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A unified nomenclature for tectonic structures on the surface of Enceladus
... Therefore, the classification of tectonic features is necessarily morphology based, with inferences about kinematics and driving mechanisms being based on perceived similarities of features to analogs on Earth (e.g., Kattenhorn and Hurford, 2009). Nahm and Kattenhorn (2015) presented a morphology-based classification scheme for tectonic features on Enceladus, based on five distinct categories of features: ridges, troughs, (Becker et al., 2016) overlain with contours of spatial density, in craters per square kilomters, for impact crater diameters >3 km (Kinczyk et al., 2017). Ancient terrain is fully enclosed within the first contour, and the higher-density contour lines show the relative lack of craters in the equatorial zones of the ancient terrain. ...
... Sets of multiple, arcuate, parallel ridges comprise the interiors of triangular wedges that extend northward away from the cusps in the tectonic boundary surrounding the SPT, and are referred to as corrugated ridge belts (Fig. 6a). Corrugated ridge belts have been interpreted as either compressional ridges Schenk and McKinnon, 2009) or tilt-block (imbricate) normal faults (Nahm and Kattenhorn, 2015). Double ridges exhibit a central trough hundreds of meters deep, flanked by ridges up to ~100 m high, and typify the eruptive cracks (tiger stripes) in the SPT (Fig. 6b). ...
... Troughs are linear to curvilinear, cracklike depressions that lack bounding ridges and are the most abundant type of tectonic feature on Enceladus (Nahm and Kattenhorn, 2015) (Fig. 6d). They appear to exhibit only opening motions, in that the features they crosscut exhibit no evidence of lateral offsets. ...
... The satellite of Saturn, Enceladus, displays a surprising amount of tectonic activity in spite of its relatively small size (Figure 1) [e.g., Crow-Willard and Pappalardo, 2015;Nahm and Kattenhorn, 2015]. Tectonic activity and geophysical and geochemical evidence indicate that a global or regional ocean is likely present underneath the ice shell [e.g., Nimmo and Pappalardo, 2016], with important implications for the astrobiological potential of the satellite [Hsu et al., 2015;Glein et al., 2015]. ...
... A high heat flow at the SPT is inferred from direct measurements and modeling of tectonic length scales [Spencer et al., 2006;Bland et al., 2012Bland et al., , 2015. The SPT as a whole is surrounded by a deformation belt, the Southern Curvilinear Terrain (SCT), originally interpreted to be a compressional feature [e.g., Porco et al., 2006], although extension and shearing are also involved in generating this terrain [Spencer et al., 2009;Nahm and Kattenhorn, 2015]. In an icy satellite with a radius of 252 km and an orbital eccentricity of only 0.0047 it is surprising to find ongoing geologic activity. ...
... The SCT, which lies along the edge of the SPT, especially in the eastern hemisphere, constitutes a mountain belt at least a kilometer high [Schenk and McKinnon, 2009] with curved, parallel ridges, and grooves originally interpreted as compressional features [Gioia et al., 2007;Spencer et al., 2009]. A newer analysis puts this interpretation in question, though, pointing out the presence of grabens near the summit of the range [Nahm and Kattenhorn, 2015]. However, as graben can develop at the summit of compressional ranges on Earth due to differences in gravitational potential energy [e.g., Molnar and Lyon-Caen, 1988], this observation does not, on its own, contradict a compressional origin for the SCT. ...
Liquid water is likely present in the interior of Enceladus, but it is still debated whether this water forms a global ocean or a regional sea and whether the present-day situation is stable. As the heat flux of Enceladus exceeds most heat source estimates, the liquid water is likely cooling and crystallizing, which results in expansion and pressurization of the sea or ocean. We determine, using an axisymmetric Finite Element Model, the tectonic patterns that pressurization of a regional sea or global ocean might produce at the surface of Enceladus. Tension is always predicted above where the ice is thinnest and generates cracks that might be at the origin of the Tiger Stripes. Tectonic activity is also expected in an annulus around the sea if the ice shell is in contact with but slips freely along the rocky core of the satellite. Cracks at the north pole are expected if the shell slips along the core or if there is a global ocean with thin ice at the pole. Water is likely injected along the base of the ice when the shell is grounded, which may lead to cycles of tectonic activity with the shell alternating between floating and grounded states and midlatitude faulting occurring at the transition from a grounded to a floating state.
... To explain this placement, some studies (Nimmo & Pappalardo, 2006;Matsuyama & Nimmo, 2008) suggest that the hotspot moved to the polar region after forming elsewhere. Indeed, various aspects of Enceladus' surface geology could reflect changes in the satellite's orientation (Crow-Willard & Pappalardo, 2015;Spencer et al. 2009;Helfenstein et al. 2010;Nahm & Kattenhorn, 2015). ...
... Both convective regimes show, in general, good agreement with the overall arrangement of geological provinces discussed above; for example, the resurfaced leading and trailing hemispheres, which used to be at the paleo-poles (Fig.2b), are superficially similar to the SPT, but differ significantly in the overall placement and nature of their terrain sub-units (Crow-Willard & Pappalardo, 2015). However, among the geological features in the provinces are structures located in the geologically named "Transitional" terrain (Crow-Willard & Pappalardo, 2015) (at S1-S2, and ) such as peculiar ropy folds, called funiscular plains, materials that are otherwise found exclusively in the SPT region (Crow-Willard & Pappalardo, 2015;Spencer et al. 2009;Helfenstein et al. 2010;Nahm & Kattenhorn, 2015), hinting at past possible activity in Enceladus' paleo-poles. These are discussed in greater detail below. ...
Many obsects in the solar system are suspected to have experience reorientation of their spin axes. As their rotation rates are slow and their shapes are nearly spherical, the formation of mass anomalies, by either endogenic of exogenic processes, can change objects' moments of inertia. Therefore, the objects reorient to align their largest moment of inertia with their spin axis. Such phenomenon is called True Polar Wander (TPW). Here we report the discovery of a global series of topographic lows on Saturn's satellite Enceladus that we interpret to show that this synchronously locked moon has undergone TPW by ~55{\deg} about the tidal axis. We use improved topographic data from the spherical harmonic expansion of Cassini limb and stereogrammetric measurements to characterize regional topography over the surface of Enceladus. We identify a group of nearly antipodal basins orthogonal to a topographic basin chain tracing a non-equatorial circumglobal belt across Enceladus' surface. We argue that the belt and the antipodal regions are fossil remnants of an earlier equator and poles, respectively. We argue that these lows arise from isostasic compensation and that their pattern reflects spatial variations in internal dynamics of the ice shell. Our hypothesis is consistent with a variety of geological features visible in Cassini images.
... These fractures are believed to be ancient TSF, currently inactive, which suggest a long geological history of tiger-stripelike activity in the SPT (Figure 1, red arrows; Patthoff & Kattenhorn, 2011). Between the TSF are intensely folded terrains called funiscular plains (Figure 2; Barr & Preuss, 2010;Nahm & Kattenhorn, 2015;Spencer & Nimmo, 2013). This structural unit is part of the inner unit (i.e., the Central South Polar Unit) proposed by Crow-Willard and Pappalardo (2015). ...
... According to Cianfarra and Salvini (2015), the increased internal angle may have resulted from a transpressional regime within the SPT. The presence of folded funiscular plains (Barr & Preuss, 2010;Nahm & Kattenhorn, 2015;Spencer & Nimmo, 2013) may confirm the prevalence of a compressional component in those blocks delimited by the TSF. The texture clusters shown in Figure 9 within these blocks show compressional and extensional zones. ...
The South Polar Terrain (SPT) of Enceladus is a site with eruptions of gas and water ice particle plumes, which indicate internal geodynamic activity. These eruptions are located along a series of tectonic structures, that is, the Tiger Stripe Fractures (TSF), which are composed of regularly spaced, linear depressions. The SPT is surrounded by sinuous chains of ridges and troughs (the Marginal Zone). To unravel the tectonics that affect the region and its evolution, we performed specific structural mapping and quantitative analyses of brittle features from remotely sensed images. The results are consistent with a block rotation model, in which several tectonic regimes coexist. The TSF are left‐lateral strike‐slip faults that bound rigid elongated blocks. The blocks rotate clockwise and are enclosed in a regional scale right‐lateral kinematic framework expressed in the Marginal Zone. These two opposite and complementary kinematic regimes induce transtensional and transpressional regimes within the SPT. An evolutionary tectonic model is proposed for the past and future evolution of the SPT. This model confirms the role of tectonic‐related kinematics in icy satellites and contributes to preparations for future missions.
... The total length of the four "Tiger Stripe" sulci 8 (Alexandria Sulcus, Baghdad Sulcus, Cairo Sulcus and Damascus Sulcus, in alphabetical order) is approximately 500-590 km (Ingersoll and Pankine 2010;Porco et al. 2006). In the SPT, sulci are either double ridges or subdued double ridges (Nahm and Kattenhorn 2015). Double ridges are typically up to 2 km wide (Porco et al. 2006), with a 500 m central depression flanked by ∼ 100 m high ridges. ...
... Subdued double ridges are typically narrower (10-100s m wide rather than km) with less pronounced flanks. Although features attributed to tectonic modification are found at other locations (approximately centred on the orbital leading and trailing hemispheres, Bland et al. 2015 for example), the SPT is the only location in which double-ridged "Tiger Stripe" sulci are found (Nahm and Kattenhorn 2015). ...
In recent decades, volcanic and cryovolcanic activity on moons within the Solar System has been recognised as an important source of cosmic dust. Two moons, Jupiter’s satellite Io and Saturn’s satellite Enceladus, are known to be actively emitting dust into circumplanetary and interplanetary space. A third moon, Europa, shows tantalising hints of activity. Here we review current observations and theories concerning the generation, emission and evolution of cosmic dust arising from these objects.
... To explain this placement, some studies (Nimmo & Pappalardo, 2006;Matsuyama & Nimmo, 2008) suggest that the hotspot moved to the polar region after forming elsewhere. Indeed, various aspects of Enceladus' surface geology could reflect changes in the satellite's orientation (Crow-Willard & Pappalardo, 2015;Spencer et al. 2009;Helfenstein et al. 2010;Nahm & Kattenhorn, 2015). ...
... Both convective regimes show, in general, good agreement with the overall arrangement of geological provinces discussed above; for example, the resurfaced leading and trailing hemispheres, which used to be at the paleo-poles (Fig.2b), are superficially similar to the SPT, but differ significantly in the overall placement and nature of their terrain sub-units (Crow-Willard & Pappalardo, 2015). However, among the geological features in the provinces are structures located in the geologically named "Transitional" terrain (Crow-Willard & Pappalardo, 2015) (at S1-S2, and ) such as peculiar ropy folds, called funiscular plains, materials that are otherwise found exclusively in the SPT region (Crow-Willard & Pappalardo, 2015;Spencer et al. 2009;Helfenstein et al. 2010;Nahm & Kattenhorn, 2015), hinting at past possible activity in Enceladus' paleo-poles. These are discussed in greater detail below. ...
Many objects in the solar system are suspected to have experienced reorientation of their spin axes. As their rotation rates are slow and their shapes are nearly spherical, the formation of mass anomalies, by either endogenic or exogenic processes, can change objects’ moments of inertia. Therefore, the objects reorient to align their largest moment of inertia with their spin axis. Such a phenomenon is called True Polar Wander (TPW). Here we report the discovery of a global series of topographic lows on Saturn's satellite Enceladus that we interpret to show that this synchronously locked moon has undergone TPW by ∼55° about the tidal axis. We use improved topographic data from the spherical harmonic expansion of Cassini limb and stereogrammetric measurements to characterize regional topography over the surface of Enceladus. We identify a group of nearly antipodal basins orthogonal to a basin chain tracing a non-equatorial circumglobal belt across Enceladus' surface. We argue that the belt and the antipodal regions are fossil remnants of earlier equator and poles, respectively. We argue that these lows arise from isostasic compensation and that their pattern reflects spatial variations in internal dynamics of the ice shell. Our hypothesis is consistent with a variety of geological features visible in Cassini images.
