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At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

Microbial mats in playa lakes and other saline habitats: Early Mars analog?

Microbial mats are cohesive benthic microbial communities which inhabit various Terra (Earth-based) environments including the marine littoral and both permanent and ephemeral (playa) saline lakes. Certain geomorphological features of Mars, such as the Margaritifer Sinus, were interpreted as ancient, dried playa lakes, presumably formed before or during the transition to the present Mars climate. Studies of modern Terran examples suggest that microbial mats on early Mars would have had the capacity to survive and propagate under environmental constraints that would have included irregularly fluctuating regimes of water activity and high ultraviolet flux. Assuming that such microbial communities did indeed inhabit early Mars, their detection during the Mars Rover Sample Return (MRSR) mission depends upon the presence of features diagnostic of the prior existence of these communities or their component microbes or, as an aid to choosing suitable landing, local exploration or sampling sites, geomorphological, sedimentological or chemical features characteristic of their playa lake habitats. Examination of modern Terran playas (e.g., the Lake Eyre basin) shows that these features span several orders of magnitude in size. While stromatolites are commonly centimeter-meter scale features, bioherms or fields of individuals may extend to larger scales. Preservation of organic matter (mats and microbes) would be favored in topographic lows such as channels or ponds of high salinity, particularly those receiving silica-rich groundwaters. These areas are likely to be located near former zones of groundwater emergence and/or where flood channels entered the paleo-playa. Fossil playa systems which may aid in assessing the applicability of this particular Mars analog include the Cambrian Observatory Hill Beds of the Officer Basin and the Eocene Wilkins Peak Member of the Green River Formation.

Bauld, John↗
NASA NTRS1990

A bubblelike coronal mass ejection flux rope in the solar wind

A resolution to the question of whether coronal mass ejections are loops or bubbles is proposed and applied to the geometrical analysis of a solar wind event detected at 1 AU by ISEE 1 and 3. The discontinuity orientations, the size determined by time of passage, and the magnetic cloud signature are fit into the topology of a flux rope loop distorted by expansion into a thick rope with comparable dimensions in both the ecliptic and meridional planes. The looped rope fills a bubblelike cavity, thus preserving both types of proposed coronal mass ejection geometries. Other interesting features of the data include an apparent separation by the rope core of bidirectionally streaming protons in the leading section from electrons in the trailing section, possible vortical flow within the magnetic cloud, and a well-defined filamentary structure behind the shock.

Crooker, N. U.↗
NASA NTRS1992

Breccia dikes from the Beaverhead Impact structure, southwest Montana

While shatter cones are generally accepted as indicators of meteorite impact, older petrologic features are not widely recognized in the geologic community. Breccia dikes are one such feature. They are found in many large impact structures occurring over an area at least as extensively as shatter cones. Breccia dikes will survive moderate degrees of metamorphism and tectonism, unlike many other microscopic features (shocked quartz grains, high-pressure polymorphs, etc.) and even large-scale features such as annular or bowl-shaped topographic features. Thus, they are important diagnostic criteria, especially for large, poorly preserved impact structures. The Beaverhead Impact structure is a recently discovered, deeply eroded impact structure in southwestern Montana. The remains of the structure are delineated by the occurrence of shatter cones, found in an area greater than 200 sq km, occurring within the Cabin thrust plate, part of the Cretaceous Sevier fold and thrust system. The distribution of shatter cones is further truncated by Tertiary normal faults. The present remains represent an allochthonous fragment of a larger structure.

