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1.
New narrow-band (100 Å) photoelectric area-scanning photometry of the Uranus disk is reported. Observations were concentrated on the two strong CH4 bands at λ 6190 and 7300 Å. Adjacent continuum regions at λ 6400 and 7500 Å were also measured for comparison. Both slit and pinhole scans were made in orthogonal directions. Disk structure in each waveband is apparent through lack of circular symmetry in the intensity distribution over the Uranus image. Polar brightening is especially prominent in the λ 7500-Å waveband. Coarse quantitative determinations of the true intensity distribution over the Uranus disk were made. For the λ 6190-Å CH4 band, Uranus exhibits a disk of essentially uniform intensity except for a hint of polar brightening. For the λ 7300-Å CH4 band, moderate limb brightening is apparent. Specifically, the true intensities at the center and limb of the planetary disk are approximately in the proportion 1:2. Extreme limb brightening, with a corresponding intensity ratio greater than 1:4, is not permitted by the observational data. 相似文献
2.
Michael J. Price 《Icarus》1978,35(1):93-98
Measurements of limb brightening on the Uranus disk within the λ7300 Å CH4 band are interpreted using an elementary inhomogeneous radiative transfer model to describe the atmosphere. A two layer model which consists of a finite, optically thin, region of conservatively scattering particles overlying a semi-infinite clear H2CH4 atmosphere satisfactorily explains the observations. The maximum optical thickness of the upper layer appears to lie in the range 0.1 to 0.2. The CH4/H2 mixing ratio in the lower layer is larger than the corresponding solar value by a factor on the order of three or greater. The results are discussed briefly in terms of current models of the Uranus atmosphere. 相似文献
3.
A photoelectric observation in the near infrared of the 10 February 1977 lunar occultation of Uranus is described and analyzed in terms of planetary radius, limb darkening, and polar brightening. Contact timings, corrected for lunar limb effects, indicate an equatorial radius of 25,700 ± 500km for the visible disk. A modified Minnaert function is used to model limb darkening and polar brightening. Least-squares fits to the observed light curve indicate that Uranus is slightly limb darkened in the passband of the observation (450 ÅA FWHM centered near 6900 ÅA) and that polar brightening is present. 相似文献
4.
We present a preliminary analysis of CH4 absorptions near 6800 Å in new high resolution spectra of Uranus. A curve of growth analysis of the data yields a rotational temperature near 100 K and a CH4/H2 ratio that is 1 to 3 times that expected for a solar type composition. The long pathlengths of CH4, apparently demanded by absorptions near 4700 Å, are qualitatively shown to be the result of line formation in a deep, predominantly Rayleigh scattering atmosphere in which continuum absorption is a strong function of wavelength. The analysis of the CH4 also yields a minimum value for the effective pressure of line formation (～ 2 atm). This value is shown to be twice that expected on Uranus if the atmosphere were predominantly H2. It is speculated that large amounts of some otherwise optically inert gas is present in the Uranus atmosphere. N2 is suggested as a possible candidate since there are cosmogonic reasons why Uranus should contain large amounts of N relative to C, He, and H, and also because the pressure-induced pure rotation spectrum of N2 could possibly account for the low brightness temperatures that have recently been observed at 33 and 350 μm. If N2 is present the planet probably possesses a surface at the 10–100 atmosphere level. 相似文献
5.
Uwe Fink Bradford A. Smith D. Chris Benner James R. Johnson H.J. Reitsema James A. Westphal 《Icarus》1980,44(1):62-71
Using a low-resolution spectrograph and a CCD array, a spectrum of Pluto from 0.58 to 1.06 μm was obtained. The spectrum had a resolution of and a signal-to-noise ratio of ～300. It showed CH4 absorption bands at 6200, 7200, 7900, 8400, 8600, 8900 and 10,000 Å. The strongest of these bands was at 8900 Å with an absorption depth of 0.23. This band was heavily saturated, compared to the weaker bands, providing proof for the gaseous origin of the observed absorptions. By applying CH4 band model parameters to our data, a total CH4 abundance of 80 ± 20 m-am was derived. This translates into a one-way abundance of 27 ± 7 m-am and a CH4 surface pressure of 1.5 × 10?4 atm. An upper limit to the total pressure of ～0.05 atm could be set. First-order calculations on atmospheric escape showed that this methane atmosphere would be stable if the mass of Pluto is increased 50% over its current value and its radius is 1400 km. Alternatively a heavier gas mixed with the CH4 atmosphere would aid its stability. The relatively large amount of gaseous CH4 observed implies that the absorption bands recently reported at 1.7 and 2.3 μm are likely due to atmospheric CH4 absorptions rather than surface frost as interpreted earlier. 相似文献
6.
