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At least 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
NASA NTRS1979

Vaporization of comet nuclei - Light curves and life times

The effects of vaporization from the nucleus of a comet are examined and it is shown that a latitude dependence of vaporization can explain the asymmetries in cometary light curves. An attempt is made to explain the observed variation in molecular production rates with heliocentric distance when employing CO2 and clathrate hydrate ice as cometary nuclei substances. The energy balance equation and the vapor pressure equations of water and CO2 are used in calculating the vaporization from a surface. Calculations were carried out from both dry-ice and water-ice nuclei, using a variety of different effective visual albedos, but primarily for a thermal infrared of 0 (emission). Attention is given to cometary lifetimes and light curves and it was determined that the asymmetry in light curves occurs (occasionally) as a 'seasonal' effect due to a variation in the angle between the comet's rotation axis and the sun-comet line.

Cowan, J. J.↗
NASA NTRS1980

Vaporization in comets - The icy grain halo of Comet West

The variation with heliocentric distance of the production rates of various species in Comet West (1975n = 1976 VI) is explained with a cometary model consisting of a CO2 dominated nucleus plus a halo of icy grains of H2O or clathrate hydrate. It is concluded that the parents of CN and C3 are released primarily from the nucleus but that the parent of C2 is released primarily from the halo of icy grains.

Ahearn, M. F.↗
NASA NTRS1981

The infrared spectrum of Rhea

The reflectance spectrum of the leading side of the Saturnian satellite Rhea in the 0.65 to 2.5 micron range is reported and used to quantify the amount of water and other materials present on the surface. Data were obtained with 1.5% spectral resolution and 3 to 5% precision by the 3.0-m Infrared Telescope Facility at Mauna Kea Observatory with a continuously scanning circular variable filter spectrometer. Water ice absorptions previously identified at 2.02, 1.65 and 1.55 microns are confirmed, and additional absorptions at 1.25 and probably 1.04 microns are identified. The spectrum of Rhea is noted to be very similar to that of the leading side of Ganymede in the 0.6 to 2.5 micron region and to laboratory spectra of water frost on ice blocks rather than an optically thick frost. Results indicate that the leading side of Rhea is at least 90 wt % water ice, and may be as much as 98 wt %. Of the remaining material, neither clathrates nor minerals is excluded.

Clark, R. N.↗
NASA NTRS1981

Kinetic inhibition of CO and N2 reduction in circumplanetary nebulae - Implications for satellite composition

In contrast to the solar nebula, the conversion of CO to CH4 and of N2 to NH3 in the circumplanetary nebulae of the Jovian planets is fast enough relative to radial mixing and nebula cooling rates that CO and N2 are minor constituents in the circumplanetary nebulae. Thus, although the Jovian planets may have accreted carbon and nitrogen from the solar nebula mainly in the form of CO and N2, these species were then reprocessed within the circumplanetary nebulae to form mainly CH4 and NH3. Satellites of the Jovian planets which accreted in sufficiently cool parts of their circumplanetary nebula are therefore predicted to retain large amounts of NH3 and CH4 in the form of clathrate hydrates and also very small but chemically important amounts of HCN.

Prinn, R. G.↗
NASA NTRS1982

Volcanism and igneous processes in small icy satellites

Evidence for the production of endogenic processes of the small Saturnian satellites by igneous activity of low melting point NH3-H2O magma heated radiogenically is presented. An initial state of the inner satellites is modeled as a homogeneous mixture of particulate silicates and ices. Conductive transport is assumed to have either combined with heat capacity in a mixture of crystalline phases, or to have been reduced by the presence of dust, defects, vitreous or amorphous phases, and clathrates. The extent to which a eutectic melt could form is calculated in terms of the volume percentage of melt, with all heating above 175 K going toward overcoming the latent heat of the eutectic mix. Mimas and Enceladus were treated as free from radiogenic heat sources, which were significant for Tethys, Dione, Rhea, and Iapetus. The migrations of inner materials to form surface structures and seal off the interiors of the moons are described.

Stevenson, D. J.↗
NASA NTRS1982

The infrared spectral properties of frozen volatiles

Since Whipple's dirty snowball model of comet nuclei, it has been generally accepted that volatile ices help to explain cometary phenomena. The infrared spectral properties of many substances that are potential candidates for frozen volatiles in the solar system are being pursued; indeed some of these frozen materials have been found in the solar system: H2O, CO2, and SO2. A review of laboratory spectra in the range 1 to 20 microns of H2O, CO2, SO2, CH4, NH3, H2S, CO, NH4HS and NH3.H2O is presented. Both reflection spectra of thick frosts and transmission spectra of thin films are shown, and their main characteristics are described. Hydrates, clathrates, and composite spectra are discussed. When it is possible to observe the nuclei of comets at close range, it may be possible to identify frozen volatiles by their infrared spectra.

