Search

Icy satellites of the outer solar system have become the primary target for planetary exploration because of their relevance to understanding of solar-system evolution and to the origin of life. Despite this importance, it remains unclear how different combinations of tectonic deformation, climate conditions, and surficial and interior processes have shaped geologically diverse paths of satellite evolution, as evident from their widely different surface morphologies. Here I address this fundamental question by conducting geological mapping of Enceladus and Titan, the two end-member icy satellites of Saturn; Enceladus has tectonic activity expressed by erupting plumes along active faults while Titan has a thick atmosphere that exerts strong control on its surface processes and hence surface morphologies. My studies on Enceladus focus on two subjects: (1) the transport time scale for nanoparticles of silica from the ocean floor to the erupting plumes and (2) the role of the non-tidal stress in controlling the phase lag of time-varying plume fluxes that share the same periodicity with the diurnal tide. I assess the transport time scale of silica particles based on experimentally determined scaling relationships for convection systems under rotation and entrainment of particles in thermally-driven convecting fluids. The physics-based analytical relationships obtained from this approach allow the establishment of the size of the silica particles to the thermal regime of the core, which in turn provides the basis for estimating the transport time scale of the particle through the ocean, which I find to be on the order of months. To assess the role of the non-tidal stress in controlling the phase lag of plume eruption on Enceladus, I conducted detailed structural mapping along geyser-hosting faults zones (i.e., the informally named tiger stripes in the literature). My mapping shows that the geysers are preferentially located at local extensional structures along overall strike-slip faults. In order to have simultaneous strike-slip fault motion and local development of extensional structures along the strike-slip faults, coeval shear and tensile failure is required. Imposing this condition and assuming that the peak-eruption time is the result of the superposed tidal and non-tidal stresses reaching the maximum tensile-stress value, I am able to use a stress-decomposition model to determine the static non-tidal stress field along geyser-hosting faults. The required non-title stress field is best explained by lateral viscous flow induced by the gradient of gravitational potential stored in an unevenly thick ice shell.My research on Titan focuses on the geomorphological response in space and time to climate change and tectonic deformation. In this end, I established the spatial distribution and temporal relationships among morphologically distinctive terrains through mapping in the South Belet and Soi Crater regions. The major finding of the work is that dunes and lakes are the youngest geomorphologic units resulting from the youngest climate condition that are superposed on top of hummocky, labyrinth, pitted, and mountainous terrains. The presence of dune fields requires aeolian transport, the lake and labyrinth terrains surface and subsurface fluid-flow activities, and the pitted terrain removal of volatile materials. The oldest mountainous terrain is best explained by early tectonic deformation. The spatial distribution of dunes and lakes is consistent with the global mapping results that climate-sensitive terrains are distributed symmetrically with respect to the equator, reflecting the symmetry of the atmosphere circulation.

Enceladus, an icy moon of Saturn, possesses an internal water ocean and jets expelling ocean material into space. Cassini investigations indicated that the subsurface ocean could be a habitable environment having a complex interaction with the rocky core. Further investigation of the composition of the plume formed by the jets is necessary to fully understand the ocean, its potential habitability, and what it tells us about Enceladus' origin. Moonraker has been proposed as an ESA M-class mission designed to orbit Saturn and perform multiple flybys of Enceladus, focusing on traversals of the plume. The proposed Moonraker mission consists of an ESA-provided platform, with strong heritage from JUICE and Mars Sample Return, and carrying a suite of instruments dedicated to plume and surface analysis. The nominal Moonraker mission has a duration of 13.5 years. It includes a 23-flyby segment with 189 days allocated for the science phase, and can be expanded with additional segments if resources allow. The mission concept consists in investigating: i) the habitability conditions of present-day Enceladus and its internal ocean, ii) the mechanisms at play for the communication between the internal ocean and the surface of the South Polar Terrain, and iii) the formation conditions of the moon. Moonraker, thanks to state-of-the-art instruments representing a significant improvement over Cassini's payload, would quantify the abundance of key species in the plume, isotopic ratios, and physical parameters of the plume and the surface. Such a mission would pave the way for a possible future landed mission., Accepted for publication in The Planetary Science Journal

