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1.
Cassini ISS astrometric observations of the inner jovian satellites, Amalthea and Thebe 总被引：1，自引：0，他引：1
We present a total of 289 new astrometric observations of the inner jovian satellites, Amalthea and Thebe, obtained using the Cassini ISS narrow angle camera. Observations were made using image sequences from 2000 December 11-12 (inbound) and 2001 January 15-16 (outbound), at phase angles of approximately 2° and 122°, respectively. Target distances were of order 284 RJ, giving a maximum resolution of approximately 100 km/pixel. Centroided line and sample values for 239 observations of Amalthea and 50 of Thebe are provided, together with estimated camera pointing information for each image. Orbit fitting using a uniformly precessing Keplerian ellipse model, taking into account the oblateness of Jupiter up to terms in J6, gave RMS fit residuals of 0.364 and 0.443 pixel for Amalthea and Thebe, respectively (equivalent to 0.450 and 0.547 arcsec). RMS residuals relative to the JPL JUP230 ephemeris were 0.306 and 0.604 pixel (equivalent to 0.378 and 0.746 arcsec), for Amalthea and Thebe. The fitted orbital parameters confirm the relatively high inclinations of these satellites (0.374°±0.002° and 1.076°±0.003°, respectively), equivalent to maximum vertical displacements above Jupiter's equatorial plane of 1188±6 and 4240±12 km, respectively, consistent with current estimates of the half-thicknesses of the Amalthea and Thebe gossamer rings [Ockert-Bell, M.E., Burns, J.A., Dauber, I.J., Thomas, P.C., Veverka, J., Belton, M.J.S., Klaasen, K.P., 1999. Icarus 138, 188-213]. 相似文献
2.
3.
Hubble Space Telescope (HST) Wide-Field Planetary Camera (WFPC2) observations at phase angles in the range α=0.26°-6.4° obtained at every opposition and near quadrature between October 1996 and December 2002 reveal the opposition effect of Enceladus. We present a photometric analysis of nearly 200 images obtained through the five broadband UVBRI filters (F336W, F439W, F555W, F675W, and F814W) and the F785LP and F1042M filters from which we generate mutually consistent solar and rotational phase curves. Our solar phase curves reveal a dramatic, sharp increase in the albedo (from 0.11 mag in the F675W filter to 0.17 mag in the F785LP filter) as phase angles decrease from 2° to 0.26°. A slight opposition effect is evident in data from the F1042M filter (λeff=1022 nm); however, the smallest phase angle currently available for observations from this filter is α=0.58°. With the addition of data from the F255W filter we demonstrate the wavelength dependence of the albedo of the trailing hemisphere from 275 to 1022 nm. Our rotation curves show that the trailing hemisphere is ∼0.06 mag brighter than the leading when observed at wavelengths between 338 and 868 nm and 0.11 mag brighter than the leading at 1022 nm. We have supplemented the phase curve from the F439W filter (λeff=434 nm) with Voyager clear filter (λeff=480 nm) observations made at larger phase angles (α=13°-43°) to produce a phase curve with the most extensive phase angle coverage possible to date. This newly expanded range of phase angles enhances the ability of the Hapke photometric model (Hapke B., 2002, Icarus 157, 523-534) to relate physical characteristics of the surface of Enceladus to the manner in which incident light is reflected from it. We present Hapke 2002 model fits to solar phase curves from each UVBRI filter as well as from the F785LP and F1042M filters. Geometric albedos derived from these model fits range from p=0.92±0.01 at 1022 nm to p=1.41±0.03 at 549 nm, necessitating an increase of about 20% from previously derived values. Our Hapke fits demonstrate that the opposition surge of Enceladus is best described by a model which combines both moderate shadow-hiding and narrow coherent backscattering components. 相似文献
4.
