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Robert Pappalardo (Ph.D. Arizona State Univ. '94, B.A. Cornell University '86) researches processes that have shaped the surfaces of icy outer planet satellites. Past research focused on Miranda, an icy satellite of Uranus; examination of ridge and trough sets on Miranda led to the conclusion that large upwellings within the satellite shaped its surface. More recent research relates to the ongoing encounters of the Galileo spacecraft with the Galilean satellites of Jupiter: Io, Europa, Ganymede, and Callisto. Ganymede research involves the nature, origin, and evolution of its bright grooved terrain, specifically the style of tectonism and implications for the satellite's geological history. Europa research includes the possibility that solid-state convection has played an important role in the satellite's history, investigation of regions of separation and spreading of the satellite's icy lithosphere, and implications of the surface geology for lithospheric properties. Overall, Europa research relates to the question of whether the satellite ever had, or presently has, a liquid water ocean beneath its icy surface. Patterson, G. W., J. W. Head, and R. T. Pappalardo. Plate motion on Europa and nonrigid behavior of the icy lithosphere: The Castalia Macula Region. J. Struct. Geol., in review. We have developed a generalized quantitative technique for determining the finite pole of rotation between two rigid plates and use it to critically examine differing reconstructions of a region surrounding a prominent dark spot on Europa, Castalia Macula. This region is located near the equator of Europa's trailing hemisphere and has been suggested as a site where crustal convergence may have occurred. Previous reconstructions of the region have indicated that a ridge set and/or band-like complex that define a collection of tectonic plates in the region accommodated surface contraction. However, a critical examination of the differences between these reconstructions has been complicated by the lack of a finite pole of rotation for the plates involved in either reconstruction. We have applied our modeling technique, coupled with a detailed examination of the morphology and cross-cutting relationships involving this ridge set and band-like complex, to determine if a unique reconstruction exists for a number of tectonic plates in this region. The cross-cutting relationships involving the ridge set also allow us to test the general assumption that plates behave rigidly on Europa. Assuming rigid behavior our results suggest that a unique reconstruction does exist indicating the ridge set accommodated surface contraction. However, analysis performed to test the assumption of plate rigidity indicates that one or more of the plates in the region did not behave rigidly. This does not rule out surface contraction along the ridge set but does indicate that a component of nonrigid behavior must be taken into account. Barr, A. C. and R. T. Pappalardo. Onset of Convection in the Icy Galilean Satellites: Influence of Rheology. J. Geophys. Res., in press. Ice I exhibits a complex rheology at temperature and pressure conditions appropriate for the interiors of the ice I shells of Europa, Ganymede, and Callisto. We use numerical methods and existing parameterizations of the critical Rayleigh number to determine the conditions required to trigger convection in an ice I shell with the stress-, temperature- and grain size- dependent rheology measured in laboratory experiments by Goldsby and Kohlstedt [2001]. The critical Rayleigh number depends on the ice grain size and the amplitude and wavelength of temperature perturbation issued to an initially conductive ice I shell. If the shells have an assumed uniform grain size 0.4 mm, deformation during initial plume growth is accommodated by Newtonian volume diffusion. If the ice grain size is between 0.4 mm and 3 cm, deformation during plume growth is accommodated by weakly non-Newtonian grain boundary sliding, where the critical ice shell thickness for convection depends on the amplitude of temperature perturbation to the -0.5 power. If the ice grain size exceeds 2 cm, convection cannot occur in the ice I shells of the Galilean satellites regardless of the amplitude or wavelength of temperature perturbation. If the grain size in a convecting ice I shell evolves to effective values greater than 2 cm, convection will cease. If the ice shell has a grain size large enough to permit flow by dislocation creep, the ice is too stiff to permit convection, even in the thickest possible ice I shell. Consideration of the composite rheology implies that estimates of the grain size in the satellites and knowledge of their initial thermal states are required when judging the convective instability of their ice I shells. Stempel, M. M., A. C. Barr, and R. T. Pappalardo, Model constraints on the opening rates of bands on Europa. Icarus, in press. A