P43A-01 INVITED
Interactions Between Neutral Gas Clouds and Plasma Near the icy satellites of Jupiter and Saturn.
Neutral gas clouds associated with icy satellites are intimately tied to the magnetospheric plasma in which they are formed and reside. Plasma interactions can create the clouds, remove material from them, and make it possible for us to observe them. At Europa, for example, energetic ions incident on the icy surface eject hydrogen and oxygen formed from the dissociation of water (Johnson et al. 1982). The hydrogen escapes, but the O2remains gravitationally bound, forming an atmosphere. This atmosphere then interacts with the thermal plasma in Jupiter's magneotpshere: the O2is dissociated by the electrons resulting in emissions from atomic oxygen which have been observed by HST and Cassini (Hall et al. 1995; Hansen et al. 2005). Charge exchange with magnetospheric ions and electron-impact ionization removes atoms and molecules from Europa's atmosphere and exosphere, and contributes fresh ions to the plasma (Saur et al. 1998; Shematovich et al 2005). At Enceladus, where 150-300 kg/s of H2O gas is supplied by the south pole plume (Hansen et al. 2006; Burger et al. 2007), charge exchange reactions between the plasma and H2O produce fresh pickup ions which slow and deflect the plasma (Tokar et al. 2006; Pontius and Hill 2006) and induce perturbations in Saturn's magnetic field (Dougherty et al. 2006; Khurana et al. 2006). The neutrals created in these charge exchange reactions either escape from Saturn entirely or are redistributed throughout the inner magnetosphere forming gas clouds which have been observed by HST and Cassini (Johnson et al. 2006). I will describe the interaction processes between the neutral atoms and molecules in icy satellite atmospheres and exospheres, and discuss differences between the processes imporant in Jupiter's magnetosphere, where the plasma content is greater than the neutral content, and Saturn's magnetosphere, which is dominated by neutrals. References: Burger et al., JGR, 2007, in press. Dougherty et al., Science, 311, 1406, 2006. Hall et al., Nature, 373, 677, 1995. Hansen et al. Icarus, 176, 305, 2005. Hansen, et al., Science, 311, 1422, 2006. Johnson et al., Nucl. Inst. Meth., 198, 829, 1982. Johnson, et al., ApJ, 644, L137, 2006. Khurana et al., JGR, 2007, submitted. Pontius and Hill, JGR, 111, A09214, 2006. Saur et al., JGR, 103, 19947, 1998. Shematovich et al., Icarus, 173, 480, 2005. Tokar et al. 2006, JGR, 311, 1409, 2006.
P43A-02
A numerical approach for simulation of the dusty-gas Enceladus' atmosphere
In addition to being the major source of neutral components in the Saturian E-ring and, ultimately, heavy ions for the inner magnetosphere, Enceladus exhibits a geological activity that recently made it an object of intensive study. Cassini flybys performed in 2005, have provided a detailed map of its surface and have shown that most of the activity occurs in a region around the South Pole of the satellite. Solid grains, entrained by the outcoming gas flow, can be potentially dangerous for spacecraft instruments. Therefore, an accurate estimation of the grain velocity and size distributions is essential for planning of future flybys. This work presents results of the development of a kinetic two-phase model of the Enceladus' atmosphere consisting of the gas and the solid grains. The model is developed based on the Direct Simulation Monte Carlo method with both phases described in terms of their velocity distributions that allows us to calculate all major macroscopic parameters within the atmosphere and treat the acceleration of the grains by the rarefied gas. In this work, we present the model results for both phases and compare them with the Cassini dust and gas observations from INMS and CDA, respectively.
P43A-03
Does Dione have a tenuous atmosphere?
Magnetic field observations collected from a close flyby of Dione by the Cassini spacecraft provide hints of a plasma-loading type interaction. The magnetic field lines drape around the moon and have a higher field strength upstream of the moon. Detailed modeling shows that this moon is a net contributor to the plasma in its vicinity. Even though the inferred mass-loading rates are quite small (< 7 gm/s), a detailed investigation shows that they cannot be explained from sputtering from the surface alone. The observations from Dione are in contrast to observations from Tethys and Rhea where the moons appear to absorb all the plasma incident upon them such that the wakes of these moons were observed to have enhanced field strengths and the magnetic field appeared to be drawn into the wakes from the flanks. The strength of ion-cyclotron waves observed in Saturn's magnetosphere also show an enhancement close to Dione's position indicating that the moon is a source of newly ionized plasma.
P43A-04
Europa's Surface Composition by Measuring Pickup Ions: Model Calculations
Using a polar orbiting spacecraft around Europa with height ~ 100 km and in situ package composed of the following instruments: 3D plasma ion composition spectrometer (3DPICS), 3D plasma electron spectrometer, hot plasma energetic particle instrument and vector magnetometer. With these on can determine Europa's surface composition in 4π steradians by measuring locally produced pickup ions. Our model calculations include Jupiter's magnetospheric interaction with Europa (see Saur et al., 1998), and a model of Europa's neutral exosphere with surface as its source term. We use measurements of Europa's surface composition from Galileo (Carlson et al., 1999) for comparison purposes. The approach is to alternate between times measuring pickup ions and times measuring plasma and magnetic field parameters along the spacecraft trajectory. By measuring the pickup ion energy, arrival direction and mass-per-charge, one can trace back in time its origin if one knows the electric field and magnetic field in 3D. In situ observations of plasma flow velocities and vector magnetic fields can be used to determine the local convective electric field (E = -VXB) along the spacecraft trajectory. Then by combining this information with models of the magnetospheric interaction with Europa one can generate 3D maps of the electric and magnetic field and compute the trajectories of the pickup ions back to their source points. The neutral exosphere model is needed to estimate production rates of pickup ions and determine if ion-neutral scattering is important. The hot plasma measurements are needed to correct for sputtering rates which can be time dependent and electron plasma observations for electron impact ionization rates. Instrument characteristics, field-of-view requirements, and modes of operation will be given. At minimum we will require the 3DPICS and vector magnetometer. Saur, J., D. F. Strobel and F. M. Neubauer, JGR, vol. 103, 19947, 1998 Carlson, R. W., R. E. Johnson and M. S. Anderson, Science, vol. 286, 97, 1999.
