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NASA NTRS1978

Forwardscattering corrections for optical extinction measurements in aerosol media. II - Polydispersions

The paper presents a parametric study of the forwardscattering corrections for experimentally measured optical extinction coefficients in polydisperse particulate media, since some forward scattered light invariably enters, along with the direct beam, into the finite aperture of the detector. Forwardscattering corrections are computed by two methods: (1) using the exact Mie theory, and (2) the approximate Rayleigh diffraction formula for spherical particles. A parametric study of the dependence of the corrections on mode radii, real and imaginary parts of the complex refractive index, and half-angle of the detector's view cone has been carried out for three different size distribution functions of the modified gamma type. In addition, a study has been carried out to investigate the range of these parameters in which the approximate formulation is valid. The agreement is especially good for small-view cone angles and large particles, which improves significantly for slightly absorbing aerosol particles. Also discussed is the dependence of these corrections on the experimental design of the transmissometer systems.

Deepak, A.↗
NASA NTRS1979

Measurements of the extinction to the molecular hydrogen emission from the Orion Nebula

Observations of the reddening of two pairs of H2 emission lines in the Orion Nebula are presented. A reddening of 0.8 + or - 0.2 mag between 2.12 and 2.42 microns is derived from observations of the intensity ratio of the v = 1-0 S(1) (2.12-micron) line and the Q(3) (2.42-micron) line and a comparison of this ratio with its unreddened value. The result is checked by a measurement of the intensity ratio of the v = 1-0 S(2) (2.03-micron) line and the Q(4) (2.43-micron) line. The total IR and visual extinctions to the emission region are inferred to be 4 + or - 1 mag (2.1 microns) and 40 + or - 10 mag (visual). Previous measurements of the H2 emission lines in Orion are corrected on the basis of these results, yielding a total H2 luminosity of approximately 2000 suns, an average column density of about 3 x 10 to the 20th per sq cm, and essentially equal rotational and vibrational temperatures (2000 K) for the H2. It is concluded that the H2 emission region in Orion lies within the molecular cloud, probably at the same depth as the IR cluster.

Beckwith, S.↗
NASA NTRS1980

Revised depolarization corrections for atmospheric extinction

Possible sources of errors in depolarization data are analyzed with special reference to the rotational Raman contribution to the optical depth. It is shown that the depolarization data now available are so accurate that their uncertainty affects the calculated Rayleigh optical depth by less than 1%, and consequently, the anomalously low extinctions occasionally reported must be due to systematic errors of measurement rather than errors in the Rayleigh optical depths.

Young, A. T.↗
NASA NTRS1980

The galactic center - 16-30 micron observations and the 18 micron extinction

Within the galactic center, ionized gas, stars, and heated dust are in close proximity to each other. Observations of the galactic center were made during two flight series on NASA aircraft flying at altitudes of more than 12 km. In both series the instrument used was a 10-channel liquid helium cooled grating spectrometer. Three 16-30 micrometers continuum spectra are presented along with two-color scans of the galactic center. No 18.71 micrometers line emission has been detected from Sgr A West. This is consistent with large dust extinction and normal S III/H II abundances. All three 16-30 micrometers continuum spectra show a broad depression centered at about 18 micrometers. This observation is attributed to absorption by cold silicate dust grains. The characteristics of a silicate model are discussed.

Mccarthy, J. F.↗
NASA NTRS1982

Impact of an asteroid or comet in the ocean and extinction of terrestrial life

Finite difference calculations describing the impact mechanics associated with a 10 to 30 km diameter silicate or water object impacting a 5 km deep ocean overlying a silicate solid planet demonstrate that from 12 to 15% of the bolide energy resides in the water. It is speculated that minimal global tsunami run-up heights on the continents would be 300-400 meters, and that such waves would inundate all low altitude continental areas, and strip and silt-over virtually all vegetation. As a result the terrestrial animal food chain would be seriously perturbed. This could in turn cause extinction of large terrestrial animals.

Ahrens, T. J.↗
NASA NTRS1982

The peculiar UV extinction of Herschel 36

The differential extinction curve of Herschel 36 was determined from International Ultraviolet Explorer data. It is quite unusual, characterized by a distinct 2200 A peak with a very low far blue end at 5 to 7 mu to the -1 power. The star appears to be an extreme member of the group Savage drew attention to, previously consisting only of theta Ori, NU Ori, sigma Sco, and rho Oph. It appears that multiple scattering effects are needed to explain the observations.

