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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 rotation period of Neptune

Photometric observations of Neptune with a highly accurate doubly differential photometer, two filters centered in the 6190-A band of methane, and two immediately adjacent continuum filters centered at 5993 and 6332 A are reported which were performed in the interval from March 29 to August 8, 1977. The principal rotation period of at least a major methane albedo feature of Neptune's deeper atmosphere is found to be 18.44 + or - 0.01 (est. error) hr, assuming that such features are stable in longitude over weeks or months. The result obtained is compared with previous determinations, and reasons for a discrepancy between the present and a previous photometric period are considered. It is suggested that the period of 18.44 hr is likely to be quite close to that of the main body of Neptune.

Slavsky, D.↗

NASA NTRS1980

Galileo's observations of Neptune

Galileo's recorded observations of Neptune are examined, including an assessment of the accuracies of the positions of Neptune and those of the Jovian satellites. It is shown that one observation may be of astrometric value. The possible error in the ephemeris of Neptune is also discussed.

Kowal, C. T.↗

NASA NTRS1981

Observations of Uranus, Neptune, and Titan by the International Ultraviolet Explorer

The ultraviolet spectra of Uranus, Neptune, and Titan do not reveal absorption features at 7-A resolution from 2100 to 3200 A. Upper limits of from 1 to 3 A are set for the equivalent widths of narrow absorptions, which corresponds to a CO/H2 mixing ratio less than 2 x 10 to the -4th in the case of Uranus. The slopes of the continuum reflectivities of Uranus and Neptune are consistent with the semi-infinite Rayleigh-Raman scattering model of Cochran, while the absolute levels are matched only if solar photometry is modified within acceptable limits. An alternative, but less satisfactory explanation of the new data is that the UV reflectivities of both Uranus and Neptune are depressed uniformly by a continuum absorber. The suggestion by Savage et al (1980) that the albedo of Uranus decreases by approximately 20% from 2200 to 1800 A is not confirmed. For Titan, the albedo decreases monotonically toward shorter wavelengths down to 2300 A. New limits are set for the pressure level in the atmosphere of Titan up to which the real, but presently unidentified, UV absorber there must extend, and for the fraction of Titan that must be covered by this absorber

Caldwell, J.↗

NASA NTRS1981

The rotation period of Neptune's upper atmosphere

The variations in the near-infrared brightness of Neptune observed during July and August 1980 show a well-defined, large-amplitude variation in Neptune's J-K color, with a period of 17.73 + or - 0.1 hour. These results are interpreted as diurnal variations resulting from the 17.73-hour rotation period of the Neptune upper atmosphere in the presence of inhomogeneous weather, and are found to qualitatively corroborate those of Cruikshank (1978). It is also noted that Smith and Slavsky (1980) report a 17.7-hour component as a secondary periodicity in their data. Variations were observed in the 5-0-micron spectral region which are in phase with the variations seen at shorter wavelengths.

Brown, R. H.↗

NASA NTRS1983

New observational constraints on the temperature inversions of Uranus and Neptune

The presence of a temperature inversion in the lower stratospheres of both Uranus and Neptune is confirmed by the 20-micron photometric data presented. It is found that the brightness temperature difference between 17.8 and 19.6 microns is 0.8 + or - 0.5 K for Uranus and 1.8 + or - 0.6 K for Neptune, implying that the temperature inversions of both planets are weaker than previously thought. Comparisons with model atmospheres suggested by Appleby (1980) imply that these temperature inversions may be understood as a consequence of heating through CH4 and aerosol absorption of sunlight. The stratospheric CH4 mixing ratio of Neptune must, however, be higher than that at the temperature minimum.

Tokunaga, A. T.↗

NASA NTRS1983

The structure of Neptune's upper atmosphere - The stellar occultation of 24 May 1981

Observations of the 24 May 1981 occulation of an uncatalogued star by Neptune made at the Cerro Tololo Inter-American Observatory have been analyzed to yield temperature profiles of Neptune's upper atmosphere for number densities near 5 x 10 to the 13th per cu cm. The mean temperatures at immersion (latitude - 56 deg) and emersion (latitude - 16 deg) obtained by numerical inversion were 140 + or - 10 K and 154 + or - 10 K, respectively. The immersion and emersion profiles are remarkably similar in overall shape, suggestive of global atmospheric layering. From the astrometry of the event, precise relative positions of Neptune and the occulted star were obtained.

French, R. G.↗

NASA NTRS1984

Restored methane band images of Uranus and Neptune

Charge-coupled device images of Uranus and Neptune taken in the 8900-A absorption band of methane are presented. The images have been digitally processed by means of nonlinear deconvolution techniques to partially remove the effects of atmospheric seeing. The restored Uranus images show strong limb brightening consistent with previous observations and theoretical models of the planet's atmosphere. The computer-processed images of Neptune show discrete cloud features similar to those reported previously by Smith, Reitsema, and Larson (1979). A time series of the restored Neptune images shows a continuous variation which may be due to the planet's rotation.