Fiske, P. S.↗
NASA NTRS2013

Do We Already have Samples of CERES H Chondrite Haliites and the CERES-HEBE Link

We investigate the hypothesis that halite grains in the brecciated H chondrites Zag and Monahans originate from Ceres. Evidence includes mineralogy of the halites consistent with formation on a large, carbonaceous, aqueously active body close to the H chondrite parent body >4 Ga ago. Evidence also includes orbital simularities between 1 Ceres and the purported H chondrite parent body (HPB) 6 Hebe, possibly facilitating a gentle transfer between the bodies. Halite grains in the Monahans and Zag Hchondrites are exogenous to the H chondrite parent body and were transported to the HPB >4 Ga ago. Examination of minerals and carboanceous materials entrained within the halites shows that the halite parent body (HaPB) is consistent with a carbonaceous body [1]. It is probably a large body due to the variety of entrained carbonaceous materials which probably accreted from multiple sources. The halite grains contain intact, HaPB-origin, ancient fluid inclusions indicating that transfer between the HaPB and the HPB was a gentle process resulting in a ΔT of <25 degC. Ejection from the HaPB may have been via cryovolcanic processes similar to those on modern-day Enceladus, which have been interpreted to include halite from spectroscopic observations. The ΔTmax to preserve the brinebearing halite restricts the impact velocity to the HPB at less than ≈350-700 m/s, depending upon the fraction of kinetic energy used heat the sample. [2-6]. Therefore the HaPB and HPB must have shared nearby orbits at the time of the HaPB-HPB transfer. Evidence presented elsewhere indicates asteroid 6 Hebe is a favored candidate for the HPB based on reflectance spectrum similarity with H chondrites and dynamical arguments [7,8]. The modern orbits of Ceres and Hebe are reasonably similar, with aphelion/perihelion of Ceres and Hebe of 2.99/2.55 and 2.91/1.94 AU, respectively. Initial calculations indicate an approximate mean infall velocity of 1.20 to 1.38 km/s. While higher than 350- 700 m/s, the orbits may have been more favorable >4 Ga ago. Additional dynamical factors need to be investigated. A combination of factors suggests Ceres as the HaPB. It is a carbonaceous body with suggestions of past aqueous activity [9], which is consistent with the mineral species found in H chondrite halites. Ceres is also a large body capable of accreting the range of carbonaceous materials noted [5]. It is relatively near to purported HPB Hebe, which is required to preserve halite fluid inclusions. The above evidence defines a hypothesized scenario featuring ejection of halite grains from Ceres onto Hebe. Halite was then entrained in H-chondrite near-surface breccias and ejected from Hebe for transport to Earth.

Fries, Marc D.↗
NASA NTRS1990

Theory of multicolor lattice gas - A cellular automaton Poisson solver

The present class of models for cellular automata involving a quiescent hydrodynamic lattice gas with multiple-valued passive labels termed 'colors', the lattice collisions change individual particle colors while preserving net color. The rigorous proofs of the multicolor lattice gases' essential features are rendered more tractable by an equivalent subparticle representation in which the color is represented by underlying two-state 'spins'. Schemes for the introduction of Dirichlet and Neumann boundary conditions are described, and two illustrative numerical test cases are used to verify the theory. The lattice gas model is equivalent to a Poisson equation solution.

Chen, H.↗
NASA NTRS2009

Construction of Low Dissipative High Order Well-Balanced Filter Schemes for Non-Equilibrium Flows

The goal of this paper is to generalize the well-balanced approach for non-equilibrium flow studied by Wang et al. [26] to a class of low dissipative high order shock-capturing filter schemes and to explore more advantages of well-balanced schemes in reacting flows. The class of filter schemes developed by Yee et al. [30], Sjoegreen & Yee [24] and Yee & Sjoegreen [35] consist of two steps, a full time step of spatially high order non-dissipative base scheme and an adaptive nonlinear filter containing shock-capturing dissipation. A good property of the filter scheme is that the base scheme and the filter are stand alone modules in designing. Therefore, the idea of designing a well-balanced filter scheme is straightforward, i.e., choosing a well-balanced base scheme with a well-balanced filter (both with high order). A typical class of these schemes shown in this paper is the high order central difference schemes/predictor-corrector (PC) schemes with a high order well-balanced WENO filter. The new filter scheme with the well-balanced property will gather the features of both filter methods and well-balanced properties: it can preserve certain steady state solutions exactly; it is able to capture small perturbations, e.g., turbulence fluctuations; it adaptively controls numerical dissipation. Thus it shows high accuracy, efficiency and stability in shock/turbulence interactions. Numerical examples containing 1D and 2D smooth problems, 1D stationary contact discontinuity problem and 1D turbulence/shock interactions are included to verify the improved accuracy, in addition to the well-balanced behavior.

