Uruk Sulcus Research Articles

Multi-phase activity on Ganymede's dark terrain: Tectonic evolution of Galileo Regio

Widespread brittle deformation characterizes the surface of Ganymede. In dark terrain regions, tectonic-related structures occur and represent linear to curvilinear kilometer-scale troughs. Such structures are called furrows and are pivotal to understand the early history of the satellite, the tectonics that affected the dark terrain, and the transition or possible future generation of light terrain. The dark terrain of Galileo Regio shows the highest density of furrows on the satellite at the resolution of Voyager and Galileo imagery. Currently, the origin of such structures is still poorly constrained and strictly related to impact cratering that occurred during Ganymede's geologic history. However, evidence from structural geology interpretation suggests an endogenic origin. In this work, we investigate the structural setting of furrows and deformation structures on Galileo Regio to understand the tectonic regimes responsible for their formation and their relative chronological evolution. Furrows that deform Galileo Regio represent evidence for a multiphase tectonic activity that affected the study area. We perform structural mapping and geostatistical analyses, which allow us to recognize a total of four systems that deform Galileo Regio at both regional- and local-scale. Our analyses allow to understand their azimuthal distribution, deformation intensity and to obtain paleo-stress tensors that show the orientations of the maximum and minimum horizontal stress axes (SHmax/Shmin). The obtained results allow to propose a tectonic model that frames five phases affecting Galileo Regio, which followed a deformation history starting from an impact origin to a tectonic remodeling. Such evolutionary phases refer to a total of three different extensional regimes with Shmin trending i) NW-SE, ii) NE-SW and iii) NNE-SSW, and two strike-slip processes related to iv) NNW-SSE and v) NW-SE trending kinematics. Several phases of the proposed model are consistent with the activity of the adjacent light terrain of Uruk Sulcus and suggest its tectonic control over the dark terrain of Galileo Regio by leading to the formation of two systems. In addition, the last phase of the model shows the current setting on Galileo Regio and suggests possible rejuvenation sites of the dark terrain by the Precursor structures, considered as key elements for the dark/light transition.

Equatorial grooves distribution on Ganymede: Length and self-similar clustering analysis

Grooves represent the evidence of tectonic activity that deformed Ganymede surface during its geologic evolution. In this work, we investigate the main characteristics of Ganymede’s groove populations on four different areas located at the equatorial region of the satellite (Uruk Sulcus, Babylon Sulci, Phrygia Sulcus and Mysia Sulci). Specifically, we analyse i) the grooves length distribution to provide a framework for their evolution and ii) the grooves self-similar clustering to infer their vertical penetration inside Ganymede icy shell. For each dataset, we find that the grooves distribution is well fitted by an exponential-law and a power-law distribution depending on the structure length. This implies the presence of confined structures in a shallow layer of the icy crust (relatively shorter, exponentially-fitted structures) and crustal-scale structures that could theoretically reach the crust-ocean interface (relatively longer, power law-fitted structures). In addition, the existence of two exponential distributions for few datasets suggest that there could be two different system of structure confined within specific mechanical crust layers.The thickness of the penetrated icy shell is retrieved through the self-similar clustering analysis and ranges between 105 and 130 ​km for the examined datasets. This value agrees with independent estimates of the icy shell thickness, ranging between 80 and 150 ​km. Moreover, our results support the hypothesis that a large number of grooves penetrate the brittle icy crust, with sets of fractures vertically confined in different mechanical layers, while the penetration of few interconnected faults underlying longer grooves may interest the whole icy crust above the liquid ocean.

