P23A-1348
A Hybrid Simulation of the Plasma Flow Around Io Coupled to a Multi-species Chemistry Model of Io's Local Interaction.
The Galileo flybys of Io have provided a number of observations that are still not completely understood i.e. the strong asymmetry of the flow and the high plasma density in the center of the wake. Past studies have tackled the problem of the local interaction of Io's corona with the plasma in the torus using different complementary approaches, each of them involving some simplifications: MHD modeling with a parameterization of the sources of ionization and collision (Linker et al., 1998; Combi et al.,1998), 2-fluid approach assuming a constant magnetic field (Saur et al., 1999) and multi-species chemistry assuming a prescribed flow of the plasma around Io (Dols et al.,2008). We couple a hybrid model of the plasma flow around Io (kinetic ions and fluid electrons) and a multi-species chemistry model (chemistry of SO2, S and O) to obtain a fully self-consistent model of the local interaction. We present results of the flow calculation, plasma production and charge exchange rates, pickup current as well as contribution of the electron beams detected close to Io and compare these results with the Galileo flyby observations.
P23A-1349
Topographic Analysis of Europa's Ridges
Ridges are the most ubiquitous surface feature on Europa. Here we examine double ridges that have two parallel, raised flanks with a continuous axial trough (referred to as a ridge pair). Characterizing ridge edifices may help us better understand the processes that drive ridge formation and evolution. Because there is no global elevation map for Europa, topography was derived from high resolution (18 to 181 m/pixel) combined stereographic and photoclinometric images to create 265 topographic profiles across 24 features of interest. Ridge topography was examined across 22 ridge pairs (12 with apparent lateral offsets) and 2 ridge complexes, in the Bright Plains, Conamara Chaos, Cilix, Argadnel Regio, Rhadamanthys Linea, and the E17DISSTR01 (northwest of Katreus Linea) areas. Topographic profiles are oriented perpendicular to the strike of each ridge pair to capture height and width variations as well as to highlight asymmetry between adjacent ridges. We characterize ridges using ridge height and width (vertical and horizontal distance from the base of the ridge flank to the ridge peak), average ridge height (average of the individual peaks in a ridge pair), total ridge width (distance between the ridge's outer flanks), and peak-to-peak (PTP) width (distance between peaks in a ridge pair). Height-to-width ratios of 44 individual ridges fall within a wide range that never exceeds 0.53, implying a maximum outer slope of 28 degrees, slightly less than the suggested angle of repose of loose granular ice (~34 degrees). Most slopes are much gentler, between 10 and 20 degrees, which are significantly smaller than those presented in a prior study undertaken early in the Galileo imaging mission. In fact, we have found that ridges can be very wide and low with outer slopes of only a few degrees, implying that very few ridge morphologies are likely to be controlled by granular flow processes down their outer slopes. The ratio of average ridge height to total ridge width has a lower limit of 0.05 that corresponds to the widest ridge examined (4.05 km). Conversely, the upper limit of this ratio is 0.13, corresponding to the highest ridge examined (362 m). This raises the possibility that tall ridges modify their morphology through time through some form of gravitational collapse, thus decreasing the height, increasing the width, and decreasing the ridge slopes. The low slopes and overall low average height to total width ratios suggest predominantly time-dependent viscoplastic gravitational collapse. Variability between ridges may be related to the mechanisms driving ridge development (whether dilational, contractional, or shear heating), which are likely to influence the rate of ridge construction as well as the temperature (and hence rheology) of the icy material involved. We also observe a distinct upper limit of 0.58 for the ratio of average ridge height to PTP width, suggesting that once an active ridge exceeds a certain height, the ridge peaks begin to geomorphically migrate apart in order to maintain a limiting slope of the inner and outer flanks through gravitational collapse (whether it be granular flow or viscoplastic). Lower ratios of average ridge height to PTP width may indicate underdeveloped ridge heights but may also be a sign of dilation across a ridge, causing a tectonic increase in PTP width. Finally, variability in ratios of average height to total width along individual ridges indicates that some ridge pairs may have evolved differently along their lengths, an effect partially related to changes in ridge orientation along the observable length and associated variability in ridge kinematics.
