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At least 541 records · Page 30
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NASA NTRS1988

Antarctic measurements of ozone, water vapor, and aerosol extinction by Sage 2 in the spring of 1987

Recent measurements of ozone, water vapor, and aerosol extinction from the spring of 1987 are presented and compared to 1985 and 1986. The observed changes to variations in meteorological conditions in the vortex for these three years are noted. March ozone data at similar latitudes for these three years will be used to investigate coupling between severity of the springtime depletion and early fall values. Researchers also investigate correlations between the measured species of water vapor, ozone, and aerosols throughout the vortex region.

Larsen, J. C.↗
NASA NTRS1988

Possible world-wide middle miocene iridium anomaly and its relationship to periodicity of impacts and extinctions

In a study of one million years of Middle Miocene sediment deposition in ODP Hole 689B in the Weddell Sea near Antarctica, a single iridium (Ir) anomaly of 44 (+ or - 10) x 10 to the 12th gram Ir per gram rock (ppt) was observed in core 6H, section 3, 50 to 60 cm, after background contributions associated with manganese precipitates and clay are subtracted. The ODP Hole 689B is 10,000 km away from another site, DSDP Hole 588B in the Tasman Sea north of New Zealand, where a single Ir anomaly of 144 + or - 7 ppt over a background of 11 ppt was found in an earlier study of 3 million years of deposition. From chemical measurements the latter deposition was thought to be impact-related. Ir measurements were made, following neutron activation, with the Iridium Coincidence Spectrometer. The age vs depth calibration curves given in the DSDP and ODP preliminary reports indicate the ages of the Iranomalies are identical, 11.7 million years, but the absolute and relative uncertainties in the curves are not known. Based on the newest age data the age estimate is 10 million years. As the Ir was deposited at the two sites at about the same time and they are one quarter of the way around the world from each other it seems likely that the deposition was world-wide. The impact of a large asteroid or comet could produce the wide distribution, and this data is supportive of the impact relationship deduced for Deep Sea Drilling Project (DSDP) 588B from the chemical evidence. If the surface densities of Ir at the two sites are representative of the world-wide average, the diameter of a Cl type asteroid containing the necessary Ir would be 3 + or - 1 km, which is large enough to cause world-wide darkness and hence extinctions although the latter point is disputed.

Asaro, F.↗
NASA NTRS1988

Impact and extinction signatures in complete Cretaceous-Tertiary (K-T) boundary sections

The Zumaya, Caravaca and Agost sections in Spain, the El Kef section in Tunisia and the Negev (Nahal Avdat) sections in Israel are among the most continuous, expanded and complete K-T boundary sections. The distribution patterns of the planktic faunas were quantitatively analyzed in closely spaced samples across the K-T boundary in these sections, in conjuction with the geochemistry, stable isotopes, mineralogy and magnetostratigraphy. Three hundred foraminiferal specimens were randomly selected and determined. Reliable estimates for the foraminiferal productivity changes across the K-T boundary and for the 1 to 2 Ma interval preceding the K-T boundary were made from the numbers of individuals/gram of sediment corrected for the sedimentation rates (calculated from magnetic reversals and lithology). No gradual or stepwise extinction is seen below the K-T boundary nor any productivity decrease. Stable isotope analyses show a warming just after deposition of the ejecta layer, not cooling as predicted by nuclear winter scenarios, although the duration of such cooling may be too short to be observed even in these complete sections. Low REE values and cpx spherules with quench textures idential to quench-textures in diagenetically altered spherules, strongly indicate an oceanic site of (one of) the impactor(s).

Smit, J.↗
NASA NTRS1988

Some volcanologic aspects of Columbia River basalt volcanism relevant to the extinction controversy