mid-ocean-ridge spreading analog is used to constrain the opening rates and brittle-ductile transition depths for two axisymmetric ridged bands on Europa. Estimates of brittle-ductile transition depth based on the morphologies of Yelland and Ino Lineae are combined with a conductive cooling model based on a mid-ocean ridge analog to estimate the opening rates and active lifetimes of the bands. This model limits local strain rates to ~10^-15 to 10^-12 s^-1, opening rates to 0.2 to 40 mm yr^-1, and active lifetimes of the bands to 0.1 to 30 Myr. If the observed structures in the outer portions of ridged bands are indeed normal faults, the estimated range for the tensile strength of ice on Europa is 0.4 to 2 MPa, consistent with nonsynchronous rotation as the dominant driving mechanism for band opening. Pappalardo, R. T., and G. C. Collins, Extensional tectonics on Ganymede as recorded by strained craters. J. Struct. Geol., 27, 827-838, 2005. High-resolution images of Jupiter's satellite Ganymede obtained by the Galileo spacecraft reveal several elliptical craters that are transected by sets of subparallel ridges and troughs oriented roughly orthogonal to the long axis of the crater, implying that these craters have been extensionally strained. We analyze five such craters, four in the ancient dark terrain and one in bright grooved terrain, inferring their strain histories by assuming each crater was originally circular. These strained craters have been elongated by amounts ranging from ~5 to ~50%. All are deformed by extension near-perpendicular to subparallel ridges and troughs, and some are also deformed by distributed simple shear parallel to these tectonic structures. The rift zones that deform these craters show extensions of ~5 up to ~180% for a well-defined fault zone that rifts Saltu crater within dark terrain of Nicholson Regio. Extensional fault zones consisting of domino-style normal faults and representing strains of several tens of percent may be common on Ganymede, and strains of this order can alter pre-existing surface features beyond recognition through the process of tectonic resurfacing. Prockter, L. M., R. T. Pappalardo, and F. Nimmo. A Shear heating origin for ridges on Triton. Geophys. Res. Lett., 32, L14202, doi:10.1029/2005GL022832, 2005. Triton and Europa each display a variety of distinctive curvilinear ridges and associated troughs. Here we propose that ridges on Triton may have formed by diurnal tidal stresses, in a manner similar to that proposed for ridge formation on Europa. The greater width of ridges on Triton is likely a consequence of the lower surface temperature and greater brittle-ductile transition depth on this body compared to Europa. The magnitudes of the stresses and heat fluxes required to generate ridges of the correct scale are comparable to likely values generated during the latter part of Triton's orbital evolution from an initial highly eccentric state. Barr, A. C., S. Zhong, and R. T. Pappalardo. Convective instability in ice I with non-Newtonian rheology: Application to the Galilean satellites. J. Geophys. Res., 109, E12008, doi:10.1029/2004JE002296, 2004. Convective instability in ice I with non-Newtonian rheology: Application to the Galilean satellites. J. Geophys. Res., 109, E12008, doi:10.1029/2004JE002296, 2004. At the temperatures and stresses associated with the onset of convection in an ice I shell of the Galilean satellites, ice behaves as a non-Newtonian fluid with a viscosity that depends on both temperature and strain rate. The convective stability of a non-Newtonian ice shell can be judged by comparing the Rayleigh number of the shell to a critical value. Previous studies suggest that the critical Rayleigh number for a non-Newtonian fluid depends on the initial conditions in the fluid layer, in addition to the thermal, rheological, and physical properties of the fluid. We seek to extend the existing definition of the critical Rayleigh number for a non-Newtonian, basally heated fluid by quantifying the conditions required to initiate convection in an ice I layer initially in conductive equilibrium. We find that the critical Rayleigh number for the onset of convection in ice I varies as a power (-0.6 to -0.5) of the amplitude of the initial temperature perturbation issued to the layer, when the amplitude of perturbation is less than the rheological temperature scale. For larger-amplitude perturbations, the critical Rayleigh number achieves a constant value. We characterize the critical Rayleigh number as a function of surface temperature of the satellite, melting temperature of ice, and rheological parameters so that our results may be extrapolated for use with other rheologies and for a generic large icy satellite. The values of critical Rayleigh number imply that triggering convection from a conductive equilibrium in a pure ice shell less than 100 km thick in Europa, Ganymede, or Callisto requires a large, localized temperature perturbation of a