P23A-1350
Characterizing the Morphology of Chaos on Europa
Chaos terrain represents a type of feature unique to Europa and covers approximately one third of the satellite's surface. Two endmember models have been proposed for its formation: one suggests it forms through melting of the surface by liquid water from the subsurface ocean; the second suggests that chaos forms from the upwelling of thermally or compositionally buoyant diapirs. The formation of chaos disrupts preexisting surfaces and it has been observed that the magnitude of this disruption varies from one feature to another. Based on the morphological characteristics of different prominent and well-imaged regions of chaos (i.e., Conamara and Murias), this feature-type has been subdivided into endmember classes (Greeley et al., 2000). Conamara chaos is defined by kilometer-scale blocks of preexisting ridged plains material that have been tilted, translated and rotated with respect to one another within a generally lower-albedo matrix of hummocky material. Approximately 60% of the preexisting terrain has been replaced with or converted into matrix material (Spaun et al., 1998). In contrast, Murias chaos appears to be comprised purely of matrix material, with no hint of blocks of preexisting material or tectonic structure (Figueredo et al. 2002). The morphological characteristics of these two types of chaos have been commonly used to establish criteria for examining formation models. However, additional distinct morphologies of chaos have been proposed and the abundance/distribution of chaos morphologies is not well known. Understanding the importance of these various morphologies could provide valuable insight regarding the formation and evolution of this unique feature-type. To that end, we have systematically mapped the global distribution of chaos using image data at resolutions from ~1 km/pixel to 10 m/pixel and covering a range of viewing geometries. From this, we have categorized variations in morphology using the relative abundance of plates within a given feature as a defining characteristic and, using this map, we examine potential trends in the distribution of chaos morphologies.
P23A-1351
Examining topographic variability within chaos terrain on Europa
Chaos terrain is a unique and prevalent surface feature on the Galilean satellite Europa that forms as a result of the disruption of subcircular regions of the satellite's surface. Evidence suggests that these features are endogenic and that they form via processes involving the interaction of a mobile substrate with a brittle surface. Based on the morphology and relative topography of prominent and well-imaged examples of chaos terrain, models have been proposed suggesting that the mobile substrate could be either liquid water or ductile ice. Using a digital elevation model (DEM) of Conamara Chaos, Schenk and Pappalardo (2004) alluded to the presence of several prominent domes within the margins of the feature. They concluded that this was best described by a formation mechanism for chaos involving the diapiric upwelling of a ductile ice substrate, with the coalescence of several individual diapirs in the shallow subsurface. To explore this result in more detail, we use Fourier analysis to examine the long-wavelength components of the topography of several regions of chaos utilizing DEMs of Europa's surface produced utilizing stereo-controlled photoclinometry. Through this analysis, we identify the presence, size, and distribution of domes within the boundaries of chaos terrain and, with this information, examine how topographic variability within chaos terrain can be used to constrain proposed formation mechanisms for this unique feature-type.
P23A-1352
Shape and size distribution of chaos areas on Europa
Chaos terrain is ubiquitous on Europa's surface, but not randomly distributed. The global distribution of chaos areas shows a significant concentration between 30° N and S latitude, decreasing dramatically at higher latitudes. The low-latitude clustering is not an artifact of recognizability, as there is a greater proportion of images with high solar incidence angle (low light) at higher latitudes. Clustering is especially marked in context of the few but vast regional chaos tracts (>15,000 km2) that occupy a substantial proportion of the equatorial region: i.e. the low latitudes have not only greater numbers but much greater areal chaos coverage. Apex-antapex asymmetry is difficult to evaluate because the Galileo longitudinal coverage is so poor; but comparison of the image swaths that follow great circles across the leading and trailing hemispheres respectively shows greater numbers of chaos areas on the leading side. In spite of the equatorial location of a few vast chaos tracts, there is no apparent relationship between chaos area size and latitude. Chaos area outlines vary from smoothly circular to extremely jagged: the irregularity index ranges from 2- 270% (based on the ratio between measured chaos area perimeter and the circumference of a circle of equal area). There is a range of shapes in all size brackets, but smaller chaos areas on average have simpler, more equidimensional shapes, and edge complexity increases for larger chaos areas. Chaos areas of ~10 km equivalent circle diameter (ECD) have outlines that are 4-90% irregular, ones ~50 km ECD are 15-180% and those >100 km ECD are 35-270% irregular. In general, chaos areas with higher irregularity indices also have a higher raft:matrix ratio. These results, while preliminary, are consistent with experimental evidence suggesting an impact origin for some chaos terrain on Europa. In particular, the relationship between shape and size parallels the results of impact experiments into ice over water, in which lower-energy impacts produce small, circular bullet-holes with few or no rafts; and higher-energy impacts generate wide-field fragmentation of the ice, producing large and highly irregular openings with abundant floating crustal blocks.