Evidence of transpressional tectonics on the Uruk Sulcus region, Ganymede

The surface of Ganymede is characterized by intense tectonic deformation. The two terrains, light and dark, show morphotectonic features, namely, grooves and furrows. These represent kilometric systems at the regional scale, interpreted as extensional structures (e.g., horst and graben systems). Currently, evidence of significant compression has not yet been recognized, leaving open the opportunity to clarify the tectonic balance of Ganymede's surface. Debate still exists on the geodynamics responsible for surface deformation, and the investigation of these features contributes to understanding the internal processes. To understand the deformation that shapes the surface of the satellite, we explore the tectonic setting of the light terrain region of Uruk Sulcus. Using structural geology methodologies by remote sensing and systematic analysis of grooves, we recognize three groove systems within the sulcus and one furrow system in the neighboring dark terrain. Uruk Sulcus is also characterized by the presence of two domains of lineaments, subtle linear texture anisotropies related to the influence of tectonic processes on morphology. Comparison of the combined results from the analyses of the recognized groove systems with the lineament domains allowed us to infer the stress field responsible for their development in the studied area. According to these results, an evolutionary tectonic scenario for Uruk Sulcus is proposed: a tectonic corridor characterized by dextral transpression, with 58% of the internal compressional component approximately N-S oriented and responsible for localized, E-W extensional features among crustal blocks of the shear zone.

Tidal stress modeling of Ganymede: Strike-slip tectonism and Coulomb failure

High-resolution Galileo data of Ganymede's complex surface provide strong and ubiquitous evidence of strike-slip motion: en echelon structures, strike-slip duplexes, laterally offset pre-existing features, and strained craters. In a previous study, we performed a detailed mapping of these strike-slip morphologies within nine regions of Ganymede: Dardanus Sulcus, Tiamat Sulcus, Nun Sulci, Byblus Sulcus, Nippur and Philus Sulci, the Transitional Terrain of Northern Marius Regio, Anshar Sulcus, Arbela Sulcus, and Uruk Sulcus. Strike-slip indicators were inferred in various combinations at each site, in addition to extensional processes. However, the prominence of strike-slip indicators suggests shearing has been an important process to the structural development of Ganymede's surface. Moreover, tidal stresses on Ganymede, under particular circumstances, may have been sufficient to induce Coulomb failure and generate strike-slip faulting. Here we investigate the role of both diurnal and non-synchronous rotation (NSR) tidal stresses on Ganymede using the numerical code SatStress. We resolve normal and shear tractions onto discrete fault segments of specified orientation and assess Coulomb failure potential for the nine inferred fault zones. Testing a range of subsurface fault depths (z = 0–5 km) and ice friction (μf = 0.2–0.6), we find that tidal stress models of combined diurnal and NSR stress readily promote Coulomb failure within each studied fault zone. High friction (μf = 0.6) limits failure depths to ∼1 km, while low friction (μf = 0.2) extends failure depths to ∼2 km, consistent with elastic thickness estimates. We also compare each fault zone's predicted shear sense to the inferred shear sense from our structural mapping efforts and find compatible senses of shear among six of the nine regions that exhibit notable fault offset and/or prevalent inferences of en echelon, duplexes, and strained craters. In addition, principal stress orientations and failure feasibility computed on a global scale suggest that combined diurnal and NSR tidal stresses promote Coulomb failure within the shallow (< 2 km) icy lithosphere across much of Ganymede. Coulomb failure is limited near the equator by large compressive NSR stresses, but stresses at mid- to high-latitudes readily promote Coulomb failure along a wide range of fault orientations. Based on these results, we infer that nonsynchronous rotation may have assisted the formation and evolution of strike-slip structures on Ganymede.

Morphological mapping of Ganymede: Investigating the role of strike-slip tectonics in the evolution of terrain types

