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1.
We have used 3-mm Saturn observations, obtained from 1965 through 1977 and with Jupiter as a reference, to derive a ring brightness temperature of 18 ± 8°K. Thebrightness temperature of the disk of Saturn is 156 ± 9° K. Part of the ring brightness (≈62K) may be accounted for as disk emission which is scattered from the rings; the remainder (12 ± 8° K we attributed to ring particle thermal emission. Because this thermal component brightness temperatures is so much less than the particle physical temperature, limits are placed on the mean size and composition of the ring particles. In particular, as found by others, the particles cannot be rocky, but must be either metallic or composed of extremely low-loss dielectric material such as water ice. If the particles are pure water ice, for example, then a simple slab model and a multiple-scattering model both give upper limits to the particle sizes of ≈ 1 m, a value three times smaller than previously available. The multiple-scattering model gives a particle single-scattering albedo at 3 mm of 0.83±0.13.  相似文献   

2.
We observed Saturn at far-infrared and submillimeter wavelengths during the Earth's March 1980 passage through the plane of Saturn's rings. Comparison with earlier spectroscopic observations by D. B. Ward [Icarus32, 437–442 (1977)], obtained at a time when the tilt angle of the rings was 21.8°, permits separation of the disk and ring contributions to the flux observed in this wavelength range. We present two main results: (1) The observed emission of the disk between 60 and 180 μm corresponds to a brightness temperature of 104 ± 2°K; (2) the brightness temperature of the rings drops approximately 20°K between 60 and 80 μm. Our data, in conjunction with the data obtained by other observers between 1 μm and 1 mm, permit us to derive an improved estimate for the total Saturnian surface brightness of (4.84 ± 0.32) × 10?4W cm?2 corresponding to an effective temperature of 96.1 ± 1.6°K. The ratio of radiated to incident power, PR/PI, is (1.46 ± 0.08)/(1 - A), where A is the Bond albedo. For A = 0.337 ± 0.029, PR/PI = 2.20 ± 0.15 and Saturn's intrinsic luminosity is LS = (2.9 ± 0.5) × 10?10L⊙.  相似文献   

3.
David Morrison 《Icarus》1974,22(1):57-65
Broad-band radiometry with a spatial resolution of 5 arc sec is presented of Saturn and its rings. The brightness temperature of the B ring is 96 ± 3°K at 20 μm and 91 ± 3°K at 11 μm. These values constrain the bolometric Bond albedo of the ring particles to be less than 0.6, thus requiring a phase integral of less than unity. From differences in the thermal emission of the ansae, I suggest that the leading side of the particles has higher albedo than the trailing side. A measured drop in temperature of the B ring following eclipse of 2.0 ± 0.5°K is consistent with radii for the ring particles of 2 cm or larger.  相似文献   

4.
We present interferometric observations of Saturn and its ring system made at the Hat Creek Radio Astronomy Observatory at a wavelength of 1.30 cm. The data have been analyzed by both model-fitting and aperture synthesis techniques to determine the brightness temperature and optical thickness of the ring system and estimate the amount of planetary limb darkening. We find that the ring optical depth is close to that observed at visible wavelenghts, while the ring brightness temperature is only 7 ± 1°K. These observational constraints require the ring particles to be nearly conservative scatterers at this wavelength. A conservative lower limit to the single-scattering albedo of the particles at 1.30-cm wavelength is 0.95, and if their composition is assumed to be water ice, then this lower limit implies an upper limit of 2.4 m for the radius of a typical ring particle. The aperture synthesis maps show evidence for a small offset in the position of Saturn from that given in the American Ephemeris and Nautical Almanac. The direction and magnitude of this offset are consistent with that found from a similar analysis of 3.71-cm interferometric data which we have previously presented (F.P. Schloerb, D.O. Muhleman, and G.L. Berge, 1979b, Icarus39, 232–250). Limb darkening of the planetary disk has been estimated by solving for the best-fitting disk radius in the models. The best-fitting radius is 0.998 ± 0.004 times the nominal Saturn radius and indicates that the planet is not appreciably limb dark at 1.30 cm. Since our previous 3.71-cm data also indicated that the planet was not strongly limb dark (F.P. Schloerb, D. O. Muhleman, and G.L. Berge, 1979a, Icarus39, 214–230), we feel that the limb darkening is not strongly wavelength dependent between 1.30 and 3.71 cm. The difference between the best-fitting disk radii at 3.71 and 1.30 cm is +0.007 ± 0.007 times the nominal Saturn radius and suggests that the planet is more limb dark at 1.30 cm than at 3.71 cm. Models of the atmosphere which have NH3 as the principal source of microwave opacity predict that the planet will be less limb dark at 1.30 cm. However, the magnitude of the effect predicted by the NH3 models is ?0.009 and only marginally different from the observed value.  相似文献   

