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1.
Topographic models of Saturn's F-Ring shepherd satellites Prometheus and Pandora were derived from the shapes of limbs and terminators in Voyager images, modified locally to accommodate large craters and ridges. The models are presented here in tabular and graphic form, including the first published maps of the satellites. The shape of Prometheus is approximated by a triaxial ellipsoid with axes of 145, 85 and 60 km. The volume is estimated to be 3.9 ± 1.0 × 105 km3, significantly smaller than previous estimates. A system of prominent ridges and valleys cross the north polar region. Prometheus appears to be less heavily cratered than the other small satellites near the edge of the rings, though this may be an artifact of the low resolution of available images. Pandora is approximated by a triaxial ellipsoid with axes of 114, 84 and 62 km. The volume is estimated to be 3.1 ± 1.0 × 105 km3. Its surface appears to be very heavily cratered.  相似文献   

2.
A topographic model of Saturn's larger co-orbital satellite Janus was derived from the shapes of limbs and terminators in Voyager images, modified locally to accommodate large craters and ridges. The model is presented here in tabular and graphic form, including the first detailed shaded relief maps of the satellite. The shape is approximated by a triaxial ellipsoid with axes of 196, 192 and 150 km. The volume is estimated to be 3.0 ± 0.5 × 106 km3, leading to a density estimate of 0.67 ± 0.10 g/cm3. The surface is heavily cratered. Several possible crater chains of uncertain significance are observed, but few prominent linear ridges and no narrow grooves.  相似文献   

3.
A topographic model of Saturn's smaller co-orbital satellite Epimetheus was derived from the shapes of limbs, terminators and shadows of the F Ring in Voyager images, modified locally to accommodate large craters and ridges. The model is presented here in tabular and graphic form, including the first detailed shaded relief maps of the satellite. The shape is approximated by a triaxial ellipsoid with axes of 144, 108 and 98 km. The volume is estimated to be 8 ± 1 × 105 km3. A prominent valley and several grooves are found on the heavily cratered surface.  相似文献   

4.
I have mapped the Saturnian satellite Hyperion using Voyager 2 images obtained in 1981 and a shape model derived from the results of Thomas et al. (1995). The results are presented in tabular and graphic form, including detailed shaded relief maps of the satellite. The shape is approximated by a triaxial ellipsoid with axes of 270, 201 and 336 km. The volume is estimated to be 9.5 ± 2.0 × 106 km3. Geological interpretations were augmented by the use of super-resolution image composites. The surface is heavily cratered. A system of scarps and an isolated mountain are interpreted as the rim and central peak of an impact crater with a diameter similar to the mean diameter of the satellite itself, the largest crater with recognizable impact morphology in relation to the size of the body yet observed in the solar system. The crater density dates that impact, not the formation of Hyperion. Grooves are identified in several images, and form part of a zone of fracturing radial to a prominent crater.University of Western Ontario  相似文献   

5.
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   

6.
Optical measurements of the diameter of Mars were made using a double-image micrometer with large refractors from 1952 to 1971. Discussion of the 90 independent series of measurements gives nine determinations of radius with an accuracy of ±7–8km for different latitudes from pole to equator. The Mariner 4, 6 and 7 occultation results and the radar results availble in 1970 added seven further determinations of comparable accuracy. All these values, within the accuracy of measurement, fit an ellipsoid with Req=3398±3km and Rpo1=3371±4km. The mean density of Mars is thus 3940±0.012g cm−3. The resulting optical oblateness of 0.0079, larger than the dynamical value of 0.0052, results in an equatorial radius excess of 9±5 km which presumably comes about by internal stresses.  相似文献   

7.
Sherman S.C. Wu 《Icarus》1978,33(3):417-440
Topographic contour maps of Mars are compiled by the synthesis of data acquired from various scientific experiments of the Mariner 9 mission, including S-band radio-occulation, the ultraviolet spectrometer (UVS), the infrared radiometer (IRR), the infrared interferometer spectrometer (IRIS) and television imagery, as well as Earth-based radar information collected at Goldstone, Haystack, and Arecibo Observatories. The entire planet is mapped at scales of 1:25,000,000 and 1:25,000,000 using Mercator, Lambert, and polar stereographic map projections. For the computation of map projections, a biaxial spheroid figure is adopted. The semimajor and semiminor axes are 3393.4 and 3375.7 km, respectively, with a polar flattening of 0.0052. For the computation of elevations, a topographic datum is defined by a gravity field described in terms of spherical harmonics of fourth order and fourth degree combined with a 6.1-mbar occulation pressure surface. This areoid can be approximated by a triaxial ellipsoid with semimajor axes of A = 3394.6 km and B = 3393.3 km and a semiminor axis of C = 3376.3 km. The semimajor axis A intersects the Martian surface at longitude 105°W. The dynamic flattening of Mars is 0.00525. The contour intercal of the maps is 1 km. For some prominent features where overlapping pictures from Mariner 9 are available, local contour maps at relatively larger scales were also compiled by photogrammetric methods on stereo plotters.  相似文献   

