Bibliothek

Notizen: [Auszug] Spacecraft and ground-based observations of Venus have revealed a geologically young and active surfacewith volcanoes, rift zones, orogenic belts and evidence for hotspots and crustal spreadingyet the processes responsible for these features cannot be identified from the available data. ...

Notizen: [Auszug] Ground-based spectroscopy of Jupiter's moon Europa, combined with gravity data, suggests that the satellite has an icy crust roughly 150 km thick and a rocky interior. In addition, images obtained by the Voyager spacecraft revealed that Europa's surface is crossed by numerous ...

Notizen: [Auszug] Images obtained by the Voyager spacecraft revealed dark, wedge-shaped bands on Europa that were interpreted as evidence that surface plates, 50–100 km across, moved and rotated relative to each other. This implied that they may be mechanically decoupled from the interior by a ...

Notizen: Abstract The geologic/morphologic map of the northern mid-to-high latitudes of Venus prepared by a Soviet science team on the basis of Venera 15/16 mission radar image coverage is analyzed and used to define six discrete assemblages of geologic/morphologic units that have well-defined geographic distributions. These assemblages have distinctive and differing geological and tectonic expressions and include: Plains Assemblage - which is dominated by lowland smooth plains and lowland rolling plains interpreted to be of volcanic origin, and a high concentration of small volcanic domes; Plains-Corona Assemblage - which is dominated by lowland smooth plains and lowland rolling plains interpreted to be of volcanic origin, at least ten coronae structures concentrated in the northern half of the region, and at least five large volcanoes, generally concentrated in the southern and western half of the region; Plains-Ridge Belt Assemblage - which is dominated by lowland smooth plains and lesser amounts of lowland rolling plains, major occurrences of ridge belts in a distinctive fan-shaped pattern, and very minor and patchy occurrences of tessera; Plains-Corona-Tessera Assemblage - which is dominated by approximately equal amounts of lowland smooth plains and lowland rolling plains, at least five coronae concentrated in the northern part of the region, a small number of large volcanoes, also in the northern part of the region, and numerous small patches of tesserae scattered throughout, and the highest abundance of small volcanic domes observed in the northern hemisphere; Tessera-Ridge Belt Assemblage — which is dominated by a few large areas (Fortuna, Laima, Tellus) and several smaller areas (Dekla, Meni) of tesserae, ridge belts generally arrayed in an angular and often orthogonal pattern different from the fan-shaped pattern of the Plains-Ridge Belt Assemblage, lowland rolling plains and lesser amounts of lowland smooth plains, and an upland rise (Bell Regio); Tessera-Mountain Belt Assemblage - which is centered on the two volcanoes Colette and Sacajawea in Lakshmi Planum, and characterized by the peripheral mountain belt/tessera pairs, with the tessera on the outboard side: Danu/Clotho (S), Akna/Atropos (W), Freyja/ltzpapalotl (N), and Maxwell/Fortuna (E). The distribution and characteristics of assemblages demonstrate that vertical and horizontal tectonic forces are operating on the crust and lithosphere of Venus in different ways in specific localized areas. Alternative models are outlined for the origin of each assemblage and the relationship between assemblages, and important unresolved questions are identified. A key to the further understanding of these assemblages is the origin of ridge belts and tessera terrain.

Notizen: Abstract Cryptomaria are mare basalt deposits hidden or obscured by superposed higher albedo material or variations in albedo. They represent a record of the earliest mare volcanism, and may be a significant volumetric contribution to the volcanic and magmatic history of the Moon. In order to assess their global distribution and significance, criteria for the identification of cryptomaria are developed and techniques for locating them are described. These criteria and techniques include the presence of dark halo craters, identification by spectral mixing analysis, identification by geochemical evidence, association with light plains units, location within basin topography, proximity to known mare, relation to mascons indicated by gravity anomalies, and identification of the source of an obscuring agent, such as crater ejecta. On the basis of these criteria and techniques, several types of cryptomare are recognized, depending on the nature of ejecta and mare materials. Cryptomaria may be formed when maria are obscured by coverings of proximal or distal basin ejecta, or by crater ejecta dusting, or when ejecta covers over basalts which lack a distinctive 1µm absorption band. Using these concepts we outline three case studies: 1) the Schiller-Schickard region adjacent to the Orientale basin, classified as a basin-ejecta cryptomare and grading from distal to proximal, with possible crater-ejecta covering occurring in the southwestern portion of the region, 2) the Balmer basin, classified as a crater-ejecta-dusting cryptomare, and 3) the Australe basin, in which two types of cryptomare were identified: a) crater-ejecta-dusting on old mare patches and b) possible distal-basin-ejecta covering even older mare material. These case studies provide criteria for the further global identification and classification of cryptomaria and stress the need for utilization of multiple criteria and data sets.

