共查询到20条相似文献，搜索用时 15 毫秒
1.
We present far-infrared observations of Saturn and Venus made within four spectral bands (31 to 38, 47 to 67, 71 to 94, and 114 to 196 μm) using a 32-cm airborne telescope during May 1977. The set of brightness temperatures obtained from Saturn is analyzed on the basis of thermal models of the atmosphere of this planet. The best agreement is obtained with an effective temperature of about 95°K for the planet itself and a ring contribution corresponding to brightness temperatures ranging from 55 to 70°K. These values of the temperature of the ring system are smaller than the ones measured at shorter wavelengths and could be indicative of a decreasing emissivity of the rings in the far infrared. 相似文献
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3.
R.R. Daniel S.K. Ghosh K.V.K. Iyengar T.N. Rengarajan S.N. Tandon R.P. Verma 《Icarus》1982,49(2):205-212
We present far-infrared observations of Saturn in the wavelength band 76–116 μm, using a balloon-borne 75-cm telescope launched on 10 December 1980 from Hyderabad, India, when B′, the Saturnicentric latitude of the Sun, was 4°.3. Normalizing with respect to Jupiter, we find the average brightness temperature of the disk-ring system to be 90 ± 3° K. Correcting for the contribution from rings using experimental information on the brightness temperature of rings at 20 μm, we find TD, the brightness temperature of the disk, to be 96.9 ± 3.5° K. The systematic errors and the correction for the ring contribution are small for our observations. We, therefore, make use of our estimate of TD and earlier observations of Saturn when contribution from the rings was large and find that for wavelengths greater than 50 μm, there is a small reduction in the ring brightness temperature as compared to that at 20 μm. 相似文献
4.
We present scans at five wavelengths between 7.8 and 25 μm north-south along Saturn's central meridian and east-west parallel to the equator through the subearth point. The brightening of Saturn's 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. 相似文献
5.
Observations of the rings of Saturn at 2–4 μm reveal the presence of a 3.6-μm peak in the infrared reflectivity. This peak is consistent with a particle size of ? 50 μm, and a composition of pure H2O ice. The quoted size may only be indicative of the textural scale of frost on the surface of larger particles. The presence of small amounts of CH4 in the form of a clathrate, however, cannot be ruled out by our measurements. 相似文献
6.
G.H. Rieke 《Icarus》1975,26(1):37-44
Infrared observations of Saturn from 5 to 40 μm are described. There is intense limb brightening at 12.35 μm over the southern polar cap. The C ring is anomalously bright at 10 and 20 μm and has bluer (hotter) colours than the A and B rings. The ring spectra have been extrapolated beyond 40 μm and subtracted from low-resolution far-infrared measurements to show that the far-infrared spectrum of the disk of Saturn is qualitatively similar to that of Jupiter and that Saturn radiates 2.5 ± 0.6 times the energy it absorbs from the Sun. 相似文献
7.
I.G. Nolt W.M. Sinton L.J. Caroff E.F. Erickson D.W. Strecker J.V. Radostitz 《Icarus》1977,30(4):747-759
We have resolved the relative rings-to-disk brightness (specific intensity) of Saturn at 39 μm (δλ ? 8 μm) using the 224-cm telecscope at Mauna Kea Oservatory, and have also measured the total flux of Saturn relative to Jupiter in the same bandpass from the NASA Learjet Observatory. These two measurements, which were made in early 1975 with Saturn's rings near maximum inclination (b′ ? 25°), determine the disk and average ring (A and B) brightness in terms of an absolute flux calibration of Jupiter in the same bandpass. While present uncertainties in Jupiter's absolute calibration make it possible to compare existing measurementsunambiguously, it is nevertheless possible to conclude the following: (1) observations between 20 and 40 μm are all compatible (within 2σ) of a disk brightness temperature of 94°K, and do not agree with the radiative equilibrium models of Trafton; (2) the rings at large tilt contribute a flux component comparable to that of the planet itself for λ ? 40 μm and (3) there is a decrease of ～22% in the relative ring: disk brightness between effective wavelengths of 33.5 and 39 μm. 相似文献
8.