8.
Using the high-quality data set of 165 images taken at 11 epochs over the 5.13 h rotation of the large C-type Asteroid 511 Davida, we find the dimensions of its triaxial ellipsoid model to be 357±2×294±2×231±50 km. The images were acquired with the adaptive optics system on the 10 m Keck II telescope on December 27, 2002. The a and b diameters are much better determined than previously estimated from speckle interferometry and indirect measurements, and our mean diameter, (abc)1/3=289±21 km, is 19% below previous estimates. We find the pole to lie within 2° of [RA=295°; Dec=0°] or in Ecliptic coordinates [λ=297°; β=+21°], a significant improvement to the pole direction. Otherwise, previous determinations of the axial ratios agree with our new results. These observations illustrate that our technique of finding the dimensions and pole of an asteroid from its changing projected size and shape is very powerful because it can be done in essentially one night as opposed to decades of lightcurves. Average departures of 3% (5 km) of the asteroid's mean radius from a smooth outline are detected, with at least two local positive-relief features and at least one flat facet showing approximately 15 km deviations from the reference best-fit ellipsoid. The facet is reminiscent of large global-scale craters on Asteroid 253 Mathilde (also a C-type) when seen edge-on in close-up images from the NEAR mission flyby. We show that giant craters (up to 150 km diameter, the size of the largest facets seen on Davida) can be expected from the impactor size distribution, without likelihood of catastrophic disruption of Davida.  相似文献   

9.
Erich Karkoschka 《Icarus》2003,162(2):400-407
Based on 87 resolved Voyager images of the five innermost satellites of Neptune, their shapes were measured and fit by tri-axial ellipsoids with the semi-axes of 48 × 30 × 26 km for Naiad, 54 × 50 × 26 km for Thalassa, 90 × 74 × 64 km for Despina, 102 × 92 × 72 km for Galatea, and 108 × 102 × 84 km for Larissa. Thomas and Veverka published a similar shape for Larissa (104 × 89 km, J. Geophys. Res. 96, 19261-19268, 1991). The other satellites had no published shapes. Using Voyager photometry of the six inner satellites by the same authors and the revised sizes, including the published size of Proteus, the reflectivity within this inner system was found to vary by about 30%. Geometric albedos in the visible are estimated between 0.07 for Naiad and 0.10 for Proteus. The rotational lightcurves of these satellites seem to be due to satellite shapes.  相似文献   

10.
Although we can observe current activity on Saturn's satellite Enceladus with Cassini, insight into past activity is best achieved (for now) through studying the impact crater distributions. Furthermore, approximation of terrain ages can only be attained through calculations using crater densities and estimations of impact rates in the saturnian system. Here we focus on what the impact crater distribution in Enceladus' heavily cratered plains can tell us about Enceladus' geologic history. We use Cassini ISS images to count craters in the heavily cratered plains on Enceladus, along with Rhea, Dione, Tethys and Mimas as references, to develop and compare their size-frequency distributions. Comparisons of our counts show that Enceladus' cratered plains distribution is unique in that it appears to have a relative deficiency of craters for diameters ?2 km and ?6 km compared to the other satellites' heavily cratered plains. Our data also indicates that the impact crater density within the cratered plains changes with latitude. Specifically, both the north and south mid-latitude regions have approximately three times higher density than the equatorial region. We hypothesize that the “missing” small and large craters in Enceladus' cratered plains is due to a combination of viscous relaxation of the larger craters, and burial of the relaxed large craters and small craters by south polar plume and possibly E-ring material. We also conclude that the spatial density distribution is not consistent with recent polar wander.  相似文献   

11.
The second zonal and the second sectorial Stokes parameters of the Moon's gravitational field and/or the polar and equatorial flattenings of the lunar triaxial level ellipsoid have been explained by the tidal and rotational distortions due to the Earth. The Epoch at which the Moon's figure formation was finished has been estimated as 1.6 × 109 y B. P. when the Earth-Moon distance was about 168 400 km and the orbital/rotational period of the Moon about 8 days.  相似文献   

