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Podolak, M.

Publications and source records attributed to Podolak, M..

30 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

The origin of Uranus - Compositional considerations

Implications of two broad classes of theories for the origin of the solar system for the composition of Uranus are examined as an approach to the testing of alternative hypotheses. Consideration is given to accretional theories of planetary formation, which predict interior structures consisting of a rock core surrounded by an envelope of H2, He, H2O, NH3 and CH4 with different ratios of H2 to ice, and to theories postulating the formation of giant gaseous protoplanets as a result of an encounter with an extended protostar, the condensation of eddies (flocules) in a cloud, with supersonic turbulence and the breakup of a ring formed in a collapsing massive solar nebula. It is shown that whereas accretional theories can easily account for the high density of Uranus relative to Jupiter and Saturn, the apparent absence of NH3, and the observed D/H ratio being about three times the solar ratio, the giant protoplanet theories do not clearly demonstrate how the compositions were achieved.

Podolak, M.↗
NASA NTRS1981

On the structure and composition of Uranus and Neptune

A series of models is presented of Uranus and Neptune in which the relative amounts of (1) rock, (2) ices, and (3) hydrogen and helium are allowed to vary. By fitting the density and the gravitational quadrupole moment, the model composition can be determined. Because of the ambiguity in the rotation periods of these planets, several possible models are presented and discussed.

Podolak, M.↗
NASA NTRS1980

Atmospheric effects on radiometric imaging from satellites under low optical thickness conditions

The interference that the atmosphere poses to analyzing the imagery taken by satellite-borne instruments is discussed, assuming a cloud-free, planar, and horizontally uniform atmosphere. An approximate explicit formula is derived for the earth-atmosphere system nadir-beam reflectivity in terms of the atmospheric parameters, object pixel reflectivity and surrounding area reflectivity, for the limiting case of an optically thin atmosphere. The concepts of the forward-scattering and the backward-scattering optical thickness are introduced, and it is shown that the atmospheric effects in a spectral band depend in a specific fashion on these atmospheric parameters and on the surface spectral reflectivity. In addition, contrast transmittance through the atmosphere, which affects the possibilities of photointerpretation, is discussed.

Otterman, J.↗
NASA NTRS1980

Dependence of the spectral surface irradiance on aerosol properties and surface reflectivity

A reduction in global surface irradiance occurs with increasing aerosol loadings when the aerosols are absorbing. For scattering aerosols, a reduction is pronounced for isotropic scattering (characteristic of small particles) but reduction is not as significant for scattering with a high anisotropy of a large forward peak (characteristic of large particles). This distinction between isotropic and anisotropic scattering becomes small or null over highly reflecting terrain; and for reflectivities higher than 0.5 and solar elevation angles close to the zenith, the global irradiance can be slightly higher for isotropic scattering than in the case of an anisotropy of a forward peak. Under such conditions, which can be encountered in reflective infrared bands over dense vegetation or over sandy deserts (close to noon, in low latitudes) the surface irradiance becomes nearly independent of the aerosol optical thickness.

Otterman, J.↗
NASA NTRS1979

Inhomogeneous models of the Venus clouds containing sulfur

Based on the suggestion that elemental sulfur is responsible for the yellow color of Venus, calculations are compared at 3.4 microns of the reflectivity phase function of two sulfur containing inhomogeneous cloud models with that of a homogeneous model. Assuming reflectivity observations with 25% or less total error, comparison of the model calculations leads to a minimum detectable mass of sulfur equal to 7% of the mass of sulfuric acid for the inhomogeneous drop model. For the inhomogeneous cloud model the comparison leads to a minimum detectable mass of sulfur between 17% and 38% of the mass of the acid drops, depending upon the actual size of the large particles. It is concluded that moderately accurate 3.4 microns reflectivity observations are capable of detecting quite small amounts of elemental sulfur at the top of the Venus clouds.

