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At least 631 records · Page 35
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NASA NTRS1993

Spatially extensive uniform stress fields on Venus inferred from radial dike swarm geometries: The Aphrodite Terra example

The high resolution and near global coverage of Magellan radar images is facilitating attempts to systematically investigate the stresses that have deformed the venusian crust. Here we continue earlier efforts to utilize approximately 170 large, radially lineated structures interpreted as dike swarms to assess the orientation of the regional maximum horizontal compressive stress (MHCS) which existed in their vicinities during emplacement. Examination of swarms near the equator reveals a link to broad scale regional structures, such as Aphrodite Terra, across distances in excess of 1000 km, suggesting the existence of first order stress fields which affect areas of more than 10(exp 6) sq km in a uniform fashion. Focusing further upon the Aphrodite Terra region, the MHCS field in the surrounding lowlands inferred from radial swarms is oriented approximately normal to the slope of the highland topography. This stress configuration appears, at a simple level, to be incompatible with that expected during either upwelling or downwelling construction of the highlands. In addition, the relatively undeformed geometry of the radial structures within the highlands implies that these dike swarm features formed more recently than their highly deformed surroundings. We conclude that the differential stresses which existed during emplacement of the dike swarms within and adjacent to the Aphrodite Terra highlands are related to the gravitational relaxation of pre-existing topography.

Grosfils, Eric B.↗
NASA NTRS1993

Ishtar deformed belts: Evidence for deformation from below?

The mountain belts of Ishtar Terra are unique on Venus. Models for their formation include mantle upwelling, mantle downwelling, and horizontal convergence. The present forms of these models are too simple to predict surface strain, topography, or gravity. More detailed models will require specific constraints as imposed by geologic relations. In order to develop specific constraints for geodynamic models, we examine the geology of Ishtar Terra as viewed in Magellan SAR imagery in an attempt to interpret regional surface strain patterns. In this paper, we present geologic and structural relations that leads us to postulate that Ishtar deformed belts result from shear forces within the mantle acting on the lithosphere, and not by horizontal forces from colliding plates. We propose that the surface strains result from differential strain and displacement of domains within the upper mantle, and that further analysis of Ishtar deformation may allow us to identify individual domains within the mantle, and to constrain displacement trajectories between domains.

Hansen, V. L.↗
NASA NTRS1992

In-flight calibration of the spectral and radiometric characteristics of AVIRIS in 1991

On 7 Mar. 1991, an in-flight calibration experiment was held at the Ivanpah Playa in southeastern California for the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) imaging spectrometer. Five AVIRIS overflights were acquired of a calibration target designated on the Ivanpah Playa surface. At the time of the overflights, the reflectance of the calibration target was measured with a field spectrometer. In addition, the atmospheric optical depths and water vapor abundance were measured from a radiometer station adjacent to the calibration target. These in-situ measurements were used to constrain the MODTRAN radiative transfer code to model the upwelling spectral radiance incident to the sensor aperture during the overflights. Analyses of this modeled radiance in conjunction with the laboratory-calibrated radiance were used to determine the spectral and radiometric calibration of AVIRIS while in flight.

Green, Robert O.↗
NASA NTRS1993

Rifting at Devana Chasma, Venus: Structure and estimation of the effective thickness of the elastic lithosphere

To understand the relationship between extension and sites on Venus interpreted to be associated with mantle upwelling, the characteristics of the northern part of Devana Chasma in Beta Regio are examined. The structure of this rift is compared to that of terrestrial continental rifts. To ascertain the degree to which the lithosphere at Beta might be thinned, estimates of lithospheric thickness are calculated using a plate flexure model. These values are compared to those determined for other parts of the planet.

Senske, D. A.↗
NASA NTRS1993

Volcanic rises on Venus: Geology, formation, and sequence of evolution

Large centers of volcanism on Venus are concentrated primarily in the equatorial region of the planet and are associated with regional topographic rises. Analysis of both radar images and geophysical data suggest that these uplands are sites of mantle upwelling. Magellan radar imaging provides a globally contiguous data set from which the geology of these regions is evaluated and compared. In addition, high resolution gravity data currently being collected provide a basis to assess the relationship between these uplands and processes in the planet's interior. Studies of the geology of the three largest volcanic highlands (Beta Regio, Atla Regio, Western Eistla Regio) show them to be distinct, having a range of volcanic and tectonic characteristics. In addition to these large areas, a number of smaller uplands are identified and are being analyzed (Bell Regio, Imdr Regio, Dione Regio (Ushas, Innini, and Hathor Montes), and Themis Regio). To understand better the mechanisms by which these volcanic rises form and evolve, we assess their geologic and geophysical characteristics.

