首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
Dale P. Cruikshank 《Icarus》1980,41(2):246-258
New JHK photometry and spectrometry (1.4–2.6 μm) are presented for Enceladus, Hyperion, Phoebe, Umbriel, Titania, and Oberon. From spectral signatures, mainly in the 2-μm region, water ice is verified on Enceladus and identified on Hyperion and the three Uranian satellites. The JHK photometry shows that Phoebe is different from all other satellites and asteroids observed thus far. The new photometry corroborates the earlier conclusion by Cruikshank et al. (1977) Astrophys. J217, 1006–1010] that the Uranian satellites, as a class, have overall surface reflectances different from other water-ice-covered satellites, and the reason for the difference remains unclear. The diameters and the masses of the Uranian satellites are reviewed in light of the probable high albedo representative of ice-covered surfaces and the new dynamical studies by Greenberg, 1975, Greenberg, 1976, Greenberg, 1978.  相似文献   

2.
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   

3.
We use the technique developed in our previous studies to determine the ratios of optical depth components τ a /τ R and τκ/τ S on the basis of the observational data on the Uranian geometrical albedo for the years 1981, 1993, and 1995. Here τ a and τ R are the aerosol and gas scattering components, τ S = τ a + τ R , and τκ is the absorption component of the effective optical depth at which the intensity of the diffusely reflected radiation is formed. The ratios turn out to be different for different years. This phenomenon is caused by the horizontal inhomogeneity of the aerosol component distribution over the Uranian disk.  相似文献   

4.
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   

5.
R.F. Knacke  T. Owen  R.R. Joyce 《Icarus》1975,24(4):460-464
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   

6.
We found a new empirical fonmula for the distance of the n-th satellite in the Jovian, Saturnian and Uranian systems, an = B1 × Bn, with just two constants b1 and B for each system. The difference between the observed distances and the values calculated according to this formula is generally less than 10%. We take the view that the satellites were formed from the accretion of planetesimals in the gas-planetesimal disk surrounding the planet, that the main component of the disk was gas so that the effect of gas drag would be very important in the above process. Our theoretical analysis shows that one type of radial perturbation in the disk will lead to instability and hence the formation of gaseous rings with enhanced density. Within these rings, the planetesimals stick together to form the satellites, and it is the form of the distribution of the rings that leads to the distance law.  相似文献   

7.
Terry Z. Martin 《Icarus》1981,45(2):427-446
A Mars average data set (MADS) has been constructed from thermal and albedo measurements of the Viking Infrared Thermal Mapper; by merging information from all longitudes; and, ensuring reasonably complete longitudinal sampling, a representation of mean Mars behavior is obtained. Brightness temperatures at 7, 9, 11, 15, and 20 μm and albedo information in the band 0.3–3.0 μm have been binned using 2° latitude strips, 24 times of day, 3 emission angle intervals, and 23 nonoverlapping Ls periods covering 1.43 Mars years starting at Ls = 84°. The MADS is ideally suited to parametric study of latitudinal, diurnal, angular, and seasonal dependences. Data are presented for surface thermal and albedo behavior in clear and dusty atmospheric conditions; the thermal response of the atmospheric temperature to a major dust storm is found to be consistent with Mariner 9 data from the 1971 storm. Examples of use of the MADS, which is available through the Mars Consortium, indicate how averaged data reveal specific surface and atmospheric phenomena.  相似文献   

8.
New reflectance spectra at 3.5% resolution have been obtained for Ariel, Titania, Oberon, and Hyperion in the 0.8- to 1.6-μm spectral region. The new spectra show no absoptions other than the 1.5-μm water-ice feature (within the precision of the data), and demonstrate extension into the 0.8- to 1.6-μm region of the 1.5- to 2.5-μm spectral similarity of Ariel to Hyperion (R. H. Brown and D. P. Cruikshank (1983). Icarus55,93-92). The new data confirm the presence of dark, spectrally bland component on/in the water-ice surfaces of the Uranian satellites, which, with some reservations, has spectral similarities to the dark substance on the leading side of Iapetus and the dark material on/in the surface of Hyperion, as well as other dark, spectrally neutral substances such as charcoal. Attempts were made to match the spectra of Ariel, Titania, and Oberon with additive reflectance mixes (areal coverage) of fine-grained water frost and various dark components such as charcoal, lampblack, and charcoal-water-ice mixtures. The results were broad limits on the amounts of possible areal coverage of a charcoal-like spectral component on the surfaces of the Uranian satellites, but the data are not of sufficient precision to conclusively determine whether the dominant mode of contaminant dispersal is areal or voluminal. The effect of highly variegated albedos on the diameters derived by R. H. Brown, D. P. Cruikshank, and D. Morrison (1982a) (Nature300, 423–425) is found to be small.  相似文献   

