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The Geological History of Enceladus
... As a result, many late reefs are superimposed upon early reefs and scatter laterally in a wide area; a single reef is small in size. Flood [18]made a study of bioherm limestone deposited in the Heron Island in 1993. They found that reef-bank deposits on the marginal gentleslope platform nowadays have similar features, i.e. small single reef size, multi-phase stacking of reefs in vertical direction, and scattered distribution in lateral direction. ...
Located in the Sichuan Basin, the Yuanba Gasfield is the deepest high-sulfur carbonate gas field among those discovered in the world. Its biohermal gas reservoir of the Upper Permian Changxing Fm is characterized by ultra depth, multi-stage small and scattered reef, thin reservoir, poor physical properties, strong heterogeneity, complex fluid distribution, and low production of vertical wells. The development of the biohermal gas reservoir is subject to many difficulties. For example, it is necessary to deepen the studies on time–space distribution laws of reef dolomite reservoirs; it is difficult to characterize small reefs precisely and predict thin reservoirs quantitatively; the deployment and optimization design of horizontal wells are influenced by multiple factors; and the difficulty for horizontal wells with long horizontal sections to run through high-quality thin reservoirs is high. In order to develop the Yuanba Gasfield efficiently, therefore, it is necessary to carry out a series of technical researches on the distribution laws and development models of biohermal reservoirs, precise characterization of small reefs, quantitative prediction of thin reservoirs, optimization design of horizontal wells in banded small reef gas reservoirs, and real-time trajectory optimization and adjustment of horizontal wells in ultra-deep thin reservoirs. These research results provide a powerful support for the development and construction of the Yuanba Gasfield. Based on these technologies, China's first ultra-deep high-sulfur large biohermal gas field was built with a mixed gas annual production capacity of 40 × 10⁸ m³. The successful commissioning of the Yuanba Gasfield lays a basis for the leading position of China in the field of high-sulfur gas field development. In addition, it is of great significance to the long-term stable gas supply in 70 cities of six provinces and two municipalities along the “Sichuan-to-East China Gas Transmission Pipeline”, as well as to the industrial structure adjustment in central–western China and the economic development along the Yangtze River.
... Ribbon Reef 5 core, in the northern GBR, suggests four to six packages of reef growth in the upper c. 96 m with the upper four successions separated by unconformities associated with glacial lowstands (see Braithwaite and Montaggioni, 2009). Flood (1993) recognised five solution unconformities within the Heron bore reefal sequence and it is assumed that latest Pleistocene aggradational reef facies of the southern GBR reefs consist of stacked highstand reef packages separated by unconformities associated with intervening lowstands. Maxwell (1968) and Hopley (1982) summarised the hydrology, climate and weather of the GBRP and Wolanski (1994) reviewed its oceanography and hydrodynamics. ...
Heron Island has been the focal point for research on the southern Great Barrier Reef Province for the last 80 years. Heron Reef is an excellent example of a lagoonal platform reef with a sand cay developed on its leeward end, displaying typical reef morphological, sedimentological and ecological zonations allowing comparison of their windward and leeward development. Limited subsurface data indicate that the total reef
section is only 150 m thick, consisting of stacked limestone packages, with a gently eastward sloping solution disconformity delineating the base of c. 15 m of Holocene reef growth. Holocene reef growth does not appear to fit the “classical” model, with evidence of much progradation on the windward margin relative to the associated leeward margin. Large dredged blocks of reefal material provide new data on the abundance of in situ framework in much of the reef and the importance of microbialite in the unification process.
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.
In order to improve our understanding of the interior structure of Saturn's small moon Enceladus, we reanalyze radiometric tracking and onboard imaging data acquired by the Cassini spacecraft during close encounters with the moon. We compute the global shape, gravity field, and rotational parameters of Enceladus in a reference frame consistent with the International Astronomical Union's definition, where the center of the Salih crater is located at −5° East longitude. We recover a quadrupole gravity field with J3 and a forced libration amplitude of 0.091° ± 0.009° (3‐σ). We also compute a global shape model using a stereo‐photoclinometry technique with a global resolution of 500 m, although some local maps have higher resolutions ranging from 25 to 100 m. While our overall results are generally consistent with previous studies, we infer a thicker 27–33 km mean ice shell, a thinner 21–26 km mean ocean thickness, and a mean core density range of 2,270–2,330 kg/m³.
Studies of psychrophilic life on Earth provide chemical clues as to how extraterrestrial life could maintain viability in cryogenic environments. If living systems in ocean worlds (e.g., Enceladus) share a similar set of 3-mer and 4-mer peptides to the psychrophile Colwellia psychrerythraea on Earth, spaceflight technologies and analytical methods need to be developed to detect and sequence these putative biosignatures. We demonstrate that laser desorption mass spectrometry, as implemented by the CORALS spaceflight prototype instrument, enables the detection of protonated peptides, their dimers, and metal adducts. The addition of silicon nanoparticles promotes the ionization efficiency, improves mass resolving power and mass accuracies via reduction of metastable decay, and facilitates peptide de novo sequencing. The CORALS instrument, which integrates a pulsed UV laser source and an Orbitrap™ mass analyzer capable of ultrahigh mass resolving powers and mass accuracies, represents an emerging technology for planetary exploration and a pathfinder for advanced technique development for astrobiological objectives. Teaser: Current spaceflight prototype instrument proposed to visit ocean worlds can detect and sequence peptides that are found enriched in at least one strain of microbe surviving in subzero icy brines via silicon nanoparticle-assisted laser desorption analysis.
Plain Language Summary
The tiger stripes are four sub‐parallel, linear depressions in the south polar region (SPR) of Saturn's moon Enceladus, which are known for their vapor plumes containing organic molecules. The nature of the tiger stripes is not fully understood and remains a subject of intense debate. Here, we propose a new model of the tiger stripes in which Enceladus' ice shell is modeled as an elastic system with frictional interfaces subjected to periodic tidal loading. We find that the diurnal tides produce a complex pattern of stress anomalies, characterized by a length scale of tens of km and the peak values exceeding 100 kPa. Friction delays the system's response to tidal loading and leads to an asymmetry between the compression and extension phases. This asymmetry results in an additional stress, which is constant in time and comparable in magnitude to the cyclic stress. The static stress field is characterized by compression in the direction perpendicular to the faults, and its magnitude is large enough to influence the evolution of the SPR on geological time scales. The total heat flow generated by friction is 0.1–1 GW, accounting for only a small fraction of the heat power emitted from the tiger stripes.
To reexamine the potential for lateral mixing over large distances (>100 km) by impact craters, a mathematical model utilizing the stable probability distribution is proposed for estimating lateral mixing efficiency on the Moon. The proposed model divides material mixing into shallow slope and steep slope regimes. Mixing in the shallow slope regime conforms to the condition that the exponent of the power law describing lunar crater size frequency is larger than the exponent of the power law describing the rim thickness plus 2; otherwise the mixing is called the steep slope regime. The model suggests that in the shallow slope regime, lateral mixing on the Moon is efficient enough to deliver 20–30% exotic components over distances greater than 100 km (e.g., highland material to the mare). The model indicates that lateral mixing conforming to the steep slope regime is not efficient if linear addition of ejecta deposits is assumed because in this regime, impact cratering is driven by small craters reworking lunar surface, and the addition of crater ejecta is an invalid assumption. If the result of the proposed model for the shallow slope regime is applied to regolith layers below the reworked zone, a significant number of “exotic” components is predicted.
