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At least 541 records · Page 30
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Thermographic imaging of disbonds in laminated structures

The presence of disbonds significantly decreases the integrity of many laminated structures used in aerospace applications. Critical for the assessment of the effect of these disbonds on structural integrity is a determination of the disbond area. Thermography is able to detect local variations in temperature after external heating resulting from subsurface delaminations. One limitation of thermography is that the heat diffuses within the laminates, causing difficulty in determining the area of the disbond from the thermographic image. A technique has been developed for reducing the thermographic image to an image of subsurface delaminations for some laminated structures. The technique clearly delineates regions of disbond and is able to resolve disbonds which are unresolvable in the thermographic image. The limits of this technique for resolving the area and separation of disbonds are investigated using both experimental measurements and numerical calculations.

Winfree, William P.↗
NASA NTRS1991

Strain accommodation beneath structures on Mars

A recent review of tectonic features on Mars shows that most of their subsurface structures can be confidently extended only a few kilometers deep (exceptions are rifts, in which bounding normal faults penetrate the entire brittle lithosphere, with ductile flow at deeper levels). Nevertheless, a variety of estimates of elastic lithosphere thickness and application of accepted failure criteria under likely conditions on Mars suggest a brittle lithosphere that is many tens of kilometers thick. This raises the question of how the strain (extension or shortening) accommodated by grabens and wrinkle ridges within the upper few kilometers is being accommodated at deeper levels in the lithosphere. Herein, the nonrift tectonic features present on Mars are briefly reviewed, along with their likely subsurface structures, and some inferences and implications are presented for behavior of the deeper lithosphere.

Golombek, M. P.↗
NASA NTRS1992

Magmatic intrusions and hydrothermal systems on Mars

We are continuing our investigation of Martian hydrothermal systems and the formation of fluvial valleys on Mars. Here we present our initial numerical modeling results of hydrothermal systems associated with magmatic intrusions on Mars. To model such hydrothermal systems, we consider single, cylindrical intrusions of 4 km height 2 km below the surface. Our preliminary results of modeling hydrothermal systems associated with magmatic intrusions in the Martian environment suggest that such systems, if associated with intrusions of several 10(exp 2) km(exp 3) or larger, in the presence of permeable, water-rich subsurface should be able to provide adequate discharges of water over the time periods needed to form fluvial valleys. However, it is important to note that the amount of water ultimately available for erosion also depends on the surface and subsurface lithology.

Gulick, V. C.↗
NASA NTRS1992

Mars: Wavelength-dependent dual polarization global scattering

During the 1988 and 1990 Mars oppositions, the first continuous-wave (CW) multi-wavelength radar observations were performed that include the entire echo in both polarizations. These observations, coordinated in subradar coverage when possible, were made with the Arecibo S-band (12.6 cm lambda) and Goldstone X-band (3.5 cm lambda) facilities. The CW spectra obtained during these oppositions have been studied using a variety of techniques to explore the spatial and wavelength dependence of both the Same-sense Circular (SC) and Opposite-sense Circular (OC) polarization returns. Earlier multi-wavelength comparisons dealt primarily with the quasispecular component of the echoes. Our work in contrast has much new information (at high signal-to-noise) for the SC 'depolarized component. The unique value of these radar observations lies in their potential for probing the subsurface scattering behavior in 'appropriate' terrain. The clearest case for wavelength dependence in the SC component is the scattering behavior over Tharsis, where the X-band features are significantly weaker than the S-band features. This hypothesis was advanced to account for the low thermal inertia of Tharsis, but also can explain the S/X differences if the layer is about 40 cm thick. In contrast to the Tharsis result, the depolarized echo from the heavily cratered terrain is actually stronger at X-band. The obvious interpretation is that more scatterers exist at the scale of the shorter wavelength, either at the surface or as a distributed subsurface scatterers. The strongest depolarized feature of the X-band spectra is associated with the south residual polar cap. The radar cross section of this feature corresponds to an equivalent full-disk albedo of unity.

