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At least 37 records · Page 2
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NASA NTRS1991

Terrestrial glacial eskers: Analogs for Martian sinuous ridges

A glacial model was introduced last year for the Argyre region, a concept which is now extended, and which was recently integrated with a Global Hydrologic Model incorporating many other aspects of Martian geology. Despite wide agreement that the Martian ridges strongly resemble glacial eskers, this hypothesis has been presented with great equivocation due to a perceived lack of other glacial landforms. Quite to the contrary, it is shown that the Martian ridges actually do occur in logical ordered sequences with many other types of characteristically glacial appearing landforms. Herein, the esker hypothesis is further supported in isolation from considerations of regional landform assemblages. It is concluded that Martian sinuous ridges are similar in every respect to terrestrial eskers: scale, morphology, planimetric pattern, and associations with other probable glaciogenic landforms. It is found that the esker hypothesis is well supported. Eskers are glaciofluvial structures, and owe their existence to large scale melting of stagnant temporate glaciers. Thus, eskers are indicators of an ameliorating climatic regime after a protracted episode of cold, humid conditions.

Kargel, Jeffrey S.↗
NASA NTRS1992

Dorsa Argentea type sinuous ridges, Mars: Evidence for linear dune hypothesis

Unusual ridges with sinuous, dendritic, and braided morphologies near the south pole of Mars as well as in the Argyre basin have recently begun to receive attention. These ridges were originally thought to be glacially formed eskers, and recent work continues to argue for this hypothesis. Indeed, an entire glacial model was built around these ridges. A brief examination of possible ridge forming processes included an esker origin but also examined other mechanisms. Of the alternative hypotheses, an origin as linear dunes is the most reasonable. The current study focuses on evidence for this mode of origin.

Ruff, Steven W.↗
NASA NTRS2012

Origin of Sinuous Channels on the SW Apron of Ascraeus Mons and the Surrounding Plains, Mars

Ascraeus Mons is one of three large shield volcanoes located along a NE-SW trending lineament atop the Tharsis Bulge on Mars. Spacecraft images, beginning with Viking in the 1970 s, revealed that the SW rift apron of Ascraeus Mons is cut by numerous sinuous channels, many of which originate from large, elongated, bowl shaped amphitheaters known as the Ascraeus Chasmata. A number of these channels can be traced onto the flatter plains to the east of the rift apron. These features have been interpreted as either fluvial [1] or volcanic [2] in origin. Most recently, it has been shown that one of the longest channels on the Ascraeus rift apron appears to transition into a roofed-over lava channel or lava tube at its distal end, and thus the entire feature is likely of a volcanic origin [2]. In addition, field observations of recent lava flows on Hawaii have shown that lava is capable of producing features such as the complex braided and anastomosing channels and streamlined islands that are observed in the Ascraeus features [2].

Schierl, Z.↗
NASA NTRS2012

A Comparison and Analog-Based Analysis of Sinuous Channels on the Rift Aprons of Ascraeus Mons and Pavonis Mons Volcanoes, Mars

The origin of sinuous channels on the flanks of the Tharsis volcanoes on Mars is debated among planetary scientists. Some argue a volcanic genesis [1] while others have suggested a fluvial basis [2-4]. The majority of the studies thus far have focused on channels on the rift apron of Ascraeus Mons. Here, however, we broadly examine the channels on the rift apron of Pavonis Mons and compare them with those studied channels around Ascraeus. We compare the morphologies of features from both of these volcanoes with similar features of known volcanic origin on the island of Hawai i. We show that the morphologies between these two volcanoes in the Tharsis province are very similar and were likely formed by comparable processes, as previous authors have suggested [5]. We show that, although the morphologies of many of the channels around these volcanoes show some parallels to terrestrial fluvial systems, these morphologies can also be formed by volcanic processes. The context of these features suggests that volcanic processes were the more likely cause of these channels.

