P31A-0953 0800h
Theoretical investigation of the structural properties of high-density H2O-ice
At extreme conditions not easily realized in the laboratory, such as low T, long observational time scales, and high P produced by static or dynamic compression, H2O-ice displays amazing structural behavior. Computer simulations can help us to understand this unusual material and we have investigated ice VIII intensively using first principles methodologies. This is a prototypical form of high-pressure ice consisting of two interpenetrating hydrogen-bond networks. Here we clarify several of its unusual properties under pressure: the correlated non-linear behavior of various physical quantities, instabilities responsible for amorphization, and dipole re-ordering. The relationship between ice VIII-like and ice XI-like structures, two prototypical forms of high and low-pressure ice, is also clarified. Research supported by NSF/EAR 0230319 and 013533 (COMPRES).
P31A-0954 0800h
Rheology of ice II at low differential stresses
Knowledge of the rheology of high-pressure phases of water ice is crucial for understanding the thermal structure and internal dynamics within medium- and large-size icy moons of the outer planets. In previous studies of ice at relatively high differential stresses, the flow laws of the several high-pressure phases were inferred to lie mainly in the dislocation creep regime. In this study, we have carried out creep experiments of fine-grained ice II at confining pressures of 200-250 MPa and temperatures of 200-220 K using a gas-medium triaxial deformation apparatus to measure rheology at lower stress conditions. We make fine-grained ice II by multiple I-II transitions at lower temperatures and larger overpressure conditions from the equilibrium boundary. Microstructures of deformed ice II aggregates were examined by cryogenic SEM. Clear differences in creep behavior between ice II made by single and triple I-II transitions were observed at stresses of 4-18 MPa and strain rates of 10$^{-8}$-10$^{-7}$ s$^{-1}$. In the former case, the stress exponent is about 5, which is consistent with the flow law previously obtained at differential stresses above 20 MPa. In the latter case, the ice II is much weaker and the stress exponent is about 2.4. Preliminary estimates of the grain size of ice II are about 50 micron and 10 micron, respectively. These experimental results imply that the grain-size sensitive creep of ice II dominates plastic strain at low stress conditions, making ice II the second phase of ice (after ice I) to manifest a change from a grain-size insensitive rheology to a weaker grain-size sensitive rheology at low stresses and finer grain sizes. The implications for icy moons are towards more convective instability, and lower internal stresses, temperatures, and/or grain sizes.
P31A-0955 0800h
Microstructure and Physical Properties of Sulfate Hydrate/Ice Eutectic Aggregates in the Binary System Sodium-Sulfate/Water at Planetary Conditions
Reflectance spectra data from Mars Odyssey, Galileo and potentially from Cassini suggest the presence of hydrated salts on numerous satellites in environments such as evaporate beds or combined with water ice. Improved mission data on these occurrences indicate that grain structures and properties of such materials merit a closer look using laboratory methods. Here we report the synthesis of a two-phase aggregate of sodium sulfate hydrate and water ice made by eutectic solidification from solution, characterization of its microstructure using cryogenic SEM, and comparison of its physical properties to those of its end-member components. Samples are crystallized from solution using a precision cryobath and seeded growth. The reaction is a "simple" one meaning that there is no solid solution formation in either of the two solid phases. The eutectic composition we studied for the sodium sulfate hydrate is 4wt% Na$_2$SO$_4$, which corresponds to about .06 volume fraction of Na$_2$SO$_4$$\cdot$10H$_2$O, mirabilite, and .94 ice I. The eutectic microstructure observed with this volume fraction, which is termed "broken lamellar", consists of fairly uniform blade-like mirabilite grains arranged in roughly parallel columns within a water ice matrix. The blades and matrix material form a lamella that alternates with lamellae of pure ice. Energy dispersive spectroscopy of these eutectic mixtures confirms the presence of the two crystalline phases. Also, we find that lamellar spacing decreases with increasing growth rate. Constant-strain-rate tests in compression are carried out in the cryogenic gas deformation apparatus at LLNL in a pressure-temperature range appropriate to the icy satellites. We report the rheology of the two-phase aggregate and compare it to the strength properties of pure water ice and pure mirabilite. With the aid of numerous studies on similar structures in the literature on metals, we analyze the deformation mechanics from the perspective of defect and crack propagation between the two phases in the eutectic structure. This ongoing investigation is the first of several planned experimental studies of sulfate-hydrate binaries with ice I that are likely to be important in the icy satellites.
