P21B-0531
The Jupiter System Observer: Probing the Foundations of Planetary Systems
Galileo's observations in the 1600's of the dynamic system of Jupiter and its moons launched a revolution in understanding the way planetary systems operate. Now, some 400 years later, the discovery of extra solar planetary systems with Jupiter-sized bodies has led to a similar revolution in thought regarding how these systems form and evolve. From the time of Galileo, the Jovian system has been viewed as a solar system in miniature, providing a laboratory to study, diverse and dynamic processes in a single place. The icy Galilean satellites provide a window into solar system history by preserving in their cratering records a chronology dating back nearly 4.5 By and extending to the present. The continuously erupting volcanoes of Io may provide insight into the era when magma oceans were common. The discovery of an internally generated magnetic field at Ganymede, one of only three terrestrial bodies to possess such a field, is a place to gain insight as to how dynamos work. The confirmation and characterization of icy satellite subsurface oceans impacts the way habitability is considered. Understanding the composition and volatile inventory of Jupiter can shed light into how planets accrete from the solar nebulae. Finally, like our sun, Jupiter influences its system through its extensive magnetic field. In early 2007, NASA's Science Mission Directorate formed four Science Definition Teams (SDTs) to formulate science goals and objectives in anticipation of the initiation of a flagship-class mission to the outer solar system (Europa, Jupiter system, Titan and Enceladus). The Jupiter System Observer (JSO) mission concept emphasizes overall Jupiter system science: 1) Jupiter and its atmosphere, 2) the geology and geophysics of the Galilean satellites (Io, Europa, Ganymede and Callisto), 3) the magnetosphere environment - both Jupiter's and Ganymede's&pand 4) interactions within the system. Focusing on the unique geology, presence of an internal magnetic field and evidence for a subsurface ocean, the final mission destination will be in orbit around Ganymede. As conceived, JSO will return a wealth of data to provide significant advancement in understanding the foundations of planetary systems.
P21B-0532
The Jupiter System Observer Mission Concept: Scientific Investigation of the Jovian System
NASA's Science Mission Directorate (SMD), in efforts to start an outer solar system flagship mission in the near future, commissioned studies of mission concepts for four high-priority outer solar system destinations: Europa, the Jovian system, Titan, and Enceladus. Our team has identified and evaluated science and mission architectures to investigate major elements of the Jovian system: Jupiter, the Galilean moons, rings, and magnetosphere, and their interactions. SMD dubbed the mission concept the "Jupiter System Observer (JSO)." This JPL-led study's final report is now complete and was submitted in August 2007. SMD intends to select a subset of these four concepts for additional detailed study, leading to a potential flagship mission new start. The study's NASA-appointed, multi-institutional Science Definition Team (SDT) identified a rich set of science objectives that JSO can address quite well. The highly capable science payload (including ~50-cm optics), an extensive tour with multiple close flybys of Io, Europa, Ganymede and Callisto, and a significant time in orbit at Ganymede, addresses a large set of Solar System Exploration Decadal Survey (2003) and NASA Solar System Exploration Roadmap (2006) high-priority objectives. With the engineering team, the SDT evaluated a suite of mission architectures and the science they enable to arrive at two architectures that provide the best science for their estimated mission costs. This paper discusses the science objectives and operational capabilities and considerations for these mission concepts, and some options available for emphasizing specific science objectives. This work was performed at JPL, APL, and other institutions under contract to NASA.