Heasley, J. N.↗

NASA NTRS1984

Properties of the Upper Tropospheres of Uranus and Neptune Derived from Observations at Visible to Near-Infrared Wavelengths

Photons at wavelengths between 0.3 and 4.5 microns penetrate the atmospheres of Uranus and Neptune to pressures between about 0.01 bar and 10 bars. This pressure range brackets the radiative convective boundary in both atmospheres and is therefore designated upper troposphere. Physical processes which govern the transfer of radiation in Uranus's and Neptune's atmospheres at these wavelengths include Rayleigh/Raman scattering by hydrogen, scattering and broadband absorption by suspended aerosol particles and absorption in discrete bands and lines by methane and hydrogen. Consequently, tropospheric properties constrained by observations at these wavelengths include optical properties and distribution of aerosol particles, methane/hydrogen ratio, and ortho/para hydrogen ratio. Recent observations of Uranus and Neptune in this spectral range, are reviewed and compared with predictions based on models of the atmospheric structures. Significant results for Uranus include the presence of an opaque lower boundary to the visible atmosphere very near the level corresponding to 2 bars pressure, and consequently a methane/hydrogen ratio no less than 3 percent.

Bergstralh, J. T.↗

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

Atmospheric Dynamics of Uranus and Neptune: Theoretical Considerations

The atmospheric dynamics of neptune and uranus are investigated. Uranus, because of its pole on orientation and low internal heat source, is in a dynamically different atmospheric regime from Jupiter and Saturn. Neptune resembles Jupiter and Saturn in orientation and internal heating, but its extremely long radiative time constant puts Neptune in a different class. Voyager observations of seasonal temperature gradients, equator to pole temperature gradients, infrared emission, Bond albedo, possible cloud structures (bands, spots, eddies), and cloud motions can be used to improve the ability to classify planetary atmospheres according to their dynamical regimes.

Andrew P Ingersoll↗

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

Magnetospheric Structures: Uranus and Neptune

Magnetospheric structures that might be encountered at Uranus and Neptune are described. Statistics indicate a sufficiently high probability to warrant consideration of their likely properties in advance of the Voyager encounters. Because the spin axis of Uranus lies nearly in the ecliptic and presently points approximately sunward, Voyager is likely to encounter the unique pole on configuration that has special theoretical significance. Corotation in the magnetospheres of Uranus and Neptune would probably exclude solar wind drive convection as an important driver of global magnetospheric dynamics, as it does at Jupiter and Saturn. The magnetospheres of Uranus and Neptune probably lack sufficient internal sources of plasma to produce significant levels of rotationally driven convection. The reported observation of auroral emission from Uranus has therefore motivated the development of an alternative model in which solar wind motion is coupled directly to the rotation of the ionosphere to establish a dynamo circuit which generates Birkeland currents and polar cap aurora. This model predicts the strength and configuration of the aurora as functions of the magnitude and polarity, respectively, of the planetary magnetic moment.

T W Hill↗

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.↗

NASA NTRS1986

1981N1 - A Neptune arc?

An object in the vicinity of Neptune detected in 1981 by simultaneous stellar occultation measurements at observatories near Tucson, Arizona, was interpreted as a new Neptune satellite. A reinterpretation suggests that it may have instead been a Neptune arc similar to one observed in 1984. The 1981 object, however, did not occult the star during simultaneous observations at Flagstaff, Arizona. This result constrains possible arc geometries.

Hubbard, W. B.↗

NASA NTRS1986

Estimates of the bolometric albedos and radiation balance of Uranus and Neptune

Models possessing an upper haze layer of finite optical depth and a lower cloud layer of infinite optical depth at discrete altitudes are used to bound the wavelength-averaged phase integrals and bolometric albedos of Uranus and Neptune. The models differ in the assumed value of the particles' single scattering phase function and the wavelength dependence of the haze optical depth. A range of phase functions, from the isotropic to those characterizing Titan, Jupiter, and Saturn atmosphere particles, are discussed. The results obtained imply that the meteorological regimes in the observable atmospheres of Uranus and Neptune may differ considerably; internal heat flux could play a much more important role for Neptune than for Uranus.

Pollack, J. B.↗

NASA NTRS1986

JHK photometry of Uranus and Neptune occultation candidate stars: 1986-1990

A report of 20 stars to be occulted by Neptune and 46 stars to be occulted by Uranus during the period 1986-1990 has been provided by Mink and Klemola (1985). This paper has the objective to present J, H, and K magnitudes for most of these stars. It is pointed out that Uranus and Neptune occultation events observed in the K band give optimal signal-to-noise ratios due to the planets' strong methane absorption at 2.2 microns. Program stars and standards were observed over six nights with the aid of the 30- and 74-in telescopes at the South African Astronomical Observatory in Sutherland. The obtained results show that several bright candidates exist for both planets. The brightest are N55 for Neptune and U37, U41, and U65 for Uranus.

Covault, C. E.↗