P31A-0956 0800h
Experimental Study of "Dirty" Ice Systems: Implications for Ganymede and Callisto
Recent studies have hypothesized that high-pressure H$_{2}$O polymorphs, specifically Ice VI and Ice VII, make up a significant portion of the interiors of both Ganymede and Callisto. Several of Jupiter's satellites have been conjectured to contain subsurface salty H$_{2}$O waters; therefore, any potential "warm" ice phases in the interior of these satellites could have interacted extensively with the salty oceans. Raman spectroscopy and synchrotron radiation has been used previously to study the bonding structure and unit cell parameters of pure Ice VII. However, no data exists on the effect of salts on the unit cell parameters and volume of solid H$_{2}$O at high pressure. To obtain more realistic data for use in planetary physics, it is important to understand the effect of impurities on H$_{2}$O at high pressure. This study measured the unit cell volumes of Ice VII formed from a 5 wt.% NaCl-H$_{2}$O solution up to 25 GPa and 800 K by using a hydrothermal diamond anvil cell and synchrotron X-ray radiation. The experiments were conducted at the GSECARS 13-BM-D beamline (Advanced Photon Source), using monochromatic X-ray radiation and an online imaging system. The diffraction data indicate that the Ice VII formed from a 5 wt.% NaCl-H$_{2}$O solution has a molar density that is systematically increased at any given pressure relative to the molar density of Ice VII formed from pure H$_{2}$O. A zero-pressure volume, bulk modulus, and pressure derivative were calculated from this data and compared with pure Ice VII data reported by others. Thermal expansivity data of the Ice VII formed from the 5 wt.% NaCl-H$_{2}$O solution will also be presented up to 21 GPa. The inclusion of NaCl into Ice VII also acts to decrease systematically the melting curve of Ice VII. The new data can be used to model the density profiles of H$_{2}$O-rich bodies.
P31A-0957 0800h
Energy transport in water rich ices
Water ice is the dominant surface component on many surfaces in the solar system. Understanding interaction of ice with organic molecules is of prime importance to many current and planned flight missions [example current missions include Mars, where the polar caps may contain organics at some depth; Cassini, where Titan should have organics in abundance; and Deep Impact, where the organics may be mined with excavation of the icy crust]. Light and other radiation impinging on these surfaces serves both as a probe of the surface composition (as measured with remote sensing and in situ instruments) and as a source of energy that modifies, often greatly modifies, the ice-organic systems. One of the major unsolved problems is the ability to reliably calculate the detailed radiation field within icy surfaces that exhibit significant scattering, such as surface frosts or extensively fractured ice. This prevents accurate determination of the composition of ice mixtures quantitatively with remote sensing techniques, and prevents accurate estimation of reaction rates and products within a mixture. The need to be able to accurately model the radiance field within icy surfaces of solar system bodies requires achieving the the capability to determine quantitatively the composition of scattering ice mixtures with remote sensing measurement and the capability to predict accurately the evolution of ice mixtures under the conditions of space weathering. We report here progress on experiments designed to advance these capabilities.
P31A-0958 0800h
Tethys: Modeling of Near Infrared Telescopic Observations
We have analyzed near-infrared telescopic spectral observations of Tethys for comparison to Cassini observations. A mixture modeling code based on Hapke theory (Hapke, 1981, 1993) has been adapted and combined with an iterative simplex algorithm (Press et al., 1992; Bauer et al., 1998) and applied to observations of Tethys. The mnodel can treat spectra as nonlinear mixtures of various grain sizes and compositions, as linear mixtures of adjacent spatial regions, or a coimbination of both. We have assembled from various sources (Ockman, 1957; Warren, 1984; Hudgins et al., 1993; Grundy and Schmitt, 1998) water ice optical constants for the near-infrared spectral range for temperatures of 70, 80, 90, and 100 K. Preliminary results indicate that telescopic spectra of Tethys are best matched by crystalline water ice with a range of grain sizes, with a small complement of dark material such as amorphous carbon.
P31A-0959 0800h
The Importance of Resolved Shear Stress and Dilation at the Instant of Cycloid Cusp Formation on Europa
Chains of arcuate fractures called cycloids or flexi are nearly ubiquitous on the ice surface of Europa and are typically among the youngest lineaments visible. Cycloids, by definition, consist of two or more arcuate segments and at least one cusp, although many cycloids may form long chains containing several segments and cusps. Cycloid segments meet at cusps, which are typically defined by a sharp kink. The current model for cycloid formation suggests that cycloids grow as tensile fractures in the diurnal tidal stress field on Europa, which constantly changes in magnitude and orientation, rotating clockwise in the southern hemisphere and counter-clockwise in the northern hemisphere. In this model, cycloids form perpendicular to the maximum tensile stress direction and grow in a curving path as the principal stresses rotate. Cycloids form cusps since a number of hours pass between the time when a segment is inactive and the time when the next segment initiates. This tensile crack model has seemingly isolated the dominant driving mechanism for cycloid growth (i.e. diurnal stress). Nonetheless, this model fails to account for the likely mechanics of crack development in the extant stress field at cycloid cusps. Our data indicate that cusp angles (the angular change from one segment to the next) are all less than $90\deg$, which geometrically necessitates resolved shear stress on an existing cycloid segment at the time of cusp formation. The only loading condition in which there would be zero resolved shear stress (for cusp angles $ < $ $45\deg$) would be if the two horizontal principal stresses coincidentally resolved equal and opposite shear stresses on the existing segment, causing the resolved shear stresses to balance out. This scenario is impossible for the average cusp angle of $56\deg$ (based on 126 measured cusps), as it would geometrically require the minimum tensile stress to be more tensile than the maximum tensile stress. The existence of resolved shear stress must therefore be accounted for in cycloid growth models. When shear stress is resolved onto a crack, tailcracks may form in the extensional quadrants. For pure strike-slip sliding, tensile stress is theoretically maximized at $70.5\deg$ from the trend of the slipping feature in the extensional quadrants. We surveyed strike-slip faults on Europa having observable offsets and associated tailcracks and measured a wide variability in tailcrack angles with respect to fault strike, averaging $54\deg$ and ranging from 30 to $80\deg$. This variability in tailcrack angle can be mathematically attributed to concurrent opening and shearing along faults on Europa. High-angle tailcracks reflect pure strike-slip motions, whereas progressively lower angle tailcracks indicate increasing ratios of opening to strike-slip motion of the crack surfaces at the instant of tailcrack development. We advocate that rotating diurnal stresses can similarly form cycloids by initiating a cycloid segment in the form of a tailcrack, thus forming a cusp. The sense of shearing that is geometrically required during cusp formation is consistent with the formation of cusps by tailcrack initiation. The formation of cycloid cusps by tailcracking is also consistent with the observation that average cycloid cusp angles are almost identical to average tailcrack take-off angles along ridge-like strike-slip faults on Europa. These results suggest that a component of opening must accompany shearing during the formation of both cycloid cusps and tailcracks along strike-slip faults, but that the mechanical development of these two features is nonetheless identical.