Wang, Wei↗
NASA NTRS2019

Outstanding Natural Occurrence of TiO2-II at the Chicxulub Crater - Anatomy of A Shock-Produced High-Pressure Polymorph

Scientific drilling of the end-Cretaceous, approximately 180 kilometers-diameter Chicxulub crater (Yucatan Peninsula, Mexico) during IODP–ICDP (Integrated Ocean Drilling Program - International Continental Scientific Drilling Program) Expedition 364 has provided new insights into the formation, shock metamorphism, structural evolution, and thermal history of peak rings in large complex impact craters. An outstanding feature in uplifted granitoid rocks of the Chicxulub peak ring is the preservation of TiO2-II, an orthorhombic high-pressure polymorph of TiO2 with an alpha-PbO2 structure, produced during the impact from rutile and/or anatase at shock pressures of approximately 12.5-17.5 gigapascals. Unlike other mostly micro- and cryptocrystalline occurrences of TiO2-II at terrestrial impact sites, ejecta deposits, and in rare ultra-high pressure metamorphic rocks, TiO2-II at Chicxulub occurs as abundant euhedral crystals greater than or equal to 70 microns in size within aggregates of altered magmatic titanite. This mode of occurrence provides an excellent opportunity to investigate the crystallography and transformation kinetics of the shock-produced high-pressure polymorph using scanning electron microscopic, micro-Raman, electron backscatter diffraction (EBSD), focused ion beam (FIB), as well as transmission-EBSD and transmission electron microscopic (TEM) techniques. Here we present refined microstructural and new crystallographic results for TiO2-II at the Chicxulub crater.

Schmieder, Martin↗
NASA NTRS1972

Lunar subsurface exploration with coherent radar.

The Apollo Lunar Sounder Experiment that is scheduled to orbit the moon on Apollo 17 consists of a three frequency coherent radar system and an optical recorder. The coherent radar can be used to measure both phase and amplitude characteristics of the radar echo. Measurement methods that are related to the phase and amplitude will be used to determine the surface profile, locate subsurface features and ascertain near surface electrical properties of the lunar surface. The key to the coherent radar measurement is a highly stable oscillator that preserves an accurate phase reference (2 or 3 electrical degrees) over a long period of time. This reference provides a means for reducing surface clutter so that subsurface features are more easily detected and also provides a means of measuring range to the surface to within a fraction of a wavelength.

Brown, W. E., Jr.↗
NASA NTRS1985

Ancient landscapes: Their nature and significance for the question of inheritance

It is widely believed that much of the world's scenery is youthful. Thornbury's assertion that little of the world's scenery is older than Tertiary and that most of it is no older than Pleistocene dies hard. Yet there is ample evidence, long recognized, that very ancient forms and surfaces (here the term surface is used in the sense of a planation surface, surface d'aplanissement or Einebnungsflache) are an integral part of the contemporary landscape, and that such features are not restricted to the low latitude regions, though they are well preserved there. Many of them were formed in environments very different from that in which they now occur and are thus inherited. Paleosurfaces of many age ranges have been recognized. They can conveniently be considered as of three types: exhumed, epigene and etch.

Twidale, C. R.↗
NASA NTRS1995

Advanced interactive display formats for terminal area traffic control

The basic design considerations for perspective Air Traffic Control displays are described. A software framework has been developed for manual viewing parameter setting (MVPS) in preparation for continued, ongoing developments on automated viewing parameter setting (AVPS) schemes. The MVPS system is based on indirect manipulation of the viewing parameters. Requests for changes in viewing parameter setting are entered manually by the operator by moving viewing parameter manipulation pointers on the screen. The motion of these pointers, which are an integral part of the 3-D scene, is limited to the boundaries of screen. This arrangement has been chosen, in order to preserve the correspondence between the new and the old viewing parameter setting, a feature which contributes to preventing spatial disorientation of the operator. For all viewing operations, e.g. rotation, translation and ranging, the actual change is executed automatically by the system, through gradual transitions with an exponentially damped, sinusoidal velocity profile, in this work referred to as 'slewing' motions. The slewing functions, which eliminate discontinuities in the viewing parameter changes, are designed primarily for enhancing the operator's impression that he, or she, is dealing with an actually existing physical system, rather than an abstract computer generated scene. Current, ongoing efforts deal with the development of automated viewing parameter setting schemes. These schemes employ an optimization strategy, aimed at identifying the best possible vantage point, from which the Air Traffic Control scene can be viewed, for a given traffic situation.