... The inventory of organic molecules and other oxidizable species identified by INMS (Waite et al., 2006 indicates that redox reactions are also a potential energy source. CIRS has observed that most of the 15 GW of thermal emission from the south polar terrain is localized on the "tiger stripes" (Howett et al., 2011), the ∼130 km long fissures (Nahm and Kattenhorn, 2015) from which the plumes erupt, though exactly what processes drive this geothermal activity is not yet clear. ...
Saturn's moon Enceladus offers a unique opportunity in the search for life and habitable environments beyond Earth, a key theme of the National Research Council's 2013-2022 Decadal Survey. A plume of water vapor and ice spews from Enceladus's south polar region. Cassini data suggest that this plume, sourced by a liquid reservoir beneath the moon's icy crust, contain organics, salts, and water-rock interaction derivatives. Thus, the ingredients for life as we know it-- liquid water, chemistry, and energy sources-- are available in Enceladus's subsurface ocean. We have only to sample the plumes to investigate this hidden ocean environment. We present a New Frontiers class, solar-powered Enceladus orbiter that would take advantage of this opportunity, Testing the Habitability of Enceladus's Ocean (THEO). Developed by the 2015 Jet Propulsion Laboratory Planetary Science Summer School student participants under the guidance of TeamX, this mission concept includes remote sensing and in situ analyses with a mass spectrometer, a sub-mm radiometer-spectrometer, a camera, and two magnetometers. These instruments were selected to address four key questions for ascertaining the habitability of Enceladus's ocean within the context of the moon's geological activity: (1) How are the plumes and ocean connected? (2) Are the abiotic conditions of the ocean suitable for habitability? (3) How stable is the ocean environment? (4) Is there evidence of biological processes? By taking advantage of the opportunity Enceladus's plumes offer, THEO represents a viable, solar-powered option for exploring a potentially habitable ocean world of the outer solar system.
... Multiple particle sizes from 0.6 − 15 µm are simulated for each source location, and data are generated on the impact flux in particles/sec/m 2 and mass deposition in mm/year across the surface of Enceladus. Initial simulated maps of surface deposition from the Enceladus plume published in Kempf et al. (2010) have received interest from the larger research community (for example, Di Sisto and Zanardi, 2016;Nahm and Kattenhorn, 2015;Scipioni et al., 2017) and, here, we provide a more complete set of maps and data with respect to source location and particle size. Using the newly generated surface data for a curtain-style plume (Spitale et al., 2015) and the ∼ 100 discrete jets proposed in Porco et al. (2014), we provide new insight into the zenith angle of plume emissions, that is, the "tilt" of the jets. ...
Since the discovery of an ice particle plume erupting from the south polar terrain on Saturn's moon Enceladus, the geophysical mechanisms driving its activity have been the focus of substantial scientific research. The pattern and deposition rate of plume material on Enceladus' surface is of interest because it provides valuable information about the dynamics of the ice particle ejection as well as the surface erosion. Surface deposition maps derived from numerical plume simulations by Kempf et al. (2010) have been used by various researchers to interpret data obtained by various Cassini instruments. Here, an updated and detailed set of deposition maps is provided based on a deep-source plume model (Schmidt et al., 2008), for the eight ice-particle jets identified in Spitale and Porco (2007), the updated set of jets proposed in Porco et al. (2014), and a contrasting curtain-style plume proposed in Spitale et al. (2015). Methods for computing the surface deposition are detailed, and the structure of surface deposition patterns is shown to be consistent across changes in the production rate and size distribution of the plume. Maps are also provided of the surface deposition structure originating in each of the four Tiger Stripes. Finally, the differing approaches used in Porco et al. (2014) and Spitale et al. (2015) have given rise to a jets vs. curtains controversy regarding the emission structure of the Enceladus plume. Here we simulate each, leading to new insight that, over time, most emissions must be directed relatively orthogonal to the surface because jets "tilted" significantly away from orthogonal lead to surface deposition patterns inconsistent with surface images. Data for maps are available in HDF5 format for a variety of particle sizes at http://impact.colorado.edu/southworth_data.
... Pit chains are type of linear structure comprise of circular to semi-circular collapsed pit rims (Martin & Kattenhorn, 2013;Martin et al., 2017;Nahm & Kattenhorn, 2015;Wyrick et al., 2004) which are partially or fully fused together to produce scalloped-edged steeply dipping troughs. Isolated pits in an alignment are considered as an initial phase of the linear structure which gradually transit into a wider trough structure (Horstman and Melosh, 1989). ...
This work evaluates volatile induced surface features on Vesta and Ceres, two of the largest asteroids present within the asteroid belt. Both the planetary objects have similar surface acceleration but different regolith nature. Vesta is a relatively dry body whereas Ceres is rich with water ice. Direct measurement of volatiles is challenging due to harsh space conditions. However, when they are mixed with regolith, it produces peculiar landforms due to melting and/or sublimation and affects the overall evolution of a planetary body. Therefore, in this study the surface features which have direct or indirect link to ice and/or volatiles are examined in order to understand the volatile distribution. For this, regional and global scale investigations related to ponded deposits, pit chains and mass wasting analysis were conducted on Vesta and Ceres. In the vicinity of Marcia and Cornelia impact craters of Vesta, two types of pond deposits were observed. Type 1 melt ponds have smooth, shallow deposits (depth <100 m) and are produced from the downslope movement of volatile bearing impact melt material. In contrast, type 2 dust ponds deposit consist of rough surface with ~200 m depth. These deposits are produced from the mobility of granular dust via infrequent high-amplitude seismic diffusivity and/or short-lived volatile outgassing activity. Due to low amounts of volatiles, the dusty material did not achieve kinetic sieving and thus do not attain typical smooth pond morphology. The findings of this study strongly support the hypothesis related to presence of low amounts of volatiles within Vesta’s regolith. To understand the volatile distribution on Ceres, the analysis of pit chains is carried out within three impact craters namely; Occator, Azacca and Urvara. Radial pit chain pattern of Occator is related to subsurface laccolith swelling of volatile rich cryomagmatic material. Linear pit chain clusters at floors of Azacca and Urvara are attributed to seasonal thermal contraction of ice layer present near the surface. Additionally, based on the pit chains depth the depicted average minimum thickness of regolith within Azacca, located at equator is ~200 m. On the contrary, within Occator and Urvara, the localized thickness is 30 m and 800 m, respectively, which is attributed to their distinct subsurface condition. Hence, this investigation favors the presence of ice layer within the subsurface layer and reveals that it is not distributed homogeneously on Ceres. Lastly, the global scale comparative examination of the mass wasting process on Vesta and Ceres shows few common and some distinct characteristics. In general, granular sliding on Vesta and flow-like movements on Ceres are observed as dominant population. Further, slides and slumping features are restricted to mid-latitudes on Ceres which implies ice-rock fractionation at regional scale. Additionally, the volatile concentration also influences the deposit mobility on Vesta and Ceres and is analyzed by estimating height, width and effective coefficient of friction; H/L. The outcome suggests that deposits become immobile at shorter distances on Vesta in comparison to Ceres (avg. distance 4.5 km and 11.2 km, respectively). The difference in morphology and mobility is related to contrast in the amounts of volatiles present within regolith of both the bodies. While comparing the effective coefficient of friction of Vesta and Ceres with planetary objects in outer solar system, the examination shows that lower temperature may have more influence on mobility. Together, all the above-mentioned studies summarize the volatile induced surface landforms and provide evidences related to their distribution on Vesta and Ceres. This work also presents the first-time comparative investigation that reveals the influence of volatile content on the morphological characteristics of Vesta and Ceres.
... Europa's inventory of geologic units includes ridged plains, craters, band material and chaos terrain (Leonard et al. 2018). At Enceladus, terrain types include troughs, scarps, chasmata, ridges, and bands (Nahm and Kattenhorn, 2015). Enceladus also famously hosts a plume, sourced from its subsurface ocean and emanating from four giant fissures in the South Polar Terrain (Schenk et al., 2018, and references therein). ...
... Due to the variety of structural features on the surface, a unified nomenclature for the structural features on Enceladus has been recently proposed by Nahm and Kattenhorn (2015) with five classes of identified tectonic structures. ...
The objective of this document is the definition of a set of cartographic and technical standards and directions to be used, adapted or -in minor form -established for GMAP. Standards proposed and mentioned in the present documents include geologic and cartographic aspects. Some of the proposed directions and standards are initial ones that are planned to be refined and/or updated throughout the Europlanet H2024RI project, to be used within the VA activities and for future sustainable European planetarymapping efforts beyond the RI.The state of the art and relevant documents are included, as well as process-specific and body-specific best practice and exemplary published cases. The approaches for two-dimensional mapping and three-dimensional geologic mapping and modelling are introduced, as well as the range of non-standard map types that are envisaged within GMAP activities. Mapping review directions are indicated, as well data sharing, distribution and discovery.Proposed standards, best practice, andtools are based on existing ones or on additional or new developments and adaptations.Appendices are included and point to either individual developments or external resources and tools that will be maintained throughout the duration of the research infrastructure, and beyond it, through sustainability.The present document is going to be a live document permanently accessible on the GMAP wiki and periodically updated in form of a deliverable.
... Features known as pit chains have been observed across the surface of Enceladus (Martin et al., 2017;Martin & Kattenhorn, 2013;Michaud et al., 2008;Nahm & Kattenhorn, 2015), as well as Earth, Mars, Venus, Phobos, and many asteroids (Gaspra, Ida, Eros, Steins, Lutetia, Vesta, Ceres; e.g., Thomas et al., 1978;Solomon et al., 1992;Veverka et al., 1994;Sullivan et al., 1996;Prockter et al., 2002;Ferrill et al., 2004Ferrill et al., , 2011Wyrick et al., 2004;Keller et al., 2010;Thomas et al., 2012;Davey et al., 2013;Buczkowski et al., 2014;Scully et al., 2017; Figure 1); pits chains have also been referred to as grooves, pit crater chains, or catena. Pit chains are linear to curvilinear alignments of circular to near-circular depressions. ...
Pit chains are distinctive geologic landforms present on planetary bodies across the solar system, which can be used to understand regolith properties and formation. If there is a direct relationship between pit diameter and the regolith depth, pit chains can be used to map regolith thickness across a planetary surface. Here, the morphology of individual pits within pit chains in northern Iceland was measured as a planetary analog to determine whether there is a direct relationship between individual pit diameters and pit depth, which is assumed to be a proxy for regolith depth. The morphology of pit chains and regolith thickness was measured in three different locations in northern Iceland, within two different substrates: unconsolidated floodplain sediments and more consolidated soils overlaying basaltic lava flows. There is a significant linear relationship between pit diameter and depth for pit chains located in the floodplain sediment, but not for pits located in basaltic materials. However, this depth‐diameter relationship does not correlate with regolith thickness. The depth of the pits in basaltic materials is more representative of the regolith depth than the pits in the floodplain sediments. If high‐resolution topography data are available, then the depth of pits in more consolidated materials can be used as a reliable estimate of regolith thickness. In more unconsolidated materials, the pit depth represents a minimum regolith depth. This ability to resolve regolith thicknesses may help resolve the processes by which regolith is emplaced and has implications for the thermal evolution of planetary bodies.