P31A-0960 0800h
Quantifying and cataloguing small-scale motions along lineaments on Europa
Small-scale normal and shear motions (approaching the limits of resolution; $ < $ a few 100 m) along lineaments on Europa are not well constrained. Previous work has not differentiated, quantified, and catalogued the small-scale motions along the lengths of lineaments of varying morphologies. In their characterizations, such work principally utilized rigid-block reconstructions which do not address any variations in motion along the lengths of lineaments. Also, these investigations typically did not consider the effect of relatively small amounts of fault-orthogonal motion, or the apparent offsets caused by convergence, if present. For example, the existence of lateral offsets along a lineament does not explicitly require that any strike-slip motion occurred at all as offsets could purely be the result of convergence. Using a technique which utilizes characteristic changes in the distribution of geometric relations of crosscutting features, small-scale motions can be inferred (whether strike-slip or fault-orthogonal), within the limits of image resolution. By measuring the total offset, the separation, and alpha (the clockwise angle between a lineament and a crosscut feature) for every crosscut feature along the length of the lineament (i.e., a range of alpha values), the actual motions can be resolved. Specifically, by using these measured quantities and a series of trigonometric equations, opening, convergence, actual strike-slip, or a combination of strike-slip and opening/convergence can be determined. Actual motions along lineaments become particularly apparent in graphs of alpha versus separation, which display different patterns depending on the displacement ratio (DR: the ratio of opening/convergence to strike-slip), which can be estimated from the graph. The accuracy of this technique is limited to DR $ < $ 3. If a crosscut feature is approximately orthogonal to a slipped lineament, the observable strike-slip component of motion would have been unaffected by convergence or opening, allowing the strike-slip offset to be measured precisely. Using this offset, the exact amount of convergence can be determined using the DR evident in the alpha versus separation graph. The true strike-slip offset can be determined very accurately using features that may be several tens of degrees from being orthogonal to a lineament, however, the accuracy decreases as DR increases. This technique can be applied to the analysis of lineaments of any morphologic type. Therefore, unraveling actual motions along lineaments can be used to evaluate likely DR values along Europan lineaments and also to evaluate current conceptual models for the formation of linear features on Europa, such as troughs, ridges, ridge complexes, and bands. Another benefit of this technique is that the distribution of motions along the length of a lineament can be catalogued, which may foster a better understanding of the mechanics of motions along lineaments (e.g., elastic vs. transform).
P31A-0961 0800h
Stresses Generated in Cooling Viscoelastic ice Shells: Application to Europa
A cooling and thickening viscoelastic ice shell overlying an ocean develops stresses due to two effects: thermal contraction of the ice due to cooling; and the expansion of the shell due to the ice-water volume change. The former effect generates near-surface compression and deeper extension; the second effect generates extension only. In both cases, stresses generally decrease with depth due to viscous creep. The resulting combined stresses are extensional except at shallow ($ < $1~km) depths in thin ice shells. These extensional stresses exceed 10~MPa for shells thicker than 20~km, and thus dominate all other likely sources of stress as long as shell cooling occurs. The dominantly extensional nature of the stresses may help to explain the puzzling lack of compression observed on Europa, Ganymede and other large icy satellites. In particular, the proposed mechanism suggests that Europa's extensional features are due to a thickening ice shell, in agreement with observed stratigraphic relationships and impact crater characteristics.