The Columbia River Basalt Group is the youngest and most thoroughly studied flood-basalt province known; information about it should be relevant to questions about the possible relation of flood-basalt volcanism to mass extinctions. The group has a total volume of about 174,000 cu km and covers an area of about 164,000 sq km. It was erupted between 17.5 and 6 Ma, as measured by K-Ar and Ar-40/Ar-39 dates. Early eruptions formed the Imnaha Basalt. More than 85 percent of the group was produced during a 1.5 my period between 17 and 15.5 Ma, forming the Grande Ronde and greatly subordinate Picture Gorge Basalts. Later flows formed the Wanapum Basalt, which includes the well-known Roza Member, and the Saddle Mountains Basalt. Linear vent systems for many of the flows are known and are located only in the eastern third of the Columbia Plateau. No systematic migration of vents occurred throughout the 11.5 my period of activity; this and other considerations make it unlikely that the province is related to a hot spot. Model calculations based on observations that little cooling occurred during flow of hundreds of kilometers suggest eruption and emplacement durations of a few days. Some voluminous flows occur in all formations, but most such flows apparently were erupted during Grande Ronde time. The eruption and emplacement of more than 1,000 cu km of 1100 C basaltic lava on the surface within several days doubtless had at least local meteorologic effects. Whether the effects were broader can at present only be hypothesized. Grande Ronde Basalt and Picture Gorge Basalts contain moderately common but thin sedimentary interbeds between flows, whereas earlier and later formations contain numerous, locally thick sediment accumulations. Volcaniclastic debris derived from extra-plateau sources commonly occurs in the testbeds.

Swanson, Donald A.↗
NASA NTRS1988

Heavy metal toxicity as a kill mechanism in impact caused mass extinctions

Heavy metals that are known to be toxic exist in carbonaceous chrondrites at abundances considerably in excess to that of the terrestrial crust. An impactor of relatively undifferentiated cosmic matter would inject into the terrestrial environment large quantities of toxic elements. The abundances of toxic metals found in the Allende CV carbonaceous chondrite and the ratio of meteoritic abundance to crustal abundance are: Cr, 3630 PPM, 30X; Co, 662 PPM, 23X; ni, 13300 PPm, 134X; se, 8.2 PPM, 164X; Os, 0.828 PPM, 166X. The resulting areal density for global dispersal of impactor derived heavy metals and their dilution with terrestrial ejecta are important factors in the determination of the significance of impactor heavy metal toxicity as a kill mechanism in impact caused mass extinctions. A 10 km-diameter asteroid having a density of 3 gram per cu cm would yield a global areal density of impact dispersed chondritic material of 3 kg per square meter. The present areal density of living matter on the terrestrial land surface is 1 kg per square meter. Dilution of impactor material with terrestrial ejecta is determined by energetics, with the mass of ejecta estimated to be in the range of 10 to 100 times that of the mass of the impactor. Because a pelagic impact would be the most likely case, the result would be a heavy metal rainout.

Wdowiak, T. J.↗
NASA NTRS1988

The Cretaceous-Tertiary boundary marine extinction and global primary productivity collapse

The extinction of marine phyto-and zoo-plankton across the K-T boundary has been well documented. Such an event may have resulted in decreased photosynthetic fixation of carbon in surface waters and a collapse of the food chain in the marine biosphere. Because the vertical and horizontal distribution of the carbon isotopic composition of total dissolved carton (TDC) in the modern ocean is controlled by the transfer of organic carbon from the surface to deep reservoirs, it follows that a major disruption of the marine biosphere would have had a major effect on the distribution of carbon isotopes in the ocean. Negative carbon isotope excursions have been identified at many marine K-T boundary sequences worldwide and are interpreted as a signal of decreased oceanic primary productivity. However, the magnitude, duration and consequences of this productivity crisis have been poorly constrained. On the basis of planktonic and benthic calcareous microfossil carbon isotope and other geochemical data from DSDP Site 577 located on the Shatsky Rise in the north-central Pacific, as well as other sites, researchers have been able to provide a reasonable estimate of the duration and magnitude of this event.

Zachos, J. C.↗
NASA NTRS1989

Impact production of CO2 by the Cretaceous/Tertiary extinction bolide and the resultant heating of the earth

Various observations and data demonstrate that sea level at the end of the Cretaceous was 150-200 m higher than at present, suggesting the possibility that the extinction bolide struck a shallow marine carbonate-rich sedimentary section. It is shown here that the impact of such a bolide (about 5 km in radius) onto a carbonate-rich terrane would increase the CO2 content of the atmosphere by a factor of two to ten. Additional dissolution of CO2 from the ocean's photic zone could release much larger quantities of CO2. The impact-induced release of CO2, by itself, would enhance atmospheric greenhouse heating and give rise to a worldwide increase in temperature from 2 K to 10 K for periods of 10,000 to 100,000 years.