... The lines and edges are two types of structures that have high curvature in a direction and low curvature in another direction. In an image with different intensity surface, the curvilinear structures correspond to the ridges and valleys within the image (Nahm and Kattenhorn 2015). The curvilinear structures can be modelled in 2D as S(t) curves which represent the characteristics of a 1D line profile as perpendicular to the curve S′(t). ...
In this paper, the main goal is to identify the sine fractures of reservoir rock automatically. Therefore, a five-step algorithm is applied on the imaging logs. The first step consists of extracting the features of the imaging log by applying the Zernike moments. In the second step, the features are learned by using sparse coding. In the third step, the imaging log is segmented by using the self-organizing map neural network and the training dataset. In the fourth step, the fracture points are extracted by Steger method. In the last step, to determine the sine parameters of fractures, the Hough transform is applied to the image fracture points. The experimental results show that the proposed algorithm is highly able to detect the fractures of the imaging logs successfully. Also, the precision of the proposed method to extract the fracture pixels is so high and it has low sensitivity to noise in the imaging logs. In this paper, the proposed algorithm has been applied on the imaging datasets of FMI and the obtained results show that the classification has better precision compared with other proposed algorithm.
... Multiple particle sizes from 0.6 − 10 µm are simulated for each source location, and data are generated on the impact rate in particles/sec and mass deposition in mm/year across the surface of Enceladus. Initial simulated maps of surface deposition from the Enceladus plume published in Kempf et al. (2010) have received interest from the larger research community (for example, Di Sisto and Zanardi, 2016; Nahm and Kattenhorn, 2015;Scipioni et al., 2017) and, here, we provide a more complete set of maps and data with respect to source location and particle size. Using the newly generated surface data for a curtain-style plume (Spitale et al., 2015) and the ∼ 100 discrete jets proposed in , we provide new insight into the zenith angle of plume emissions, that is, the "tilt" of the jets. ...
Since the discovery of an ice-particle plume erupting from the south polar terrain on Saturn's moon Enceladus (Porco et al., 2006; Spahn et al., 2006; Spencer et al., 2006), the geophysical mechanisms driving its activity have been the focus of substantial scientific research. The pattern and deposition rate of plume material on Enceladus' surface is of interest because it provides valuable information about the dynamics of the ice particle ejection as well as the surface erosion. Surface deposition maps derived from numerical plume simulations by Kempf et al. (2010) have been used by various researchers to interpret data obtained by various Cassini instruments. Here, an updated and detailed set of deposition maps is provided based on a deep-source plume model (Schmidt et al., 2008), for the eight ice-particle jets identified in Spitale and Porco (2007), the updated set of jets proposed in Porco et al. (2014), and a contrasting curtain-style plume proposed in Spitale et al. (2015). Methods for computing the surface deposition are detailed, and the structure of surface deposition patterns is shown to be consistent across changes in the production rate and size distribution of the plume. Images are also provided of the surface deposition structure originating in each of the four Tiger Stripes. Finally, the differing approaches used in Porco et al. (2014) and Spitale et al. (2015) have given rise to a jets vs. curtains controversy regarding the emission structure of the Enceladus plume. Here we simulate each, leading to new insight that, over time, most emissions must be directed relatively orthogonal to the surface because jets "tilted" significantly away from orthogonal lead to surface deposition patterns inconsistent with surface images.
... Its 287 ridged plains mark a zone of convergence and folding 500 km wide, with a crater-based 288 age of 10 to 100 Ma (Kargel and Pozio 1996). Geologic activity in the southern polar 289 region is documented in the form of water vapor and ice particles emanating from 290 fractures known informally as ''tiger stripes'' (Nahm and Kattenhorn, 2015 Titan is the largest satellite and the largest Tholin-rich body in the Solar System, 302 with far more hydrocarbons than found on Earth (Lorenz et al., 2008). Its soft icy 303 hydrocarbon-rich surface does not preserve faults well but the SAR instrument onboard 304 ...
To better understand Earth's present tectonic style–plate tectonics–and how it may have evolved from single plate (stagnant lid) tectonics, it is instructive to consider how common it is among similar bodies in the Solar System. Plate tectonics is a style of convection for an active planetoid where lid fragment (plate) motions reflect sinking of dense lithosphere in subduction zones, causing upwelling of asthenosphere at divergent plate boundaries and accompanied by focused upwellings, or mantle plumes; any other tectonic style is usefully called “stagnant lid” or “fragmented lid”. In 2015 humanity completed a 50+ year effort to survey the 30 largest planets, asteroids, satellites, and inner Kuiper Belt objects, which we informally call “planetoids” and use especially images of these bodies to infer their tectonic activity. The four largest planetoids are enveloped in gas and ice (Jupiter, Saturn, Uranus, and Neptune) and are not considered. The other 26 planetoids range in mass over 5 orders of magnitude and in diameter over 2 orders of magnitude, from massive Earth down to tiny Proteus; these bodies also range widely in density, from 1000 to 5500 kg/m3. A gap separates 8 silicate planetoids with ρ = 3000 kg/m3 or greater from 20 icy planetoids (including the gaseous and icy giant planets) with ρ = 2200 kg/m3 or less. We define the “Tectonic Activity Index” (TAI), scoring each body from 0 to 3 based on evidence for recent volcanism, deformation, and resurfacing (inferred from impact crater density). Nine planetoids with TAI = 2 or greater are interpreted to be tectonically and convectively active whereas 17 with TAI
... 3 0 1 0 0 2 W m −2 for a DSS with CBE-appear coincident with one or two curved scarps associated with sharp elevation changes (up to ~1 km (ref. 22 )) and the bounding radar-bright V-shaped discontinuities located at the eastern and western ends of the swath (Fig. 2c,e). The broken-up appearance of these discontinuities in the SAR image and their blue colour in Cassini Imaging Science Subsystem (ISS) images suggest the presence of coarsegrained ice, like in the active sulci, and thus recent activity 23 . ...
Saturn’s moon Enceladus is an active world. In 2005, the Cassini spacecraft witnessed for the first time water-rich jets venting from four anomalously warm fractures (called sulci) near its south pole1,2. Since then, several observations have provided evidence that the source of the material ejected from Enceladus is a large underground ocean, the depth of which is still debated3, 4, 5, 6. Here, we report on the first and only opportunity that Cassini’s RADAR instrument7,8 had to observe Enceladus’s south polar terrain closely, targeting an area a few tens of kilometres north of the active sulci. Detailed analysis of the microwave radiometry observations highlights the ongoing activity of the moon. The instrument recorded the microwave thermal emission, revealing a warm subsurface region with prominent thermal anomalies that had not been identified before. These anomalies coincide with large fractures, similar or structurally related to the sulci. The observations imply the presence of a broadly distributed heat production and transport system below the south polar terrain with ‘plate-like’ features and suggest that a liquid reservoir could exist at a depth of only a few kilometres under the ice shell at the south pole. The detection of a possible dormant sulcus further suggests episodic geological activity.
... Groove widths and spacings are relatively homogeneous, as is typical of extensional fracture sets in a uniform thickness brittle layer. Scalloped edges along many grooves are consistent with pit chain morphologic evolution, whereby initially isolated pits merge along strike to create a continuous feature, as is described across the solar system [Horstman and Melosh, 1989;Ferrill et al., 2004;Wyrick et al., 2004;Nahm and Kattenhorn, 2015]. Initially isolated pits in loose regolith align above an underlying dilational crack, into which regolith progressively drains. ...
Phobos, the innermost satellite of Mars, displays an extensive system of grooves that are mostly symmetric about its sub-Mars point. Phobos is steadily spiraling inward due to the tides it raises on Mars lagging behind Phobos’ orbital position and will suffer tidal disruption before colliding with Mars in a few tens of millions of years. We calculate the surface stress field of the deorbiting satellite and show that the first signs of tidal disruption are already present on its surface. Most of Phobos’ prominent grooves have an excellent correlation with computed stress orientations. The model requires a weak interior that has very low rigidity on the tidal evolution time scale, overlain by an ~10–100 m exterior shell that has elastic properties similar to lunar regolith as described by Horvath et al. (1980).
... The inventory of organic molecules and other oxidizable species identified by INMS (Waite et al., 2006 indicates that redox reactions are also a potential energy source. CIRS has observed that most of the 15 GW of thermal emission from the south polar terrain is localized on the "tiger stripes" (Howett et al., 2011), the ∼130 km long fissures (Nahm and Kattenhorn, 2015) from which the plumes erupt, though exactly what processes drive this geothermal activity is not yet clear. ...
Saturn's moon Enceladus offers a unique opportunity in the search for life and habitable environments beyond Earth, a key theme of the National Research Council's 2013-2022 Decadal Survey. A plume of water vapor and ice spews from Enceladus's south polar region. Cassini data suggest that this plume, sourced by a liquid reservoir beneath the moon's icy crust, contain organics, salts, and water-rock interaction derivatives. Thus, the ingredients for life as we know it-- liquid water, chemistry, and energy sources-- are available in Enceladus's subsurface ocean. We have only to sample the plumes to investigate this hidden ocean environment. We present a New Frontiers class, solar-powered Enceladus orbiter that would take advantage of this opportunity, Testing the Habitability of Enceladus's Ocean (THEO). Developed by the 2015 Jet Propulsion Laboratory Planetary Science Summer School student participants under the guidance of TeamX, this mission concept includes remote sensing and in situ analyses with a mass spectrometer, a sub-mm radiometer-spectrometer, a camera, and two magnetometers. These instruments were selected to address four key questions for ascertaining the habitability of Enceladus's ocean within the context of the moon's geological activity: (1) How are the plumes and ocean connected? (2) Are the abiotic conditions of the ocean suitable for habitability? (3) How stable is the ocean environment? (4) Is there evidence of biological processes? By taking advantage of the opportunity Enceladus's plumes offer, THEO represents a viable, solar-powered option for exploring a potentially habitable ocean world of the outer solar system.
This study presents a comprehensive assessment of the geomorphology, crater distributions, and tectonic structures within Enceladus' cratered terrains. We analyzed the distributions of impact craters and tectonic structures in seven regions of interest to inform an interpretation of the geological history of this terrain in the context of Enceladus' global evolution. We found that the tectonic structures, including both ancient, subdued troughs and young, narrow fractures, point to a cratered terrain that not only experienced early tectonic modification but also shows evidence of recent geological activity. Ancient troughs present in the equatorial cratered terrains are similar in scale and orientation to troughs present in the Leading and Trailing Hemisphere Terrains, an observation that supports possible non‐synchronous rotation of the ice shell. A dearth of impact craters in the equatorial regions as identified previously does not hold for craters <3 km in diameter in the anti‐Saturnian hemisphere. The anomalous presence of excess small craters in this region could be due to secondary or sesquinary impacts from a catastrophic event occurring at Enceladus or a neighboring moon. Finally, narrow fractures are pervasive across the cratered terrains and are most commonly oriented parallel or sub‐parallel to the most proximal cratered terrain boundary. This directionality of pervasive recent fracturing could be related to the vertical movement of an isostatically uncompensated ice shell. Enceladus' cratered terrains provide insight into the long‐term evolution of the satellite, an important component to assessing its role in Solar System evolution and its potential for habitability.