A model of the atmospheric structure of Uranus is presented which differs from previous types of models in two important respects: (1) The CH4/H2 ratio is sufficiently large that CH4 is saturated to large depths in the Uranian atmosphere. (2) The internal energy flux is small compared with that due to solar heating. Because of the small internal flux, the thermal flux decreases rapidly with depth and the atmosphere is radiative to large optical depths. A CH4 droplet cloud forms where the atmosphere finally becomes convective due to the internal flux. The model is shown to be in reasonable agreement with published observations of the H2 quadrupole 3-0 and 4-0 bands, the visible (4000–6000 Å) CH4 bands, and the infrared emission spectrum. 相似文献
7.
Narrow-waveband (100 Å) photoelectric slit-scan photometry of the Neptune disk is reported. Observations were concentrated within the strong CH4 band at λ7300 Å. For comparison, measurements were also made within a continuum waveband at λ6800 Å. Point spread function data were obtained in both colors. Qualitative estimates of the true intensity distribution over the Neptune disk were made. Within the λ6800-Å continuum band, Neptune appears as an essentially uniform disk. Within the λ7300 Å CH4 band, the planet exhibits strong limb brightening. Our results appear to require the presence of an optically thin layer of brightly scattering aerosol particles high in the Neptune atmosphere. 相似文献
8.
UBV pinhole scans of the Saturn disk have been made with a photoelectric area-scanning photometer. Limb profiles, spaced parallel to the equator, were obtained over the entire southern hemisphere of the planet. Saturn was found to exhibit strong limb brightening in the ultraviolet, moderate limb brightening at blue wavelengths, and strong limb darkening in the visual region of the spectrum. Latitudinal variations in the disk profiles were found. In general, the degree of limb brightening decreases towards the polar region. Pronounced asymmetry is apparent in the disk profiles in each color. The sunward limb is significantly brighter than the opposite limb. This asymmetry depends on phase angle; approaching zero at opposition, it reaches a maximum near quadrature. Our observations are interpreted using an elementary radiative transfer model. The Saturn atmosphere is approximated by a finite homogeneous layer of isotropically scattering particles overlying a Lambert scattering haze or cloud layer. The reflectivity of the haze or clouds is a strongly dependent function of wavelength. Our best-fitting model consists of a clear H2 layer of column density ～31 km-am above the haze or clouds; the maximum permitted H2 column density is ～46 km-am. The H2 column density above the equatorial region appears to be less than at temperate latitudes. The phase-dependent asymmetry in the disk profiles is a natural consequence of the scattering geometry. Our results are consistent with current knowledge of the Saturn atmosphere. 相似文献
9.
An error in the Hayes and Belton (1977), Icarus32, 383–401) estimate of the rotation period of Neptune is corrected. If Neptune exhibits the same degree of limb darkening as Uranus near 4900 Å, the rotation period is 15.4 ± 3 hr. This value is compatible with a recent spectroscopic determination of Munch and Hippelein (1979) who find a period of 11.2?1.2+1.8 hr. However, if, as indirect evidence suggests, the law of darkening on Neptune at these wavelengths is less pronounced than on Uranus, then the above estimates may need to be lengthened by several hours. Recent photometric data are independently analyzed and are found to admit several possible periods, none of which can be confidently assumed to be correct. The period of Neptune most probably falls somewhere in the range 15–20 hr. The Hayes-Belton estimate of the period of Uranus is essentially unaffected by the above-mentioned error and remains at 24 ± 4 hr. All observers agree that the rotation period of Uranus is longer than that of Neptune. 相似文献
10.
The scattering and absorption properties of Axel dust were investigated by means of Mie theory. We find that a flat distribution of particle radii between 0 and 0.1 μm, and an imaginary part of the index of refraction which varies as λ?2.5 produce a good fit to the variation of Titan's geometric albedo with wavelength (λ) provided that τext, the extinction optical depth of Titan's atmosphere at 5000 Å, is about 10. The real part of the complex index is taken to be 2.0. The model assumes that the mixing ratio of Axel dust to gas is uniform above the surface of Titan. The same set of physical properties for Axel dust also produces a good fit to Saturn's albedo if τext = 0.7 at 5000 Å. To match the increase in albedo shortward of 3500 Å, a clear layer (containing about 7 km-am H2) is required above the Axel dust. Such a layer is also required to explain the limb brightening in the ultraviolet. These models can be used to analyze the observed equivalent widths of the visible methane bands. The analysis yields an abundance of the order of 1000 m-am CH4 in Titan's atmosphere. The derived CH4/H2 mixing ratio for Saturn is about 3.5 × 10?3 or an enhancement of about 5 over the solar ratio. 相似文献
11.
