Enceladan Tectonics: Ice and Isostasy in Action
Saturn's moon Enceladus is the smallest body in the solar system known to be geologically active. Extensive, energetic resurfacing processes are ongoing and it possesses a system of geysers at its South Pole that supply material to Saturn's E-ring. The South Polar Terrain (SPT) is the youngest region on Enceladus and its contacts with the older cratered and grooved plains to the north are delineated by a variety of complex geologic features that include mountain ranges and massive grabens. On Earth, new lithosphere is created at spreading centers and consumed at subduction zones, a process enabled by differences in composition, density, thickness and mineral properties between continental and oceanic crust. However, the Enceladan lithosphere is made entirely of water ice, so any newly formed crust would have the same composition but lower density due to higher temperature (being more recently solidified), making subduction and consequently spreading, as we understand it on Earth, unlikely. Geometrically, the absence of fold-thrust belts and transform faults, and the widespread presence of normal faulting and extensional structures, implies extension without corresponding shortening elsewhere. This is not possible in a conventional (terrestrial) plate tectonic regime, as surface area is not conserved; an alternate explanation is required. Topographic features associated with density contrasts between old and new terrain that are diagnostic of terrestrial spreading centers are also not observed on Enceladus. We hypothesize that the orogenic zone surrounding the SPT is an extensional phenomenon, broadly analogous to terrestrial basin and range topography, formed by the "calving" of blocks at the periphery of the SPT. Superficially resembling the seracs in a glacial icefall, these tilted ice blocks remain essentially stationary, while the basal detachment (possibly a listric normal fault) progresses outward from the SPT through time, effectively marking the expanding transition from the thin, warm ice of the SPT basin to the cold, thick ice of the northern plains. As the heat affected zone spreads outward over time, more proximal blocks undergo gradual viscoelastic relaxation until they are topographically indistinguishable from the complexly-oriented ridged units associated with the "tiger-stripe" sulci. We interpret these ridges as cryovolcaniclastic levees, based on recent high-resolution Cassini-ISS imagery. The presence of a solid-liquid phase transition below the south polar terrain provides a mechanism for producing a basin over a positive thermal anomaly, as a consequence of the unusual behaviour of water ice compared to silicate rock on Earth. We conclude that features observed on Enceladus are inconsistent with terrestrial-style plate tectonic spreading, and represent a style of tectonism peculiar to bodies with icy lithospheres.
Tidally Driven Strike-Slip Fault Activity at Enceladus's Tiger Stripes
Straddling the south polar region of Enceladus, the four principal tiger stripe fractures are a likely source of tectonic activity and plume generation. Here we investigate tidally driven stress conditions at the tiger stripe fractures through a combined analysis of shear and normal diurnal tidal stresses. We compute Coulomb failure conditions to assess likely failure location, timing, and direction (right- vs. left-lateral slip) throughout the Enceladus orbital cycle and explore a suite of model parameters that inhibit or promote shear failure at the tiger stripes. We find that low coefficients of friction (μf = 0.1-0.2) and shallow fracture depths (2-4 km) permit shear failure along the tiger stripe faults, and that right- and/or left-lateral slip responses are possible. We integrate these conditions into a 3D time-dependent fault dislocation model to evaluate tectonic displacements and stress variations at depth during a tiger stripe orbital cycle. In this model, the sequence of stress accumulation and subsequent fault slip varies as a function of fault location and orientation, frictional coefficient, and fault depth. We estimate shear stress accumulation of ∼70 kPa prior to fault failure, which can generate strike-slip displacements on the order of ∼0.5 m in the horizontal direction and ∼5 mm in the vertical direction per slip event. Tectonic activity inferred from these analyses positively correlates with observed plume activity and temperature anomalies at the tiger stripes, however in detail some regions of our model do not strongly match the observations. In these regions, future work will tune the model, varying frictional and fault geometry parameters, to best simulate the available plume and temperature anomaly data.