This study systematically characterizes normal faults in tephra rings at Weizhou and Xieyang volcanic islands in southern China. Detailed field observations, measurements and analyses indicate that these faults were likely formed during phreatomagmatic eruptions based on the following evidence: i) faults in tephra rings are closely spaced and fault cores commonly have no fault gouge, debris and secondary infill, ruling out long-term faulting mechanisms in extensional settings; ii) faults are usually accompanied by soft-sediment deformation structures, indicating most volcanic layers were in soft during faulting; iii) faulted tephra deposits are consistently thicker on the downdropped hanging-wall than on the uplifted footwall, indicating that the faults formed during the deposition of the tephra; iv) unlike late-stage strike-slip faults, which crop out in many places on the two islands, the normal faults generally occur adjacent to the volcanic craters, implying local control. Repeated phreatomagatic eruptions produced alternating and unstable ash and lapilli deposits on the tephra rings. These layers failed during the eruptions, resulting in the formation of syn-eruptive normal faults in the tephra rings.
Abstrakt W Układzie Słonecznym odkryto do tej pory 213 księżyców [15]. Wiele z nich wykazuje niezwykłe cechy i zachowania, które czynią je nie mniej interesującymi od planet. Jeśli chodzi o oddziaływanie grawitacyjne Słońce ma w Układzie Słonecznym dominującą rolę. Jednak sześć planet zdołało wokół siebie utworzyć własne, miniplanetarne systemy, w których na orbitach znajdują się ich naturalne satelity. Każda z tych planet utrzymuje na orbicie od jednego do kilkudziesięciu księżyców, mimo iż Słońce stara się je "uwolnić". Brak satelitów wokół Merkurego i Wenus jest spowodowane ich niewielką odległością od Słońca i wynika z jego grawitacji. Za pasem asteroid, oddzielającym planety skaliste od planet gazowych grawitacja słoneczna jest słabsza i z tego powodu planety zewnętrzne cechuje liczebność orbitujących wokół nich różnorodnych księżyców, na które mogą silnie oddziaływać. Wpływ planet olbrzymich na okrążające satelity sprawia, że niektóre z nich są stale aktywne geologicznie.
Celem niniejszej pracy jest analiza procesów zachodzących na Europie-księżycu Jowisza i Enceladusie-księżycu Saturna na podstawie informacji o własnościach fizycznych i budowie wewnętrznej tych księżyców oraz oddziaływań pomiędzy tymi satelitami, ich macierzystymi planetami i innymi księżycami.
For many years, planetary science has been taught as part of the astronomy curriculum, from a very physics-based perspective, and from the framework of a tour of the Solar System - body by body. Over the past decades, however, spacecraft exploration and related laboratory research on extraterrestrial materials have given us a new understanding of planets and how they are shaped by geological processes. Based on a course taught at the University of Tennessee, Knoxville, this is the first textbook to focus on geologic processes, adopting a comparative approach that demonstrates the similarities and differences between planets, and the reasons for these. Profusely illustrated, and with a wealth of pedagogical features, this book provides an ideal capstone course for geoscience majors - bringing together aspects of mineralogy, petrology, geochemistry, volcanology, sedimentology, geomorphology, tectonics, geophysics and remote sensing.
For many years, planetary science has been taught as part of the astronomy curriculum, from a very physics-based perspective, and from the framework of a tour of the Solar System - body by body. Over the past decades, however, spacecraft exploration and related laboratory research on extraterrestrial materials have given us a new understanding of planets and how they are shaped by geological processes. Based on a course taught at the University of Tennessee, Knoxville, this is the first textbook to focus on geologic processes, adopting a comparative approach that demonstrates the similarities and differences between planets, and the reasons for these. Profusely illustrated, and with a wealth of pedagogical features, this book provides an ideal capstone course for geoscience majors - bringing together aspects of mineralogy, petrology, geochemistry, volcanology, sedimentology, geomorphology, tectonics, geophysics and remote sensing.
Conceptual models for the evolution of dilatant faults in volcanic rift settings involve a step-wise growth pattern, involving upward propagation of subsurface faults, and surface monocline formation, which are breached by subvertical, open faults. Immature, discontinuous normal faults are considered representative of the early stages of mature, linked faults that accommodate extensional strains. We consider the evolution of surface-breaking normal faults using a comparison of the distribution and geometry of normal faults from two volcanic rift zones: the Koaʻe fault system, Hawaiʻi, and the Krafla fissure swarm, NE Iceland. Field mapping highlights similarities to current predicted geometries, but also prominent differences that are not reconciled by current models. Variable deformation styles record magma supply changes within the rift zones, which drive local strain rate gradients. Building on existing studies, we present a conceptual model of fault growth that accounts for spatial and temporal changes in strain rate within the deforming regions. We propose that faults in separate rift systems may not advance through the same stages of evolution and that faults within individual rift systems can show differing growth patterns. Variations in surface strains may be indicative of subsurface magmatic system changes, with important implications for our understanding of volcano-tectonic coupling.
Enceladus is the first outer solar system body on which pit chains have been positively identified. We map the global distribution of pit chains and show that pit chains are among the youngest tectonic features on Enceladus's surface, concentrated in the cratered plains centered on Enceladus's Saturnian and anti-Saturnian hemispheres. Pit chains on Enceladus are interpreted as the surface expressions of subsurface dilational fractures underlying a cover of unconsolidated material, which we infer to be a geologically young cover of loose regolith that mantles the surface of Enceladus. A widespread layer of regolith may act to insulate the surface, which has implications for the thermal state of Enceladus's ice shell. The widespread distribution of pit chains across the cratered plains indicates that this ancient surface has recently been tectonically active.
Despite a decade of intense research the mechanical origin of the tiger-stripe fractures (TSF) and their geologic relationship to the hosting South Polar Terrain (SPT) of Enceladus remain poorly understood. Here we show via systematic photo-geological mapping that the semi-squared SPT is bounded by right-slip, left-slip, extensional, and contractional zones on its four edges. Discrete deformation along the edges in turn accommodates translation of the SPT as a single sheet with its transport direction parallel to the regional topographic gradient. This parallel relationship implies that the gradient of gravitational potential energy drove the SPT motion. In map view, internal deformation of the SPT is expressed by distributed right-slip shear parallel to the SPT transport direction. The broad right-slip shear across the whole SPT was facilitated by left-slip bookshelf faulting along the parallel TSF. We suggest that the flow-like tectonics, to the first approximation across the SPT on Enceladus, is best explained by the occurrence of a transient thermal event, which allowed the release of gravitational potential energy via lateral viscous flow within the thermally weakened ice shell.
Introduction: Brittle deformation on the icy satellites can be the result of numerous processes. On Earth, many of these processes are related in some way to plate tectonics; however, plate-like motions are rare on Europa [1]. A dominant driving force may be tidal deformation, which caused the majority of the fracturing on Europa [2-7] and possibly in the south polar region of Enceladus [8-9]. Such deformation is prevalent where the ice shell responds to the oscillations of tidal bulges above a liquid layer on any icy satellite having an orbital eccentricity [4]. Surface fracturing can also be driven by endogenic processes such as diapiric uplift [10], spreading due to gravitational collapse, folding and warping of the ice shell [11], flexure alongside a surface load [12], and impact events [13].
Regardless of the source of stress in a deforming ice shell, another type of tectonic deformation that may plate a significant contributor to the strain history and surface morphology is that due to shearing effects. Shearing of a pre-existing structure (whether it be a discrete crack or a weak zone of finite width) loaded by any source of differential stress, may induce locally perturbed, high magnitude stress fields that cause localized deformation [14]. We outline the mechanics of secondary tectonic deformation due to shearing and provide examples of its significance in the tectonic history of Europa. Similar deformation could potentially be found on other icy satellites, particularly if there is a significant source of stress to drive shearing, such as from tidal forcing.
Secondary Tectonic Deformation: When a pre-existing discontinuity is reactivated by horizontal shear stresses, resultant lateral motions turn the discontinuity into a strike-slip fault. For the case of a constant maximum compressive principal stress direction acting at some oblique angle to a fault, motion occurs when the Coulomb failure criterion is met: t ³ msn, where t is shear stress, sn is normal stress, and m is the coefficient of static friction. Tidal stresses on a satellite with orbital eccentricity rotate during the course of the orbit, so the mechanics of motion along the fault may vary during the day [15]. Tensile stresses may cause a discontinuity to open during the orbit, in which case there is no frictional resistance to shear motion. Evidence for both dilational shear motion and frictional shear motion has been described on Europa [14].
Because sheared lineaments on icy satellites must have a finite length, linear elastic fracture mechanics predicts that concentrations of stress occur at the tips of the shearing discontinuities. In fact, the entire region adjacent to a strike-slip fault experiences a perturbation to the regional stress field, resulting in localized zones of extension and compression arranged antisymmetrically about each fault tip (Fig. 1a). Localized deformation in these zones of increased stress is referred to as secondary tectonic deformation and may include fracturing and crustal thinning in extensional quadrants, and folding, pressure solution, or crustal thickening in compressional quadrants.
Fig. 1. (a) Quadrants of locally increased extension (blue colors) and compression (orange colors) adjacent to a right-lateral fault. (b) Tailcracks form at fault tips and propagate into the extensional quadrants. The maximum compressive stress is s3. Tailcrack angles are shown as qt. Both (a) and (b) are for a right-lateral fault. The left-lateral case is the mirror image.
Application to Europa: Shearing of lineaments and secondary tectonic deformation have played an important role in Europan tectonics.
Tailcracks. Stress concentrations at fault tips may induce secondary cracks called tailcracks that propagate into the extensional quadrants (Fig. 1b). The angle of the tailcrack (qt) is commonly around 70¡ but may be less if there is a component of opening along the fault during shearing [14]. Tailcracks have been identified on Europa [14, 16-17] and should be relatively easy to identify on any icy satellite (Fig. 2).
Fig. 2. Tailcracks at the SE tip of Agenor Linea, which experienced concomitant dilation and shear [14].
Anti-cracks. These are very subtle contractional features that form in the compressional quadrants at the tips of a shearing lineament. They have been described at Argadnel Regio [18] but are uncommon.
Cycloidal cracks. Although cycloids on Europa have been shown to trace out the changing direction of the maximum tensile diurnal tidal stress during the orbit [5], there is a period of time during which crack growth ceases while stresses continue to rotate. During this time, shear stresses are resolved onto the tip region of the arrested cycloid segment. Cycloid cusp angles and geometries are compatible with having formed by a tailcrack process, thus initiating a new cycloid segment that then propagates into the extensional quadrant driven onwards by the tidal stresses [19].
Cusp angles are thus analogous to tailcrack angles and must similarly be dictated by the exact ratio of shear-to-normal stress (t/sn) resolved onto the cycloid tip at the instant of cusp growth. Our analysis of Europan cycloids in the northern trailing hemisphere reveals that it is always possible to find a point in the orbit at which the required t/sn ratio occurs needed to account for measured cusp angles (Fig. 3) [20]. This point in the orbit occurs later than when the maximum tension is achieved, implying that new cycloid segments are only able to form due to the effects of shearing and tailcrack development at the tip of a previously formed segment. Hence, initial cycloid growth is likely triggered by shearing along, and cracking away from, an older lineament.
Crustal contraction. Shearing of a pre-existing lineament produces shear heating that may be responsible for thermal upwelling and the construction of ridge ramparts to either side of a central crack [21].
Our analysis of ridges showing strike-slip offsets reveals that they could not have formed purely due to lateral motions. Instead, apparent offsets were also produced than can only be reconciled with crustal convergence at ridges during shearing and heating [22].
Fig. 3. Shear stress (dashed curve) and normal stress (solid curve) resolved onto a cycloid tip where a cusp developed. The gray area represents the point in the orbit where the ratio of the stresses was exactly right for the cusp to form. The vertical gray line is the point at which the the tensile principal stress is maximized.