P21B-0533
The Science Goal and Objectives of Europa Explorer
Europa's icy surface is believed to hide a global subsurface ocean. The moon is sparsely cratered, implying that it is most likely geologically active today. The molecular constituents of life have rained onto Europa throughout solar system history, are created by radiation chemistry at its surface, and may pour from vents at the ocean floor. With water, energy, and biogenic elements, Europa appears to have the "ingredients" necessary for life. An orbital Europa mission is the first step in understanding the satellite's potential habitability. The Europa Science Definition Team has developed the goal, objectives, and investigations of the Europa Explorer mission, using input from the previous science definition teams and community groups. The NRC planetary science Decadal Survey summarizes the inherent motivation for Europa exploration by its fundamental science question: Where are the habitable zones for life in the solar system, and what are the planetary processes responsible for producing and sustaining habitable worlds? Europa exploration should permit hypothesis testing while enabling discoveries and emphasizing themes of habitability and processes. It follows that the recommended overall goal of the Europa Explorer mission is: Explore Europa and investigate its habitability. Flowing from this goal are five Priority 1 objectives: (A) Europa's Ocean: Characterize the ocean and deeper interior; (B) Europa's Ice Shell: Characterize the ice shell and any subsurface water, and the nature of surface-ice-ocean exchange; (C) Europa's Chemistry: Determine global surface compositions and chemistry, especially as related to habitability; (D) Europa's Geology: Understand the formation of surface features, including sites of recent or current activity, and identify and characterize candidate sites for future in-situ exploration; (E) Europa's External Environment: Characterize the magnetic environment and moon-particle interactions. A single Priority 2 objective is: (F) Europa's Neighborhood: Determine how the components of the Jovian system operate and interact, leading to potentially habitable environments in icy moons. For each objective, several investigations are associated, and all can be rigorously addressed from orbit with the Europa Explorer mission.
P21B-0534
Tectonic Patterns of Reoriented and Despun Planetary Bodies: Application to Enceladus
We find analytic solutions for the stresses associated with distortions of biaxial or triaxial planetary figures. Distortions of biaxial figures may be driven by variations in rotation rate, rotation axis orientation, or the combination of both. Distortions of triaxial figures may be driven by the same mechanisms and/or variations in tidal axis orientation for tidally deformed satellites. While the magnitude of the resulting stresses depends on the adopted elastic and physical parameters, the expected tectonic pattern is independent of these parameters for these mechanisms. We consider the tectonic pattern on Saturn's moon Enceladus. The global scale tectonic pattern on this satellite has been interpreted as due to a rotation rate increase (Porco et al. 2006), perhaps due to a suitably oriented impact. In this scenario, the flattening increases, forcing the tidal bulge to expand and the polar regions to contract. We show that the observed tectonic pattern is more easily explained by a large reorientation (~90°) of the rotation axis roughly around the tidal axis, than by spin-up. In the spin-up scenario, extension occurs centred on the sub- and anti-Saturnian points and strike-slip faulting centered on the leading and trailing hemispheres. In the reorientation scenario, these patterns are reversed, and are more consistent with the available geological observations (Kargel and Pozio 1996, Porco et al. 2006, Helfenstein et al. 2007). Furthermore, the latitude of the predicted polewards transition to compressional stresses is comparable to that of the observed south polar terrain margin. Reorientation of ~90° may be driven by mass redistribution associated with an internal load (Nimmo & Pappalardo 2006), ice shell thickness variations (Ojakangas and Stevenson 1989), or an equatorial large impact (Melosh 1975). Given Enceladus' orbital evolution constraints, the effect of despinning due to tidal disspation on the predicted tectonic pattern is negligible.
P21B-0535
Modeling the interaction of Enceladus's plume with Saturn's magnetosphere
Enceladus is now known to be the principal source of particles for the E ring, the neutral torus and the populations of charged particles in the magnetosphere of Saturn. Remote sensing and in situ observations show that a water vapor and dust plume located at the south pole of Enceladus has a source strength exceeding 100 kg/s. Most of the neutral material from the plume escapes local plasma pick-up and charge exchange processes and populates the neutral torus of Saturn. In this presentation, we will study the local interaction of the water plume with plasma from the magnetosphere of Saturn. We use three dimensional Biot-Savart and hybrid models of the interaction to understand the neutral escape and ionization processes near Enceladus. A comparison of the model results with magnetic field and plasma observations is provided.
P21B-0536
Is it possible to crack in the absence of tension?
