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At least 109 records · Page 6
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NASA NTRS1978

Far-infrared observations of Uranus, Neptune, and Ceres

During a single flight of our 102 cm balloon-borne telescope and 40-250 micron photometer we observed Uranus, Neptune, Ceres, Venus, Mars, and Saturn. Effective temperatures for Uranus (58.5 plus or minus 2 K) and Neptune (59.7 plus or minus 4 K) were determined, thus confirming the absence of a substantial internal heat source for Uranus and the presence of one for Neptune. Ceres has a brightness temperature of 195 plus or minus 12 K at an effective wavelength of 60 microns.

Stier, M. T.↗
NASA NTRS1978

The rings of Uranus - Results of the 10 April 1978 occultation

Observations of the April 10, 1978, stellar occultation by the rings of Uranus are presented. Nine rings were observed, and their radii and widths are calculated. Rings eta, gamma, and delta are found to be most likely circular and coplanar, in agreement with previous analyses; the remaining rings are either noncircular or slightly inclined. The width of the epsilon ring is a linear function of its radius from the center of Uranus, projected onto the satellites' orbital plane; this suggests that it forms one continuous noncircular ring. The optical-depth profile of the epsilon ring has not changed significantly since March 1977. A model of this ring which fits all available observations adequately is that of a uniformly precessing Keplerian ellipse coplanar with the satellites' orbits. This model permits predictions of the radius and width of the epsilon ring for future occultations. The precession rate is used to determine J2 for Uranus, on the assumption that precession is caused solely by the planetary oblateness and not by satellite-ring interactions.

Nicholson, P. D.↗
NASA NTRS1979

Methane band limb-brightening on Uranus

Area scanner measurements of the brightness distributions of Uranus at 6300 and 7250 A are presented. At the former wavelength, which is characteristic of continuum radiation, the observed limb-darkening is consistent with the results of Stratoscope observations. At the latter wavelength, in the center of a strong methane absorption band, the planet shows substantial limb-brightening as has been reported by other investigators. The limb-brightening and geometric albedo of Uranus in this methane band can be explained in terms of a clear atmospheric model in which the methane partial pressure above the saturation level is equal to its equilibrium vapor pressure. This conclusion is relatively insensitive to the methane mixing ratio below the saturation level. The presence of a high-altitude, conservatively, isotropically scattering haze of optical thickness 0.1, such as that previously proposed as a possible cause of the 7250-A limb-brightening, would produce a geometric albedo at this wavelength that is substantially larger than the value observed for Uranus.

Pilcher, C. B.↗
NASA NTRS1979

The infrared spectra of Uranus, Neptune, and Titan from 0.8 to 2.5 microns

The paper combines the results of two projects that significantly augment previous studies: exploratory IR spectroscopic observations of Uranus, Neptune, and Titan using a 4-m telescope; and new long-path laboratory comparison spectra of methane at abundances appropriate for these planetary spectra. The observations of these previously unexplored spectral regimes provide new insights into the composition and structures of these atmospheres. The spectra of Uranus, Neptune, and Titan are discussed and analyzed from several complementary points of view. A number of specific topics are discussed in detail: methane abundance determinations, limitations in using the 3nu3 CH4 band at high abundances, upper limits to a number of molecules, implications from the pressure-induced spectrum of H2, and consequences of Rayleigh scattering in Uranus's upper atmosphere.

Fink, U.↗
NASA NTRS1980

The radius and ellipticity of Uranus from its occultation of SAO 158687

From occultation timings obtained from the Kuiper Airborne Observatory and from Cape Town for Mar. 10, 1977 occultation of SAO 158687 by Uranus, the equatorial radius, Re, of the planet has been determined to be 26,228 + or - 30 km and its ellipticity epsilon = 1 - Rp/Re = 0.033 + or - 0.007. These values refer to the 1.0 x 10 to the 14th/cu cm number-density level, under the assumption that the upper atmosphere is composed of H2 and He with a mean molecular weight mu = 2.20. The dominant source of uncertainty is the position of the center of the ring system, which was used to define the center of Uranus in our analysis. A rotation rate of 12.8 + or - 1.7 hours for the planet is implied by our value for the ellipticity, under the assumption that Uranus is in hydrostatic equilibrium below the 1.0 x 10 to the 14th/cu cm number density level.

Elliot, J. L.↗
NASA NTRS1980

Application of methane band-model parameters to the visible and near-infrared spectrum of Uranus

Laboratory band-model absorption coefficients of CH4 were used to calculate the Uranus spectrum from 5400 to 10,400 A. A good fit for both strong and weak bands for the Uranus spectrum over the entire wavelength interval was achieved; three atmospheric models were employed: (1) a reflecting layer model; (2) a homogeneous scattering layer model; and (3) a clear atmosphere sandwiched between two scattering layers. The spectrum for the reflecting layer model shows serious discrepancies but indicates that large amounts of CH4 are necessary to reproduce the Uranus spectrum. Both scattering models gave reasonably good fits; the homogeneous model requires a particle scattering albedo of at least 0.998 and an abundance per scattering mean free path of about 1 km-am. For the sandwich model, a continuum single scattering albedo of 0.995 was derived for the upper scattering layer, and the scattering optical depth variable wavelength was consistent with Rayleigh scattering.