Conclusions: Shearing of lineaments on Europa has contributed to the tectonic deformation through the creation of strike-slip faults and associated development of secondary tailcracks and anti-cracks, the initiation of cycloid segments, and the accommodation of crustral contraction along ridges. Similar deformation could conceivably occur on other icy satellites.
References: [1] Patterson, G. W. et al. (2006) JSG, 28, 2237–2258. [2] Helfenstein, P., Parmentier, E. M. (1985) Icarus, 61, 175-184. [3] McEwen, A. S. (1986) Nature, 321, 49-51. [4] Greenberg, R. et al. (1998) Icarus, 135, 64-78. [5] Hoppa, G. V. et al. (1999) Science, 285, 1899-1902. [6] Figueredo, P. H., Greeley, R. (2000) JGR, 105, 22,629-22,646. [7] Kattenhorn, S. A. (2002) Icarus, 157, 490-506. [8] Porco, C. C. et al. (2006) Science, 311, 1393-1401. [9] Hurford, T. A. et al. (2007) Nature, in press. [10] Collins, G. C. et al. (2000) JGR, 105, 1709–1716. [11] Prockter, L. M., Pappalardo, R. T. (2000) Science, 289, 941-943. [12] Billings, S. E., Kattenhorn, S. A. (2005) Icarus, 177, 397-412. [13] Melosh. H. J. (1989) Impact Cratering: A Geologic Process. [14] Kattenhorn, S. A. (2004) Icarus, 172, 582-602. [15] Hoppa, G. V. et al. (1999) Icarus, 141, 287-298. [16] Prockter, L. M. et al. (2000) JGR, 105, 9483-9488. [17] Schulson, E. M. (2002) JGR, 107, doi:10.1029/2001JE001586. [18] Kattenhorn, S. A., Marshall, S. T. (2006) JSG, 28, 2204-2221. [19] Marshall, S. T., Kattenhorn, S. A. (2005) Icarus, 177, 341-366. [20] Groenleer, J. M., Kattenhorn, S. A. (2006) Eos, Trans. AGU, 87, P31D-08. [21] Nimmo, F., Gaidos, E. (2002) JGR, 107, 1-8. [22] Vetter, J. C., Kattenhorn, S.A. (2005), LPSC, XXXVI, abstract #1053.
Introduction: Pit chains are linear troughs made up of circular to elliptical depressions [1] and are distinguishable from impact craters (which have raised crater rims), impact ejecta, or flow features [1,2]. They commonly tend to be cone or bowl shaped collapse depressions with circular to elliptical plan view shapes and, in some cases, exhibit flat floors [3] (Fig. 1). Often forming in parallel sets, pit chains are thought to be the result of unconsolidated regolith drainage into dilational normal faults within extensional tectonic settings [1,2]. Pit chains have been identified in many locations including Mars [1,4], Phobos [5], and Earth [6] (see review by [3]), and more recently have been identified in the outer solar system on Enceladus [2]. On Enceladus specifically, pit chains are concentrated within the cratered terrains of Enceladus's Saturnian and anti-Saturnian hemispheres.
Pit chains may serve as an important tool for probing the distribution of regolith depths on the surface of Enceladus. It has been experimentally determined by [5] that a correlation exists between average pit spacing along a line of pits and the regolith thickness, independent of the angle of repose [5]. Additional experimental work on pit chains consider their formation along preexisting, unsegmented faults [1,5]. However, faults are typically segmented; therefore, it is important to understand pit chain formation as fault segments join together. If the correlation between pit spacing and regolith depth is to be used to estimate regolith depth across Enceladus's cratered terrains, the effect of the segmented nature of nascent faults on the regolith depth proxy must be constrained. Here we consider the effect of initial fault geometries on the relationship between pit formation, geometry, and regolith thickness.
Previous Experimental Models: The model set up by [1] to investigate the evolution of pit chains consisted of two base plates beneath two rigid wooden blocks overlain by two material layers. The rigid wooden blocks, in combination with a layer of cohesive silica powder, represented a vertical fault segment. This vertical fault segment is then linked to a 65° dip detachment fault. As the two baseplates begin to move apart, a tabular void is created along the vertical portion of the fault. The sandpack layer represents regolith with little cohesion and therefore drains into the created void.
Experiments by [5] established a very similar analog model to that of [1]. Rather then looking at the evolution of pit chain formation [1], the models of [5] were constructed to explain pit chain morphologies and geometric relations assuming pits formed due to regolith drainage into an underlying open fracture. The work of [5] used three different regolith materials: expanding vermiculite, no. 20 mesh silica sand, and glass spheres 0.7 mm in diameter. The study found a direct relationship between overlying regolith depth and pit chain spacing.
Experiment Model: Our initial experiment aims to understand the effect of initial segmented fault geometries on the evolution of pit chains. Pit chains will form along fault segments, but as dilation continues and segments begin to merge, will the pits simply get larger, thus maintaining their average spacing, or will new pits nucleate and change the average pit spacing? These initial questions will be answered by first attempting to replicate the observed pit chain formation and evolution as seen by [1,5] with our new experimental set up model (Fig. 2). Our analog model construction differs slightly from [1] and [5] because we will be using a pure dilational fault fracture with no sense of normal motion along the fault. Future experiments will study the temporal evolution of fault populations under moderately oblique and highly oblique distributed extension.
Our proposed experimental set up consists of a la- tex sheet, at the base of our cohesive material layer, that is connected to one fixed and one moving baseplate. The latex sheet is used to help create a fault population within our cohesive layer later in our experiments. [1] and [5] did not use a base latex layer in their experiment setups. Above our latex sheet and aluminum baseplates will be two material layers. The cohesive powder or clay layer is used to represent a material layer that can support steep slopes in hopes of maintaining our dilational fault tabular void throughout our experiment. We will experimentally determine which material is better (cohesive powder, clay, or both) for the purpose of our study. The sandpack layer will represent our unconsolidated regolith. A fracture will be induced in the clay or cohesive powder before the experiment takes place to ensure dilation initiates at this locality and in a controlled geometry. As dilation increases, material from the sandpack layer should progressively drain into our induced fracture. This early experiment will involve perpendicular motion of the moving wall in relation to our induced fracture. The experiment aims to build upon work by [1] and [5] by incorporating more complex fracture geometries and growth patterns.
Further experiments will be expanded by involving moderately oblique and highly oblique distributed extension. Similar experiments were performed by [7] when considering the effect of oblique extension on fault populations in a clay layer. Our experiment will differ by incorporating a layer of sandpack material above the clay or cohesive powder. This sandpack layer will represent the regolith on Enceladus. By ultimately incorporating oblique extension in our experiment we hope to induce fault populations in an en echelon pattern within our cohesive powder or clay layer. Our regolith layer should then drain into these fault populations. This will allow us to observe the impact of fault segmentation on pit chain spacing and geometry.
Discussion: Our modified experimental set up of pure dilation and a variety of fault geometries will allow us to to determine if segmented fault systems result in larger or more numerous pits. We hope to experimentally compare the different pit chain geometries associated with a simple linear dilational fracture (similar to Ferrill et al. [1]) and in an en echelon fault pattern. With further experimentation we would also like to see if regolith properties such as cohesion effects the pit chain geometries and spacing associated with an en echelon fault pattern.
References: [1] Ferrill D. A. and Wyrick D. Y., GSA Today, 4-12, 2004; [2] Michaud et al. (2008) 39th LPSC Abs. #1678. [3] Wyrick et al., Journal Geophysical Research, 20, 2004; [4] Wyrick et al. (2011) 42th LPSC Abs. #1536; [5] Horstman K. C. and Melosh H. J., JGR, 12433-12441, 1989; [6] Ferrill D. A. and Wyrick D. Y., GSA, 133-142, 2011; [7] Schlische et al., Journal of Structural Geology, 910-925, 2009.
Introduction: Pit chains are linear troughs comprised of circular to elliptical depressions [1], and are unique to Enceladus in the outer solar system [2] (Fig. 1). Pits are distinguishable from impact craters, because they lack raised crater rims, impact ejecta, or other flow features [1,2]. Formation of pit chains on Mars is closely associated with regions of extension [1,3]; motion along high-angle normal faults causes drainage of overlying loose regolith into the resultant dilational space along the fault plane, causing pit chains to form in the regolith [1]. Such dilational faults are common on Earth, Venus, and on small bodies like Phobos, Eros, Gaspra and Ida (review by [4]). Previously, [2] used high resolution Cassini ISS data to map pit chains on Enceladus. Isolated primarily within the old cratered terrains, [2] concluded that, like on Mars, pit chains on Enceladus are formed by drainage of loose material into a void formed through dilational faulting.
Pit chains may prove to be a valuable proxy to measure the depths of surficial regolith, providing a powerful tool to probe planetary surfaces and how they are modified by regolith accumulation. Understanding the evolution of pit chain formation is critical to the development of such a proxy, as well as revealing the early stages of extensional faulting on icy surfaces.
Measuring regolith depths: The spacing of individual pits (pit center to pit center) within a pit chain has been used as a proxy for measuring regolith depth [2,5]. Such a proxy was experimentally determined by [5], who found a nearly one-to-one correlation between pit spacing and regolith depth. Using this method, [2] measured an average regolith depth on Enceladus of 250 m ± 20 m. We propose to apply the pit spacing proxy of [5] throughout Enceladus's cratered terrains to examine the spatial distribution of regolith depth. Fig. 2 shows a small region of Enceladus's cratered terrains, which show three distinct fracture sets, with crosscutting relationships that suggest they formed at different times in geological history. Thus, it is important that pit spacings be measured within individual fracture sets because each set of pit chains was formed at a different period of geologic time and thus represent potentially disparate regolith depths. We also aim to test the validity of the pit spacing proxy, which is based on experiments that assume pit chains formed above continuous fractures with uniform opening along the fault trace. The existing proxy also assumes that the pit spacing should be constant for a given regolith thickness; however, no mechanism has been described to explain why the initiation locations of pits along a dilating fault should be somehow controlled by the regolith thickness.
Based on both terrestrial field observations and spacecraft images of solid planetary surfaces, fractures are known to be highly segmented, with the maximum amount of opening occurring near the center of the fracture, and decreasing toward its tips. Near-surface dilation of high-angle normal faults is expected to produce identical patterns, given the analogous decrease in fault slip from the fault center to the tips. As the fractures propagate, fracture segments merge, creating fracture patterns that are much more complex than a single, continuous crack. Given the potential issues with the pit spacing proxy, we explore an alternate technique that uses pit diameter and an estimated angle of repose to trigonometrically resolve the depth of the regolith.
As loose regolith begins to drain into an underlying cavity along a dilating fault, a pit starts to form at the surface immediately above the drainage point. With ongoing drainage, the pit widens and deepens, with a maximum diameter necessarily controlled by regolith depth and the angle of repose. Hence, pits of variable diameter along a fault likely reflect a sequence of pit development. Variable diameters across a region may reflect changes in regolith thickness. If pits form close together, they may ultimately merge, requiring that the peripheries be carefully examined to see evidence of merged cones. In such cases, the pit-to-pit spacing is not a proxy for regolith thickness.
Geologic history of pit chain formation: We have established a systematic history of pit chain formation (Fig. 2) within regions outside of the south-polar terrain (SPT) that are suggestive of a rotation of the stress field trough time, analogous to evidence from fractures within the SPT [6]. Mapped fracture sets containing pit chains are predominantly within Enceladus's cratered terrains and (consistent with results from [2]), based on crosscutting relationships, appear to be the youngest features on the surface. Individual sets of pit chains can therefore provide a preserved record of pit chain formation through time, with each set potentially forming at different stages of regolith thickness accumulation.