Smith, S. M.↗
NASA NTRS1979

On inhomogeneous scattering models of Titan's atmosphere

Podolak and Danielson (1977) discussed the homogeneous model of Titan's atmosphere with the dust and gas uniformly mixed. They were able to reproduce the equivalent widths of the CH4 bands at 4680 and 5430 A but failed to agree with the observations in the near infrared. The present paper develops an inhomogeneous model which has the dust and gas essentially separated; this model agrees fairly well with the near-IR data of Younkin (1974) but does not agree with the data of Lutz et al. (1976) for the 4860-A band and it must be adjusted to match their data for the 5430-A band.

Podolak, M.↗
NASA NTRS1978

The albedo of Titan

It is shown that a two-layer model of Titan's atmosphere is required in order to match laboratory data on methane absorption to ground-based IR observations of Titan. An excellent fit to the observations is obtained with a model where the dust in Titan's atmosphere is confined to a region containing 50 m-am of methane and located above an atmosphere with 1.95 km-am of methane, with the amount of dust being the value required for an extinction optical depth of 5 at 0.5 micron. The surface reflectivity required by this model is about 40% to 50%, which is typical for both dirty snow and clouds. The possibility is discussed of distinguishing between two scenarios for the observed surface of Titan (a true surface or just cloud tops) on the basis of limb-darkening curves obtained at many wavelengths with an orbiting telescope or a flyby mission.

Podolak, M.↗
NASA NTRS1977

Axel dust on Saturn and Titan

The observed sharp drop in the geometric albedo of Titan shortward of 6000 A (also observed for Jupiter and Saturn) has been tentatively attributed to the presence of small absorbing particles referred to as Axel dust. The properties of Axel dust particles are studied, with the effect of scattering included through Mie theory, in an attempt to obtain a physically plausible set of parameters that properly characterizes the dust. Quantitative characteristics of the dust are described, scattering and absorption properties of various particle size distributions are calculated, and fits are made to observations of Titan and Saturn. It is found that: (1) a flat distribution of particle radii up to 0.1 micron and an imaginary part of the refractive index that varies as the -2.5 power of wavelength produce a good fit for Titan provided the extinction optical depth of the satellite's atmosphere is about 10 at 5000 A; (2) a clear layer of about 7 km-am hydrogen is required above the dust for a good fit to Saturn with the same dust properties; and (3) the inclusion of dust significantly increases the computed abundance of methane over the value given by reflecting-layer models.

Podolak, M.↗
NASA NTRS1976

Methane rich models of Uranus

A series of models of Uranus is computed assuming that Uranus consists of a core of rocky material surrounded by a convecting envelope rich in H2O, NH3, and CH4. It is found that good fits are obtained to the observed parameters when the CH4:H2 ratio is of the order of 0.1. It is suggested that the rotational period of Uranus is roughly 18 h.

Podolak, M.↗
NASA NTRS1975

Further investigations of Jupiter models

It is noted that previously calculated models of Jupiter, in which the H/He ratio is assumed to be solar, required the inclusion of considerable additional mass in the form of volatilized condensates in the atmosphere and excess mass in the central core. Several additional models are considered in this paper which take into account the stricter constraints imposed by Pioneer 10 measurements of Jupiter's gravitational moments. These measurements have indicated that the mass of the excess volatilized condensates (assumed to be water) relative to the core mass (assumed to be rock) exceeds the relevant solar ratio. The present models are tested for sensitivity to variations in the H/He ratio, in the softness of the equation of state for water, in the treatment of the internal adiabat, in departures from an internal adiabat, and in temperature at the one-bar level. The preliminary results indicate that the ratio of excess water to rock in Jupiter is considerably in excess of the solar value.

Podolak, M.↗
NASA NTRS1974

Models of the giant planets

Models of the giant planets were constructed based on the assumption that the hydrogen to helium ratio is solar in these planets. This assumption, together with arguments about the condensation sequence in the primitive solar nebula, yields models with a central core of rock and possibly ice surrounded by an envelope of hydrogen, helium, methane, ammonia, and water. These last three volatiles many be individually enhanced due to condensation at the period of core formation. Jupiter was found to have a core of about 40 earth masses and a water enhancement in the atmosphere of about 7.5 times the solar value. Saturn was found to have a core of 20 earth masses and a water enhancement in the atmosphere of about 25 times the solar value. Rock plus ice constitute 75-85% of the mass of Uranus and Neptune.

Podolak, M.↗