9.
New high-resolution spectra in the 0.33 to 0.92 μm range of Iapetus, Hyperion, Phoebe, Dione, Rhea, and three D-type asteroids were obtained on the Palomar 200-inch telescope and the double spectrograph. The spectra of Hyperion and the low-albedo hemisphere of Iapetus can both be closely matched by a simple model that is the linear admixture of the spectrum of a medium-sized, high-albedo icy saturnian satellite and D-type material. Our results support an exogenous origin to the dark material on Iapetus; furthermore, this material may share a common origin and a similar means of transport with material on the surface of Hyperion. The recently discovered retrograde satellites of Saturn (Gladman et al., Nature412, 163-166) may be the source of this material. The leading sides of Callisto and the Uranian satellites may be subjected to a similar alteration mechanism as that of Iapetus: accretion of low-albedo dust originating from outer retrograde satellites. Phoebe does not appear to be related to either Iapetus or Hyperion. Separate spectra of the two hemispheres of Phoebe show no identifiable global compositional differences.  相似文献   

10.
Near-infrared spectrophotometry at 5% resolution shows Miranda to have a water-ice surface. Estimates of Miranda's albedo made from the depth of its 2.0-μm absorption band suggest that its visual geometric albedo is likely to be between 10 and 70%, which when combined with the satellite's visual magnitude, yields a diameter of 500 ± 225km. There is some evidence that suggests the visual geometric albedo of Miranda may be ≥0.3, which implies that its diameter may lie near the lower end of the estimated range. With these results all the Uranian satellites are now known to have water-ice surfaces.  相似文献   

11.
In this paper, we analyze the results of ground-based and space-born photometric observations of the major satellites of Uranus—Miranda, Ariel, Umbriel, Titania, and Oberon. All sets of photometric observations of the satellites available in the literature were examined for uniformity and systematic differences and summarized to a unified set by wavelength ranging from 0.25 to 2.4 μm. This set covers the interval of phase angles from 0.034° to 35°. The compound phase curves of brightness of the satellites in the spectral bands at 0.25, 0.41, 0.48, 0.56, 0.75, 0.91, 1.4, and 1.8 μm, which include a pronounced opposition surge and linear part, were constructed. For each satellite, the geometric albedo was found in different spectral bands taking into account the brightness opposition effect, and its spectral dependence was studied. It has been shown that the reflectance of the satellites linearly depends on the wavelength at different phase angles, but has different spectral gradients. The parameters of the phase functions of brightness, including the amplitude and the angular width of the brightness opposition surge, the phase coefficient, and the phase angle at which the nonlinear increase in brightness starts, were determined and their dependences on wavelength and geometric albedo were analyzed. Our investigations show that, in their optical properties, the satellites Miranda and Ariel, Titania and Oberon, and Umbriel present three types of surfaces. The observed parameters of the brightness opposition effect for the Uranian satellites, some ice satellites of Jupiter and Saturn, and the E-and S-type asteroids are analyzed and compared within the framework of the coherent backscattering and mutual shadowing mechanisms.  相似文献   