Salt-rich icy particles within Saturn’s E-ring are thought to originate from frozen aerosolized droplets of the salty seawater of Enceladus’ subsurface ocean. They are ejected into space, through fractures in the moon’s south polar region, within a plume of gas and ice particles. Some of these icy particles can escape the gravity of Enceladus thanks to their high velocities, and become E-ring particles, which typically have relatively young (no more than 200 years) ages. Therefore, it is believed that the salt-rich grains reflect the composition of the subsurface ocean water. The in situ mass spectra of E-ring and plume icy particles, produced by the Cosmic Dust Analyzer (CDA) impact ionization mass spectrometer onboard the Cassini spacecraft, reveal significant compositional diversity within the salt-rich population. Understanding the compositions of dissolved substance, including salt, silica, and organics, within the grains, and thus in the ocean, can provide crucial constraints for geochemical models of Enceladus’ subsurface ocean environment. The research presented in this thesis investigates the compositional diversity of the in situ mass spectra and hence the salt-rich icy particles and attempts to quantify the variations in grain composition with the aid of analogue mass spectra generated in the laboratory. In this work, the in situ mass spectra and thus the salt-rich icy particles are classified into various compositional groups and subtypes. The major subtype distributions of E-ring spectra/particles with respect to distance to Saturn, impact speed and size have been studied and the contrast between the E-ring population and plume population examined. The major findings are: The `sodium-rich and potassium-poor' spectra (and hence icy particles) are the majority among both E-ring and plume salt-rich populations, while `potassium-rich and sodium-poor' spectra and `both sodium and potassium-rich' spectra are rare. Carbonate and chloride-containing spectra are mutually exclusive in most cases. The `sodium-rich and potassium-poor' spectra (and hence icy particles) are further classified into Subtypes A, B, H, C, D, E and L according to the presence and relative amplitudes of carbonate and chloride related peaks. Among them Subtypes A, B, C, D and L are the majorities in E-ring spectra. A part of ice particles producing Subtype A and Subtype L spectra at lower impact speed could produce Subtype B spectra at higher impact speed. While abundances of the major sodium subtypes are comparable in the E-ring salt-rich populations, the ultra carbonate-rich subtype, which is named as Subtype A, spectra are much more frequent than the spectra of other subtypes in the plume salt-rich population, which implies that the majority of the ultra carbonate rich (Subtype A) ice particles do not have enough energy to escape from the gravity of Enceladus and reach the E-ring or that the E-ring grains are older and rapidly lose their carbonate signature. The spatial distribution of the subtypes and relative peak amplitudes implies the carbonate and chloride compounds in the ice particles are lost slowly with time. The impact speed distributions of the subtypes and relative peak amplitudes for the spectra of E-ring salt-rich ice particles imply higher impact speeds tend to break up carbonate molecules and thus lower the amplitudes of carbonate related peaks. To quantify the grains' compositions, a Laser Induced Liquid Beam Ion Desorption (LILBID) technique was used to desorb and ionize a wide range of Enceladean ocean-like solutions containing dissolved salts. The resulting ions were then measured by a reflectron-type time of flight mass spectrometer. Since the laser desorption mechanism simulates the ice grain impact process occurring on the CDA target, spectra produced in the laboratory from a large range of well-characterized salt solutions can be used to determine the CDA-applicable spectral appearances of substances within the ice grains emitted from Enceladus’ ocean. Firstly, the composition parameter space, with NaOH, NaCl, and Na2CO3 as endmembers, for the four major subtypes of salt-rich ice particle were explored via spectra mimicking. After this, I reconstructed the mass spectra from chloride-rich and carbonate-rich Enceladus ocean analogues, into which trace concentrations of a wide range of geochemically-relevant salts, silica and organics were been dissolved, to evaluate their detection limits for CDA impact ionization mass spectra of the frozen ocean droplets. ocean droplets., Es wird angenommen, dass die salzhaltigen Eispartikel im E-Ring des Saturns von gefrorenen aerosolisierten Tröpfchen des salzigen unterirdischen Ozeanwassers von Enceladus stammen. Sie werden in einem Plume aus Gas und Eispartikeln durch Spalten in der südlichen Polarregion des Mondes in den Weltraum geschleudert. Aufgrund ihrer Geschwindigkeit können einige dieser eisigen Partikel Enceladus‘ Schwerkraft entkommen und werden zu E-Ring-Teilchen, die typischerweise ein relativ junges Alter (nicht mehr als 200 Jahre) haben. Daher wird angenommen, dass die salzreichen Teilchen die Zusammensetzung des unterirdischen Ozeans Wasser widerspiegeln. Die in-situ-Massenspektren von E-Ring- und Plume-Eispartikeln, die mit dem Cosmic Dust Analyzer (CDA), einem Einschlagsionisations-Massenspektrometer an Bord der Cassini-Raumsonde erstellt wurden, zeigen eine erhebliche Vielfalt in der Zusammensetzung der salzhaltigen Partikel. Das Verständnis der Zusammensetzungen der in den Eisteilchen (und damit im Ozean) gelösten Substanzen, einschließlich Salze, Siliziumdioxid und organische Komponenten, kann entscheidende Hinweise für geochemische Ozeanmodelle für Enceladus liefern. Diese Arbeit untersucht die Vielfältigkeit der in situ Massenspektren der salzreichen Eispartikel und versucht, die Variationen in der Zusammensetzung der Eispartikel mit Hilfe von Analogexperimenten im Labor zu quantifizieren. Die in situ Massenspektren und damit die salzreichen Eispartikel werden anhand ihrer Zusammensetzung in verschiedene Gruppen und Subtypen eingeteilt. Die Verteilung der wichtigsten Subtypen der E-Ring-Spektren/Partikel hinsichtlich deren Abstand zu Saturn, Einschlagsgeschwindigkeit und Größe wurden untersucht. Außerdem wurde der Unterschied zwischen der E-Ring- und der Plume-Population untersucht. Die wichtigsten Ergebnisse sind: Die „natriumreichen und kaliumarmen“ Spektren (und damit Eisteilchen) machen den größten Teil sowohl der E-Ring- als auch der Plume-Population aus, während sowohl die „kaliumreichen und natriumarmen“ Spektren als auch die „natriumreichen und kaliumreichen“ Spektren selten sind. Carbonat- und chloridhaltige Spektren schließen sich in den meisten Fällen gegenseitig aus. Die „natriumreichen und kaliumarmen“ Spektren (und damit Eisteilchen) werden basierend auf den relativen Amplituden von carbonat- und chloridbezogenen Peaks weiter in die Untertypen A, B, H, C, D, E und L unterteilt. Von diesen Untertypen sind A, B, C, D und L die häufigstens im E-Ring. Ein Teil der Eispartikel, die Subtyp A und Subtyp L Spektren bei niedrigen Einschlagsgeschwindigkeiten erzeugt, könnte bei höheren Einschlagsgeschwindigkeiten Spektren des Subtyps B erzeugen. Die Häufigkeiten der wichtigsten Natrium-Subtypen innerhalb der salzreichen E-Ring-Populationen sind vergleichbar. Die Spektren des ultrakarbonatreichen Subtyps A sind viel häufiger als die Spektren der anderen Subtypen in der salzreichen Population des Plumes zu finden, was bedeutet, dass die Energie der Mehrheit der ultrakarbonatreichen reichen (Subtyp A) Eispartikel nicht ausreicht, um der Schwerkraft von Enceladus zu entkommen, sodass sie den E-Ring erreichen könnten. Möglich ist auch, dass die E-Ring-Teilchen älter sind und rasch ihre karbonatsignatur verlieren. Die räumliche Verteilung der Subtypen und der relativen Peakamplituden legt nahe, dass die Karbonat- und Chlorid-Komponenten in den Eispartikeln mit der Zeit verloren gehen. Sowohl die Verteilung der Einschlagsgeschwindigkeiten der Subtypen als auch die der relativen Peakamplituden in den Spektren der salzreichen E-Ring Eispartikel implizieren, dass höhere Einschlagsgeschwindigkeiten tendenziell die Karbonatmoleküle aufbrechen und somit die Amplituden der karbonatbezogenen Peaks verringern. Um die Zusammensetzung der Eisteilchen zu quantifizieren, wurde eine laserinduzierte Flüssigstrahl-Desorptions (LILBID) Methode verwendet, um ein breites Spektrum von Enceladusozean-ähnlichen Lösungen zu desorbieren und zu ionisieren. Die enstandenen Ionen wurden mit einem Reflektron-Flugzeit-Massenspektrometer gemessen. Da der Laser-Desorptionsmechanismus den Einschlag der Eispartikel auf das CDA-Instrument simuliert, können im Labor erzeugte Massenspektren von einer großen Anzahl gut charakterisierter Salzlösungen verwendet werden, um deren spektrales Erscheinungsbild in CDA-Massenspektren zu bestimmen. Zunächst wurde der Parameterraum (NaOH, NaCl und Na2CO3 als Endglieder) für die vier bedeutendsten Untertypen von salzreichen Eispartikeln untersucht, indem die Massenspektren dieser Untertypen im Labor nachgestellt wurden. Danach rekonstruierte ich die Massenspektren von chloridreichen und karbonatreichen Enceladus-Ozean-Analoga, in denen Spurenkonzentrationen von einer Vielzahl von geochemisch relevanten Salzen, Siliziumdioxid und organischen Substanzen gelöst wurden, um deren Nachweisgrenzen für die CDA-Einschlagsionisationsmassenspektren der gefrorenen Ozeantröpfchen zu bestimmen.