5.
Interferometric observations of Saturn and its rings made at the Owens Valley Radio Observatory at a wavelength of 3.71 cm ar fit to models of the Saturn brightness structure. The models have allowed us to estimate the brightness temperatures and optical thicknesses of the A, B, and C rings as well as the brightness temperature of the planetary disk. The most accurate results are the ratios of the ring temperatures to the planet temperature of 0.030 ± 0.012, 0.050 ± 0.010, and 0.040 ± 0.014 for the A, B, and C rings, respectively. The best estimates of the ring optical thicknesses are τA = 0.2 ± 0.1, τB = 0.9 ± 0.2, and τC = 0.1 ± 0.1. The actual brightness temperatures, which are affected by the absolute calibration errors, are Tplanet = 178 ± 8, TA = 5.2 ± 2.0, TB = 9.1 ± 1.8, and TC = 7.1 ± 2.6°K. The particle single-scattering albedo that would be most consistent with the observations is slightly less than one, but probably greater than 0.95. The observations are consistent with particles which conservatively scatter the thermal emission from Saturn to the Earth and emit no thermal emission of their own. The 3.71-cm optical depths which we have estimated are very close to the visible wavelength optical depths. This similarity indicates that the ring particles must be at least a few centimeters in size, although we feel that the particles may well be much larger than this in view of the closeness of the visible and microwave optical depths. Particles which are nearly conservative scatterers at our wavelength and at least a few centimeters in size must be composed of a material which is either a very good reflector of microwaves or a very poor absorber of them. At this time, water ice seems to be the most likely candidate since it is a very poor absorber of microwaves and has been detected in the rings spectroscopically.  相似文献   

6.
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   

7.
Uwe Fink  Harold P. Larson 《Icarus》1975,24(4):411-420
Reflection spectra of water ice from 1 to 4 μm are presented as a function of temperature. It is found that a feature at 6056 cm?1 changes its intensity sufficiently that it can be used as a spectroscopic measure of the ice temperature. A temperature calibration curve of this feature down to 55 K is developed and is used to determine ice temperatures for the Galilean satellites Europa (95±10 K), Ganymede (103±10 K), and the rings of Saturn (80±5 K). The ice temperatures for the Galilean satellites are lower than their measured brightness temperatures, which can be explained by a higher albedo of the ice covered regions relative to the rest of the satellite and possibly a concentration of the ice near the polar caps.  相似文献   

8.
The spectrum of Saturn was measured from 80 to 350 cm?1 (29 to 125 μm) with ≈6-cm?1 resolution using a Michelson interferometer aboard NASA's Kuiper Airborne Observatory. These observations are of the full disk, with little contribution from the rings. For frequencies below 300 cm?1, Saturn's brightness temperature rises slowly, reaching ≈111°K at 100 cm?1. The effective temperature is 96.8 ± 2.5°K, implying that Saturn emits 3.0 ± 0.5 times as much energy as it receives from the Sun. The rotation-inversion manifolds of NH3 that are prominent in the far-infrared spectrum of Jupiter are not observed on Saturn. Our models predict the strengths to be only ≈2 to 5°K in brightness temperature because most of the NH3 is frozen out; this is comparable to the noise in our data. By combining our data with those of an earlier investigation when the Saturnicentric latitude of the Sun was B′ = 21.2°, we obtain the spectrum of the rings. The high-frequency end of the ring spectrum (ν > 230 cm?1) has nearly constant brightness temperature of 85°K. At lower frequencies, the brightness temperature decreases roughly as predicted by a simple absorption model with an optical depth proportional to ν1.5. This behavior could be due to mu-structure on the surface of the ring particles with a scale size of 10 to 100 μm and/or to impurities in their composition.  相似文献   