A review of visual and photographic data on the appearance of Uranus indicates that markings frequently occur on the planet. The featureless images obtained by the Stratoscope II balloon telescope are possibly the result of the broad spectral band that was used. Difference, or ratio, picture techniques which enhance color or polarization contrasts are proposed as the basis for Uranus imagery on the '79 MJU Mission. An attempt is made to predict the aspect of Uranus at high resolution on the basis of what is currently known about the Uranus atmosphere. The planet should have no visible surface, the tops of a thick NH3 cloud layer should exist near the 3–4 bar level and there is a very uncertain possibility of a thin, broken CH4 cloud layer near 300 mbar. It is proposed that if the choice of an MJU imaging system rests on Uranus objectives alone (i.e., excluding the satellites) then the system should emphasize photo-polarimetric observations between 5500 and 10 000 Å. If, however, the total mission objectives are the basis of choice then a high resolution imaging system, based on the Mariner Jupiter-Saturn system, but including a solid state silicon array would be a more suitable choice. The performance of such a system at Uranus is analyzed. 相似文献
12.
Charge-coupled device images of Uranus and Neptune taken in the 8900-Å 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 discreted cloud features similar to those reported previously by B. A. Smith, H. J. Reitsema and S. M. Larson (1979 Bull. Amer. Astron. Soc.11, 570). A time series of the restored Neptune images shows a continuous variation which may be due to the planet's rotation. 相似文献
13.
The near-infrared (0.65–2.5μm) spectral albedo of Jupiter and Saturn with 1.5% spectral resolution is presented for the center of disk and for the limb. There is a distinct difference in the continuum slope between Jupiter and Saturn which may be attributed to a difference in the dust content or composition of the two atmospheres. There is an indication of limb brightening in the deepest CH4 bands on Saturn. No limb brightening is found for Jupiter. 相似文献
14.
L. Trafton 《Icarus》1980,44(1):53-61
The presence of CH4 ice on Pluto implies that Pluto may have a substantial atmosphere consisting of heavy gases. Without such an atmosphere, sublimation of the CH4 ice would be so rapid on a cosmogonic time scale that either such an atmosphere would soon develop through the exposure of gases trapped in the CH4 ice or else the surface CH4 ice would soon be all sublimated away as other, more stable, ices became exposed. If such stable ices were present from the beginning, the existence of CH4 frosts would also imply that Pluto's present atmosphere contains a remnant of its primordial atmosphere. 相似文献
15.
The spectrum of Titan from 4800 to 11 000 Å has many CH4 absorption bands which cover a range of intensities of several orders of magnitude. Yet even the strongest of these bands in Titan's spectrum has considerable residual central intensity. Some investigators have concluded that these strong CH4 bands must be highly saturated, but recent laboratory measurements of the bands made at room temperature show that curve-of-growth saturation is very small. At the presumed low pressures and temperatures in Titan's atmosphere, we show that saturation is very dependent on the band model parameters. However, in either a simple reflecting layer model or in a homogeneous scattering model saturation cannot be the principal cause of the filling in of these strong CH4 bands if our best estimates of the band model parameters are correct. We find that an inhomogeneous scattering model atmosphere with fine “Axel dust” above most ot the CH4 gas is needed to fill in the band centers. The calculated spectrum of one particular model of this class is compared to observations of Titan. Our essential conclusion is that Titan does have most of its scattering particles above most of the CH4 gas which has an abundance of at least 2 km-am. This large abundance of CH4 is necessary to produce the 6420-Å feature recently discovered in Titan's spectrum. 相似文献
16.