O'Keefe, John D.↗
NASA NTRS1989

A Theoretical Study of Propagation and Extinction of Nonsteady Spherical Flame Fronts

Freely propagating expanding spherical flame fronts are studied analytically using the method of activation energy asymptotics. Effects of Lewis number (Le), curvature, heat loss, and thermal expansion on propagation rates and extinction conditions are examined. The behavior of spherical flame fronts is found to be determined primarily by the values of Le and a heat-loss parameter. Results are compared to recent experimental observations of spherical flame fronts in mixtures near the flammability limits; for Le less than 1, it is found that many of the features of these experimental results are described by the analysis presented here. For Le greater than 1, however, unrealistic dynamic behavior is predicted due to the nature of diffusive-thermal instabilities under these conditions.

Ronney, P. D.↗
NASA NTRS1989

Bio-markers and the search for extinct life on Mars

In order to predict what biomarkers could be used on Mars, several biomarkers, or key signatures, of extinct life on earth are identified. Some of these biomarkers which may be applicable to Mars include reduced carbon and nitrogen compounds, CO3(2-), SO4(2-), NO3(-), Mg, Mn, Fe, and the isotopic ratios of C, N, and S. It is suggested that a fully equipped Mars rover might be able to perform analyses to measure most of these biomarkers while on the Martian surface.

Schwartz, D. E.↗
NASA NTRS1989

The abundances of atomic carbon and carbon monoxide compared with visual extinction in the Ophiuchus molecular cloud complex

Emission from the 492 GHz lines of C I have been observed toward six positions in the Ophiuchus molecular cloud complex for which accurate visual extinctions are available. The column density of C I increases with A(V) to greater than 2 x 10 to the 17th/sq cm at 100 mag, the column-averaged fractional abundance reaches a peak of about 2.2 x 10 to the -5th for A(V) in the range 4-11 mag and the column-averaged abundance ratio of C I to CO decreases with A(V) from about 1 at 2 mag to greater that about 0.03 at 100 mag. These results imply that, while C I is not the primary reservoir of gaseous carbon even at cloud edges, its fractional abundance remains high for at least 10 mag into the cloud and may be significant at even greater depths.

Frerking, Margaret A.↗
NASA NTRS1989

Far-infrared emission in the Rho Ophiuchi region - A comparison with molecular gas emission and visual extinction

The dust characteristics of a 40 arcmin x 40 arcmin section of the Rho Ophiuchi molecular clouds complex revealed by the IRAS satellite are compared with both molecular line and deep star count data for the region. The data reveal the cloud to be clearly visible at 12 and 25 microns, as well as at 60 and 100 microns. Modeling the infrared emission from the cloud as being due to two populations of dust grains, the short-wavelength emission is found not to reflect the internal structure of the molecular cloud. The 60 micron opacity determined using a single grain emission model correlates reasonably well with (C-13)O column density and visual extinction up to roughly 5 mag. The cloud edges appear to be heated by the interstellar radiation field. The far-infrared luminosity to mass ratio for the region is determined to be 0.9 solar luminosity/solar mass, a value much smaller than the average ratio for the inner Galaxy.

Jarrett, T. H.↗
NASA NTRS1989

Radiative transfer theory for inhomogeneous media with random extinction and scattering coefficients

The small-angle scattering approximation of the scalar radiative transfer equation (RTE) is examined for the case where the extinction and scattering coefficients have a component that is a deterministic function of position along the propagation path and a component that is a random function of position transverse to the propagation direction. It is found that the resulting stochastic RTE can be reduced to a system of two stochastic integrodifferential equations for the average and fluctuating components of the radiant intensity. Two transfer equations are obtained describing the average radiant intensity and the spatial correlation function of the intensity fluctuations. The average intensity equation is then solved and applied to a simple propagation scenario. An approximate solution is also derived for the equation giving the correlation function. The developed equations can be applied to problems involving short wavelength electromagnetic wave propagation through media possessing the variable characteristics of turbulence and turbidity, such as plasmas, the atmosphere, and the ocean.

Manning, Robert M.↗
NASA NTRS1989

Effects of H2O, CO2, and N2 air contaminants on critical airside strain rates for extinction of hydrogen-air counterflow diffusion flames

Dish-shaped counterflow diffusion flames centered by opposing laminar jets of H2 and clean and contaminant O2/N2 mixtures in an argon bath at 1 atm were used to study the effects of contaminants on critical airside strain. The jet velocities for both flame extinction and restoration are found for a wide range of contaminant and O2 concentrations in the air jet. The tests are also conducted for a variety of input H2 concentrations. The results are compared with those from several other studies.