12.
By correlating the 1:25,000,000 geologic map of Mars of Scott and Carr (1977) with 4- to 10-km-diameter crater density data from Mariner 9 images, the average crater density for 23 of the equatorial geologic-geomorphic units on Mars was computed. The correlation of these two data sets was accomplished by digitizing both the crater density data and geologic map at the same scale and by comparing them in a computer. This technique assigns the crater density value found in the corresponding location on the geologic data set to a discrete computer file assigned each of the 23 geologic units. By averaging the crater density values accumulated in each file, an “average” crater density for each geologic unit was obtained. Condit believes these average crater density values are accurate indicators of the relative age of the geologic units considered. The statistical validity of these average values is strongest for the geologic units of the largest areal extent. The relative ages as obtained from the average crater density values for the seven largest geologic units, from youngest to oldest, are: Tharsis volcanic material, 21 ± 4 craters/106km2; smooth plains material, 57 ± 14 craters/106km2; rolling plains material, 66 ± 16 craters/106km2; plains materials, 80 ± 17 craters/106km2; ridged plains material, 128 ± 25 craters/106km2; hilly and cratered material, 137 ± 38 craters/106km2; and cratered plateau material, 138 ± 27 craters/106km2.  相似文献   

13.
Utilizing the topographic model of Jovian moon Amalthea (Stooke, 1994) and supposing that its mass density is constant we derived its basic geometrical and dynamical characteristics. For calculations the harmonic model of topography of the degree and order 18 was selected. The model appears to fit the entire surface to a mean accuracy of a few hundred meters, except in the regions localized around longitudes 0° and 180°. On the basis of the harmonic expansion of the topography we estimated the volume (V = 2.43 ± 0.02 km3) and the mean radius of topographyr 0 = (79.7 ± 0.2) km. Generalized moments of inertia up to the order 2, principal moments of inertia and orientation of the principal axes with respect to the original reference frame were also calculated. The results show that although Amalthea has extremely irregular shape it may be treated dynamically as an almost symmetric body (B C). Finally, the set of the Stokes coefficients up to the degree and order 9 was evaluated. The results are verified by direct numerical integration.  相似文献   

14.
Analytic expressions for the semimajor and semiminor axes and an orientation angle of the ellipse projected by a triaxial ellipsoid (an asteroid) and of the ellipse segment cast by a terminator across the ellipsoid as functions of the dimensions and pole of the body and the asterocenteric position of the Earth and Sun are derived. Applying these formulae to observations of the Earth-approaching asteroid 433 Eros obtained with the speckle interferometry system of Steward Observatory on December 17–18, 1981, and January 17–18, 1982, the following dimensions are derived: (40.5 ± 3.1 km) × (14.5 ± 2.3 km) × (14.1 ± 2.4 km) Eros' north pole is found to lie within 14° of RA = 0h16m Dec. = +43° (ecliptic longitude 23°, latitude +37°). Other than knowing the rotation period of Eros, these results are completely independent of any other data, and in the main confirm the results obtained in the 1974–1975 apparition by other methods. These dimensions, together with a lightcurve from December 18, 1981, lead to a geometric albedo of 0.156 ± 0.010. A series of two-dimensional power spectra and autocorrelation functions of the resolved asteroid clearly show it spinning in space.  相似文献   

15.
Measurements of the high‐energy gamma‐ray flux emanating from asteroid 4 Vesta by the Dawn Gamma‐Ray and Neutron Detector (GRaND) have revealed variability in the near‐surface elemental composition of the Vestan surface. These observations are consistent with the presence of large (≥8 × 104 km2) regions with distinct, HED‐like elemental compositions. The results agree broadly with other global measurements, such as the macroscopic neutron absorption cross section and spectral reflectance‐derived mineralogic maps. Two distinct regions with eucrite‐like elemental compositions have been identified, the first located primarily within the Lucaria and Marcia quadrangles and the second within Oppia quadrangle. The former region is collocated with some of the oldest, most heavily cratered terrain on Vesta. The interior of the 500 km diameter Rheasilvia impact basin is found to have a composition that is consistent with diogenite‐like material. Taken together, these observations support the hypothesis that Vesta's original crust was composed of basaltic outflows in the form of eucritic‐like material and that the Rheasilvia‐basin‐forming impact exposed lower‐crustal, diogenite‐like material. These measurements also constrain the maximum amount of mesosiderite‐like material to <10% for each 15 × 15° surface element.  相似文献   