NASA’s Cassini spacecraft spent 13 years exploring the Saturn system, including 23 targeted flybys of the small, geologically active moon Enceladus. These flybys provided a wealth of image data from Cassini’s Imaging Science Subsystem. To improve the usability of the Enceladus data set, we created a new, global photogrammetric control network for Enceladus that enabled compilation of a versatile cartographic package to support geologic mapping and other investigations. The network used 586 images in four image filters with a pixel scale generally between 50 and 500 m per pixel and a phase angle less than 120° and consisted of 10,362 tie points and 173,704 individual image measures, averaging nearly 17 measures per tie point. Least squares bundle adjustment resulted in a root-mean-square residual of 0.45 pixel, corresponding to root-mean-square ground point uncertainties of 66, 51, and 46 m in latitude, longitude, and radius, respectively. Using our geodetic control network, we created new global image mosaics, coregistered flyby mosaics to support geologic mapping, and updated pointing kernels for every image used in the solution. These products, including the updated pointing kernels, are available to the community through NASA’s Planetary Data System Imaging Annex. The bundle adjustment solution also yielded independently determined shape information, resulting in radii within the stated uncertainty of International Astronomical Union values. The challenges of the data set, and the technical methodology described here are applicable to bodies imaged during multiple flybys with variable viewing and illumination geometry, including other midsized satellites of Saturn, and the Europa Clipper mission.