Benner, D. C.↗
NASA NTRS1982

The rotation of Uranus

The determination of the rotation rate of Uranus based on available observational evidence is discussed. Previous determinations of the rotation rate since the work of Lowell and Slipher (1912), which tended to converge on a period of 10.8 days until the mid 1970s, most likely due to mutual reinforcement between unreliable results, are reviewed. Recent independent determinations based on (1) the use of theoretical interior models together with observations of oblateness and the gravitational moment J2; (2) periodic brightness fluctuations; and (3) spectrographic measurements of Doppler shifts are then presented which yield a weighted mean value of 16.31 + or - 0.27 h for the rotational period of Uranus. It is noted that the detection of the motion of surface features across the disk by future high-resolution imaging and Voyager 2 observations may allow a direct determination of the rotational period of Uranus.

Goody, R. M.↗
NASA NTRS1982

Rings of Uranus - A review of occultation results

The structure and orbits of the nine confirmed rings of Uranus are reviewed based on occultation observations from ring discovery in March 1977 through April 1981. Observations of stellar occultations by Uranus have revealed the rings to be characterized by narrow widths (up to 100 km) and sharp edges, with possible internal structure, and to have orbits fitting the coplanar precession model, and showing a decreasing eccentricity with semimajor axis. Occultation results have also allowed the determination of the J2 and J4 coefficients of the Uranian gravity potential, the coordinates of the ring plane pole, planetary equatorial radius and ellipticity and the rotation period. The sharp edges and narrow widths of the rings may be explained by small satellites orbiting near the rings, and the uniform precession of three of the rings is attributed to self-gravity. Future plans for Uranus ring observations include further occultation studies, the Voyager 2 fly-by, and Space Telescope data.

Elliot, J. L.↗
NASA NTRS1982

Possible occulations by satellites of Uranus and Neptune - 1983-1985

Predictions are presented for 15 possible occulations by the satellites of Uranus and Neptune from 1983 through 1985. Umbriel, the third satellite of Uranus, might occult a 10.4-mag star (Hyd-20 deg 51699) on 25 March 1983 which will be occulted by Uranus 14 hr earlier. Uncertainties in star positions and ephemerides of planets and satellites are quite large in comparison to the size of these bodies, and these predictions are to be taken as possibilities only.

Mink, D. J.↗
NASA NTRS1983

Tentative confirmation of an aurora on Uranus

There have been three recent reports of the detection of Lyman-alpha radiation (due to atomic hydrogen, H, at wavelength 1,216 A) from Uranus by means of the International Ultraviolet Explorer (IUE) satellite. The interpretations of these results differ. Two reports conclude that there is a strong aurora on Uranus, but the third concludes that the source is resonance scattering of solar Lyman-alpha. This paper reports the detection of emission features due to molecular hydrogen, H2, near 1,600 A. This detection is near the limit of the IUE sensitivity. If it is real, the detection of H2 emission strongly supports the conclusion that Uranus has an aurora comparable in strength with those of the inner two giant planets, Jupiter and Saturn. The first published IUE spectra of the Saturn aurora are also presented.

Caldwell, J.↗
NASA NTRS1983

Aurora on Uranus - A Faraday disc dynamo mechanism

A mechanism is proposed whereby the solar wind flowing past the magnetosphere of Uranus causes a Faraday disk dynamo topology to be established and power to be extracted from the kinetic energy of rotation of Uranus. An immediate consequence of this dynamo is the generation of Birkeland currents that flow in and out of the sunlit polar cap with the accompanying production of polar aurora. The power extracted from planetary rotation is calculated as a function of planetary dipole magnetic moment and the ionospheric conductivity of Uranus. For plausible values of ionospheric conductivity, the observed auroral power requires a magnetic moment corresponding to a surface equatorial field of the order of 4 Gauss, slightly larger than the value 1.8 Gauss given by the empirical 'magnetic Bodes law'.

Hill, T. W.↗
NASA NTRS1984

Near Infrared Imaging of Uranus and Neptune

Imaging of Uranus and Neptune in the deep methane absorption band at 890nm is used to detect high altitude atmospheric hazes and to search for possible undiscovered close in satellites. The appearances of Uranus and Neptune are very different from one another and Uranus seems to be changing with time. The Neptune images show rotation in the direct sense.