P31A-0962 0800h
Origin and Evolution of Castalia Macula, An Anomalous Young Depression on Europa
The Castalia Macula region on Europa was comprehensively imaged by the Galileo spacecraft on several orbits, at both local and regional resolutions and with different illumination geometries. Using the various data sets in combination allows us to map the geology, topography, and color of this area in greater detail than perhaps any other site on Europa. Castalia Macula consists of unusually dark and reddish material, most of which is confined to a broad topographic depression 350 m deep. This depression is located between two large uplifted domes 900 and 750 m high, to the north and south, respectively. The dome to the north of Castalia Macula has the highest elevation of any features yet studied on Europa. The Castalia Macula dark plains deposit covers a region about 600 km^2,$ forming a heart-shaped feature approximately~30 km in diameter. The preservation of ridges at the bottom of Castalia Macula indicates that the dark material may be less than a few tens of meters thick. However, it must initially have been fluid and deep enough to flood ridged plains and some double ridges in the middle and at the edges of the Castalia depression. It appears that dark material initially filled the depression to a certain depth but most of this material was subsequently removed via drainage, resulting in a dark stain up to the old equipotential surface. A small area of chaos existed before the formation of Castalia Macula and the domes, implying at least two episodes of deformation in this area. Superposition and topographic relationships indicate that the Castalia Macula dark plains deposit predates both the southern and northern domes, and secondary craters show that the Macula was emplaced before the impact that formed the very young crater Pwyll. Although older than Pwyll, it nevertheless appears that Castalia Macula and the domes are comparatively young, on the basis of relative albedo, color, and lack of crosscutting features. Thus Castalia Macula could provide an ideal place to sample material that has recently been erupted from the subsurface, and may have been in communication with Europa's ocean. In addition, the surface of Castalia Macula is relatively smooth and flat compared to the majority of Europa's terrain, and so could be a relatively low-risk place to set down a spacecraft. These factors combine to make Castalia Macula a very attractive site for a future Europa lander.
P31A-0963 0800h
Correlation of Regional and Global Scale Geology on Europa
Image data obtained by the Galileo spacecraft provide a global view of Europa at a scale of 1-km/pixel while regional scale (~100-m/pixel) data provided only limited coverage. Regional and global scale geologic studies in the vicinity of Manann'an crater provide detailed unit characterizations, which provide a means to compare results based on different dataset resolutions and to make extrapolations between different parts of the satellite. The study area contains extensive outcrops of chaos materials that are characterized by various degrees of disruption, ranging from complete removal of regional plains to areas comprised of outcrops of rafted regional plains and more recent extruded deposits. In addition to widespread occurrences of chaos materials, localized cryovolcanic deposits are pervasive. Results show that local cryovolcanic activity in this area is characterized by blocky deposits and low-albedo, smooth material, suggesting eruption of low-viscosity fluids onto the surface. Larger, coherent, volcanic provinces show evidence of multiple eruptive centers and episodes of activity. We identify multiple sites of upwelling, an observation that suggests crustal thinning, "melt rough" and extrusion, rapid refreezing at the surface, and significant foundering of older plains-related "rafts." In some locations, upwelling and lateral motion has resulted in the piling up of rafts with distal fracturing and faulting possibly associated with subsurface return flow. Tectonic activity is manifested by both compressional and extensional structures. A broad, low, rise that is surmounted by en echelon segments of sinuous fractures cuts across the area. We interpret this rise to have formed under regional compression. Extension-related features are manifested by a variety of morphologies ranging from single and double ridges to ridge and broad "band" complexes. Tectonic reconstruction of some of the band materials suggests processes associated with simple lateral crustal displacement to more complex activity associated with multiple episodes of activity and reactivation of older structures. The most recently identified centers of tectonic activity appear to be associated with extensive, narrow, single, through-going fractures.
P31A-0964 0800h
Measuring Strain Across Fault Zones on Ganymede
Ganymede, the largest satellite of Jupiter, is transected by numerous fault zones. Galileo images have shown more than five craters cut by these fault zones. Due to their initial near-circular shape, these craters make ideal strain markers for the surface of Ganymede. We have developed two methods for analyzing strained craters. One method is used for craters cut by a fault zone which is narrow with respect to the crater diameter, calculating strain based on the displacement of best-fit centers to intact crater rim segments. The second method, for craters which are pervasively faulted, calculates the strain based on the ellipticity and orientation of the best fit ellipse to intact crater rim segments. While previous planetary studies have examined simple extension based on the elongation of strained craters, our methods are able to calculate the relative roles of both simple extension and simple shear in the fault zones. Our results show that many of the fault zones that morphologically resemble tilt-block normal faults exhibit 50% to 180% extensional strain. Other fault zones which exhibit subdued morphology consistent with narrow graben show more modest extensional strains of 5% to 15%. Two of the craters have well over a kilometer of offset due to simple shear. The crater-based strain measurements allow us to test other strain measurement techniques on Ganymede fault zones. In a region of grooved terrain with Galileo stereo coverage, extensional strain of 50% was estimated based on assumptions about fault geometry from the images. Directly comparing strain measurements from craters to measurements from fault geometry allows us to refine our fault geometry assumptions, with the goal of being able to estimate strain wherever we have high resolution image data for Ganymede. If high extensional strains are indeed common on Ganymede, then the lack of contractional structures on Ganymede becomes more problematic. Understanding the amount of strain represented by fault zones on Ganymede, and their evolution through time, is critical for understanding the underlying driving mechanisms of Ganymede tectonics.
