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1 Icarus 205 (2010) Contents lists available at ScienceDirect Icarus journal homepage: Segmented lineaments on Europa: Implications for the formation of ridge complexes and bright bands G. Wesley Patterson a, *, James W. Head b a Applied Physics Laboratory, Johns Hopkins University, MP3-E106, Johns Hopkins Road, Laurel, MD 20723, USA b Department of Geological Sciences, Brown University, Providence, RI 02912, USA article info abstract Article history: Received 25 September 2007 Revised 25 June 2009 Accepted 21 July 2009 Available online 23 August 2009 Keywords: Europa Geological processes We describe several segmented lineaments on Europa s surface. These lineaments are extensive, stretching for 100s 1000s of km, and have ridge complex or bright band morphologies. The geometries of the segmented portions of these features are diagnostic of the remote normal and shear stress environment in which they formed and, therefore, constrain ridge complex and bright band formation mechanisms. Analysis of four ridge complexes indicates that they formed in a remote normal stress environment that was tensile and isotropic (or nearly so) and that these lineaments may have formed in a manner more analogous to bands on Europa than to ridges. The stress environment associated with these ridge complexes may also explain the anastomosing nature of their interior morphology. Analysis of two bright bands indicate that one formed in a remote normal stress environment that was tensile and the other was reactivated under a combination of remote tensile normal stress and remote sinistral shear stress. Aspects of the morphologies of these features also indicate that bright bands likely have complex deformation histories that can include multiple episodes of reactivation. Ó 2009 Elsevier Inc. All rights reserved. 1. Introduction The Galilean moon, Europa is a differentiated body with an outer layer km in thickness that is predominately H 2 O (Anderson et al., 1998; Schubert et al., 2004). Evidence suggests this layer consists of a global liquid water ocean (Khurana et al., 1998; Kivelson et al., 2000) overlain by a layer of water ice that is likely kilometers to 10s of kilometers thick (Pappalardo et al., 1999; Schenk, 2002; Greeley et al., 2004). Europa s visible surface represents the outermost portion of this water ice shell and it is extensively fractured by linear features of a wide range of length scales (100s of m to >1000 km). The formation and modification of these fractures has been attributed to stresses caused by diurnal tides, nonsynchronous rotation, and/or polar wander. Europa s proximity to Jupiter and its resonance with Io and Ganymede lead to diurnal and nonsynchronous stresses. Diurnal stresses vary on the 3.55-day timescale of the satellite s orbit and this stress mechanism has been used to model the formation of a variety of linear and cycloidal features observed on Europa s surface (Helfenstein and Parmentier, 1985; Greenberg et al., 1998; Hoppa et al., 1999a). Diurnal tides also impose torques on the satellite and, with the presence of a global subsurface ocean acting as a decoupling layer, Europa s water ice shell is free to rotate nonsynchronously with respect to its interior (Greenberg and Weidenshilling, * Corresponding author. Fax: / address: Gerald.Patterson@jhuapl.edu (G. Wesley Patterson). 1984; Ojakangas and Stevenson, 1989a). This mechanism has been used to model the formation of many prominent global-scale surface features (McEwen, 1986; Leith and McKinnon, 1996; Geissler et al., 1998). It is also supported by regional-scale mapping efforts, which have observed systematic variations in the orientations of co-located features with respect to relative age (e.g., Geissler et al., 1998; Figueredo and Greeley, 2000, 2004; Kattenhorn, 2002). Finally, it has been suggested that variations in the thickness of Europa s ice shell from equator to pole could lead to true polar wonder (Ojakangas and Stevenson, 1989b; Leith and McKinnon, 1996). Based on the orientation and distribution of various features mapped from available image data, it has been suggested that 30 (Sarid et al., 2002) to80 (Schenk et al., 2008) of true polar wander may have occurred. These stress mechanisms, acting alone or in concert, have fractured Europa s outer brittle ice shell and produced a wide variety of surface features. Two prominent examples of such features are ridge complexes and bright bands. These features extend for 100s 1000s of km across the surface of the satellite and have been often used to evaluate models of global surface stresses (e.g., Helfenstein and Parmentier, 1985; McEwen, 1986; Leith and McKinnon, 1996; Greenberg et al., 1998; Geissler et al., 1998; Figueredo and Greeley, 2000, 2004; Schenk et al., 2008). Ridge complexes are linear features that are kilometers wide and can be traced for 100s of km across Europa s surface. In the literature, they have been referred to as ridge complexes, complex ridges, or triple bands (Lucchitta and Soderblom, 1982; Greenberg /$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi: /j.icarus

