The small (approximately 500 km in diameter) satellite Enceladus is moving near the equatorial plane and deep in the gravity field of its parent planet Saturn. Owing to tidal interaction with its parent, Enceladus has adopted a pronounced 3-axial ellipsoidal shape and is tidally locked, with rotational and orbital periods of about 1.37 days. As the equator of Saturn is inclined to the planet’s orbital plane, Enceladus, like most of the other satellites of Saturn, undergoes pronounced seasons. This paper gives a summary of the current status as well as shortcomings of our current knowledge regarding Enceladus’ geodetic and dynamic parameters.

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.

The Saturnian satellite Tethys exhibits geological and spectral properties, whose appearance, nature and spatial distribution partly mirror those identified on the neighboring satellites Dione and Rhea or fit to the picture how spectral surface properties are expected to change from one satellite to the other within the inner Saturnian system. However, we also identified spectral variations that are unique in the Saturnian system. Whereas geologically young surface features are characterized by pure H2O-ice composition with relatively large particles, which match the particle sizes measured for fresh surface features also on Dione and Rhea, geologically old weathered regions are dominated by submicron-sized ice particles. Our investigations confirm that the Odysseus impact event did not cause the formation of Tethys' extended graben system Ithaca Chasma. On the contrary, Odysseus might be responsible for the N-S trending 'icy' bands that mark Tethys' surface in the center of its leading and trailing hemisphere. (C) 2016 Elsevier Inc. All rights reserved.

Cassini's Visual and Infrared Mapping Spectrometer (VIMS) offers the first spatially resolved hyperspectral data of the Saturnian satellites [1] allowing a detailed comparison of their spectral properties including their spatial distribution across the satellite's surface. Additionally, images acquired by the Cassini ISS cameras [2] offer the opportunity to study any relationships between the spectral variations to geological and morphological surface features. This is essential to understand the origin of the major and especially minor surface compounds and/or their physical properties. Either, the derived spectral variations are closely related to the surface geology of the specific satellite, or they can be explained by the interaction of the surface material and the space environment (including the planets magnetosphere). Thus, the analysis of the icy satellites spectral properties not only further our understanding of the icy satellites evolution but also complete our view about the whole planetary system.

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. (C) 2014 Elsevier Inc. All rights reserved.

Since 2004 the Cassini spacecraft performed numerous targeted and non-targeted flybys at the major Saturnian satellites. During these flybys the Visual and Infrared Mapping Spectrometer (VIMS) onboard the Cassini spacecraft [1] detects the spectral properties of the satellites’ surfaces in the wavelength range from 0.35 to 5.1 μm with a spectral resolution that allows the mapping of the distribution of major surface compounds like H2O ice as well as discov-ery/identification of minor and/or trace compounds like CO2 as shown for several Saturnian satellites [2-6]. Mapping of these compounds revealed a distinct distribution of most of these surface compounds either related to geological surface features/processes or to weathering of the uppermost surface layer. In order to further our understanding of the Satutnian system we now performed an analysis of the spectral surface properties of Tethys in context of the surface geology as well as topography based on simultaneously ac-quired Cassini ISS images.

Based on control point calculations it is shown that Enceladus experiences librations forced by Dione.

On Cassini’s 121st orbit, the onboard ISS camera acquired high-resolution (15-30 m/pxl) images in Enceladus’ south polar province. The imaging sequence was specifically designed to study one of the source regions of Enceladus’ erupting plumes, Baghdad Sulcus. To facilitate the analysis, we derived a digital elevation model in an active section (76°S/323°E) across Baghdad Sulcus. The model reveals that there is a V-shaped trough up to 500 m deep in the center of this section, with flanking slopes of 30° (SW-facing) and > 32° (NE-facing, this slope is in shadow). The slopes do approach angle of repose, but the morphology on the SW slope (blocky terrain with lineation patterns and even benches at angles to the maximum slope) suggests that this is not a slope undergoing angle-of-repose control. The trough, therefore, may owe its shape primarily to faulting, with only some modification by deposition of icy particles by the plume-forming gas. Blocky covering, which includes block sizes of up to 50 m, is not restricted to the trough but also occurs at about the same size and frequency distribution away from it. This suggests that the blocks are not related to the venting process, which concentrates in the trough. Rather, the association of the blocky surfaces with multiple patterns of lineations (presumably fractures and faults) suggests they are outcrops of fault-related ice blocks or lithified detritus undergoing some form of erosion. A potential erosion process may include seismic shaking. The V-shaped trough is partly accompanied by an elevated flanking ridge, which is indicative for rift zones and hints at an extensional origin of Baghdad Sulcus. Alternatively, fault-block rotation at large strains could have led to the elevated ridge.

The surface morphology of icy moons is affected by several processes implicating exchanges between their subsurfaces and atmospheres (if any). The possible exchange of material between the subsurface and the surface is mainly determined by the mechanical properties of the lithosphere, which isolates the deep, warm and ductile ice material from the cold surface conditions. Exchanges through this layer occur only if it is sufficiently thin and/or if it is fractured owing to tectonic stresses, melt intrusion or impact cratering. If such conditions are met, cryomagma can be released, erupting fresh volatile-rich materials onto the surface. For a very few icy moons (Titan, Triton, Enceladus), the emission of gas associated with cryovolcanic activity is sufficiently large to generate an atmosphere, either long-lived or transient. For those moons, atmosphere-driven processes such as cryovolcanic plume deposition, phase transitions of condensable materials and wind interactions continuously re-shape their surfaces, and are able to transport cryovolcanically generated materials on a global scale. In this chapter, we discuss the physics of these different exchange processes and how they affect the evolution of the satellites’ surfaces.

The surfaces of the Solar System's icy satellites show an extraordinary variety of morphological features, which bear witness to exchange processes between the surface and subsurface. In this paper we review the characteristics of surface features on the moons of Jupiter, Saturn, Uranus and Neptune. Using data from spacecraft missions, we discuss the detailed morphology, size, and topography of cryovolcanic, tectonic, aeolian, fluvial, and impact features of both large moons and smaller satellites.

Since the arrival of the Cassini spacecraft at Saturn in June 2004, the Visual and Infrared Mapping Spectrometer has obtained new spectral data of the icy satellites of Saturn in the spectral range from 0.35 to 5.2 μm. Numerous flybys were performed at Saturn's second largest satellite Rhea, providing a nearly complete coverage with pixel-ground resolutions sufficient to analyze variations of spectral properties across Rhea's surface in detail. We present an overview of the VIMS observations obtained so far, as well as the analysis of the spectral properties identified in the VIMS spectra and their variations across its surface compared with spatially highly resolved Cassini ISS images and digital elevation models. Spectral variations measured across Rhea's surface are similar to the variations observed in the VIMS observations of its neighbor Dione, implying similar processes causing or at least inducing their occurrence. Thus, magnetospheric particles and dust impacting onto the trailing hemisphere appear to be responsible for the concentration of dark rocky/organic material and minor amounts of CO2 in the cratered terrain on the trailing hemisphere. Despite the prominent spectral signatures of Rhea's fresh impact crater Inktomi, radiation effects were identified that also affect the H2O ice-rich cratered terrain of the leading hemisphere. The concentration of H2O ice in the vicinity of steep tectonic scarps near 270°W and geologically fresh impact craters implies that Rhea exhibits an icy crust at least in the upper few kilometers. Despite the evidence for past tectonic events, no indications of recent endogenically powered processes could be identified in the Cassini data.