The ongoing eruption of water vapor and ice on Enceladus and proposed past eruptions of liquid water on Europa have led to discussion about the feasibility of cracking a planetary ice shell. We use a boundary element method to model crack propagation in an elastic layer subjected to tension and hydrostatic compression. We consider the presence of a region at the base of the ice shell in which the far-field extensional stresses vanish due to viscoelastic relaxation, impeding the propagation of fractures towards the subsurface ocean. The maximum extent of fracture propagation can be limited by hydrostatic pressure or by the presence of the unstressed basal zone, depending on its thickness. Our simulation results indicate that Enceladus' ice shell may be cracked if it is capable of supporting ~1-3 MPa tension and is less than ~30 km thick. Europa's ice shell may only be cracked under 1-3 MPa tension if it is about 1 km thick.
P21B-0537
Electromagnetic Induction Sounding of the Interior of Enceladus in Relation to the Observed Southern Plume Using Magnetic Field Measurements on Cassini Flyby Orbits and Future Missions
This study is motivated by the possibility of an electrolytically conducting body of water of unknown dimensions below the surface of Enceladus needed in some models to explain the observed plume in the south polar region.This body of water can in principle be investigated by electromagnetic induction techniques.Assuming internal models ranging from a local bubble of electrolyte to a global ocean we estimate the magnetic signature after a discussion of possible inducing magnetic fields.
P21B-0538
A numerical study of dust distribution in the Enceladus' atmosphere
Enceladus, which is the major source of gas and dust particles in the E-ring of Saturn, became an object of considerable interest after the discovery of its geological activity during the 2005 Cassini flybys. The measurements performed during the flybys have indicated a significant compositional and spatial variation in the gas phase and a presence of dust particles of a micrometer size in the Enceladus' atmosphere. It is believed that the highly nonuniform gas production observed during the flybys is associated with the geological activity and is released by the active geysers located in the south polar region. In addition to being the source of gas detected by the in situ measurments, the geysers produce dust grains seen by the remote sensing instruments. The numerical study presented in this work is performed with a newly developed kinetic model of the Enceladus' atmosphere developed on the basis of the Direct Simulation Monte-Carlo method. A multi vent gas source for the geysers' gas production into the atmosphere has been modeled. In addition to the gas phase, a flow of dust grains is also considered. The dynamics of dust particle flow is determined by its momentum exchange with the surrounding gas environment. The unique element of the model is the self-consistent treatment of a multispecie 3D flow of gas and dust grains with the dust and gas phases being coupled through sublimation/recondensation and momentum exchange. The water number and column density are calculated for conditions that are compared with the available Cassini INMS and UVIS data.
P21B-0539
The effect of near-surface heating on the underlying convection pattern with application to Enceladus
Rapid strike-slip motion is predicted to be a consequence of diurnal tidal stresses in most satellites of the outer solar system with short orbital timescales [1]. Such motion can lead to near-surface heating through friction or viscous dissipation [2]. Here we discuss the effect of near-surface shear heating on convection in the underlying ice shells of icy satellites [3], with a focus on Enceladus and a possible origin of the south polar thermal anomaly [4]. We present models of convection in spherical ice shells including both spatially variable volumetric tidal heating [5] and regional shear heating localized in the top 5 km at either the pole or the equator. We observe that the presence of the near-surface heating strongly controls the convective pattern, increasing the wavelength, and promoting the formation of a hot upwelling beneath the shear zone. Our results suggest that localized near- surface heating may result in a degree-1 convective planform in an ice shell of a thickness that may be appropriate for a differentiated Enceladus (d < 0.36 Rsat). The near-surface heating and convection pattern will produce a localized heat flow anomaly. The upwelling beneath the shear zone also produces a few hundred meters of long-wavelength dynamic topography. The ℓ=2 component of the topography may cause reorientation of the satellite [6]. [1] Hoppa, G., B. R. Tufts, R. Greenberg, and P. Geissler, Icarus, 141, 287-298, 1999. [2] Nimmo, F., E. Gaidos, JGR, 107, 5021, 2002. [3] Han, L., A. P. Showman, LPSC XXXVIII, #2277, 2007. [4] Spencer, J. R., et al., Science, 311, 1401-1405. [5] Tobie, G., A. Mocquet, C. Sotin, Icarus, 177 534-549. [6] Nimmo, F., R. T. Pappalardo, Nature, 441, 614-616.