Harmon, J. K.↗
NASA NTRS1992

Is ground ice stable near the Martian equator?

Determining the present distribution of subsurface water ice on Mars will undoubtedly be a prime objective for future spacecraft missions. Past theoretical studies have predicted that Martian ground ice should be stable to evaporation only at latitudes poleward of approximately +/- 50 deg. Presented here are a new set of calculations which show that bright surficial dust deposits can protect subsurface ice deposits from evaporation at much lower latitudes than previously expected.

Paige, D. A.↗
NASA NTRS1990

Remote sensing science for the Nineties; Proceedings of IGARSS '90 - 10th Annual International Geoscience and Remote Sensing Symposium, University of Maryland, College Park, May 20-24, 1990. Vols. 1, 2, & 3

Various papers on remote sensing (RS) for the nineties are presented. The general topics addressed include: subsurface methods, radar scattering, oceanography, microwave models, atmospheric correction, passive microwave systems, RS in tropical forests, moderate resolution land analysis, SAR geometry and SNR improvement, image analysis, inversion and signal processing for geoscience, surface scattering, rain measurements, sensor calibration, wind measurements, terrestrial ecology, agriculture, geometric registration, subsurface sediment geology, radar modulation mechanisms, radar ocean scattering, SAR calibration, airborne radar systems, water vapor retrieval, forest ecosystem dynamics, land analysis, multisensor data fusion. Also considered are: geologic RS, RS sensor optical measurements, RS of snow, temperature retrieval, vegetation structure, global change, artificial intelligence, SAR processing techniques, geologic RS field experiment, stochastic modeling, topography and Digital Elevation model, SAR ocean waves, spaceborne lidar and optical, sea ice field measurements, millimeter waves, advanced spectroscopy, spatial analysis and data compression, SAR polarimetry techniques. Also discussed are: plant canopy modeling, optical RS techniques, optical and IR oceanography, soil moisture, sea ice back scattering, lightning cloud measurements, spatial textural analysis, SAR systems and techniques, active microwave sensing, lidar and optical, radar scatterometry, RS of estuaries, vegetation modeling, RS systems, EOS/SAR Alaska, applications for developing countries, SAR speckle and texture.

Source record↗
NASA NTRS1992

Buoyant thermocapillary flow with nonuniform supra-heating. I - Liquid-phase behavior. II - Two-phase behavior

The present computational study of transient heat transfer and fluid flow in a circular pool of n-decane which is undergoing central radiative heating from above gives attention to the volumetric absorption of the radiation incident on the pool surface. The first part of this study notes that buoyancy influences the number and recirculation rates of the subsurface vortices by stabilizing hot subsurface fluid above the colder core fluid; this affects the liquid surface temperature profile and in turn governs the velocity profile that is due to thermocapillarity. In the second part, the effects of gas-liquid phase coupling, variable density and thermophysical properties, and vaporization are considered.

Schiller, David N.↗
NASA NTRS1992

Al Umchaimin depression, Western Iraq: An impact structure?

Al Umchaimin, in Arabic 'hiding place' or 'place of ambush', is located at latitude 32 degrees 35.5 N and longitude 39 degrees 25 E. The nearly circular depression averages 2.75 km in diameter and is 33-42 m deep. It is floored with fine-grained, clay-rich deposits, estimated to be 36 m thick, the surface of which shows well-developed desiccation fissures or mudcracks when dry. Because of its nearly circular planimetric shape and its apparent isolation from other surface and subsurface features, it has been considered by some to be a possible meteorite impact structure and by others to be a surface collapse feature that originated following removal of magma from the subsurface as the magma extruded elsewhere. Al Umchaimin was listed in the U.S. Geological Survey tabulation of 110 structures worldwide for which a meteorite impact origin had been suggested. It was placed in Category 6 Structures for which more data are required for classification. It is concluded that, on the basis of the studies that have been made of Al Umchaimin and on the basis of the brief site visit made, Al Umchaimin probably is not an impact structure but most likely resulted from the enlargement and coalescence of sink holes and eventual collapse of the roof material into the resulting cavity.