Some problems in determining the orbits of inner satellites associated with the complex behavior of the target function, which is strongly ravine and which possesses multiple minima in the case of the satellite orbit is determined based on fragmentary observations distributed over a rather long time interval, are studied. These peculiarities of the inverse problems are considered by the example of the dynamics of the inner Jupiter satellites: Amalthea, Thebe, Adrastea, and Metis. Numerical models of the satellite motions whose parameters were determined based on ground-based observations available at the moment to date have been constructed. A composite approach has been proposed for the effective search for minima of the target function. The approach allows one to obtain the respective evaluations of the orbital parameters only for several tens of iterations even in the case of very rough initial approximations. If two groups of observations are available (Adrastea), a formal minimization of the target function is shown to give a solution set, which is the best solution from the point of view of representation of the orbital motion, which is impossible to choose. Other estimates are given characterizing the specific nature of the inverse problems. 相似文献
5.
Galileo was the first artificial satellite to orbit Jupiter. During its late orbital mission the spacecraft made two passages through the giant planet’s gossamer ring system. The impact-ionization dust detector on board successfully recorded dust impacts during both ring passages and provided the first in-situ measurements from a dusty planetary ring. During the first passage—on 5 November 2002 while Galileo was approaching Jupiter—dust measurements were collected until a spacecraft anomaly at 2.33RJ (Jupiter radii) just 16 min after a close flyby of Amalthea put the spacecraft into a safing mode. The second ring passage on 21 September 2003 provided ring dust measurements down to about 2.5RJ and the Galileo spacecraft was destroyed shortly thereafter in a planned impact with Jupiter. In all, a few thousand dust impacts were counted with the instrument accumulators during both ring passages, but only a total of 110 complete data sets of dust impacts were transmitted to Earth. Detected particle sizes range from about 0.2 to 5 μm, extending the known size distribution by an order of magnitude towards smaller particles than previously derived from optical imaging [Showalter, M.R., de Pater, I., Verbanac, G., Hamilton, D.P., Burns, J.A., 2008. Icarus 195, 361-377; de Pater, I., Showalter, M.R., Macintosh, B., 2008. Icarus 195, 348-360]. The grain size distribution increases towards smaller particles and shows an excess of these tiny motes in the Amalthea gossamer ring compared to the Thebe ring. The size distribution for the Amalthea ring derived from our in-situ measurements for the small grains agrees very well with the one obtained from images for large grains. Our analysis shows that particles contributing most to the optical cross-section are about 5 μm in radius, in agreement with imaging results. The measurements indicate a large drop in particle flux immediately interior to Thebe’s orbit and some detected particles seem to be on highly-tilted orbits with inclinations up to 20°. Finally, the faint Thebe ring extension was detected out to at least 5RJ, indicating that grains attain higher eccentricities than previously thought. The drop interior to Thebe, the excess of submicron grains at Amalthea, and the faint ring extension indicate that grain dynamics is strongly influenced by electromagnetic forces. These findings can all be explained by a shadow resonance as detailed by Hamilton and Krüger [Hamilton, D.P., Krüger, H., 2008. Nature 453, 72-75]. 相似文献
6.
V. K. Rosenbush 《Solar System Research》2005,39(4):312-321
Systematic and uniform sets of photometric and polarimetric observations of comet 1P/Halley have been analyzed. The phase dependence of brightness for comet Halley was obtained at phase angles α ranging from 1.4° ≤ α ≤ 65°. The following parameters were determined: the amplitude of the opposition effect Δm = 0.75m ± 0.06m; the half-width at a half-maximum of intensity HWHM = 6.4° ± 1.6°; the linear phase coefficient β = 0.0045 ± 0.0001 mag/deg for α from 30° ≤ α ≤ 65°; and the phase angle at which a nonlinear increase in brightness starts, α opp ≈ 31°. For the first time, the phase-angle dependence was obtained for the color of the dust of comet Halley: the color index BC-RC systematically decreases with increasing phase angle. Such a phase behavior of the dust color can be caused by the decrease in the mean size of dust particles that occurs when the comet approaches the Sun. For comet Halley, the negative polarization branch is almost symmetric; the minimal value of polarization is P
min = −1.54% at a phase angle αmin = 10.5°, and the inversion angle is αinv = 21.7°. A comparative analysis of the phase functions of brightness and polarization has been performed for the cometary dust and atmosphereless bodies. Among the latter are low-albedo asteroids of the P and C types (102 Miriam and 47 Aglaja, respectively), as well as Deimos; high-albedo objects, such as the E-type asteroid 64 Angelina and the icy satellite of Jupiter Europa; and the Moon with its intermediate albedo. The possibility of a weak depression in the negative polarization branch of comets Halley and 47P/Ashbrook-Jackson at phase angles smaller than 2° is discussed.__________Translated from Astronomicheskii Vestnik, Vol. 39, No. 4, 2005, pp. 353–363.Original Russian Text Copyright © 2005 by Rosenbush. 相似文献
7.