Extension-driven faulting is a dominant tectonic process on the icy moons but has not been explicitly described or characterized on Enceladus despite the high frequency of normal faults. Enceladus provides a unique opportunity to examine the evolution of normal faulting on icy surfaces. Pit chains are surface expressions of nascent normal faults [1,3] and the pervasively faulted, tectonically resurfaced leading and trailing hemispheres are tectonic plains of highly evolved normal fault systems.
Pit chains appear to be the youngest features in the region, cutting across all other features within the cratered terrains [1,2]. However, the lack of observed pit chains within the heavily fractured terrains of the leading and trailing hemispheres makes it difficult to determine the relative age of pit chains with respect to these other fractures. Further observations will resolve this conundrum if pit chains are located near and across the boundaries between these terrains.
More evolved pit chains show a series of merged pits within a chain and will eventually take on the appearance of a linear trough with scalloped edges, which are the only remnants of pits [3]. This pattern has been produced experimentally by [3], who note that as a crack continues to dilate, pits will begin to merge with one another. Pit chains that represent different stages of dilation were observed by [2] and matched experiments by [3,7]. However, both experiments and observations lacked a temporal component that can be deduced from crosscutting relationships between different sets of pit chains. Are fully merged pit chains relatively older than chains of isolated pits? Alternatively, if older pit chains have been rendered inactive, and mantling of the cratered terrains by plume and E ring material has been ongoing, do older pit chains take on a more subdued appearance?
Discussion: We will present the results of two independent pit chain proxies, and the respective inferred regolith depth. To develop an acceptable pit chain proxy, the formation of pit chains must be thoroughly understood. Thus, we will ultimately consider the spatial distribution of pit chains and their relative age relationships in both the SPT and Enceladus's tectonized terrains. We will also explore the relative ages of pit chains in various stages of formation (isolated pits through to fully merged pit chains). These results will further our understanding of the modification of Enceladus's terrain by the mantling of plume derived regolith and its dissection by normal faulting.
References: [1] Wyrick et al. (2004) JGR 109. [2] Michaud et al. (2008) 39th LPSC Abs. #1678. [3] Ferrill et al. (2004) GSA Today, 4-12. [4] Wyrick et al. (2010) 41st LPSC Abs., #1413. [5] Horstman K.C. & Melosh H.J. (1989) JGR, 12433-12441. [6] Patthoff & Kattenhorn (2011) GRL, 38, L18201. [7] Miller et al. (2012) 43rd LPSC Abs. #2925.
Acknowledgements: [This work is funded by NESSF Grant #NNX11AP30H. We also thank Geoff Collins and Danielle Wyrick for productive discussions pertaining to this work.
Pit chains on Mars have been documented to be closely associated with extension along high-angle, buried normal faults. Drainage of overlying loose regolith into the dilational space along the fault plane results in aligned pits in the regolith. Pit chains have also been observed on Earth, Venus, and small bodies Phobos, Eros, Gaspra, Ida, and Vesta. Enceladus is the only body in the outer solar system where pit chains have been positively identified, and therefore the only icy body to exhibit a demonstrable layer of loose regolith. Pit chains on Enceladus form predominantly within the cratered terrains on the Saturn and anti-Saturn hemispheres, and form multiple sets of parallel pit chains that share the same relative age, orientation, and morphology. Pits provide a useful proxy to measure the thickness of surficial regolith and thus to probe planetary surfaces that have been modified by regolith accumulation. A previously published proxy experimentally determined a one-to-one correlation between average pit spacing along a single pit chain and regolith depth, although why such a correlation would occur is unclear. We compare the pit spacing proxy to a new proxy that infers regolith depths from the maximum pit diameter within an individual chain, which is physically controlled by the regolith thickness and angle of repose. We initially estimated a range of angles of repose (20°-40°), but this resulted in a wide spread of possible regolith depths at each pit. We refined our approach by accurately measuring the angle of repose at each pit using an established methodology. Assuming circular pits, the sun angle, shadow length, and pit diameter were used to calculate the angle of repose. Our results reveal a global spatial variability in angles of repose, with lower angles (<50°) in the cratered terrains, and higher angles (50°-80°) in tectonized terrains. Thus, surface material is more competent in the tectonized terrains, perhaps explaining the dearth of pits. Regolith thickness in the cratered terrains is variable (~50-1000 m). Higher values occur in the tectonized terrains (1000-3500 m), but only near the boundary with the south polar terrain (SPT). Enceladus's regolith is derived primarily from fall-back from the SPT plumes, and models suggest that regolith will preferentially accumulate thicker deposits nearer to the SPT.
Introduction: Large-scale tectonic deformation in icy shells can manifest itself as fractures that form in response to stresses caused by a range of mechanisms including true polar wander, despinning, changes in volume caused by freezing or thawing of a subsurface ocean, orbital recession/decay, diurnal tides, and nonsynchronous rotation (NSR) [1,2]. Icy shells often preserve this record of tectonic deformation as patterns of fractures, in this case pit chains, which can be used to identify the sources of stress.
For example, pit chains are linear troughs comprised of circular to elliptical depressions [3], and are unique to Enceladus in the outer solar system [4] (Fig. 1). Pits are distinguishable from impact craters, because they lack raised crater rims, impact ejecta, or other flow features [3,4]. Formation of pit chains on Mars is closely associated with regions of extension [3,4]; motion along high-angle normal faults causes drainage of overlying loose regolith into the resultant dilational space along the fault plane, causing pit chains to form in the regolith [3]. Such dilational faults are common on Earth, Venus, and on small bodies like Phobos, Eros, Gaspra and Ida (review by [6]). Previously, [4] used high resolution Cassini ISS data to map pit chains on Enceladus. Isolated primarily within the old cratered terrains, [4] concluded that, like on Mars, pit chains on Enceladus are formed by drainage of loose material into a void formed through dilational faulting. A second possibility is that pit chains simply form above dilational cracks (like terrestrial joint sets) below an icy regolith.
We can use the extensive geologic record of tectonic deformation on Enceladus's surface [7,8] to identify the source of stress that produced these fractures. For example, the established geologic history of the South Polar Terrain (SPT) involves four systematic fracture sets suggestive of a relative change in the stress field through time caused by the rotation of the ice shell due to NSR [7]. The high heat production along the tiger stripes is attributed to geologic activity driven by diurnal tidal stresses [7, 9, 10]. It is fitting therefore that we test whether the pattern of pit chains in the cratered terrains is consistent with results from the SPT. We utilize the viscoelastic stress modeling program SatStressGUI [8,9] to produce a theoretical global NSR stress field by making reasonable assumptions about the rheological properties of the ice shell and the source of stress [9, 11, 12]. We test whether the predicted fracture patterns produced with SatStressGUI match the observed fracture patterns to thus infer the likelihood that NSR stresses were also responsible for fracture formation outside the SPT.
Establishing Fracture Histories: Pit chains have been suggested to be some of the youngest features on Enceladus outside of the SPT [4]. [8] used crosscutting relationships to demonstrate that they may, in fact, be the youngest features on Enceladus's surface other than the tiger stripes. Globally extensive fracture sets are mostly likely to form in response to a global stress field. If the stress field changes, new fractures may form in a different orientation dictated by the orientation and magnitude of the new stress field, and the strength of the ice shell. Rather than Enceladus's stress field changing with respect to Saturn, NSR changes the position of Enceladus's ice shell through faster than synchronous rotation relative to the solid interior, allowing for multiple fracture sets with distinct orientations to form within the same region at different points in time. Detailed fracture mapping can resolve the different sets, and their relative age relationships (Figs. 1, 2). Pit chains were found mostly within cratered terrains (see also [4]) but can also be found within the tectonized terrains of the trailing hemisphere. We used crosscutting relationships to determine the relative ages of adjacent fractures. Additionally, fracture orientation and morphology were used to place fractures within distinct fracture sets.
Results: Detailed fracture mapping of Enceladus's cratered terrains have resolved at least 6 pit chain sets on both the Saturn and anti-Saturn hemispheres (Figs. 1, 2). Both sets are offset from 0° and 180° longitude by ~30° to the west. There are a similar number of fracture sets on each hemisphere, producing similar fracture patterns. Moreover, sets with similar orientations on each hemisphere also share similar places within the relative age sequence. Fig. 3 shows the breakdown of pit chain sets. Black lineaments represent the expected orientations of theoretical extensional fractures based on SatStressGUI [13, 14]. To produce all fracture sets, Enceladus's ice shell must have rotated a minimum of 115° ([7] suggest a minimum of 153° of rotation in the SPT during the observable fracture sequence).
Conclusion: In agreement with [7], we find that the patterns of pit chains in Enceladus's cratered terrains show systematic changes in fracture orientation through time. The mapped fracture patterns are consistent with NSR of a floating ice shell over a global liquid ocean.
References: [1] Kattenhorn & Hurford (2009), in Europa, UA Press, 199-236. [2] Collins et al., (2009) in Solar System Tectonics, Cambridge U. Press. [3] Wyrick et al. (2004) JGR 109. [4] Michaud et al. (2008) 39th LPSC Abs. #1678. [5] Ferrill et al. (2004) GSA Today, 4-12. [6] Wyrick et al. (2010) 41st LPSC Abs., #1413. [7] Patthoff & Kattenhorn (2011) GRL, 38, L18201. [8] Martin & Kattenhorn (2013), 44th LPSC, Abs. 2047. [9] Nimmo et al. (2007) Nature, 447, 289-291. [10] Roberts & Nimmo, (2008) GRL, 35, L09201. [11] Smith-Konter & Pappalardo, (2008), Icarus, 198, 435-451. [12] Olgin et al., (2011) GRL, 38 L02201. [13] Wahr et al., (2009) Icarus 200, 188-206. [14] Kay & Kattenhorn (2010) LPSC Abs. 2046.
Acknowledgements: This work is funded by NESSF Grant #NNX11AP30H.
Figure 1. (see PDF file) Systematic fracture sets of pit chains on Enceladus's anti-Saturn facing hemisphere.
Figure 2. (see PDF file) Systematic fracture sets of pit chains on Enceladus's Saturn facing hemisphere.
Figure 3. (see PDF file) The position and orientations of the position of the NSR stresses remain fixed with respect to Saturn. The ice shell will rotate slightly faster than synchronous causing a progressive eastward migration.
Cross-sectional shapes / Suggest dorsa are compressed: / Icy wrinkle ridge. .
As Voyager 1 flew through the Saturn system it returned photographs revealing many new and surprising characteristics of this complicated community of bodies. Saturn's atmosphere has numerous, low-contrast, discrete cloud features and a pattern of circulation significantly different from that of Jupiter. Titan is shrouded in a haze layer that varies in thickness and appearance. Among the icy satellites there is considerable variety in density, albedo, and surface morphology and substantial evidence for endogenic surface modification. Trends in density and crater characteristics are quite unlike those of the Galilean satellites. Small inner satellites, three of which were discovered in Voyager images, interact gravitationally with one another and with the ring particles in ways not observed elsewhere in the solar system. Saturn's broad A, B, and C rings contain hundreds of "ringlets," and in the densest portion of the B ring there are numerous nonaxisymmetric features. The narrow F ring has three components which, in at least one instance, are kinked and crisscrossed. Two rings are observed beyond the F ring, and material is seen between the C ring and the planet.