1] Multiple end-member spectral mixture analysis (MESMA) was applied to the Clementine UVVIS global 1 km multispectral data set, and the resulting highland material fraction image was used to investigate highland contamination of mare surfaces by impact cratering. MESMA decomposes each pixel with the number of end-members fewer than the number of the spectral bands of Clementine UVVIS data. This allows the use of variable end-member combinations to accommodate global spectral variance. A 13-end-member set of lunar soil components that spans the lunar spectral diversity was selected from the UVVIS data and included 1 mature and 4 fresh highland soils, 4 mature and 2 fresh mare soils, and 2 soils of dark mantling materials. This set was applied to the lunar data through MESMA, and an aggregated global highland abundance was derived. Comparison of the highland fraction results from MESMA and traditional spectral mixture analysis (SMA) demonstrated that MESMA better accommodates the compositional variation of mare soils, resulting in a greater accuracy in the measurement of highland. With the derived products, we investigated highland contamination in Mare Nectaris, Mare Fecunditatis, and Mare Crisium. These mare surfaces are proximal to the large craters Theophilus, Taruntius, Langrenus, and Proclus and provided an opportunity to investigate highland contamination by large impacts. The analyses indicated that Theophilus impact resulted in highland contamination of 20–80% on most of Mare Nectaris, while Taruntius and Langrenus impacts caused 5–40% contamination of Mare Fecunditatis. The trend of minimal highland abundance in Mare Fecunditatis correlates poorly with basalt thickness, suggesting an efficient lateral mixing due to the large craters Taruntius and Langrenus. Additionally, Proclus, combining with other craters, was responsible for highland contamination in most of the Mare Crisium surface, and some basalt classification in this mare reflects highland contamination of the spectral properties. MESMA significantly reduces uncertainty in calculating global composition variations due to mixing, and these analyses demonstrate that large impacts played a dominant role in delivering highland materials onto three mare surfaces. Citation: Li, L., and J. F. Mustard, Highland contamination in lunar mare soils: Improved mapping with multiple end-member spectral mixture analysis (MESMA), J. Geophys. Res., 108(E6), 5053, doi:10.1029/2002JE001917, 2003.
Isotopic analyses of mineral fractions and whole rocks from the ferroan anorthosite 62236 yield a Sm-Nd isochron with an age of 4.29 ± 0.06 Ga and an initial εNd143 value of +3.1 ± 0.9. We have also measured εNd142 anomalies of +0.25 on two fractions of 62236. These values are higher than the value of −0.1 predicted if 62236 was derived from a chondritic source at 4.29 Ga, but are consistent with the positive initial εNd143 value. The Sm-Nd isotopic composition of 62236 has been modified by the capture of thermal neutrons such that the 147Sm/144Nd, 143Nd/144Nd, and 142Nd/144Nd ratios measured on the mineral fractions and whole rocks must be corrected. The corrections do not significantly alter the Sm-Nd isotopic results determined on 62236. Despite the fact that the Ar-Ar and Rb-Sr isotopic systematics of 62236 have been reset by impact metamorphism at 3.93 ± 0.04 Ga, the Sm-Nd systematics appear to have been unaffected. The Sm-Nd isotopic systematics of 62236 provide several constrains on models of lunar crustal differentiation provided they have not been reset since crystallization. First, the relatively young age of 62236, as well as the old ages determined on several crustal plutonic rocks of the Mg-suite, require multiple sources of magmas on the Moon very early in its history. Second, positive εNd143 values determined on all analyzed ferroan anorthosites suggest that they were derived from sources depleted in light rare earth elements. And third, models based on initial εNd143 and present-day εNd142 values suggest that the source of 62236 was depleted in light rare earth elements at ∼4.46 Ga. In order to reconcile these observations with the lunar magma ocean model (1) the magma ocean must have existed for a very short period of time, and may have had a sub-chondritic Nd/Sm ratio, and (2) the youngest ferroan anorthosites, such as 62236, cannot be cumulates from the magma ocean, but must form by other processes.
Introduction: The south polar thermal anomaly and plume activity [1,2] has dominated work focused on Enceladus. Little is known about the geologic history of Enceladus's surface beyond the south polar terrain (SPT) and minimal detailed fracture mapping has been attempted outside of the SPT. Morphologically distinct geological units have been mapped [3,4,5], revealing that adjacent terrains can vary widely. Heavily fractured terrains are centered on the leading and trailing hemispheres, separated by cratered terrains on the saturnian and anti-saturnian hemispheres. Understanding local heterogeneities of fracture and terrain types is critical for gaining perspective on Enceladus's global geologic history.
We aim to understand localized changes in fracture orientation seemingly caused by local perturbations in the stress field by craters within the cratered terrains [6,7,8], and whether or not these changes are dependent on crater size. Our results will begin to elucidate why some craters affect fractures but not others, and provide insights into the causes of local heterogeneities within the regional stress regimes on Enceladus.
Many fracture sets appear to be influenced by nearby craters. The mutually parallel trend of such fractures changes with increasing proximity to a crater, converging towards the crater in a somewhat radial pattern. The fractures then cut through and beyond the crater, returning to the original orientation of the fracture set at some distance past the crater (Fig. 1). On average, interacting fracture sets extend 10s of kilometers from crater centers. Craters that perturb fractures are both simple and complex craters, with some craters showing various stages of relaxation [8,9]. Fractures that experience reorientation by a crater appear relatively young, and many of them are pit chains, thought to be some of the youngest features on Enceladus [10]. Preliminary observations suggest that craters may influence fractures as far as ~5 crater diameters away from the crater center. Future work will quantify this observation over a range of crater sizes.
Crater-fracture interactions have been previously noted [6,7,8] and several formation mechanisms have been presented. Reorientation of fracture growth may be due to topographic loading by the crater rim [6]. This is not a favored mechanism, as craters appear to perturb fracture growth over large distances (~5 crater diameters), and rim topography may not be sufficient to influence local stresses at such a distance. Crater depth, controlled by relaxation, may create the proper conditions for fracture reorientation [8]. Another possibility is a source of internal pressure underneath the crater, such as thermally mobilized ice due to an impact, which would cause parallel fracture sets to reorient radially to the crater.
Methods: In our analysis, craters were selected where they were overprinted by one or more fractures. Because the influence of crater size was the target of this study, a wide range of crater sizes were examined where crosscutting fractures were observed, totaling 41 craters. Crater diameters were measured and each crater was assigned a 1 or 0 denoting whether or not crosscutting fractures experienced re-orientation as they passed across the crater.
Figure 1: Crater-fracture interaction morphologies. a Centered at -3°, 136°. b Centered at 2°, 162°. c Centered at -23°, 156°. d Centered at 34°, 160°. e Centered at -34°, 152°. a and e show crater-fracture interactions where no fracture reorientation occurs. Craters in b, c and d show reorientation of fractures but to varying degrees of orientation change.
Crater-Fracture Morphologies: Interactions between craters and fractures take on two morphologies: fractures that crosscut craters with no orientation change (Fig. 1a,e), and fractures that crosscut craters with a quantifiable orientation change (Fig. 1b,c,d). Fracture sets that demonstrate reorientation show a modification of parallel sets that converge towards the crater center. This can be a subtle change (Fig. 1d) or more pronounced (Fig. 1b). Fig. 1c shows two craters of nearly the same size interacting very differently with fractures; radial fractures emanate from the northern crater (which, when viewed from a larger spatial extent do not return to their original parallel orientation), while a parallel set crosscuts the southern crater. The same phenomenon was noted by [8] who suggest that the relaxation state of the crater may play a role in how fractures are perturbed.
Results: Fig. 2 shows the spatial distribution of craters on a global scale, classified by whether or not fractures reorient as they interact with the crater. There may be localized clustering of craters within the same group 1 or 0 (showing fracture reorientation and no change, respectively); however, the sample size must be increased to verify this result.
