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At least 1,045 records · Page 58
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NASA NTRS1975

Venus - Vertical structure of stratospheric hazes from Mariner 10 pictures

An 'optical barometer' technique for determining the altitudes of haze in the stratosphere of Venus is presented wherein the Rayleigh-scattering component is derived by comparing orange and UV brightness profiles for nearby Mariner 10 television-picture pairs. The derived scale height for CO2 gas is 4.2 km, corresponding to a temperature of 200 K, in good agreement with radio occultation data. The optical barometer yields a pressure of 4 mb for the level at which the slant-path optical depth at the limb is unity. This level corresponds to a distance from the center of Venus equal to 6131 km, which is accurate to within 1 km provided that there is no appreciable contribution to the brightness by Rayleigh-scattering aerosols which mimic CO2 gas. It is possible that the limb haze layering observed between 6130 and 6140 km could be correlated with temperature inversions detected by the Mariner 5 radio-occultation experiment. A model is proposed wherein the concentration of particles increases rapidly with an effective scale height of about 2 km as one descends about 10 km from the limb haze to the main polarization cloud deck.

Oleary, B.↗
NASA NTRS1976

Particles, environments and possible ecologies in the Jovian atmosphere

The eddy diffusion coefficient is estimated as a function of altitude, separately for the Jovian troposphere and mesosphere. Complex organic molecules produced by the Ly alpha photolysis of methane may possibly be the absorbers in the lower mesosphere which account for the low reflectivity of Jupiter in the near ultraviolet. The optical frequency chromophores are localized at or just below the Jovian tropopause. Candidate chromophore molecules must satisfy the condition that they are produced sufficiently rapidly that convective pyrolysis maintains the observed chromophore optical depth. The condition is satisfied if complex organic chromophores are produced with high quantum yield by NH3 photolysis at less than 2,300 A. Jovian photoautotrophs in the upper troposphere satisfy this condition well, even with fast circulation, assuming only biochemical properties of comparable terrestrial organisms. An organism in the form of a thin, gas filled balloon can grow fast enough to replicate if (1) it can survive at the low mesospheric temperatures, or if (2) photosynthesis occurs in the troposphere.

Sagan, C.↗
NASA NTRS1976

Remarks on the overall distribution of hydrogen in the galactic disk

Several current problems concerning the overall distribution of hydrogen in the galaxy are discussed in general terms. These problems include the degree of saturation characterizing low-latitude emission observations of HI, and the optical-depth corrections to the derived column and volume densities; the amount of fine-scale velocity and spatial structure diluted by the instrumental limitations of the presently available surveys; and the general problem of detailed mapping of the HI in the galaxy. Comparison is made between the distribution of HI and that of CO and several other galactic tracers. The galactic disk as defined by atomic hydrogen has a diameter fully twice as large as that defined by the ionized and molecular states of hydrogen, as well as by other molecules, supernova remnants, pulsars, gamma radiation, synchroton radiation, and the youngest stars. It is also less confined to the galactic equator than most of the other constituents. The degree of small-scale structure apparent in the molecular observations is much greater than in the HI observations. Parameters describing the small-scale structure were determined using Monte Carlo techniques to simulate the observations.

Burton, W. B.↗
NASA NTRS1976

Multiple scattering in planetary atmospheres

Certain simple procedures for solving radiative transfer problems in planetary atmospheres are reviewed: the similarity relations relating isotropic to anisotropic scattering, asymptotic results relating known solutions for semi-infinite layers to desired solutions for optical depths, and expansions relating known solutions for conservative scattering to desired solutions for the nearly conservative case. The complications introduced by atmospheric inhomogeneity, surface reflection, and spectral features are also discussed.

Irvine, W. M.↗
NASA NTRS1975

Mariner 9 ultraviolet spectrometer experiment - Scattering properties of Hellas

A least-squares analysis of 709 Mariner 9 UV spectra obtained over Hellas from Revolution 140 to 214 (Jan. 22, 1972, to Feb. 28, 1972) showed the extinction optical depth of the atmosphere above Hellas to be 0.5 plus or minus 0.2. This is evidence of lingering dust in the Hellas basin. An atmospheric model, combining dust and Rayleigh scattering, was used to make an apparent pressure map of Hellas. Anomalously high pressures are interpreted as clouds developing over the southern part of Hellas near the time of autumnal equinox. The blue ratio (reflectance at 2680 A divided by reflectance at 3050 A) showed a 20% increase from Revolution 40 to Revolution 74, attributed to the decay of the dust storm. Thereafter, the blue ratio remained essentially constant through Revolution 214, which implies that the rate of clearing in Hellas was much slower than that planetwide.

Ajello, J. M.↗
NASA NTRS1975

The rocket-ultraviolet spectrum of the planetary nebula NGC 7027

An ultraviolet spectrum of NGC 7027, obtained with a rocket-borne telescope, is analyzed. Absolute values are presented for the observed fluxes, and upper limits are given for the strongest predicted lines that were not observed. The results are corrected for interstellar extinction using the observed and calculated line ratios between H-beta and the hydrogenic recombination line of He II at 1640 A. The corrected C IV resonance line at 1549 A is found to be in good agreement with the intensity calculated from models, but the intercombination line of C III at 1909 A is found to be too bright by a factor of 10. This discrepancy is reduced to a factor of 4 by taking dielectric recombination into account and is eliminated by using the solar carbon abundance, which implies attenuation in the C IV line. It is shown that no appreciable number of absorbing grains can exist in the C IV-producing region of the nebula since the optical depth is of the order of 10,000 at the line center.