Bradford A Smith↗
NASA NTRS1984

Interior structure of Uranus

Key measurements are discussed which are diagnostic of Uranus' interior structure and evolutionary history, and reviews their present status. Typical interior models have chondritic cores, but have the bulk of their mass in an envelope consisting of ice component, principally H2O. The total amount of free H2 in the planet cannot exceed approximately 1 to 2 earth masses. Measurements of the gravitational moments of Uranus are beginning to be accurate enough to constrain models, but are limited in utility by uncertainty in the rotation period. Discussed is evidence that the outermost planetary layers have a gravitationally significant quantity of denser material (ice component?) in addition to H2 and He. The He/H ratio and the deuterium abundance in the atmosphere may be diagnostic of the planet's previous evolutionary history. It is argued that the planet's interior is likely to now be at a temperature approximately 10(3) deg K. Uranus' interior with Neptune's in a number of ways, considering heat flow, degree of internal differentiation, and possible magnetic field.

Hubbard, W. B.↗
NASA NTRS1984

Rotational Properties of Uranus and Neptune

An assessment of recent research bearing on the rotational properties of Uranus and Neptune is given. It is concluded that these properties are best described by the following estimates; URANUS: Direction of pole = alpha (1950) = 256:72 delta (1950) = -15:04 with an absolute uncertainty of about 0:2, Period = 15 to 17 hours; Sense of spin = Retrograde; NEPTUNE: alpha (1950) = 297:8; delta (1950) = + 41:2, with an uncertainty of about 4 degrees towards the pole of Triton's orbit, Period = 18.2 + or - 0.4 hours, Sense of spin = Prograde. There is a clear signature of large scale, zonal, atmospheric flows in observations of the atmosphere of Neptune. Wind velocities are at least as great as 109 m.sec-1. There is no evidence pro or con for atmospheric motions in the Uranus case.

Belton, M. J. S.↗
NASA NTRS1984

Oblatenesses of Uranus and Neptune

The oblateness of a planet is closely related to its rotation rate and internal mass distribution, and is therefore an important indicator of gross planetary structure. Analysis of Stratoscope II images of Uranus yields epsilon = 0.022 + or - 0.001, and stellar occultation observations yield epsilon = 0.024 + or - 0.003. Because of the current pole on aspect of Uranus, it is unlikely that a significantly more accurate value can be determined by stellar occultations before Voyager 2 encounters Uranus in January, 1986. Neptune's oblateness has been determined from stellar occultation observations made in 1968 and 1983. The 1968 observations yield an oblateness of 0.021 + or - 0.004. A recent determination of Neptune's oblateness using both the 1968 and 1983 observations is consistent with this value. Space Telescope observations of several stellar occultations by Neptune could provide a significantly more accurate determination of the oblateness before the Voyager 2 encounter in 1990.

French, R. G.↗
NASA NTRS1984

Absolute spectrophotometry of Titan, Uranus, and Neptune 3500-10,500 A

The present absolute measurements of Titan, Uranus and Neptune geometric albedo spectra in the 3500-10,500 A range have a resolution of about 7 A, together with high SNR, in virtue of the exceptional effeciency of the spectrograph and Reticon detector employed. The high precision and spectral resolution of the data, which are in excellent agreement with the Uranus albedo measurements of Lockwood et al. (1983), make possible quantitative measurements of the effects of Raman scattering by H2 in the Uranus and Neptune atmospheres.

Neff, J. S.↗
NASA NTRS1985

Atmospheric temperature profiles of Uranus and Neptune

Far-infrared spectrophotometry of Uranus and Neptune in the 30-55 micron spectral range is presented. The measurements in the present six independent spectral bands allow the derivation of atmospheric temperature profiles for these planets. Both planets are found to have tropopause temperatures near 53 K, with Neptune having a stronger stratospheric temperature inversion than Uranus. Effective temperatures of 57.7 + or - 1.8 K and 58.2 + or - 1.9 K are obtained for Uranus and Neptune, respectively, confirming the large internal heat source in Neptune.

Moseley, H.↗
NASA NTRS1985

Bolometric albedos of Titan, Uranus, and Neptune

The energy budgets of Titan, Uranus, and Neptune are reevaluated using new observational data on energy input as well as unpublished data on energy output. The bolometric geometric albedo of each object was determined, and preliminary determinations of the phase functions were used to compute the Bond albedos and effective temperatures. The values for the latter are 83 + or - 2 K for Titan, 57 + or - 2 K for Uranus, and 47 + or - 2 K for Neptune. The effective temperature of Titan is greater than the observed brightness temperatures in the thermal infrared region of the spectrum, indicating that the emissivity is less than unity for this part of the spectrum. An internal luminosity of (3.9 + or 1.1) x 10 to the 15th W is found for Neptune, and an upper limit of (0.6 + or - 1.4) x 10 to the 15th W is found for Uranus.

Neff, J. S.↗