Voyager 2 images of the southern hemisphere of Uranus indicate that submicrometer-size haze particles and particles of a methane condensation cloud produce faint patterns in the atmosphere. The alignment of the cloud bands is similar to that of bands on Jupiter and Saturn, but the zonal winds are nearly opposite. Nine of the newly discovered small satellites (40 to 165 kilometers in diameter) orbit between the rings and Miranda; the tenth is within the ring system. Two of these small objects may gravitationally confine the epsilon ring. Oberon and Umbriel have heavily created surfaces resembling the ancient cratered highlands of Earth's moon, although Umbriel is almost completely covered with uniform dark material, which perhaps indicates some ongoing process. Titania and Ariel show crater populations different from those on Oberon and Umbriel; these were probably generated by collisions with debris confined to their orbits.
Global structural mapping of high resolution Cassini images of Enceladus reveals a richly varied surface. Most notable are three main regions of deformation each containing multiple structural units. In addition to the well known “South Polar Terrain” (SPT), there are two other large regions of deformation that we term “Leading Hemisphere Terrain” (LHT) and “Trailing Hemisphere Terrain” (THT). Each of these three terrains includes a circumferential belt that encloses one or more other structurally deformed units. Areal extents range from about 80,000 km2 (SPT) to 195,000 km2 (LHT), or 160 to 250 km equivalent circular radius. Based on relative crater densities, the THT is inferred to be older than the LHT; the geologically active SPT is the youngest. The overall similarities in shape and dimension of the three tectonized terrains suggest similar formational processes, plausibly related to broad loading of a thin elastic shell. A viable scenario is that each tectonized terrain formed above a large-scale region of warm upwelling ice, with subsequent downwarping triggered by cooling and/or subsurface melting. However, differences in morphological detail suggest that the specific evolution of each tectonized terrain has been different.
We produced a geologic map of the Av-9 Numisia quadrangle of asteroid Vesta using Dawn spacecraft data to serve as a tool to understand the geologic relations of surface features in this region. These features include the plateau Vestalia Terra, a hill named Brumalia Tholus, and an unusual “dark ribbon” material crossing the majority of the map area. Stratigraphic relations suggest that Vestalia Terra is one of the oldest features on Vesta, despite a model crater age date similar to that of much of the surface of the asteroid. Cornelia, Numisia and Drusilla craters reveal bright and dark material in their walls, and both Cornelia and Numisia have smooth and pitted terrains on their floors suggestive of the release of volatiles during or shortly after the impacts that formed these craters. Cornelia, Fabia and Teia craters have extensive bright ejecta lobes. While diogenitic material has been identified in association with the bright Teia and Fabia ejecta, hydroxyl has been detected in the dark material within Cornelia, Numisia and Drusilla. Three large pit crater chains appear in the map area, with an orientation similar to the equatorial troughs that cut the majority of Vesta. Analysis of these features has led to several interpretations of the geological history of the region. Vestalia Terra appears to be mechanically stronger than the rest of Vesta. Brumalia Tholus may be the surface representation of a dike-fed laccolith. The dark ribbon feature is proposed to represent a long-runout ejecta flow from Drusilla crater.
Within the outer solar system, normal faults are a dominant tectonic feature; however, strike-slip faults have played a role in modifying the surfaces of many icy bodies, including Europa, Ganymede, and Enceladus. Large-scale tectonic deformation in icy shells develops in response to stresses caused by a range of mechanisms including polar wander, despinning, volume changes, orbital recession/decay, diurnal tides, and nonsynchronous rotation (NSR). Icy shells often preserve this record of tectonic deformation as patterns of fractures that can be used to identify the source of stress responsible for creating the patterns. Previously published work on Jupiter's moon Europa found that right-lateral strike-slip faults predominantly formed in the southern hemisphere and left-lateral strike-slip faults in the northern hemisphere. This pattern suggested they were formed in the past by stresses induced by diurnal tidal forcing, and were then rotated into their current longitudinal positions by NSR. We mapped the distribution of strike-slip faults on Enceladus and used kinematic indicators, including tailcracks and en echelon fractures, to determine their sense of slip. Tailcracks are secondary fractures that form as a result of concentrations of stress at the tips of slipping faults with geometric patterns dictated by the slip sense. A total of 31 strike-slip faults were identified, nine of which were right-lateral faults, all distributed in a seemingly random pattern across Enceladus's surface, in contrast to Europa. Additionally, there is a dearth of strike-slip faults within the tectonized terrains centered at 90°W and within the polar regions north and south of 60°N and 60°S, respectively. The lack of strike-slip faults in the north polar region may be explained, in part, by limited data coverage. The south polar terrain (SPT), characterized by the prominent tiger stripes and south polar dichotomy, yielded no discrete strike-slip faults. This does not suggest that the SPT is devoid of shear: previous work has indicated that the tiger stripes may be undergoing strike-slip motions and the surrounding regions may be experiencing shear. The fracture patterns and geologic activity within the SPT have been previously documented to be the result of stresses induced by both NSR and diurnal tidal deformation. As these same mechanisms are the main controls on strike-slip fault patterns on Europa, the lack of a match between strike-slip patterns on Europa and Enceladus is intriguing. The pattern of strike-slip faults on Enceladus suggests a different combination of stress mechanisms is required to produce the observed distributions. We will present models of global stress mechanisms to consider how the global-scale pattern of strike-slip faults on Enceladus may have been produced. This problem will be investigated further by measuring the angles at which tailcracks have formed on Enceladus. Tailcracks produced by simple shear form at 70.5° to the fault. Any deviation from this angle indicates some ratio of concomitant shear and dilation, which may provide insights into elucidating the stresses controlling strike-slip formation on Enceladus.
We present the first comprehensive examination of the geysering, tidal stresses, and anomalous thermal emission across the south pole of Enceladus and discuss the implications for the moon's thermal history and interior structure. A 6.5 yr survey of the moon's south polar terrain (SPT) by the Cassini imaging experiment has located ~100 jets or geysers erupting from four prominent fractures crossing the region. Comparing these results with predictions of diurnally varying tidal stresses and with Cassini low resolution thermal maps shows that all three phenomena are spatially correlated. The coincidence of individual jets with very small (~10 m) hot spots detected in high resolution Cassini VIMS data strongly suggests that the heat accompanying the geysers is not produced by shearing in the upper brittle layer but rather is transported, in the form of latent heat, from a sub-ice-shell sea of liquid water, with vapor condensing on the near-surface walls of the fractures. Normal stresses modulate the geysering activity, as shown in the accompanying paper; we demonstrate here they are capable of opening water-filled cracks all the way down to the sea. If Enceladus' eccentricity and heat production are in steady state today, the currently erupting material and anomalous heat must have been produced in an earlier epoch. If regional tidal heating is occurring today, it may be responsible for some of the erupting water and heat. Future Cassini observations may settle the question.
Enceladus’ stripes are hot; the land in between? Maybe not. Forming ropy plains therein, requires litho quite thin, so the heat flux must‘ve been quite a lot.
Jupiter's second Galilean satellite, Europa, is a Moon-sized body with an icy shell and global ocean approximately 100 km thick surrounding a rocky interior. Its surface displays extensive tectonic activity in a geologically recent past. Europa's most ubiquitous surface features, double ridges, have a central trough flanked by two raised edifices. Double ridges can extend hundreds of kilometers and appear genetically related to cracks formed in the Europan ice shell. The origin of the raised flanks has been the center of much debate and many models have been proposed. There are also ridges without a central trough, single ridges. These ridges are far less common than their double ridge counterparts. However, there are locations where along-strike changes in ridge type appear to occur. We explore an elastic model in which the ridges form in response to crystallization of a liquid water intrusion. In our model, liquid water fills tension cracks that open in the Europan crust in response to tidal stress or perhaps overpressure of a subsurface ocean. The crack would be long and essentially continuous, similar to dikes on Earth, explaining the remarkable continuity and lack of segmentation of Europan ridges. The freezing of the water would cause a volume expansion, compressing and buckling the adjacent crust. We find that the geometry of the intruding water body controls the shape of the resulting ridges, with single ridges forming above sill-like intrusions and double ridges above dike-like intrusions. In order to match the ridge heights observed for double ridges we would need approximately 1.5 km(2) of water intruded at a shallow depth in the ice shell, potentially over the course of multiple events. Deeper intrusions result in a broader, lower amplitude ridge than shallow intrusions.
Inside Enceladus
Saturn's moon Enceladus has often been the focus of flybys of the Cassini spacecraft. Although small—Enceladus is roughly 10 times smaller than Saturn's largest moon, Titan—Enceladus has shown hints of having a complex internal structure rich in liquid water. Iess et al. (p. 78 ) used long-range data collected by the Cassini spacecraft to construct a gravity model of Enceladus. The resulting gravity field indicates the presence of a large mass anomaly at its south pole. Calculations of the moment of inertia and hydrostatic equilibrium from the gravity data suggest the presence of a large, regional subsurface ocean 30 to 40 km deep.
Geology: dark terrain, bright grooved terrain, polar caps, impact
structures; Surface composition: ice, dark material, other molecules;
Regolith properties: photometric properties, thermal and radar
characteristics; Interior evolution and global history.
At low differential effective stress and with minimum principal
effective stress near zero or tensile, rocks fail in several modes and
with variable failure angles. Under these conditions mechanical
stratigraphy exerts a significant influence on initial dip of normal
faults. Less competent layers fail in shear mode along fractures that
approximate the failure angle predicted by a standard rock mechanics
analysis. Deformation of more competent layers, which is driven in part
by interaction with the more rapidly deforming incompetent layers,
produces hybrid mode failure in which failure angles are smaller than in
shear mode. Analyses of small-displacement (<1 m displacement) normal
faults cutting Cretaceous carbonate strata in west Texas (USA) indicate
that fault geometries resulting from this effect commonly display steep
segments where the fault traverses more competent beds. Displacement on
these faults has caused dilation of steeper segments. Analogous steep
dilational fault segments host ore deposits in the North Pennine
Orefield, England. Dilatant segments along faults within carbonates of
the Cretaceous age Edwards Group near San Antonio, Texas (USA) have been
enlarged by groundwater flow, and are important permeability and shallow
groundwater infiltration pathways.
[1] Crosscutting relationships of tectonic lineaments on Europa record the history of surface deformation. We mapped the displacement and orientation of older features crosscut by two types of lineaments: bands and double ridges. These measurements allow us to determine both the strike-perpendicular and strikeparallel displacement along investigated features. Double ridges record both ridge-perpendicular contraction and expansion, with a mean of 0.16 ± 0.06 km of contraction based on the analysis of sixteen double ridges. Bands record expansion, with a mean of 3.33 ± 0.27 km for the six bands analyzed, but with perpendicular displacement less than their apparent morphologic widths of 324 km. The implied global surface strain for double ridges (including those that expand) and bands is 2.22 ± 0.76% contraction and 7.60 ± 3.7% expansion, respectively. Double ridges thus may accommodate part of the surface expansion recorded by bands. Most current models for double ridges do not predict contraction. The models that satisfy the observations for bands are “slow spreading” models, cryovolcanism, and folding.
Rupes Recta, also known as the ‘Straight Wall’, is an individual normal fault located in eastern Mare Nubium on the nearside of the Moon. Age and cross-cutting relationships suggest that the maximum age of Rupes Recta is 3.2 Ga, which may make it the youngest large-scale normal fault on the Moon. Based on detailed structural mapping and throw distribution analysis, fault nucleation is interpreted to have occurred near the fault centre, and the fault has propagated bidirectionally, growing northwards and southwards by segment linkage. Forward mechanical modeling of fault topography gives a best-fitting fault dip of approximately 858, and suggests that Rupes Recta accommodated approximately 400 m of maximum displacement and extends to a depth of around 42 km. The cumulative driving stresses required to form Rupes Recta are similar in magnitude to those that formed normal faults in Tempe Terra, Mars. The spatial and temporal association with Rima Birt, a sinuous rille to the west of Rupes Recta, suggests a genetic relationship between both structures and implies regional extension at the time of formation.