Fig. 3 shows the distribution of crater size within crater groups 1 and 0. The mean of crater group 1 is 10.4 km (with crater diameters ranging from 2.2-9.7 km) and the mean of crater group 0 is 6.1 km (with crater diameters ranging from 5.9-20.5 km). This data suggests that there is no concrete threshold at which crater size will begin to influence fracture growth, but rather a continuum of crater sizes in the range of 6-8 km. No dependence on crater diameter was found by [8] but it is unclear how rigorously this dependence was examined.
Discussion: We interpret our observations to suggest that there is a dependence on crater size in the reorientation of fracture growth in the vicinity of the craters examined here. No explicit threshold of crater diameter must be achieved to result in a perturbation of the local stress field in the crater vicinity, although a transition occurs in the crater size range of 6-8 km, beyond which fractures are consistently perturbed by the craters. This suggests a complex interaction between craters and fractures, with crater size being one parameter within a larger, more complex context.
What makes larger craters more capable of perturbing local stress fields than smaller craters? If an internal source of pressure beneath a crater is thermally mobilized ice induced by impact, a smaller impact crater may not sustain enough heat to affect fracture growth at some later time. What cannot be determined is the amount of time that lapsed between crater and fracture formation, which will vary between craters. The proximity of a newly formed crater to a simultaneously propagating fracture set may explain why some smaller craters (5-6 km) are able to reorient fractures where larger craters (8-9 km) cannot: smaller craters forming near propagating fracture sets may retain their heat long enough to perturb its local stress field and modify fracture growth.
Future Work: To further understand the extent of influence of craters on fractures on Enceladus, accurate measurements of the change of fracture orientation as a result of the craters' influence will be completed. We will examine whether such changes are also influenced by regional stress fields related to tidal deformation of the ice shell. The radial extent of each crater's influence will be measured to determine if larger craters (which seemingly are more capable of reorienting fractures) have a correspondingly wider influence on fracture growth and the spatial extent of the local stress perturbation.
References: [1] Porco et al. (2006) Science, 311, 1393-1401. [2] Spencer et al. (2006) Science, 311, 1401-1405. [3] Kargel & Pozio (1996) Icarus, 119, 385-404 [4] Crow-Willard & Pappalardo (2010) 41st LPSC Abs. #2715. [5] Crow-Willard & Pappalardo (2010) 42nd DPS Abs. #25.03. [6] Miller et al. (2007), Ices, Oceans and Fire: LPI Contribution No. 1357, p.95-96. [7] Barnash et al. (2006) 38th DPS Abs. #24.06. [8] Bray et al. (2007) 38th LPSC, Abs. #1873. [9] Kirchoff & Schenk (2009) Icarus, 206, 656-668. [10] Michaud et al. (2008) 39th LPSC Abs. #1678.
Acknowledgements: Funded by NASA OPR Grant # NNX08AQ94
Figure 2: Spatial distribution of craters examined in this study. Blue indicates crater-fracture interactions where no fracture reorientation is observed, and red indicates where fractures have undergone reorientation.
Figure 3: The distribution of crater size within groups 1 and 0 and their means (10.4 km and 6.1 km, respectively). There is no absolute threshold at which crater size dominates fracture reorientation, but a transition occurs within the 6-8 km crater diameter range.
Introduction: There are a variety of ways in which fractures disrupt craters on planetary surfaces (Fig. 1) including those on Ganymede [1], Venus, and Dione. The way in which fractures interact with impact craters on Enceladus however, appears to be a unique style of deformation in the solar system. On Enceladus, the mutually parallel trend of some fractures appears to change with increasing proximity to a crater, converging towards the crater in a somewhat radial pattern [2, 3, 4]. Fractures then cut through and continue beyond the crater, returning to the original orientation of the fracture set at some distance past the crater. Both simple and complex craters have been observed to perturb fractures [4,5]. There appears to be no dependence of fracture reorientation due to the relaxation state of the crater [4]. Fractures that experience reorientation by a crater appear relatively young, and many of them are pit chains, thought to be some of the youngest features on Enceladus [6].
Several mechanisms have been proposed for crater- induced fracture reorientation [2,3,4]. Reorientation of fractures may be due to topographic loading by the crater rim [2]. This is not our favored mechanism, as we observe craters that perturb fracture growth over large distances, and rim topography may not be sufficient to influence local stresses at such a distance. Crater depth, controlled by relaxation, may play a role in fracture orientation; only craters of a certain depth relative to the depth of the fracture may cause a fracture to reorient [4]. We explore the possibility of a source of internal pressure underneath the crater, such as thermally mobilized ice due to an impact, which would cause parallel fracture sets to reorient radially to the crater. Measuring the amount of reorientation that has occurred, and the maximum distance at which a crater can reorient fractures, is necessary for further examination of this hypothesis.
Characterization of these interactions is important for understanding what is driving a craterÕs ability to reorient fractures. Understanding local heterogeneities of fracture and terrain types is also critical for gaining perspective on Enceladus's global geologic history.
Previously, [7] reported a possible crater size dependency on fracture reorientation. We present here continued analysis of fracture reorientations by presenting initial measurements of the change in average fracture orientation with increasing distance from the crater. We also introduce a new possibility that some apparently reoriented fractures are the result of intertwined fracture sets overprinting older craters which warrants further examination.
Radius of Influence: Craters of all sizes were found to reorient fractures; however, craters greater than 7 km in diameter in particular always reorient fractures [7]. It follows therefore, that larger craters might also influence fracture orientations at greater distances. It is also expected that average fracture orientation will converge on the orientation of the back- ground fracture set with increasing distance from the crater. We refer to the maximum distance at which a crater influences fracture orientation as the Radius of Influence (Fig. 2).
Five craters previously determined by [7] to induce fracture reorientation were selected for preliminary analysis. 1 km long fracture segments within a single fracture set were mapped around each selected crater. Average fracture orientations were measured within buffer zones concentric about each crater at increasing distances scaled to crater size. For each of the five craters, there is no pattern of change in average fracture orientation with increasing distance from the crater. There is no constant change of azimuth converging on the orientation of the background fracture set as initially hypothesized. An increased sample size of craters is needed to verify this result. Additionally, the average fracture orientation within each buffer zone must be compared with the average orientation of the background fracture set to determine the radius of influence for each crater.
Detailed Fracture Mapping: Enceladus's cratered terrains, which are commonly overlooked as being extensively fractured, are dominated by fractures and pit chains that appear very young. Detailed fracture mapping has been carried out in a variety of locations within the cratered terrains (Fig. 3,4). Fractures are mapped based on crosscutting relationships, fracture orientation, and fracture morphology, resulting in identification of dominant fracture sets. In the absence of crosscutting relationships, fracture orientation and morphology become the next most important fracture characteristics used to determine which set a fracture belongs to. In the case of Fig. 3, it is possible that the apparently reoriented fractures are within two separate fracture sets that have formed after the crater. Fig. 4 shows a similar scenario but highlights the difficulties of fracture mapping. A fracture that appears to be reoriented towards the crater (arrowed), has no crosscutting relationships to help determine the set to which it belongs. Also, the fracture cannot be resolved to pass through the crater, and thus orientation and morphology are used to place it within the green fracture set. These fracture maps highlight the importance of measuring average fracture orientation within the same fracture set, but also brings to question crater-induced fracture reorientation in all apparent cases.
Discussion: Previous work by [7] determined reorientation by measuring the difference between fracture orientation and the average orientation of the background fracture set. We present here two examples where detailed fracture mapping suggests that fractures which have apparently undergone reorientation may belong to different fracture sets, which may postdate the crater. The inconclusive results from initial measurements of the radius of influence show that continued analysis is required.