Bohlin, R. C.↗
NASA NTRS1976

Far-infrared photometry of NGC 1068

The detection of far-IR emission from the galaxy NGC 1068 in four passbands spanning the spectral range from 28 to 320 microns is reported. The observations were made with a 61-cm airborne IR telescope and a liquid-helium-cooled multifilter radiometer. It is found that the flux density between 38 and 100 microns increases with increasing wavelength to a maximum value of 454 (plus or minus 94) Jy, but decreases rapidly for wavelengths greater than 100 microns. The total flux is estimated to be about 3 by 10 to the -11th power W/sq m, which corresponds to a bolometric luminosity of 3.7 by 10 to 11th power times the solar luminosity at a distance of 20 Mpc. The nuclear M/L ratio is shown to be no greater than about 0.003, indicating that the scale, optical depth, or both, of a thermal far-IR source in NGC 1068 would have to be larger than those of the extended IR sources around the nuclei of our Galaxy, NGC 253, or M82.

Telesco, C. M.↗
NASA NTRS1976

A tensor formulation of the equation of transfer for spherically symmetric flows

A tensor formulation of the equation of radiative transfer is derived in a seven-dimensional Riemannian space such that the resulting equation constitutes a divergence in any coordinate system. After being transformed to a spherically symmetric comoving coordinate system, the transfer equation contains partial derivatives in angle and frequency, as well as optical depth due to the effects of aberration and the Doppler shift. However, by virtue of the divergence form of this equation, the divergence theorem may be applied to yield a numerical differencing scheme which is expected to be stable and to conserve luminosity. It is shown that the equation of transfer derived by this method in a Lagrangian coordinate system may be reduced to that given by Castor (1972), although it is, of course, desirable to leave the equation in divergence form.

Haisch, B. M.↗
NASA NTRS1976

High-resolution far-infrared observations of the galactic center

A map at 53 microns with 17-arcsec resolution and three-color observations at 53, 100, and 175 microns with 30-arcsec beams of Sgr A are presented. Sagittarius A is resolved into two main sources, one associated with the cluster of strong 10-micron sources and another about 45 arcsec to the southwest coincident with a weak 10-micron source. The dust temperature peaks near the strong 10-micron sources, but the 100-micron and 175-micron fluxes and the far-infrared optical depth are greatest near the southwest source. The amount of dust required to explain the far-infrared emission is comparable to that observed in absorption in the near-infrared.

Harvey, P. M.↗
NASA NTRS1976

The vertical distribution of water vapor in the atmosphere of Mars

Calculations are performed of the vertical distribution of water vapor and condensate in an adiabatic atmosphere on Mars taking into account turbulent diffusion and terminal velocity. The distributions are found to be substantially different when terminal velocity is included. The eddy-diffusion coefficient in the troposphere cannot be much greater than 100,000 sq cm/sec if optical depths are to be kept low enough to be consistent with observations. Processes in the boundary layer are also discussed. It is concluded that virtually all the water vapor is to be found in the lowest 6-10 km and that the lowest 2 km should have a greater concentration than the rest of that layer. Some observational tests of these ideas and conclusion can be performed by the Viking missions to Mars.

Hess, S. L.↗
NASA NTRS1975

On the determination of haze levels from Landsat data

The paper describes two methods for determining haze levels (specified by haze optical depth at a wavelength of 0.5 microns) from Landsat multispectral scanner data. The channel correlation method relates the haze level to the y-intercept of the regression line through a plot of the data in the plane of the multispectral channels MSS 4 and MSS 5. The minimum value method relates haze level to the minimum value of individual lines in the MSS 4 data set.

Potter, J. F.↗
NASA NTRS1976

Far infrared polarization of dust clouds

The recently observed 10-micron polarization of the Orion Nebula and the Galactic Center suggests that far infrared polarization may be found in these objects. Estimates are made of the degree of far infrared polarization that may exist in the Orion Nebula. The effects of radiation transfer are briefly considered and it is concluded that polarization observations over a range of wavelengths will be useful in deducing the optical depth structure of dust clouds.

Dennison, B.↗
NASA NTRS1976

Infrared observations of ices and silicates in molecular clouds

Spectrophotometric observations from 2 to 4 microns and from 8 to 13 microns of several infrared sources associated with molecular clouds are reported. Narrow absorption features at 3.08 microns, attributed to interstellar ices, appear in all sources with a molecular cloud in the intervening line of sight. All sources showing ice absorptions also show broad absorption features, attributed to cold silicates, from 8 to 13 microns. The observed ice absorption profiles are all quite similar; however, they do not fit in detail Mie theory predictions of extinction for pure H2O or NH3 ices. The ratio of ice-to-silicate optical depths is found to vary, with most sources showing a ratio in the range 0.1-0.4. The ratio of visual extinction to ice absorption is found to increase rapidly from inside to outside the molecular cloud in NGC 2024.

