W. Macy

The diameter and reflectance of Triton

Abstract New image-tube spectra of Triton are analyzed for a determination of the reflectance of the satellite between 0.32 and 0.74 μm. Comparison of the violet reflectance of Triton with that of terrestrial minerals, lunar samples, and meteorites, gives evidence that the satellite surface is composed largely of rocky material having the same sources of violet opacity (mineral charge transfer and crystal field transitions). New radiometric observations set a stringent upper limit to the satellite radius ( r ⩽ 2600 km) and a lower limit to the geometric albedo ( p v ⩾ 0.19). The albedo can be somewhat higher and still within the range allowed by a rocky surface. No useful constraints can be put on the mean density of Triton because of remaining uncertainties in the radius and the mass. The image-tube spectra show no evidence of gaseous absorption in the methane bands, though a stronger band has been found in the infrared at 2.3 μm (Cruikshank and Silvaggio, 1979, in press; the near-infrared photometric colors may be affected by the CH 4 band. Rayleigh scattering computations of a potential inert atmospheric component of Triton appear to preclude the presence of large quantities of nitrogen and the noble gases.

Inhomogeneous models of the atmosphere of Saturn

Abstract Analyses of ultraviolet, visible, and near-infrared spectra of Saturn lead to an inhomogeneous atmospheric model, having a clear gas layer which lies above an absorbing particle layer which lies above an ammonia haze layer. The boundary between the clear layer and the absorbing particle layer is at a pressure of 0.2 atm in the equatorial region and 0.3 atm in the temperate region. The boundary between the absorbing particle layer and the haze layer is at the radiative-convective boundary. Observations of ammonia absorption lines indicate that sunlight penetrates the haze to the ammonia sublimation level at a depth of 1.1 atm. Absorbing particles cause the observed decrease in reflectivity from visible to ultraviolet wavelengths. Consideration of the wavelength variation of Mie scattering parameters leads to an upper limit of about 0.2 μm for the particle radii and a particle number density of 10 3 cm −3 . Some possible particle compositions are discussed. Comparison of computed 3-0 and 4-0 band hydrogen quadropole line equivalent widths with observed values leads to a haze layer optical thickness above the ammonia sublimation level of approximately 10. Equivalent widths computed for an equilibrium distribution of states agree better with observed values than those computed for a normal distribution. Methane 3 ν 3 band manifold equivalent widths are in best agreement with measured equivalent widths for a CH 4 /H 2 abundance ratio of 2 × 10 −3 , which is 4.5 times the solar C/H ratio.

Interpretation of the 6818.9-Å methane line in terms of inhomogeneous scattering models for Uranus and Neptune

Abstract We have obtained high-resolution spectra of Uranus and Neptune in the methane transition near 6800 A, and in particular, the 6818.9A feature. Calculated equivalent widths for this line using recently proposed models of the atmospheres of these two planets indicate that the C/H ratio is greater than or equal to 5 × 10 −3 below the CH 4 saturation level. This value is 12 times the solar mixing ratio. The half-widths of the computed line profiles are in agreement with the observed half-widths. Therefore, it is unnecessary to introduce an unidentified constituent with an abundance comparable to H 2 , postulated recently by Belton and Hayes, and by Bergstrahl, to account for the observed line broadening.

Mixing ratios of methane, ethane, and acetylene in Neptune's stratosphere

Abstract Matching computed spectra for the ν 4 band of methane, the ν 9 band of ethane, and the R branch of the ν 5 band of acetylene to observed spectra for Neptune suggests mixing ratios of CH 4 /H 2 ∼ 10 −3 −10 −2 , C 2 H 6 /H 2 ∼ 10 −6 , and C 2 H 2 /H 2 ∼ 10 −8 in the stratosphere.

The H2 4-0 S(0, 1, and 2) quadrupole features in Jupiter

Abstract We have detected the H 2 S 4 (2) feature in Jupiter. Using new low-temperature laboratory measurements for NH 3 , we have conducted a detailed analysis to determine an improved equivalent width for the Jovian H 2 S 4 (0) line. We have found good agreement of the equivalent widths computed from published models with the observed H 2 4−0 line equivalent widths for Jupiter. The effective line formation temperature of ∼150°K at the averaged disk center of Jupiter found from the nominal S 4 (2)/ S 4 (0) ratio is consistent with an H 2 ortho-para ration for equilibrium tropospheric H 2 in Jupiter.

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.

Infrared scans of Saturn

Abstract We present scans at five wavelengths between 7.8 and 25 μm north-south along Saturns central meridian and east-west parallel to the equator through the subearth point. The brightening of Saturns South Pole at 12.7 μm was more enhanced in 1977 than in 1978 due to the 5° greater declination of the polar axis in 1977. There is a plateau in the Southern Hemisphere between −30 and −60° latitude in the 7.8 and 12.7 μm scans. The apparent temperature of the rings decreased as Saturn approached the equinox. We find generally that the strongest ring emission arises from the C ring.

Infrared scans of Jupiter

Abstract We present spatial scans at eight wavelengths between 7.8 and 24 μm along Jupiters meridian and along the Equatorial Zone, the North Equatorial Belt, and the South Tropical Zone. Some features of these scans are differences in brightness temperatures between the Great Red Spot and the surrounding South Tropical Zone, a higher temperature at high northern latitudes than high southern latitudes, equal or possibly higher temperatures of zones than belts at 7.8 μm in contrast to higher temperatures of belts at other observed wavelengths, very strong limb darkening at 8.9 μm possibly due to a large scale height or a nonuniform distribution of solid NH 3 particles, and inhomogenities within belts and zones.

On the clouds of Uranus

Several models for the atmosphere of Uranus are considered. If the H2 abundance is less than 250 km-am and the internal heat source is only a few percent of the total emitted energy then the cloud at the base of the atmosphere may be composed of solid CH4 particles, while if the H2 abundance is greater than 250 km-am or if the internal heat source is near the current upper limit of 35% of the total emitted energy the cloud at the base of the atmosphere may be composed of either solid NH3 or H2S particles.

Measurements of the H2 4-0 quadrupole bands of Uranus and Neptune

It is found that the equivalent widths of the lines of the 4-0 H2 quadrupole band on Uranus and Neptune are substantially smaller than the values found by some previous observers. An analysis of the results based on a range of atmospheric models yields H2 abundances of 240 + or - 60 km-amagats for Uranus and greater than approximately 200 km amagats for Neptune.