Grunwald, Arthur J.↗
NASA NTRS2005

System for Processing Coded OFDM Under Doppler and Fading

An advanced communication system has been proposed for transmitting and receiving coded digital data conveyed as a form of quadrature amplitude modulation (QAM) on orthogonal frequency-division multiplexing (OFDM) signals in the presence of such adverse propagation-channel effects as large dynamic Doppler shifts and frequency-selective multipath fading. Such adverse channel effects are typical of data communications between mobile units or between mobile and stationary units (e.g., telemetric transmissions from aircraft to ground stations). The proposed system incorporates novel signal processing techniques intended to reduce the losses associated with adverse channel effects while maintaining compatibility with the high-speed physical layer specifications defined for wireless local area networks (LANs) as the standard 802.11a of the Institute of Electrical and Electronics Engineers (IEEE 802.11a). OFDM is a multi-carrier modulation technique that is widely used for wireless transmission of data in LANs and in metropolitan area networks (MANs). OFDM has been adopted in IEEE 802.11a and some other industry standards because it affords robust performance under frequency-selective fading. However, its intrinsic frequency-diversity feature is highly sensitive to synchronization errors; this sensitivity poses a challenge to preserve coherence between the component subcarriers of an OFDM system in order to avoid intercarrier interference in the presence of large dynamic Doppler shifts as well as frequency-selective fading. As a result, heretofore, the use of OFDM has been limited primarily to applications involving small or zero Doppler shifts. The proposed system includes a digital coherent OFDM communication system that would utilize enhanced 802.1la-compatible signal-processing algorithms to overcome effects of frequency-selective fading and large dynamic Doppler shifts. The overall transceiver design would implement a two-frequency-channel architecture (see figure) that would afford frequency diversity for reducing the adverse effects of multipath fading. By using parallel concatenated convolutional codes (also known as Turbo codes) across the dual-channel and advanced OFDM signal processing within each channel, the proposed system is intended to achieve at least an order of magnitude improvement in received signal-to-noise ratio under adverse channel effects while preserving spectral efficiency.

Tsou, Haiping↗
NASA NTRS2004

Some Improvements in Signal-Conditioning Circuits

Two documents present wide-ranging discussions of some issues in the design and operation of signal-conditioning circuits. The first document focuses on active low-pass filter circuits that contain resistors, capacitors, and operational amplifiers. It describes design and operational problems encountered previously, deficiencies of prior designs, and four design improvements to overcome the deficiencies. These improvements are as follows: 1. An offset-calibration feature in which an electronic switch isolates a filter capacitor in order to preserve its voltage during a calibration performed to measure the offset voltage of the operational amplifier; 2. Configuring a pair of complementary operational amplifiers to prevent latchup and decrease the degree of nonlinearity in overall response; 3. Minimizing distortion by taking the filter output from the operational-amplifier output nodes instead of from one of the other nodes as in prior designs; and 4. Providing for switching different feedback resistors to change filter break frequencies. The second document addresses topics in the architecture of signal-conditioning and multiplexing circuitry. Improvements are described as being made with respect to greater compactness, increased flexibility in accommodating a variety of inputs, improvements in filter performance, simplification of wiring, and reconfigurability of designs.

