Cassini Observes the Active South Pole of Enceladus Porco, C. C; Helfenstein, P; Thomas, P. C ...
Science (American Association for the Advancement of Science),
03/2006, Volume:
311, Issue:
5766
Journal Article
Peer reviewed
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 approximately55°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.
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Functionally graded steel/tungsten layers may be used as interlayers in the first wall of future fusion reactors to balance thermally-induced stress peaks in the steel‑tungsten joint. In this work, a ...modified water-stabilized atmospheric plasma spraying set-up is used to deposit uniform and functionally graded steel/tungsten coatings at elevated substrate temperatures. Uniform coatings were used to characterise individual sublayers of graded coatings in detail. The thermal expansion, thermal conductivity, Young's modulus and yield strength of the layers are promising for the application in steel‑tungsten joints and can be explained by microstructural observations. Only at a substrate preheating temperature of 900 °C the formation of intermetallic precipitates during deposition was observed.
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► Tempel 1 has two types of terrain, rough pitted and smoother, flow-like materials. ► The smooth materials are collected in low regions. ► Changes observed between the Deep Impact and Stardust-NExT ...encounters involve scarp retreat. ► Timescales for formation of the surface’s topography appear to be tens to perhaps hundreds of orbits.
The nucleus of comet Tempel 1 has been investigated at close range during two spacecraft missions separated by one comet orbit of the Sun, 5½years. The combined imaging covers ∼70% of the surface of this object which has a mean radius of 2.83±0.1km. The surface can be divided into two terrain types: rough, pitted terrain and smoother regions of varying local topography. The rough surface has round depressions from resolution limits (∼10m/pixel) up to ∼1km across, spanning forms from crisp steep-walled pits, to subtle albedo rings, to topographic rings, with all ranges of morphologic gradation. Three gravitationally low regions of the comet have smoother terrain, parts of which appear to be deposits from minimally modified flows, with other parts likely to be heavily eroded portions of multiple layer piles. Changes observed between the two missions are primarily due to backwasting of scarps bounding one of these probable flow deposits. This style of erosion is also suggested by remnant mesa forms in other areas of smoother terrain. The two distinct terrains suggest either an evolutionary change in processes, topographically-controlled processes, or a continuing interaction of erosion and deposition.
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Deep Impact images of the nucleus of Comet Tempel 1 reveal pervasive layering, possible impact craters, flows with smooth upper surfaces, and erosional stripping of material. There are at least 3 ...layers 50–200 m thick that appear to extend deep into the nucleus, and several layers 1–20 m thick that parallel the surface and are being eroded laterally. Circular depressions show geographical variation in their forms and suggest differences in erosion rates or style over scales >1 km. The stratigraphic arrangement of these features suggests that the comet experienced substantial periods of little erosion. Smooth surfaces trending downslope suggest some form of eruption of materials from this highly porous object. The Deep Impact images show that the nucleus of Tempel 1 cannot be modeled simply as either an onion-layer or rubble pile structure.
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► Stardust-NExT extended image coverage of Tempel 1 to 70% of the surface. ► One-third of the surface is covered by smooth, flow-like deposits. ► Irregular pits cover two-thirds of the surface. ► ...Surface changes were detected in isolated areas. ► A 50-m depression has been located at the DI impact site.
On February 14, 2011 Stardust-NExT (SN) flew by Comet Tempel 1, the target of the Deep Impact (DI) mission in 2005, obtaining dust measurements and high-resolution images of areas surrounding the 2005 impact site, and extending image coverage to almost two thirds of the nucleus surface. The nucleus has an average radius of 2.83±0.1km and a uniform geometric albedo of about 6% at visible wavelengths. Local elevation differences on the nucleus reach up to 830m. At the time of encounter the spin rate was 213° per day (period=40.6h) and the comet was producing some 130kg of dust per second. Some 30% of the nucleus is covered by smooth flow-like deposits and related materials, restricted to gravitational lows. This distribution is consistent with the view that the smooth areas represent material erupted from the subsurface and date from a time after the nucleus achieved its current shape. It is possible that some of these eruptions occurred after 1609 when the comet’s perihelion distance decreased from 3.5AU to the current 1.5AU. Much of the surface displays evidence of layering: some related to the smooth flows and some possibly dating back to the accretion of the nucleus. Pitted terrain covers approximately half the nucleus surface. The pits range up to 850m in diameter. Due to their large number, they are unlikely to be impact scars: rather they probably result from volatile outbursts and sublimational erosion. The DI impact site shows a subdued depression some 50m in diameter implying surface properties similar to those of dry, loose snow. It is possible that the 50-m depression is all that remains of an initially larger crater. In the region of overlapping DI and SN coverage most of the surface remained unchanged between 2005 and 2011 in albedo, photometric properties and morphology. Significant changes took place only along the edges of a prominent smooth flow estimated to be 10–15m thick, the margins of which receded in places by up to 50m. Coma and jet activity were lower in 2011 than in 2005. Most of the jets observed during the SN flyby can be traced back to an apparently eroding terraced scarp. The dust instruments detected bursts of impacts consistent with a process by which larger aggregates of material emitted from the nucleus subsequently fragment into smaller particles within the coma.