Collins, A.↗
NASA NTRSDate not supplied

Warm-Based Glacial Landforms and Past Climate Signals: Icelandic Eskers as Proxies for Small Sinuous Ridges on Mars

The surface of Mars exhibits plentiful geomorphic evidence for glacial processes, but the history of the cryosphere on Mars remains debated [e.g., 1, 2]. Recent hydrological modeling predicts that warm-based ice sheets on Mars may have resulted in subglacial incised channels (e.g., valley networks) and eskers [3]. Evidence on Mars for warm-based glaciation includes candidate moraines, bedrock channels, and candidate eskers [e.g., 4-6]. Eskers are sinuous ridges comprising glaciofluvial sediments deposited by meltwater flowing through tunnels at the bed of warm-based large glaciers [e.g., 7, 8]. They provide direct evidence of basal melting of ice and preserve glaciofluvial sediments. Warm-based glacial transport is primarily driven by meltwater and can chemically weather the underlying bedrock. An alternative signature of past climate on Mars may thus be the geochemical record, due to alteration by interactions with glacial meltwater, preserved in esker sediments and strata. At present, the sedimentology and stratigraphy of terrestrial eskers is poorly understood. This study will combine sedimentology, geochemistry, and geomorphology to better constrain the unique properties of eskers resulting from warm-based glaciation on Mars-like substrates by leveraging a natural laboratory in Iceland.

A. M. Rutledge↗
NASA NTRS2012

Origins of Sinuous and Braided Channels on Ascraeus Mons, Mars - A Keck Geology Consortium Undergraduate Research Project

Water has clearly played an important part in the geological evolution of Mars. There are many features on Mars that were almost certainly formed by fluvial processes -- for example, the channels Kasei Valles and Ares Vallis in the Chryse Planitia area of Mars are almost certainly fluvial features. On the other hand, there are many channel features that are much more difficult to interpret -- and have been variously attributed to volcanic and fluvial processes. Clearly unraveling the details of the role of water on Mars is extremely important, especially in the context of the search of extinct or extant life. In this project we built on our recent work in determining the origin of one channel on the southwest rift apron of Ascraeus Mons. This project, funded by the Keck Geology Consortium and involving 4 undergraduate geology majors took advantage of the recently available datasets to map and analyze similar features on Ascraeus Mons and some other areas of Mars. A clearer understanding of how these particular channel features formed might lead to the development of better criteria to distinguish how other Martian channel features formed. Ultimately this might provide us with a better understanding of the role of volcanic and fluvial processes in the geological evolution of Mars.

de Wet, A. P.↗
NASA NTRS2000

Channels and Valleys

More than 200 channels and valleys have been identified on the Magellan images of Venus. These are classified, on the basis of morphology. as simple channels (including sinuous rilles, simple channels with flow margins. and canali), complex channels (with or without flow margins), compound channels, and valley networks (including rectangular, labyrinthic, and pitted or irregular networks). Sinuous rilles closely resemble their lunar counterparts. Canali are exceptional for their remarkably constant width along very extenuated flow paths, exceeding 500 km. One of the compound channels, the outflow complex of Kallistos Vallis. extends over 1200 km and is up to 30 km wide. Venusian channels are globally distributed, but each class has a preferential topographic association. The canali are developed on the volcanic plains. while sinuous rilles occur at higher elevations, associated with volcanic complexes and coronae. Both canali and sinuous rilles have been deformed by post-emplacement tectonism. Highly fluid lavas, erupted at sustained, high discharges seem best to explain many of the channel features, particularly for the canali and the compound channels. Explanation of the canali morphologies may also involve unusual low-viscosity lavas, perhaps of exotic composition.

Baker, Victor R.↗
NASA NTRS1978

Regional deformation of mare surfaces

The possible tectonic significance of large topographic irregularities within Mare Serenitatis is discussed, and the characteristics of sinuous rilles are considered as possible indicators of structural deformation. It is shown that most sinuous rilles have been structurally warped and that their slopes lie within the laminar flow range. The sinuous rilles may have been formed in the domain of turbulent flow, if the ratios of meander wavelength to channel width can be taken as indicators. Some surface tectonic deformation occurring before the broad uplift of the basin center is suggested by the surface topography and radar reflectors in the Mare Serenitatis.