P43A-05
Water ice Crystallinity and Grain Sizes on Dione
Using data from the Cassini Visual and Infrared Mapping Spectrometer (VIMS), we examine the properties of the dark and wispy terrains found on Dione, and provide interpretations to their varying content. Mapping of a "crystallinity factor" on the surface of Dione, using the shape of the 3.1 micron Fresnel reflection peak, indicates that the dark areas are more amorphous than the surrounding terrain and the interweaving wispy streaks. Alternatively, these effects could be produced by grain size variations, if the dark terrain consists of sub-micron sized particles. We also report preliminary analysis of grain sizes on Dione, suggesting that the wispy streaks and the bright background regions contain ice particles of similar size and the dark terrain contains smaller particles. We discuss implications for these results, including an exogenous source for the dark material and possible geologic activity on Dione, which could produce fissures supplying fresh crystalline ice, as in the case of Enceladus.
P43A-06
3-D Modeling of Landscape-Modifying Processes on Icy Planetary Surfaces
We will evaluate the sequence and extent of various landform-modifying erosional and volatile redistribution processes that have shaped the Galilean satellites using a 3-D model that simulates the following surface and subsurface processes: 1) sublimation and re-condensation of volatiles; 2) development of refractory lag deposits; 3) radiative heating/cooling of the surface (including reflection, emission, and shadowing by other surface elements); 4) thermal diffusion; and 5) vapor diffusion. All processes will be modeled as a function of slope, orientation, albedo, diurnal (or geothermal) heat variations, and the thermo-physical properties of the substrate. The model will provide explicit simulations of landform development and thusly will predict the topographic and volatile evolution of the surface and final landscape form as constrained by DEMs.
P43A-07
Serpentinization-driven Hydrothermal Systems on Ocean Planets and Icy Moons
The ferromagnesian silicate minerals olivine and clinopyroxene are dominant in planetary mantles, and similar
assemblages likely also typify the subsurface lithologies of icy moons endowed with rocky interiors, such as
Jupiter's Europa. Water is also common in the Solar System. Liquid water may persist to the present day on
Mars, Europa, Callisto, Enceladus and Titan.
Within the P-T window applicable to ocean/seafloor interaction (10-200 MPa, 273-700 K), the presence of water
causes olivine and clinopyroxene to be unstable with respect to the serpentine minerals (antigorite, lizardite and
chrysotile). The ensuing hydration reaction, termed 'serpentinization', essentially acts to re-equilibrate the
nascent water-deficient high-temperature state that attended planetary formation to the water-saturated low-
temperature state that characterizes the planetary seafloor environment. Importantly, thermodynamic
considerations require that this process is accompanied by the release of both (i) heat energy resulting from the
exothermic nature of the reaction; and (ii) H2 gas resulting from unlike FeMg-1 partitioning in the
reactants and products of the reaction. Because of their potential to provide heat energy, nutrients and electron-
donors for extraterrestrial metabolism in the absense of sunlight, and act as crucibles for Fischer-Tropsch-type
(FTT-) synthesis of hydrocarbons, serpentinization-driven hydrothermal systems are of considerable interest to
astrobiology.
By assuming a bulk peridotitic composition and applying new insights on cracking depth, we constrain the
potential heat- and H2 flux of extraterrestrial serpentinization over time. We further examine how different
kinetic considerations affect the longevity of such systems. In the absence of crustal rejuvenation and under our
assumed ideal conditions, serpentinization through progressive cracking persists on planetary timescales and
generates heat on a globally averaged basis at a fraction of a percent of present-day terrestrial radiogenic
heating, whilst producing hydrogen at rates of 109-1010 molecules cm-2s-1. These
values lie at the limiting extreme capable of sustaining life on Earth. We argue that the absence of macronutrient
delivery, specifically phosphorus and electron acceptors (CO2, NO3-, etc.), may further be inhibitive
of biology under these conditions. Serpentinization accompanying the initial onset of ocean/seafloor interaction,
on the other hand, enjoys much shorter lifetimes on the order of 106 - 108 years, depending mostly on
temperature and fluid accessibility. Although concomitant heat and hydrogen production is on the order of that
encountered in hydrothermal systems on Earth today, such systems may be prohibitively short-lived to evolve and
sustain biology.
http:earthweb.ess.washington.edu/~jelte/FirstLife.html
P43A-08
Early Formation of the Saturnian System?
Recent observations have provided clues that the Saturnian satellites formed early enough so that short-lived radiogenic isotopes accreted with them and played an important role in their thermophysical evolution. Using the date when calcium-aluminum inclusions (CAIs), as seen in some meteorites, were created as our time reference, we have deduced times of formation for Iapetus, Mimas, and Enceladus. These consistently fall in the interval between 2 and 5 My after CAIs were formed. Using these dates in the initial conditions for thermophysical models yield scenarios that can explain Iapetus' shape, and solve the Cold-Mimas / Hot-Enceladus paradox. The results are also consistent with interior models proposed for Titan and interpretations of Rhea's gravity field measurements. These results also have implications for the age of Saturn and the formation of the outer solar system. Acknowledgements: This work was carried out at the Jet Propulsion Laboratory-California Institute of Technology, under contract to NASA.