Notizen: Abstract During the latter part of the last century, a profound change took place in our perception of the Earth. First, this change was holistic: Plate tectonic theory provided a unifying theme that seems to explain disparate observations about the Earth and how it works, and lets us see the Earth as a planet. Secondly, actually seeing the Earth from the Moon, and exploring the other planets provided additional perspectives on our own home planet and hastened the decline of scientific terracentrism. Thirdly, learning that the uniqueness of the Moon in terms of size and aspects of its chemistry may be due to its derivation from the Earth as the result of a giant impact, provided a concrete filial link. Finally, the geological record revealed by exploration of the Moon and planets has provided us with the missing chapters in the dynamic history of the Earth. We now know that gargantuan impact basins formed in Earth's formative years and that impact events are likely to be the cause of many punctuations in Earth's biological evolution. Perspectives on ancient tectonic activity are provided by Mercury, Venus, Mars, and the Moon, and show that the Earth has changed considerably since its youth. Widely varying volcanic eruption styles are seen on the planets, providing insight into how puzzling rocks from early Earth history formed. The composition of planetary atmospheres has revealed the unusual nature of Earth's, and its link to the evolution of life. The atmospheres of the planets have undergone radical changes with time, providing clues to Earth's history and destiny. Fundamentally different hydrological cycles on Earth, Venus, Europa and Mars, and evidence for significant changes with time, have provided insight into Earth's history. The probable presence of oceans on Europa and Mars has changed our thinking about the origin and evolution of life on Earth. We no longer think of the Earth in isolation. Instead, Earth is now perceived of as a member of a family of planets, each of which provides important missing information and perspective on the other, and together reveal the fabric of the history of the Solar System. Future exploration and perspectives will place our Solar System in the context of all of the others.

Notizen: Abstract The Solid State Imaging (SSI) experiment on the Galileo Orbiter spacecraft utilizes a high-resolution (1500 mm focal length) television camera with an 800 × 800 pixel virtual-phase, charge-coupled detector. It is designed to return images of Jupiter and its satellites that are characterized by a combination of sensitivity levels, spatial resolution, geometric fiedelity, and spectral range unmatched by imaging data obtained previously. The spectral range extends from approximately 375 to 1100 nm and only in the near ultra-violet region (∼ 350 nm) is the spectral coverage reduced from previous missions. The camera is approximately 100 times more sensitive than those used in the Voyager mission, and, because of the nature of the satellite encounters, will produce images with approximately 100 times the ground resolution (i.e., ∼ 50 m lp-1) on the Galilean satellites. We describe aspects of the detector including its sensitivity to energetic particle radiation and how the requirements for a large full-well capacity and long-term stability in operating voltages led to the choice of the virtual phase chip. The F/8.5 camera system can reach point sources of V(mag) ∼ 11 with S/N ∼ 10 and extended sources with surface brightness as low as 20 kR in its highest gain state and longest exposure mode. We describe the performance of the system as determined by ground calibration and the improvements that have been made to the telescope (same basic catadioptric design that was used in Mariner 10 and the Voyager high-resolution cameras) to reduce the scattered light reaching the detector. The images are linearly digitized 8-bits deep and, after flat-fielding, are cosmetically clean. Information ‘preserving’ and ‘non-preserving’ on-board data compression capabilities are outlined. A special “summation” mode, designed for use deep in the Jovian radiation belts, near Io, is also described. The detector is ‘preflashed’ before each exposure to ensure the photometric linearity. The dynamic range is spread over 3 gain states and an exposure range from 4.17 ms to 51.2 s. A low-level of radial, third-order, geometric distortion has been measured in the raw images that is entirely due to the optical design. The distortion is of the pincushion type and amounts to about 1.2 pixels in the corners of the images. It is expected to be very stable. We discuss the measurement objectives of the SSI experiment in the Jupiter system and emphasize their relationships to those of other experiments in the Galileo project. We outline objectives for Jupiter atmospheric science, noting the relationship of SSI data to that to be returned by experiments on the atmospheric entry Probe. We also outline SSI objectives for satellite surfaces, ring structure, and ‘darkside’ (e.g., aurorae, lightning, etc.) experiments. Proposed cruise measurement objectives that relate to encounters at Venus, Moon, Earth, Gaspra, and, possibly, Ida are also briefly outlined. The article concludes with a description of a ‘fully distributed’ data analysis system (HIIPS) that SSI team members intend to use at their home institutions. We also list the nature of systematic data products that will become available to the scientific community. Finally, we append a short ‘historical’ note outlining the responsibilities and roles of institutions and individuals that have been involved in the 14 year development of the SSI experiment so far.