Scott, D. H.↗
NASA NTRS1987

Preliminary results of geologic and remote sensing studies of Rima Mozart

In order to better understand the processes responsible for the formation of lunar sinuous rilles, a study of Rima Mozart was conducted using a variety of geologic, photographic, and remote sensing data. The apparent source of this rille is located in a highlands unit of known composition and it is hypothesized that thermal and mechanical erosion played an important role in the formation of Rima Mozart. Excellent photographic, topographic, multispectral, and radar data exist for this rille. The preliminary results of an analysis of this data are presented. Photographic data indicates the presence of two volcanic source vents for Rima Mozart: Kathleen and Ann. It is suggested that Rima Mozart, like many other lunar sinuous rilles, was most likely formed by a combination of events. Rima Mozart does follow a pre-existing, dominant NW/SE structural trend suggesting the influence of structural features on the rille, however, the tectonic influence is not the sole source for the formation of the rille, as suggested by the presence of the two source vents and the spatter around Ann. It is suggested that the rille formation began with an explosive eruption at Kathleen which later calmed down to a pulsating, high volume, low-viscosity lava flow. The rapid effusion rate of the magma as well as its high temperature and turbid nature helped carve the sinuous rille into the fractured and structurally weak Apennine Bench Formation underneath. Similar eruptions and subsequent flows were also created at Ann and joined to the main channel by a NE-trending secondary rille.

Coombs, C. R.↗
NASA NTRS1990

On the inviscid acoustic-mode instability of supersonic shear flows. I - Two-dimensional waves

The same previously utilized methods to study acoustic-mode instability in supersonic boundary layers are applied to free shear layers, and new calculations are derived for boundary layers with cooling and suction. The linear inviscid stability theory is employed to calculate spatial amplification rates at Mach 3 for the sinuous and varicose modes of a single wake flow and a single jet flow, each made up of the same mixing-layer profile plus a central region of uniform flow. It is shown that along with sequences of sinuous and varicose unstable modes clearly identifiable as acoustic modes, both of these flows, unlike the boundary layer, have a lowest sinuous mode that is the most unstable.

Mack, Leslie M.↗
NASA NTRS1993

Geomorphic processes in the Argyre-Dorsa Argentea region of Mars

Among many indications of possible ancient Martian galciation are sinuous eskerlike ridges in southern Argyre Planitia and the Dorsa Argentea region. But, in photogeology, other interpretations are always possible, and what appears eskerlike to one set of eyes may appear quite different to another. Interpretations of these ridges are about as numerous as observers, who collectively have suggested nine distinct hypotheses. Martian sinuous ridges have been interpreted as wrinkle ridges, lava flows, igneous dikes, clastic dikes, linear sand dunes, spits or bars, examples of inverted stream topography, or glacial crevasse fill. With Mars Observer en route to Mars, the prospects for a narrowing of the debate are bright. The esker hypothesis will gain support if Mars Observer images show that the ridges contain boulders, that the ridges are layered and contain channel structures, that the ridges are modified by thermokarst, or that the ridges occur in close, logical associations with other glacial landforms such as flutes, oriented grooves, and moraines. In the meantime, the evidence presented below bolsters the esker hypothesis, challenges certain alternative ideas, and draws a tentative geomorphic connection between the sinuous ridges of Argyre and those of Dorsa Argentea.

Kargel, J. S.↗
NASA NTRS1972

Are lunar rilles inverted eskers.

A proposed mechanism for the formation of lunar rilles argues that escaping volatiles (CO2 and H2O) could produce a sinuous channel between an impermeable basement surface and a developing surficial permafrost layer. Concurrent with the development of the subsurface channel would be the production of a large amount of fragmented ice mixed with rock debris at the surface above and along the sides of the sinuous subsurface channel. Eventual sublimation of the ice in the surficial material down to and perhaps including the subsurface would develop a feature at the surface that would be morphologically very similar to a valley produced by running water.

Helsley, C. E.↗
NASA NTRS1974

On the development of noise-producing large-scale wavelike eddies in a turbulent jet

The development of large-scale wavelike eddies in a two-dimensional turbulent jet was studied. The basic mean flow develops from a mixing region type with an initial specified boundary layer thickness into a fully developed jet. The role of the varicose and sinuous modes as these develop in a growing mean flow is brought out. In general, it was found, for a given frequency parameter, the varicose mode has a shorter streamwise lifetime than the sinuous mode. The latter, for lower frequency ranges, persists past the end of the potential core only to be subject to dissolution by the more enhanced fine scale turbulent activity in that region.

Merkine, L. O.↗
NASA NTRS1975

On the development of noise-producing large-scale wavelike eddies in a plane turbulent jet

The paper studies the development of large-scale wavelike eddies in a two-dimensional turbulent jet, extending earlier work on the mixing region (Liu, 1974). The basic mean flow developes from one of mixing-region type with an initially specified boundary-layer thickness into a fully developed jet. This study brings out the role of the varicose and sinuous modes as they develop in a growing mean flow. In general, it is found that, for a given frequency parameter, the varicose mode has a shorter streamwise lifetime than the sinuous mode. For lower frequencies, the latter persists past the end of the potential core only to become subject to dissipation by the enhanced fine-scale turbulent activity in that region.