2 G. Wesley Patterson, J.W. Head / Icarus 205 (2010) et al., 1998; Figueredo and Greeley, 2000, 2004). Distinguishing characteristics that have been described include having a linear to curvilinear trend, no obvious bilateral symmetry, and an internal morphology that resembles a collection of anastomosing/sinuous double ridges (e.g., Greenberg et al., 1998; Figueredo and Greeley, 2000). It has also been observed that ridge complexes can have bright appearing interiors that are flanked by deposits that are dark at visible wavelengths (Lucchitta and Soderblom, 1982). These lower relative albedo deposits can extend for up to 10 km on either side of a ridge complex and may be continuous along the flanks of the complex or patchy and diffuse, forming discrete subcircular regions (Belton et al., 1996). The formation of ridge complexes has been attributed to the successive buildup of individual ridges over time (Greenberg et al., 1998; Geissler et al., 1998; Figueredo and Greeley, 2000; Manga and Sinton, 2004) or by volumetric deformation (Aydin, 2006). Bright bands are prominently observed features that are up to 10s of kilometers in width and, as with ridge complexes, can be traced for 100s of km across Europa s surface. Three such features have been identified in the available image data for Europa; Agenor, Corick, and Katreus Linea. They are readily distinguished from other features by their high relative albedo with respect to the terrain that surrounds them. From Voyager image data (km/pixel), Lucchitta et al. (1981) proposed that bright bands formed as the result of a dike intrusion of clean, icy material. Based on the enigmatic appearance of Agenor Linea, its proximity to Argadnel Regio, and the fact that its margins could not be easily reconstructed, Schenk and McKinnon (1989) suggested that bright bands might be contractional in nature. During the Galileo mission, portions of Agenor Linea were imaged at high resolution (10s of m/ pixel) to characterize its interior morphology. With this data, Prockter et al. (2000) mapped three subparallel swaths of material, distinguishable on the basis of relative albedo and morphology, within Agenor. They identified short, left-stepping en echelon structures close to the margins of each swath, along with transpressional and transtensional structures within the curvilinear portions of the band. They interpreted these structures as indications that the curvilinear portions of Agenor represented restraining or releasing bends and proposed that this bright band likely formed through a combination of dextral (right-lateral) shear and extension. Analysis of a collection of tailcracks associated with the eastern terminus of Agenor Linea (Kattenhorn, 2004) also suggested that, at some time during its formation, this bright band was subject to dextral shear stresses. That analysis further concluded that a portion of Katreus Linea underwent dextral shear as well. In contrast, work done involving the bright band Corick Linea, using a planar reconstruction of potential offset features imaged at lower resolution (km/pixel), was used to suggest that the formation of bright bands is primarily the result of surface contraction (Greenberg, 2004). We have observed that several ridge complexes and bright bands are segmented (Fig. 1) and have compared the interactions between the segmented portions of these lineaments to mechanical processes observed in association with fracture systems on Earth. These processes lead to specific geometries and, as a result, provide important information regarding the stress environment in which they formed. Terrestrial studies involving theoretical, experimental, and field work have provided a basic framework with which local and remote stress environments associated with the formation of such geometries can be assessed (e.g., Cottrell and Rice, 1980; Pollard et al., 1982; Sempere and Macdonald, 1986; Olson and Pollard, 1989; Cruikshank et al., 1991; Thomas and Pollard, 1993). This framework has been used to determine the stress environment leading to the formation and/or evolution of a number of structural features on Europa (Michalski and Greeley, 2002; Schulson, 2002; Kattenhorn, 2004). In similar fashion, we have examined these segmented lineaments in an effort to constrain the stress environment in which ridge complexes and bright bands formed and/or evolved. 2. Observations In this section, we describe the characteristics of the segmented portions of the ridge complexes and bright bands we have identified (Fig. 1). These include general morphology and segment orientation, offset, and, overlap (where present). All data used were of the highest resolution available (0.075 to 1 km/pixel) and, where possible, multiple observations of the same feature were used in the analysis. The images we have used are in Mercator or orthographic projection and, at the scale and precision that we report measurements, angles and distances within an image are preserved. Fig. 1. Portion of a Mercator projected global image mosaic of Europa (57 S to 57 N latitude). Lineaments described in this analysis are labeled and their extent is indicated (dashed lines). The segmented region associated with each lineament is indicated with a circle and the locations of high resolution images that include Asterius and Cadmus lineae are shown (boxes).