References: [1] Pappalardo & Collins (2005) J of Struc. Geol. 27, 827-838. [2] Miller et al. (2007), Ices, Oceans and Fire: LPI Contribution No. 1357, p.95-96. [3] Barnash et al. (2006) 38th DPS Abs. #24.06. [4] Bray et al. (2007) 38th LPSC, Abs. #1873. [5] Kirchoff & Schenk (2009) Icarus, 206, 656-668. [6] Michaud et al. (2008) 39th LPSC Abs. #1678. [7] Martin & Kattenhorn (2011), 42nd LPSC Abs. #2666.
Acknowledgements: This work was funded by NASA OPR Grant #NNX08AQ94 and NESSF Grant #NNX11AP30H.
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.
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.
Near the tiger stripes / Boulders are distributed / Exponentially.
We use limb profiles to quantify the long-wavelength topography of the
Saturnian satellites. The degree 2 shapes of Mimas, Enceladus, and
Tethys are not consistent with hydrostatic equilibrium. We derive 2-D
topographic maps out to spherical harmonic degree 8. There is a good
correlation with topography derived from stereo techniques. If
uncompensated, topography at degree 3 and higher is large enough to be
detectable during close spacecraft flybys. If not properly accounted
for, this topography may bias estimates of a satellite's degree 2
gravity coefficients (which are used to determine the moment of
inertia). We also derive a one-dimensional variance spectrum (a measure
of how roughness varies with wavelength) for each body. The
short-wavelength spectral slope is -2 to -2.5, similar to
silicate bodies. However, unlike the terrestrial planets, each satellite
spectrum shows a reduction in slope at longer wavelengths. If this break
in slope is due to a transition from flexural to isostatic support, the
globally averaged elastic thickness Te of each satellite may
be derived. We obtain Te values of ≥5 km, 1.5-5 km,
≈5 km, and ≥5 km for Tethys, Dione, Rhea, and Iapetus,
respectively. For Europa, we obtain Te ≈ 1.5 km. These
estimates are generally consistent with estimates made using other
techniques. For Enceladus, intermediate wavelengths imply Te
≥ 0.5 km, but the variance spectrum at wavelengths greater than 150
km is probably influenced by long-wavelength processes such as
convection or shell thickness variations. Impact cratering may also play
a role in determining the variance spectra of some bodies.
Enceladus's south polar thermal anomaly and water-rich plumes suggest the existence of a subsurface ocean, which is overlain by an ice shell of uncertain thickness. Our objective is to constrain Enceladus's ice shell thickness, through assessment of tidally driven Coulomb failure of Enceladus's tiger stripe faults. We find that thin to moderate ice shell thicknesses (<40 km) support shear failure along the tiger stripes, assuming low ice coefficients of friction (0.1-0.3) and shallow fault depths (<3 km). These results are marginally consistent with the minimum ice shell thickness which can permit convection within Enceladus's ice shell. A plausible scenario is one in which the heat loss and tectonic style of Enceladus has changed through time, with convection initiating in a thick ice shell, and tiger stripe activity commencing as the ice shell thinned.
Solid CO2 surface deposits were reported in Enceladus’ South Polar Region by Brown et al. (2006). They noted that such volatile deposits are temporary and posited ongoing replenishment. We present a model for this replenishment by expanding on the Matson et al. (2012) model of subsurface heat and chemical transport in Enceladus. Our model explains the distributions of both CO2 frost and complexed CO2 clathrate hydrate as seen in the Cassini Visual and Infrared Mapping Spectrometer (VIMS) data. We trace the journey of CO2 from a subsurface ocean. The ocean-water circulation model of Matson et al. (2012) brings water up to near the surface where gas exsolves to form bubbles. Some of the CO2 bubbles are trapped and form pockets of gas in recesses at the bottom of the uppermost ice layer. When fissures break open these pockets, the CO2 gas is vented. Gas pocket venting is episodic compared to the more or less continuous eruptive plumes, emanating from the “tiger stripes”, that are supported by plume chambers. Two styles of gas pocket venting are considered: (1) seeps, and (2) blowouts.
The presence of CO2 frost patches suggests that the pocket gas slowly seeped through fractured, cold ice and when some of the gas reached the surface it was cold enough to condense (i.e., T ∼70 to ∼119 K). If the fissure opening is large, a blowout occurs. The rapid escape of gas and drop in pocket pressure causes water in the pocket to boil and create many small aerosol droplets of seawater. These may be carried along by the erupting gas. Electrically charged droplets can couple to the magnetosphere, and be dragged away from Enceladus. Most of the CO2 blowout gas escapes from Enceladus and the remainder is distributed globally. However, CO2 trapped in a clathrate structure does not escape. It is much heavier and slower moving than the CO2 gas. Its motion is ballistic and has an average range of about 17 km. Thus, it contributes to deposits in the vicinity of the vent. Local heat flow indicates that gas pockets can be located as deep as several tens of meters below the surface. Gas pockets can be reused, and we explore their life cycle.
Cassini stereo-derived topography reveals an exceptionally high-standing sawtooth-shaped ridge in Enceladus’ Samarkand Sulcus. Over a length of 100 km and of a width of 10 km, it reaches elevations of up to 1750 m, which makes it the highest ridge observed on Enceladus so far. Flank slopes reach 40°. The morphology of the ridge suggests that it formed first by rift flank-uplift caused by extension, but sinistral shear and compression later modified the shape. This modification has in particular emplaced small-scale fragments sticking out of the surface and creating a (previously enigmatic) pattern of black spots on the sun facing side of the ridge. Modelling of uplift related lithospheric flexure yields an effective elastic thickness (Te) of 0.36 km (E=1 GPa) at the time of formation, similar to results obtained in Harran Sulcus [1]. Considering the ridge as a load on the lithosphere at present-day, we obtain a lower limit on Te of 1.5 km. Within an asteroid/comet based impact chronology the ridge is 3.6/0.7 Gy old.
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.
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.
Images of the icy Saturnian satellites Mimas, Enceladus, Tethys, Dione, Rhea, Iapetus, and Phoebe, derived by the Voyager and Cassini cameras are used to produce new local high-resolution image mosaics as well as global mosaics [http://ciclops.org, http://photojournal.jpl.nasa.gov]. These global mosaics are valuable both for scientific interpretation and for the planning of future flybys later in the ongoing Cassini orbital tour. Furthermore, these global mosaics can be extended to standard cartographic products.
During the development of continental rifts, strain accommodation shifts from border faults to intra-rift faults. This transition represents a critical process in the evolution of rift basins in the East African Rift, resulting in the focusing of strain and, ultimately, continental breakup. An analysis of fault and fluid systems in the younger than 7 Ma Natron and Magadi basins (Kenya-Tanzania border) reveals the transition as a complex interaction between plate flexure, magma emplacement, and magmatic volatile release. Rift basin development was investigated by analyzing fault systems, lava chronology, and geochemistry of spring systems. Results show that extensional strain in the 3 Ma Natron basin is primarily accommodated along the border fault, whereas results from the 7 Ma Magadi basin reveal a transition to intra-rift fault-dominated strain accommodation. The focusing of strain into a system of intra-rift faults in Magadi also occurred without oblique-style rifting, as is observed in Ethiopia, and border fault hanging-wall flexure can account for only a minor portion of faulting along the central rift axis (~12% or less). Instead, areas of high upper crustal strain coincide with the presence of hydrothermal springs that exhibit carbon isotopes and N2-He-Ar abundances indicating mixing between mantle-derived (magmatic) fluids and air saturated water. By comparing the distribution of fault-related strain and zones of magmatic fluid release in the 3 Ma Natron and 7 Ma Magadi basins, we present a conceptual model for the evolution of early-stage rifting. In the first 3 m.y., border faults accommodate the majority of regional extension (1.24-1.78 mm yr⁻¹ in Natron at a slip rate ranging 1.93-3.56 mm yr⁻¹), with a significant portion of intra-rift faulting (38%-96%) driven by flexure of the border fault hanging wall. Fluids released from magma bodies ascend along the border fault and then outward into nearby faults forming in the flexing hanging wall. By 7 m.y., there is a reduction in the amount of extension accommodated along the border fault (0.40-0.66 mm yr⁻¹ in Magadi at a slip rate ranging from 0.62 to 1.32 mm yr⁻¹), and regional extension is primarily accommodated in the intra-rift fault population (1.34-1.60 mm yr⁻¹), with an accompanying transition of magmatic volatile release into the rift center. The focusing of magma toward the rift center and concomitant release of magmatic fluids into the flexing hanging wall provides a previously unrecognized mechanism that may help to weaken crust and assist the transition to intra-rift dominated strain accommodation. We conclude that the flow of magmatic fluids within fault systems plays an important role in weakening lithosphere and focusing upper crustal strain in early-stage continental rift basins prior to the establishment of magmatic segments.