Very low values of the radio brightness temperature of the rings of Saturn indicate that their high refar reflectivity is not simply due to a gain effect in the backscattering direction. These two sets of observations are consistent with the ring particles having a very high single scattering albedo at radio wavelenghts, with multiple scattering effects being important. Comparison of scattering calculations for ice and silicate particles with the radio and radar observations imply a mean particle radius of ～1 cm. The ice bands observed in the rings' near-infrared reflectivity spectra are formed by scattering within a microstructure on the surface of the ring particles, with the scattering centers being 25–125 μm in size. The Poynting-Robertson effect has caused a significant spiraling-in of the ring particles, probably resulting in a broadening of the rings. The inferred mean size is consistent with a model in which meteoroid impacts have caused a substantial reduction in the mean particle size from its initial value. 相似文献
9.
The backscattered reflectivity of Jupiter's ring has been previously measured over distinct visible and near infrared wavelength bands by a number of ground-based and spaceborne instruments. We present spectra of Jupiter's main ring from 2.21-2.46 μm taken with the NIRSPEC spectrometer at the W.M. Keck observatory. At these wavelengths, scattered light from Jupiter is minimal due to the strong absorption of methane in the planet's atmosphere. We find an overall flat spectral slope over this wavelength interval, except for a possible red slope shortward of 2.25 μm. We extended the spectral coverage of the ring to shorter wavelengths by adding a narrow-band image at 1.64 μm, and show results from 2.27-μm images over phase angles of 1.2°-11.0°. Our images at 1.64 and 2.27 μm reveal that the halo contribution is stronger at the shorter wavelength, possibly due to the redder spectrum of the ring parent bodies as compared with the halo dust component. We find no variation in main ring reflectivity over the 1.2°-11.0° phase angle range at 2.27 μm. We use adaptive optics imaging at the longer wavelength L′ band (3.4-4.1 μm) to determine a 2-σ upper limit of 22 m of vertically-integrated I/F. Our observing campaign also produced an L′ image of Callisto, showing a darker leading hemisphere, and a spectrum of Amalthea over the 2.2-2.5 and 2.85-3.03 μm ranges, showing deep 3-μm absorption. 相似文献
10.
Dennis B. Ward 《Icarus》1977,32(4):437-442
The spectrum of Saturn and its rings between 45 and 115 μm has been measured at an average resolving power of 14 from the NASA Lear Jet. The combined brightness temperature of the rings and planetary disk decreases beyond 65 μm, in disagreement with previous results. A brightness temperature of 65 ± 10°K is obtained for the planetary disk in the 80–110-μm wavelength range if a large-particle, constant-emissivity model is assumed for the rings. The possible effects of small particles in the rings are briefly considered. 相似文献
11.
Infrared photometry of Titan, Saturn, and Saturn's Rings at 3.5, 4.9, 17.8, and 18.4 μm is reported. Comparison of the albedo of Titan in the 4.9 μm “window” with the albedo of the rings and with laboratory spectra suggests that frost, possibly water ice, could be a major constituent. If thick clouds are present they must be very dark at 4.9 μm. The 17.8 and 18.4 μm data are not consistent with a clear, dense molecular hydrogen atmosphere. 相似文献
12.
We have observed Rhea (S5) at 1.6 μm and 2.2 μm at Mt. Wilson using the Caltech photometer on the 1.52m and 2.54m telescopes. The infrared spectral reflectances relative to 0.55μm are 0.8 (±0.1 p.e.) at 1.65μm and 0.6 (±0.1 p.e.) at 2.2μm. Such absorption bands in the near infrared are not consistent with spectra of most rocks or minerals; even carbonaceous chondritic materials have nearly flat reflectances over this spectral region. Frosts, however, have strong absorption bands in the 1–3μm region. In particular, the broadband infrared reflectances of Rhea are similar to those of the Galilean satellites Europa (J2) and Ganymede (J3) and also the rings of Saturn (all of which are known from high resolution scans to have water frosts on their surfaces). The broadband photometry does not have sufficient resolution to identify the frost species: but Rhea's low density, high albedo and relatively flat reflectance from 0.3μm to 1.1μm as well as the low infrared reflectances reported here are consistent with the presence of water ice on Rhea's surface. 相似文献
13.
The wavelength dependence of the reflectivity of Io indicates the presence of two materials on the surface of this satellite of Jupiter. These materials are sulfur and an unspecified material (R1) which shows a wavelength dependence of its reflectivity for 0.3 μm < λ < 1.0 μm similar to the non-H2O frost spectrum of the rings of Saturn. A 60/40 admixture of these two spectra matches the observed reflection spectrum of Io from 0.3μm–3 μm, if the spectrum of R1 is featureless for λ > 1 μm. Sulfur will give rise to a posteclipse brightening. The variation with wavelength of the temperature dependence of the reflectivity of sulfur will allow an observational confirmation of the presence of sulfur on Io. The material R1 should show a large geometrical albedo. The translucency of sulfur is consistent with the polarization-phase curve to Io. The material R1 is also required to be translucent. The thermal conductivity of a cooled sulfur powder under vacuum was measured and found to agree with the value determined for the upper layer of Io from observations at 10 μm. It is shown that this agreement is not necessarily meaningful. 相似文献
14.