P31A-0965 0800h
Tectonic history of the icy satellites: Discussions on the internal evolution and its surface manifestation
Most eminent feature of tectonic activity on the icy satellites is extensional signatures, which have been formed by tensional stress. The examples are band-shaped terrain on Europa, and groove on Ganymede. Various origins for the stress have been discussed. The plausible candidates are solid state convection, tidal deformation, and volume change associated with internal structural evolution. Convective stress (e.g. Squyres and Croft [1986]) and tidal stress (e.g. Greenberg et al. [1998]) have been evaluated as an order of 0.1 MPa at maximum, which is insufficient to induce the surface fracture. Among the source of the internal volume change, differentiation and thermal stress (due to temperature change) could have induced large volume expansion but these must have been exerted only at very early stage of the satellites. So the traces of tectonics would be erased, and these processes are not likely to contribute to the present features. Here we have focused on the possibility of phase changes (i.e. the solidification of liquid water) as a source of the internal volume change. The volume increase due to the solidification of liquid water to the ice-Ih is as large as about 10%, which is much larger than that due to the temperature change. To evaluate stress associated with the solidification of liquid water, it is necessary to determine thermal/structural evolution of the satellites. In this work, the evolution of the internal structure for Europa and Ganymede case have been numerically solved by considering heat transfer based on the mixing length theory. Starting from the water-covered silicate-iron core, the solidification of the liquid layer proceeds from the surface and the bottom of the layer, which is treated as Stefan problem. In the Europan case, the solidification only proceeds from the surface because of its small size and negative pressure gradient of the melting curve. Associated with this structural evolution, we have evaluated the stress accumulation by this phase change. The icy lithosphere is modeled as a viscoelastic medium. The stress accumulation is governed by competition between solidification rate and viscous relaxation. We have found the distinct difference in history of the internal structure and the surface stress between Europa and Ganymede. In Europa, the ice shell grows slowly and the internal liquid layer survives until today. Slow solidification rate is mostly due to the negative slope of the melting curve of ice Ih. Phase change from liquid water to ice Ih can create sufficient tensile stress to induce the surface fracture. In Ganymede, the liquid layer solidifies rapidly due to growth of ice shell and high-pressure ice layer, so the liquid layer has disappeared within an order of 0.1 Gyr. Phase change from liquid water to high-pressure ice with volume reduction is dominant, so compressional stress is generated at the surface. Surface tectonics of Ganymede, which has formed grooved terrain, may have been controlled due to another event rather than the solidification of liquid layer.
P31A-0966 0800h
Layering and Double-Diffusion Style Convection in Europa's Ocean
We assess the effects of fluid composition and depth-dependent hydrostatic pressure on dynamics in Europa's ocean for aqueous Na2SO4 and MgSO4. We observe a salinity- and pressure-dependent check on buoyancy in putative upwellings, which may act as mechanism for storing heat in the ocean's base. For either sodium or magnesium, a small excess in salinity of an initially buoyant parcel of water, even for low average ambient salinity, causes upwellings to lose buoyancy before reaching the base of the overlying ice. The result is a two-layer convecting system with a characteristic lifetime dependent on the properties of Europa's ocean, including the balance of heat flow through the system and details of pressure effects on thermal expansion and volumes of mixing. When volume of mixing is neglected plume rise remains dependent on temperature and pressure effects alone. Stratification develops as bottom water continually acquires heat and salt. The added mass of the salt dominates over the thermal expansion. As the rise in temperature becomes sufficient to overcome the salinity effect, upwellings rise to a terminal height determined by the pressure dependence of thermal expansion of the fluid, and by differences in the fluid's temperature and salinity. This regime is similar to double-diffusive convecting systems observed in the Red Sea and Lake Vanda over the last forty years. In the Red Sea, boundary layers separating convecting zones have been observed to move upward as the lower layer acquires heat and salt. By analogy using reasonable parameters for Europa's ocean, we estimate a time scale on the order of 10 Myr for the upward progress of a lower convecting layer saturated with MgSO4, which could correlate with recently inferred change in surface alteration style over the last 70-80 Myr.
P31A-0967 0800h
Mass Anomalies on Ganymede
The discovery of mass anomalies on Ganymede was reported this summer (Anderson et al., Science 305, 989 (2004)). We report here on a more detailed characterization of the source of the anomalies. In order to reduce the noise evident in the Doppler residuals previously used, we applied a variable-width Gaussian filter to the time series. The filter width in the time domain increases with the spacecraft altitude, reducing the noise before and after closest approach. The smoothed Doppler data were numerically differentiated and the resulting accelerations along the line of sight were fit with a multiple point-mass model. The variable-width filter reveals a previously obscured positive acceleration feature about 300 s before closest approach. Rather than two or three mass points as reported in Anderson et al., we find that five mass points provide a much improved fit to the data, including the new acceleration feature before closest approach. Two of the five masses are near the previous masses from the two-point fit, and are in good agreement with their mass values. There is a positive anomaly at about 60$^{o}$ north latitude and a negative anomaly at about 24$^{o}$ north latitude. We conclude that the two-point fit reveals two major anomalies on Ganymede, but misses three more revealed by the five-point fit. Further, the mass anomalies can be divided into two groups. Three of the five masses could indicate a single broad anomaly under the outgoing flyby trajectory centered roughly at 45$^{o}$ north latitude and 18$^{o}$ west longitude. The other two masses could indicate a single extended anomaly under the incoming trajectory centered roughly at 20$^{o}$ north latitude and 173$^{o}$ west longitude. We also include results on placing the five masses at different depths from zero to 1450 km below the surface. A good fit is obtained at any depth from surface to rock-ice interface at about 800 km depth, but the fit deteriorates at greater depth. It is highly unlikely mass anomalies exist within Ganymede's ice shell. We prefer either the near surface or the rock-ice interface. The rock-ice interface is attractive based on rigidity arguments, and the suggestion of two major extended anomalies is even more striking at greater depth. In order to fit the acceleration data, the anomalies must be six or seven times more massive at the 800 km depth than at the surface.