第4回「あかり」国際会議 (2017年10月17-20日. 東京大学), 文京区, 東京, The 4th AKARI International Conference: The Cosmic Wheel and the Legacy of the AKARI archive: from galaxies and stars to planets and life (October 17-20, 2017. The University of Tokyo), Bunkyo-ku, Tokyo, Japan, Saturn's E ring was discovered in 1996 and it is associated with materials ejected out (geysers) from particular areas (Tiger Stripes) around the south pole of Enceladus. Our study about activities of the geysers and their influences on E ring was conducted by making use of Cassini data of the ejected plasma and dust. The suitable data were recorded by the Cassini detectors on three events, i.e. March, August, and October, 2008. Moreover, ephemeris data of Enceladus, Cassini, and Saturn are used to figure out orbital configurations across the events. Mean-motion resonance between Enceladus and Dione occurs in equatorial configuration, so this may play an important contribution on Tiger Stripes compared to those caused by other satellites. Analyzing of the plasma and dust data reveals no regular pattern covering Enceladus and the data vary caused by the geysers. Number of ejected dust and mass increases for the three events, while that of plasma decreases. Variation of the data indicates no regular material ejection., 形態: カラー図版あり, Physical characteristics: Original contains color illustrations, 資料番号: AA1730026049, レポート番号: JAXA-SP-17-009E