Strike-slip faulting is typically characterized by lateral offsets on icy satellites of the outer solar system. However, strike-slip faults on Enceladus lack these typical lateral offsets and instead are marked by the presence of tailcracks or en echelon cracks. These features are used here to develop the first near-global distribution of strike-slip faults on Enceladus. Strike-slip faults on Enceladus fall into three broad categories: tectonic terrain boundaries, reactivated linear features, and primary strike-slip faults. All three types of strike-slip faults are found predominantly, or within close proximity to, the antipodal cratered terrains on the Saturnian and anti-Saturnian hemispheres. Stress modeling suggests that strike-slip faulting on Enceladus is not controlled by nonsynchronous rotation, as on Europa, suggesting a fundamentally different process driving Enceladus's strike-slip faulting. The motion along strike-slip faults at tectonic terrain boundaries suggests large-scale northward migration of the ice shell on the leading hemisphere of Enceladus, occurring perpendicular to the opening direction of the tiger stripes in the south polar terrain.
We apply histogram analysis, photogeological methods, and tidal stress modeling to Porco et al.'s survey of 101 Enceladus South Polar Basin geysers and their three-dimensional orientations to test if the jet azimuths are influenced by their placement relative to surface morphology and tectonic structures. Geysers emplaced along the three most active tiger stripe fractures (Damascus Sulcus, Baghdad Sulcus, and Cairo Sulcus) occur in local groupings with relatively uniform nearest-neighbor separation distances (∼5 km). Their placement may be controlled by uniformly spaced en echelon Riedel-type shear cracks originating from left-lateral strike-slip fault motion inferred to occur along tiger stripes. The spacing would imply a lithosphere thickness of ∼5 km in the vicinity of the tiger stripes. The orientations of tilted geyser jets are not randomly distributed; rather their azimuths correlate with the directions either of tiger stripes, cross-cutting fractures, or else fine-scale local tectonic fabrics. Diurnal tidal stress modeling suggests that periodic changes of plume activity are significantly affected by cross-cutting fractures that open and close at different times than the tiger stripes that they intersect. We find evidence of sub-kilometer scale morphological modification of surface geological features surrounding geysers from sublimation-aided erosion, and ablation, and scouring. We propose that the simultaneous crushing and shearing action of periodic transpressional tidal stress on ice condensing on the inside walls of geyser conduits is the mechanism that extrudes the peculiar, paired narrow ridges known as “shark fins” that flank the medial tiger stripe fissures. We present a gallery of high-resolution image mosaics showing the placement of all the jets in their source region and consequently their geological context.
Several planetary satellites apparently have subsurface seas that are of
great interest for, among other reasons, their possible habitability. The
geologically diverse Saturnian satellite Enceladus vigorously vents liquid
water and vapor from fractures within a south polar depression and thus must
have a liquid reservoir or active melting. However, the extent and location of
any subsurface liquid region is not directly observable. We use measurements of
control points across the surface of Enceladus accumulated over seven years of
spacecraft observations to determine the satellite's precise rotation state,
finding a forced physical libration of 0.120 0.014{\deg} (2{\sigma}).
This value is too large to be consistent with Enceladus's core being rigidly
connected to its surface, and thus implies the presence of a global ocean
rather than a localized polar sea. The maintenance of a global ocean within
Enceladus is problematic according to many thermal models and so may constrain
satellite properties or require a surprisingly dissipative Saturn.
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.
Miranda, an icy moon of Uranus, is one of the most visually striking and enigmatic bodies in
the solar system. Three polygonal-shaped regions of intense deformation, dubbed “coronae,” dominate the surface of Miranda. Here we use numerical methods to show that sluggish-lid convection in Miranda’s ice shell, powered by tidal heating, can simultaneously match the global distribution of coronae, the concentric deformation pattern, and the estimated heat flow during formation. The expected rheological conditions in Miranda’s ice shell lead to the devel- opment of low-order convection that produces surface deformation patterns similar to those observed. We find that satellite core size strongly controls convection geometry and that low- order convection patterns are much more stable for core radii less than half the satellite radius.
Observations of the south pole of the Saturnian moon Enceladus revealed large rifts in the south-polar terrain, informally called 'tiger stripes', named Alexandria, Baghdad, Cairo and Damascus Sulci. These fractures have been shown to be the sources of the observed jets of water vapour and icy particles and to exhibit higher temperatures than the surrounding terrain. Subsequent observations have focused on obtaining close-up imaging of this region to better characterize these emissions. Recent work examined those newer data sets and used triangulation of discrete jets to produce maps of jetting activity at various times. Here we show that much of the eruptive activity can be explained by broad, curtain-like eruptions. Optical illusions in the curtain eruptions resulting from a combination of viewing direction and local fracture geometry produce image features that were probably misinterpreted previously as discrete jets. We present maps of the total emission along the fractures, rather than just the jet-like component, for five times during an approximately one-year period in 2009 and 2010. An accurate picture of the style, timing and spatial distribution of the south-polar eruptions is crucial to evaluating theories for the mechanism controlling the eruptions.
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 have mapped the locations of over 100,000 ice blocks across the south polar region of Saturn’s moon Enceladus, thus generating the first quantitative estimates of ice-block number density distribution in relation to major geological features. Ice blocks were manually identified and mapped from twenty of the highest resolution (4–25 m per pixel) Cassini Imaging Science Subsystem (ISS) narrow-angle images using ArcGIS software. The 10–100 m-diameter positive-relief features are marginally visible at the resolution of the images, making ice-block identifications difficult but not impossible. Our preliminary results reveal that ice blocks in the southern hemisphere are systematically most concentrated within the geologically active South Polar Terrain (SPT) and exhibit peak concentrations within 20 km of the tiger-stripe fractures as well as close to the south pole. We find that ice blocks are concentrated just as heavily between tiger-stripe fractures as on the directly adjacent margins; although significant local fluctuations in ice-block number density do occur, we observe no clear pattern with respect to the tiger stripes or jet sources. We examine possible roles of several mechanisms for ice-block origin, emplacement, and evolution: impact cratering, ejection from fissures during cryovolcanic eruptions, tectonic disruption of lithospheric ice, mass wasting, seismic disturbance, and vapor condensation around icy fumeroles. We conclude that impact cratering as well as mass wasting, perhaps triggered by seismic events, cannot account for a majority of ice-block features within the inner SPT. The pervasiveness of fracturing at many size scales, the ubiquity of ice blocks in the inner SPT, as well as the occurrence of linear block arrangements that parallel through-cutting crack networks along the flanks of tiger stripes indicate that tectonic deformation is an important source of blocky-ice features in the SPT. Ejection during catastrophic cryovolcanic eruptions and condensation around surface vents, however, cannot be ruled out. Further, sublimation processes likely erode and disaggregate ice blocks from solid exposures of ice, especially near the warm tiger-stripe fractures. The relative paucity of blocks beyond the bounds of the SPT, particularly on stratigraphically old cratered terrains, may be explained in part by mantling of the surface by fine particulate ice grains that accumulate over time.