P31A-0968 0800h
Io's Tidal Dissipation and Longitudinal Drift
Previously, Io's tidal heating has been calculated assuming synchronous rotation and a Keplerian orbit. However, Io's orbit is significantly non-Keplerian (due to Jupiter's oblateness), and Io's rotation may include significant departures (librations) from the synchronous state, which introduce additional terms to the tidal potential. To quantify the effect of these additional tidal modes on the dissipation within Io, the dynamical evolution of Io has been studied by coupling the orbital dynamics to rotation and tidal deformation. Io's heat flow is computed utilizing both a simplified (Q parameterization) model of dissipation, and a complete solution of the tidal deformation in a layered, viscoelastic body. We will compare the Keplerian, synchronous case with the unrestricted case to determine the sensitivity of the inferred internal structure of Io to the dynamical assumptions. We will also study the phenomenon of longitudinal drift or asynchronous rotation of Io. Longitudinal drift can occur in a deformable body because a degree-two deformation combined with a rotation can leave the shape of the body unchanged but result in a slow drift of surface features relative to perfect synchronous rotation. Orbital eccentricity causes longitudinal drift only in one direction, inclination can induce drift in either direction. We have determined that the drift rate scales with the square of the orbital eccentricity and is controlled by the relaxation time (or viscosity) of the body. We carry out a systematic numerical and analytical study to quantify the rate of longitudinal drift as a function of orbital parameters and internal structure. We will also look for the signature of longitudinal drift by analysis of the distribution of mountains on Io as a function of longitude. If volcanic activity destroys mountains (as suggested by the anti-correlation of volcanoes and mountains on Io), then we should observe progressive degradation of the mountain population as the longitudinal drift carries them across the sub- and anti-jovian volcanic regions (where tidal heating is a maximum).
P31A-0969 0800h
Europa's Alfv\'en Wing: Shrinking and Displacement by an Induced Magnetic Field
An Alfv\'en wing (AW) is created by the interaction of a conducting body with a flowing magnetized plasma. In this presentation we investigate the AW created by the moon Europa in the Jovian magentosphere. During its 7 years of duty the Galileo spacecraft has visited Europa 11 times, out of which 4 flybys were crossing the Europan AW. We have studied the properties of the AW using the magnetic field data, with the moon embedded in different background magnetic field and plasma parameters. We have found the predicted shrinking and displacement of the AW by the induced magentic field in Europa. We also show that the characteristics of the AW can be used to get information about the local plasma parameters.
P31A-0970 0800h
Simple Particle Modeling of the Varying Structure of the Io Inner Torus
Observations show that the inner Io torus has both short and long term variations in its structure and position. Generally the dusk side of the torus is closer to Jupiter than the dawn side, and the radial location of various features in the inner torus show longitudinal variations with time. This highly dynamic inner torus structure is likely due to several magnetospheric effects, most importantly mass loading at Io. Large volcanic events can alter magnetospheric conditions and change the radial configuration of the inner torus. Using simple particle modeling of ion pickup at Io, we explore the variations in inner torus structure that can result from changing pickup conditions at Io as well as specific sources of mass loading on Io (i.e. known volcanoes).
P31A-0971 0800h
Mapping the Velocity Distribution of Neutral Sodium Near Io
We report on two dimensional velocity maps of neutral sodium near Io obtained on 20 November, 1999, using the National Solar Observatory McMath-Pierce stellar spectrograph. The stellar spectrograph was utilized in high spectral resolution (R = 150,000) mode with a 10x10 element image slicer array with arc second spatial resolution. The sodium emission was uncommonly bright on this night, which, combined with excellent seeing conditions, allowed a rare opportunity for us to generate images of the distinctly different velocity populations of sodium for that night. We observed the changes in these populations over a period of about three hours following emergence of Io from eclipse by Jupiter. We will present line profiles and imaging from this night that show several distinct velocity resolved populations at different locations near Io, and discuss the mechanisms that may give rise to them. These observations were part of our continuing observations of the increases observed in sodium emissions following emergence of Io from eclipse, and we will summarize the current status of these observations. This work was supported by the NASA Planetary Astronomy Program and the National Solar Observatory, Tucson, Arizona.