《Icarus》1987,72(1):69-78
Observations of the Uranian rings were made in several color filters by the Voyager Imaging Science experiment in January 1986 for the purpose of determining the color of the rings. Selected images were taken through the Violet (λ = 0.41 μm), Clear (λ = 0.48 μm), and Green (λ = 0.55 μm) filters of the Voyager 2 narrow angle camera. The results of the analysis are consistent with the α, β, η, γ, δ, and ϵ rings being very dark, with flat spectra throughout the visible, and are comparable to the latest Voyager results showing a lack of color for the Uranian satellites. The general lack of color in the ring/satellite system of Uranus is remarkably different than the case of the distinctly reddish systems of Jupiter and Saturn. The unique combination of low absolute reflectivity and flat spectrum which characterizes the Uranian rings supports the concept that the Uranian ring material is compositionally distinct from either the Si- and S-rich Jovian ring and inner satellites, or the water-ice-rich rings and inner satellites of Saturn. Of all cosmically abundant materials, the candidate which best matches the low brightness and flat spectrum of the Uranian rings is carbon. 相似文献
15.
We present adaptive optic images of Uranus obtained with the 10-m W. M. Keck II telescope in June 2000, at wavelengths between 1 and 2.4 μm. The angular resolution of the images is ∼0.06-0.09″. We identified eight small cloud features on Uranus's disk, four of which were in the northern hemisphere. The latter features are ∼1000-2000 km in extent and located in the upper troposphere, above the methane cloud, at pressures between 0.5 and 1 bar. Our data have been combined with HST data by Hammel et al. (2001, Icarus153, 229-235); the combination of Keck and HST data allowed derivation of an accurate wind velocity profile. Our images further show Uranus's entire ring system: the asymmetric ? ring, as well as the three groups of inner rings (outward from Uranus): the rings 6+5+4, α+β, and the η+γ+δ rings. We derived the equivalent I/F width and ring particle reflectivity for each group of rings. Typical particle albedos are ∼0.04-0.05, in good agreement with HST data at 0.9 μm. 相似文献
16.
Philip D. Nicholson Matthew M. Hedman Mark R. Showalter Jeffrey N. Cuzzi Fabrizio Capaccioni Gary B. Hansen Pierre Drossart Bonnie J. Buratti Angioletta Coradini 《Icarus》2008,193(1):182-212
Soon after the Cassini-Huygens spacecraft entered orbit about Saturn on 1 July 2004, its Visual and Infrared Mapping Spectrometer obtained two continuous spectral scans across the rings, covering the wavelength range 0.35-5.1 μm, at a spatial resolution of 15-25 km. The first scan covers the outer C and inner B rings, while the second covers the Cassini Division and the entire A ring. Comparisons of the VIMS radial reflectance profile at 1.08 μm with similar profiles at a wavelength of 0.45 μm assembled from Voyager images show very little change in ring structure over the intervening 24 years, with the exception of a few features already known to be noncircular. A model for single-scattering by a classical, many-particle-thick slab of material with normal optical depths derived from the Voyager photopolarimeter stellar occultation is found to provide an excellent fit to the observed VIMS reflectance profiles for the C ring and Cassini Division, and an acceptable fit for the inner B ring. The A ring deviates significantly from such a model, consistent with previous suggestions that this region may be closer to a monolayer. An additional complication here is the azimuthally-variable average optical depth associated with “self-gravity wakes” in this region and the fact that much of the A ring may be a mixture of almost opaque wakes and relatively transparent interwake zones. Consistently with previous studies, we find that the near-infrared spectra of all main ring regions are dominated by water ice, with a typical regolith grain radius of 5-20 μm, while the steep decrease in visual reflectance shortward of 0.6 μm is suggestive of an organic contaminant, perhaps tholin-like. Although no materials other than H2O ice have been identified with any certainty in the VIMS spectra of the rings, significant radial variations are seen in the strength of the water-ice absorption bands. Across the boundary between the C and B rings, over a radial range of ∼7000 km, the near-IR band depths strengthen considerably. A very similar pattern is seen across the outer half of the Cassini Division and into the inner A ring, accompanied by a steepening of the red slope in the visible spectrum shortward of 0.55 μm. We attribute these trends—as well as smaller-scale variations associated with strong density waves in the A ring—to differing grain sizes in the tholin-contaminated icy regolith that covers the surfaces of the decimeter-to-meter sized ring particles. On the largest scale, the spectral variations seen by VIMS suggest that the rings may be divided into two larger ‘ring complexes,’ with similar internal variations in structure, optical depth, particle size, regolith texture and composition. The inner complex comprises the C and B rings, while the outer comprises the Cassini Division and A ring. 相似文献
17.