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•Small satellites in different dynamical positions have different surface morphologies.•Ring-arc embedded moons are likely equilibrium forms of mean density ∼300kgm−3.•Trojan moons may receive ...materials from larger satellites.
More than a dozen small (<150km mean radius) satellites occupy distinct dynamical positions extending from within Saturn’s classical rings to the orbit of Dione. The Cassini mission has gradually accumulated image and spectral coverage of these objects to the point where some generalizations on surface morphology may be made. Objects in different dynamical niches have different surface morphologies. Satellites within the main rings display equatorial ridges. The F-ring shepherding satellites show structural forms and heavily cratered surfaces. The co-orbitals Janus and Epimetheus are the most lunar-like of the small satellites. Satellites occupying libration zones (Trojan satellites) have deep covering of debris subject to downslope transport. Small satellites embedded in ring arcs are distinctively smooth ellipsoids that are unique among small, well-observed Solar System bodies and are probably relaxed, effectively fluid equilibrium shapes indicative of mean densities of about 300kgm−3.
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The sizes and shapes of six icy saturnian satellites have been measured from Cassini Imaging Science Subsystem (ISS) data, employing limb coordinates and stereogrammetric control points. Mimas, ...Enceladus, Tethys, Dione and Rhea are well described by triaxial ellipsoids; Iapetus is best represented by an oblate spheroid. All satellites appear to have approached relaxed, equilibrium shapes at some point in their evolution, but all support at least 300 m of global-wavelength topography. The shape of Enceladus is most consistent with a homogeneous interior. If Enceladus is differentiated, its shape and apparent relaxation require either lateral inhomogeneities in an icy mantle and/or an irregularly shaped core. Iapetus supports a fossil bulge of over 30 km, and provides a benchmark for impact modification of shapes after global relaxation. Satellites such as Mimas that have smoother limbs than Iapetus, and are expected to have higher impact rates, must have relaxed after the shape of Iapetus was frozen.
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We consider the hypothesis that the layering observed on the surface of Comet 9P/Tempel 1 from the Deep Impact spacecraft and identified on other comet nuclei imaged by spacecraft (i.e., 19P/Borrelly ...and 81P/Wild 2)
is ubiquitous on Jupiter family cometary nuclei and is an essential element of their internal structure. The observational characteristics of the layers on 9P/Tempel 1 are detailed and considered in the context of current theories of the accumulation and dynamical evolution of cometary nuclei. The works of Donn Donn, B.D., 1990. Astron. Astrophys. 235, 441–446, Sirono and Greenberg Sirono, S.-I., Greenberg, J.M., 2000. Icarus 145, 230–238 and the experiments of Wurm et al. Wurm, G., Paraskov, G., Krauss, O., 2005. Icarus 178, 253–263 on the collision physics of porous aggregate bodies are used as basis for a conceptual model of the formation of layers. Our hypothesis is found to have implications for the place of origin of the JFCs and their subsequent dynamical history. Models of fragmentation and rubble pile building in the Kuiper belt in a period of collisional activity (e.g., Kenyon, S.J., Luu, J.X., 1998. Astron. J. 115, 2136–2160; 1999a. Astron. J. 118, 1101–1119; 1999b. Astrophys. J. 526, 465–470; Farinella, P., Davis, D.R., Stern, S.A., 2000. In: Mannings, V., Boss, A.P., Russell, S.S. (Eds.), Protostars and Planets IV. Univ. of Arizona Press, Tucson, pp. 1255–1282; Durda, D.D., Stern, S.J., 2000. Icarus 145, 220–229) following the formation of Neptune appear to be in conflict with the observed properties of the layers and irreconcilable with the hypothesis. Long-term residence in the scattered disk Duncan, M.J., Levison, H.F., 1997. Science 276, 1670–1672; Duncan, M., Levison, H., Dones, L., 2004. In: Festou, M., Keller, H.U., Weaver, H.A. (Eds.), Comets II. Univ. of Arizona Press, Tucson, pp. 193–204 and/or a change in fragmentation outcome modeling may explain the long-term persistence of primordial layers. In any event, the existence of layers places constraints on the environment seen by the population of objects from which the Jupiter family comets originated. If correct, our hypothesis implies that the nuclei of Jupiter family comets are primordial remnants of the early agglomeration phase and that the physical structure of their interiors, except for the possible effects of compositional phase changes, is largely as it was when they were formed. We propose a new model for the interiors of Jupiter family cometary nuclei, called the
talps or “layered pile” model, in which the interior consists of a core overlain by a pile of randomly stacked layers. We discuss how several cometary characteristics—layers, surface texture, indications of flow, compositional inhomogeneity, low bulk density low strength, propensity to split, etc., might be explained in terms of this model. Finally, we make some observational predictions and suggest goals for future space observations of these objects.