P21B-0540
Organic History and Ice-Rock Decoupling on Enceladus
The Cassini detection of methane, propane and acetylene in the Enceladus plume, and condensed organic compounds (OC) on the south polar region imply an organic-bearing interior of the moon. At least a few wt. % of C is expected in rocks from which Enceladus accreted. By analogy with carbonaceous chondrites, the majority of accreted OC was in a polymer in which polyaromatic groups are linked by O-, N-, and S-bearing aliphatic units. If accreted, cometary-type materials also delivered CO2, CO(?), methanol, ethane, ethene, acetylene, and condensed OC. Subsequent water ice melting and hydrothermal processes driven by decay of short-lived radionuclides led to dissolution of CO, CO2 and methanol in water and transformations of the polymer and cometary OC. CO converted to formic acid, carbonate species, methanol and methane. Hydrous pyrolysis and oxidation of the polymer partially liberated aromatic molecules and led to the formation of O-bearing OC (carboxylic and amino acids, alcohols). Increase in temperature favored oxidation of OC to carbonate species and N2, and led to graphitization of the polymer. Despite net oxidation of OC driven by H2 escape, mineral- catalyzed Fisher-Tropsch like synthesis of hydrocarbons and methane occurred in H2-rich niches. As a result, an array of aromatic, aliphatic, and N-, O-, S-bearing OC, and methane was delivered into a primordial water ocean in hydrothermal fluids. Highly soluble OC (acids, alcohols) made multiple passes through hydrothermal systems causing further oxidation of OC in rocks and solutions. In contrast, hydrocarbons exolved from cold oceanic water and formed an organic layer below the ice shell. Subsequent cooling of ocean-entering fluids and ocean freezing from above led to further separation and accumulation of OC. Some OC was trapped in ice, and methane formed clathrates. After freezing of salt eutectic brines, the light oil (a solution/mixture of ethane, propane, butane, ethene, acetylene, methanol, toluene etc.) remained unfrozen and decoupled the ice shell from underlying salt deposits and rocks. Even after oil solidification, if it occurred, the organic layer had a lower viscosity than salts and ice. An uneven pressure and/or topography at the ice-salt boundary could have led to preferential oil (and salt?) accumulation below the south polar region. Throughout history (and today), the uneven oil-rich layer could have favored tidal motions and heat generation at the bottom of the ice shell.
P21B-0541
Enceladus Surface Classification Using Cassini Visual infrared Mapping Spectrometer Data
Surface fractures (called "tiger stripes") near the south pole of Enceladus have been found to be spatially correlated with the highest temperatures measured in the region (Porco et al 2006). These features are also believed to be the source of an ice plume observed from the Cassini orbiter. As proposed by Nimmo (2007), shear heating from tidally driven lateral fault motion suggests an explanation for both the formation of the surface fractures and the associated plumes. One possible approach to determining the rate of deformation is to compare changes in the areas of the fractures over a period of time. This study employed a maximum likelihood multispectral classification method for identifying terrain types in the southern region of Enceladus using data from Cassini's Visual and Infrared Mapping Spectrometer. It was found that the mid infrared bands between 24.9 to 26.5 microns produce greater contrast between differentiating terrain than bands in the visual spectrum. The maximum likelihood classifier allowed a reasonable level of classification accuracy and an estimate of the open fracture area to be obtained.