International audience; Titan was a mostly unknown world prior to the Cassini spacecraft’s arrival in July 2004. We review the major scientific advances made by Cassini’s Titan Radar Mapper (RADAR) during 13 years of Cassini’s exploration of Saturn and its moons. RADAR measurements revealed Titan’s surface geology, observed lakes and seas of mostly liquid methane in the polar regions, measured the depth of several lakes and seas, detected temporal changes on its surface, and provided key evidence that Titan contains an interior ocean. As a result of the Cassini mission, Titan has gone from an uncharted world to one that exhibits a variety of Earth-like geologic processes and surface-atmosphere interactions. Titan has also joined the ranks of “ocean worlds” along with Enceladus and Europa, which are prime targets for astrobiological research.

Studies of Saturn's magnetosphere with the Cassini mission have established the importance of Enceladus as the dominant mass source for Saturn's magnetosphere. It is well known that the ionosphere is an important mass source at Earth during periods of intense geomagnetic activity, but lesser attention has been dedicated to study the ionospheric mass source at Saturn. In this paper we describe a case study of data from Saturn's magnetotail, when Cassini was located at ≃ 2200 h Saturn local time at 36 R S from Saturn. During several entries into the magnetotail lobe, tailward flowing cold electrons and a cold ion beam were observed directly adjacent to the plasma sheet and extending deeper into the lobe. The electrons and ions appear to be dispersed, dropping to lower energies with time. The composition of both the plasma sheet and lobe ions show very low fluxes (sometimes zero within measurement error) of water group ions. The magnetic field has a swept‐forward configuration which is atypical for this region, and the total magnetic field strength is larger than expected at this distance from the planet. Ultraviolet auroral observations show a dawn brightening, and upstream heliospheric models suggest that the magnetosphere is being compressed by a region of high solar wind ram pressure. We interpret this event as the observation of ionospheric outflow in Saturn's magnetotail. We estimate a number flux between (2.95 ± 0.43) × 109 and (1.43 ± 0.21) × 1010 cm−2 s−1, 1 or about 2 orders of magnitude larger than suggested by steady state MHD models, with a mass source between 1.4 ×102 and 1.1 ×103 kg/s. After considering several configurations for the active atmospheric regions, we consider as most probable the main auroral oval, with associated mass source between 49.7 ±13.4 and 239.8 ±64.8 kg/s for an average auroral oval, and 10 ±4 and 49 ±23 kg/s for the specific auroral oval morphology found during this event. It is not clear how much of this mass is trapped within the magnetosphere and how much is lost to the solar wind., Key Points Tailward observation of ionospheric plasma in the magnetotail lobeEvidence for simultaneous reconnection in the plasma sheetFirst observational estimate at Saturn of mass source rate from the ionosphere to the magnetotail

Between 2004 and 2017, spectral observations have been gathered by the Visual and Infrared Mapping Spectrometer (VIMS) on-board Cassini (Brown et al., 2004) during 23 Enceladus close encounters, in addition to more distant surveys. The objective of the present study is to produce a global hyperspectral mosaic of the complete VIMS data set of Enceladus in order to highlight spectral variations among the different geological units. This requires the selection of the best observations in terms of spatial resolution and illumination conditions. We have carried out a detailed investigation of the photometric behavior at several key wavelengths (1.35, 1.5, 1.65, 1.8, 2.0, 2.25, 2.55 and 3.6 ${\mu}$m), characteristics of the infrared spectra of water ice. We propose a new photometric function, based on the model of Shkuratov et al. (2011). When combined, corrected mosaics at different wavelengths reveal heterogeneous areas, in particular in the terrains surrounding the Tiger Stripes on the South Pole and in the northern hemisphere around 30{\deg}N, 90{\deg}W. Those areas appear mainly correlated to tectonized units, indicating an endogenous origin, potentially driven by seafloor hotspots., Comment: Erratum: Fix south polar grid longitudes in Figures 9 and 11