Senske, D. A.↗
NASA NTRS1993

Parga and Hecate Chasmata, Venus: Structure, volcanism and models of formation

Linear zones of deformation in the equatorial zone of Venus characterized by extension and volcanism have previously been identified in Pioneer Venus and Arecibo data. Two of these zones, Parga and Hecate Chasmata, are seen in Magellan data to be characterized by systems of fractures along which lie numerous coronae and corona-like features. The relationship between tectonic deformation, corona formation, and volcanism in Parga and Hecate Chasmata is examined using Magellan image and altimetry data. We examine several hypotheses for the origin of these zones, including extension and upwelling and delamination or subduction. In a companion abstract, we examine a section of Hecate Chasma in detail.

Stofan, Ellen R.↗
NASA NTRS1994

Ocean-ice interaction in the marginal ice zone

Ocean ice interaction processes in the Marginal Ice Zone (MIZ) by wind, waves, and mesoscale features, such as upwelling and eddies, are studied using ERS-1 Synthetic Aperture Radar (SAR) images and ocean ice interaction model. A sequence of SAR images of the Chukchi Sea MIZ with three days interval are studied for ice edge advance/retreat. Simultaneous current measurements from the northeast Chukchi Sea as well as the Barrow wind record are used to interpret the MIZ dynamics.

Liu, Antony K.↗
NASA NTRS1993

MOPITT - A gas correlation spectrometer for tropospheric Earth observations

MOPITT (Measurement of Pollution in the Troposphere) is an instrument that will fly on the EOS AM-1 Polar Orbiting Platform, to be launched in mid-1998. The instrument accurately measures the concentration of carbon monoxide (CO) and methane (CH4) gas in the troposphere, using novel and conventional gas correlation spectroscopy techniques. These techniques are used to measure both the column amount and the vertical profile of the two gases in the troposphere. The instrument is nadir viewing and has cross-track scanning ability. It measures upwelling IR radiation through the atmosphere in the 4.6-micron and 2.3-micron bands for CO detection, and 2.2-micron for CH4. The instrument performance is enhanced using Stirling cycle mechanical coolers to maintain the IR detectors at less than 100 K. MOPITT has a total of eight detector channels to measure the total column of atmospheric CH4 gas to a precision of 1 percent, and to measure CO profiles to a precision of 10 percent. The instrument is self-calibrating to ensure data validity using internal targets and an external cold space view.

Drummond, James R.↗
NASA NTRS1994

Vertical distribution of CH4 and N2O over the tropical site Hyderabad

Vertical distribution profiles of N2O and CH4 have been measured from Hyderabad, India using a balloon-borne cryogenic air sampler. The samples have been analyzed using gas chromatographic techniques. Results for two balloon flights made in 1987 and 1990 show effects of tropical characteristics like higher tropopause and upwelling motion due to Hadley circulation. These profiles also exhibit perturbations around 25 km height, which are likely to be due to dynamical effects. A comparison with the SAMS data show that the SAMS values for both these gases are higher by a factor of about 1.5 to 2 around 30 km height.

Lal, Shyam↗
NASA NTRS1994

The 1990 vertical distribution of two important halons (F-12B1 and F-13B1) in the tropics

The first vertical profiles of F-12B1 and F-13B1 had been obtained in the tropical troposphere and stratosphere by us in 1987. The measurement of these substances responsible for almost the entire anthropogenic contribution to the stratospheric BrO(x) budget is important in the tropics, as tropical upwelling provides their injection along with that of other pollutants, into the stratosphere. To ascertain the trends of these distributions and foster the data, the 1987 experiment was repeated in April 1990. Like 1987, the MPAE cryogenic whole air sampler was launched on a balloon from Hyderabad, India (17.5 deg N), and 14 samples were collected between 10 and 35 km altitude. The results obtained by means of GC and GC-MS analyses showed that the atmospheric abundance of both F-12B1 and F-13B1 is increasing at a fast rate, respectively by about 15 percent and 10 percent per year. From 1987 to 1990, F-12B1 and F-13B1 tropospheric mixing ratios have been growing from 1.2 and 1.3 ppt to 1.8 and 1.7 ppt, respectively. The vertical profiles will be discussed.