P21B-0542
Hot Spot of Enceladus: Role of Thermal Convection and Tidal Internal Heating in the Ice Shell
The south pole of Enceladus is geologically active at the present time, with elevated surface temperatures, fractures, and jets of water and fine grained particles. We explore whether thermal convection can occur in Enceladus' ice shell and the spatial localization of tidal heating within convective plumes. To determine whether concentrated dissipation can occur in convective plumes, we develop a two-dimensional model to compute the volumetric dissipation rate for an idealized, vertically oriented, isolated convective plume obeying a Maxwellian viscoelastic compressible rheology. We apply the model to the Enceladus ice shell, and we investigate the consequences for partial melting and resurfacing processes. Calculations by us and others have demonstrated that thermal convection can occur in the ice shell of Enceladus under a range of conditions. Here, we show that tidal heating is strongly temperature dependent in a convective ice plume and could produce elevated temperatures and local partial melting in the ice shell of Enceladus. Our calculation provides the first quantitative verification of the hypothesis by Sotin et al. (2002) and others that the tidal dissipation rate is a strong function of temperature inside a convective plume. Localized tidal heating in a thermal plume could explain the concentrated activity at the south pole and its associated heat transport (2-7 GW).
P21B-0543
The vertical structure of Saturn's E ring as a consequence of the Enceladus plumes
Before Cassini dynamical models of Saturn's E ring failed to reproduce its peculiar vertical vertical structure as seen by earth-bound observations. After the discovery of an active ice-volcanism in the south pole area of Saturn's icy moon Enceladus the relevance of these particles for the vertical ring structure was rapidly realised. However, ad-hoc models for the plume particle injection predict too a small vertical ring thickness and overestimate the amount of the injected dust. Here we report on numerical simulations of the injection of plume particles into the ring. Furthermore, we performed long-term simulations to investigate how the initial dynamical properties of the injected dust determines the vertical ring profile. We show that only plume particles with injection speeds in excess of an effective escape speed larger than the three body escape speed of Enceladus are populating the E ring. The resulting vertical ring profile matches the measurements by the Cassini dust instrument CDA and is consistent with edge-on images obtained by the Cassini camera ISS.
P21B-0544
Range-Doppler processing of Saturn's Icy Satellites using the Cassini RADAR Scatterometer
The Cassini RADAR has obtained disk-integrated 2.2-cm reflectivity measurements for a number of Saturn's major icy satellites (Enceladus, Tethys, Dione, Rhea, Iapetus, Hyperion, Phoebe, and Mimas) [1, 2]. In these observations, the RADAR instrument operates in scatterometer mode, where the low receiver bandwidth of 117 kHz helps to minimize thermal noise. Transmitting a narrow bandwidth pulsed tone further reduces the noise variance [3]. In spite of these precautions to minimize noise, the instrument is often operating at distances as high as 400,000 km, and the signal-to-noise ratio is so low that it is impossible to detect the signal within the individual echoes (which are recorded in time as real 8-bit voltage samples); thus, the echo powers are accumulated in the frequency domain to produce a measurable signal [1, 3]. Yet, in a few observations, the SNR is estimated to be high enough for range compression and a pulsed chirp signal is thus transmitted, allowing us to divide the coarse disk reflectivities into fine annular rings. If the signal is strong enough, we attempt to further discriminate the echo into cells by separating the return into Doppler bins. To date, there are six observations that support higher resolution processing: Rhea (Orbit 18 and 22), Enceladus (Orbit 3), Dione (Orbit 16), Hyperion (Orbit 15), and Iapetus (Orbit B). Here, we present the preliminary results of this processing, obtaining finer resolution radar returns of these bodies than ever before, with the exception of the forthcoming Iapetus SAR imaging flyby, expected to achieve 2-12 km surface resolution [2, 4]. [1] Ostro et al. 2006, Icarus 183, 479-490. [2] Ostro et al. 2007, this conference. [3] West et al. 2007, IEEE TGARS, in preparation. [4] West et al. 2007, this conference.