Merrill, K. M.↗
NASA NTRS1977

Remarks on the overall distribution of hydrogen in the galactic disk

The degree of saturation characterizing low altitude emission observations of H(I) and the optical depth corrections to the derived column and volume densities are discussed. The amount of fine-scale velocity and spatial structure diluted by the instrumental limitations of the presently available surveys are described. The general problem of detailed mapping of H(I) in the galaxy is explored. Comparison is made between the distribution of H(I) and that of CO and several other galactic tracers. Atomic hydrogen is unique in its distribution, instead of being typical of many Population I constituents. As defined by atomic hydrogen, the galactic disk has a diameter fully twice as large as that defined by the ionized and molecular states of hydrogen, as well as by other molecules, supernova remnants, pulsars, gamma-radiation, synchroton radiation, and the youngest stars. It is also less confined to the galactic equator than most of the other constituents. The degree of small scale structure apparent in the molecular observations is much greater than that in the H(I) observations.

Burton, W. B.↗
NASA NTRS1976

The surface of Mars - The view from the Viking 2 lander

Viking 2 lander began imaging the surface of Mars at Utopia Planitia on September 3, 1976. The surface is a boulder-strewn reddish desert cut by troughs that probably form a polygonal network. A plateau can be seen to the east of the spacecraft, which for the most probable lander location is approximately the dirction of a tongue of ejecta from the crater Mie. Boulders at the lander 2 site are generally more vesicular than those near lander 1. Fines at both lander sites appear to be very fine-grained and to be bound in a duricrust. The pinkish color of the sky, similar to that observed at the lander 1 site, indicates suspension of surface material. However, the atmospheric optical depth is less than that at the lander 1 site. After dissipation of a cloud of dust stirred during landing, no changes other than those stemming from sampling activities have been detected in the landscape. No signs of large organisms are apparent at either landing site.

Mutch, T. A.↗
NASA NTRS1976

The delta-Eddington approximation for radiative flux transfer

Simple approximations, like the Eddington, are often incapable of coping with the highly asymmetric phase functions typical of particulate scattering. A simple yet accurate method called the delta-Eddington approximation is proposed for determining monochromatic radiative fluxes in an absorbing-scattering atmosphere. In this method, the governing phase function is approximated by a Dirac delta function forward scatter peak and a two-term expansion of the phase function. The fraction of scattering into the truncated forward peak is taken proportional to the square of the phase function asymmetry factor, which distinguishes the delta-Eddington approximation from others of similar nature. The transmission, reflection, and absorption predicted by the delta-Eddington approximation are compared with doubling method calculations for realistic ranges of optical depth, single-scattering albedo, surface albedo, sun angle and asymmetry factor. The approximation is shown to provide an accurate and analytically simple parameterization of radiation to replace the empirism currently encountered in many general circulation and climate models.

Joseph, J. H.↗
NASA NTRS1976

The emission-line spectrum of a sunspot in the far-ultraviolet

The emission-line spectrum between 1200 and 1817 A from a sunspot in McMath region 12510 near the solar center is discussed. The spectrum was obtained by the normal-incidence spectrograph on Skylab. The principal results are: (1) the widths of emission lines originating in the chromosphere and lower transition region over the sunspot are much narrower than those previously reported for a polar coronal hole observed above the limb and a quiet chromospheric network observed near the solar center, indicating that the mass motions in the sunspot are less than in these other regions; (2) the sunspot spectrum, aside from the narrow widths of emission lines, is similar to spectra from the chromospheric network boundary. The intensities of lines in the sunspot are much enhanced relative to the network interior. From the full-width at half-maximum of the 1207-A Si III line, an optical depth at line center of 3.6 is deduced. Comparison with Parker's (1974) theory of sunspots shows that, if the enhancement of emission lines is due to enhanced transport of hydromagnetic waves generated in the sunspot convective zone, the mode of the waves is predominately Alfvenic.

Cheng, C.-C.↗
NASA NTRS1977

Sodium D-line emission from Io - A second year of synoptic observation from Table Mountain Observatory

Measurements of sodium D-line emission from Io are reported which were made during synoptic patrols of the 1975 and 1974 apparitions. Corrected equivalent widths of the D lines are plotted against the satelliite's orbital phase reckoned from superior heliocentric conjunction, and quantitative results of selected patrol observations are provided. The results indicate that: (1) resonant scattering is most likely the dominant emission mechanism; (2) the mechanism which injects neutral sodium into the space immediately around Io underwent no major secular variation in the period between the two apparitions; (3) a less pronounced asymmetry between the peak emission rates at 90-deg and 270-deg orbital phases is present in the selected data; and (4) the measured D2/D1 intensity ratios are consistent with a value of 1.6 + or - 0.3, which corresponds to a value of the order of unity for the maximum optical depth possible for resonant scattering the D1 line center. It is suggested that an unexplained scatter of about 20% in the general emission levels is due to a variation in the ionization rate.

Bergstralh, J. T.↗