12.
Multiaperture K photometry and 2.0- to 2.5-μm spectrophotometry of Uranus and its ring system are presented. The photometric results are used, together with a previously published measurement, to set limits on the geometric albedos of Uranus and the rings at ～2.2 μm: (0.74 ± 0.02) × 10(su?4) ≤ pK (Uranus) ≤ (1.5 ± 0.3) × 10?4, and (2.7 ± 0.6) × 10?2 ≤ pK (rings) ≤ (3.4 ± 0.1) × 10?2. Reflectance spectra of Uranus and Uranus plus rings show features in the planet's spectrum which are attributed to gaseous CH4 absorption, and a 2.20-μm feature in the combined spectrum which may be due to the rings. This feature is tentatively identified with either the 2.26-μm absorption feature of NH3 frost, or the 2.2-μm OH band exhibited by certain silicate minerals. The results of JHK photometry of Uranus' satellite, Ariel (U1), indicate that the infrared colors of this object are very similar to those of the satellites U2, U3, and U4.  相似文献   

13.
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   

14.
Material of low geometric albedo (pV?0.1) is found on many objects in the outer Solar System, but its distribution in the saturnian satellite system is of special interest because of its juxtaposition with high-albedo ice. In the absence of clear, diagnostic spectral features, the composition of this low-albedo (or “dark”) material is generally inferred to be carbon-rich, but the form(s) of the carbon is unknown. Near-infrared spectra of the low-albedo hemisphere of Saturn's satellite Iapetus were obtained with the Visible-Infrared Mapping Spectrometer (VIMS) on the Cassini spacecraft at the fly-by of that satellite of 31 December 2004, yielding a maximum spatial resolution on the satellite's surface of ∼65 km. The spectral region 3-3.6 μm reveals a broad absorption band, centered at 3.29 μm, and concentrated in a region comprising about 15% of the low-albedo surface area. This is identified as the CH stretching mode vibration in polycyclic aromatic hydrocarbon (PAH) molecules. Two weaker bands attributed to CH2 stretching modes in aliphatic hydrocarbons are found in association with the aromatic band. The bands most likely arise from aromatic and aliphatic units in complex macromolecular carbonaceous material with a kerogen- or coal-like structure, similar to that in carbonaceous meteorites. VIMS spectra of Phoebe, encountered by Cassini on 11 June 2004, also show the aromatic hydrocarbon band, although somewhat weaker than on Iapetus. The origin of the PAH molecular material on these two satellites is unknown, but PAHs are found in carbonaceous meteorites, cometary dust particles, circumstellar dust, and interstellar dust.  相似文献   

15.
The rings and satellites discovered by Voyager 2 in the Uranian system confirm the existence of small satellites and rings foreseen before their discoveries and close to the locations suggested by an exponential distance relation found by the method of comparison of magnitude order of successive distances ratios of the large Uranian satellites. A new exponential distance relation is deduced, fitting the distances of the large satellites and groups of rings and new small satellites with a spacing ratio of 1.456. Some characteristics of an hypothetical parent body for the new inner satellites are also deduced.  相似文献   

16.
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   

17.
Olav L. Hansen 《Icarus》1975,26(1):24-29
Infrared (1.5–5 μm) albedos and rotation curves of the Galilean satellites have been obtained. The data suggest that the rotational variation in the infrared is less than ±10% for all four satellites. While no conclusion about rotational variation could be reached for Io, the 1.57 μm data for the outer three satellites marginally suggest phase correlation with the visual variation. The geometric albedos obtained are in general agreement with earlier results. For Io, the absorption feature near 1.5 μm found by Pilcher et al. (1972) is confirmed, thus contradicting the flat spectrum measured by Fink et al. (1973). Io and Ganymede were observed in the 1.57 μm bandpass as they reappeared from eclipse. The curve for Io shows a slight (<10%) overshoot similar to those sometimes reported for visual measurements. This result is based on a single reappearance, and is extremely tentative.  相似文献   

