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At least 127 records · Page 7

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NASA NTRS1978

Geoid Anomalies and the Near-Surface Dipole Distribution of Mass

Although geoid or surface gravity anomalies cannot be uniquely related to an interior distribution of mass, they can be related to a surface mass distribution. However, over horizontal distances greater than about 100 km, the condition of isostatic equilibrium above the asthenosphere is a good approximation and the total mass per unit column is zero. Thus the surface distribution of mass is also zero. For this case we show that the surface gravitational potential anomaly can be uniquely related to a surface dipole distribution of mass. Variations in the thickness of the crust and lithosphere can be expected to produce undulations in the geoid.

Turcotte, D. L.↗

NASA NTRS1980

Thermography for estimating near-surface soil moisture under developing crop canopies

Previous investigations of thermal infrared techniques using remote sensors (thermography) for estimating soil water content have been limited primarily to bare soil. Ground-based and aircraft investigations were conducted to evaluate the potential for extending the thermography approach to developing crop canopies. A significant exponential relationship was found between the volumetric soil water content in the 0-4 cm soil layer and the diurnal difference between surface soil temperature measured at 0230 and 1330 LST (satellite overpass times of NASA's Heat Capacity Mapping Mission - HCMM). Surface soil temperatures were estimated using minimum air temperature, percent cover of the canopy and remote measurements of canopy temperature. Results of the investigation demonstrated that thermography can potentially be used to estimate soil temperature and soil moisture throughout a complete growing season for a number of different crops and soils.

Heilman, J. L.↗

NASA NTRS1980

Anomalous region on Mars - Implications for near-surface liquid water

An anomalous region has been identified on Mars from the 1971 and 1973 earth-based Goldstone radar data. This region is characterized by coincident very high radar reflectivities and unusual smoothness. Very few realistic surface morphologies can generate such return-signals. Liquid water within about 50-100 cm of the surface is one possibility, an interpretation strengthened by an apparent seasonal variation in surface reflectivity for the same locality.

Zisk, S. H.↗

NASA NTRS1980

Lunar near-surface shear wave velocities at the Apollo landing sites as inferred from spectral amplitude ratios

The horizontal-to-vertical amplitude ratios of the long-period seismograms are reexamined to determine the shear wave velocity distributions at the Apollo 12, 14, 15, and 16 lunar landing sites. Average spectral ratios, computed from a number of impact signals, were compared with spectral ratios calculated for the fundamental mode Rayleigh waves in media consisting of homogeneous, isotropic, horizontal layers. The shear velocities of the best fitting models at the different sites resemble each other and differ from the average for all sites by not more than 20% except for the bottom layer at station 14. The shear velocities increase from 40 m/s at the surface to about 400 m/s at depths between 95 and 160 m at the various sites. Within this depth range the velocity-depth functions are well represented by two piecewise linear segments, although the presence of first-order discontinuities cannot be ruled out.

Horvath, P.↗

NASA NTRS1981

The role of near surface outflow in maintaining convective activity

Continuous measurements of horizontal velocity, temperature and humidity at the surface over a dense network of stations were used to examine the behavior, at the surface, of a large number of convective storms. Feedback between storm outflow at the surface and the generation of convective cells is demonstrated. The outflow is shown in the surface divergence fields once rain begins, with characteristics common to storms in distinctly different meteorological environments, such as summertime Florida and Illinois. This commencement precedes the occurence of rainfall and radar echo in the location of the cell by some tens of minutes up to more than 100 mins. Given surface measurements with a spacing of no more than 10 km, the direction of propagation, timing of the future occurence of rainfall and the intensity of the rainfall generated in the cell can be determined.

Garstang, M.↗

NASA NTRS1982

Correction of lunar seismograms for instrumental and near-surface effects and constraints on the velocity structure of the lunar interior

Long-period lunar seismograms were studied with the aim of identifying consistent sets of direct shear and secondary wave arrivals, thus constraining the velocities in the lunar mantle and the depths of the velocity discontinuities. Two velocity models were used to locate the natural impacts and the shallow moonquakes and to obtain the travel time residuals. Seismic sections were made of the radial, transverse, and vertical components of ground motion for impacts, shallow, and deep moonquakes in order to search for consistent sets of secondary wave arrivals. No conclusive set of secondary arrivals could be recognized on the seismic sections and thus the velocities and depths of the velocity discontinuities cannot be severely constrained by secondary arrivals. It is likely that the crust is thinner than 50 km and that a first-order discontinuity separates the upper and lower crust at a depth of between 20 and 30 km.

Horvath, P.↗

NASA NTRS1984

Remote sensing of near surface humidity over north Pacific

Humidity soundings at 12 midocean stations of small islands and weather ships in the north Pacific from 7 deg N to 57 deg N during a 9-year period from 1972 to 1980 were used to study the variation of columnar water vapor W (as measured by spaceborne sensors) in relation to the variation of surface level specific humidity Q (as required in the determination of air-sea moisture and latent heat exchanges). It was found that a simple regression can be used to specify monthly mean Q from W to an accuracy of about 0.0008, corresponding to about 20 W/sq m in latent heat flux. The regression accounts for both temporal and spatial variations of Q and W. Better accuracy can be achieved by using regional regressions. The study affirms the potential of spaceborne sensors in providing global monitoring of air-sea moisture and heat exchanges.