Underwood, James R., Jr.↗
NASA NTRS1992

Determining stress states using dike swarms: The Lauma Dorsa example

Initial examination of the Magellan coverage of Venus has revealed between 150 and 300 large, radially lineated landforms distributed across the planet's surface. Where the lineaments have been examined in detail, the majority fail to exhibit signatures indicative of relief at or above the resolution of the radar; however, when the sense of topographic relief may be ascertained, the lineaments commonly appear as fissures or flat-floored trenches interpreted as graben. Individual lineaments can display graben, fissure, and zero relief behavior along their length, suggesting either that these differences are a function of the resolution of the radar, or that the morphological distinctions are real but somehow genetically linked. In many instances, radial lineaments exhibiting these characteristics are directly associated with surface volcanism, including flanking and terminal flows, superimposed shield domes and pit chains, and central, calderalike topographic lows. These observable characteristics, as well as theoretical studies and comparison with similar terrestrial features, have led to the working hypothesis that many of the radial fracture systems on Venus are the surface manifestation of subsurface dikes propagating laterally from a central magma source. If this interpretation is correct, studies of terrestrial dikes suggest that the lineament directions, with localized exceptions and barring subsequent deformation, should be perpendicular to the orientation of the least compressive stress at the time of their formation. To test this hypothesis, we briefly examine a radial fracture system (63.7 degrees N, 195 degrees E) located between two deformation belts in Vinmara Planitia, and verify that the lineaments to the east behave in the expected manner. We have also chosen this feature, however, because of its proximity to Lauma Dorsa to the west. On the basis of Venera 15/16 data, both compressional and extensional origins for this deformation belt have been proposed. By examining the stratigraphy and applying our interpretation that the fracture system is linked to the presence of subsurface dikes, we present an independent evaluation of the stress state associated with Lauma Dorsa, and thus contribute to the assessment of its origin.

Grosfils, Eric B.↗
NASA NTRS1991

Mars Aquarius Mission and Titan Explorer

The Mars Aquarius Mission is designed to carry out several scientific studies of the surface and subsurface of Mars with an emphasis on locating subterranean water. This mission is a precursor to a manned mission to Mars. A manned mission will require an extended stay on the planet's surface, and an accessible source of water will greatly simplify life support requirements. Using data from previous Mars observations, four sites were selected as possible locations of subsurface water. The Aquarius spacecraft carries one penetrator for each of these sites.

Dulla, Brian↗
NASA NTRS1993

Geographic variations in the thermal and diffusive stability of ground ice on Mars

Time-dependent models of the thermal and diffusive behavior of the Martian regolith are developed to investigate the stability of ground ice within the top several meters of the regolith. The geographic distribution of thermal inertia and albedo as well as the latitudinal variation in insolation are included in calculations of surface and subsurface temperatures between +/- 60 deg latitude. Ground ice is found to be stable where the annual mean surface and subsurface temperatures were below the atmospheric frost point. Calculations of temperatures at high and low obliquity suggest that ground ice would be stable globally at high obliquity and would not be stable between +/- 60 deg latitude at low obliquity. The time scales for condensation of ground ice are found to be comparable to that of orbital oscillations, suggesting that the present geographic distribution of ground ice may depend on the orbital history of Mars.

Mellon, Michael T.↗
NASA NTRS1993

A model for the hydrologic and climatic behavior of water on Mars

An analysis is carried out of the hydrologic response of a water-rich Mars to climate change and to the physical and thermal evolution of its crust, with particular attention given to the potential role of the subsurface transport, assuming that the current models of insolation-driven change describe reasonably the atmospheric leg of the planet's long-term hydrologic cycle. Among the items considered are the thermal and hydrologic properties of the crust, the potential distribution of ground ice and ground water, the stability and replenishment of equatorial ground ice, basal melting and the polar mass balance, the thermal evolution of the early cryosphere, the recharge of the valley networks and outflow, and several processes that are likely to drive the large-scale vertical and horizontal transport of H2O within the crust. The results lead to the conclusion that subsurface transport has likely played an important role in the geomorphic evolution of the Martian surface and the long-term cycling of H2O between the atmosphere, polar caps, and near-surface crust.