few kelvins to tens of kelvins to soften the ice and therefore may require tidal dissipation in the ice shell. Schenk, P.M. and R.T. Pappalardo. Topographic variations in chaos on Europa: Implications for diapiric formation. Geophys. Res. Lett., 31, L16703, doi:10.1029/2004GL019978, 2004. Disrupted terrain, or chaos, on Europa, might have formed through melting of a floating ice shell from a subsurface ocean [Carr et al., 1998; Greenberg et al., 1999], or breakup by diapirs rising from the warm lower portion of the ice shell [Head and Pappalardo, 1999; Collins et al., 2000]. Each model makes specific and testable predictions for topographic expression within chaos and relative to surrounding terrains on local and regional scales. Highresolution stereo-controlled photoclinometric topography indicates that chaos topography, including the archetypal Conamara Chaos region, is uneven and commonly higher than surrounding plains by up to 250 m. Elevated and undulating topography is more consistent with diapiric uplift of deep material in a relatively thick ice shell, rather than melt-through and refreezing of regionally or globally thin ice by a subsurface ocean. Vertical and horizontal scales of topographic doming in Conamara Chaos are consistent with a total ice shell thickness >15 km. Contact between Europa's ocean and surface may most likely be indirectly via diapirism or convection. Pappalardo, R. T., and A.C. Barr, The origin of domes on Europa: The role of thermally induced compositional diapirism, Geophys. Res. Lett., 31, L01701, doi:10.1029/2003GL019202, 2004. The surface of Jupiter's moon Europa is peppered by topographic domes, interpreted as sites of intrusion and extrusion. Diapirism is consistent with dome morphology, but thermal buoyancy alone cannot produce sufficient driving pressures to create the observed dome elevations. Instead, diapirs may initiate by thermal convection that induces compositional segregation. Exclusion of impurities from warm upwellings allows sufficient buoyancy for icy plumes to create the observed surface topography, provided the ice shell has a small effective elastic thickness (~0.2 to 0.5 km) and contains low-eutectic point impurities at the few percent level. This model suggests that the ice shell may be depleted in impurities over time. Head, J. W., R. Pappalardo, G. Collins, M. J. S. Belton, B. Giese, R. Wagner, H. Breneman, N. Spaun, B. Nixon, G. Neukum, and J. Moore. Evidence for Europa-like resurfacing styles on Ganymede. Geophys. Res. Lett., 10.1029/2002GL015961, 2002. Very high-resolution imaging and stereo topographic data obtained during the Galileo G28 encounter with Ganymede show 1) evidence for Europa-like, crustal spreading and resurfacing to form portions of the bright terrain, and 2) bright terrain that appears smooth at Voyager resolution (and thus a strong candidate for cryovolcanism) but instead is tectonically deformed and lacks embayment relationships when viewed at high resolution. In contrast to previous views, these new data show that tectonism has been the dominant process in shaping some very smooth areas and that Ganymede appears to have experienced Europa-like crustal spreading during its previous history. Jones, K. B., J. W. Head, R. T. Pappalardo, and J. M. Moore, Morphology and origin of palimpsests on Ganymede from Galileo observations, Icarus, 164, 197-212, 2003. Palimpsests are large, circular, low-relief impact scars on Ganymede and Callisto. These structures were poorly understood based on Voyager-era analysis, but high-resolution Galileo images allow more detailed inspection. We analyze images of four Ganymedean palimpsests targeted by Galileo: Memphis and Buto Faculae, Epigeus, and Zakar. Ganymedean craters and Europan ring structures are used as tools to help better understand palimpsests, based on morphologic similarities. From analysis of Galileo images, palimpsests consist of four surface units: central plains, an unoriented massif facies, a concentric massif facies, and outer deposits. Using as a tie point the location in these structures where secondary craters begin to appear, outer deposits of palimpsests are analogous to the outer ejecta facies of craters; the concentric massif facies of palimpsests are analogous to the pedestal facies of craters; and the unoriented massif facies and central plains are analogous to crater interiors. These analogies are supported by the presence of buried preexisting structure beneath the outer two and absence of buried structure beneath the inner two units. Our observations indicate that palimpsest deposits represent fluidized impact ejecta, rather than cryovolcanic deposits or ancient crater interiors. Nimmo, F., R. T. Pappalardo, and B. Giese. On the origins of band topography, Europa. Icarus, 166, 21-32. We use stereo-derived topography of extensional bands on Europa to show that these features can be elevated by 100-150 m with respect to the surroundings, and that the positive