Shuler, Robert L.↗
NASA NTRS1993

Parabolic features and the erosion rate on Venus

The impact cratering record on Venus consists of 919 craters covering 98 percent of the surface. These craters are remarkably well preserved, and most show pristine structures including fresh ejecta blankets. Only 35 craters (3.8 percent) have had their ejecta blankets embayed by lava and most of these occur in the Atla-Beta Regio region; an area thought to be recently active. parabolic features are associated with 66 of the 919 craters. These craters range in size from 6 to 105 km diameter. The parabolic features are thought to be the result of the deposition of fine-grained ejecta by winds in the dense venusian atmosphere. The deposits cover about 9 percent of the surface and none appear to be embayed by younger volcanic materials. However, there appears to be a paucity of these deposits in the Atla-Beta Regio region, and this may be due to the more recent volcanism in this area of Venus. Since parabolic features are probably fine-grain, wind-deposited ejecta, then all impact craters on Venus probably had these deposits at some time in the past. The older deposits have probably been either eroded or buried by eolian processes. Therefore, the present population of these features is probably associated with the most recent impact craters on the planet. Furthermore, the size/frequency distribution of craters with parabolic features is virtually identical to that of the total crater population. This suggests that there has been little loss of small parabolic features compared to large ones, otherwise there should be a significant and systematic paucity of craters with parabolic features with decreasing size compared to the total crater population. Whatever is erasing the parabolic features apparently does so uniformly regardless of the areal extent of the deposit. The lifetime of parabolic features and the eolian erosion rate on Venus can be estimated from the average age of the surface and the present population of parabolic features.

Strom, Robert G.↗
NASA NTRS1977

Endogenic modification of impact craters on Mercury

The presence of internally modified impact craters on Mercury's surface may be used to evaluate the possibility of Mercurian volcanism. Such craters are similar to the floor-fractured and mare-filled craters observed on the moon. Mariner-10 images show that most such craters occur, as on the moon, near plains-filled basins. Color-ratio images have indicated that some Mercurian craters manifest red plains materials on their floors. These features may be associated with lava analogous to mare basalts in some lunar craters, or with compositionally distinct subsurface material preserved within the impact crater. Several basins manifest photometric contrasts between basin exteriors and basin-filling plains. Dark haloes are observed around some impact craters superimposed on the interior plains. This suggests the excavation of compositionally distinct material. Some possible endogenic features are discerned, despite the poor surface resolution, such as irregular rimless depressions. It is felt that volcanism may have occurred on Mercury, and that in some areas it may be similar to that of the lunar Mare Australe region.

Schultz, P. H.↗
NASA NTRS1985

The Pongola structure of southeastern Africa - The world's oldest preserved rift?

Rocks of the Pongola Supergroup form an elongate belt in the Archean Kaapvaal Craton of southern Africa. Because these rocks exhibit many features that are characteristic of rocks deposited in continental rifts, including rapid lateral variations in thickness and character of sediments, volcanic rocks that are bimodal in silica content, coarse, basement derived conglomerates and thick sequences of shallow water sedimentary facies associations, it is suggested that the Pongola Supergroup was deposited in such a rift. The age of these rocks (approximately 3.0 Ga) makes the Pongola structure the world's oldest well-preserved rift so far recognized, and comparison of the Pongola Rift with other rifts formed more recently in earth history reveals striking similarities, suggesting that the processes that formed this rift were not significantly different from those that form continental rifts today.

Burke, K.↗
NASA NTRS1985

Volcano/ground ice interactions in Elysium Planitia, Mars

The occurrence within Elysium Planitia of meltwater deposits, possible pseudocraters, collapse features within troughs, and outflow channels indicates that a layer of subsurface volatiles existed at the time of volcanic activity within this area. The pseudocraters are interpreted to be indicators of near-surface volatiles, while meltwater deposits and the degree of preservation of trough walls and floors are thought to signify greater volatile depths. A latitudinal variation in the distribution of these features indicates either that the depth to the volatile layer increased from less than about 50 m at 35 deg N to greater than 600 m at 24 deg N, or that an ice wedge that existed at 35 deg N thinned to nonexistence at 24 deg N. Braided distributary channel systems within the chaotic terrain north of Elysium Planitia show that ephemeral lakes were repeatedly created and drained at this locality. The existence of volatiles contemporaneous with volcanic activity permits a search to be made for explosively generated landforms predicted to exist by previous theoretical models. Morphological evidence for strombolian, vulcanian and plinian eruptions is lacking within western Elysium Planitia; there are no identifiable cinder cones, pyroclastic flow deposits, or mantled areas indicative of large airfall deposits at an image resolution of 50-150 m/pixel. However, the pseudocraters indicate that small-scale phreatomagmatic activity may have taken place.