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.
We use images acquired by the Cassini Imaging Science Subsystem (ISS) to investigate the temporal variation of the brightness and height of the south polar plume of Enceladus. The plume's brightness peaks around the moon's apoapse, but with no systematic variation in scale height with either plume brightness or Enceladus' orbital position. We compare our results, both alone and supplemented with Cassini near-infrared observations, with predictions obtained from models in which tidal stresses are the principal control of the eruptive behavior. There are three main ways of explaining the observations: (1) the activity is controlled by right-lateral strike slip motion; (2) the activity is driven by eccentricity tides with an apparent time delay of about 5 hr; (3) the activity is driven by eccentricity tides plus a 1:1 physical libration with an amplitude of about 08 (3.5 km). The second hypothesis might imply either a delayed eruptive response, or a dissipative, viscoelastic interior. The third hypothesis requires a libration amplitude an order of magnitude larger than predicted for a solid Enceladus. While we cannot currently exclude any of these hypotheses, the third, which is plausible for an Enceladus with a subsurface ocean, is testable by using repeat imaging of the moon's surface. A dissipative interior suggests that a regional background heat source should be detectable. The lack of a systematic variation in plume scale height, despite the large variations in plume brightness, is plausibly the result of supersonic flow; the details of the eruption process are yet to be understood.
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.
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.
Among the many Cassini ISS (Imaging Science Subsystem) images of
Enceladus are a few severely-underexposed, motion-blurred images that
were acquired on “boresight-drag” events on the closest
flybys. During boresight-drags, ISS is statically aimed at a point that
intercepts the predicted path of Enceladus’ across the sky. The
ISS Narrow angle (NAC) and Wide Angle (WAC) cameras are repeatedly
triggered together in hope of serendipitously capturing a close-up
“BOTSIM” image-pair of the body as it passes. Because the
events are so fast, the surface footprints and lighting geometry cannot
be predicted in advance - a cascade of images are just quickly shuttered
at the minimum 5 msec exposure. On each of four boresight-drags, surface
images were captured. However, the two most recent (image-pair
W/N1669812043 in November 2010 and W/N1713106405 in April 2012,
respectively) were poorly illuminated -- three of four images only in
Saturnshine. Despite their poor signal quality, they are rare images of
Enceladus’ surface obtained with spatial resolutions better than a
few meters/pixel. Careful use of Fourier filtering and spatial
reconstruction techniques was needed to eliminate image noise and
residual electronic banding that was not removed during routine
radiometric calibration of the images. Fourier motion debluring
techniques were then applied to correct for significant motion smear.
Images W/N1669812043 (55.1°N, 20.2°W) are in old cratered
terrain, inside a prominent 23 km sized impact crater along the rise of
its updomed floor. They show a system of parallel ~250m wide mesas
trending around the dome’s circumference. Smooth detritus
inundates mesas and valleys near the dome summit and the mesa surfaces
are otherwise mantled with regolith that is finely cratered down to the
~2 m/pixel NAC resolution limit. W/N1713106405 (66.9°S, 29.5°W)
show the chaotically fractured margin of the active South Polar Terrain
- an area divided by parallel ridges and troughs with relatively smooth
flanks and valley floors. Quasi-linear arrangements of ice-blocks, each
block tens of meters or smaller, are found mostly near ridge-tops.
Enceladus exhibits a strong tectonic contrast between its south polar
terrain (SPT), which is young and geologically active, and its northern
hemisphere, which is relatively ancient and stable. Previous global
three‒dimensional (3‒D) spherical models of convection exhibit
patterns that are symmetrical around the equator and fail to explain the
formation of a hemispheric dichotomy. Here we present global 3‒D
spherical models of convection in Enceladus' ice shell to show that
convection in Enceladus' ice shell with plasticity and irregular core
geometry can self‒consistently generate a hemispheric dichotomy in
tectonics and heat flux. With a spherical core, convection produces
global overturning, which cannot explain the regional confinement of
Enceladus' current tectonic activity to the SPT. Models with appropriate
nonspherical core geometry and plasticity tend to produce overturning
confined to the SPT or regional overturning in different regions at
different times, which can explain the tectonic dichotomy and local age
differences on Enceladus. Our models predict heat flows up to 5-10
gigawattts (GW) during active episodes, consistent with Cassini
observations.
The width and temperature of the active fissures on Saturn's satellite
Enceladus provide key observable constraints on physical models of these
geyser-like eruptions. We analyze a sequence of high spatial resolution
near-infrared spectra acquired with VIMS at 0.025 s intervals during a
74 km altitude flyover of the South Pole of Enceladus by the Cassini
spacecraft on 14 April 2012 UTC. A thermal-emission spectrum covering 3-
to 5-μm wavelengths was detected as the field of view crossed one of
the four major fissures, Baghdad Sulcus, within 1 km of 82.36S latitude
and 28.24W longitude. We interpret this spectrum as thermal emission
from a linear fissure with temperature 197 ± 20 K and width 9 m.
At the above wavelengths, the spectrum is dominated by the warmest
temperature component. Looking downward into the fissure at only 13°
from the vertical, we conclude that our results measure the temperature
of the interior fissure walls (and the H2O vapor) at depths
within 40 m of the surface.
[1] The thermal dichotomy of Enceladus suggests an asymmetrical structure in its global heat transfer. So far, most of the models proposed that obtained such a distribution have prescribed an a priori asymmetry, i.e. some anomaly in or below the south polar ice shell. We present here the first set of numerical models of convection that yield a stable single-plume state for Enceladus without prescribed mechanical asymmetry. Using the convection code StagYY in a 2D-spherical annulus geometry, we show that a non-Newtonian ice rheology is sufficient to create a localized, single hot plume surrounded by a conductive ice mantle. We obtain a self-sustained state in which a region of small angular extent has a sufficiently low viscosity to allow subcritical to weak convection to occur due to the stress-dependent part of the rheological law. We find that the single-plume state is very unlikely to remain stable if the rheology is Newtonian, confirming what has been found by previous studies. In a second set of numerical simulations, we also investigate the first-order effect of tidal heating on the stability of the single-plume state. Tidal heating reinforces the stability of the single-plume state if it is generated in the plume itself. Lastly, we show that the likelihood of a stable single-plume state does not depend on the thickness of the ice shell.
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 long wavelength surface topography of Enceladus shows depressions
about 1 km in depth and ˜102 km wide. One possible
cause of this topography is spatially variable amounts of compaction of
an initially porous ice shell, driven by spatial variations in heat
flux. Here, we show that the heat flux variations associated with
convection in the shell can quantitatively match the observed features.
We develop a simple model of viscous compaction that includes the effect
of porosity on thermal conductivity, and find that an initial shell
porosity of at least 20-25% is required to develop the observed
topography over ˜1 Ga. This mechanism produces topographic
depressions, not rises, above convective upwellings, and does not
generate detectable gravity anomalies. Unlike transient dynamic
topography, it can potentially leave a permanent record of ancient
convective processes in the shallow lithospheres of icy satellites.
