摘要:

The following reports summarize a number of post‐Jupiter aspects of the first phase of the Voyager Mission, a major element in NASA's program of exploration of the outer planets. The objectives of this phase included comparative studies of the Jovian and Saturnian systems and studies of the inter‐planetary medium at increasing distances from the sun. Scientific reports on the mission through the Jupiter encounters may be found in the Journal of Geophysical Research (86, 8123–8841, 1981). With the completion of the mission through Saturn and with both spacecraft continuing to function well, a second mission phase was added with the objectives of exploring Uranus and investigating the interplanetary medium to beyond 20 AU. The Uranus flyby has been chosen so that a third phase can include the exploration of Neptune and extend the investigation of the interplanetary medium to distances beyond 40 AU and at distances of more than 20 AU above the ecl

ISSN:0148-0227    

DOI:10.1029/JA088iA11p08639

年代:1983

数据来源: WILEY

摘要:

On August 25, 1981, the Voyager 2 photopolarimeter system observed a stellar occultation by Saturn's rings. We present a brief description of this experiment along with details of the data reduction. The occultation results are given in tabular and graphical form at a resolution of 60 km. Histograms of the frequency of optical depth show dominantly unimodal distributions in each of the classical ring elements. The frequency distribution of the entire ring system shows three modes at τ ≈ 0.08, τ ≈ 0.5, and τ

ISSN:0148-0227    

DOI:10.1029/JA088iA11p08643

年代:1983

数据来源: WILEY

摘要:

Voyager 2 images provide a basis for detailed study of the morphology and circulation of Saturn's northern mid‐latitudes. Both Saturn's large‐scale cloud bands and the distribution of its local cloud features have a characteristic zonal organization. The region between 30°N and 45°N contains two oppositely directed jets in close proximity, with many bright, active features in the westward jet, and an unusual ribbon‐like wave feature encircling the planet in the eastward jet. Several of the smaller features within the westward jet do not remain at fixed latitudes and interact with each other. One group of v‐shaped features is found to have periods of high activity correlated with the passage of a cyclonic bright spot. The ribbon wave was Fourier analyzed to determine its spectral composition. The greatest power is near wave number 9, with significant additional peaks appearing at planetary wave numbers 19, 25–27, 35–38, and 47–51. The phase velocity increases with wave number but is not well described by a Rossby‐Haurwitz dispersion relation. The curvature of the mean wind profile obtained from cloud tracking indicates that the westward jet exceeds the standard barotropic instability condition, while the eastward jet marginally exceeds the deep‐circulation instability condition of Ingersoll and Pollard (1982). The rms eddy velocities on Saturn are less than half as large as those

ISSN:0148-0227    

DOI:10.1029/JA088iA11p08650

年代:1983

数据来源: WILEY

摘要:

The temperature and composition of the upper atmosphere of Saturn have been inferred from Voyager 2 ultraviolet spectrometer occultation measurements made by observing the sun and the star δ‐Scorpii while they were being occulted by Saturn. The observations analyzed here provide atmospheric parameters from 2900 km down to 960 km above the 1‐bar level referred to the equator. The temperature in the model simulation of the data is 420±30 K down to about 1600 km. Below 1600 km the temperature decreases with a variable lapse rate down to 120±30 K near the methane homopause located at 1010±40 km. A constant temperature at 120 K is applied in the model for the region of the methane homopause down to 960 km where the present analysis terminates. Column amounts of H2and H were measured from 2900 km down to about 1100 km, giving densities of [H2] ∼ 1.0×108cm−3and [H] ∼ 5.5×106cm−3near the exobase at 2500 km. Near the methane homopause the H2density is [H2] = 1.2 × 1012cm−3with a CH4number density mixing ratio of 6.0×10−5. The eddy diffusion coefficient in the vicinity of the methane homopause is estimated to be ∼5.0×106cm² s−1. The atomic hydrogen density profile suggests a downward H flux of ∼1.8×109cm−2s−1from near the exobase down to

ISSN:0148-0227    

DOI:10.1029/JA088iA11p08667

年代:1983

数据来源: WILEY

摘要:

We have reduced and tabulated photometry and polarimetry data at 2640 and 7500 Å observed by the Voyager 2 photopolarimeter experiment. Spatially resolved limb‐to‐terminator scans across Saturn's Equatorial Zone from 12° to 68° phase angle provide information on the altitude distribution of UV absorbing hazes and the phase function and polarizing properties of stratospheric and tropospheric aerosols. Limb‐to‐terminator scans across the northern hemisphere at 10° phase angle are used to study altitude variations of the tropospheric cloud at several latitudes. For the Equatorial Zone we find (1) the UV photometry and polarimetry are best fit by Rayleigh's phase matrix. (2) A stratospheric haze of small particles is allowed as long as the optical depth is near unity or less and the center of the haze layer is in the 30‐ to 70‐mbar region. A diffuse haze fits better than a thin layer, and the aerosol/gas mixing ratio diminishes above 10 mbar. The vertical distribution and optical depth of the haze differ significantly from models proposed by others. To be in agreement with ground based and other spacecraft data, the haze optical depth is about 0.4 at 2640 Å and decreases by a factor of 10 or more at 6400 Å. If the haze aerosol scattering properties are similar to those for spheres with mean radius 0.1 μm, their imaginary refractive index is 0.4 or larger at 2640 Å and the total column density above the tropopause is 109cm−2. (3) UV contrasts between belts and zones are interpreted as altitude variations in the top of the tropospheric cloud. The altitudes derived here for three latitudes agree with altitudes derived from ground‐based methane band studies and analyses of polarization from Pioneer 11. A high altitude absorber is abundant in the polar regions. (4) At 7500 Å, the phase function of tropospheric aerosols in the Equatorial Zone is described by a synthetic two‐term Henyey‐Greenstein function withg1= 0.54 ± 0.11,g2= −0.47 ± 0.08, ƒ = 0.87 ± 0.03, and ω = 0.986. The single scattering albedo in the North Equatorial Belt is ω = 0.967. The Equatorial Zone tropospheric aerosols are positively polarizing at all

