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At least 73 records · Page 4

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NASA NTRS1981

Inference of dust opacities for the 1977 Martian great dust storms from Viking Lander 1 pressure data

The tidal heating components for the dusty Martian atmosphere are computed based on dust optical parameters estimated from Viking Lander imaging data, and used to compute the variation of the tidal surface pressure components at the Viking Lander sites as a function of season and the total vertical extinction optical depth of the atmosphere. An atmospheric tidal model is used which is based on the inviscid, hydrostatic primitive equations linearized about a motionless basic state the temperature of which varies only with height, and the profiles of the tidal forcing components are computed using a delta-Eddington approximation to the radiative transfer equations. Comparison of the model results with the observed variations of surface pressure and overhead dust opacity at the Viking Lander 1 site reveal that the dust opacities and optical parameters derived from imaging data are roughly representative of the global dust haze necessary to reproduce the observed surface pressure amplitudes, with the exception of the model-inferred asymmetry parameter, which is smaller during the onset of a great storm. The observed preferential enhancement of the semidiurnal tide with respect to the diurnal tide during dust storm onset is shown to be due primarily to the elevation of the tidal heating source in a very dusty atmosphere.

Zurek, R. W.↗

NASA NTRS1982

Sulfuric acid vapor and other cloud-related gases in the Venus atmosphere - Abundances inferred from observed radio opacity

It is suggested that the absorbing characteristics of sulfuric acid vapor appear to reconcile what had been thought to be an inconsistency among measurements and deductions regarding the constituents of the Venus atmosphere and radio occultation, radar reflection, and radio emission measurements of its opacity. Laboratory measurements of sulfuric acid, sulfur dioxide, water vapor, and carbon dioxide are used to model relative contributions to opacity as a function of height in a way that is consistent with observations of the constituents and absorbing properties of the atmosphere. It is concluded that sulfuric acid vapor is likely to be the principal microwave absorber in the 30-50 km altitude range of the middle atmosphere of Venus.

Steffes, P. G.↗

NASA NTRS1983

The effect of heavy element opacity on the structure of H II regions

In the modeling of H II regions, there generally has been universal dismissal of opacity due to elements other than H and He. It is found that in certain cases, the trace elements do contribute substantially to the opacity, modifying the ionization equilibrium directly and having feedback on the thermal structure. When important, this effect will reduce the volume where some lines from high ionization states are produced and will decouple transition zones that have traditionally been assumed to be coextensive. The possible reconciliation of some recent problems where observations and theory appeared to be inconsistent with one another are discussed.

Rubin, R. H.↗

NASA NTRS1984

Soft X-ray opacity in hot and photoionized gases

New calculations are presented of the soft X-ray opacity of gas having cosmic elemental abundances, but in a variety of ionization states. Coronal ionization equilibrium is considered over the range of temperatures 10 to the 4th to 3 x 10 to the 8th K and photoionization equilibrium with power-law and thermal bremsstrahlung spectra over a wide range of ionization parameters. The results are displayed in illustrative figures. For temperatures and ionization parameters commonly found in astrophysical plasmas, the opacity (particularly to photons of equal to or less than 1 keV) can be diminished by substantial factors relative to the state in which all atoms are neutral. As the dominant ionization stages change, the positions and contrasts of prominent K-edges also change.

Krolik, J. H.↗

NASA NTRS1986

Molecular clouds in galaxies with different Z - Fragmentation of diffuse clouds driven by opacity

Molecular clouds are formed from diffuse interstellar clouds when the external ultraviolet radiation field is prevented from penetrating into the cloud. The opacity is provided mainly by dust grains and the required column density to the cloud center is larger than about 5 x 10 to the 20th (solar Z/Z)/sq cm. This high-opacity criterion could have a significant impact on the radial trends observed in spiral galaxies, and on the distinctions between spiral and dwarf irregular galaxies.

Franco, Jose↗

NASA NTRS1988

Constraints on the microwave opacity of gaseous methane and water vapor in the Jovian atmosphere

Gaseous NH3's microwave absorption in the Jovian atmosphere appears too great to be due to a solar abundance of this gas. The additional capacity of microwave absorption is presently sought in measurements of the microwave absorption of CH4 and H2O under simulated Jovian conditions at 2.25 GHz, 8.5 GHz, 21.7 GHz; due to large error bars, measurements represent upper limits on the microwave opacity generated by H2O and CH4. The results obtained are consistent with theoretical expressions for microwave opacity in a Jovian atmosphere at the specified frequencies. The presence of an NH3 abundance exceeding the solar level by a factor of 1.5 is indicated by test results.

Jenkins, Jon M.↗

NASA NTRS1990

Observation of a strong inverse temperature dependence for the opacity of atmospheric water vapor in the mm continuum near 280 GHz

Results are presented of the field measurements of atmospheric opacity at 278 GHz (9.3/cm) conducted at the McMurdo Station (Antarctica) during the austral springs of 1986 and 1987, in conjunction with balloon measurements of water vapor profile and total column density, showing a strong inverse temperature dependence when normalized to precipitable water vapor. The value of measured opacity per mm of precipitable water vapor (PWV) is roughly two times greater at -35 C than at -10 C and three times greater than measurements at +25 C reported by Zammit and Ade (1981). Various theories proposed to explain excess absorption in continuum regions are reviewed.