Clifford, Stephen M.↗
NASA NTRS1993

Evaluation of a distributed catchment scale water balance model

The validity of some of the simplifying assumptions in a conceptual water balance model is investigated by comparing simulation results from the conceptual model with simulation results from a three-dimensional physically based numerical model and with field observations. We examine, in particular, assumptions and simplifications related to water table dynamics, vertical soil moisture and pressure head distributions, and subsurface flow contributions to stream discharge. The conceptual model relies on a topographic index to predict saturation excess runoff and on Philip's infiltration equation to predict infiltration excess runoff. The numerical model solves the three-dimensional Richards equation describing flow in variably saturated porous media, and handles seepage face boundaries, infiltration excess and saturation excess runoff production, and soil driven and atmosphere driven surface fluxes. The study catchments (a 7.2 sq km catchment and a 0.64 sq km subcatchment) are located in the North Appalachian ridge and valley region of eastern Pennsylvania. Hydrologic data collected during the MACHYDRO 90 field experiment are used to calibrate the models and to evaluate simulation results. It is found that water table dynamics as predicted by the conceptual model are close to the observations in a shallow water well and therefore, that a linear relationship between a topographic index and the local water table depth is found to be a reasonable assumption for catchment scale modeling. However, the hydraulic equilibrium assumption is not valid for the upper 100 cm layer of the unsaturated zone and a conceptual model that incorporates a root zone is suggested. Furthermore, theoretical subsurface flow characteristics from the conceptual model are found to be different from field observations, numerical simulation results, and theoretical baseflow recession characteristics based on Boussinesq's groundwater equation.

Troch, Peter A.↗
NASA NTRS1993

High-resolution seismic reflection survey at the Manson crater, Iowa

Approximately 17.4 km of high-resolution reflection data were acquired along an east-west radius of the Manson Impact Structure (MIS) to delineate the shallow (upper 300 m) subsurface structural configuration. The geometry of the shallow structure is poorly known due to a 30-90 m thick Pleistocene till cover. The resolution of the new seismic data is roughly 5-10 times that of existing Vibroseis data. Data quality varies rapidly along the line from exceptional to poor, due primarily to velocity variations associated with the geological complexity of the area. Preliminary results indicate subsurface structural blocks previously envisioned to be several hundreds of meters in size are actually an order of magnitude smaller and more complex. A seismogram-by-seismogram analysis is necessary to confidently identify intricate stratigraphic and structural relationships seen on preliminary CDP sections, as numerous faults, diffractions, and complicated reflection patterns create potential pitfalls.

Keiswetter, D. A.↗
NASA NTRS1993

Equatorial ground ice on Mars: Steady-state stability

Current Martian equatorial surface temperatures are too warm for water ice to exist at the surface for any appreciable length of time before subliming into the atmosphere. Subsurface temperatures are generally warmer still and, despite the presence of a diffusive barrier of porous regolith material, it has been shown by Smoluchowski, Clifford and Hillel, and Fanale et al. that buried ground ice will also sublime and be lost to the atmosphere in a relatively short time. We investigate the behavior of this subliming subsurface ice and show that it is possible for ice to maintain at a steady-state depth, where sublimation and diffusive loss to the atmosphere is balanced by resupply from beneath by diffusion and recondensation of either a deeper buried ice deposits or ground water. We examine the behavior of equatorial ground ice with a numercial time-marching molecular diffusion model. In our model we allow for diffusion of water vapor through a porous regolith, variations in diffusivity and porosity with ice content, and recondensation of sublimed water vapor. A regolith containing considerable amounts of ice can still be very porous, allowing water vapor to diffuse up from deeper within the ice layer where temperatures are warmer due to the geothermal gradient. This vapor can then recondense nearer to the surface where ice had previously sublimed and been lost to the atmosphere. As a result we find that ice deposits migrate to find a steady-state depth, which represents a balance between diffusive loss to the atmosphere through the overlying porous regolith and diffusive resupply through a porous icy regolith below. This depth depends primarily on the long-term mean surface temperature and the nature of the geothermal gradient, and is independent of the ice-free porosity and the regolith diffusivity. Only the rate of loss of ground ice depends on diffusive properties.