Liu, W. T.↗

NASA NTRS1987

Mars low albedo regions: Possible map of near-surface

A freeze/thaw desorption mechanism is proposed in certain low albedo areas which could be the factor that instigated dust storms. It is widely accepted that the bulk of the episodic gas evolution (not necessarily the oxygen release) experienced during the humidification process in the Viking Gas Exhange Experiment (GEX) was caused by a familiar process in which more polar H2O molecules replace large quantities of other preadsorbed gas molecules on adsorption sites. The author suggests that a similar process could produce high pore pressures in soil that could disrupt the soil and eject dust at high velocity. The author also argued that association of sites of dust storms initiated with high thermal inertial areas may simply reflect repeated dust depletion.

Huguenin, R. L.↗

NASA NTRS1988

Relationships between near-surface plankton concentrations, hydrography, and satellite-measured sea surface temperature

Sea surface temperatures (SSTs) mapped by IR satellite images and in situ hydrographic measurements off the west coast of British Columbia for early-winter and midsummer periods were correlated with in situ measurements of surface chlorophyll and zooplankton concentration. Correlations between winter log(e) transformed zooplankton concentrations and SSTs demonstrated that IR satellite imagery could explain 49 percent of the sampled zooplankton concentration variance. A least-squares-fit nonlinear equation showed that satellite-measured SST patterns explained 72 percent of the log(e) transformed chlorophyll variance. However, summer zooplankton concentrations were not consistently related to satellite temperature patterns.

Thomas, A. C.↗

NASA NTRS1988

Studies of ocean radar backscatter and near surface winds relationship during FASINEX

The relationship between radar backscatter and surface wind was investigated using data of two separate studies conducted using a set of airborne radar backscatter measurements and the associated in-situ measurements obtained during the Frontal Air-Sea Interaction Experiment (FASINEX) on February 21, 1986. The results showed that the radar backscatter cross section, sigma(0), of the ocean correlates best with the friction velocity, rather than with the equivalent neutral stability wind, U(N), indicating that the existing model of sigma(0) vs U(N) is inadequate.

Li, F.↗

NASA NTRS1990

Measuring and modeling near-surface reflected and emitted radiation fluxes at the FIFE site

Information is presented pertaining to the measurement and estimation of reflected and emitted components of the radiation balance. Information is included about reflectance and transmittance of solar radiation from and through the leaves of some grass and forb prairie species, bidirectional reflectance from a prairie canopy is discussed and measured and estimated fluxes are described of incoming and outgoing longwave and shortwave radiation. Results of the study showed only very small differences in reflectances and transmittances for the adaxial and abaxial surfaces of grass species in the visible and infrared wavebands, but some differences in the infrared wavebands were noted for the forbs. Reflectance from the prairie canopy changed as a function of solar and view zenith angles in the solar principal plane with definite asymmetry about nadir. The surface temperature of prairie canopies was found to vary by as much as 5 C depending on view zenith and azimuth position and on the solar azimuth. Aerodynamic temperature calculated from measured sensible heat fluxes ranged from 0 to 3 C higher than nadir-viewed temperatures. Models were developed to estimate incoming and reflected shortwave radiation from data collected with a Barnes Modular Multiband Radiometer. Several algorithms for estimating incoming longwave radiation were evaluated and compared to actual measures of that parameter. Net radiation was calculated using the estimated components of the shortwave radiation streams, determined from the algorithms developed, and from the longwave radiation streams provided by the Brunt, modified Deacon, and the Stefan-Boltzmann models. Estimates of net radiation were compared to measured values and found to be within the measurement error of the net radiometers used in the study.

Blad, Blaine L.↗

NASA NTRS1992

The thermal stability of near-surface ground ice on Mars

Results of thermal calculations are presented which show that observed geographic variations in the thermal and reflectance properties of Martian soils significantly affect subsurface temperatures. The results indicate that in certain regions, ground-ice deposits could exist much closer to the surface and much closer to the equator than previously thought. These deposits could be a valuable resource for future human exploration.

Paige, David A.↗

NASA NTRS1992

Chemical variation and zoning of olivine in lunar dunite 72415 - Near-surface accumulation

Electron microprobe remeasurements have been used to reevaluate the range of olivine compositions, including CaO, in lunar dunite sample 72415 and compare that range with olivines in established plutonic rocks. Olivines from ferroan anorthosite 62237 and samples from the Stillwater intrusion were analyzed; literature data for other lunar and terrestrial plutonic samples were used for comparisons. The analyses show that the lunar dunite 72415 contains a range of olivine compositions, that the olivines are zoned, and that they have CaO abundances that are consistent with shallow hypabyssal rather than deep plutonic accumulation.