P23A-1353
Exospheric signatures of alkalis abundances in Europa's regolith
Sputtering of Europa's surface material by Jupiter's magnetospheric plasma results in a strong coupling between the moon's exosphere and its surface content (Johnson 2001). In particular, the presence of alkalis in Europa's exosphere (Brown and Hill 1996, Brown 2001, Brown 2004, Leblanc et al 2005; Cassidy et al. 2008), sputtered from potential surface salts embedded in the icy regolith, supports the hypothesis of a geologically young surface associated with a subsurface ocean. We have carried out test-particle simulations of the ejection of sodium and potassium atoms from the icy matrix of Europa, by both magnetospheric ions and electrons sputtering and desorption stimulated by UV solar photons (hereafter noted PSD). We show that a minimal surface source term of sodium of 3x106cm-2.s-1 is required to produce the average emission intensities observed at different positions of Europa's orbit around Jupiter, in good agreement with Leblanc et al 2002. We also obtain that PSD alone can not account for the emission brightness variations reported in Leblanc et al (2005), as suggested earlier. Instead we propose that a plasma transient corresponding to a global increase of the flux of particles impacting the surface by a factor of about 8 compared to the ambient energetic ions and electrons flux during 10 hours would be sufficient to explain the observed emission enhancement (Cipriani et al 2008). At altitudes typically lower than 500km, we observe that the surface content produces clear asymmetries of the exospheric density from trailing to leading hemispheres consistent with the Cassini observations of Europa in eclipse (Cassidy et al. 2008). We investigate those asymmetries and relate them to signatures of surface processes such as PSD, or surface inhomogeneities and density gradients of sodium. Of particular importance is the estimated Na/K ratio at Europa, whose estimated values close to 20 support the hypothesis of endogenic processes releasing sodium bearing materials at Europa's surface (Johnson, 2001). Our model of Europa's exosphere and surface content, constrained by previous observations, is also used to study variations of the Na/K ratio in Europa's exosphere with respect to surface inhomogeneities. This is carried out in order to test the feasibility of characterizing surface-composition from an orbiter.
P23A-1354
The EJSM Jupiter-Europa Orbiter: Planning Payload
In the decade since the first return of Europa data by the Galileo spacecraft, the scientific understanding of Europa has greatly matured leading to the formulation of sophisticated new science objectives to be addressed through the acquisition of new data. The Jupiter-Europa Orbiter (JEO) is one component of the proposed multi-spacecraft Europa Jupiter System Mission (EJSM) designed to obtain data in support of these new science objectives. The JEO planning payload, while notional, is used to quantify engineering aspects of the mission and spacecraft design, and operational scenarios required to obtain the data necessary to meet the science objectives. The instruments were defined to demonstrate the viability of meeting the measurement objectives, performing while in the background radiation environment, and the ability to meet stringent planetary protection requirements. The actual instrument suite would ultimately be the result of an Announcement of Opportunity (AO) selection process carried out by NASA. The JEO planning payload consists of a notional set of remote sensing instruments, fields-and-plasma instruments, and both X-band and Ka band telecommunications systems which provide Doppler and range data for accurate orbit reconstruction. For JEO, the sensor portions of the instruments are located on the nadir facing deck of the spacecraft while a shared chassis houses the electronics portion of the instruments making optimal use of radiation shielding mass. A spacecraft supplied 10 meter boom is deployed for use by the JEO Magnetometer. All instruments are co-aligned and nominally nadir pointing for simplification of spacecraft operations. Instrument articulation required for target motion compensation, limb viewing or other purposes will be implemented within the instrument. Spacecraft telemetry and telecommand interfaces are nominally Spacewire for high-bandwidth instruments and Mil-Std-1553 for low-bandwidth instruments. Instrument power is provided by a 28 volt bus.