9.
Four-color photographic photometry of Saturn for the 1977–1979 apparitions has been analyzed to determine the dependence of ring brightness on wavelength, solar phase angle, ring particle orbital phase angle (azimuthal effect), declination of the Earth relative to the ring plane (tilt angle), and radial distance from Saturn. Azimuthal brightness variations up to ±20% relative to the ansae are clearly apparent for the maximum of ring A, but are not detectable for ring B or the outer portion of ring A. The shape of the intensity (I) versus orbital phase angle (θ) curve varies with ring tilt (B) and probably with wavelength, and shows 180° symmetry. As characterized by its slope near the ansae, this curve suggests that the azimuthal effect increases as B decreases from 26 to ≈11°. The phase curves l(α) for the ansae show very little dependence on ring tilt (26° > B > 6°), on wavelength, or on radial distance from Saturn; possibly the curves are somewhat steeper at the smallest tilt angles and for ring A relative to ring B. The radial profile of both rings becomes flatter with decreasing tilt angle and with decreasing wavelength. The latter effect is a natural result of the classical, many-particle-thick ring model.  相似文献   

10.
We present a second epoch of Very Large Array Saturn observations taken in February 1997 spanning wavelengths 1.3-21 cm. These observations complement earlier observations at Saturn's autumnal equinox in November 1995. In this epoch, however, we generally have better signal-to-noise ratios and the ring inclination of the present observations was −5.0°, whereas the previous observations were made with ring inclination +2.7°.Our observations confirm the latitudinal structure on the saturnian disk as seen at 2.0, 3.6, and 6.1 cm. We also see some latitudinal structure at 1.3 cm for the first time. The details of this structure have changed dramatically from those reported by I. de Pater and J. R. Dickel (1991, Icarus94, 474-492) for the 1980s and are consistent with those seen in F. van der Tak et al. (1999, Icarus142, 125-147). The most prominent features are a pair of brightness enhancements just inside the edges of the Equatorial Zone.The rings do not show the east-west asymmetry seen in our previous epoch, perhaps indicative of a viewing angle effect on the scattering properties of the rings. The radial trend in brightness in the ansae is generally consistent with that expected from optical depth variations and increasing distance from the source of scattered light. In particular the increased optical depth towards the center of the C ring is evident. Azimuthal variation in brightness in the C ring shows the forward scattering expected of Mie scattering. By contrast, the A and B rings show little or no azimuthal variation.We present Monte Carlo simulations of the ring brightness under the assumptions of isotropic and Mie scattering. These are the first synthetic maps of Saturn which can be directly compared to the images we obtained. Neither model fits all the data well. However, a hybrid model combining isotropic and Mie scattering does fit well. We interpret the consistency with isotropic scattering in the outer rings as an indication that near-field effects may be important. This in turn implies geometrically thin rings, as predicted by dynamical simulations of these rings.  相似文献   

11.
Observations of Saturn's satellites and external rings during the 1980 edge-on presentation were obtained with a focal coronograph. A faint satellite traveling in the orbit of Dione and leading it by 72° has been detected, together with the two inner satellites already suspected (cf. J. W. Fountain and S. M. Larson, 1978,Icarus36, 92–106). The external ring has been observed on both east and west sides; it may extend up to ?8.3 Saturn radii, and appears structured.  相似文献   

12.
Early ground-based and spacecraft observations suggested that the temperature of Saturn's main rings (A, B and C) varied with the solar elevation angle, B′. Data from the composite infrared spectrometer (CIRS) on board Cassini, which has been in orbit around Saturn for more than five years, confirm this variation and have been used to derive the temperature of the main rings from a wide variety of geometries while B′ varied from near −24° to 0° (Saturn's equinox).Still, an unresolved issue in fully explaining this variation relates to how the ring particles are organized and whether even a simple mono-layer or multi-layer approximation describes this best. We present a set of temperature data of the main rings of Saturn that cover the ∼23°—range of B′ angles obtained with CIRS at low (α∼30°) and high (α≥120°) phase angles. We focus on particular regions of each ring with a radial extent on their lit and unlit sides. In this broad range of B′, the data show that the A, B and C rings’ temperatures vary as much as 29-38, 22-34 and 18-23 K, respectively. Interestingly the unlit sides of the rings show important temperature variations with the decrease of B′ as well. We introduce a simple analytical model based on the well known Froidevaux monolayer approximation and use the ring particles’ albedo as the only free parameter in order to fit and analyze this data and estimate the ring particle's albedo. The model considers that every particle of the ring behaves as a black body and warms up due to the direct energy coming from the Sun as well as the solar energy reflected from the atmosphere of Saturn and on its neighboring particles. Two types of shadowing functions are used. One analytical that is used in the latter model in the case of the three rings and another, numerical, that is applied in the case of the C ring alone. The model lit side albedo values at low phase are 0.59, 0.50 and 0.35-0.38 for the A, B and C rings, respectively.  相似文献   