Emmons, Louisa K.↗

NASA NTRS1991

Modeling of Jupiter's millimeter wave emission utilizing laboratory measurements of ammonia (NH3) opacity

The techniques used to make laboratory measurements of the millimeter wave opacity from gaseous ammonia under simulated Jovian conditions are described. The results are applied to a radiative transfer model, which is used to compute Jupiter's millimeter wavelength emission. The absorptivity of gaseous NH3 is measured to reduce one of the largest uncertainties in modeling Jupiter's millimeter wave emission. Several other millimeter wave opacity sources are examined. New expressions are given for computing the absorptivity of NH3, H2O, cloud condensates, and pressure-induced absorption. Jupiter's reliable millimeter wavelength observations are compared with synthetic emission spectra which utilize these new absorption expressions.

Joiner, Joanna↗

NASA NTRS1992

Mars dust and cloud opacities and scattering properties

We have recently completed an analysis of the visible emission-phase function (EPF) sequences obtained with the solar-band channel of the Infrared Thermal Mapping (IRTM) instrument onboard the two Viking Orbiters. Roughly 100 of these EPF sequences were gathered during the 1977-1980 period, in which the total broadband (.3-3.0 microns) reflectances of the atmosphere/surface above specific locations on Mars were measured versus emission angle as the spacecraft passed overhead. A multiple scattering radiative transfer program was employed to model the EPF observations in terms of the optical depths of dust/clouds, their single scattering albedos and phase functions, and the Lambert albedos and phase coefficient of the underlying surfaces. Due to the predominance of atmospheric scattering at large atmospheric pathlengths and/or large dust opacities, we were able to obtain strong constraints on the scattering properties of dust/clouds and their opacities for a wide range of latitudes, longitudes, and seasons on Mars.

Clancy, R. T.↗

NASA NTRS1992

New dust opacity maps from Viking IR thermal mapper data

Mapping of dust opacity of the Martian atmosphere, using the silicate-induced absorption of 9 micron radiation, was performed with the Viking Infrared Thermal Mapper (IRTM) data for several local dust storms and in a global sense. We present here the first results from an effort to extend the earlier mapping work to the period of the 1977b major storm, and to concentrate attention on the details of opacity behavior during the initial phases of the 1977a and b storms.

Martin, T. Z.↗

NASA NTRS1991

Metal opacities and convective core overshooting in Population I stars

New stellar opacities computed by Rogers and Iglesias are found to lead to revised evolutionary models for stars of intermediate-to-high mass that show remarkably close agreement with observed stars, under otherwise standard physical assumptions. Most or all of the earlier discrepancies, except for the problem of predicted apsidal motion constants that are perhaps still too large, appear to have been resolved by using these new opacities with their increased metal line contribution. Thus, the implied interior metallicity of massive stars in the solar neighborhood now becomes 'normal'. Furthermore, the effective distance of convective overshooting beyond the classical convective core boundary is confirmed to be small and probably less than 0.2 times the local pressure scale height.

Stothers, Richard B.↗

NASA NTRS1993

New dust opacity mapping from Viking Infrared Thermal Mapper data

Global dust opacity mapping for Mars has been carried forward using the approach described by Martin (1986) for Viking IR Thermal Mapper data. New maps are presented for the period from the beginning of Viking observations, until Ls 210 deg in 1979 (1.36 Mars years). This range includes the second and more extensive planet-encircling dust storm observed by Viking, known as storm 1977b. Improvements in approach result in greater time resolution and smaller noise than in the earlier work. A strong local storm event filled the Hellas basin at Ls 170 deg, prior to the 1977a storm. Dust is retained in equatorial regions following the 1977b storm far longer than in mid-latitudes. Minor dust events appear to raise the opacity in northern high latitudes during northern spring. Additional mapping with high time resolution has been done for the periods of time near the major storm origins in order to search for clues to the mechanism of storm initiation. The first evidence of the start of the 1977b storm is pushed back to Ls 274.2 deg, preceding signs of the storm in images by about 15 hours.

Martin, Terry Z.↗

NASA NTRS1993

Temporal and spatial mapping of surface albedo and atmospheric dust opacity on Mars

The Mariner 9 and Viking provided abundant evidence that eolian processes are active over much of the surface of Mars. Past studies have demonstrated that variations in regional albedo and wind-streak patterns are indicative of sediment transport through a region, while thermal inertia data (derived from the Viking Infrared Thermal Mapper (IRTM) dataset) are indicative of the degree of surface mantling by dust deposits. The visual and thermal data are therefore diagnostic of whether net erosion or deposition of dust-storm fallout is taking place currently and whether such processes have been active in a region over the long term. These previous investigations, however, have not attempted to correct for the effects of atmospheric dust loading on observations of the martian surface, so quantitative studies of current sediment transport rates have included large errors due to uncertainty in the magnitude of this 'atmospheric component' of the observations. We have developed a radiative transfer model that allows the atmospheric dust opacity to be determined from IRTM thermal observations. Corrections for the effects of atmospheric dust loading on observations of surface albedo can also be modeled. This approach to determining 'dust-corrected surface albedo' incorporates the atmospheric dust opacity, the single-scattering albedo and particle phase function of atmospheric dust, and the bidirectional reflectance of the surface, and it accounts for variable lighting and viewing geometry.