The Cassini Imaging Science Subsystem (ISS) acquired many
high-resolution images (<1 km/pixel) of the Icy Saturnian satellites
during the nominal mission of the Cassini spacecraft between 2004 and
2008. These images were used to create high-resolution mosaics of these
satellites. The Cassini mission is expected to continue till 2017 and
high-resolution images of the first three years of the extension were
used to improve the mosaics, especially in the Northern parts which were
not illuminated during the nominal mission. These improved mosaics were
the baseline for new versions of the atlases of Mimas, Enceladus, and
Dione described in this paper. These new atlases supersede the previous
versions from 2006 (Enceladus) and 2008 (Mimas and Dione), and include
the official names of additional features, proposed by the Cassini
imaging team, approved by the International Astronomical Union (IAU).
The new atlases are available to the public through the Imaging Team's
website and the Planetary Data System (PDS).
The south polar terrain (SPT) of Saturn's moon Enceladus is a
mysteriously active region that exhibits intriguing tectonic signatures
and widespread fracturing. The central region of the nearly-circular SPT
is depressed into the surface by a few hundred meters and bounded by a
ring of cliffs roughly 1 km high. In this study, we investigate whether
this depression and surrounding mountainous uplift is consistent with
the morphology of terrestrial rift basins and the possibility that the
SPT could have formed during a tectonic event analogous to those of such
rift basins on Earth. Using three mechanical models of basin formation,
we compare our predicted topography of the SPT with observed topography
of the region. The first of three models we consider assumes crustal
stretching by factor β, and predicts a basin depth of roughly 600
m, closely matching previously published estimates of the depth at the
SPT. Models of extension and compression, assuming an elastic response
in the ice crust, predict best-fit mountain uplift of roughly 1820 m and
1130 m, respectively. Our preferred model suggests that the icy shell in
the SPT has been stretched, but the extension is (partially) balanced by
compression along the edges of the basin leading to the uplift of the
mountains along the boundary, thereby implying that the SPT may have a
tectonic origin analogous to that of a terrestrial basin.
Images of Vesta taken by the Dawn spacecraft reveal large-scale linear
structural features on the surface of the asteroid. We evaluate the
morphology of the Vesta structures to determine what processes caused
them to form and what implications this has for the history of Vesta as
a planetary body. The dimensions and shape of these features suggest
that they are graben similar to those observed on terrestrial planets,
not fractures or grooves such as are found on smaller asteroids. As
graben, their vertical displacement versus length relationship could be
evaluated to describe and interpret the evolution of the component
faults. Linear structures are commonly observed on smaller asteroids and
their formation has been tied to impact events. While the orientation of
the large-scale Vesta structures does imply that their formation is
related to the impact events that formed the Rheasilvia and Veneneia
basins, their size and morphology is greatly different from
impact-formed fractures on the smaller bodies. This is consistent with
new analyses that suggest that Vesta is fully differentiated, with a
mantle and core. We suggest that impact into a differentiated asteroid
such as Vesta could result in graben, while grooves and fractures would
form on undifferentiated asteroids.
Enceladus' cratered terrains contain large numbers of unusually shallow
craters consistent with deformation by viscous relaxation of water ice
under conditions of elevated heat flow. Here we use high-resolution
topography to measure the relaxation fraction of craters on Enceladus
far from the active South Pole. We find that many craters are shallower
than expected, with craters as small as 2 km in diameter having
relaxation fractions in excess of 90%. These measurements are compared
with numerical simulations of crater relaxation to constrain the minimum
heat flux required to reproduce these observations. We find that
Enceladus' nominal cold surface temperature (70 K) and low surface
gravity strongly inhibit viscous relaxation. Under such conditions less
than 3% relaxation occurs over 2 Ga even for relatively large
craters (diameter 24 km) and high, constant heat fluxes
(150 mW m-2). Greater viscous relaxation occurs if
the effective temperature at the top of the lithosphere is greater than
the surface temperature due to insulating regolith and/or plume
material. Even for an effective temperature of 120 K, however, heat
fluxes in excess of 150 mW m-2 are required to
produce the degree of relaxation observed. Simulations of viscous
relaxation of Enceladus' largest craters suggest that relaxation is best
explained by a relatively short-lived period of intense heating that
decayed quickly. We show that infilling of craters by plume material
cannot explain the extremely shallow craters at equatorial and higher
northern latitudes. Thus, like Enceladus' tectonic terrains, the
cratered regions of Enceladus have experienced periods of extreme heat
flux.
Europa, a satellite of Jupiter, is one of the most intriguing worlds in the Solar System. Its dearth of impact craters and plethora of surface morphologies point to a dynamic evolution of its icy shell in geologically recent times. Double ridges are a common landform and appear to have formed over a significant fraction of the satellite’s observed geologic history. Thus, understanding their formation is critical to unraveling Europa’s history, and many models have been proposed to explain their creation. A clue to the formation of ridges may lie in evidence for flexure of the lithosphere in response to a load imposed by the ridge itself (marginal troughs and subparallel flanking fractures). When this flexure has been modeled, a simple elastic lithosphere has typically been assumed; however, the generally thin lithospheres suggested by these models require very high heat flows that are inconsistent with Europa’s expected thermal budget (of order 1 W m−2 vs. of order 10 mW m−2). Each of the proposed formational models, however, predicts a thermal anomaly that may facilitate the flexure of Europa’s lithosphere. Here, we simulate this flexure in the presence of these anomalies, as a means to evaluate the different models of ridge formation. We find that nearly all models of double ridge formation are inconsistent with the observation of flexure (specifically the flanking fractures), except for a cryovolcanic model in which the growing ridge is underlain by a cryomagmatic sill that locally heats and thins the lithosphere.
Analyses of landforms on Rhea are used to define 13 types of geomorphic features, interpreted to be of tectonic and volcano-tectonic origin. All types except one seem to be extensional in origin. They occurred after the formation of population I craters and are contemporaneous with a major resurfacing event. The troughs, grabens, grooves, pit chains, scarps, and other lineaments vare “pure” extensional features, while the ridges are volcanic features formed in an extensional stress field. This global surface extension was followed by an era of global compression which produced megaridges and megascarps. These compressional features are not numerous enough to fit the theoretical stresses predicted by the typical thermal models of Rhea. All the extensional landforms appear to form a global grid pattern. This planetary wide grid is similar in direction to the theoretical pattern of a tidally distorted planet. That suggests either an orbital variation very early in the history of Rhea or the existence of an unknown process which has oriented the stress field during the extensional period.
The plausibility of invoking a lithospheric instability mechanism to account for the grooved terrains on Ganymede, Encedalus, and Miranda is presently evaluated in light of the combination of a simple mechanical model of planetary lithospheres and asthenospheres with recent experimental data for the brittle and ductile deformation of ice. For Ganymede, high surface gravity and warm temperatures render the achievement of an instability sufficiently great for the observed topographic relief virtually impossible; an instability of sufficient strength, however, may be able to develop on such smaller, colder bodies as Encedalus and Miranda. 15 refs.
We present evidence that compression has occurred along doublet ridges on Europa, a model proposed, but not previously demonstrated.
Three distinct and intersecting sets of graben exist near the Martian volcano Alba Patera. One set formed as a response to the evolution of the Alba Patera edifice. A second set includes mainly linear structures widely interpreted to have formed in response to very large-scale regional stresses. The members of the third set contain numerous pit craters, and we propose that these craters formed as a result of volcanic activity above long regional dikes emanating from a source to the south within the Tharsis region. The pit craters are of two morphologies: small (
Current models for the formation of the abundant large closed depressions of Valles Marineris have serious shortcomings. Purely tectonic mechanisms are inconsistent with the morphology of many depressions, and removal of large quantities of ground ice from the canyon walls is inconsistent with the observed strength of the walls. Accordingly, some alternatives are offered. One possibility involves decay of ice-rich bodies occupying partially sediment-choked ancient graben that predated the overlying cratered and ridged plains. Other possibilities involve the removal of massive equatorial carbonate deposits storing much of the planet's CO2 inventory, generated during greenhouse conditions on early Mars. Solution by carbonic acid derived from the atmosphere (analogous to terrestrial karst) requires extensive recycling of the available water supply. Solution by various groundwater acids, possibly derived from the Tharsis magmas, requires less water, especially if only the smaller closed depressions are due to carbonate decay. Alternatively, volume loss due to decarbonation of carbonate during early high heat flow or a later Tharsis-related heat pulse can produce extensive collapse, especially if the carbonates have high silica content.
Multilayered viscoelastic models of Encedalus are presently used to model its tidal heating, whose theoretical maximum violates an upper bound based on Saturn's Q. These multilayered models, which are demonstrably thermally stable at the current eccentricity with an about 10-km thick lithosphere, require a combination of near-surface insulation, subsurface solar heating, or anomalously low lithospheric conductivity, to accomodate both the dynamic and the geological constraints. The two- and three-layer models considered in detail suggest that the thermal and dynamical evolution of Encedalus may have been a very straightforward one that involved only Dione in a 2:1 resonance.
Analog modeling demonstrates that dilational faulting in cohesive rock beneath cohesionless material will produce pit chains, troughs, and grabens similar to those observed on Mars.
(1) Dione has ridge-bounded high-standing plains; (2) Rhea has a N-S belt of well-defined graben and extensional faults at ~270° that are co-incident with its "wispy terrain"; and (3) Tethys' plains unit boundary (at least in the first region we examined)
The morphology of the grabens and fractures on the flanks of Pavonis Mons indicates an underlying concentric dike swarm related to the rift zones. We address the origin of the required radial extension and the general evolution of martian shields.
Pit crater chains exist on a range of planetary bodies --- from small asteroids to icy moons to large terrestrial planets --- raising important questions about formation mechanisms and near-surface crustal properties of solid bodies in our solar system.
We analyze graben in N Tharsis to deduce the geometries of underlying giant dikes. The shapes and sizes of associated pit craters are used to find how much dike magma was erupted in spatially localized but violently explosive eruptive events.
Analysis of 2008 Cassini Composite Infrared Spectrometer (CIRS) 10 to 600 cm-1 thermal emission spectra of Encleadus shows that for reasonable assumptions about the spatial distribution of the emission and the thermophysical properties of the solar-heated background surface, which are supported by CIRS observations of background temperatures at the edge of the active region, the endogenic power of Enceladus' south polar terrain is 15.8 ± 3.1 GW. This is significantly higher than the previous estimate of 5.8 ± 1.9 GW. The new value represents an improvement over the previous one, which was derived from higher wave number data (600 to 1100 cm-1) and was thus only sensitive to high-temperature emission. The mechanism capable of producing such a high endogenic power remains a mystery and challenges the current models of proposed heat production.
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.
Triton, with a diameter of ≡2700 km, is Neptune's only planet-class satellite. The complexity of Triton's surface and the variety of surface features is unequaled among the satellites of the solar system. From a geologic viewpoint, some of Triton's features have apparently familiar morphologies and general interpretative agreement exists. However, many of its landforms have novel morphologies and geologic settings, which have given rise to a number of innovative and competing interpretations. The first portion of this chapter describes Triton's surface in primarily nongenetic terms. The authors then review various models and speculations regarding geologic processes that have operated on Triton, followed by an interpretive stratigraphy and geologic history.