《Planetary and Space Science》1999,47(8-9):1077-1100
The geometric albedos of Uranus and Neptune, inferred from archived Hubble Space Telescope observations and from the ground-based measurements of Karkoschka, 1994, are modeled in the wavelength range 2200–4200 Å. The radiative transfer model, which includes Rayleigh–Raman scattering and Mie scattering by haze particles, aims at reproducing the fine structure of the geometric albedos at a resolution of 2–10 Å. The steep variation of the total optical depth allows to investigate the influences of both the stratospheric and tropospheric haze layers and that of the deep tropospheric cloud, although their relative importance is difficult to estimate accurately. Using the haze models of Baines et al., 1995, the optical properties of the Mie scatterers are inferred. The haze material on Uranus is characterized by a slowly decreasing imaginary index of refraction: ni varies from about 0.10 to 0.01–0.02 between 2200 and 4200 Å. Below 3000 Å, the absorptivity of Neptuness haze material is comparable to that on Uranus or slightly lower (ni ∼ 0.03–0.10). Above 3000 Å, it exhibits a steeper decrease (from 0.30 to 0.003). The main source of uncertainty at longer wavelengths is the reflectivity of the underlying (H2S ?) cloud. At shorter wavelengths, molecular scattering strongly dominates Mie scattering and the determination of the absorptivities is estimated to be accurate within a factor of 2. For Neptune, there is an additional uncertainty due to the inability of the initial haze model to provide a fit to the observed albedo. The Baines et al. model was modified by multiplying the number-densities of the hydrocarbons haze layers by a factor of 2.5–4.8, making it more consistent with the results of Pryor et al., 1992. For Uranus, these results suggest a darkening of the southern hemisphere since the Voyager epoch, in agreement with recent HST imaging. As a whole, the Neptunian haze seems to be more transparent than that of Uranus, possibly owing to the more turbulent dynamical state of the troposphere. Longwards of 3000 Å, the inferred absorptivities are consistent with laboratory measurements on tholins produced from CH4–H2 gas mixtures (Khare et al., 1987). The para-H2 mole fraction on both planets is constrained from the strength of a prominent H2 Raman feature at 2853 Å. On Uranus, at latitudes between 45 and 75°S and in the 50–500 mbar pressure range, the best agreement is obtained with an equilibrium para-H2 distribution. On Neptune, there is an indication of a slight departure from equilibrium in the same pressure range at mid-southern latitudes. Although this new method is significantly less accurate, its results are consistent with those of previous investigations based on the analysis of H2 quadrupole lines (Baines et al., 1995) and of the Voyager IRIS spectra (Conrath et al., 1998). 相似文献
17.
Laboratory band-model absorption coefficients of CH4 are used to calculate the Uranus spectrum from 5400 to 10,400 Å. A good fit of both strong and weak bands for the Uranus spectrum over the entire wavelength interval is achieved for the first time. Three different atmospheric models are employed: a reflecting layer model, a homogeneous scattering layer model, and a clear atmosphere sandwiched between two scattering layers. The spectrum for the reflecting layer model exhibits serious discrepancies but shows that large amounts of CH4 (5–10 km-am) are necessary to reproduce the Uranus spectrum. Both scattering models give reasonably good fits. The homogeneous model requires a particle scattering albedo and an abundance per scattering mean free path . The parameters derived from the sandwich layer model are: forsb the upper scattering layer a continuum single scattering albedo () of 0.995 and a scattering optical depth variable with wavelength consistent with Rayleigh scattering; for the clear layer they are a CH4 abundance (a) of 2.2 km-am and an effective pressure (p) ? 0.1 atm; for the lower cloud deck a Lambert reflectivity (L) of 0.9 resulted. A severe depletion of CH4 in the upper scattering layer is required. An enrichment of CH4/H2 over the solar ratio by a factor of 4–14 in the lower atmosphere is, however, indicated. 相似文献
18.
V.G. Teifel 《Icarus》1983,53(3):389-398
Modeling of the geometric albedo of Uranus in and near prominent methane absorption bands between 0.5 and 0.9 μm indicates that the visible atmosphere probably consists of a thin aerosol haze layer (τscat ? 0.3?0.5; ωH ? 0.95) above an optically thick, semi-infinite Rayleigh scattering atmosphere. A significant depletion of methane gas above the haze layer is indicated. The mixing ratio of methane in the lower atmosphere is consistent with a value of CH4/H2 ? 3 × 10?3, comparable to those derived for Jupiter and Saturn. 相似文献
19.
R.E. Danielson 《Icarus》1977,30(3):462-478
Models of the interior of Uranus (Podolak, 1976) suggest that the abundances of such substances as CH4 are greatly enhanced with respect to solar abundances of heavy elements. Such enhancement leads to a new type of model atmosphere for Uranus, which agrees with observation if the internal energy flux is small (?10%) compared with the absorbed solar energy. An important feature of the models is the presence of a cloud of CH4 droplets whose top is at a temperature of ?90°K and a pressure of ?4atm. Above the cloud, the atmosphere is stable because of the rapid decrease of the thermal flux with depth. Being saturated, most of the observable gaseous CH4 is near the cloud; the CH4 abundance above the cloud, of the order of 5 km-am, is a very sensitive function of the cloud-top temperature. 相似文献
20.
Orthogonal narrow-band (100 Å) photoelectric slit scan photometry of Uranus has been used to infer the basic two-dimensional structure of the disk within the 7300-Å methane band. Numerical image reconstruction and restoration techniques have been applied to quantitatively estimate the degrees of polar and limb brightening on the planet. Through partial removal of atmospheric smearing, an effective spatial resolution of approximately 0.9 arcsec has been achieved. Peak polar, limb, and central intensities on the disk are in the respective proportions 3:2:1. In addition, the bright polar feature is displaced from the geometric pole towards the equator of the planet. 相似文献