Voyager imaging observations provide new photometric data on Saturn's satellites at large phase angles (up to 133° in the case of Mimas) not observable from Earth. Significant new results include the determination of phase integrals ranging from 0.7 in the case of Rhea to 0.9 for Enceladus. For Enceladus we find an average geometric albedo pv = 1.04 ± 0.15 and Bond albedo of 0.9 ± 0.1. The data indicate an orbital lightcurve with an amplitude of 0.2 mag, the trailing side being the brighter. For Mimas, the lightcurve amplitude is probably less than 0.1 mag. The value of the geometric albedo of Mimas reported here, pv = 0.77 ± 0.15 (corresponding to a mean opposition magnitude V0 = +12.5) is definitely higher than the currently accepted value of about 0.5. For Dione, the Voyager data show a well-defined orbital lightcurve of amplitude about 0.6 mag, with the leading hemisphere brighter than the trailing one. 相似文献
8.
We simulate the production and orbital evolution of escaping ejecta due to cometary impacts on Io. The model includes the four Galilean satellites, Amalthea, Thebe, Jupiter's gravitational moments, Saturn and the Sun. Five scenarios are examined: an impact at the apex, the sub-jovian point, the anti-jovian point, the antapex, and at the south pole of Io. We estimate that on average a cometary impact injects thrice its mass (in the form of Io surface material) into jovicentric orbit. The majority of the escaping debris comes back to Io, but a sizeable fraction (between 5.0 and 8.7%) manages to reach Europa, and a smaller fraction Ganymede (between 1.5 and 4.6%). Smaller fractions reached Amalthea Thebe, Callisto, and Jupiter itself. For million year time scales, the mass transfer to Europa is estimated as 1.8-3.1×1014 g/Myr. The median time for transfer of ejecta from Io to Europa is ∼56 years. 相似文献
9.
New 0.8- to 2.4-μm spectral observations of the leading and trailing hemispheres of the uranian satellite Ariel were obtained at IRTF/SpeX during 2002 July 16 and 17 UT. The new spectra reveal contrasts between Ariel’s leading and trailing hemispheres, with the leading hemisphere presenting deeper H2O ice absorption bands. The observed dichotomy is comparable to leading-trailing spectral asymmetries observed among jovian and saturnian icy satellites. More remarkably, the trailing hemisphere spectrum exhibits three narrow CO2 ice absorption bands near 2 μm. This discovery of CO2 ice on one hemisphere of Ariel is its first reported detection in the uranian system. 相似文献
10.