The heavily fractured surface of Ganymede displays many morphologically distinctive regions of inferred distributed shear and strike-slip faulting that may be important to the structural development of its surface. To better understand the role of strike-slip tectonism in shaping this complex icy surface, we perform detailed mapping at nine sites using Galileo and Voyager imagery, noting key examples of strike-slip morphologies where present. These four morphological indicators are: en echelon structures, strike-slip duplexes, laterally offset pre-existing features, and possible strained craters. We map sites of both light, grooved terrain (Nun Sulci, Dardanus Sulcus, Tiamat Sulcus, Uruk Sulcus, and Arbela Sulcus), and terrains that are transitional from dark to light terrains (Nippur and Philus Sulci, Byblus Sulcus, Anshar Sulcus, and the Transitional Terrain of Northern Marius Regio). At least one, if not more, of the four strike-slip morphological indicators are observed at every site, suggesting strike-slip tectonism is indeed important to Ganymede's evolutionary history. Byblus Sulcus is the only mapped site where the presence of strike-slip indicators is limited to only a few en echelon structures; every other mapped site displays examples of at least two types, with Arbela Sulcus containing candidate examples of all four. In addition, quantification of morphological characteristics suggests related rotation between sites, as evidenced by the predominant NW/SE trend of mapped features within the light terrain present in five different sites (Nun, Tiamat, Uruk, Nippur/Philus, Byblus, and Anshar Sulcus). Moreover, incorporation of strike-slip tectonism with pre-existing observations of extensional behavior provides an improved, synoptic representation of Ganymede's tectonic history.

Mass anomalies on Ganymede

Radio Doppler data, generated with NASA's Galileo spacecraft during its second encounter with Jupiter's moon Ganymede, are used to infer the locations and magnitudes of mass anomalies on Ganymede. We construct models for both surface and buried anomalies. With only one flyby and no global coverage, a solution for mass anomalies cannot be uniquely determined. However, we are able to constrain acceptable solutions for mass anomalies to four broad regions—a near polar region and three that are roughly equatorial. If the mass anomalies are constrained to lie at the surface, the centers of the regions are located near the coordinates (77° N, 333° W), (36° N, 0° W), (33° N, 130° W), and (7° N, 194° W). If the mass anomalies are located at the deep ice–rock interface 800 km below the surface, the regions' centers are approximately (65° N, 17° W), (32° N, 30° W), (37° N, 175° W), and (15° N, 211° W). For both models, the regions are up to a few thousand kilometers across. The magnitude of mass anomalies on the surface is on the order of 10 17 kg. Mass anomalies at the ice–rock interface are on average no more than an order of magnitude larger (10 18 kg). There are two positive and two negative mass anomalies in both the surface and ice–rock interface models. One of the positive mass anomalies at the surface is associated with Galileo Regio. The other positive surface mass anomaly is located at high northern latitudes with no obvious geological association. Negative surface mass anomalies lie near Uruk Sulcus and between Perrine Regio and Barnard Regio near Sicyan Sulcus and Phrygia Sulcus. The locations of the ice–rock interface mass anomalies lie approximately radially below the surface anomalies. Positive mass anomalies at the surface could be associated with the silicate-rich ice or accumulated silicate layers of the dark regions. Negative mass anomalies at the surface could be associated with the relatively clean, low-lying ice of sulci. Alternatively, Ganymede's mass anomalies could be associated with the topography or other mass concentrations at the deep ice–rock interface.

Topographic wavelengths of Ganymede groove lanes from Fourier analysis of Galileo images

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.

The Distribution of Bright and Dark Material on Ganymede in Relationship to Surface Elevation and Slopes

The unique high-resolution digital terrain data set available for Ganymede's Uruk Sulcus area and Galileo Regio is analyzed by quantitative image analysis techniques to identify relationships between surface brightness, elevation, and slopes. Broad and narrow ridges and grooves of the Uruk Sulcus terrain model are found to be aligned with bright and dark lineament patterns visible in images. These prominent lineaments represent areas covered by bright and dark albedo material, not effects of photometric shading. There is a strong correlation between material locations and elevation: bright material is found on topographic highs, whereas dark material is located in depressions; the relative abundance of the dark material increases with lower elevation. In Galileo Regio, we find that bright material is also located on slopes facing north. We suggest that the bright material in Uruk Sulcus and Galileo Regio was emplaced or exposed during or after formation of the terrain in its present location, whereas the dark material was eroded and redistributed down-slope. Effects of net solar illumination may play a role in the peculiar azimuthal distribution of bright and dark material on slopes.