3 530 G. Wesley Patterson, J.W. Head / Icarus 205 (2010) Fig. 2. Context image taken from USGS controlled photo mosaic of Europa (map I-2757) that includes portions of the prominent ridge complex Belus Linea and double ridge Rhadamanthys Linea. Segments of Belus Linea are labeled BL 1 and BL Ridge complexes The segmented lineaments Belus, Euphemus, and Asterius/Cadmus Linea are described here (Fig. 1). Belus Linea stretches for 2400 km across Europa s trailing hemisphere from 24 N, 192 W to 5 S, 275 W. Also found in the trailing hemisphere, Asterius Linea stretches for 2200 km from 16 S, 301 W to 37 N, 237 W and Cadmus Linea stretches for 1800 km from 35 N, 239 W to 20 N, 166 W. Euphemus Linea extends for 1500 km across the satellite s leading hemisphere from 3 S, 70 W to 25 S, 15 W. Morphological similarities between the lineaments at resolutions up to 100s of m/ pixel indicate that these segmented fracture systems can be classified as ridge complexes Belus linea This lineament is segmented at 21 N, 214 W (Fig. 1). The two segments of Belus, BL 1 and BL 2, have trends in this region that are misaligned by 14 with respect to each other (Fig. 2). When these segments are 75 km apart, the trend of BL 1 changes by 23 in a clockwise direction and the trend of BL 2 changes by 8 in a clockwise direction. The result is that the two segments diverge from each other on essentially parallel trends. BL 1 and BL 2 then overlap and the trend of BL 2 eventually changes again, but in a counterclockwise direction. This leads BL 2 to converge toward BL 1 (Fig. 2). Higher resolution (100 m/pixel) images of this discontinuity along Belus were acquired during the E14 encounter of the Galileo spacecraft (Fig. 3). From these data, we can see the anastomosing double ridge morphology that is characteristic of ridge complexes. We also see that the changes in trend of BL 1 and BL 2 as they approach each other and diverge are not abrupt in nature, but occur gradually over tens of kilometers. After the clockwise change in direction of BL 2, the two segments maintain a nearly constant offset of 20 km over a length of 35 km. Beyond that point, BL 2 gradually converges toward BL 1 over a distance of 25 km and intersects it at an acute angle (Fig. 3). This leads to an overlap of 60 km between these two segments of Belus Linea Asterius/Cadmus lineae Asterius and Cadmus both appear to terminate in a region centered around 35 N, 241 W (Fig. 1). This region was observed at 1.5 km/pixel during the G1 orbit of the Galileo spacecraft (Fig. 4) and the limited superposition relationships available suggest that both lineaments are similar in relative age with respect to the surrounding terrain. At this image resolution, they are each on the order of 5 km wide with bright interiors and dark flanks, analogous to characteristics of Belus at equivalent resolutions (Fig. 2). Asterius and Cadmus were also imaged elsewhere on Europa (Fig. 1) at resolutions of 100s of m/pixel and, in these data, it is clear that they also both have the characteristic anastomosing double ridge morphology associated with ridge complexes (Fig. 5). The average trends of Asterius and Cadmus Linea in the region where they each terminate (A/CL 1 and A/CL 3, respectively) are misaligned by 15 with respect to each other (Fig. 4). Between these terminations, a lineament 60 km in length is present that has a strikingly similar trend and morphology to that of Asterius and Cadmus. The similar characteristics of this lineament to both of these features and its location between them indicate that Asterius and Cadmus lineae may be two segments of an otherwise continuous ridge complex. In other words, we suggest that the 60 km long lineament may represent a right-stepping en echelon segment between Asterius and Cadmus Linea and that these ridge complexes may be part of a continuous feature stretching for over 4000 km across Europa s trailing hemisphere. Under this premise, we describe this fracture system as consisting of three segments (A/ CL 1, A/CL 2, and A/CL 3 ) separated at 34 N, 243 W and 36 N, 239 W (Fig. 4). Segments A/CL 1 and A/CL 2 have trends that are misaligned by not more than 1 with respect to each other. Given the resolution of the available image data for this region, it appears that these two segments do not overlap and instead terminate when they are 30 km apart (Fig. 4). A/CL 2 and A/CL 3 have trends that are initially misaligned by 16 with respect to each other. When these segments are 60 km apart, the trend of A/CL 3 changes by 22 in a counterclockwise direction before terminating 35 km later. Here too, the two segments do not appear to overlap (Fig. 4) Euphemus linea This lineament was imaged at 1 km/pixel during the I25 encounter of the Galileo mission and is segmented at 20 S, 29 W (Fig. 1). At the resolution Euphemus Linea was imaged, the characteristically anastomosing double ridge interior morphology associated with ridge complexes is not observed (Fig. 6). However, its morphology at 1 km/pixel is consistent with that of Belus, Asterius,

4 G. Wesley Patterson, J.W. Head / Icarus 205 (2010) Fig. 3. (Top) Mosaic of images taken of Belus Linea during the 14ESTRPBND01 Galileo observation at a resolution of 75 m/pixel and shown in orthographic projection. (Bottom) Sketch map of the region with segments of Belus labeled BL 1 and BL 2. Light gray polygons with solid outlines indicate prominent features in the region. Light gray polygons with a dash-dot outline represent the extent of a region of low albedo material associated with Belus. Darker Gray polygons represent the complex ridge Belus Linea. Inflection points indicating changes in propagation path of the two segments are shown with filled circles. Distances and angles described in the text were measured from these points. and Cadmus at a similar image resolution (Figs. 2 and 4). Based on this similarity, we conclude that Euphemus Linea is likely a ridge complex as well. The two segments of Euphemus, EL 1 and EL 2, have trends that are initially misaligned by 6 with respect to each other. When these segments are 170 km apart, the trend of EL 1 changes by 18 in a counterclockwise direction and the trend of EL 2 changes by 9 in a counterclockwise