This chapter presents a short overview of physical processes in a specific plasma region (a so-called plume) found by the Cassini orbiter in the vicinity of the icy moon Enceladus. In particular, we summarize highlights of scientific results which have been performed by German research groups in GFZ German Research Centre for Geosciences, Potsdam and Technical University of Braunschweig as part of the Special Priority Program “PlanetMag” of the Deutsche Forschungsgemeinschaft between 2013 and 2016. The highlights cover mostly those theoretical and numerical studies in which measurements of different Cassini instruments could be compared with theory or where the phenomenon has a diagnostic application. Dust charging, role of dust size distributions, and dust charge fluctuations are discussed. The findings are used for interpretations of the Cassini Plasma Spectrometer data and improve constraints on the dust characteristics. Then the main forces and dust dynamics are discussed in conditions relevant for the near-Enceladus environment. It is also examined how the charged dust can affect the plasma shielding length which is of importance for the reliable Cassini Langmuir probe measurements. Considering the dust grains as heavy negative ion species, the electric conductivity tensor is modified, whose elements are the key quantities for understanding the magnetic field perturbations registered by the Cassini Magnetometer during Enceladus flybys.

Alfvén wings are known to form when a conducting or mass-loading object slows down a flowing plasma in its vicinity. Alfvén wings are not expected to be generated when an inert moon such as Rhea interacts with Saturn's magnetosphere, where the plasma impacting the moon is absorbed and the magnetic flux passes unimpeded through the moon. However, in two close polar passes of Rhea, Cassini clearly observed magnetic field signatures consistent with Alfvén wings. In addition, observations from a high-inclination flyby (Distance > 100 RRh) of Rhea on 3 June 2010 showed that the Alfvén wings continue to propagate away from Rhea even at this large distance. We have performed three-dimensional hybrid simulations of Rhea's interaction with Saturn's magnetosphere which show that the wake refilling process generates a plasma density gradient directed in the direction of corotating plasma. The resulting plasma pressure gradient exerts a force directed toward Rhea and slows down the plasma streaming into the wake along field lines. As on the same field lines, outside of the wake, the plasma continues to move close to its full speed, this differential motion of plasma bends the magnetic flux tubes, generating Alfvén wings in the wake. The current system excited by the Alfvén wings transfers momentum to the wake plasma extracting it from plasma outside the wake. Our work demonstrates that Alfvén wings can be excited even when a moon does not possess a conducting exosphere.

Using a particle-in-cell electrostatic simulation, we examine the conditions that allow low-energy ions, like those produced in the Enceladus plume, to be attracted and trapped within the sheaths of negatively charged dust grains. The conventional wisdom is that all new ions produced in the Enceladus plume are free to get picked up (i.e., accelerated by the local E field to then undergo vB acceleration). However, we suggest herein that the presence of submicron-charged dust in the plume impedes this pickup process since the local grain electric field greatly exceeds the corotation E fields. The simulations demonstrate that cold ions will tend to accelerate toward the negatively charged grains and become part of the ion plasma sheath. These trapped ions will move with the grains, exiting the plume region at the dust speed. We suggest that Cassini's Langmuir probe is measuring the entire ion population (free and trapped ions), while the Cassini magnetometer detects the magnetic perturbations associated with pickup currents from the smaller population of free ions, with this distinction possibly reconciling the ongoing debate in the literature on the ion density in the plume.

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.