P31A-0972 0800h
Impact Craters on Jupiter's Icy Moons as Astrobiological Targets
Impact craters are prime astrobiological targets on icy moons with possible subsurface oceans. Craters are natural probes to subsurface materials; hence may sequester biologic signatures such as whole organisms, fossils, biochemicals, biomarkers, and biotextures. Craters expose a stratigraphic record. The closer to the crater center, the more deep-seated is the excavated material. All craters provide samples of the crust in the walls, overturned ice, and ejecta. Larger craters excavate deep interior ice in the central peaks, which may arise at the interface between solid ice and liquid water. The largest craters possess concentric rings, and flat floors that may be frozen water from under the icy shell that might expose information about the water column. Craters immediately deliver sub-surface materials to the surface, in contrast to endogenic processes (e.g. Europa) that likely operate over days to thousands or more years. The slower endogenic processes allow any extant life proactively to migrate from their habitats to other marginal regions or to die and degrade. Craters also offer unique habitats. A sub-surface "lens" of melt remains beneath the crater floor for up to thousands of years after impact. Life could rapidly exploit and abandon these habitats as they form and disappear, so these sub-surface melt-lenses may serve as rich - albeit temporary - locales of biological activity or repositories for fossils as they freeze. Although Europa displays the best evidence for a global ocean of the three icy Galilean satellites, Ganymede and Callisto also may have liquid water beneath their icy crusts. Ganymede, at least, bears evidence of an extensive geological history, which may have provided energy and habitats for biological processes. Though briny oceans (if they exist) are deeper beneath the icy crusts on Ganymede and Callisto than on Europa, the investigative principle largely remains the same. However, excavation to the deeply buried water requires larger and less frequent, and thus generally older, impacts. The surfaces of the larger, older craters have been processed by subsequent impacts, sputtering, and/or tectonic activity, all of which mask clues to any excavated biological signatures.
P31A-0973 0800h
Chemical Kinetics of Polycyclic Aromatic Hydrocarbons in Comet Impacts
Polycyclic aromatic hydrocarbons (PAHs) are stable, robust organic compounds that would have been an important constituent of the early atmospheres of terrestrial planets. These strongly-bound molecules readily absorb ultraviolet light and may play a role in aerosol formation. PAHs are one of the predominant carriers of carbon in interstellar space, after CO. They are common in carbonaceous chondrites, and quite likely in comets as well. Impacts of volatile-rich planetesimals such as carbonaceous chondrites and comets would have been common during the late stages of planet formation. Theoretical studies of impact chemistry typically assume that the chemical composition of the post-impact material is given by thermodynamic equilibrium at 2000 K. These calculations also typically ignore the formation of aromatic compounds because the closure of the first aromatic ring is kinetically inhibited, although thermodynamically favorable at the temperatures and pressures of an impact fireball. Do the PAHs present in a comet or asteroid survive impact? If so, how are these PAHs modified during impact? To address these questions, we model the chemical kinetics of PAH survival, formation, growth and destruction within a parameter space consisting of impact fireball cooling timescales, pressures, temperatures, C/O ratios and other factors. The chemistry of PAHs has been well studied under conditions present in plug flow reactors and sooting flames (P $\approx$ 1atm, T$\geq$ 1000 K). We hope that our results will motivate more experimental investigation of reaction mechanisms and rate coefficients for a broader range of temperatures and pressures than those heretofore studied for industrial applications. This work has been supported by the NASA Astrobiology Institute's Virtual Planetary Laboratory and the Institute for Geophysics and Planetary Physics at Lawrence Livermore National Laboratory.
P31A-0974 0800h
Estimates of Production Rates for Comets C/2001 Q4 (NEAT) and C/2002 T7 (LINEAR) Derived from Polar UVI Observations
During April and May of 2004, the Ultraviolet Imager (UVI) onboard the Polar spacecraft was in the unique position to observe two comets near their perihelion: C/2001 Q4 (NEAT) and C/2002 T7 (LINEAR). Using its suite of four far ultraviolet filters, UVI is able to observe atomic oxygen emissions at 130.4 nm and atomic carbon emissions at 156.1 nm and 165.7 nm. The photon flux is directly related to the production rates for these atoms. From the oxygen and carbon production rates, we are able to estimate the production rates of CO, OH, and water. Initial analysis shows that both comets are very dim in the far ultraviolet and are not easily detected by UVI. We are able to derive upper limits to the production rates. To our knowledge, this is the first report of observations of these comets in the far ultraviolet region.
P31A-0975 0800h
CASIM : A Multi-fluid MHD Cometary Atmosphere Simulator
We present new results of our Cometary Atmosphere Simulation (CASIM) code that is presently under development to model the interaction of a cometary atmosphere with the solar wind. Several high-resolution 2-D simulations for a Halley-type comet are shown. In the current simulations we investigate more precisely the evolution of the interaction between the cometary atmosphere and the solar wind starting from the early stages of the comet outgassing ($\sim$5. a.u.) to the peak of its activity ($\sim$1.5 a.u.). Our code is based on the solution of the multi-fluid MHD equations using an efficient adaptively refined cartesian mesh solver that provides a very high resolution over a large space domain (simulation domain $\sim$ 10^8 km, highest resolution $\sim$ 25 m). The multi-fluid approach we chose leads to a more accurate representation of the cometary atmosphere since additional details of the interaction between the neutral gas and the plasma, and their resulting structures are revealed. In particular, the multi-fluid representation of the ion population gives an improved view of the coupling between heavy and light ions and the resulting coma boundaries at different spatial scales. This research is sponsored by NASA and the National Academies.