Geological Mapping of Tectonized Terrains in the Trailing Hemisphere of Enceladus
Saturn's moon Enceladus has a currently active South Polar Terrain (SPT) that is intensely tectonized. Other portions of the surface of Enceladus, specifically the trailing hemisphere, have also been intensely tectonized, inviting comparisons to the SPT. Through geological mapping, we recognize seven different geological units and their relative ages on the trailing hemisphere. From oldest to youngest, they are: (1) heavily cratered terrain, at the northern edge of the tectonized region; (2) moderately cratered terrain, to the northeast and northwest of the tectonized region; (3) finely striated ridge and trough terrain, which make up the bulk of Sarandib and Diyar Planitiae; (4) boundary curved terrain, which is similar in shape to the southern curved terrain that comprises the northern edge of the SPT, but with more subdued topography, and is composed of Samarkand, Hamah, and Harran Sulci; (5) ridged terrain, composed of the Cufa Dorsa and Ebony Dorsum, which probably formed through deformation of older finely striated ridge and trough terrain; (6) terrain with linear, widely spaced, smooth depressions, comprising the southern portion of the trailing hemisphere's tectonized region; (7) southern curved terrain of Cashmere Sulci, which forms the northern boundary of the SPT. Fractures that are younger than or contemporaneous with the SPT's southern curved terrain (including Labtayt Sulci and Khorasan Fossa) cut across the trailing hemisphere. We will present a geological map of the region, along with interpretations of the stratigraphy and geological history that our mapping implies. We will address geological and age comparisons relative to the SPT, with implications for whether similar or different processes have shaped the SPT and the tectonized trailing hemisphere.
High-Resolution Observations of Enceladus' Endogenic Thermal Radiation in 2008
Extended Search for Energetic Sodium Ions in Saturn's Magnetosphere
Instruments on the Cassini spacecraft discovered that Enceladus, an icy satellite of Saturn, ejects plumes of gas, mainly water vapor, and icy grains from so called "tiger stripes" near its South pole. Ice particles emitted from Enceladus form Saturn's large E ring. Both grains and gas can be sources of energetic charged particles through photoionization and/or charged particle impact. Extant sunlight and high-energy charged particle flux allow both processes. Once atoms and molecules are ionized and subsequently accelerated, they can quickly traverse large portions of a planetary magnetosphere before being detected. Sodium (Na) is considered a necessary tracer for a liquid water ocean on Enceladus. Water in contact with Enceladus' rocky interior should become salty. A high-sensitivity telescopic search using ground-based observatories failed to detect sodium emissions near Enceladus [Schneider et al., 2007]. On the other hand, in nearly all in situ measurements of E ring ice particle composition, Cassini's Cosmic Dust Analyzer (CDA) finds sodium in varying concentrations [Postberg et al., 2008]. In this study, we use measurements from MIMI/CHEMS, the Charge-Energy-Mass Spectrometer, to extend our analysis of ion composition in the range >31-220 keV/e taking full note of extant backgrounds in the magnetosphere [Christon et al., 2008]. CHEMS, one of three sensors comprising the MIMI investigation on Cassini, determines the energy, mass, and charge state of ions. In our earlier study, for mid- 2004 through 2006, we found an upper limit for the number ratio of Na+1 ions relative to the more abundant water-group W+1 (O+1, OH+1, and H2O+1) ions, Na+1/W+1 ∼ 0.0005-0.0010. Our present extended and refined analysis uses data collected for mid-2004 to early- 2009 in the near-equatorial, R < 20 Rs, magnetosphere to better define the in situ measurement of sodium with improved statistics and to examine the relative abundance of other low charge state heavy ion species. References: Christon et al. (2008), Eos Trans. AGU, 89(53), Fall Meet. Suppl., abst. P23B-1383. Postberg et al. (2008), Eos Trans. AGU, 89(53), Fall Meet. Suppl., abst. P14A-03. Schneider et al, (2007), Eos Trans. AGU, 88(52), Fall Meet. Suppl., abst. P11F-08.
Dusty plasma of E-ring and Enceladus observed by Cassini Langmuir probe
During the three Enceladus plume and associated E-ring encounter of the Cassini spacecraft, the ion/electron number densities and the temperature data are obtained by the RPWS Langmuir probe (LP). We found that the electron/ion densities increase as Cassini approaches the E-ring. As Cassini encounters the E-ring within the distance from the equator Z = ∼ ± 20,000 km, the electron density gradually decrease while the ion density is still increasing. This signature seems to be due to the negatively charged dust as recently reported by Wahlund et al. (2005, 2009). Furthermore, in this region the ion velocity is close to the Keplarian speed. Such observed signatures have been detected throughout the plasma disk near the E-ring. There are no Langmuir probe observations indicating an ionosphere around Enceladus. In the Enceladus plume, on the other hand, both the ion and electron density became very large (up to a few 105 cm-3 for the ions and at least few 104 cm-3 for the electrons). However, the number densities of the electrons are still significantly lower than the ion number densities, except possibly in the densest parts. That is, the charged dusts are still important in the plume region. The ion speed is departed from the Keplarian speed, which seems to be consistent with the plasma flow in the plume region. We discuss the dust-plasma interaction and electrodynamics in the plasma disk near the E-ring and in the Enceladus plume region.