direction. The result is that the two segments diverge from each other. However, unlike with the segments of Belus Linea, EL 1 and EL 2 remain slightly misaligned. EL 1 terminates 90 km after it changes direction while the trend of EL 2 changes again in a clockwise direction after 185 km and converges toward EL 1 at an acute angle. This leads to an overlap between these two segments of 135 km. The offset between EL 1 and EL 2 diminishes as they overlap each other but averages 34 km. Given the resolution of the available image data for this region, it is not clear if the two segments intersect but it appears probable (Fig. 6) Bright bands In this section, we describe the segmented lineaments Katreus and Corick Linea (Fig. 1). These fracture systems have previously been discussed in association with the extensively studied bright band Agenor Linea (Greenberg, 2004; Kattenhorn, 2004). Katreus Linea stretches across Europa s southern trailing hemisphere for 800 km from 39 S, 206 W to 32 S, 244 W. Corick Linea stretches for 1500 km across the satellite s northern leading hemisphere from 25 N, 0 W to 10 N, 46 W Katreus linea This lineament is segmented at 37 S, 220 W(Fig. 1) and the two segments, KL 1 and KL 2, have trends in this region that are misaligned by 9 with respect to each other (Fig. 7). When these segments are 90 km apart, the trends of KL 1 and KL 2 diverge, changing in a counterclockwise direction by 23 and 21, respectively. Approximately 200 km to the southeast, KL 2 changes direction by 20 in a counterclockwise direction before terminating 60 km later (Fig. 7). These two termini of KL 2 are anti-symmetric with respect to each other. Higher resolution (220 m/pixel) images of the segmented portion of Katreus were acquired during the E17 encounter of the Galileo spacecraft (Fig. 8). From these data, it is apparent that the changes in trend of KL 1 and KL 2, as they approach each other and diverge, are more abrupt than we observe for the segmented regions associated with the ridge complexes Belus and Euphemus Linea. After diverging, the segments maintain a distance of nearly

5 532 G. Wesley Patterson, J.W. Head / Icarus 205 (2010) Fig. 4. Image that includes portions of the ridge complexes Asterius and Cadmus Linea taken from USGS controlled photo mosaic of Europa (map I-2757). Segments of these complexes are labeled A/CL 1, A/CL 2, and A/CL 3. Segmented regions are indicated with arrows and inflection point indicating a change in propagation path along A/CL 3 is shown with a filled circle. Distances and angles described in the text were measured from this point. 50 km from each other and each appear to have propagated for 50 km before terminating. KL 1 and KL 2 do not overlap. Unlike the other segmented lineaments we have described, it appears that KL 1 and KL 2 both bifurcate near where the two segments diverge from each other (Fig. 8). The bifurcated segment of KL 1 stretches for 20 km and is essentially aligned with the trend of KL 2 before it diverged from KL 1. The bifurcated segment associated with KL 2 diverges from the trends of both KL 1 and KL 2 at a high angle and appears to transition from a bright band to double ridge morphology. Kattenhorn (2004) described this other portion of KL 2 as a tailcrack that formed as a result of dextral shear along Katreus Linea. It is apparent from these image data that the surface of Europa in the vicinity of Katreus and Agenor Linea is disrupted by the presence of mottled terrain; i.e., terrain that has been disrupted in situ, leaving plates of preexisting material surrounded by a matrix material (Lucchitta and Soderblom, 1982; Pappalardo et al., 1998; Greeley et al., 1998; Spaun et al., 1998). This terrain has destroyed portions of Katreus and Agenor (Fig. 8), suggesting that its formation post-dates both lineae. One other prominent structural feature clearly superposes Katreus Linea (Onga Linea Fig. 8), but there are no other observable superposition relationships involving these two features in the available image data. We interpret this to suggest that these lineae are relatively young and likely formed/evolved coevally or nearly so Corick linea The segmented portion of Corick is located at 15 N, 27.5 W (Fig. 1). The two segments, CL 1 and CL 2, have a perpendicular separation of 115 km and trends that are initially misaligned by 4 as they approach each other (Fig. 9). When the two segments have a parallel separation of 80 km, they each change direction. CL 1 initially changes direction by 36 in a counterclockwise sense and converges toward CL 2. As it does so, CL 1 changes direction by an additional 18 counterclockwise before intersecting CL km later. The segment CL 2 initially changes direction by 21 in a clockwise sense and diverges from CL 1. After 30 km, CL 2 changes direction again by 77 in a counterclockwise sense. This leads it to converge toward CL 1 and it appears to intersect that segment at a high angle after 150 km. The two segments maintain a near constant separation of 60 km in the region where they overlap. Similar to what is observed in association with the two segments of Katreus Linea, CL 1 appears to bifurcate at the location where it initially changes direction (Fig. 9). The bifurcated segment of CL 1 has a trend that differs by 38 in a clockwise sense and it stretches for at least 350 km. 3. Discussion and interpretation The mechanical response to changes in stress environment involving isolated fractures and interacting fracture systems have been examined using theory (Cottrell and Rice, 1980; Pollard et al., 1982; Sempere and Macdonald, 1986; Cruikshank et al., 1991), experiment (Olson and Pollard, 1989; Thomas and Pollard, 1993), and observation (Macdonald et al., 1984, 1987; Cruikshank et al., 1991; Martinez et al., 1997). This work has demonstrated that the geometry of the resulting fracture, or set of fractures, is sensitive to the direction and magnitude of the normal and shear stresses acting on the system. Such geometries are observed, on Earth, over scales ranging from joints in hand sample to mid-ocean ridges (Sempere and Macdonald, 1986, and references therein). In this section, we use that robust body of work to provide insight into the stress environment responsible for the formation and/or evolution of the segmented lineaments we have described. We then use this information to help constrain mechanisms proposed for the formation of ridge complexes and bright bands on Europa. When describing the orientations of remote stresses acting on lineaments in this section we assume a coordinate system that is fixed to the trend of those lineaments being examined. Therefore, stresses acting in the r xx direction refer to lineament-parallel orientations and stresses acting in the r yy direction refer to lineament-perpendicular orientations. When describing the magnitudes of stresses and their comparison from one feature to another, we do so in a strictly relative sense. We feel this is justi-