P21B-0545
Impact Crater Distributions on the Saturnian Satellites From Cassini ISS Imaging - Implications for Geologic Histories and Ages
Although we can observe any current activity, such as the plumes on Enceladus, with the Cassini orbiter, insight into past activity on the Saturnian satellites can only be achieved (for now) through studying the impact crater distributions. Previous work by the authors (Kirchoff et al., LPSC XXXVII, #2089, 2007) has indicated that the distribution on Enceladus is particularly intriguing. This is not only because of the relatively recent resurfacing of the ridged terrain implied by the lower crater density, but also the difference of the distribution on Enceladus' heavily cratered terrain when compared to the heavily cratered terrains of Rhea, Mimas and Dione. When the crater size distribution data is plotted in a R-plot we find that Enceladus' distribution has steeper drop offs at both the small (< 2 km) and large (> 9 km) diameter range. This implies that Enceladus is "missing" smaller and larger craters when compared to Rhea, Mimas and Dione. Several hypotheses can be formulated to account for this interesting difference in the distributions. The first is simply that the heavily cratered terrain on Enceladus has been impacted by a different population than these other bodies. This idea has been presented before by several authors (e.g., Plescia & Boyce, Nature 301, 666-670, 1983), and has typically been referred to as "Population 2". Population 2 is believed to be composed of planetocentric debris created by ejecta from large impact basins such as Odysseus on Tethys. The second is that impact craters on Enceladus have been undergoing some type of endogenic modification. For example, the smallest craters could be erased by burial from plume material (Kirchoff et al., LPSC XXXVII, #2089, 2007) and larger craters by relaxation (e.g., Smith et al., LPSC XXXVII, #2237, 2007). We will test these hypotheses (and possibly formulate new ones) by analyses of the impact crater distributions on Enceladus, Dione, Rhea, Tethys, Iapetus and Phoebe. To test the endogenic hypothesis, we will also look at the spatial distribution of impact craters in the heavily cratered terrain on Enceladus and topography of the craters.
P21B-0546
High Resolution Dione Atlas derived from Cassini-ISS images
The Cassini Imaging Science Sub-system (ISS) acquired many high-resolution im-ages (< 1 km/pixel) during a close targeted flyby of Dione in October 2005 and two non-targeted flybys in December 2004 and April 2007. We combined 44 high-resolution clear filter images with lower-resolution coverage and two images taken by Voyager cameras to produce a high-resolution global semi-controlled mosaic of Dione. This global mosaic is the baseline for a high-resolution Dione atlas that con-sists of 15 tiles mapped at a scale of 1:1,000,000. The nomenclature used in this atlas was proposed by the Cassini imaging team and is awaiting validation by the IAU. The atlas will be made available to the public through CICLOPS (http://ciclops.org) and PDS (http://pds.jpl.nasa.gov).
P21B-0547
Relationships of Dione's physical and chemical surface properties to geological and morphological surface features
During orbit DI 016 (October 2005) Cassini's Visual and Infrared Mapping Spectrometer (VIMS) [1] acquired spatially resolved spectral data of the Saturnian satellite Dione with pixel ground resolutions up to 2 km per pixel. Additionally, the Imaging Science Subsystem (ISS) [2] observed the satellite synchronously to VIMS with pixel ground resolutions up to 15 m per pixel. Based on high resolution VIMS data we mapped the chemical and physical properties across Dione's surface. In order to study the relationships between the spectral characteristics of the surface material and geological processes it is essential to relate the spectral information to geological and geomorphological surface features. Therefore we reprojected and mosaicked the resulting VIMS maps [3] and registered them to the simultaneously acquired ISS images. Resulting VIMS/ISS maps show pronounced spectral differences which can be correlated with geological units and are found in the vicinity of stratigraphically younger impact craters and tectonic features (troughs and "wispy streaks") indicating exposed water ice from underneath. Globally abundant cratered plains are well distinguishable from the tectonically deformed regions and show the highest influence of dark material [4]. Although ISS data indicate different subunits of the cratered plains with respect to morphology and ages [5], these subunits are indistinguishable in the VIMS data. In contrast their spectral characteristics appear to be only related to hemispherical differences. [1] Brown R.H. et al., SSR, 115, 111-168, 2004. [2] Porco C.C. et al., SSR, 115, 363-497, 2004. [3] Jaumann et al., PSS, 54, 1146-1155, 2006. [4] Clark R.N. et al. (Icarus, in press). [5] Wagner R. et al., LPSC XXXVII, abstract 1805.