Notizen: Abstract Lakshmi Planum is distinctive and unique on the surface of Venus as an expansive (~2 × 106km2), relatively smooth, flat plateau containing two large shield volcanoes and abundant volcanic plains in the midst of a region of extreme relief. It rises 3–5 km above the datum and is surrounded on all sides by bands of mountains interpreted to be of compressional tectonic origin. The major units mapped on Lakshmi are volcanic edifices, smooth, ridged and grooved plains units, and structural units referred to as ridged terrain. Three styles of volcanism are observed to dominate the surface of Lakshmi. Distributed effusive volcanism is associated with extensive plains deposits and many of the small shields, domes and cones mapped within the plateau. Centralized effusive volcanism is primarily associated with the paterae, Colette and Sacajawea, and their circumferential low-shield-forming deposits. The precise origin and evolution of these unusually large and complex structures is not understood, although a catastrophic, explosive origin is unlikely. Pyroclastic volcanism may be represented by a unit referred to as the “diffuse halo”. The origin and evolution of Lakshmi Planum is closely related to its compressional tectonic environment; volcanism on Lakshmi has occurred synchronously with tectonism in the surrounding orogenic belts. A model for the origin and evolution of Lakshmi Planum consisting of a continuous sequence of convergence and horizontal shortening of crustal segments against a preexisting block of tessera seems best able to account for the elevation, plateau shape and irregular polygonal outline of Lakshmi, as well as the presence of ridged terrain and its resemblance to tessera. Volcanism on Lakshmi is proposed to be the result of basal melting of a thickened crustal root. According to this model, the origin and evolution of Lakshmi Planum has consisted of the following sequence of events: (1) formation of a large, elevated block of tessera surrounded by low-lying plains; (2) convergence and underthrusting of crustal segments to produce peripheral mountain ranges, thickening, and uplift of the plateau; and (3) basal melting of the thickened crust and underthrust material and surface volcanism that occurred synchronously with continued edge deformation.

Notizen: Abstract Ridge belts, composed of closely spaced individual ridges 5–20 km wide, form sinuous patterns 30–400 km wide and 200–2000 km long in the plains of northern Venus. They are not homogeneously distributed, but occur primarily in two regions: between 0 ° E and 90 ° E ridge belts are associated with large blocks of tessera, and have a cumulative length of about 13,200 km; and between 150 ° E and 250 ° E, the ridge belts form a fan-shaped pattern and have a total cumulative length of about 25,800 km. Most ridge belts trend within 10 ° of N-S. Five morphologic components exist within the ridge belts: (1) broad ridges, which have no sharp crest and usually occur individually in the plains: (2) discontinuous ridges, with short ridge segments less than 20 km long; (3) paired ridges, with closely spaced ridges (less than 10 km apart) that never merge; (4) parallel ridges, with widely spaced (10–50 km), less prominent ridges; and (5) anastomosing ridges, in which ridges splay at angles up to 30 °. Subtle cross-strike lineaments cut the ridge belts at angles of 30–90 ° to the ridge belt, and augen-shaped plains are often present in anastomosing ridges. We examine the relationships between the components, plains, cross-strike lineaments, and augen-shaped plains in five ridge belts. Broad arches similar to the arches associated with wrinkle ridges on the Moon, Mars and Mercury appear in all of the ridge belts examined. Through studying each of these components individually and in the context of five specific ridge belts, we conclude that these ridge belts formed by compressional forces. The ridge belts form a continuum of deformation, from the simple broad arches (Nephele Dorsa), representing small amounts of shortening, through asymmetric ridge belts in the plains (Pandrosa Dorsa) and adjacent to tessera (Kamari Dorsa), to ridge belts in troughs representing underthrusting (Ausra and Lukelong Dorsa). Underthrusting is also observed along the borders of Lakshmi Planum, associated with Freyja and Danu Montes. The interpreted compressional origins for the ridge belt components suggests that many of the other ridge belts are of compressional origin, although complex origins (involving a combination of extension, shear, and/or compression) for some ridge belts cannot be ruled out. Global high resolution data from the Magellan mission will permit global mapping of the characteristics and distribution of ridge belts and allow further tests for their origin and evolution.