6 G. Wesley Patterson, J.W. Head / Icarus 205 (2010) Fig. 5. (a) A portion of Asterius Linea, found in Galileo observation E6ESDRKLIN01 at a resolution of 220 m/pixel. (b) Portion of Cadmus Linea found in Galileo observation 15ESREGMAP01 at a resolution of 220 m/pixel. fied by the fact that all of the segmented regions described are of similar scale (within approximately a factor of 2 Fig. 10) and that applied stress magnitudes should be fairly consistent across the surface Observed geometries and inferred stress environments Overlapping veers When opening mode fractures are not isolated, as with developing fracture systems consisting of closely spaced echelon segments, adjacent fractures in the system can mechanically interact as they propagate toward each other. Such interaction produces non-uniform, mixed mode loading, and the resulting propagation paths of the segments overlap and can become significantly curved (Sempere and Macdonald, 1986; Cruikshank et al., 1991; Thomas and Pollard, 1993). One fracture veering toward another in this fashion is relatively common and indicates the controlling influence of local fracture-tip stresses over short distances (e.g., Pollard et al., 1982; Pollard and Aydin, 1984, 1988). However, it has been shown that remote normal stresses play an important and distinguishable role in the resulting geometry, as well (Olson and Pollard, 1989; Cruikshank et al., 1991; Thomas and Pollard, 1993). The overlap of fracture segments and their gradual curvature toward each other are defining attributes of this type of interaction and we will refer to segmented regions of fracture systems with these attributes as overlapping veers. Another attribute that has been described in association with overlapping veers is a characteristic ratio of fracture overlap to offset (Macdonald et al., 1984, 1987; Sempere and Macdonald, 1986; Martinez et al., 1997). This ratio is a measure of the semi-major and semi-minor axis of an overlap zone between two interacting fractures. The range of observed ratios for overlapping veers on Earth is from 2.3:1 to 4.6:1 (Sempere and Macdonald, 1986), although it has been demonstrated that it does not vary significantly from 3:1 (Macdonald et al., 1984). Pollard and Aydin (1984) suggest that this ratio is the result of the decrease in fracture propagation force, for a given initial offset, as the magnitude of overlap increases such that the fracture can no longer propagate after some critical distance. As discussed in Section 2, the segmented portions of Belus, Euphemus, and Corick lineae each overlap and the segments of these features have trends that change gradually and converge toward each other. Further, the segmented portions of these features have overlap to offset ratios of 3:1, 4:1, and 2.5:1 respectively. Based on these attributes, we interpret the segmented portions of these lineae as overlapping veers. This particular geometry has not been previously identified in association with structural features on Europa and, as previously mentioned, this geometry has specific implications for the remote principal stress environment in which it formed. Modeling and experiment have shown that the relative curvature of mechanically interacting fractures depends on the magnitude of the remote differential stress and the fracture spacing (Olson and Pollard, 1989; Cruikshank et al., 1991; Thomas and Pollard, 1993). In these studies, remote shear stresses were zero so the normal stresses, r xx and r yy, were principal stresses. This means simply that the differential stress can be determined by the difference between the largest and smallest normal stresses. In this scenario, if the remote normal stresses are isotropic (r xx = r yy ) and the differential stress is then zero, interacting fractures exhibit an initial divergence in trend followed by convergence and an asymptotic approach toward each other (Fig. 11a). If the remote differential stress is tensile, the curvature of the interacting fractures is exaggerated and the fracture tips approach the neighboring fracture plane at a near perpendicular angle (Fig. 11b). Alternatively, a remote differential stress that is compressive eliminates the initial divergence between interacting fractures and suppresses their curvature (Fig. 11c). The spacing of interacting fractures affects the magnitude of the remote differential stress necessary to exaggerate or suppress their curvature (Thomas and Pollard, 1993). The geometry of the segmented portions of Belus and Euphemus lineae are strikingly similar, although there is approximately a factor of two difference in scale between them (Fig. 10). A comparison with the idealized geometries shown in Fig. 11 indicates that the remote normal stresses, r xx and r yy, acting on these lineaments as their segments interacted were likely isotropic to differentially tensile. Further, it has been demonstrated that, regardless of fracture spacing, segment intersection is only achieved in cases where the r xx and r yy stresses are both tensile (Thomas and Pollard, 1993). Available image data shows that the segments of Belus Linea do intersect (Fig. 3) and, while image data that covers the segmented portion of Euphemus Linea is not sufficient to definitively determine whether EL 1 and EL 2 intersect (Fig. 6), its similarity to the segmented portion of Belus suggests that it is likely. This argues that the interactions of these segments occurred in a remote stress environment where r xx and r yy were both tensile.