Pellett, G. L.↗
NASA NTRS1990

Images in the rocket ultraviolet - Star formation and extinction in the M51 system

UV images of M51 with up to 15 arcsec resolution were obtained by a rocket-borne telescope. The bandwidth was 970 A with maximum response at 2650 A. The two most prominent features of M51 in the UV are the bright central region and a region of intense star formation about 2.5 NE of the nucleus toward the companion NGC 5195. This complex is the source of 20 percent of the total UV flux in M51. The companion is much less prominent in the UV than in optical bands. Spiral arms show much higher contrast in UV and U bands than in the R band. The nuclear region in the UV band shows clumped emission from probable star-forming regions, possibly associated with the inner Lindblad resonance. These regions are probably the UV counterparts of FIR sources discovered in an EW scan across the nucleus. UV/U colors of most bright H II regions with known extinction are consistent with O star spectra reddened by amounts estimated from radio and Balmer line measurements. However, some of the brightest H II regions have redder UV/U colors, probably indicating the presence of cooler stars which contribute in the U band. CO and IR observations indicate that the companion NGC 5195 may be a reddened starburst in spite of its faintness in the UV.

Bohlin, Ralph C.↗
NASA NTRS1990

Neutral atomic absorption lines and far-UV extinction: Possible implications for depletions and grain parameters

Researchers examine nine lines of sight within the Galaxy and one in the Large Magellanic Cloud (LMC) for which data on both neutral atomic absorption lines (Snow 1984; White 1986; Welty, Hobbs, and York 1989) and far UV extinction (Bless and Savage 1972; Jenkins, Savage, and Spitzer 1986) are available, in order to test the assumption that variations in gamma/alpha will cancel in taking ratios of the ionization balance equation, and to try to determine to what extent that assumption has affected the aforementioned studies of depletions and grain properties.

Welty, Daniel E.↗
NASA NTRS1989

The detection of a broad interstellar extinction feature near 1700 A

A statistical examination of 126 extinction curves has revealed the presence of a second broad absorption feature similar in nature to the 2200 A feature. The feature is centered on wavelength 1706 A, has a full half-width of 350 A, and a mean central height of 0.21 magnitudes. The strength of the feature increases with E(B-V) supporting an interstellar origin, and on average it is 18 times weaker than the 2200 A feature.

Carnochan, David J.↗
NASA NTRS1991

Near-infrared observations of primitive asteroids and a possible extinct comet

Photometry was obtained simultaneously in the near-infrared and in the visible, of several D-type asteroids, thought to be composed of the most primordial material in the asteroid belt. These objects have been previously classified as D-type based on their low albedo and broadband colors in the visible region of the spectrum. One of the objects observed is 944 Hidalgo, which is possibly an extinct comet, based on its orbital elements. The photometry further constrains the surface mineralogy and is a test of whether differences exist between these objects in different regions of the asteroid belt. Refining the knowledge of the surface composition of these objects will allow a better measure of the amount of thermal processing which has taken place in these so-called primitive asteroids. Spectral slope is assumed to be sensitive to surface composition. There is some variability in spectral slope among those objects classified as D-type, with indications of increasing slope into the near-infrared with heliocentric distance. The spectral features are quite subtle as expected for these low albedo objects, but there is possible evidence of subtle features in some of the D-type asteroids and possibly some active comets. Further analysis of these spectra and the implications for the outer region of the asteroid belt is presented.

Howell, E. S.↗
NASA NTRS1991

Counterflow diffusion flames of hydrogen, and hydrogen plus methane, ethylene, propane, and silane vs. air - Strain rates at extinction

Five coaxial tubular opposed jet burners (OJBs) with tube diameter D(T) of 1.8-10 mm and 5 mm conical nozzles were used to form dish-shaped counterflow diffusion flames centered by opposing laminar jets of nitrogen and hydrocarbon-diluted H2 versus air in an argon-purged chamber at 1 atm. Area-averaged air jet velocities at blowoff of the central flame, U(air), characterized extinction of the airside flame as functions of input H2 concentration on the fuelside. A master plot of extensive U(air) data at blowoff versus D(T) shows that U(air) varies linearly with D(T). This and other data sets are used to find that nozzle OJB results for U(air)/diameter average 4.24 + or - 0.28 times larger than tubular OJB results for the same fuel compositions. Critical radial velocity gradients consistent with one-dimensional stagnation point boundary theory and with plug flow inputs are estimated. The results compare favorably with published numerical results based only on potential flow.

Pellett, G. L.↗