P23A-1355
Current Status of the EJSM Jupiter Europa Orbiter Flagship Mission Design
NASA and ESA have embarked on a joint study of a mission to Europa and the Jupiter system with orbiters
developed by NASA, ESA, and possibly JAXA. An international Joint Jupiter Science Definition Team (JJSDT)
is defining the science content for the Jupiter Europa Orbiter (JEO) mission study run by NASA and for the
Jupiter Ganymede Orbiter (JGO) mission study run by ESA. Engineering teams for both missions are working
closely with the JJSDT to define mission concepts that optimize science, cost, and risk. The NASA-led JEO
mission addresses a scientifically rich subset of the complete EJSM science goals and is designed to stand
alone or in conjunction with the ESA-led JGO. This paper focuses on the NASA-led JEO mission and will
describe the concept in the context of a standalone mission.
An orbital mission to Europa is driven by the desire to investigate an astrobiological archetype for icy satellite
habitability, with a putative warm, salty, water ocean with plausible energy sources. Additionally, JEO will
explore the Jupiter system to better understand how Europa's possible habitability is related to the formation
scenario of the other Jovian satellites. The JEO mission will perform 2.5–3 years of Jupiter system science,
including encounters with Io, Ganymede and Callisto, before insertion into orbit around Europa for a
comprehensive set of science campaigns lasting for up to one year. This paper will highlight the JEO mission
design and implementation concept. The work reported was sponsored by the National Aeronautics and
Space Administration.
http://opfm.jpl.nasa.gov
P23A-1356
Current Status of the Jupiter Europa Orbiter (JEO): Science and Science Implementation
The Jupiter-Europa Orbiter (JEO) is one component of the proposed multi-spacecraft Europa Jupiter System Mission (EJSM). The overarching goal of JEO is to explore Europa to investigate its habitability. Europa is believed to shelter an ocean between its geodynamically active icy shell and its rocky mantle, where the conditions for habitability may be fulfilled. With a warm, salty, water ocean and plausible chemical energy sources, Europa is the astrobiological archetype for icy satellite habitability. It is also a geophysical wonderland of interrelated ice shell processes that are intimately related to the ocean and tides, and of complex interactions among its interior, surface, atmosphere, and magnetospheric environments. A mission to Europa has been studied for a decade and has strong links to and recommendations from NASA reports. The conditions at Europa are well-understood, and JEO is prepared for the radiation environment at Europa. Europa science is mature, and hypotheses are well-formed. Five broad investigations have been defined to address the overarching goal: the Ocean, the Ice Shell, Chemistry, Geology and the Jupiter System. Measuring Europa's tides provides a simple and definitive test of the existence of an internal ocean – and the ocean and ice shell can be studied and characterized. Composition and chemistry form the linkages that enable understanding Europa's potential for life and habitability in the context of geologic processes, probe the interior structure, and record the evolution of the surface under the influence of internal and external processes. The search for recent or current geologic activity is important for understanding the origin of landforms, and especially significant for understanding Europa's potential for habitability. Understanding the Jupiter system as a whole is critical for placing Europa in its context as a member of the Jovian satellite system and for understanding the origin and evolution of the system, including Jupiter.
P23A-1357
How Can My Spacecraft Land in an Ice Volcano? and other puzzles in planetary protection
Current missions to the outer planets are returning exciting data about some icy bodies that support
hypotheses suggesting liquid water may be present within, and perhaps even near the surfaces, of objects
previously assumed to be frozen and inert. Ongoing mission concept studies are developing plans for new
spacecraft that would investigate some of these objects in detail, in part motivated by the speculation that
where there is water, there may be life. When investigating places where we will search for life, it is very
important to ensure that spacecraft are both clean and sterile before launch, since it would be a distinct
embarrassment to 'discover life' that is subsequently recognized to be something the discovering spacecraft,
or a previous one, had brought from Earth.