7 534 G. Wesley Patterson, J.W. Head / Icarus 205 (2010) Fig. 6. Image that includes portion of the ridge complex Euphemus Linea taken from USGS controlled photo mosaic of Europa (map I-2757). Segments of this complex are labeled EL 1 and EL 2. Segmented region is indicated with an arrow and inflection points indicating changes in the propagation path of the two segments are shown with filled circles. Distances and angles described in the text were measured from these points. Fig. 7. Image that includes portion of the bright band Katreus Linea taken from USGS controlled photo mosaic of Europa (map I-2757). Segments of this complex are labeled KL 1 and KL 2. The observed termini of KL 1 and KL 2 are indicated with arrows. Inflection points indicating changes in the propagation path of the two segments are shown as white circles. Distances and angles described in the text were measured from these points. The segmented portion of Corick Linea is similar in scale to that of Euphemus Linea. However, the perpendicular separation between the segments of Corick is greater than any of the other lineaments examined and the curvature of the interacting segments of the former is significantly exaggerated with respect to both Belus and Euphemus (Fig. 10). The exaggerated curvature indicates that the segments of Corick Linea interacted in a remote stress environment that was differentially tensile with a r xx stress component that was markedly more tensile then the r yy component (e.g., Fig. 11b). Reinforcing this interpretation, the image data that covers the segmented portion of Corick Linea shows that CL 1 and CL 2 do intersect (Fig. 9). As previously discussed, this indicates that the interaction between the segments of Corick Linea occurred in a remote stress environment where r xx and r yy were both tensile (Thomas and Pollard, 1993). The difference in spacing suggests that the remote stress magnitudes necessary to promote interaction between the segments of the bright band Corick were greater than those of the ridge complexes Belus and Euphemus Kinks When dormant fractures are reactivated under mixed mode loading, secondary fractures can form at the termini of the parent fracture. These secondary fractures are commonly referred to as kinks or tailcracks and propagate in a direction that is, at least initially, abruptly different from the parent fracture (Cruikshank et al., 1991). Kinks can form along isolated fractures and, where found in association with segmented fracture systems, do not necessarily have propagation paths that lead to overlap of the fracture segments. Kinks can propagate in straight or curved paths, depending on the relative magnitudes of the remote normal and shear stresses involved and, when kinks are present at both termini of a fracture, they propagate in anti-symmetric directions (Cruikshank et al., 1991). Lineaments on Europa with analogous geometries have been observed (Schulson, 2002; Kattenhorn, 2004; Marshall and Kattenhorn, 2005). The segmented portions of Asterius/Cadmus and Katreus lineae are similar in scale (Fig. 10) and, as described in Section 2.1, each have termini that change direction abruptly by 20. Our interpretation of the available image data indicates that the segments of Asterius/Cadmus Linea do not overlap (Figs. 4 and 10). Available image data indicates the segments of Katreus Linea also do not overlap (Figs. 8 and 10) and the termini of KL 2 appear to propagate in anti-symmetric directions (Fig. 7). Based on these attributes, we interpret the visible terminus of A/CL 3 and the termini of KL 1 and KL 2 to be kinks.

8 G. Wesley Patterson, J.W. Head / Icarus 205 (2010) Fig. 8. (Top) Portion of image mosaic that includes the bright bands Katreus and Agenor Linea acquired during 17ESREGMAP01 Galileo observation at a resolution of 220 m/pixel. (Bottom) Sketch map of the region with prominent linear features labeled and mottled terrain that post-dates the formation of Agenor and Katreus Linea shown as gray polygons. Segments of Katreus are labeled KL 1 and KL 2 and bifurcated portions of both segments are shown with arrows. The geometry of a kink has specific implications for the remote stress environment in which it formed. The direction a kink propagates with respect to its parent fracture indicates the direction of the remote shear stress that was applied during its formation (Cruikshank et al., 1991). That is, a kink that turns clockwise with respect to the direction of the parent fracture is the result of a remotely applied dextral shear stress and, conversely, a counterclockwise direction indicates remote sinistral (left-lateral) shear. The angular difference between the trend of a kink and its parent fracture is related to the ratio of the remote shear stress responsible for the kinking to a combination of the remote stress in the r yy direction and the pressure within the fracture (Cottrell and Rice, 1980; Cruikshank et al., 1991). This latter combination of terms can also be referred to as the deviatoric stress in the fracture perpendicular direction or s yy. Finally, the curvature of a kink can be related to the relative magnitudes of the remotely applied normal stresses (Fig. 12). Cruikshank et al. (1991) show that for a remote normal stress environment that is isotropic and tensile, a kink will propagate in a straight path (Fig. 12a). A kink will diverge from the parent fracture if the r xx and r yy stress components are tensile and r xx > r yy (Fig. 12b). Conversely, a kink will converge toward parallelism with the parent fracture if there is a strong compressive stress in the r xx direction (Fig. 12c). The kinks associated with Asterius/Cadmus and Katreus Linea each have propagation paths oriented counterclockwise with respect to their parent fractures. This indicates that each formed under the influence of a remote sinistral shear stress. The 20 angular difference between the propagation paths of each kink with respect to its parent fractures indicates that the magnitude of the shear stresses involved in their formation was 4 5 times less than the s yy component of the deviatoric stress (Table 1 Cruikshank et al., 1991). The straight propagation path of A/CL 3 suggests the remote normal stresses acting on Asterius/Cadmus lineae were isotropic and tensile (Fig. 12a). The propagation paths of the kinks associated with Katreus Linea appear straight, at lower resolutions (Fig. 7), indicating isotropic and tensile remote normal stresses as well. At higher resolutions (Fig. 8), it appears that some curvature of the kinks associated with Katreus may be present. Such curvature is indicative of a remote normal stress environment in which r yy is tensile and larger in magnitude than r xx. In this case, the slight curvature suggests that r xx is still tensile as well Implications for ridge complexes and bright bands Ridge complexes The component ridges within a ridge complex are morphologically similar to that of individual double ridges on the surface of Europa and features that appear transitional between double ridges and ridge complexes have been described (Belton et al., 1996; Geissler et al., 1998). This suggests that these two morphol- Fig. 9. Corick Linea shown at a resolution of 1 km/pixel (Galileo observation 25ESGLOBAL01). Segments are labeled CL 1 and CL 2. Segmented region and bifurcated portion of CL 1 are indicated with arrows.