Lee, S. W.↗

NASA NTRS1993

Temporal and spatial mapping of atmospheric dust opacity and surface albedo on Mars

The Mariner 9 and Viking missions provided abundant evidence that eolian processes are active over much of the surface of Mars. Past studies have demonstrated that variations in regional albedo and wind streak patterns are indicative of sediment transport through a region, while thermal inertia data (derived from the Viking Infrared Thermal Mapper (IRTM) datasets) are indicative of the degree of surface mantling by dust deposits. We are making use of the method developed by T. Z. Martin to determine dust opacity from IRTM thermal observations. We have developed a radiative transfer model that allows corrections for the effects of atmospheric dust loading on observations of surface albedo to be made. This approach to determining 'dust-corrected surface albedo' incorporates the atmospheric dust opacity, the single-scattering albedo and particle phase function of atmospheric dust, the bidirectional reflectance of the surface, and accounts for variable lighting and viewing geometry.

Lee, S. W.↗

NASA NTRS1994

More missing stellar opacity?

Observational data for Population I stars have shown that blue loops on the Hertzsprung-Russell (H-R) diagram form for stellar masses as low as approximately 4 solar mass. However, current state-of-the-art stellar models, unlike the older ones that were based on smaller opacities, fail to loop out of the red-giant region during core helium burning for masses less than 7 solar mass. A possible explanation is that the currently used Livermore opacities need to be further increased, by at least 70%, at temperatures characteristic of the base of the outer convection zone, around 1 x 10(exp 6) K. Indeed, no other suggested remedy seems to yield a blue loop at the lowest observed loop luminosities.

Stothers, Richard B.↗

NASA NTRS1999

Water Ice Cloud Opacities and Temperatures Derived from the Viking IRTM Data Set

The degree to which water ice clouds play a role in the Mars climate is unknown. Latent heating of water ice clouds is small and since most hazes appeared to be thin (tau less than or = 1) their radiative effects have been neglected. Condensation likely limits the vertical extent of water vapor in the water column and a lowering of the condensation altitude, as seen in the northern spring and summer, could increase the seasonal exchange of water between the atmosphere and the surface. It has been suggested that water ice cloud formation is more frequent and widespread in the aphelic hemisphere (currently the northern). This may limit water to the northern hemisphere through greater exchange with the regolith and through restricted southward transport of water vapor by the Mars Hadley circulation. In addition, it has been suggested that water ice cloud formation also controls the vertical distribution of atmospheric dust in some seasons. This scavenging of dust may Continuing from the IRTM cloud maps, derived cloud opacities and cloud temperatures for several locations and seasons will be presented. Sensitivities to cloud particle sizes, surface temperature, and dust opacity will be discussed.

TamppariL. K.↗

NASA NTRS2004

Research on Spectroscopy, Opacity, and Atmospheres

I propose to continue providing observers with basic data for interpreting spectra from stars, novas, supernovas, clusters, and galaxies. These data will include allowed forbidden line lists both laboratory and computed, for the first five to ten ions of all atoms and for all relevant diatomic molecules. I will eventually expend to all ions of the first thirty elements to treat far UV end X-ray spectra, and for envelope opacities. I also include triatomic molecules providing by other researchers. I have made CDs with Partridge and Schwanke's water data for work on M stars.The luna data also serve as input to my model atmosphere and synthesis programs that generated energy distributions, photometry, limb darkening, and spectra that can be used for planning observations and for fitting observed spectra. The spectrum synthesis programs produce detailed plots with the line identified. Grids of stellar spectra can be used for radial velocity-, rotation-, or abundance templates and for population synthesis. I am fitting spectra of bright stars to test the data and to produce atlases to guide observer. For each star the whole spectrum is computed from the UV to the far IR. The line data, opacities, models, spectra, and programs are freely distributed on CDs and on my web site and represent a unique resource for many NASA programs.

Oliversen, Ronald↗

NASA NTRS2005

The Effects of Atmospheric Opacity on the Seasonal Variation of Martian Surface Temperature

The daily and seasonal variation of surface temperature is a central element in the description of martian climate. Surface thermal inertia and albedo are critical boundary inputs for simulating surface temperature in Mars general circulation models (MGCMs). Thermal inertia (TI) is also of intrinsic interest as it may be related to regolith properties such as particle size and surface character and so high spatial resolution is desirable. The recent mapping of TI at very high (0.25 deg) spatial resolution was achieved by fitting a thermal model to surface temperature observations obtained over a broad range of several martian years. However, varying atmospheric opacity (dust and water ice clouds) can significantly influence the estimated TI field and this effect was not fully compensated for. Opacity leads to an increase in morning temperature and a decrease in afternoon temperature, thus increasing the apparent thermal inertia.

Wilson, R. J.↗