topography sometimes extends beyond the band margins. Lateral variations in shell thickness cannot maintain the observed topography for timescales greater than ~0.1 Myr. Lateral density variations can maintain the observed topography indefinitely; mean density contrasts of 5 and 50 kgm^3 are required for shell thicknesses of 20 and 2 km, respectively. Density variations caused by temperature contrasts require either present-day heating or that bands are young features (< 1 Myr old). Stratigraphic analyses suggest that these mechanisms are unlikely. The observation that bands form from ridges may be explained by an episode of shear-heating on ridges weakening the ridge area, and leading to strain localization during extension. Fracture porosity is likely to persist over Myr timescales in the top one-third to one-quarter of the conductive part of the ice shell. Lateral variations in this porosity (of order 20%) are the most likely mechanism for producing band topography if the ice shell is thin (Å 2 km); porosity variations of 2% or less are required if the shell is thicker (Å 20 km). If the ice shell is thick, lateral variations in salt content are a more likely mechanism. Warm ice will tend to lose dense, low-melting temperature phases and be buoyant relative to colder, salt-rich ice. Thus, lateral density variations will arise naturally if bands have been the sites of either localized heating or upwelling of warm ice during extension. Nimmo, F., B. Giese, and R. T. Pappalardo, Estimates of Europa's ice shell thickness from elastically-supported topography. Geophys. Res. Lett., 30(5), 10.1029/2002GL016660, 2003. The thickness of Europa's solid ice shell is uncertain, and has important implications for Europa's habitability and thermal history, and the design of future spacecraft missions. Here we obtain an estimate of the ice shell thickness from observations of a plateau SW of Cilix impact crater. Stereo topographic profiles suggest that the plateau is flexurally supported, with an effective elastic thickness t_e of 6 (+5/-2) km. For a conductive temperature profile this t_e value implies a solid ice shell thickness of 15 (+20/-9) km; if the shell is convecting, this estimate is a lower bound. Combined with independent estimates, we infer a probable shell thickness of Å25 km. The shell thickness is likely to be uniform over the entire satellite. Pappalardo, R.T. and R. Greeley. A review of the origins of subparallel ridges and troughs: Generalized morphological predictions from terrestrial models. J. Geophys. Res., 100, 18985-19007, 1995. The morphologies of ridges and troughs on Earth and the processes involved in their formation are reviewed for application to sets of subparallel ridges and troughs on outer planet satellites. We consider: tension fracturing, normal faulting, thrust faulting, buckle folding, strike-slip tectonism, fissure eruption, wall diapirism, surface folding of a flow, and near-surface (laccolithic) intrusion. The landform characteristics and geologic associations predicted of each process are generalized to be independent of planet-specific parameters. We find that the combinations of landforms derived from each process are unique, making each distinguishable in the idealized case. Morphological parameters most diagnostic of formational process are ridge and trough cross-sectional shape, termination style, and incipient character. The characteristics described are useful in constraining the origin of ridge and trough terrains observed in Voyager images and will aid in analysis of high-resolution images from the Galileo and Cassini spacecraft. Pappalardo, R.T. and R.J. Sullivan. Evidence for separation across a gray band on Europa. Icarus, 123, 557-567, 1996. Thynia Linea, a gray band on Europa, is found to be a ~25 km wide and >900 km long region of lithospheric separation which has been infilled by relatively dark material. Roughly a dozen older features and the cuspate segments that form its outline appear to have been displaced across its width. Displacement azimuths indicate a best fit pole of opening near 79¡S, 200¡W. However, displacement magnitude decreases toward either end of the gray band, indicating that it is more akin to a "tear" in a nonrigid europan lithosphere. Opening was in response to NW-SE directed tensile stress, in accord with the stress predictions of nonsynchronous rotation. Observations of Thynia Linea are consistent with a laterally mobile brittle lithosphere, decoupled from ductile or liquid material below, as previously suggested to account for opening of wedge-shaped bands in Europa's antijovian region. Lithospheric separation and contemporaneous emplacement of new material offers a possible volcano-tectonic scenario for resurfacing Europa. If this process is ongoing, resurfacing can be accomplished on a time scale consistent with the satellite's surface age if one gray band or zone of wedge-shaped bands forms every ~10^3-10^4 yr and becomes unrecognizable