13.
Steven W. Squyres 《Icarus》1980,44(2):502-510
Surface temperatures and ice evaporation rates are calculated for Ganymede and Callisto as a function of latitude, time of day, and albedo. The model uses surface thermal properties determined by eclipse radiometry (Morrison and Cruikshank, 1973Icarus18 224–236) and albedos determined from photometrically decalibrated Voyager images. Daytime temperatures on Callisto are roughly 8°K warmer than those in Ganymede's cratered terrain and 11°K warmer than those in Ganymede's grooved terrain. Diurnal mean ice evaporation rates are high enough on both bodies that the surface material probably consists of a very low density lag deposit of primarily silicate dust overlying a denser regolith of silicates and ice. The difference in temperature between Ganymede and Callisto is not great enough to account for the lack of bright polar caps on Callisto. This lack seems instead to reflect a real deficiency in the amount of available H2O frost relative to Ganymede. The temperature difference between Ganymede's grooved and cratered terrains also cannot account for the strong concentration of bright ray craters in grooved terrain. This concentration suggests instead that an internal geologic process has enriched the grooved terrain in ice relative to the cratered terrain.  相似文献   

14.
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   

15.
Martha S. Hanner 《Icarus》1980,43(3):373-380
The zodiacal light brightness and measured spatial density of the interplanetary dust lead to a mean geometric albedo of 0.24 for the dust particles near 1 AU; whereas the composition of collected micrometeroids suggests a geometric albedo ?0.1. The data do not support the very low albedo (?0.01) proposed by A. F. Cook [Icarus33 (1978), 349–360]. The evidence is against a change in the mean particle albedo between 0.1 and 2 AU. Beyond 2 AU the data are unclear and a change in albedo is not ruled out.  相似文献   

16.
We have obtained 5-μm brightness temperatures and brightness temperature upper limits for Uranus and Neptune which are substantially lower than those of Jupiter and Saturn and which correspond to a geometric albedo of approximately 0.01, in agreement with results reported by F. C. Gillet and G. H. Rieke (1977, Astrophys. J.218, L141–L144). Phospine and CH3D, which are observed at 5 μm on Jupiter and Saturn, are discussed as possible sources of opacity at 5 μm in the atmospheres of Uranus and Neptune.  相似文献   

17.
Titan was observed in four broad passbands between 35 and 150 μm. The brightness temperature in this interval is roughly constant at 76 ± 3°K. Integrating Titan's spectrum from 5 to 150 μm yields an effective temperature of 86 ± 3°K. Both the bright and dark hemispheres of Iapetus were observed in one broadband filter with λe ～ 66 μm. The brightness temperatures for these two sides of Iapetus are 96 ± 9°K and 114 ± 10°K, respectively. The bright-side Bond albedo is calculated to be 0.61?0.22+0.16.  相似文献   

18.
Full-disk and high-resolution measurements recorded during the Voyager 1 flyby of Saturn by the radiometer of the infrared instrument, IRIS, indicate a geometric albedo of 0.242 ± 0.012, which is lower than previous estimates. The given error is largely due to uncertainties in systematic corrections; random effects are small. Combining this measurement with the Pioneer-derived phase integral yields a Bond albedo of 0.342 ± 0.030. Infrared spectra recorded at the same time by the Michelson interferometer, along with a model extrapolation to wavenumbers not covered by the instrument, yield an effective temperature of 95.0 ± 0.4°K. As in the case of the radiometer, random instrumental errors are small, and the quoted error in the effective temperature reflects primarily uncertainties in systematic corrections. The rings of Saturn significantly affect both the short- and long-wavelength fluxes. From these measurements the internal heat flux of Saturn is 2.01 ± 0.14 10?4W cm?2, and the energy balance, defined as the ratio of total emitted to total absorbed energy, is 1.78 ± 0.09.  相似文献   

19.
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   

20.
Observations of the planet Saturn at wavelengths of 49.5 and 94.3 em are reported. The equivalent disk brightness temperatures were found to be 400 ± 65°K and 540 ± 110°K, respectively. It is suggested that the enhanced portion of the spectrum of the disk brightness temperature favours the idea that the observed long wavelength radiation comes from the planet's atmosphere.However, the possibility of a magnetic field associated with Saturn is not rejected by the observations. Part of the excess temperature could be attributed to weak synchrotron emission coming from a region outside the ring system.  相似文献   

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