ISSN:0148-0227    

DOI:10.1029/JA088iA11p08679

年代:1983

数据来源: WILEY

摘要:

Observations of Titan's whole disk polarization at 2460 and 7500 Å are presented and analyzed in terms of model scattering atmospheres. If the Titan aerosols are spherical or nearly spherical, no single combination of refractive index and size distribution is able to fit data at both wavelengths. However, a vertically inhomogeneous distribution suggested by Tomasko and Smith (1980), characterized by a size gradient with altitude, fits the data at 2640 Å moderately well but must be modified at intermediate and large optical depths to fit the 7500‐Å data. Results for synthetic phase functions indicate that the single scattering polarization must be 70% or larger in the UV and 78% or larger in the near‐IR at 90° phase angle, depending on the phase function. If the correct phase function is similar to that for 0.5‐μm‐radius spheres, the UV single‐scattered polarization must be 84% and the near‐IR single‐scattered polarization must be over 90%. Such large polarizations are impossible for 0.5‐μm‐radius spheres but may be possible for nonspherical particles with effective radii near 0.5 µm, although the existence of nonspherical particles with the scattering properties required by these and other observations h

ISSN:0148-0227    

DOI:10.1029/JA088iA11p08699

年代:1983

数据来源: WILEY

摘要:

A sharp stratospheric emission feature at 667 cm−1in the Voyager infrared spectra of Titan is associated with the ν2Qbranch of CO2. A coupling of photochemical and radiative transfer theory yields an average mole fraction above the 110 mbar level of, with most of the uncertainty being due to imprecise knowledge of the vertical distribution. CO2is found to be in a steady state, with its abundance being regulated principally by the ∼72 K cold trap near the tropopause and secondarily by the rate at which water‐bearing meteoritic material enters the top of the atmosphere. An influx of water about 0.4 times that at the top of the terrestrial atmosphere is consistent with a combination of the observed CO2abundance and a steady state CO mole fraction of 1.1×10−4; the theoretical value for CO is close to the value observed by Lutz et al. (1983), although there are large margins for error in both numbers. If steady state conditions for CO prevail, little information is available regarding the evolution of Titan's a

ISSN:0148-0227    

DOI:10.1029/JA088iA11p08709

年代:1983

数据来源: WILEY

摘要:

The diffusion problem of light gases in the atmosphere of Titan is reassessed after Voyager encounter. The pre‐Voyager theoretical work of Hunten (1973a) is completed by the use of parameters derived from Voyager encounter: the radial distances of turbopause level (3500 km) and critical level (4100 km), the exospheric temperature (186 K) and the volume mixing ratio of 2×10−3for H2. An analytical expression of the solution of the diffusion equation is given for a spherical geometry and an isothermal atmosphere. It is used to predict an altitude profile of H2in the upper atmosphere of Titan. The modification of this profile by a possible return flow of H2from a torus to the atmosphere of Titan is addressed, and is found negligible since there is observational evidence from Voyager that the H2density is less than 100 cm−3in the torus, at large distance from Titan. The H2concentration at the exobase level is 3.1×105cm−3, and the thermal escape of H2is nearly equal to the diffusion limiting flux of 3.4×109molecules cm−2s−1at the turbopause level. Photodissociation of CH4has to provide for an equal constan

ISSN:0148-0227    

DOI:10.1029/JA088iA11p08716

年代:1983

数据来源: WILEY

摘要:

Limits on the physical properties of the scattering haze near the top of Titan's atmosphere are derived from data obtained from seven high‐phase‐angle images from Voyager 1 and 2. From the ratio of the intensities observed at two different high phase angles, an estimate can be made of the forward scattering lobe of the single‐scattering phase function. Comparing the forward scattering estimate with diffraction lobes from particles of different radii, we conclude that the average radius of the particles found in the upper few tenths of an optical depth exceeds 0.19 μm. Judging from data observed at four different phase angles, the haze particles probably have a refractive index near 1.6 and a mean size of ∼0.5 μm, if the widths of their diffraction peaks are close to those for equal volume spheres. However, the highly polarizing nature of the particles over a broad wavelength band pass [Tomasko and Smith, 1982; West et al. this issue] combined with their forward scattering behavior makes it very unlikely that the particles are spherical. The nonsphericity contributes to the uncertainty about the radii of the particles, but it is thought that the average radius is several tenths of

ISSN:0148-0227    

DOI:10.1029/JA088iA11p08721

年代:1983

数据来源: WILEY

摘要:

Control networks of Saturnian satellites of Tethys and Dione have been established photogrammetrically from pictures taken by the two Voyager spacecraft during their flybys. Coordinates of 110 points on Tethys and 126 points on Dione are listed; selected points are identified on U.S. Geological Survey maps of the satellites, and many are identified by name. Measurements of these points were made on six pictures from Voyager 1 and 21 from Voyager 2 for Tethys and on 27 pictures from Voyager 1 and 1 from Voyager 2 for Dione. The longitude systems on the satellites have been defined by craters on their surfaces. The mean radii have been determined as 524±5 km for Tethys and 559±5 km for Dion

ISSN:0148-0227    

DOI:10.1029/JA088iA11p08729

年代:1983

数据来源: WILEY