Fink, U.↗
NASA NTRS1984

Advanced onboard storage concepts for natural gas-fueled automotive vehicles

The evaluation of several advanced concepts for storing natural gas at reduced pressure is presented. The advanced concepts include adsorption on high surface area carbon, adsorption in high porosity zeolite, storage in clathration compounds, and storage by dissolution in liquid solvents. High surface area carbons with high packing density are the best low pressure storage mediums. A simple mathematical model is used to compare adsorption storage on a state of the art carbon with compression storage. The model indicates that a vehicle using adsorption storage of natural gas at 3.6 MPa will have 36 percent of the range, on the EPA city cycle, of a vehicle operating on a compression storage system having the same physical size and a peak storage pressure of 21 MPa. Preliminary experiments and current literature suggest that the storage capacity of state of the art carbons could be improved by as much as 50 percent, and that adsorption systems having a capacity equal to compression storage at 14 MPa are possible without exceeding a maximum pressure of 3.6 MPa.

Remick, R. J.↗
NASA NTRS1983

Hydrodynamic models of the cometary atmosphere

Self-consistent transonic multifluid solutions for the dynamical and thermal structure of the two-phase dusty gas atmosphere of a comet containing a dirty H2O-clathrate ice nucleus have been obtained. This was done by solving a simultaneous set of differential equations representing conservation of number density, momentum, and energy, as well as the transfer of solar radiation responsible for the major photolytic processes and nuclear heating. The results presented include velocity and Mach number profiles of heavy species and the velocity of dust within the subsonic and supersonic regions, temperature profiles of the dust and gas in the supersonic region, and radial profiles of electron and ion densities in the collision-dominated region.

Mendis, D. A.↗
NASA NTRS1985

Giant Planets and Their Satellites: Cosmochemistry, Evolution and Structure

The following topics were discussed: surface state of Titan; statistical mechanical treatment of clathrate hydrates; planetary dynamos; formation of the giant planets; the e-ring; Uranian satellites and atmosphere; 10 sulfur lakes; episodic tidal evolution; resurfacing of Ganymede; and the origin of the Moon.

Stevenson, D. J.↗
NASA NTRS1985

On the Origin of Triton and Pluto

Lyttleton's (1936) hypothesis that Triton and Pluto originated as adjacent prograde satellites of Neptune is evaluated, and it is shown that with the presently accepted masses of Triton and Pluto-Charon, the momentum and energy exchange required to sell Triton on a retrograde orbit is impossible. The Pluto-Charon system could not have acquired its present angular momentum state during an ejection event unless a physical collision was involved, which is quite unlikely. The simplest hypothesis is that Triton and Pluto are independent representatives of large outer solar system planetesimals. Triton is simply captured, with spectacular consequences that include runaway melting of interior ices and release to the surface of clathrated CH4, CO, and N2. Condensed remnants of this protoatmosphere could account for features in Triton's unique spectrum.

Mckinnon, W. B.↗
NASA NTRS1984

On the origin of Triton and Pluto

Lyttleton's (1936) hypothesis that Triton and Pluto originated as adjacent prograde satellites of Neptune is evaluated, and it is shown that with the presently accepted masses of Triton and Pluto-Charon, the momentum and energy exchange required to set Triton on a retrograde orbit is impossible. The Pluto-Charon system could not have acquired its present angular momentum state during an ejection event unless a physical collision was involved, which is quite unlikely. The simplest hypothesis is that Triton and Pluto are independent representatives of large outer solar system planetesimals. Triton is simply captured, with spectacular consequences that include runaway melting of interior ices and release to the surface of clathrated CH4, CO, and N2. Condensed remnants of this protoatmosphere could account for features in Triton's unique spectrum.

Mckinnon, W. B.↗
NASA NTRS1986

The D to H ratio on Titan and the planets: Implications for origin and evolution of planetary atmospheres

Measurements of deuterated methane show that Titan's atmosphere is enriched by at least several times in deuterium compared to the major planets. Potential causative factors for this enrichment are condensation to form tropospheric methane clouds, fractionation occuring over a hypothetical CH4-C2H2 ocean and between the ocean and the clathrate crust beneath, fractionation which occurred during the formation of Titan and fractionation occuring as a result of the evolution of Titan's atmosphere. The greater part of the observed fractionation is probably derived from the formation of Titan and the subsequent evolution of Titan atmosphere driven by photochemistry. The latter process is developed here for the first time. The D/H ratio in a planetary atmosphere is one readily available measure of the origin and evolution of the hydrogen bearing volatiles on the planet. Comparison between D/H ratio in the inner solar system and the outer solar system may pose important constraints on current theories.

Pinto, J. P.↗
NASA NTRS1985

The atmospheres of icy bodies

Ices are considered as the major volatile-carrying solid in the outer solar system. As such, they play a major role in forming atmospheres of icy satellites and the comas of comets and they have probably contributed to the enrichment of heavy elements in outer planet atmospheres. A possible role for clathrate hydrates in causing the activity of distant comets and sub-surface processes on icy satellites seems worthy of further study.