The Local Topography of Uruk Sulcus and Galileo Regio Obtained from Stereo Images

High resolution (<100 m/pixel) stereo images obtained by the Galileo SSI camera are analyzed using photogrammetric techniques to derive local Digital Terrain Models (DTMs) of Ganymede's surface. The models cover areas of 22 × 56 km in Uruk Sulcus and 63 × 102 km in Galileo Regio and have spatial resolutions of 500 m/pixel and 1000 m/pixel, respectively. For Uruk Sulcus, the DTM features a wave-like topography with wavelengths of 2–6 km and amplitudes of up to 500m. Individual topographic highs show asymmetric shapes with slopes of up to 20° and terracing. The terrain model in Galileo Regio has isolated knobs, furrows of up to 10 km in width, and ridges of up to 1 km in height. Surface roughness in Galileo Regio at DTM scales is markedly higher than in Uruk Sulcus. Bowl-shaped craters are identified in the models with depth to diameter ratios of about 1:12 (Uruk Sulcus) and 1:9 (Galileo Regio). This is in approximate agreement with data for simple craters measured in Voyager images. The large complex 20-km crater Ea is found to have a central dome 300 m above the crater floor, reaching more than half of the crater rim's height. These unique topographic data, at far higher resolution than what could be derived from Voyager images, will provide new insights into the formation of Ganymede's surface.

Formation of Ganymede Grooved Terrain by Sequential Extensional Episodes: Implications of Galileo Observations for Regional Stratigraphy

High-resolution Galileo images of grooved terrain on Ganymede have revealed details of the morphology of grooves and their stratigraphic relationships. Based on lower resolution Voyager images, complex areas of grooved terrain in Uruk Sulcus were interpreted in the context of structurally isolated grooved polygons which were resurfaced by cryovolcanism and individually deformed by local processes. In Galileo images of grooved polygon boundaries, the faults comprising the groove lanes between the polygons are observed to truncate features within the polygons, and incipient fractures from these groove lanes modify the edges of the polygons, indicating that these groove lanes are younger than the polygons. In the Uruk Sulcus region, tectonic resurfacing by the formation of younger sets of grooves appears to be obscuring older patterns of deformation. The grooved polygons are thus remnants of terrain affected by older episodes of deformation isolated from each other by areas of more recent deformation. The stratigraphy of grooves can be determined for small areas covered by high-resolution Galileo images (∼100 m/pixel) and then generalized to larger areas imaged by Voyager at lower resolution (∼1 km/pixel). This method has been applied to the regions around Uruk Sulcus and Nippur Sulcus, resulting in two preliminary conclusions about the nature of grooved terrain deformation: (1) the style of deformation has changed through time and (2) the orientation of least compressive stress has changed through time. Throughout the mapped region, the stratigraphically oldest grooves are closely spaced, subdued structures which indicate least compressive stress oriented NW–SE, while the youngest grooves exhibit morphology consistent with tilt block normal faulting, with least compressive stress oriented NE–SW. These observations are consistent over an area of 1.6 million square kilometers and therefore do not favor highly localized or low-strain mechanisms for grooved terrain formation, such as the cooling of cryovolcanic flows or the surface expression of diapirs. Instead, the formation of grooved terrain on Ganymede may be dominated by global expansion due to tidal heating and/or differentiation, possibly organized into coherent regional patterns by transient low-order convection or changes in the satellite's figure.

Grooved Terrain on Ganymede: First Results from Galileo High-Resolution Imaging

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 crosscutting 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.

The role of extensional instability in creating Ganymede grooved terrain: Insights from Galileo High‐Resolution Stereo Imaging

Galileo stereo images covering about 1500 km² of Uruk Sulcus on Ganymede have revealed two scales of ridges; (1) large‐scale ridges and troughs spaced ∼6 km apart, corresponding to the “grooves” seen in Voyager images, and (2) small‐scale ridges spaced hundreds of meters apart superimposed on the large‐scale ridges. We interpret the small‐scale ridges to be the result of tilt‐block normal faulting of the surface brittle layer, while the large‐scale ridges may be due to necking of the brittle layer over a ductile substrate. The geometry of the tilt blocks revealed in Galileo images leads to a minimum estimation of 51% to 58% extensional strain in the area. The strain estimate, when incorporated into a model for the formation of grooved terrain by necking of a brittle layer undergoing extension, leads us to estimate a thermal gradient of ∼20 K/km and a strain rate of ∼10−14 s−1 during groove formation.