Notizen: Abstract Magellan radar image data of Sapas Mons, a 600 km diameter volcano located on the flanks of the Arla Rise, permit the distinction of widespread volcanic units on the basis of radar properties, morphology, and spatial and inferred temporal relations, each representing a stage or phase in the evolution of the volcano. Six flow units were identified and are arranged asymmetrically about the volcano. Although there is some evidence for overlapping of units, the stratigraphy clearly indicates a younging upwards sequence. The estimated volume of this 2.4 km high volcano is 3.1 × 104 km3, which is comparable to the largest Hawaiian shield (Mauna Loa, 4.25 × 104 km3), but it is significantly less than an estimated volume for the entire Hawaiian-Emperor chain (1.08 × 106 km3) and less than the lower diameter (100 × 150 km) island of Hawaii (11.3 × 104 km3). Although it is difficult to clearly identify a single lava flow, estimates of apparent single flow volumes range from 4 km3 (for an average unit 5 flow of 3.4 km width, 10 m thickness, and 121 km length) to almost 59 km3 (for a 17.8 km wide, l0 m thick, 330 km long unit 1 flow). Estimates of total volumes for the units show that four of the six flow units have volumes that are within a factor of 1.2 of each other, one unit is approximately three times more voluminous, and the latest unit has a very small volume. Flows within a given unit are very distinct relative to flows in other units with respect to average lengths, aspect ratio, radar brightness, and planimetric outline. There is a weak distinction in rms slope between units and emissivity is correlated with altitude, not unit boundaries. A pair of 25 km diameter scalloped-margin domes occur at the summit and are the source of the last stage of eruptions on Sapas; steep fronts and high aspect ratios suggest that associated flows may have had a high viscosity. Graben form a circumferential structure 75–100 km in diameter surrounding the summit domes and are interpreted to be indicative of subsidence over a central magma reservoir. Radial fractures with associated small edifices cut the lower flanks of the edifice but are not observed within the summit ring of graben; these are interpreted to be the expression of near-surface dykes and may have been emplaced during a period of enhanced activity that correlates with the most voluminous flow unit. Unlike at Hawaii, however, these dykes and small edifices do not seem to be the source of significant flank eruptions. Although some effusive activity may have accompanied their emplacement, the majority of lava flows at Sapas appear to be radial to a single, near-summit point located between the two summit domes. Calculated effusion rates range from 1.5 × 103 m3/s to 3.1 × 105 m3/s; these values suggest that rates were high compared with the Earth and decreased with time. These rates, and the volumes calculated, give eruption durations for the various units that range from 18 days to over 20 years. If eruption is caused by the influx of magma from depth and rupture of an overpressurized chamber, this suggests a variable flux over the history of the volcano. The late-stage eruptions which formed the summit domes are interpreted to be the result of fractional crystallization and/or volatile build-up in the chamber, following a period of decreased supply from depth. Local topography and gravity, as well as regional geology support the presence of a mantle plume at Sapas. The similar properties of large volumes of magma over the total history of the volcano, as well as the prolonged period of magma supply and gradual waning, are consistent with a plume origin. These inferences and the observations allow us to characterise the history of the volcano as follows: arrival of the mantle plume caused uplift of topography and surrounding plains formation: continued supply of smaller volumes of material permitted construction of the edifice; development of a magma reservoir (predicted by theory to form at shallow depths) modified eruption characteristics by permitting storage and homogenization of magma; unbuffered conditions prevailed for the majority of eruptions, producing flows of similar volumes but decreasing flow lengths; a period early on of enhanced supply led to buffered chamber conditions, resulting in the eruption of the voluminous flow unit and the emplacement of many lateral dykes; evacuations from the chamber and cooling towards the last stages caused distributed summit collapse and formation of the ring graben; and finally the gradual waning of supply allowed evolution of the magma which produced the late-stage, possibly viscous flows and dome construction. Preliminary observation of Sapas and two other volcanoes at different elevations suggests that altitude-dependent chamber development and growth may influence the complexity of lava flows and determine the existence of collapse calderas. Many features at Sapas are representative of large volcanoes on Venus and thus Sapas Mons is a good example of a typical plume-associated edifice. Sapas differs in many ways from Kilauea, a terrestrial type shield volcano, but these differences can be understood in the context of the Venus environment.