Mellon, Michael T.↗
NASA NTRS1993

Mars brine formation experiment

The presence of water-soluble cations and anions in the Martian regolith has been the subject of speculation for some time. Viking lander data provided evidence for salt-cemented crusts on the Martian surface. If the crusts observed at the two Viking landing sites are, in fact, cemented by salts, and these crusts are globally widespread, as IRTM-derived thermal inertia studies of the Martian surface seem to suggest, then evaporite deposits, probably at least in part derived from brines, are a major component of the Martian regolith. The composition of liquid brines in the subsurface, which not only may be major agents of physical weathering but may also presently constitute a major deep subsurface liquid reservoir, is currently unconstrained by experimental work. A knowledge of the chemical identity and rate of production of Martian brines is a critical first-order step toward understanding the nature of both these fluids and their precipitated evaporites. Laboratory experiments are being conducted to determine the identity and production rate of water-soluble ions that form in initially pure liquid water in contact with Mars-mixture gases and unaltered Mars-analog minerals.

Moore, Jeffrey M.↗
NASA NTRS1993

Water cycle research associated with the CaPE hydrometeorology project (CHymP

One outgrowth of the Convection and Precipitation/Electrification (CaPE) experiment that took place in central Florida during July and August 1991 was the creation of the CaPE Hydrometeorology Project (CHymP). The principal goal of this project is to investigate the daily water cycle of the CaPE experimental area by analyzing the numerous land and atmosphere in situ and remotely sensed data sets that were generated during the 40-days of observations. The water cycle comprises the atmospheric branch. In turn, the atmospheric branch comprises precipitation leaving the base of the atmospheric volume under study, evaporation and transpiration entering the base, the net horizontal fluxes of water vapor and cloud water through the volume and the conversion of water vapor to cloud water and vice-versa. The sum of these components results in a time rate of change in the water and liquid water (or ice) content of the atmospheric volume. The components of the land branch are precipitation input to and evaporation and transpiration output from the surface, net horizontal fluxes of surface and subsurface water, the sum of which results in a time rate of change in surface and subsurface water mass. The objective of CHymP is to estimate these components in order to determine the daily water budget for a selected area within the CaPE domain. This work began in earnest in the summer of 1992 and continues. Even estimating all the budget components for one day is a complex and time consuming task. The discussions below provides a short summary of the rainfall quality assessment procedures followed by a plan for estimating the horizontal moisture flux.

Duchon, Claude E.↗
NASA NTRS1993

Vertical transport of water in the Martian boundary layer

We are continuing our examination of the transport of H2O through the martian boundary layer, and we have written a one-dimensional numerical model of the exchange of H2O between the atmosphere and subsurface of Mars through the planetary boundary layer (PBL). Our goal is to explore the mechanisms of H2O exchange, and to elucidate the role played by the regolith in the local H2O budget. The atmospheric model includes effects of Coriolis, pressure gradient, and frictional forces for momentum, as well as radiation, sensible heat flux, and advection for heat. The model differs from Flasar and Goody by use of appropriate Viking-based physical constants and inclusion of the radiative effects of atmospheric dust. We specify the pressure gradient force or compute it from a simple slope model. The subsurface model accounts for conduction of heat and diffusion of H2O through a porous adsorbing medium in response to diurnal forcing. The model is initialized with depth-independent H2O concentrations (2 kg M(exp -3)) in the regolith, and a dry atmosphere. The model terminates when the atmospheric H2O column abundance stabilizes at 0.1 percent per sol.

Zent, Aaron P.↗