Efforts to prevent this from happening have been in place since the beginning of the space age, and the
activity is termed planetary protection. NASA's planetary protection policy sets limits on the contamination of
extraterrestrial bodies by terrestrial microorganisms that are intended to minimize or prevent contamination
resulting from spaceflight missions. Requirements for permissible contamination of specific target objects are
set and refined based on the best scientific advice available at the time the mission is planned. In 2000, the
Space Studies Board of the National Research Council released a study that provided recommendations on
preventing the forward contamination of Europa. In 2007 the Planetary Protection Subcommittee of the
NASA Advisory Council advised NASA to adopt similar requirements for all liquid water bodies that might be
present or induced by spacecraft on or within icy moons and other icy bodies. The Europa requirement that
was adopted by NASA uses a probabilistic approach, such that spacecraft sent to a location potentially
containing liquid water must demonstrate a probability less than 1x10-4 per mission of contaminating any
liquid water body with one single viable terrestrial organism. A number of factors enter into the equation for
calculating this probability, including at least bioload at launch, probability of survival during flight, probability
of reaching the surface of the body, and probability of surviving to encounter a liquid water body. This
presentation will discuss the status of current and anticipated planetary protection considerations for
missions to Europa and other icy bodies of the outer Solar System.
REFERENCES:
"Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, Including
the Moon and Other Celestial Bodies."
Entered into force after ratification by the Congress, October 10, 1967
Space Studies Board, National Research Council, Evaluating the Biological Potential in Samples Returned
from Planetary Satellites and Small Solar System Bodies, National Academy Press, Washington, D.C., 1998.
Space Studies Board, National Research Council, Preventing the Forward Contamination of Europa, National
Academy Press, Washington, D.C., 2000.
Planetary Protection Subcommittee, Recommendations to the NASA Advisory Council, from the meeting held
the 6-8 August ,2007, in Cocoa Beach, FL.
http://planetaryprotection.nasa.gov
P23A-1358
The Formation of Ganymede's Grooved Terrain: Importance of Strain Weakening
Nearly two-thirds of Ganymede's surface consists of relatively bright, young, tectonically deformed terrain dubbed grooved terrain. The grooved terrain consists of sets of parallel, undulatory ridges and troughs with peak to trough amplitudes of several hundred meters and periodic spacings that range from 3 to 10~km. The low slopes and periodic spacing of the grooves suggest that they formed via unstable extension of the ice lithosphere [e.g. Fink and Fletcher 1981, LPS XII; Pappalardo et al. 1998, Icarus 135]. Application of analytical models of unstable extension to Ganymede suggest that large amplitude grooves with appropriate wavelengths can form if the lithosphere is in pervasive brittle failure and if the lithospheric thermal gradient was relatively high (~45K km-1) [Dombard and McKinnon 2001, Icarus 154]; however, numerical models of unstable extension struggle to produce topographic amplitudes consistent with Ganymede's grooves (maximum amplitudes are a factor of five less than typical large amplitude grooves) [Bland and Showman 2007, Icarus 189]. The difficulties in producing large amplitude deformation may be overcome by the inclusion of strain weakening in models of groove formation. Strain weakening effects account for a material's tendency to strain more easily as viscous and/or plastic deformation accumulates, and as strain localizes in shear zones or along faults. When included in models of terrestrial extension, such effects can increase deformation amplitudes by up to several orders of magnitude [e.g. Fredericksen and Braun 2001, EPSL 188; Behn et al. 2002, EPSL 202]. Here we present the results of simulations of Ganymede's groove formation that include various strain weakening processes. Incorporation of a simple damage rheology, in which the yield strength of the ice lithosphere decreases as plastic strain accumulates, permits a factor of three increase in the amplitude of the simulated grooves, generating topography of 200~m or more. Such groove amplitudes are consistent with the lower-end of the range of observed groove amplitudes. More sophisticated strain weakening rheologies are likely to further increase deformation amplitudes. This work is supported by NASA PG&G.