P31A-0976 0800h
Using a Physico-Chemical Coma Model to Understand Observations of Comets
Analyses of observations of the comets, Hale-Bopp and Hyakutake, are performed using a global coma model. The relevant physico-chemical processes are identified in order to provide the framework within which observations of these comets can be understood and inferences about their composition can be made. The latest results detailing these processes are discussed concentrating on the collision-dominated inner coma and including the temperature and velocity structure and photo- and gas-phase chemistry (e.g., gas and electron energetics, electron impact reactions). Special consideration is given to the determination of parent species (e.g., C$_2$H$_2$, C$_2$H$_6$, C$_3$H$_4$) from the plethora of molecules and atoms seen in recent comets, concentrating on observations of C$_2$, C$_3$, S$_2$, and NS. The model is useful for identifying important physical and chemical processes for analyzing observations and in situ measurements of comets, to plan spacecraft encounters with these objects, and to learn lessons from past investigations for improving future models. We acknowledge funding from the NSF Planetary Astronomy and SwRI Internal Research programs.
P31A-0977 0800h
{\it Deep Interior}: Radio Reflection Tomographic Imaging of Earth-Crossing Asteroids
Near-Earth Objects (NEOs) present an important scientific question and an intriguing space hazard. They are scrutinized by a number of large, dedicated groundbased telescopes, and their diverse compositions are represented by thousands of well-studied meteorites. A successful program of NEO spacecraft exploration has begun, and we are proposing {\it Deep Interior} as the next logical step. Our mission objective is to image the deep interior structure of two NEOs using {\bf radio reflection tomography} (RRT), in order to explore the record of asteroid origin and impact evolution, and to test the fundamental hypothesis that these important members of the solar system are rubble piles rather than consolidated bodies. {\bf Asteroid Interiors.} Our mission's RRT technique is like a CAT scan from orbit. Closely sampled radar echoes yield volumetric maps of mechanical and compositional boundaries, and measure interior material dielectric properties. {\bf Exteriors.} We use color imaging to explore the surface expressions of unit boundaries, in order to relate interior radar imaging to what is observable from spacecraft imaging and from Earth. Gravity and high fidelity geodesy are used to explore how interior structure is expressed in shape, density, mass distribution and spin. {\bf Diversity.} We first visit a common, primitive, S-type asteroid. We next visit an asteroid that was perhaps blasted from the surface of a differentiated asteroid. We attain an up-close and inside look at two taxonomic archetypes spanning an important range of NEO mass and spin rate. Scientific focus is achieved by keeping our payload simple: {\bf Radar.} A 30-m (tip-to-tip) cross-dipole antenna system operates at 5 and 15-MHz, with electronics heritage from JPL's MARSIS contribution to Mars Express, and antenna heritage from IMAGE and LACE. The 5-MHz channel is designed to penetrate $ > 1$ km of basaltic rock, and 15-MHz penetrates a few 100 m or more. They bracket the diversity of solar system materials that we are likely to encounter, and are richly complementary. {\bf Imaging.} We fly a 100% redundant camera whose primary function is to provide accurate navigation and geodesy in support of radio reflection tomography. This camera also yields stereo color imaging for geology and RRT-related compositional analysis. We image the interiors of two frequently Earth-crossing asteroids: {\bf 1999 ND43} is a yet-unnamed S-type. Its diameter ($~$0.5 km) and spin period (11.4 hr) are known, from which initial mapping plans are derived. It is probably a very common asteroid type. {\bf Nyx} is one of the best-examined asteroids of its size ($~$1 km). Its faster spin period (4.4 hr) and shape are known. A V-type spheroid with distinct basaltic composition, Nyx is either a differentiated small planet with crust, mantle and core, or more likely, a fragment or assemblage of fragments ejected from the crust of a larger planetoid.
P31A-0978 0800h
Spitzer/IRS Observations of Asteroids, Centaurs, and Kuiper Belt Objects
We will present thermal emission spectra of asteroids, Centaurs, and Kuiper Belt objects from the Infrared Spectrograph (IRS) on the Spitzer space telescope. To date, 15 of the 42 asteroids and 11 of the 17 KBOs/Centaurs in our programs have been observed. The asteroids observed so far sample the S, C, P, D, and M taxonomic classes and include Near Earth, Main Belt, and Trojan asteroids. Emissivity spectra are created by dividing the measured spectral energy distributions (SEDs) by models of the thermal continua. We use either a standard thermal model or a more advanced thermophysical model (includes thermal inertia, obliquity, and surface roughness), as applicable for each asteroid. These models are able to describe the SEDs very well. The asteroid spectra typically include the full range of IRS (5.2 to 38 $\mu$m) and have spectral resolutions of 64-128 and $\sim$600 for the low and high resolution modules, respectively. The most striking feature in these asteroid spectra is an emission plateau that extends over the range $\sim$ 9.0 - 11.5 $\mu$m. This feature is strongest in the low albedo Trojan asteroids, but is also present, though weaker, in some (but not all) of the other asteroids. We compare these new data with laboratory spectra of meteorites and minerals. The KBO/Centaurs are colder and farther away, so they are fainter and their SEDs peak at longer wavelengths than the asteroids. Accordingly, most of these are only observable with the longer wavelength IRS modules ($\sim$15 - 38 $\mu$m with R $\sim$ 64-128), and the resulting sensitivity is lower. For some of the KBO/Centaurs, the IRS data provide the first constraints on size and albedo, and we find albedos are commonly higher than anticipated. We will discuss potential implications of these new data for surface composition and physical and thermal properties of these objects.