Merkine, L.-O.↗
NASA NTRS1975

Shoreline configuration and shoreline dynamics: A mesoscale analysis

The author has identified the following significant results. Atlantic coast barrier island shorelines are seldom straight, but rather sinuous. These shoreline curvatures range in size from cusps to capes. Significant relationships exist between the orientation of shoreline segments within the larger of these sinuous features and shoreline dynamics, with coefficients ranging up to .9. Orientation of the shoreline segments of Assateague Island (60 km) and the Outer Banks of North Carolina (130 km) was measured from LANDSAT 2 imagery (1:80,000) and high altitude aerial photography (1:120,000). Long term trends in shoreline dynamics were established by mapping shoreline and storm-surge penetration changes.

Dolan, R.↗
NASA NTRS1977

The Procellarum volcanic complexes - Contrasting styles of volcanism

Three major volcanic complexes have long been recognized in the Oceanus Procellarum region. Detailed study shows that the complexes share some characteristics and also display major differences which provide clues to eruption style. The Rumker Hills occupy 5000 sq km in northern Procellarum and are apparently Imbrian-Eratosthenian in age; they are dominated by domes, suggesting relatively low effusion rates. The Aristarchus Plateau-Prinz/Harbinger region occupies 40,000 sq km in central Procellarum and is predominantly Imbrian in age; it is dominated by large sinuous rilles and associated dark mantling deposits of probable pyroclastic origin, suggesting relatively high eruption rates. The Marius Hills occupy 35,000 sq km in south-central Procellarum and appear to be predominantly Eratosthenian in age; they are dominated by low domes, steep domes, cones, and sinuous rilles, suggesting variable eruption rates and possible different volatile contents associated with eruption conditions that produced each type of feature. The volcanic complexes, particularly Aristarchus and Marius, appear to be the sources for much of the central Procellarum mare fill.

Whitford-Stark, J. L.↗
NASA NTRS1984

Volcanism in Elysium Planitia, Mars

Geomorphic mapping revealed that the three volcanic constructs within Elysium Planitia (Hecates Tholus, elysium Mons and Albor Tholus) are very different in their overall morphology and represent three distinct types of martian volcano. Hecates Tholus was found to possess the most likely possible example of a young, explosively generated, air fall deposit, while the volume of magma erupted from Elysium Mons appears to have been orders of magnitude larger than that erupted from Albor Tholus. A primary aim of the regional geological analysis of Elysium Planitia is to further understand the volcanic and tectonic evolution of the area by the identification and interpretation of individual lava flows and their source vents. Lava flow size, spatial distribution, flow direction and the stratigraphic relationships of these lava flows to adjacent structural features were all measured. The topographic form of Elysium Mons has totally controlled the flow direction of lava flows within Elysium Planitia. Lava flows from Elysium Mons can be traced for distances of 150 to 250 km in a radial direction from the volcano. Parasitic vents located beyond the recognizable volcanic construct also conform to this radial pattern. A second unusual characteristic of the Elysium Planitia region is the high frequency of occurrence of sinuous channels that are morphologically similar to lunar sinuous rilles.

Mouginis-Mark, P. J.↗
NASA NTRS1984

Effects of Non-equilibrium Solidification on the Material Properties of Brick Silicon for Photovoltaics

Silicon ingot growth technologies like the Ubiquitous Crystallization Process (UCP) are solidified within a shaping crucible. The rate at which heat can be lost from this crucible minus the rate at which heat is input from an external source determines the rate at which crystallization will occur. Occasionally, when the process parameters for solidification are exceeded, the normally large multi-centimeter grain size material assocated with the UCP will break down into regions containing extremely small, millimeter or less, grain size material. Accompanying this breakdown in grain growth is the development of so called sinuous grain boundaries. The breakdown in grain growth which results in this type of small grain structure with sinuous boundaries is usually associated with the rapid crystallization that would accompany a system failure. This suggests that there are limits to the growth velocity that one can obtain and still expect to produce material that would possess good photovoltaic properties. It is the purpose to determine the causes behind the breakdown of this material and what parameters will determine the best rates of solidification.

Regnault, W. F.↗