P23A-1359
MgSO4-H2O system at High pressure and its implication for the internal structure and evolution of Ganymede
Cryovolcanism and active geological alterations have been discovered in many icy satellites. Not only the surface activities but also the possibility of the subsurface ocean in Galilean satellites have been discussed by Shoemaker et al. (1982) and Kargel (1991). In the previous studies (Prockter 2001, Fortes 2007) existence of subsurface ocean has been suggested based on the observed data such as surface structures of alteration and gravity observations which includes the momentum of inertia. We can adopt the three- layered model for Ganymede (Mueller et al. 1988) composed of Fe or FeS inner core, Silicate outer core, and volatile-rich icy mantle. We calculated the density profile for the three-layered model based on the mixture of C1 chondrite Orgueil meteorite and H2O. We constructed a new model which has a deeper icy mantle with the density larger than pure ice. The infrared data of the Galileo survey (McCord et al. 1999,2001) found a large amount of sulfates such as MgSO4•6 H2O, MgSO4•7 H2O. Thus we estimated that the volatile-rich icy mantle contains not only pure ice but also a heavy MgSO4 component with several weight percents. We investigated the phase relations of MgSO4- H2O system under high pressure in order to discuss the internal structure of Ganymede. The sample which consisted of 0 to 30 weight percent of MgSO4 was put in diamond anvil cell with external heating device. The experiments performed up to 5GPa and 600K. This condition is a bit higher than the bottom of icy mantle (around 350K and 2GPa) (Prentice 2000, Sohl 2001). Under our experimental conditions, we found various high pressure phases in MgSO4- H2O system such as Ice VI and VII, and sulfates hydrates such as MgSO4•1H2O, MgSO4•6H2O, and MgSO4•7H2O depending on pressure and temperature conditions. Viscosity and density of the eutectic liquid at high pressure were measured using the falling sphere method with diamond anvil cell.We will present our experimental results and discuss the evolution of the internal structure of Ganymede based on the experiments.
P23A-1360
Constraints on the Location, Magnitude, and Dimensions of Ganymede's Mass Anomalies
Previously, radio Doppler data, generated with NASA's Galileo spacecraft, were utilized to infer the locations and magnitudes of mass anomalies on the satellite. These earlier models used point masses to fit the line- of-sight acceleration obtained from the Doppler data. However, the point-mass solutions cannot provide the vertical and horizontal extent of the anomalous mass concentrations. We report here on a new study using spherical cap disks to model Ganymede's mass anomalies. The spherical cap disk models not only provide the locations and magnitudes of the mass anomalies, but also their vertical and horizontal dimensions. Although four or five point masses were required to fit the line-of-sight acceleration data, our new models show that three disks can reproduce all the major acceleration features. The three-disk solution results in a positive mass anomaly in Galileo Regio (53.0° N, 127.0° W) and two negative mass anomalies in the bright terrain on either side of the regio at (22.0° N, 87.0° W) and (49.0° N, 219.0° W). The magnitudes of these mass anomalies are on the order of 1018 kg and the diameters are a few thousand kilometers. The positive anomaly is about 100 meters thick and both negative anomalies have a thickness of less than a kilometer. Additionally, we explore the viability of mass anomalies at the satellite's surface using the new information provided by the disk models to make a comparison between the sizes of the anomalies and the regions of dark and bright terrain on Ganymede. We find that the dimensions of the mass anomalies likely can be explained by concentrations of rock in Galileo Regio and rock-free ice in the sulci. These results confirm that mass anomalies may reside on or near Ganymede's surface and that positive mass anomalies are correlated to areas of dark terrain and negative mass anomalies to areas of bright terrain.