We report on Adaptive Optics observations of the satellite of Asteroid 121 Hermione with the ESO-Paranal UT4 VLT and the Keck AO telescopes. The binary system, belonging to the Cybele family, was observed during two observing campaigns in January 2003 and January 2004 aiming to confirm its trajectory and accurately determine its orbital elements. A precessing Keplerian model was used to describe the motion of S/2002 (121) 1. We find that the satellite of Hermione revolves at a=768±11 km from the primary in P=2.582±0.002 days with a roughly circular and prograde orbit (e=0.001±0.001, i=3±2° w.r.t. equator primary). These extensive astrometric measurements enable us to determine the mass of Hermione to be 0.54±0.03×1019 kg and its pole solution (λ0=1.5°±2.00, β0=10°±2.0 in ecliptic J2000). Additional Keck AO observations taken close to the asteroid opposition in December 2003 give us direct insight into the structure of the primary which presents a bilobated shape. Since the angular resolution is limited to the theoretical angular resolution of the telescope (43 mas corresponding to a spatial resolution of 80 km), two shape models (called snowman and peanut) are proposed based on the images which were deconvolved with MISTRAL deconvolution process. Assuming a purely synchronous orbit and knowing the mass of the primary, the peanut shape composed of two separated components is quite unlikely. Additionally the J2 calculated from the analysis of the secondary orbit is not in agreement with the peanut model, but close to the snowman shape. The bulk density of the primary as derived from the observed size of the snowman shape is estimated to ρ∼1.8±0.2 g/cm3 implying a porosity ∼14% for this C-type asteroid, corresponding to a fractured asteroid. Considering the IRAS diameter, the density is lower (ρ=1.1±0.3 g/cm3) leading to a high porosity (p=30-60%) with a nominal value of p=48%, which indicates a completely loose rubble-pile structure for the primary. Further work is necessary to better constrain the size, shape, and then internal structure of Hermione's primary. 相似文献
11.
We present new observations of Jupiter's ring system at a wavelength of 2.2 μm obtained with the 10-m W.M. Keck telescopes on three nights during a ring plane crossing: UT 19 December 2002, and 22 and 26 January 2003. We used conventional imaging, plus adaptive optics on the last night. Here we present detailed radial profiles of the main ring, halo and gossamer rings, and interpret the data together with information extracted from radio observations of Jupiter's synchrotron radiation. The main ring is confined to a 800-km-wide annulus between 128,200 and 129,000 km, with a ∼5000 km extension on the inside. The normal optical depth is 8×10−6, 15% of which is provided by bodies with radii a?5 cm. These bodies are as red as Metis. Half the optical depth, τ≈4×10−6, is attributed to micron-sized dust, and the remaining τ≈3×10−6 to grains tens to hundreds of μm in size. The inward extension consists of micron-sized (a?10 μm) dust, which probably migrates inward under Poynting-Robertson drag. The inner limit of this extension falls near the 3:2 Lorentz resonance (at orbital radius r=122,400 km), and coincides with the outer limit of the halo. The gossamer rings appear to be radially confined, rather than broad sheets of material. The Amalthea ring is triangularly shaped, with a steep outer dropoff over ∼5000 km, extending a few 1000 km beyond the orbit of Amalthea, and a more gradual inner dropoff over 15,000-20,000 km. The inner edge is near the location of the synchronous orbit. The optical depth in the Amalthea ring is ∼5×10−7, up to 20% of which is comprised of macroscopic material. The optical depth in the Thebe ring is a factor of 3 smaller. 相似文献
12.
We present the first Earth-based images of several of the individual faint rings of Uranus, as observed with the adaptive optics system on the W.M. Keck II telescope on four consecutive days in October 2003. We derive reflectivities based on multiple measurements of 8 minor moons of Uranus as well as Ariel and Miranda in filters centered at wavelengths of 1.25(J), 1.63(H), and 2.1(Kp) μm. These observations have a phase angle of 1.84°-1.96°. We find that the small satellites are somewhat less bright than in observations made by the HST at smaller phase angles, confirming an opposition surge effect. We calculate albedoes for the ring groups and for each ring separately. We find that the ε ring particles, as well as the particles in the three other ring groups, have albedoes near 0.043 at these phase angles. The equivalent depths of some of the individual rings are different than predicted based upon ring widths from occultation measurements (assuming a constant particle ring brightness); in particular the γ ring is fainter and the η ring brighter than expected. Our results indicate that q, the ratio of ε ring intensity at apoapse vs. periapse, is close to 3.2±0.16. This agrees well with a model that has a filling factor for the ε ring of 0.06 (Karkoschka, 2001, Icarus 151, 78-83). We also determine values of the north to south brightness ratio for the individual rings and find that in most cases they are close to unity. 相似文献
13.