Encedalus' plume of charged dust has been shown to strongly modify the plasma structures. Therefore, we analyze the magnetic signatures of dust to constrain the dust plume. For the first time, the mutual feedback between the nanograins and their plasma environment is investigated. Our model combines plasma simulations by means of the A.I.K.E.F. (Adaptive Ion-Kinetic Electron-Fluid) code with Monte Carlo simulations of the 3-D profiles of the gas and dust plumes. Data from several instruments of Cassini are considered: the neutral plume model is in good agreement with INMS data, whereas theoretical predictions of the peak ion density are compared against CAPS data, and properties of the dust are obtained by comparing our results with MAG data from the recent E14–E19 flybys. Our main results are (1) due to the ion-neutral chemistry, H3O+ is the predominant ion species within the plume; (2) the high nanograin densities observed by CAPS require an increased effective ionization frequency to fulfill quasi-neutrality; (3) the nanograin pickup current makes only a minor contribution to the current systems; i.e., the major contribution of the dust arises from electron absorption; (4) the pickup of nanograins is clearly visible in the magnetic field signatures, even including the distant encounter E15; (5) MAG data indicate a southward extension of the charged dust plume of at least 4 Enceladus radii; (6) the modification of the currents by the nanograins is responsible for the surprising fact that Cassini did not detect a region with a reduced magnetic field strength.

The Cassini Imaging Science Subsystem (ISS) acquired many high-resolution images (

The Cassini Imaging Science Subsystem (ISS) acquired 377 high-resolution images ( http://ciclops.org/maps ).

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.

The surface of Enceladus consists almost completely of water ice [1,2]. The band depths of water ice absorptions are sensitive to the size of particles [2,3,4] covering the surface. The Visual and Infrared Mapping Spectrometer [5] observed Enceladus with high spatial resolution during multiple Cassini fly-bys. Based on these data we measured the band depths of water ice absorptions over Enceladus' surface and mapped their distribution. The spatial resolution of VIMS is sufficient to distinguish three major geologic units: heavily cratered terrain, fractured and ridged terrain and complex tectonically deformed regions of troughs and ridges [6], which include the south pole region. Surface ages, as derived from the impact flux models of [7,8] indicate the cratered terrain being oldest while the Sulci the youngest unit [2,9]. From the distribution of particle sizes we can conclude that the largest particle diameters are those inside the tectonically deformed regions, with a decrease in size departing from the fractures. These occur not only at the south pole but also in older tectonic regions [2]. The basic correlation between particle diameter, geologic unit and age suggests the following relative stratigraphic sequence [2,10]: (1) Formation of a primary crust (heavily cratered terrain); (2) Mechanical weathering of the surface particles by microimpacts and sputtering during the last 4 billion years; (3) Tectonic disruption of the surface and deposition of new material with large particles. Although this newly deposited material has undergone mechanical weathering of the particles by microimpacts and sputtering, these particles are larger due to a shorter exposure time; (4) Recent deposition of larger particles in the south polar region. If the larger particles in the tectonically deformed regions have of the same cryovolcanic origin as at the south pole, the volcanic activity must have a temporal evolution. However, there are still different possibilities to explain this observation [2]: (1) The eruption zones may have moved from north to south. (2) Cryovolcanic eruptions might have occurred across the entire surface and later shrunk to a small zone at the south pole, which would be indicative of a probable decrease in internal heat transfer. (3) The intensity of cryovolcanic eruptions had a different temporal evolution at different locations with maximum activity in the south polar region.

We observe geologically young cycloidal segments in different places on the surface of Saturn’s moon Enceladus. These features likely have formed as tension cracks with their form being con-trolled by diurnal variation of tides, as suggested for Europa. For this model to work the ice must be weak and, to allow for sufficient tidal amplitude, there must be a fluid layer below the icy surface. Thus, rather than being confined to just southern latitudes, our observations hint at the presence of fluid layers in also other areas on Enceladus, potentially at a global sub-suface ocean in recent times.

We study the interaction of Saturn's small, icy moon Enceladus and its plume with the corotating magnetospheric plasma by means of a 3D hybrid simulation model, which treats the ions as individual particles and the electrons as a massless, charge-neutralizing fluid. We analyze systematically how Enceladus’ internal conductivity and plasma absorption at the surface as well as charge exchange and pick-up in the plume contribute to the overall structure of the interaction region. Furthermore, we provide a comparison of our simulation results to data obtained by the Cassini magnetometer instrument. The major findings of this study are: (1) the magnetic field diffuses through the solid body of Enceladus almost unaffected, whereas plasma absorption gives rise to a symmetric depletion wake downstream of the moon; (2) due to the small gyroradii of the newly generated plume ions, the pick-up tail possesses a 2D structure; (3) the magnetic field lines drape around the plume, which triggers an Alfven wing system that dominates the structure of Enceladus’ plasma environment. Inside the plume itself, a magnetic cavity is formed; (4) besides the reproduction of the key features of the observed magnetic field signatures, evidence for variability in the locations of the active jets and in the total gas content of the plume are shown.