9 536 G. Wesley Patterson, J.W. Head / Icarus 205 (2010) Fig. 10. Sketches indicating the geometries of the segmented lineaments examined. They are all shown here at the same scale and all have been reoriented approximately E W so they can be more easily compared. ogies should share a common formation mechanism and has led to a general assumption that ridge complexes form by the successive buildup of individual double ridges (Geissler et al., 1998; Greenberg et al., 1998; Figueredo and Greeley, 2000). This inference has been incorporated into models of ridge formation (Tufts et al., 2000; Nimmo and Gaidos, 2002) and has been demonstrated as viable using wax model experiments (Manga and Sinton, 2004). It has also been proposed that ridge complexes may represent volumetric zones of deformation and form under unique circumstances with respect to double ridges (Aydin, 2006). As we have shown, the segmented lineaments Belus and Euphemus are best described as ridge complexes and the geometry of the segmented portions of these lineae are consistent with that of overlapping veers. For this particular geometry, the combined effects of locally induced shear stresses resolved onto the segments because of the nature of their interaction and remotely applied normal stresses control the propagation paths of each segment (Pollard et al., 1982; Pollard and Aydin, 1984, 1988; Olson and Pollard, 1989; Cruikshank et al., 1991; Thomas and Pollard, 1993). The particular geometries associated with these segmented lineae are characteristic of a stress environment in which the applied remote normal stresses, r xx and r yy, are tensile and isotropic (Figs. 10 and 11a). We have also shown that the segmented lineament Asterius/ Cadmus Linea can be best described as a ridge complex and that one of its segments (A/CL 3 ) is kinked. For this geometry, the path of propagation is controlled by the combined effects of remotely applied shear and normal stresses (Cottrell and Rice, 1980; Cruikshank et al., 1991). The linear propagation path of the kink in segment A/CL 3 suggests that the stresses acting in the r xx and r yy directions with respect to the linea were isotropic and tensile when it formed (Figs. 10 and 12a). The counterclockwise direction in which the kink propagates with respect to the parent fracture indicates that the remote shear stress acting on A/CL 3 was sinistral. The 22 kink angle associated with the segment indicates that the magnitude of the remote shear stress when the kink formed was 4 5 times less than the s yy component of the deviatoric stress (Cruikshank et al., 1991). Fig. 11. Cartoon illustrating idealized geometries of overlapping veers given several different remote normal stress orientations and magnitudes. (a) Geometry indicative of an isotropic and tensile remote normal stress environment. (b) Geometry indicative of a remote normal stress environment that is tensile with r xx higher in magnitude then r yy. (c) Geometry indicative of a remote normal stress environment with a r xx component that is compressive. Adapted from work by Olson and Pollard (1989) and Cruikshank et al. (1991).

