the general trend of each cycloid, but the fits worsened with each additional arc and did not fit the cusp locations well. Hurford et al. (2007) used a different set of mechanical parameters for each arc of a cycloid, improving fits but requiring many more free parameters. In addition, they found that stress due to NSR improved fits. In both studies, the formation longitude of the cycloid was used as a fit parameter because observed cycloids could not be fit at their current locations; NSR could move features eastward over time. Despite the successes of cycloid modeling in the southern hemisphere, applying the same model to equatorial cycloids failed to produce even mediocre fits, most likely because the effects of obliquity were not considered. Inclusion of obliquity and the direction of the spin pole increase the parameter space and break symmetries in the stress field, so searching for fits by hand, as was the procedure in past work, becomes unwieldy. Therefore, the goal of this study is to automate the process of finding and evaluating fits and to examine the effects of obliquity on southern hemisphere cycloids. We have successfully built an automated cycloid simulator, which searches the parameter space and relies on a quantitative measure of goodness-of-fit when evaluating model cycloids. The resulting fits are much improved over those of Hoppa et al. (2001). They are almost as good as the fits of Hurford et al. (2007) but with far fewer parameters, as we used the same parameters for all arcs of a cycloid. For comparison, the Hurford et al. fit to the cycloid Cilicia required 20 free parameters; we achieved almost as good a fit with only seven free parameters. In addition, our work has shown that good fits require obliquity but not stress from NSR. However, the rotation itself, as opposed to stress it imparts, may still be important as we have yet to fit cycloids at their current longitudes.
P31C-03
Dynamics of Ganymede's Magnetopause as Observed by the Galileo Spacecraft and Comparisons with Global MHD Simulations
The Galileo spacecraft visited Ganymede six times passing through different regions of the moon's magnetosphere. On the two low latitude passes on the upstream side, the G8 and G28 flybys, the spacecraft detected large amplitude waves in the magnetic field at the magnetopause crossings. In particular, the wave amplitude was larger during the G8 pass in which the spacecraft passed closer to the nose of the upstream magnetopause than it did on the G28 pass. Whether the observed oscillations were spatial structures or resulted from temporal variations of the magnetopause is not clear although it has been proposed to be surface waves caused by the Kelvin-Helmholtz instabilities on the magnetopause (Kivelson et al., 1998). We have conducted three-dimensional MHD simulations, which model the interaction between the Jovian plasma and Ganymede's magnetosphere, that enable us to investigate the properties of Ganymede's magnetopause boundaries. We examine the temporal variation of the system in detail and find that for the conditions during the G8 pass, the magnetopause identified in our simulation oscillates in regions consistent with the locations where large amplitude oscillations were seen in the magnetometer data. Bursty plasma flows associated with magnetic reconnection occurring near the upstream equator are observed in the vicinity of the magnetopause in our simulations. Because the ambient Jovian magnetic field is always nearly anti- parallel to Ganymede's intrinsic field at the equator, we attribute the observed boundary oscillations to temporal variations associated with magnetic reconnection at the magnetopause. We have performed similar analyses on other passes and will present comparisons with the Galileo observations.
P31C-04
The Astounding Symmetry of Worlds: Polar Wander on Enceladus, Europa and Beyond
The Space Age has produced an amazing string of surprises in our Solar System. The most recent of these is the dawning realization that rotational stability of planets is not what it was once thought. Evidence is mounting that true polar wander (TPW: the slow shifting of planetary bodies [or their outer zones] with respect to their rotational axes) is a common process. Reorientation events have been proposed for Mars, Miranda, Enceladus, and Europa. Theoretical considerations indicate that many of the icy satellites can be reoriented under the influence of a significant mass anomaly (such as a large impact depression). The physical evidence of such events is not always obvious. The prime evidence for TPW on Enceladus is the location of volcanic activity at the south pole. Reorientation fracture patterns have not yet been identified, but the distribution of ancient cratered terrains is remarkably symmetric about the rotation pole, while crater density is symmetric about the equator, leaving an ambiguous record wrt TPW. The origin of these patterns is unclear but indicate that resurfacing geography is controlled in some manner. In contrast, a startling global symmetry of Europa's global scale lineaments and geologic units (vaguely hinted at by Voyager maps) is clear if TPW is invoked. Europa may be the most dramatic example of TPW known to date.
P31C-05
Remarkable Opposition Surges on the Satellites of Saturn Revealed by the Cassini Spacecraft
Observations of planetary satellites over a range of viewing geometries offer important clues to the physical properties of their surfaces. Spacecraft observations enable a dramatic expansion of such observations because of new wavelengths and viewing geometries not attainable from Earth. The four year Cassini tour has obtained nearly a full excursion in solar phase angle (the angle between the observer, the object and the sun) from 0 to 180 degrees. The Cassini Visible and Infrared Mapping Spectrometer (VIMS) and the Imaging Science Subsystem (ISS) measured opposition phase curves of the major icy satellites between 0.2 and 5.1 microns. The moons all exhibit a remarkable opposition surge as they become fully illuminated, increasing in brightness by about 30 percent in the last degree. This phenomenon is wavelength dependent and appears to be accentuated by the presence of small particles from the E-ring. Also, bright regions tend to exhibit greater surges. Modeling of the observations enables the fluffiness of the surface, and thus the collisional and depositional history of the surface, to be characterized. The measurements at very small solar phase angles (less than degree) yield clues regarding optical phenomena such as coherent backscatter. Work funded by the NASA Cassini Data Analysis Program.