P23A-1361
Ultraviolet Spectroscopy of the Surfaces of the Inner Icy Saturnian Satellites
The Cassini mission has provided a unique opportunity to make high-resolution, multi-spectral measurements of Saturn's icy moons, to investigate their surface compositions, processes and evolution. Here we present results from the Ultraviolet Imaing Spectrograph (UVIS). This instrument allows for the first measurements of the icy satellites in the extreme ultraviolet (EUV) to far-ultraviolet (FUV) wavelength range. The icy satellites of the Saturn system exhibit a remarkable amount of variability: Dark, battered Phoebe orbiting at a distant 200 RS, black-and-white Iapetus, the wispy streaks of Dione, cratered Rhea and Mimas, bright Tethys and geologically active Enceladus. Phoebe, Iapetus and Hyperion all orbit largely outside Saturn's magnetosphere, while the inner icy satellites Mimas, Enceladus, Dione Tethys and Rhea all orbit within the magnetosphere. Furthermore, the inner icy satellites all orbit within the E-ring – so the extent of exogenic effects on these icy satellites is wide-ranging. We present an overview of UVIS results from Tethys, Dione, Mimas, Enceladus and Rhea, focusing on surface investigations. We expect that the UV signatures of these icy satellites are strongly influenced not only by their water ice composition, but by external effects and magnetospheric environments. We study the FUV reflectance spectra to learn about the surface composition, map out water ice grain size variations, investigate effects of coating by E-ring grains, examine disk-resolved and hemispheric compositional and brightness variations, and investigate the presence of radiation products. This is new work: FUV spectra of surfaces have not been well-studied in the past. Spectra of the inner icy moons have been used to better develop spectral models, to further understand existing lab data of water ice and to help with understanding instrument performance. Analysis is challenged by a lack of laboratory data in this wavelength region, but intriguing results are being found. We find that the FUV albedo is a critical tie- point to understand the composition of these moons -- important absorptions occur in the NUV-visible region. We present disk-integrated hemispherical reflectance spectra, and show that while Tethys and Dione exhibit strong UV leading-trailing differences, Mimas, Enceladus and Rhea do not. In the UV, Mimas is nearly as bright as Enceladus. Tethys is surprisingly dark in the UV. The visible-wavelength leading-trailing hemisphere albedo differences can be attributed to coating by E-ring grains; in the UV, a process appears to darken the trailing hemisphere of Tethys. We also investigate disk-resolved Enceladus spectra to understand spectral differences between the south polar tiger stripe region and elsewhere on the surface.
P23A-1362
Investigating the Origin of the Iapetus Ridge
The Cassini spacecraft has revealed a unique ridge system on Iapetus. The ridge is 1600 km in length and 18 km in height. It is linear and located along 1/3 of the equator. Since the discovery of the ridge in January 2005, three prominent theories have been postulated on the origin of this very unique system. These theories include formation by ring accretion (Ip, 2005), despin and surface relaxation (Porco et al., 2005), and convection (Czechowski et al., 2008). Giese et al., (2008) has provided substantial evidence against the collapsed ring model, which suggests that the ridge did not form from ring accretion. The remaining two theories were analyzed in this research, mainly focusing on the convection theory. We investigated the possibility of despinning and surface relaxation forming the ridge. Based on the work done by Melosh (1977), despinning produces one of two specific tectonic features, neither of which is a ridge along the equator. Since the ridge cannot be described by the accepted features of despinning, it is unlikely that the ridge was a result of this. Finally, we conducted extensive analysis on several elements of the convection theory, including the size of the convection cells and the results of isostacy on Iapetus. It was concluded that these crucial components of the theory are not consistent with observations and it is therefore highly unlikely that the ridge was formed by convection. This research has displayed extensive evidence against all three theories, implying that the origin of the Iapetus ridge remains a mystery.
P23A-1363
Small Satellites, Asteroids, and Comets: Surface Expressions of Internal Structures
High-resolution images of the surfaces of small (r<200 km) satellites, asteroids, and cometary nuclei are now sufficiently numerous to record trends in the expressions of internal structures. Recent Cassini images are particularly useful in this survey. Epimetheus, Pandora, and Phobos show patterns of grooves best interpreted as expressions of tidal stress-induced fractures. Objects that are subject largely to impact rather than tidal stresses -- Gaspra, Ida, Eros, Phoebe, and Hyperion -- show evidence of fractures in local, rather than global, patterns. Wild-2 and Tempel-1 have active, albeit very different surfaces, and most fracture evidence may have been lost by sublimation or, in the case of Tempel-1,by both erosion and burial. These groove patterns and individual morphologies allow inferences of body strengths, regolith thicknesses, and some specific events in their geologic histories. We also predict properties and surface appearances of other asteroids and satellites.