10 G. Wesley Patterson, J.W. Head / Icarus 205 (2010) Fig. 12. Cartoon illustrating idealized geometries of kinks given the combination of a remote dextral shear stress and several different normal stress orientations and magnitudes. (a) Geometry indicative of an isotropic and tensile remote normal stress environment. (b) Geometry indicative of a remote normal stress environment that is tensile with r xx higher in magnitude then r yy. (c) Geometry indicative of a remote normal stress environment with a r xx component that is compressive. Adapted from work by Cruikshank et al. (1991). The isotropic and tensile remote normal stress environment associated with the segmented portions of these lineae may provide an explanation for the anastomosing nature of the component ridges that comprise a ridge complex. Based on terrestrial field examples, it has been suggested that fractures interpreted to have formed in a nearly isotropic stress field where the driving pressure exceeded the remote differential stress exhibit an anastomosing geometry with numerous curving overlaps and crack intersections (Olson and Pollard, 1989). This is also supported by experimental and modeling work which found that propagation paths generated in isotropic and tensile remote normal stress environments deviated from ideal paths and that these deviations could not be readily corrected without the controlling influence of a remote differential stress during continued propagation (Thomas and Pollard, 1993). Based on previous work involving kinked lineaments on Europa (Kattenhorn, 2004), the propagation path of A/CL 3 appears to suggest that ridge complexes form in a manner more analogous to that of bands then of ridges. This work found that kinks associated with ridges initially propagated at high angles with respect to their parent fracture (median of 53 ) and that the propagation path then converged toward the trend of the parent fracture (e.g., Fig. 12c). In contrast, they found that kinks associated with bands initially propagated at lower angles (median of 30 ) and that the propagation path diverged away from the trend of the parent fracture (e.g., Fig. 12a and b). Kattenhorn (2004) suggested that this difference in median kink angle and geometry was indicative of differences in formation mechanism. This led them to suggest that ridges were more compatible with a formation mechanism involving shear heating (Nimmo and Gaidos, 2002) and bands were more compatible with a formation mechanism involving tidal walking (Hoppa et al., 1999b). Based on this hypothesis, the ridge complex segment A/CL 3 has a kink angle and geometry that would suggest it too was more compatible with the tidal walking mechanism. Given that we only have one example of a kinked ridge complex, we do not consider this a robust result but it is suggestive and warrants mention Bright bands As previously mentioned, opposing mechanisms have been proposed for the formation of bright bands on Europa. Inferences based on potential planar reconstructions and the morphologies of associated features observed at resolutions on the order of a km/pixel have been used to suggest that Corick and Agenor accommodated compression (Lucchitta et al., 1981; Schenk and McKinnon, 1989; Greenberg, 2004). Alternatively, analysis of secondary features associated with Agenor and Katreus Linea, and observed in higher resolution image data, have shown that strike-slip motion and extension appear to be responsible for the formation and evolution of these lineaments (Prockter et al., 2000; Kattenhorn, 2004). Our analysis supports that latter explanation for the formation of bright bands. As we have shown, the segmented portion of Corick Linea can be best described as a set of overlapping veers and the particular geometry associated with the interaction of these segments is characteristic of a stress environment in which the applied remote normal stresses, r xx and r yy, are both tensile and the magnitude of the r xx component is significantly greater than that of the r yy component (Figs. 10 and 11b). This scenario would appear to exclude compression as a contributing factor in the formation of this bright band. This does not exclude the possibility that Corick, and other bright bands, may have accommodated compression as they evolved. Indeed, the bifurcation of CL 1 argues that Corick has a complex deformation history (Fig. 9) and there is evidence to suggest that Katreus Linea does as well. We have interpreted the termini of the KL 1 and KL 2 segments of Katreus Linea as kinks. The propagation paths of the kinks associated with these segments appear linear to slightly curved in the direction of the parent fracture. They also each propagate at a low angle (20 ) and in a counterclockwise sense with respect to the trend of the parent fracture (Figs. 7 and 10). These characteristics suggest that the kinks formed as the result of a remotely applied sinistral shear stress along Katreus Linea and that the r xx and r yy components of stress were tensile and either isotropic or

11 538 G. Wesley Patterson, J.W. Head / Icarus 205 (2010) had a r xx component with a lower magnitude than the r yy component when it formed (Figs. 10 and 12). They also indicate that the magnitude of the remote shear stress when the kinks formed was 4 less than the s yy component of the deviatoric stress (Table 1 Cruikshank et al., 1991). Similar to the CL 1 segment of Corick Linea, both segments of Katreus Linea are bifurcated in the region where they are segmented (Fig. 8). The bifurcated portion of KL 2 has been previously analyzed and it was proposed to have formed as a result of a remotely applied dextral shear stress along Katreus Linea; i.e., it is a kink as well but formed in a different stress environment (Kattenhorn, 2004). Kinks forming along KL 2 that are indicative of dextral and sinistral remote shear stresses indicate that Katreus Linea has a complex deformation history. Further, it suggests that, provided stress mechanisms such as nonsynchronous rotation or polar wander are responsible for the formation of bright bands, these are long-lived features, remaining active or being periodically active as Europa s decoupled icy shell rotates with respect to its interior. 4. Conclusions We have examined several ridge complexes and bright bands that are segmented (Fig. 1) and have compared the interactions between the segmented portions of these lineaments to mechanical processes observed in association with analogous features on Earth. These processes lead to specific geometries and those geometries provide important information regarding the remote stress environment in which they formed. We have used this information to interpret the remote normal and shear stress environment these ridge complexes and bright bands formed/evolved in and to constrain the mechanism by which they formed. Our results indicate that the segmented portions of the ridge complexes Belus, Euphemus, and Asterius/Cadmus lineae formed in a remote normal stress environment that was tensile and isotropic (or nearly so) and that these lineaments may have formed in a manner more analogous to bands on Europa then ridges. This remote stress environment may also explain the anastomosing nature of component double ridges that comprise ridge complexes. Our results also indicate that the bright band Corick Linea formed in a remote normal stress environment that was tensile in the r xx and r yy directions and that the bright band Katreus Linea was reactivated under a combination of remote tensile normal stress and remote sinistral shear stress. Bifurcations associated with the segments of each bright band suggest these features have complex deformation histories that can include multiple episodes of reactivation. 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