P31C-06
Hybrid simulation of moon-magnetosphere interactions at Saturn: Titan and Dione
The interaction of Saturn's moons Titan and Dione with the corotating magnetospheric plasma has been studied by using a three-dimensional hybrid simulation model (kinetic ions and a charge-neutralizing electron fluid). In the case of Titan, the satellite's dense ionosphere represents the major obstacle for the impinging plasma flow. In this presentation, we shall focus on Titan's plasma interaction during the Cassini T32 encounter. During this flyby, the moon left Saturn's magnetosphere in the subsolar region of its orbit and interacted with the shocked solar wind plasma in Saturn's magnetosheath. The magnetic field direction almost reversed during the flyby. The density and temperature of the impinging plasma flow showed dramatic changes as well. By means of real-time simulations, it is illustrated how Titan's plasma environment is affected by such disruptive changes in the upstream conditions. Magnetic field observations during the only Cassini flyby in 2005 reveal some hints towards the existence of a tenuous atmosphere around Saturn's third-largest moon Dione. In our simulation study, we consider the case of a completely inert obstacle as well as the inclusion of a weak ionosphere around Dione. The overall topology of the interaction region is discussed as a function of the total mass-loading rate. We also estimate the mass-loading rate which is suitable for explaining Cassini MAG observations near Dione.
P31C-07
Implications From Ithaca Chasma for the Thermal and Orbital History of Tethys
Flexural modeling of Ithaca Chasma on Tethys suggests that the elastic thickness and the surface heat flux
were 5-7~km and 18-30~mW/m2 respectively when the feature formed (~4 Ga B.P.) [1]. A global
extrapolation of this heat flux suggests that as Ithaca Chasma formed, Tethys was emitting between 60 to
100~GW of power. While tidal dissipation has been included in thermal models of other icy satellites (e.g.
Europa, Enceladus), tidal heating has not been hitherto considered relevant to Tethys. Because Tethys'
eccentricity is near-zero at present, we present paleo-eccentricities required for Tethys to have been tidally
heated to the extent suggested by Ithaca Chasma. The eccentricities range from 0.001 to 0.02, depending
on the interior structure of Tethys. A liquid water ocean may have existed on Tethys, although our models do
not require this to match the geological observations. The orbital circularization timescales are short
compared to the inferred interval between Solar System formation and Ithaca Chasma formation. Therefore,
rather than the elevated eccentricity being primordial, it is likely that Tethys and Dione were involved in a
paleo-resonance and subsequently evolved to their current configuration. However, the equilibrium tidal
heating value for a Tethys-Dione resonance is substantially less than that observed at Ithaca Chasma,
similar to the present-day situation at Enceladus [2].
[1] Giese et. al, Icarus, 2007
[2] Meyer and Wisdom, Icarus, 2007
P31C-08
The interior of Iapetus: Constraints provided by the solar wind interaction
Iapetus is Saturn's third largest and most distant regular satellite. This moon is best known for its unique hemispheric albedo asymmetry and the kilometers-high ridge circling its equator, but observations made during Cassini's 2007 flyby show another of the moon's unusual features. The spacecraft approached Iapetus from downstream and stayed to the side until it had passed upstream, never having entered the moon's geometric wake. The interplanetary magnetic field's (IMF) orientation was such that during the approach, Cassini's trajectory took it towards a region where it would be magnetically connected to the moon. During this time, the Cassini plasma wave spectrometer observed low-frequency waves that were unique to the Iapetus encounter and strongly correlated with the spacecraft's distance from the moon. When Cassini passed into the space where the IMF should have magnetically connected it to the moon, the magnetometer observed a strong perturbation of the magnetic field. When we extrapolate that perturbation towards Iapetus, we find that the IMF rotated to completely avoid intersecting the moon. This suggests that the solar wind plasma was deflected by Iapetus. We have analyzed the perturbation and we find that it can be caused by neither an inert body nor a mass-loading source. Such a perturbation could be caused by either an internal, electrically-conducting ocean or remanent magnetization. Iapetus' density of 1100 kg m-3 allows for the possibility of an interior ocean and current models of Iapetus' interior allow for the possibility of a rocky core 500-700 km across (Castillo-Rogez et al., 2008). That rocky core could support the needed remanent magnetization, but the difficulty is in finding a mechanism to provide the source magnetic field.