Singh, O. N.↗
NASA NTRS1994

SeaWiFS technical report series. Volume 21: The heritage of SeaWiFS. A retrospective on the CZCS NIMBUS Experiment Team (NET) program

The Sea-viewing Wide Field-of-view Sensor (SeaWiFS) mission is based on the scientific heritage of the Coastal Zone Color Scanner (CZCS), a proof-of-concept instrument carried on the National Aeronautics and Space Administration (NASA) NIMBUS-7 environmental satellite for the purpose of measuring upwelling radiance from the ocean surface. The CZCS mission provided the first observations of ocean color from space, and over the mission lifetime of 1978-1986, allowed oceanographers an initial opportunity to observe the variable patterns of global biological productivity. One of the key elements of the CZCS mission was the formation of the CZCS NIMBUS Experiment Team (NET), a group of optical physicists and biological oceanographers. The CZCS NET was designated to validate the accuracy of the CZCS radiometric measurements and to connect the instrument's measurements to standard measures of oceanic biological productivity and optical seawater clarity. In the period following the cessation of CZCS observations, some of the insight and experience gained by the CZCS NET activity has dissipated as several proposed follow-on sensors failed to achieve active status. The Sea WiFS mission will be the first dedicated orbital successor to CZCS it in turn precedes observations by the Moderate Resolution Imaging Spectroradiometer (MODIS) of the Earth Observing System (EOS). Since the CZCS NET experience is an important model for Sea WiFS and MODIS surface truth efforts, this document is intended to provide a comprehensive review of the validation of oceanographic data for the first orbital ocean color sensor mission. This document also summarizes the history of the CZCS NET activities. The references listed in the Bibliography are a listing of published scientific research which relied upon the CZCS BET algorithms, or research which was conducted on the basis of CZCS mission elements.

Acker, James G.↗
NASA NTRS1994

Implications of summertime marine stratocumulus on the North American climate

This study focuses on the effects of summertime stratocumulus over the eastern Pacific. This cloud is linked to the semi-permanent sub-tropical highs that dominate the low-level circulation over the Pacific and Atlantic. Subsidence on the eastern flank of these highs creates an inversion based about 800 m above sea level that caps moist air near the surface. This air overlies cool waters driven by upwelling along the coastal regions of North America. Strong surface north-westerlies mix the boundary layer enough to saturate the air just below the capping inversion. Widespread stratocumulus is thus formed. All calculations were carried out using the GENESIS general circulation model that was run at MSFC. Among the more important properties of the model is that it includes radiative forcing due to absorption of solar radiation and the emission of infrared radiation, interactive clouds (both stratocumulus and cumulus types), exchanges of heat and moisture with the lower boundary. Clouds are interactive in the sense that they impact the circulation by modifying the fields of radiative heating and turbulent fluxes of heat and moisture in the boundary layer. In turn, clouds are modified by the winds through the advection of moisture. In order to isolate the effects of mid- and high-latitude stratocumulus, two runs were made with the model: one with and the other without stratocumulus. The runs were made for a year, but with perpetual July conditions, i.e., solar forcing was fixed. The diurnal solar cycle, however, was allowed for. The sea surface temperature distribution was fixed in both runs to represent climatological July conditions. All dependent variables were represented at 12 surfaces of constant sigma = p/p(sub O), where p is pressure and p(sub O) is surface pressure. To facilitate analysis, model output was transformed to constant pressure surfaces. Structures no smaller in size than 7.5 degrees longitude and 4.5 degrees in latitude were resolved. Smaller features of the circulation were parameterized. The model thus captures synoptic- and planetary-scale circulation features.

Clark, John H. E.↗
NASA NTRS1993

Estimation of aerosol optical depth and additional atmospheric parameters for the calculation of apparent reflectance from radiance measured by the Airborne Visible/Infrared Imaging Spectrometer

The Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) measures spatial images of the total upwelling spectral radiance from 400 to 2500 nm through 10 nm spectral channels. Quantitative research and application objectives for surface investigations require inversion of the measured radiance of surface reflectance or surface leaving radiance. To calculate apparent surface reflectance, estimates of atmospheric water vapor abundance, cirrus cloud effects, surface pressure elevation, and aerosol optical depth are required. Algorithms for the estimation of these atmospheric parameters from the AVIRIS data themselves are described. From these atmospheric parameters we show an example of the calculation of apparent surface reflectance from the AVIRIS-measured radiance using a radiative transfer code.