Mouginis-Mark, P. J.↗
NASA NTRS1991

The biogeochemistry of microbial mats, stromatolites and the ancient biosphere

Stromatolites offer an unparalleled geologic record of early life, because they constitute the oldest and most abundant recognizable remains of microbial ecosystems. Microbial mats are living homologs of stromatolites; thus, the physiology of the microbiota as well as the processes which create those features of mats (e.g., biomarker organic compounds, elemental and stable isotopic compositions) which are preserved in the ancient record. Observations of the carbon isotopic composition (delta C-13) of stromatolites and microbial mats were made and are consistent with the hypothesis that atmospheric CO2 concentrations have declined by at least one to two orders of magnitude during the past 2.5 Ga. Whereas delta C-13 values of carbonate carbon average about 0 permil during both the early and mid-Proterozoic, the delta C-13 values of stromatolitic organic matter increase from an average of -35 between 2.0 and 2.6 Ga ago to an average of about -28 about 1.0 Ga ago. Modern microbial mats in hypersaline environments have delta C-13 values typically in the range of -5 to -9, relative to an inorganic bicarbonate source at 0 permil. Both microbial mats and pur cultures of cyanobacteria grown in waters in near equilibrium with current atmospheric CO2 levels exhibit minimal discrimination against C-13. In contrast, hot spring cyanobacterial mats or cyanobacterial cultures grown under higher CO2 levels exhibit substantially greater discrimination. If care is taken to compare modern mats with stromatolites from comparable environments, it might be possible to estimate ancient levels of atmospheric CO2. In modern microbial mats, a tight coupling exists between photosynthetic organic carbon production and subsequent carbon oxidation, mostly by sulfate reduction. The rate of one process fuels a high rate of the other, with much of the sulfate reduction occurring within the same depth interval as oxygenic photosynthesis. Other aspects of this study are presented.

Desmarais, D. J.↗
NASA NTRS1990

Macromolecular differentiation of Golgi stacks in root tips of Arabidopsis and Nicotiana seedlings as visualized in high pressure frozen and freeze-substituted samples

The plant root tip represents a fascinating model system for studying changes in Golgi stack architecture associated with the developmental progression of meristematic cells to gravity sensing columella cells, and finally to "young" and "old", polysaccharide-slime secreting peripheral cells. To this end we have used high pressure freezing in conjunction with freeze-substitution techniques to follow developmental changes in the macromolecular organization of Golgi stacks in root tips of Arabidopsis and Nicotiana. Due to the much improved structural preservation of all cells under investigation, our electron micrographs reveal both several novel structural features common to all Golgi stacks, as well as characteristic differences in morphology between Golgi stacks of different cell types. Common to all Golgi stacks are clear and discrete differences in staining patterns and width of cis, medial and trans cisternae. Cis cisternae have the widest lumina (approximately 30 nm) and are the least stained. Medial cisternae are narrower (approximately 20 nm) and filled with more darkly staining products. Most trans cisternae possess a completely collapsed lumen in their central domain, giving rise to a 4-6 nm wide dark line in cross-sectional views. Numerous vesicles associated with the cisternal margins carry a non-clathrin type of coat. A trans Golgi network with clathrin coated vesicles is associated with all Golgi stacks except those of old peripheral cells. It is easily distinguished from trans cisternae by its blebbing morphology and staining pattern. The zone of ribosome exclusion includes both the Golgi stack and the trans Golgi network. Intercisternal elements are located exclusively between trans cisternae of columella and peripheral cells, but not meristematic cells. In older peripheral cells only trans cisternae exhibit slime-related staining. Golgi stacks possessing intercisternal elements also contain parallel rows of freeze-fracture particles in their trans cisternal membranes. We propose that intercisternal elements serve as anchors of enzyme complexes involved in the synthesis of polysaccharide slime molecules to prevent the complexes from being dragged into the forming secretory vesicles by the very large slime molecules. In addition, we draw attention to the similarities in composition and apparent site of synthesis of xyloglucans and slime molecules.

NASA Program Space Biology↗