By performing 3D simulations of thermal convection and tidal dissipation, we investigated the effect of tidal heating on the onset of convection in Enceladus’s ice shell. We considered a composite non-Newtonian rheology including diffusion, grain-size-sensitive and dislocation creeps, and we defined an effective tidal viscosity reproducing the dissipation function as predicted by the Andrade rheology. For simulations with no or moderate tidal heating, the onset of convection requires ice grain sizes smaller than or equal to 0.5-0.6 mm. For simulations including significant tidal heating (>10−6 W m−3), the critical grain size for the onset of convection is shifted up to values of 1-1.5 mm. Whatever the width of the internal ocean, convection is initiated in the polar region due to enhanced tidal dissipation at high latitudes. For a given eccentricity value, the onset of convection depends on the ocean width, as tidal flexing and hence tidal heat production is controlled by the ocean width. For heating rates larger than 5-9 × 10−7 W m−3, we systematically observe the occurrence of melting in our simulations, whatever the grain size and for both convecting and non-convecting cases. Grain sizes smaller than 1.5 mm, required to initiate convection, may be obtained either by the presence of a few percent of impurities limiting the grain growth by pinning effects or by the increase of stress and hence dynamic recrystallization associated with tidally-induced melting events.
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.
Observations of Enceladus have revealed active jets of material erupting
from cracks on its south polar surface. It has previously been proposed
that diurnal tidal stress, driven by Enceladus' orbital eccentricity,
may actively produce surface movement along these cracks daily and thus
may regulate when eruptions occur. Our analysis of the stress on jet
source regions identified in Cassini ISS images reveals tidal stress as
a plausible controlling mechanism of jet activity. However, the evidence
available in the published and preliminary observations of jet activity
between 2005 and 2007 may not be able to solidify the link between tidal
stress and eruptions from fissures. Ongoing, far more comprehensive
analyses based on recent, much higher resolution jetting observations
have the potential to prove otherwise.
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.
The eruptive plumes and large heat flow (∼15 GW) observed by Cassini in the South Polar Region of Enceladus may be expressions of hydrothermal activity inside Enceladus. We hypothesize that a subsurface ocean is the heat reservoir for thermal anomalies on the surface and the source of heat and chemicals necessary for the plumes. The ocean is believed to contain dissolved gases, mostly CO2 and is found to be relatively warm (∼0 °C). Regular tidal forces open cracks in the icy crust above the ocean. Ocean water fills these fissures. There, the conditions are met for the upward movement of water and the dissolved gases to exsolve and form bubbles, lowering the bulk density of the water column and making the pressure at its bottom less than that at the top of the ocean. This pressure difference drives ocean water into and up the conduits toward the surface. This transportation mechanism supports the thermal anomalies and delivers heat and chemicals to the chambers from which the plumes erupt. Water enters these chambers and there its bubbles pop and loft an aerosol mist into the ullage. The exiting plume gas entrains some of these small droplets. Thus, nonvolatile chemical species in ocean water can be present in the plume particles. A CO2 equivalent-gas molar fraction of ∼4 × 10−4 for the ocean is sufficient to support the circulation. A source of heat is needed to keep the ocean warm at ∼0 °C (about two degrees above its freezing point). The source of heat is unknown, but our hypothesis is not dependent on any particular mechanism for producing the heat.
The intense activity at the south pole of Enceladus hints at an internal water reservoir. However, there is no direct evidence of liquid water at present and its long-term stability in the interior remains problematic. By modeling heat production and transfer in the ice shell in a spherical geometry, we show that tidal heating naturally leads to a concentration of convective hot upwellings in the south polar region, favoring the preservation of liquid water at depth. We show that large volumes of water are produced within the ice shell at the south pole during periods of elevated orbital eccentricity (3–5 times the present-day value). Strong lateral variations in the melt production and crystallization rates result in stress concentration in the south polar region, thus providing an explanation for the tectonic activity observed today. We predict that an internal ocean may be sustained over the long term as the consequence of repeated periods with elevated orbital eccentricity, leading to episodic melting and resurfacing events.
Enceladus exhibits a strong hemispheric dichotomy of tectonism and heat flux, with geologically young, heavily tectonized terrains and a high heat flux in the South Polar Terrain (SPT) and relatively ancient terrains with presumably lower heat fluxes over the rest of the satellite. To understand the convective pattern and its relationship with surface tectonics, we present three-dimensional numerical models of convection in Enceladus’ ice shell including basal heating and tidal heating. Our thermal boundary conditions exhibit no north–south asymmetries, but because the tectonism at the SPT may weaken the ice there, we impose a mechanically weak lithosphere within the SPT. The weakening is parameterized by adopting a reduced viscosity contrast within the SPT. Without such a weak zone, convection (if any) resides in stagnant-lid mode and exhibits no hemispheric dichotomy. In the presence of such an SPT weak zone, however, we find vigorous convection in the ice underneath the SPT, with convective plumes rising close to the surface. In contrast, only stagnant lid convection, or no convection at all, occurs elsewhere over the satellite. Away from the SPT, the heat flux in our models is small (5–10 mW m−2) and the surface strains are small enough to imply surface ages >109 years. Within the SPT, however, our models yield peak heat fluxes of ∼70–200 mW m−2, implying heat flows integrated across the SPT of up to 5 GW, similar to that inferred from Cassini thermal observations. The surface strains in our models are high enough near the south pole to cause intense tectonism and imply surface ages of ∼106–107 years, consistent with age estimates of the SPT.
Observations of Enceladus by the Cassini spacecraft indicate that this tiny Saturnian moon is geologically active, with plumes of water vapor and ice particles erupting from its southern polar region. This activity suggests that tidal dissipation has become spatially localized, perhaps due to a compositional, rheological, and/or thermal anomaly in its ice shell. Here we examine the role that solid-state convection may have played in Enceladus' prolific activity by creating a suitable rheological and thermal anomaly. We find convection can only initiate in the pure water ice I shell of a differentiated Enceladus if the ice grain size is less than 0.3 mm, which is quite small, but may be realistic if non-water-ice impurities (and/or tidal stresses) keep grains from growing. This grain-size restriction becomes more severe for lower basal ice temperatures, which implies that any ammonia present has not become strongly concentrated in a thin basal ocean (while convection occurs). For a maximally thick pure ice shell and underlying ocean, convective heat flows are ~7-11 mW m-2 for ice grain sizes of 0.1-0.3 mm, compared with the ~100 mW m-2 measured for Enceladus' south polar terrain from Cassini CIRS observations. Thus whereas solid-state convection may be a prerequisite for Enceladus' geological activity, the observed heat flow requires strong tidal dissipation within the convecting region, and possibly, that convection reaches the surface.
Enceladus' south polar region has a large heat flux that is spatially associated with cryovolcanic and tectonic activity. Tidal dissipation and vigorous convection in the underlying ice shell are possible sources of heat, however, prior predictions of the heat flux carried by stagnant lid convection are too low to explain the observed heat flux. The high heat flux and cryovolcanic/tectonic activity in the region suggest that near- surface ice has become rheologically and mechanically weakened enough to permit convective plumes to reach close to the surface. If the yield strength of Enceladus' lithosphere is less than 1 to 10 kPa, convection may occur in the "mobile" lid regime, characterized by large heat fluxes and large horizontal velocities in the near-surface ice. Ice shells convecting in this regime of behavior have heat fluxes comparable to that observed by CIRS. If this style of convection is occurring, the south polar terrain should be spreading horizontally with v ~ 1 to 10 mm yr-1 and should be resurfaced in 0.1 to 10 Myr. This estimated age is comparable to age estimates of 0.5 Myr based on crater counts from Cassini imaging. Maxwell viscoelastic tidal dissipation in such an ice shell is not capable of generating enough heat to balance convective heat transport. Tidal heat is likely generated in the near-surface along faults as suggested by Nimmo et al., Nature, (2007). It is also possible that tidal dissipation within the ice shell occurs by other processes not accounted for by the canonical Maxwell dissipation model.
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.