Ryder, Graham↗

NASA NTRS1994

Use of acoustic wave travel-time measurements to probe the near-surface layers of the Sun

The variation of solar p-mode travel times with cyclic frequency nu is shown to provide information on both the radial variation of the acoustic potential and the depth of the effective source of the oscillations. Observed travel-time data for waves with frequency lower than the acoustic cutoff frequency for the solar atmosphere (approximately equals 5.5 mHz) are inverted to yield the local acoustic cutoff frequency nu(sub c) as a function of depth in the outer convection zone and lower atmosphere of the Sun. The data for waves with nu greater than 5.5 mHz are used to show that the source of the p-mode oscillations lies approximately 100 km beneath the base of the photosphere. This depth is deeper than that determined using a standard mixing-length calculation.

Jefferies, S. M.↗

NASA NTRS1995

Scales of variability of bio-optical properties as observed from near-surface drifters

A drifter equipped with bio-optical sensors and an automated water sampler was deployed in the California Current as part of the coastal transition zone program to study the biological, chemical, and physical dynamics of the meandering filaments. During deployments in 1987 and 1988, measurements were made of fluorescence, downwelling irradiance, upwelling radiance, and beam attenuation using several bio-optical sensors. Samples were collected by an automated sampler for later analysis of nutrients and phytoplankton species compositon. Large-scale spatial and temporal changes in the bio-optical and biological properties of the region were driven by changes in phytoplankton species composition which, in turn, were associated with the meandering circulation. Variance spectra of the bio-optical paramenters revealed fluctuations on both diel and semidiurnal scales, perhaps associated with solar variations and internal tides, respectively. Offshore, inertial-scale fluctuations were apparent in the variance spectra of temperature, fluorescence, and beam attenuation. Although calibration samples can help remove some of these variations, these results suggest that the use of bio-optical data from unattended platforms such as moorings and drifters must be analyzed carefully. Characterization of the scaled of phytoplankton variability must account for the scales of variability in the algorithms used to convert bio-optical measurments into biological quantities.

Abbott, Mark R.↗

NASA NTRS1999

Near Surface Vapor Bubble Layers in Buoyant Low Stretch Burning of Polymethylmethacrylate

Large-scale buoyant low stretch stagnation point diffusion flames over solid fuel (polymethylmethacrylate) were studied for a range of aerodynamic stretch rates of 2-12/ sec which are of the same order as spacecraft ventilation-induced stretch in a microgravity environment. An extensive layer of polymer material above the glass transition temperature is observed. Unique phenomena associated with this extensive glass layer included substantial swelling of the burning surface, in-depth bubble formation, and migration and/or elongation of the bubbles normal to the hot surface. The bubble layer acted to insulate the polymer surface by reducing the effective conductivity of the solid. The reduced in-depth conduction stabilized the flame for longer than expected from theory neglecting the bubble layer. While buoyancy acts to move the bubbles deeper into the molten polymer, thermocapillary forces and surface regression both act to bring the bubbles to the burning surface. Bubble layers may thus be very important in low gravity (low stretch) burning of materials. As bubbles reached the burning surface, monomer fuel vapors jetted from the surface, enhancing burning by entraining ambient air flow. Popping of these bubbles at the surface can expel burning droplets of the molten material, which may increase the fire propagation hazards at low stretch rates.

Olson, Sandra L.↗

NASA NTRS2003

Production Rates of Noble Gases in the Near-Surface Layers of Europa by Energetic Charged Particles and the Potential for Determining Exposure Ages

The surface of Europa is expected to be extremely active, undergoing tectonic and/or tidal geological activity and sputtering/ deposition, as well as impact cratering. Determination of the actual age of the surface at one or more places would greatly simplify trying to sort out what processes are occurring, and at what rate. If there is K present, as the spectral and compositional modeling discussed predict, it should be possible, in principle, to determine K-Ar crystallization ages. Whether or not there is K present, a consideration of the environment suggests we can determine an energetic particle exposure age if we can make in situ measurements of the abundances of major elements and of noble gas isotopes. This requires instrumentation that is within reach of current technology. In this paper, we calculate production rates for noble-gas isotopes in a simplified Europan surface, to quantify the amount of light noble gases produced by exposure to energetic particles.

Swindle, T. D.↗

NASA NTRS2004

Distribution of Near-Surface Ground Ice on Mars

During the course of this grant we made significant progress in our understanding of martian ground ice in areas of theoretical and laboratory research. We recently completed a manuscript comparing a new map of ground-ice stability with leakage neutron data for the southern hemisphere . In this manuscript we incorporated several improvements to a model of ground ice stability. We quantitatively compared measured epithermal-neutron flux with predicted ice-table burial depth and found that the observed distribution of ground ice is consistent with ice in diffusive equilibrium with the atmosphere. In addition to this theoretical study we have adapted an experimental apparatus, designed for measuring ice condensation in antarctic soils , to martian permafrost.

Mellon, Michael T.↗