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At least 631 records · Page 35
All sourcesNASA NTRSDOE OSTI
NASA NTRS1972

Search for extinct natural radioactivity of Pb205 via thallium-isotope anomalies in chondrites and lunar soil.

Thallium and Pb204 contents were determined by stable-isotope-dilution analysis in 16 chondrites, one achondrite, and Apollo 11 and 12 lunar fines. Meteoritic thallium contents vary over a large range, 0.02 to 100 ppb, corresponding to the fact that thallium is a highly fractionated volatile element. Lunar thallium contents are less than 5 ppb. The Tl205/Tl203 ratio was determined in most of the samples, with precision ranging from 0.03% to several percent depending mainly on the amount of thallium present. No variations from the terrestrial ratio were observed. The chondritic isochron slope for Pb205 (13.8-m.y. half-life) is less than or equal to 0.00009 (99% confidence level), corresponding to an interval of at least 60 m.y. and possibly exceeding 120 m.y. between the termination of s-process nucleosynthesis and the lead-thallium fractionations.

Huey, J. M.↗
NASA NTRS1972

Structure in the interstellar extinction curve.

Using photographic techniques, it was attempted to obtain spectra of the interstellar material at a resolution of about 5 A. Three classes of structures are discussed. It is pointed out that the optical observations can be extended into the UV with a resolution of 0.4 A using the OAO-C telescope. The far UV observations would make it possible to conduct an extended search for new diffuse features, in order to evaluate the significance of the apparent absence of any features short of 4,430 A.

York, D. G.↗
NASA NTRS1973

Investigations of atmospheric extinction using direct solar radiation measurements made with a multiple wavelength radiometer.

A multiple wavelength solar radiometer designed for the purpose of measuring atmospheric optical depth at discrete wavelengths through the visible region is described. Experimental techniques, including sample observations, are presented for obtaining atmospheric optical depth from radiometer measurements. These techniques apply for conditions where the optical depth is either temporally variant or invariant during the course of a day. The influence of the aerosol size distribution on optical depth is investigated. Theoretical calculations of the wavelength dependency of the aerosol optical depth contribution are presented for several representative aerosol size distributions. Methods are also presented for estimating the aerosol size distribution and aerosol mass loading from multiwavelength optical depth measurements.

Shaw, G. E.↗
NASA NTRS1975

Extinct radioactivities - Trapped residuals of presolar grains

The xenon anomalies trapped in meteorites and the moon may have first been trapped in circumstellar grains formed in or outside of postexplosive stars. In that case, the initial solar nebula need not have contained most of their radioactive progenitors, and this would necessitate major revision of the history of solar system formation.

Clayton, D. D.↗
NASA NTRS1975

Analysis of organic matter in sediments and meteorites and paleochemical studies of extinct and contemporary life forms

The insoluble organic materials present in the algal mats at Laguna Mormona, Baja California were studied. A series of six identical sediments collected from Mono lake which were stored under different conditions was investigated to see if any changes are observed in the lipid distribution patterns as a result of differences in sample storage conditions. Bacteria strains from Mono Lake sediments were cultured in bulk quantities and the sterol fractions from them were isolated and analyzed. Results add further support to the utility of the sterols as a chemotaxonomical tool in distinguishing and classifying these bacteria.

Calvin, M.↗
NASA NTRS1977

Application of modified Twomey techniques to invert lidar angular scatter and solar extinction data for determining aerosol size distributions

Polarization properties of the angularly scattered laser light from a volume of air are used to determine the size distribution of the aerosol particles within the volume by the use of appropriate inversion techniques. Similar techniques are employed to determine a mean size distribution of the particulates within a vertical column through the atmosphere from determinations of the aerosol optical depth as a function of wavelength. In both of these examples, a modification of an inversion technique originally described by Twomey has been employed. Details of this method are presented as well as results from actual measurements employing bistatic lidar and solar radiometer.

Herman, B. M.↗
NASA NTRS1978

Retrieving the complex refractive index of atmospheric aerosols from ratios of solar spectral extinction measurements

The technique proposed by Fymat (1976) for retrieving the complex refractive index of atmospheric aerosols using narrowband spectral transmission ratios, taken within an overall narrow spectral interval, is investigated in the case of modelled polydispersions of rural, maritime-continental, maritime-sea spray and meteoric dust aerosols. It is confirmed that for not too broad size distributions most of the information comes from a narrow size range of 'active' aerosols so that, under these circumstances, the refractive index components can indeed be retrieved essentially independently of the size distribution. For 0.1% accurate data in three colors, the technique can provide the real and imaginary components of the index respectively within 0.07% and 0.3% accuracy.

Fymat, A. L.↗
NASA NTRS1979

Interstellar extinction and ultraviolet flux distribution of Scorpius X-1

Measurements of absorption at 2200 A for Sco X-1 indicate an E(B-V) of approximately 0.28. The distance to Sco X-1 is estimated to be 350 pc while the X-ray luminosity is thought to be 5 times 10 to the 36th ergs/sec. It is found that the de-reddened ultraviolet flux distribution closely resembles a blackbody at 26,000 K. Brightness variations and their relation to flickering phenomena are considered.

Wu, C.-C.↗
NASA NTRS1979

Emission and extinction of ground and vapor-condensed silicates from 4 to 14 microns and the 10 micron silicate feature

Emission and absorption spectra from 4 to 14 microns of ground and laser-vaporized olivine and enstatite silicates are compared with the 10-micron emission feature of the Orion Trapezium. The agreement in band center and shape between the amorphous laser-vaporized olivine sample and the Trapezium feature suggests that amorphous silicate grains of approximately olivine composition may be a major constituent of interstellar dust. Differences between the emission and absorption spectral profiles (absorption plus scattering) show characteristics that could be used as a sensitive probe of the morphology of interstellar grain systems when high signal-to-noise ratio (30-100) observational spectra become available.

Stephens, J. R.↗