F.C. Witteborn J.B. Pollack J.D. Bregman J.H. Goebel B.T. Soifer R.C. Puetter R.J. Rudy S.P. Willner 《Icarus》1981,45(3):653-660
A spectrum of Saturn obtained from the Kuiper Airborne Observatory exhibits an emission peak at 6.8 μm attributed to ethane, but is otherwise dominated by absorption from 5.3 to 7.2 μm. While the large absorption in this spectral region is consistent with the presence of ammonia gas or ammonia ice, or both, such an explanation is inconsistent with the lack of a major absorption near 3.0 μm. 相似文献
18.
John Caldwell 《Icarus》1975,25(3):384-396
Broadband filter photometry from 2100 to 4300 Å has been obtained by OAO-2 for the following objects: The Galilean satellites; Titan; the rings of Saturn; and three asteroids. Agreement with independent ground-based photometry in the region of overlap is good. The previously known decrease in reflectivity from visual to ground-based ultraviolet wavelengths continues to 2590 Å for all these objects. Europa's reflectivity continues to decline towards 2110 Å, and the rings' reflectivity levels off from 2590 to 2110 Å. Other targets were too faint at 2110 Å to be measured reliably by OAO-2.The low ultraviolet albedo of Titan has important implications for that planet's atmospheric structure (Caldwell, Larach, and Danielson, 1973; Danielson, Caldwell, and Larach, 1973; Caldwell, 1974b). The ultraviolet reflectivity of Saturn's rings is suggestive of a two-component system, one being pure H2O particles. The ultraviolet albedos of the Galilean satellites are consistent with existing upper limits for atmospheric abundances, but require either that former estimates of the fractional coverage of H2O frost are too high, an unlikely circumstance, or that the frost has been darkened by some external agent in the space environment. 相似文献
19.
We present spectrophotometry in the 27–41 μm spectral region for icy satellites of Saturn (Tethys, Dione, Rhea, Iapetus, and Hyperion) and Jupiter (Europa, Ganymede, and Callisto). The 3.6-μm reflectance peak characteristic of fine-grained water ice is observed prominently on the satellites of Saturn, faintly on the leading side of Europa, and not all on Ganymede, Callisto, or the dark side of Iapetus. The spectral reflectances of these icy satellites may be affected by their equilibrium surface temperatures and magnetospheric effects. 相似文献
20.
The sizes, composition, and number of particles comprising the rings of Saturn may be meaningfully constrained by a combination of radar- and radio-astronomical observations. In a previous paper, we have discussed constraints obtained from radar observations. In this paper, we discuss the constraints imposed by complementary “passive” radio observations at similar wavelengths. First, we present theoretical models of the brightness of Saturn's rings at microwave wavelengths (0.34–21.0 cm), including both intrinsic ring emission and diffuse scattering by the rings of the planetary emission. The models are accurate simulations of the behavior of realistic ring particles and are parameterized only by particle composition and size distribution, and ring optical depth. Second, we have reanalyzed several previously existing sets of interferometric observations of the Saturn system at 0.83-, 3.71-, 6.0-, 11.1-, and 21.0-cm wavelengths. These observations all have spatial resolution sufficient to resolve the rings and planetary disk, and most have resolution sufficient to resolve the ring-occulted region of the disk as well. Using our ring models and a realistic model of the planetary brightness distribution, we are able to establish improved constraints on the properties of the rings. In particular, we find that: (a) the maximum optical depth in the rings is ～ 1.5 ± 0.3 referred to visible wavelengths; (b) a significant decrease in ring optical depth from λ3.7 to λ21.0 cm allows us to rule out the possibility that more than ～30% of the cross section of the rings is composed of particles larger than a meter or so; this assertion is essentially independent of uncertainties in particle adsorption coefficient; and (c) the ring particles cannot be primarily of silicate composition, independently of particle size, and the particles cannot be primarily smaller than ～0.1 cm, independently of composition. 相似文献