Hecht, J.↗
NASA NTRS1982

E/B - V/ extinction values for 24 planetary nebulae derived from IUE data

Excellent agreement was found for E(B - V) values for nine planetary nebulae observed in common by IUE and ANS, derived from a strong 2200A absorption feature. E(B - V) values for seven additional objects with moderately strong 2200A absorption were determined. A third group of eight planetary nebulae for which the 2200A absorption feature is marginal or absent had E(B - V) extinctions determined from the observed He II I(1640A)/I(4686A) ratio. A comparison with optical and radio data is included.

Feibelman, W. A.↗
NASA NTRS1982

Determination of surface albedos and aerosol extinction characteristics from satellite imagery

A procedure is developed for calculating atmospheric extinction characteristics (optical thickness, scattering height, single scattering albedo) and surface albedo from radiometric images made at satellite altitudes. The procedure - a fast, computerized method - is suitable for the high-volume processing of satellite imagery data and thus can be used to map temporal and spatial distributions of aerosol parameters. Based on an analytical approximate solution to the equation of radiative transfer in a plane parallel atmosphere, the procedure is primarily applicable to the 0.4-micron to 0.8-micron wavelength range for solar zenith angles from 10 deg to 60 deg, surface albedos between 0.03 and about 0.5, and atmospheric optical thicknesses from 0.2 to 2. With a step-like change in the surface albedo (for example, a seashore or river bank or other similar change of terrain), both surface reflectivity and atmospheric optical thickness can be derived from radiance measurements. In this case, the resultant optical thickness is not based on a known surface reflectivity and is in essence independent of the radiometer calibration.

Kaufman, Y. J.↗
NASA NTRS1983

Environmental effects of an impact-generated dust cloud - Implications for the Cretaceous-Tertiary extinctions

A model of the evolution and radiative effects of a debris cloud from a hypothesized impact event at the Cretaceous-Tertiary boundary suggests that the cloud could have reduced the amount of light at the earth's surface below that required for photosynthesis for several months and, for a somewhat shorter interval, even below that needed for many animals to see. For 6 months to 1 year, the surface would cool; the oceans could cool only a few degrees Celsius at most, but the continents might cool a maximum of 40 Kelvin. Extinctions in the ocean may have been caused primarily by the temporary cessation of photosynthesis, but those on land may have been primarily induced by a combination of lowered temperatures and reduced light.

Pollack, J. B.↗
NASA NTRS1983

Observations of the extinction and excitation of the molecular hydrogen emission in Orion

Observations have been made of two quadrupole purely rotational transitions of molecular hydrogen, the 0-0 S(7) and S(5) lines, and of several previously unobserved vibration-rotation hydrogen lines in the Orion molecular cloud. The line ratios indicate a 2.1-micron extinction of about 2 mag. A comparison of the populations of three vibrational levels shows a distribution of excitation temperatures between 1000 and 3000 K. These values are consistent with the theory that hydrogen is thermally excited in shock-heated gas which cools after passage of the shock front. The total luminosity of the hot molecular hydrogen is 200 plus or minus 80 solar luminosity, and the mass of the emitting gas is of the order 0.05 solar mass. The mass and cooling times of the hot hydrogen indicate that at least 10 solar mass of the material has been heated over the lifetime of the outflow.

Beckwith, S.↗
NASA NTRS1983

Intensity and extinction irregularities in the H2 emission from Orion

Fourteen lines of the v = 1-0 and v = 2-1 2 micron H2 quadrupole spectrum were measured in a series of ground-based spectroscopic observations of four locations within the Orion molecular cloud-H2 emission complex. Beam sizes of 7 and 10 arcsec were used, with a resolution of 1.8/cm. The results indicate that there are differences in the extinction between regions of as much as 20 mag. In particular there appears to be a ridge of material near the object IRS 2 (= IRc 9), a previously suggested source of activity. The 2-1 S (3) line intensity is substantially stronger at Peak 2 than the present models predict, and this observation is the first indication that there may be a non-Boltzmann distribution within the vibrational levels of shock-excited H2. A detection of the 1-0 Q (6) line is also reported, a feature which was previously absent in spectra of the Orion cloud.

Smith, H. A.↗
NASA NTRS1982

Retrieval of aerosol characteristics from scattering and extinction measurements

The present investigation is concerned with a versatile inversion code which is capable of retrieving profiles of aerosol size distributions and concentrations from multiwavelength satellite measurements of either the scattered radiance from the earth's sunlit atmosphere in both the limb-viewing mode and downward-viewing mode, or the extinction of direct solor radiation by the earth's atmosphere. The inversion code in its present form is a composite of three subcodes. The inversion code is composed of several module algorithms for treating the inverse problem in radiative transfer through a scattering and absorbing aerosol atmosphere, which is vertically inhomogeneous, spherical, and which takes into account multiple scattering and surface albedo effects. The inversion code has been applied and tested for a number of remote sensing techniques. Examples are given concerning the retrieval results for the vertical profiles of aerosol size distribution and concentation.