Owen, T.↗
NASA NTRS1986

D to H ratio and the origin and evolution of Titan's atmosphere

Potential causative factors for the unusually large ratio of CH3D to CH4 reported in the stratosphere of Titan (6 x 10 to the -4th) compared to that of Saturn (8.7 x 10 to the -5th) or Jupiter (6.7 x 10 to the -5th) are examined. With uncertainty in the observations and data reduction taken into account, an atmospheric enrichment factor of three or more is calculated for Titan relative to the planets. This enrichment factor is seen to be accounted for by condensation to form tropospheric methane clouds, fractionation occurring over a hypothetical CH4-C2H6 ocean and between the ocean and the clathrate crust beneath, fractionation which occurred during the formation of Titan, and fractionation as a result of the evolution of Titan's atmosphere. It is concluded that the greater part of the observed fractionation is probably derived from the last two of these mechanisms. The mechanisms studied do not account for the observed CH3D/CH4 value of 1.7 x 10 to the -3rd obtained by Kim and Caldwell (1982).

Pinto, J. P.↗
NASA NTRS1986

On the influx of small comets into the earth's upper atmosphere. I - Observations. II - Interpretation

The atmospheric dayglow in the atomic oxygen emissions at 130.4 nm observed by the spin-scan photometer of the Dynamics Explorer 1 satellite is analyzed. Large, transient decreases in dayglow intensities of 5-20 percent are detected; the decreases referred to as an atmospheric hole or dark spot covers an area of 2000 sq km and 10 atmospheric holes/min occur in the dayside upper atmosphere. The diurnal variation of the occurrence rate is examined; the relation between the variation and the influx of extraterrestrial objects is studied. These cometlike objects are composed of water snow or clathrate in the form of a fluffy aggregate. The effects caused by the impact of the comet with the earth's atmosphere are investigated.

Frank, L. A.↗
NASA NTRS1986

Mobilization of cryogenic ice in outer solar system satellites

Mechanisms to explain the mobilization of ice on the Uranian satellites Miranda and Ariel at the very low temperatures prevailing on those bodies are considered. A form of pressure solution creep is proposed in which very fine-grained water ice or clathrate hydrate is mobilized by a small amount of intergranular cryogenic fluid (CH4, CO, or N2). Viscosities as low as 10 to the 12th P are possible for a limited time, sufficient to allow flooding of rift valleys and perhaps even substantial lateral flows (glaciers).

Stevenson, D. J.↗
NASA NTRS1987

Radio science with Voyager 2 at Uranus - Results on masses and densities of the planet and five principal satellites

Phase-coherent Doppler data generated by the Deep Space Network with the radio communication system during the Voyager 2 encounter with Uranus in January 1986, optical navigation data generated by the Voyager Navigation Team with the Voyager 2 imaging system, and ground-based astrometric data obtained over an 8-yr period are compiled and analyzed to determine the masses and densities of Uranus and its principal satellites. The data-analysis procedures are explained in detail, and the results are presented in tables and graphs. The mean density of Uranus is found to be 1.285 + or - 0.001 g/cu cm, whereas the mean uncompressed mass of all five satellites is 1.48 + or - 0.06 g/cu cm, or 0.10 g/cu cm above the density expected for a homogeneous solar mix of rock, H2O and NH3 ice, and CH4 as clathrate hydrate. This difference is tentatively attributed to the presence of 15 mass percent of pure graphite, which would provide the thermal conductivity required to keep the satellites cold and undifferentiated.

Anderson, J. D.↗
NASA NTRS1988

Collisions with ice-volatile objects: Geological implications

The collision of the Earth with extra-terrestrial ice-volatile bodies is proposed as a mechanism to produce rapid changes in the geologic record. These bodies would be analogs of the ice satellites found for the Jovian planets and suspected for comets and certain low density bodies in the Asteroid belt. Five generic end-members are postulated: (1) water ice; (2) dry ice: carbon-carbon dioxide rich, (3) oceanic (chloride) ice; (4) sulfur-rich ice; (5) ammonia hydrate-rich ice; and (6) clathrate: methane-rich ice. Due to the volatile nature of these bodies, evidence for their impact with the Earth would be subtle and probably best reflected geochemically or in the fossil record. Actual boloids impacting the Earth may have a variable composition, generally some admixture with water ice. However for discussion purposes, only the effects of a dominant component will be treated. The general geological effects of such collisions, as a function of the dominant component would be: (1) rapid sea level rise unrelated to deglaciation, (2) decreased oceanic pH and rapid climatic warming or deglaciation; (3) increased paleosalinities; (4) increased acid rain; (5) increased oceanic pH and rapid carbonate deposition; and (6) rapid climatic warming or deglaciation.

Wilde, P.↗