Several large ridges, or "dorsa,'' located within a tectonically deformed region of the trailing hemisphere of Enceladus, have remained poorly understood since they were imaged in Feb. 2005. In map view these 2.5 km wide ridges can bifurcate in a branching manner, and in profile they appear to be somewhat trapezoidal or "boxy'' in shape. Geological mapping of the trailing hemisphere's Sarandib and Diyar Planitiae (Crow-Willard and Pappalardo, 2009) suggests the dorsa cut across and deform older striated terrain, which consists of small-scale (200 m wide) ridges and troughs. A single high-resolution (65 m/pixel) Cassini image captures the western portions of the Cufa Dorsa, where ridges of the striated terrain are visible on the southern flanks. This relationship is inconsistent with a previously suggested cryovolcanic origin, and there is no evidence of surrounding embayment. Instead, the relationships suggest tectonic uplift to form the dorsa. We propose that these ridges were formed by thrust faulting; in particular, the Cufa Dorsa suggest formation above south-dipping master thrusts, as either fault-bend folds or more likely as fault-propagation folds. The high-resolution image reveals small ( 300 m) irregular crenulations atop the ridges, and shadow roughness indicates significant roughness at sub-pixel scale. The crenulations appear analogous to those atop wrinkle ridges on the Moon and Mars, and to the Yakima Ridges of eastern Washington state, which form via high-level back thrusts in layered materials above a relatively flat décollement. The Cufa Dorsa terminate to the west against a prominent trough, which may have served as a transcurrent fault that permitted north-south contraction. Bifurcation of the Cufa Dorsa is consistent with three-dimensional straining, if the dorsa resulted from multi-directional contraction. Perhaps a thermal uplift that initialized trailing hemisphere tectonic deformation subsequently cooled and collapsed to form the dorsa.
Stereo-derived topographic mapping of ∼50% of Enceladus reveals at least 6 large-scale, ovoid depressions (basins) 90–175 km across and 800-to-1500 m deep and uncorrelated with geologic boundaries. In contrast, the south polar depression is larger and apparently shallower and correlates with active resurfacing. The shape and scale of the basins is inconsistent with impact, geoid surface deflections, or with dynamically supported topography. Isostatic thinning of Enceladus' ice shell associated with upwellings (and tidally-driven ice melting) can plausibly account for these basins. Thinning implies upwarping of the base of the shell of ∼10–20 km beneath the depressions, depending on total shell thickness; loss of near-surface porosity due to enhanced heat flow may also contribute to basin lows. Alternatively, the basins may overly cold, inactive, and hence denser ice, but thermal isostasy alone requires thermal expansion more consistent with clathrate hydrate than water ice.
Enceladus has a protracted history of impact cratering, cryovolcanism, and extensional, compressional, and probable strike–slip faulting. It is unique in having some of the outer Solar System's least and most heavily cratered surfaces. Enceladus' cratering record, tectonic features, and relief elements have been analyzed more comprehensively than done previously. Like few other icy satellites, Enceladus seems to have experienced major lateral lithospheric motions; it may be the only icy satellite with global features indicating probable lithospheric convergence and folding. Ridged plains, 500 km across, consist of a central labyrinthine ridge complex atop a broad dome surrounded by smooth plains and peripheral sinuous ridge belts. The ridged plains have few if any signs of extension, almost no craters, and an average age of just 107to 108years. Ridge belts have local relief ranging from 500 to 2000 m and tend to occur near the bottoms of broad regional troughs between swells. Our reanalysis of Peter Thomas' (Dermott, S. F., and P. C. Thomas, 1994, The determination of the mass and mean density of Enceladus from its observed shape,Icarus,109, 241–257) limb profiles indicates that high peaks, probably ridge belts, also occur in unmapped areas. Sinuous ridges appear foldlike and are similar to terrestrial fold belts such as the Appalachians. If they are indeed folds, it may require that the ridged plains are mechanically (perhaps volcanically) layered. Regional topography suggests that folding may have occurred along zones of convective downwelling. The cratered plains, in contrast to the ridged plains, are heavily cratered and exhibit extensional structures but no obvious signs of compression. Cratered plains contain a possible strike–slip fault (Isbanir Fossa), along which two pairs of fractures seem to have 15km of right-lateral offset. The oldest cratered plains might date from shortly after the formation of the saturnian system or the impact disruption and reaccretion of Enceladus. Another area of cratered plains has modified craters (e.g., Ali Baba and Aladdin), which some workers have explained by anomalous heat flow and viscous relaxation; lateral shear and shield-building volcanism also may have been important. A young rift-like structure (northern Samarkand Sulci) has few craters and a concentration of cracks or grabens and flattened, flooded, and rifted craters. Pit chains and cratered domes suggest explosive volcanism. Smooth plains may have formed by cryovolcanic equivalents of flood-basalt volcanism. Pure H2O would be difficult to extrude through an icy crust and is cosmochemically improbable as a cryovolcanic agent. Density relations rule out eutectic brine lavas on Enceladus, but NH3–H2O volcanism is possible. Current steady-state tidal dissipation may cause melting of ammonia hydrate at a depth of ∼25 km if the crust is made of ammonia hydrate or ∼100 km if it is made of water ice.
Subparallel ridges and troughs in the outer belt of Arden Corona, on the Uranian satellite Miranda, are interpreted as tilt blocks formed by extension and normal faulting. Fault scarps generally face outward from the corona, exposing dark material in the subsurface. Reconstruction of faults along a deep rift zone bounding the corona suggests initial dips of ~50°. Local extension reaches ~70%, extremely high in comparison to previous estimates of strain on icy satellites. A rise adjacent to the rift zone is modeled as flexural and indicates an effective elastic lithospheric thickness of ~2km at the time of flexure. The assumption that faulting has significantly weakened the lithosphere suggests a mechanical lithosphere thickness of ~5 to 10 km. Corresponding thermal gradients in a frictionally controlled ice lithosphere are ~8 to 20 K km-1, and lithospheric tensional strength is ~0.4 to 1.8 MPa. Normal faulting in Arden Corona indicates that internal upwelling likely formed the corona, and the outward facing direction of faults is consistent with such a model. An upwelling origin of Miranda's coronae eliminates the need to invoke catastrophic breakup and reaccretion of the satellite as an explanation for its surface geology.
High-resolution mosaics of the leading-hemisphere of Enceladus were acquired for the first time by Cassini's ISS Narrow Angle Camera (NAC) during two close flybys, one on November 21, 2009 and another on May 18, 2010, respectively. Additional imaging of the leading-hemisphere is planned for a close (2800 km) flyby on August 13, 2010 (orbit 136). Low resolution imaging obtained earlier in the Cassini mission showed that the leading hemisphere is heavily modified by tectonism, but the extent to which the tectonic styles on the leading hemisphere might be similar to those elsewhere on Enceladus was unclear. The new mosaics show that the leading side is subdivided into distinct geological provinces that exhibit different cratering histories and diverse tectonic styles. The highly tectonised terrains are bounded by a prominent broad annulus of grooved and striated terrains that ranges from about 60 km to over 140 km in width. It surrounds a complex arrangement of tectonic structures, including a conspicuous province near 30°N, 90°W of curvilinear ridges and approximately orthogonal-trending ridged-troughs. Among the most significant new findings is a region near 10°S, 60°W of terrain that is covered by a distinct pattern of rounded, rope-like sub-parallel ridges. These ridges, in appearance and scale, are remarkably similar to ropy (funiscular) plains materials that previously have been found only in the South Polar Terrain region adjacent to active tiger stripes. We suggest that the pattern of ropy ridges on the leading hemisphere arose from a similar style of tectonic deformation that produced the South Polar funiscular plains - a terrain that is closely related to possible folding and tectonic spreading associated with the tiger stripes. These features may thus record an ancient episode of South Polar style tectonism and volcanism near the equator.