The sizes and shapes of six icy Saturnian satellites have been measured from Cassini Imaging Science Subsystem (ISS) data, employing limb coordinates and stereogram-metric control points. Mimas, Enceladus, Tethys, Dione and Rhea are well described by triaxial ellipsoids; Iapetus is best represented by an oblate spheroid. The ISS acquired many high-resolution images (

Enceladus is the most astonishing and remarkable moon that has been found in the Saturn system. In 2005, when the spacecraft Cassini discovered Enceladus geologically active South Pole, more studies and research started to be conducted. In the South Pole of this little icy moon, there are plumes that emanate water vapor, methane and other hydrocarbons into space. Scientists and researchers have suggested biotic or abiotic source for those species. A part of this thesis work is to study and make experiments to come to a conclusion about the source of these elements. Here, three suggested experimental simulations will be conducted, which are of thermogenic methane production, Fischer-Tropsch synthesis and the growth of methanogenic organisms in simulated Enceladus conditions. The data that were collected from the experiments would be compared to the data from the Cassini spacecraft. As a result, the comparison showed that the methane and other hydrocarbons in the Enceladus plume were more likely to be products of abiotic thermogenic. The second part of this project is to calculate the energy cost to send a spacecraft to Enceladus in order to collect some samples from the plume and bring them back to earth. The calculations showed that the mission time would be 12.2 years, when the minimum velocity change (DV) was used. Thus, the cost of DV has been calculated for the halved mission time, which was more than double the lowest. Validerat; 20101217 (root)

Cassini has identified a geologically active province at the south pole of Saturn's moon Enceladus. In images acquired by the Imaging Science Subsystem (ISS), this region is circumscribed by a chain of folded ridges and troughs at approximately 55 degrees S latitude. The terrain southward of this boundary is distinguished by its albedo and color contrasts, elevated temperatures, extreme geologic youth, and narrow tectonic rifts that exhibit coarse-grained ice and coincide with the hottest temperatures measured in the region. Jets of fine icy particles that supply Saturn's E ring emanate from this province, carried aloft by water vapor probably venting from subsurface reservoirs of liquid water. The shape of Enceladus suggests a possible intense heating epoch in the past by capture into a 1:4 secondary spin/orbit resonance.

Cassini’s Imaging Science Subsystem (ISS) coverage obtained to date shows that the Saturnianmoon Enceladus has evolved through a long and complex tectonic history[1]. The most recent episodes of major tectonic activity are identified by a peculiar hemispheric pattern of fractures and lineations that exhibit a remarkable symmetry relative to the satellite’s spin-axis. These tectonic patterns appear to be closely tied to a unique, geologically active region known as the South Polar Terrain (SPT) province, where ongoing eruptions of water plumes were recently discovered by Cassini[2] The detailed shapes and symmetry of the young tectonic patterns, detailed below, suggest that they formed in response to a global change in rotational figure. We investigate the extent to which the morphology, geographic placement, and orientations of these tectonic patterns are consistent with axial shortening of Enceladus’ spin axis, as suggested by Porco et al.[2].

Observations of Saturn's satellite Enceladus using Cassini's Visual and Infrared Mapping Spectrometer instrument were obtained during three flybys of Enceladus in 2005. Enceladus' surface is composed mostly of nearly pure water ice except near its south pole, where there are light organics, CO 2 , and amorphous and crystalline water ice, particularly in the region dubbed the “tiger stripes.” An upper limit of 5 precipitable nanometers is derived for CO in the atmospheric column above Enceladus, and 2% for NH 3 in global surface deposits. Upper limits of 140 kelvin (for a filled pixel) are derived for the temperatures in the tiger stripes.