We performed photometry of Cassini Visual Infrared Mapping Spectrometer observations of Iapetus to produce the first phase integrals calculated directly from solar phase curves of Iapetus for the leading hemisphere and to estimate the phase integrals for the trailing hemisphere. We also explored the phase integral dependence on wavelength and geometric albedo. The extreme dichotomy of the brightness of the leading and trailing sides of Iapetus is reflected in their phase integrals. Our phase integrals, which are lower than the results of Morrison et al. (Morrison, D., Jones, T.J., Cruikshank, D.P., Murphy, R.E. [1975]. Icarus 24, 157-171) and Squyres et al. (Squyres, S.W., Buratti, B.J., Veverka, J., Sagan, C. [1984]. Icarus 59, 426-435), have profound implications on the energy balance and volatile transport on this icy satellite. 相似文献
14.
Interplanetary dust grains entering the Jovian plasmasphere become charged, and those in a certain size range get magneto-gravitationally trapped in the corotating plasmasphere. The trajectories of such dust grains intersect the orbits of one or more of the Galilean satellites. Orbital calculations of micron sized dust grains show that they impact the outermost satellite Callisto predominantly on its leading face, while they impact the inner three — Io, Europa and Ganymede — predominantly on the trailing face. These results are offered as an explanation of the observed brightness asymmetry between the leading and trailing faces of the outer three Galilean satellites. The albedo of Io is likely to be determined by its volcanism. 相似文献
15.
M. Noland J. Veverka D. Morrison D.P. Cruikshank A.R. Lazarewicz N.D. Morrison J.L. Elliot J. Goguen J.A. Burns 《Icarus》1974,23(3):334-354
Six-color photometric observations made during Saturn's 1972/73 opposition enable us to separate the solar phase and orbital phase contributions to the observed light variations of Iapetus, Titan, Rhea, Dione and Tethys. Titan shows no orbital variations, but has phase coefficients which range from negligible values in the infrared to 0.014mag/deg in the ultraviolet. Rhea has a bright leading side, a light curve amplitude of about 0.2mag, which increases toward short wavelengths, and surprisingly large phase coefficients, which increase from 0.025mag/deg in the red to 0.037mag/deg in the ultraviolet. Combined with other available information, this behavior suggests a very porous, texturally complex surface layer. Dione also has a leading side which is a few tenths of a magnitude brighter than the trailing side, but the light curve amplitude has little wavelength dependence and the phase coefficients are significantly smaller than those of Rhea, suggesting a less intricate surface texture. The leading side of Tethys is probably a few tenths of a magnitude brighter than the trailing side. Our Iapetus observations generally supplement the earlier work by Millis. The phase coefficients of the bright (trailing) side are typically ～0.03mag/deg and are not strongly wavelength dependent; the dark (leading) side coefficients are large (～0.05 mag/deg) and increase at shorter wavelengths, indicating a very porous and intricate surface texture. The light curve amplitude shows a slight increase at shorter wavelengths, suggesting an increasing contrast between the dark and bright materials. The spectral reflectance curves we derive for the satellites are in agreement with the spectrophotometry of McCord, Johnson, and Elias. 相似文献
16.
Vasilij G. Shevchenko Vasilij G. Chiorny Yury N. Krugly Igor A. Tereschenko 《Icarus》2008,196(2):601-611
The results of photometric observations of eight main-belt asteroids with low surface albedo are presented. The magnitude-phase dependences including low phase angles (<1 deg) have been obtained for Asteroids 76 Freia (down to phase angle 0.1 deg, P-type), 190 Ismene (0.3 deg, P-type), 303 Josephina (0.2 deg, C-type), 309 Fraternitas (0.1 deg, C-type), 313 Chaldaea (0.1 deg, C-type), 444 Gyptis (0.8 deg, P-type), 615 Roswitha (0.1 deg, C-type), and 954 Li (0.03 deg, FCX-type). The behavior of brightness in the range of opposition effect is found to be practically linear for 190 Ismene with amplitude of opposition effect only 0.03 mag. Amplitudes of the opposition effect for other asteroids are close to a mean for this type. The obtained data allowed us also to determine the rotation periods of asteroids: 303 Josephina (12.497±0.001 h), 309 Fraternitas (11.205±0.005 h), 615 Roswitha (4.422±0.001 h) and 954 Li (7.207±0.002 h). The color indexes B-V, V-R and R-I have been determined for some asteroids. 相似文献
17.