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9.
The geology of 433 Eros Robinson, M. S.; Thomas, P. C.; Veverka, J. ...
Meteoritics & planetary science,
December 2002, Volume:
37, Issue:
12
Journal Article
Peer reviewed
Open access
— The global high‐resolution imaging of asteroid 433 Eros by the Near‐Earth Asteroid Rendezvous (NEAR) Shoemaker spacecraft has made it possible to develop the first comprehensive picture of the ...geology of a small S‐type asteroid. Eros displays a variety of surface features, and evidence of a substantial regolith. Large scale facets, grooves, and ridges indicate the presence of at least one global planar structure. Directional and superposition relations of smaller structural features suggest that fracturing has occurred throughout the object. As with other small objects, impact craters dominate the overall shape as well as the small‐scale topography of Eros. Depth/diameter ratios of craters on Eros average ∼0.13, but the freshest craters approach lunar values of ∼0.2. Ejecta block production from craters is highly variable; the majority of large blocks appear to have originated from one 7.6 km crater (Shoemaker). The interior morphology of craters does not reveal the influence of discrete mechanical boundaries at depth in the manner of craters formed on lunar mare regolith and on some parts of Phobos. This lack of mechanical boundaries, and the abundant evidence of regolith in nearly every high‐resolution image, suggests a gradation in the porosity and fracturing with depth. The density of small craters is deficient at sizes below ∼200 m relative to predicted slopes of empirical saturation. This characteristic, which is also found on parts of Phobos and lunar highland areas, probably results from the efficient obliteration of small craters on a body with significant topographic slopes and a thick regolith. Eros displays a variety of regolith features, such as debris aprons, fine‐grained “ponded” deposits, talus cones, and bright and dark streamers on steep slopes indicative of efficient downslope movement of regolith. These processes serve to mix materials in the upper loose fragmental portion of the asteroid (regolith). In the instance of “ponded” materials and crater wall deposits, there is evidence of processes that segregate finer materials into discrete deposits. The NEAR observations have shown us that surface processes on small asteroids can be very complex and result in a wide variety of morphologic features and landforms that today seem exotic. Future missions to comets and asteroids will surely reveal still as yet unseen processes as well as give context to those discovered by the NEAR Shoemaker spacecraft.
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The nature of ponded deposits on Eros Robinson, M. S; Thomas, P. C; Veverka, J ...
Nature (London),
09/2001, Volume:
413, Issue:
6854
Journal Article
Peer reviewed
One of the surprises of the NEAR-Shoemaker mission was that Eros's surface exhibits a wide variety of landforms, which are indicative of a global covering of loose fragmental debris. At one extreme ...in roughness is the Shoemaker Regio area, which is characterized by a high density of boulders up to 100 m across, slumps, slides, and finer blanketing material. At the other extreme are distinctive, flat deposits that appear smooth down to a resolution of 1.2 cm per pixel. Here we report the results of global mapping and colour analysis of these smooth deposits. They have formed most efficiently in restricted areas, and appear to be the result of deposition of finer material sorted from the upper portion of the asteroid's regolith. The smooth deposits constitute a family of features with a range of morphologies, but all appear to be the result of sedimentation. The geography of the deposits is consistent with some predicted aspects of photoelectric sorting, but these exotic transport and depositional mechanisms are not well understood. Deposits with the properties seen on Eros have no obvious analogues in previous lunar or asteroid data.
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