P21B-0548
Using 3D MHD Simulations to Improve the Internal Magnetic Field Model of Ganymede
Ganymede, the biggest moon in the solar system, has an intrinsic magnetic field. The internal field is strong enough to form a mini magnetosphere, embedded within Jupiter's giant magnetosphere, that interacts with the corotating Jovian plasma. We have conducted a series of three dimensional MHD simulations to understand the magnetic configuration of Ganymede's magnetosphere using several field and particle data sets from Galileo as boundary conditions. In general, the magnetic field configuration resolved in the MHD simulation has shown satisfying agreement with the Galileo observations for all six close encounters. The magnetopause currents are well resolved in our high resolution simulations, producing sharp rotations in the field orientation consistent with the observations. In these simulations, we have used the internal field model from Kivelson et al. [2002] in which Ganymede's internal dipole field was fitted by using observations from three out of the six close encounters under the assumption that the magnetopause currents contributed a constant magnetic field. We will use the more accurate description of field perturbations of the external currents extracted from our MHD simulations to refine Ganymede's internal field model and to assess the inductive response.
P21B-0549
Sill Injection on Icy Satellites Inferred from Ridge-Induced Flexure on Europa
There have been observations of active jetting from the south pole of Enceladus and evidence of past extrusions on other icy satellites, indicating that cryovolcanism is an important process in the outer solar system. In volcanic systems on Earth, the amount of subsurface magmatism generally exceeds the amount of surface volcanism. Given that the "magma" is denser than the "country rock," it is thus reasonable to expect subsurface cryomagmatism, although evidence has not been forthcoming. Here, we report that flexure caused by ridges, long, quasi-linear features ubiquitous over the face of the Europa, may indicate sill injection on this icy satellite. Studies of this flexure using simple elastic models determined effective elastic thicknesses of order 100 m to 1 km, implying very high heat flows (of order 1 W m-2). Tidal dissipation and radiogenic heating within Europa probably cannot globally supply such high heat flows, suggesting there exists a mechanism to augment locally the thermal state. While there are several possibilities, we consider sill injection. We employ a thermomechanical finite-element model with an elastoviscoplastic rheology, and using the depth of neutral buoyancy as a guide, we simulate the thermal consequences of a sill and couple it to a mechanical simulation for ridge-induced flexure. The heat from this intrusion conducts through the ice and locally thins the lithosphere. For a case fit by an elastic plate 350 m thick (which translates into a heat flow of ~700 mW m-2), we find a good fit to the data with a sill 1 km deep, 12 km wide, and persisting for 100 kyr. Notably, the regional heat flow is an order of magnitude lower (70 mW m-2). Thus, indications of flexure at ridges may provide evidence of pervasive subsurface cryovolcanism on Europa and, by extension, on other icy satellites.
P21B-0550
A Self-consistent Approach to Io's Local Interaction With the Torus: a Hybrid Code Coupled to a Multi-species Chemistry Model.
Io's local interaction with the plasma torus has been extensively studied with a variety of complementary approaches: MHD (Linker et al., 1998; Combi et al.,1998), 2-fluid modeling (Saur et al., 1999) and multi-species chemistry (Dols et al., submitted to JGR). We present a self-consistent, multi-species chemistry model of this interaction. We compute the ionization and charge-exchange rates in Io's corona (~6 RIo) with a physical chemistry model that includes S, O, SO and SO2 and use these results as input into a hybrid code to compute a self-consistent plasma flow around Io. We iterate several times between the chemistry and the hybrid models to reach a stationary solution. We will present preliminary results showing the development of an asymmetry in the flow and its effect on the plasma production, ion mass production rates and pick-up current. The results will be compared to the Galileo observation on its flyby in Io's wake.