18.
We report on mid-resolution (R∼2000) spectroscopic observations of Titan, acquired in November 2000 with the Very Large Telescope and covering the range 4.75-5.07 μm. These observations provide a detailed characterization of the CO (1-0) vibrational band, clearly separating for the first time individual CO lines (P10 to P19 lines of 13CO). They indicate that the CO/N2 mixing ratio in Titan’s troposphere is 32±10 ppm. Comparison with photochemical models indicates that CO is not in a steady state in Titan’s atmosphere. The observations confirm that Titan’s 5-μm continuum geometric albedo is ∼0.06, and further indicates a ∼20% albedo decrease over 4.98-5.07 μm. Nonzero flux is detected at the 0.01 geometric albedo level in the saturated core of the 12CO (1-0) band, at 4.75-4.85 μm, providing evidence for backscattering on the stratospheric haze. Finally, emission lines are detected at 4.75-4.835 μm, coinciding in position with lines from the CO(1-0) and/or CO(2-1) bands. Matching them by thermal emission would require Titan’s stratosphere to be much warmer (by ∼ 25 K at 0.1 mbar) than indicated by the methane 7.7-μm emission and the Voyager radio-occultation. We show instead that a nonthermal mechanism, namely solar-excited fluorescence, is a more plausible source for these emissions. Improved observations and laboratory measurements on the vibrational-translational relaxation of CO are needed for further interpretation of these emissions in terms of a CO stratospheric mixing ratio.  相似文献   

19.
As part of a continuing series of experiments on the production of dark reddish organic solids, called tholins, by irradiation of cosmically abundant reducing gases, the synthesis from a simulated Titanian atmosphere of a tholin with a visible reflection spectrum similar to that of the high altitude aerosols responsible for the albedo and reddish color of Titan has been reported Sagan and Khare, 1981, Sagan and Khare, 1982, Orig. Life. 12, 280) and [C. Sagan, B. N. Khare, and J. Lewis, in press. In Saturn (M. S. Matthews and T. Gehrels, Eds.), Univ. of Arizona Press, Tucson]. The determination of the real (n) and imaginary (k) parts of the complex refractive index of thin films of such tholin prepared by continuous D.C. discharge through a 0.9 N2/0.1 CH4 gas mixture at 0.2 mb are reported. For 250 A? ≤ γ ≤ 1000 μm, n and k have been determined from a combination of transmittance, specular reflectance, interferometric, Brewster angle, and ellipsometric polarization measurements; experimental uncertainties in n are estimated to be ±0.5, and in k ± 30%. Values of n(?1.65) and k (?0.004 to 0.08) in the visible range are consistent with deductions made by ground-based and spacecraft observations of Titan. Maximum values of k (?0.8) are near 1000 Å, and minimum values (?4 × 10?4) are near 1.5 μm. Many infrared absorption features are present in k(γ), including the 4.6-μm nitrile band.  相似文献   

20.
Mid-infrared extinction coefficients of five natural amorphous silicates and seven synthetic glasses were measured. Three bands at about 10, 12, and 20 μm were seen for all the measured samples. The quantities of these bands are found to have good correlations with the SiO2 content of the samples. The correlations are the most remarkable for the 10 μm band. As the SiO2 content decreases, the peak wavelengthλ m shifts to longer side, the peak heightK m decreases and the full width of half maximumW increases. A quantityλ m K m W is constant within 15%. Empirical formula $$\lambda_m (\mu m) = {11.10-2.30 x 10^-2} {[SiO_2 wt.\%]} \pm 0.15$$ and $$W(\mu m) = {5.14-4.68 x 10^- 2} {[SiO_2 wt.\%]} \pm 0.30$$ are obtained for the measured samples. Therefore, the correlation is present between the 10 μm peak wavelengthλ m and peak widthW for amorphous silicates. The change in peak widthW is remarkable compared the change in peak wavelengthλ m as the SiO2 content varies. For the 12 μm band the correlations with the SiO2 content are not so good. A tendency that theλ m shifts to the red and theK m lowers as the decreasing SiO2 content are found. For the samples with SiO2 content less than 50% the 12 μm band cannot recognized as the peak. For the 20 μm band, theλ m is almost independent on SiO2 content and theK m lowers with decreasing SiO2 content. The results are compared with the observed 10 μm band of the astronomical objects. A method to estimate the SiO2 content of astronomical grain materials is proposed and 48±8% SiO2 wt.% is found corresponding to the peak wavelength of 9.7 μm and the peak width of 2.5–3.0 μm of typical celestial objects.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号