Deepak, A.↗
NASA NTRS1983

Comment on the extinct paradox

The extinction paradox is a contradiction between geometrical optics results which predict that at high frequencies the scattering cross section of an object should equal its geometrical cross section and rigorous scattering theory which shows that at high frequencies the scattering cross section approaches twice the geometrical cross section of the object. Confusion about the reason for this paradox persists today even though the nature of the paradox was correctly identified many years ago by Brillouin. The resolution of the paradox is restated and illustrated with an example, and then the implications to the interpretation of scattering cross sections are identified.

Levine, D. M.↗
NASA NTRS1983

Barium isotopes in Allende meteorite - Evidence against an extinct superheavy element

Carbon and chromite fractions from the Allende meteorite that contain isotopically anomalous xenon-131 to xenon-136 (carbonaceous chondrite fission or CCF xenon) at up to 5 x 10 to the 11th atoms per gram show no detectable isotopic anomalies in barium-130 to barium-138. This rules out the possibility that the CCF xenon was formed by in situ fission of an extinct superheavy element. Apparently the CCF xenon and its carbonaceous carrier are relics from stellar nucleosynthesis.

Lewis, R. S.↗
NASA NTRS1982

Current status of the impact theory for the terminal Cretaceous extinction

An iridium abundance anomaly at the Cretaceous/Tertiary (C/T) boundary has been disclosed at 36 sites throughout the globe. The impact of a large extraterrestrial object is now widely accepted as the best explanation of this anomaly. Au/Ir and Pt/Ir ratios from two C/T boundary clays indicate a type I carbonaceous chondrite composition for the impacting object, for which a sea floor impact with subsequent subduction has been calculated to have a 20 percent probability. The extinction's 'killing mechanism' has not yet been established, but both temperature changes and darkness due to atmospheric dust are probable contributors. In view of the rapid dispersal of dust in ballistic trajectories, the rapid settling of heavy, coagulated dust particles, the effects of darkness on phytoplankton, and plant records, it is suggested that darkness would have lasted a few months, rather than the originally hypothesized few years.

Alvarez, W.↗
NASA NTRS1984

Terrestrial mass extinctions, cometary impacts and the sun's motion perpendicular to the galactic plane

An analysis of the data of Raup and Sepkoski (1984) is reported which suggests that the dominant cyclicity in major marine extinctions during at least the past 250 Myr is 30 + or - 1 Myr, with the standard deviation of an individual episode being + or - 9 Myr. This terrestrial cycle is found to be strongly correlated with the time needed for the solar system to oscillate vertically about the galactic plane. It is argued that galactic triggering or forcing of terrestrial biological crises may arise as a result of collisions or close encounters of the solar system with clouds of gas and dust, which would gravitationally perturb the solar system's family of comets and thereby increase the flux of comets and meteorites near the earth, leading to large-body impacts. A dominant cyclicity of 31 + or - 1 Myr is found for the observed age distribution of impact craters on earth, the phase of this cycle agreeing with the shown by the major biological crises.

Rampino, M. R.↗
NASA NTRS1984

Mass extinctions and missing matter

The possible influence of 'invisible matter' on the solar system's comet halo, and therefore on quasi-periodic cometary bombardment of the earth and consequent mass extinctions, is briefly addressed. Invisible matter consisting of small or cold interstellar molecular clouds could significantly modulate the comet background flux, while invisible matter consisting of a large population of old, dead stars with a relatively small galactic concentration probably could not. It is also shown that the downward force exerted by the Galaxy will perturb the halo, but will not produce any periodicity.

Stothers, R. B.↗
NASA NTRS1985

Low-excitation Herbig-Haro objects and interstellar extinction

Previous work on low-excitation Herbig-Haro (HH) objects has confirmed the existence of a strongly-enhanced hydrogen two-photon continuum which is produced by low-velocity shocks in neutral material. The theoretical two-photon energy distribution was compared with that observed for HH 43. With the addition of optical (3300 lambda 7000A) data to the ultraviolet (1200 lambda 3100A) data obtained with the I.U.E, the wavelength dependence of interstellar extinction toward HH 43 has been determined.

Schwartz, R. D.↗