with age. This would imply that features on Europa brighten with age, as through continuous deposition of frost onto the surface. Pappalardo, R.T., S.J. Reynolds, and R. Greeley. Extensional tilt blocks on Miranda: Evidence for an upwelling origin of Arden Corona. J. Geophys. Res., 102, 13369-13379, 1997. Subparallel ridges and troughs in the outer belt of Arden Corona, on the Uranian satellite Miranda, are interpreted as tilt blocks formed by extension and normal faulting. Fault scarps generally face outward from the corona, exposing dark material in the subsurface. Reconstruction of faults along a deep rift zone bounding the corona suggests initial dips of ~50¡. Local extension reaches ~70%, extremely high in comparison to previous estimates of strain on icy satellites. A rise adjacent to the rift zone is modeled as flexural and indicates an effective elastic lithospheric thickness of ~2 km at the time of flexure. The assumption that faulting has significantly weakened the lithosphere suggests a mechanical lithosphere thickness of ~5 to 10 km. Corresponding thermal gradients in a frictionally controlled ice lithosphere are ~8 to 20 K km^-1, and lithospheric tensional strength is ~0.4 to 1.8 MPa. Normal faulting in Arden Corona indicates that internal upwelling likely formed the corona, and the outward facing direction of faults is consistent with such a model. An upwelling origin of Miranda's coronae eliminates the need to invoke catastrophic breakup and reaccretion of the satellite as an explanation for its surface geology. Pappalardo, R.T., J.W. Head, G.C. Collins, R.L. Kirk, G. Neukum, J. Oberst, B. Giese, R. Greeley, C.R. Chapman, P. Helfenstein, J.M. Moore, A. McEwen, B.R. Tufts, D.A. Senske, H.H. Breneman, and K. Klaasen. Grooved terrain on Ganymede: First results from Galileo high-resolution imaging. Icarus, 135, 276-302, 1998. High-resolution Galileo imaging has provided important insight into the origin and evolution of grooved terrain on Ganymede. The Uruk Sulcus target site was the first imaged at high resolution, and considerations of resolution, viewing geometry, low image compression, and complementary stereo imaging make this region extremely informative. Contrast variations in these low-incidence angle images are extreme and give the visual impression of topographic shading. However, photometric analysis shows that the scene must owe its character to albedo variations. A close correlation of albedo variations to topography is demonstrated by limited stereo coverage, allowing extrapolation of the observed brightness and topographic relationships to the rest of the imaged area. Distinct geological units are apparent across the region, and ridges and grooves are ubiquitous within these units. The stratigraphically lowest and most heavily cratered units ("lineated grooved terrain") generally show morphologies indicative of horst-and-graben style normal faulting. The stratigraphically highest groove lanes ("parallel ridged terrain") exhibit ridges of roughly triangular cross-section, suggesting that tilt-block style normal faulting has shaped them. These extensional-tectonic models are supported by cross-cutting relationships at the margins of groove lanes. Thus, a change in tectonic style with time is suggested in the Uruk Sulcus region, varying from horst and graben faulting for the oldest grooved terrain units to tilt block normal faulting for the latest units. The morphologies and geometries of some stratigraphically high units indicate that a strike-slip component of deformation has played an important role in shaping this region of grooved terrain. The most recent tectonic episode is interpreted as right-lateral transtension, with its tectonic pattern of two contemporaneous structural orientations superimposed on older units of grooved terrain. There is little direct evidence for cryovolcanic resurfacing in the Uruk Sulcus region; instead tectonism appears to be the dominant geological process that has shaped the terrain. A broad wavelength of deformation is indicated, corresponding to the Voyager-observed topography, and may be the result of ductile necking of the lithosphere, while a finer scale of deformation probably reflects faulting of the brittle near-surface. The results here form a basis against which other Galileo grooved terrain observations can be compared. Pappalardo, R.T., M.J.S. Belton, H.H. Breneman, M.H. Carr, C.R. Chapman, G.C. Collins, T. Denk, S. Fagents, P.E. Geissler, B. Giese, R. Greeley, R. Greenberg, J.W. Head, P. Helfenstein, G. Hoppa, S.D. Kadel, K.P. Klaasen, J.E. Klemaszewski, K. Magee, A.S. McEwen, J.M. Moore, W.B. Moore, G. Neukum, C.B. Phillips, L.M. Prockter, G. Schubert, D.A. Senske, R.J. Sullivan, B.R. Tufts, E.P. Turtle, R. Wagner, and K.K. Williams. Does Europa have a subsurface ocean? Evaluation of the geological evidence. J. Geophys. Res., 104, 24015-24055, 1999. It has been proposed that Jupiter's satellite Europa currently possesses a global