Near-infrared spectra, 0.65–2.5 μm, are presented for Tethys, Dione, Rhea, Iapetus, and Hyperion. Water ice absorptions at 2.0, 1.5, and 1.25 μm are seen in the spectra of all five objects (except the 1.25-μm band was not detected in spectra of Hyperion) and the weak 1.04-μm ice absorption is detected on the leading and trailing sides of Rhea, and the trailing side of Dione. Upper limits to the 1.04-μm ice band depth are <0.3% for the leading side of Dione; <0.7% for the leading side of Iapetus, and the trailing side of Tethys; <1% on the trailing side of Iapetus; and <5% on the leading side of Tethys. The leading-trailing side ice band depth differences on Saturn's satellites are similar to those for the Galilean satellites, indicating possible surface modification by magnetospheric charged particle bombardment. Limits are determined for the amount of particulates, trapped gases, and amonium hydroxide on the surface. The surfaces of Saturn's satellites (except the dark side of Iapetus) are nearly pure water ice, with probably less than about 1 wt% particulate minerals. The ice could be clathrates with as much as a few weight percent trapped gases. The upper limit of amonium hydroxide depends on the spectral data precision and varies from ～ 1 wt% NH3 for the leading side of Rhea to ～ 10 wt% NH3 for Dione. 相似文献
18.
Philip J. Stooke 《Earth, Moon, and Planets》1992,56(2):123-139
A topographic model of Amalthea (JV) was derived from the shapes of limbs and terminators in Voyager images, modified locally to accommodate large craters and ridges. The model is presented in tabular and graphic form, including the first detailed shaded relief maps of the satellite. The shape is very irregular, with radii varying between about 53 and 151 ± 5 km. The minimum value occurs in a deep crater at the south pole. The volume is estimated to be 2.5 ± 0.5 × 106km3. A prominent groove or valley extends some 150 km across the trailing side. High albedo, spectrally distinct markings are mapped and found to have a less obvious relationship with relief than previously suggested. 相似文献
19.
CCD observations of Mercury were obtained with the large angle spectrometric coronograph (LASCO) on the solar and heliospheric observatory spacecraft, near superior and inferior solar conjunctions. Whole disk photometry was extracted from the orange and blue filter images and transformed to V magnitudes on the UBV system. The LASCO data were combined with ground-based, V-filter photometry acquired at larger elongation angles. The resulting photometric phase function covers the greatest span of angles to date and is the first wide-range function to be obtained since the era of visual observation. We analyzed the data using a polynomial fit and a Hapke function fit, and derived the following photometric results. Mercury's fully lit brightness, adjusted to a distance of 1.0 AU from the Sun and observer, was found to be V=−0.694(±0.030), which is more luminous than previously measured. The corresponding geometric albedo is 0.142(±0.005). The phase integral is 0.478(±0.005) and resulting spherical albedo is 0.068(±0.003). The upper limit of a possible rotational brightness variation is about 0.05 magnitude. Mercury's brightness surges by more than 40% between phase angles 10 and 2°, while the illuminated fraction of the disk increases by less than 1%. A set of coefficients for Hapke's function that fit most of the phase curve includes h=0.065±0.002 indicating that Mercury and the Moon have similar regolith compaction states and particle size distributions, and θ-bar=16°±1° implying a macroscopically smoother surface than the Moon. However, we found other solutions that fit the observations nearly as well with significantly smaller and larger values of h, and with values of θ-bar around 25°. The wide range for θ-bar is due to the inability of the model to fit the photometry obtained at large phase angles. 相似文献
20.
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