Dark halo craters and the thickness of grooved terrain on Ganymede

Dark halo craters in grooved terrain on Ganymede represent potential probes of the subsurface geology. These craters are surrounded by a broad, diffuse, low albedo annulus, or halo. Halo radius is linearly proportional to crater radius, indicating that the halo is related to the continuous ejecta deposit of the crater and is not removed by natural aging processes, such as regolith development and exposure to the space environment. The halos have orange/violet albedo ratios that are intermediate between ratios associated with grooved terrain and cratered terrain. We propose that halos result from the incorporation of cratered terrain material into crater ejecta. In Uruk Sulcus, only craters greater than ∼12 km in diameter have halos, indicating that the cratered terrain material there forms a stratigraphic horizon, buried at depth in the grooved terrain, into which only larger craters can excavate. This depth can be calculated from the transition diameter by reconstructing the transient crater. We calculate a depth of excavation, corresponding to the depth of burial of cratered terrain, of 1.0 to 1.6 km. This shallow burial is inconsistent with deep subsidence and burial, stoping into the interior, or lateral spreading apart of ancient cratered terrain units, but is consistent with a filled graben model for grooved terrain, where relatively clean ice has been emplaced over downdropped blocks of cratered terrain.

Spectral analysis of groove spacing on Ganymede

Grooves on Ganymede are interpreted as extensional tectonic features whose regular spacing is an indicator of the thickness of a brittle surface layer, or “lithosphere,” at the time of deformation. We have performed a statistical analysis of groove spacing on Ganymede, taking Fourier transforms of 157 photometric profiles across sets of grooves and examining the resultant power spectra for peaks representing the dominant topographic periodicities. The distribution of groove spacing on Ganymede is approximately lognormal, with a minimum of ∼3.5 km, a maximum of ∼17 km, and a mean of 8.4 km. There is no correlation of groove spacing with latitude or longitude. The most striking feature of the geographic distribution is that while groove spacing tends to be quite constant within a given groove set, it can vary substantially from one groove set to another in a geographic region. An exception is the Uruk Sulcus region, with a mean groove spacing of 6.5 km and very little variability. We examine several models that relate the observed groove spacing to lithospheric thicknesses. The most important conclusion is that lithospheric thickness varied significantly from one groove set to the next while the grooves were being formed. We examine four possible causes for the heterogeneous distribution: variation in time of formation during a period in which the global mean heat flow was changing, variation in strain rate from one groove set to the next, local excursions in geothermal gradient caused by variations in the vigor of underlying convective upwellings, and local excursions in geothermal gradient caused by resurfacing events. The first three appear viable, while the fourth requires very rapid groove formation due to the short cooling times involved.

A geometric analysis of surface deformation: Implications for the tectonic evolution of Ganymede

A visual nonalignment of the furrows and the circularity of impact craters are used to study surface deformation on Ganymede. The furrow system is examined to test the hypothesis that lateral motion has taken place between areas of dark terrain. Results show that while lateral motion cannot be ruled out, it may not be required to explain the geometry of the system. Initial nonconcentricity of the furrows or an early period of penetrative deformation shortly after furrow formation could also account for the present configuration. Centers of curvature of the furrows in Galileo and Marius Regiones are numerically determined and it is shown that if lateral movement did occur, it is not possible to determine the amount of displacement. The axial ratios of impact craters in the Uruk Sulcus region which separates Galileo and Marius Regiones are determined and show that large scale shear deformation has not occured in that area since bright terrain was emplaced. Deformation of impact craters within Galileo Regio suggests that Ganymede's lithosphere has behaved rigidly throughout most of the satellite's evolution. The shapes and orientations of impact craters in dark terrain around wedges of bright terrain are used to place an upper limit on the amount of extension associated with bright terrain formation.