subsurface ocean of liquid water. Galileo gravity data verify that the satellite is differentiated into an outer H2O shell about 100 km thick but cannot determine the current physical state of this shell (liquid or solid). Here we summarize the geological evidence regarding an extant subsurface ocean, concentrating on Galileo imaging data. We describe and assess nine pertinent lines of geological evidence: 1) impact morphologies; 2) lenticulae; 3) cryovolcanic features; 4) pull-apart bands; 5) chaos; 6) ridges; 7) surface frosts; 8) topography; and 9) global tectonics. An internal ocean would be a simple and comprehensive explanation for a broad range of observations; however, we cannot rule out the possibility that all of the surface morphologies could be due to processes in warm, soft ice with only localized or partial melting. Two different models of impact flux imply very different surface ages for Europa; the model favored here indicates an average age of ~50 Myr. Searches for evidence of current geological activity on Europa, such as gas/dust plumes or surface changes, have yielded negative results to date. The current existence of a global subsurface ocean, while attractive in explaining the observations, remains inconclusive. Future geophysical measurements are essential to determine conclusively whether or not there is a liquid water ocean within Europa today. Pappalardo, R.T., J.W. Head, R. Greeley, R.J. Sullivan, C. Pilcher, G. Schubert, W. Moore, M.H. Carr, J.M. Moore, M.J.S. Belton, and D.L. Goldsby. Geological evidence for solid-state convection in Europa's ice shell. Nature, 391, 365-368, 1998. Models of the interior of Jupiter's satellite Europa indicate that tidal interactions with Jupiter might produce enough heat to maintain a subsurface liquid water layer1; moreover, recent spacecraft images show Europa to be sparsely cratered, suggesting that its ice-rich surface is geologically youthful and that the moon might be active today5. Thus, critical issues regarding the geology and physical state of Europa include its style of resurfacing, its mechanism of heat loss, and whether it has had in the past or has today a liquid water ocean. Here we report on the morphology and geological interpretation of pits, domes, and spots discovered in new Galileo high-resolution images of Europa's surface. The morphology and structure of these features suggest vertical deformation of the surface, localized heating, and possibly magmatism. We interpret the features as the surface manifestation of diapirs, relatively warm ice masses that have risen buoyantly through the subsurface. A candidate formational process is thermally induced solid-state convection, predicted to occur within an ice shell on Europa if it overlies a liquid water layer Patel, J.G., R.T. Pappalardo, J.W. Head, G.C. Collins, H. Hiesinger, and J. Sun. Topographic Wavelengths of Ganymede Groove Lanes from Fourier Analysis of Galileo Images. J. Geophys. Res., 104, 24057-24074, 1999. Galileo images have shown that grooved terrain on Ganymede consists of pervasive ridges and grooves at a variety of spatial scales, which complicates visual interpretation. We use Fourier analysis to separate complex surface deformation into its component dominant wavelengths (closely correlated to topographic wavelengths) to determine spatial relationships within and among grooved terrain units. We analyze groove lanes in four Galileo target sites (Uruk Sulcus, Byblus Sulcus, Tiamat Sulcus, and Nicholson Regio), spanning a range of resolutions and lighting geometries, and we find multiple dominant wavelengths in each. Fourier analysis of the complexly deformed Uruk Sulcus shows both similarities and differences in wavelength distribution among its tectono-stratigraphic subunits (a range of 0.5 to 6 km, with a concentration at 1.2 km); favorable comparison is made to a stereo-derived topographic model. Of the dominant wavelengths displayed by Byblus Sulcus (~1, 3.3, and 10 km), the longest wavelength is revealed by profiles across both high- and low-resolution images with very different lighting geometries. Tiamat Sulcus displays different dominant wavelengths north (5 to 10 km) and south (3 to 5 km) of the orthogonally trending Kishar Sulcus. Groove lanes in Nicholson Regio are significantly different from the other sites because they are isolated within dark terrain. Fourier analysis of these dark terrain groove lanes shows dominant wavelengths (~2.1, 3.2, and 8.0 km) that are similar to those in lanes of more typical grooved terrain. This suggests that the tectonic style and lithospheric characteristics in this portion of Ganymede's dark terrain were similar to those in bright grooved terrain at the time of deformation. Our results support the hypothesis that longer topographic wavelengths in Ganymede's groove lanes formed by means of extensional necking of the lithosphere, while multiple shorter wavelengths formed by normal faulting of the brittle