P21B-0551
Study of H2O/CO2 Ices of Astrophysical and Planetary Interest
Ices of mixtures of H2O and CO2 can be found in different astrophysical environments, like cometary nuclei or making part of solar system bodies. Water and carbon dioxide can associate in different ways in the solid state, depending on the temperature, relative concentration and process of formation of the ice. Infrared spectroscopy provides a sensitive tool to study different structures of these systems. We have prepared ices of H2O and CO2 by admitting the corresponding gases into a high-vacuum, low-temperature chamber, where the solids are formed by deposition on a substrate. The samples are studied by transmission and reflection-absorption infrared spectroscopy, and the gas content in the chamber is monitored using mass spectrometry. The ices are formed with varying amounts of water and carbon dioxide, and their spectra are taken at a range of temperatures of astrophysical relevance. Four deposition schemes are used, according to whether the gases are admitted sequentially or simultaneously. Our study concentrates on the CO2 spectral features, whose changes reveal details of the sample characteristics and type of water-carbon dioxide association. A crystalline type of CO2 is dominant in sequentiallly prepared samples at low temperature and high carbon dioxide relative concentration, and is evaporated when heating at 105 K. On the ohter hand, a non crystalline CO2 which enters the amorphous water structure, remains embedded until the temperature of water crystallization, ~ 165 K, and may be the dominant phase in co-deposited samples of low carbon dioxide relative content.
P21B-0552
The relevance of recently revised thermophysical properties of important volatiles for planetary science and exploration of the outer solar system
Water (-ice) and certain other volatiles (e.g., carbon dioxide, carbon monoxide, methane) play an important role in the solar system (small bodies, icy moons). Thermophysical properties for wide ranges of temperature and pressure are needed for simulations and in order to describe the formation/thermal evolution of those bodies and the environment for space probes. Here we discuss and revise those properties from recent literature and derive precise and physically sound correlation equations. For water ice, we compile the most precise, up-to date measurements with the widest possible range in temperature and pressure, from which suitable approximations may be generated for the desired purpose and accuracy. A new thermodynamically consistent formulation for ice Ih is used; except for the thermal conductivity, these are now the most reliable and accurate data available. Differences for other modifications of ice are discussed.
P21B-0553
Photolysis Study of Cosmic Ices and Proposed Reflectance Measurements
We have carried out laboratory simulation studies on the spectral identification of IR absorption features produced through vacuum ultraviolet photon-induced chemical reactions in several ice systems relevant to icy satellites of planetary systems and grains in interstellar medium. Several ice systems containing some of the most abundant molecules, namely, H2O, CH4, CO, CO2, CH3OH, and NH3, in cosmic environments are studied. We have also measured the production yields for the photolyzed products and the destruction yields of the parent molecules in the ice samples. A tunable intense synchrotron radiation light source available at the National Synchrotron Radiation Research Center, Hsinchu, Taiwan, was employed to provide the required photons. Typical photon-induced reaction products include radicals (CH2, CH3, C2H3, C2H5), light hydrocarbons (C2H2, C2H4, C2H6, C3H8), carbon-containing compounds (suboxides), alcohols (CH3OH, C2H5OH), and others like HCO, H2CO, XCN, and simple amino acids. In addition to the ongoing simulation studies we plan to implement an in-situ reflectance measurement of a given ice sample before and after photolysis in the spectral region between 0.3 and 2.5 micron. The chosen samples will be relevant to surface materials of our Solar System. The results to be obtained will provide valuable data to our understanding of space weathering for the surfaces of airless bodies of our Solar System. Detailed results of our ongoing work and the proposed implementation will be presented. This research is based on work supported by NSF grant AST-0604455.