Green, Robert O.↗
NASA NTRS1993

A proposed update to the solar irradiance spectrum used in LOWTRAN and MODTRAN

The calibrated upwelling radiance spectra measured by AVIRIS are increasingly being analyzed with radiative transfer codes. Analysis of AVIRIS data with the LOWTRAN and MODTRAN radiative transfer codes has led to indications of an error in the solar irradiance spectra used by these codes. This paper presents evidence for the error and proposed update to the solar irradiance spectra used by LOWTRAN and MODTRAN.

Green, Robert O.↗
NASA NTRS1993

Airborne imaging of tropospheric emission at millimeter and submillimeter wavelengths

In September 1993, the NASA Millimeter-wave Imaging Radiometer (MIR) flew on board the NASA ER-2 high-altitude aircraft during CAMEX, and obtained the first wideband millimeter- and submillimeter-wavelength images of tropospheric emission. The MIR is a cross-track radiometer with channels at 89, 150, 183 +/- 1, 3, 7, 220, and 325 +/- 1, 3, 8 GHz. This set provides upwelling brightness information at the two strong rotational water vapor lines at 183.310 and 325.153 GHz and three nearby atmospheric transmission windows. The wideband MIR images of convective raincells reveal unique cloud and precipitation mapping capabilities that are not available from lower frequency microwave channels. Comparisons between the 183 and 325 GHz spectra also reveal differential brightness temperature modes that are related to cloud water.

Wang, J. R.↗
NASA NTRS1995

SeaWiFS technical report series. Volume 26: Results of the SeaWiFS Data Analysis Round-Robin, July 1994 (DARR-1994)

The accurate determination of upper ocean apparent optical properties (AOP's) is essential for the vicarious calibration of the sea-viewing wide field-of-view sensor (SeaWiFS) instrument and the validation of the derived data products. To evaluate the role that data analysis methods have upon values of derived AOP's, the first Data Analysis Round-Robin (DARR-94) workshop was sponsored by the SeaWiFS Project during 21-23 July, 1994. The focus of this intercomparison study was the estimation of the downwelling irradiance spectrum just beneath the sea surface, E(sub d)(0(sup -), lambda); the upwelling nadir radiance just beneath the sea surface, L(sub u)(0(sup -), lambda); and the vertical profile of the diffuse attenuation coefficient spectrum, K(sub d)(z, lambda). In the results reported here, different methodologies from four research groups were applied to an identical set of 10 spectroradiometry casts in order to evaluate the degree to which data analysis methods influence AOP estimation, and whether any general improvements can be made. The overall results of DARR-94 are presented in Chapter 1 and the individual methods of the four groups are presented in Chapters 2-5. The DARR-94 results do not show a clear winner among data analysis methods evaluated. It is apparent, however, that some degree of outlier rejection is required in order to accurately estimate L(sub u)(0(sup -), lambda) or E(sub d)(0(sup -), lambda). Furthermore, the calculation, evaluation and exploitation of confidence intervals for the AOP determinations needs to be explored. That is, the SeaWiFS calibration and validation problem should be recast in statistical terms where the in situ AOP values are statistical estimates with known confidence intervals.

Hooker, Stanford B.↗
NASA NTRS1995

Global climatology and variability of potential new production estimated from remote sensing of sea-surface temperature

During this project we have collected numerous shipboard data-bases of oceanic nitrate and silicate versus temperature for both equatorial and coastal upwelling regions. These cruises all have accompanying N-15 measurements of new production. The inverse relationships between nutrients and temperatures have been determined and are being used to obtain surface nutrient fields from sea surface temperatures measured remotely by satellite borne sensors- i.e. AVHRR data from NOAA satellites contained in the MCSST data set for the world ocean provided by the University of Miami. The images and data derived from space in this way show the strong seasonal fluctuations and interannual el Nino fluctuations of the nitrate field. the nitrate data has been used to make estimates of new production for the equatorial pacific which are compared with shipboard measurements when available. The importance of silicate as a nutrient driving new production and the ratio of nitrate to silicate has been discovered to be crucial to better understand the causes of new production variability, so we have added these parameters to our study and have begun to make estimates of these for the equatorial Pacific, derived from the weekly averaged sea surface temperatures (SSTs).

Dugdale, Richard C.↗