lithosphere, in both bright and dark terrains. The similar wavelengths of deformation seen in several groove lanes in both bright and dark terrain suggest similarity in lithospheric thickness, composition, and mechanical structure at these disparate sites. A global process (such as differentiation) could be responsible for creating a similar planet-wide strain and thermal regime during the time of grooved terrain formation. Prockter, L.M. and R.T. Pappalardo. Folds on Europa: Implications for Crustal Cycling and Accommodation of Extension. Science, 289, 941-943, 2000. Regional-scale undulations with associated small-scale secondary structures are inferred to be folds on Jupiter's moon Europa. Formation is consistent with stresses from tidal deformation, potentially triggering compressional instability of a region of locally high thermal gradient. Folds may compensate for extension elsewhere on Europa and then relax away over time. Prockter, L. M., J. W. Head III, R. T. Pappalardo, J. G. Patel, R. J. Sullivan, A. E. Clifton, B. Giese, R. Wagner, and G. Neukum, Morphology of Europan bands at high resolution: A mid-ocean ridge-type rift mechanism. J. Geophys. Res., 107(E5), 10.1029/2000JE001458, 2002. We utilize imaging data from the Galileo spacecraft to investigate band formation on one of Jupiter's moons, Europa. Bands are polygonal features first observed in Voyager data close to Europa's anti-Jovian point and represent areas where preexisting terrain has been pulled apart, allowing new material to move up into the gap. We examine the detailed morphology of several bands imaged at different resolutions and lighting geometries. We identify several distinct morphological characteristics, including central troughs, hummocky textures, and ridge and trough terrains, some of which are common among the bands studied. In many cases, bands have initiated along segments of one or more preexisting double ridges, ubiquitous within Europa's ridged plains. Distinctive morphological features and high standing topography imply that the bands formed from compositionally or thermally buoyant ice, rather than liquid water. Comparisons between Europan band morphologies and features found on terrestrial mid-ocean ridges reveal several similarities, including axial troughs, subcircular hummocks, normal faults, and indications of symmetrical spreading. We conclude that terrestrial mid-ocean ridge rifting is a good analogy for Europan band formation. If a terrestrial seafloor-spreading model is applicable to Europan bands, we speculate that band morphologies might be related to the relative rate of spreading of each band. Bands may have contributed significantly to the resurfacing of Europa. Europan bands we examine predate (but do not postdate) lenticulae and related features, implying that the style of resurfacing on Europa has changed over recent geological time in these regions. Spaun, N. A., R. T. Pappalardo, and J. W. Head. Evidence for shear failure in forming near-equatorial lineae on Europa, J. Geophys. Res., 108(E6), 10.1029/2001JE001499, 2003. Global stress models for Europa are unable to readily explain the orientations and intersection angles of lineae in the equatorial region of Europa's trailing hemisphere if lineae originate a tension cracks. Our analysis and mapping of two equatorial, trailing regions reveals that lineae are predominantly oriented NE and NW, while EW lineae are relatively rare; this is contrary to predictions of existing stress and formation models. The measured orientations are consistent with an origin by shear failure. The studied regions of Europa are located near the point of maximum differential stress and minimum surface tensile stresses, indicating that shear faulting may dominate over tension fracturing. Several distinct types of lineae are recognized and their relative abundance is inferred to have changed with time, consistent with linea formation models that suggest an evolutionary sequence from simple troughs to complex ridges. The opening of crevasse-like tensile fractures is not required for the generation of all lineae; the observations are consistent with ridge formation models where troughs (formed in tension or shear) experience shear heating due to tidal deformation, allowing warm ice to buoyantly uplift, creating ridges. The stratigraphic relationships indicate the following: ridged plains formed first, followed by continued formation of a wide range of lineae, and lastly emplacement of lenticulae and continued linea formation. This sequence is consistent with an early thin, brittle lithosphere that thickened with time and was subject to diurnal tides during ~30û-90û of nonsynchronous rotation. Ultimately the thickened shell underwent thermally induced solid-state convection, producing lenticulae. The young surface age of Europa implies that this entire stratigraphic sequence was emplaced in the geologically recent past.
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