16.
This paper describes the evolution of landforms in Atlantis and Gorgonum basins, using a geomorphologic approach which interprets landform distribution and hierarchy. Rather than looking at the distribution of large-area (>106 km2) geologic sequences, this study focuses on interpreting the local-scale (<103 km2) cratered terrains, tectono-structural basins, and local manifestation of exogenic processes. Specifically, the evolution of fluvio-lacustrine landforms is interpreted as being functionally subordinated to the evolution of the cratered terrains and to the tectono-structural modifications of the landscape. Results show that three major phases of landscape evolution in Atlantis and Gorgonum basins can be identified: (a) major impact cratering during the heavy-bombardment period; (b) tectonic displacements in response to volcano emplacement in the Tharsis region, and simultaneous landform creation by fluvial and lacustrine processes; and (c) exogenic modification of the older landforms through weathering and eolian processes. Our results show that the smaller morphological features, which form on the older geological units, are not necessarily old themselves and can in fact be relatively recent (e.g. Amazonian). The main implication of these results is that martian morphology did not form only during a period immediately following the heavy bombardment, as commonly postulated, but rather that landform evolution continued throughout the entire martian history.  相似文献   

17.
Dimension of the Earth's General Ellipsoid   总被引：1，自引：0，他引：1  
The problem of specifying the Earth's mean (general)ellipsoid is discussed. This problem has been greatly simplified in the era of satellite altimetry, especially thanks to the adopted geoidal geopotential value, W0 = (62 636 856.0 ± 0.5) m2 s-2.Consequently, the semimajor axis a of the Earth's mean ellipsoid can be easily derived. However, an a priori condition must be posed first. Two such a priori conditions have been examined, namely an ellipsoid with the corresponding geopotential that fits best W0 in the least squares sense and an ellipsoid that has the global geopotential average equal to W0. It has been demonstrated that both a priori conditions yield ellipsoids of the same dimension, with a–values that are practically identical to the value corresponding to the Pizzetti theory of the level ellipsoid: a = (6 378 136.68 ± 0.06) m.  相似文献   

18.
The morphology and distribution of ridges and scarps on Mars in the ± 30° latitude belt were investigated. Two distinct types of ridges were recognized. The first is long and linear, resembling mare ridges on the Moon; it occurs mostly in plains areas. The other is composed of short, anastomosing segments and occurs mostly in ancient cratered terrain and intervening plateaus. Where ridges are eroded, landscape configurations suggest that they are located along regional structures. The age of ridges is uncertain, but some are as young as the latest documented volcanic activity on Mars. The origins of ridges are probably diverse-they may result from wrinkling due to compression or from buckling due to settling over subsurface structures. The similar morphologic expressions of ridge types of various origins may be related to a similar deformation mechanism caused by two main factors: (1) most ridges are developed in thick layers of competent material and (2) ridges formed under stresses near a free surface.  相似文献   

19.
《Icarus》1986,67(2):251-263
511 Davida was observed with the technique of speckle interferometry at Steward Observatory's 2.3-m telescope on May 3, 1982. Assuming Davida to be a featureless triaxial ellipsoid, based on five 7-min observations its triaxial ellipsoid dimensions and standard deviations were found to be (465 ± 90) × (358 ± 58) × (258 ± 356) km. This shape is close to an equilibrium figure (a gravitationally shaped “rubble pile?”) suggesting a density of 1.4 ± 0.4 g/cm3. Simultaneously with the triaxial solution for the size and shape of Davida, we found its north rotational pole to lie within 29° of RA = 19h08m, Dec = +15° (λ = 291°, β = +37°). If Davida is assumed to be a prolate biaxial ellipsoid, then its dimensions were found to be (512 ± 100) × (334 ± 39) km, with a north pole within 16° of RA = 10h52m, Dec = +16° (λ = 322°, β = +32°). We derive and apply to Davida a new simultaneous amplitude-magnitude (SAM)-aspect method, finding, from photometric data only, axial ratios of a/b = 1.25 ± .02, b/c = 1.14 ± .03, and a rotational pole within 4° of λ = 307°, β = +32°. We also derive a (weighted) linearized form of the amplitude-aspect relation to obtain axial ratios and a pole. However, amplitudes must be known to better than .01 if the b/c or a/c ratios are desired to better than 10%. Combining the speckle and SAM results, we find for the Gehrels and Tedesco phase function a geometric albedo of .033 ± .009 and for the Lumme and Bowell function .041 ± .011, for a unified model of 437 × 350 × 307 km. Differences between the photometric and speckle axial ratios and poles are probably due to the effects of albedo structure over the asteroid; details on individual lightcurves support this conclusion.  相似文献   

20.
Similarity is found in crater densities on the most heavily cratered surfaces throughout the solar system. This is hypothesized to result from a steady-state “saturation equilibrium” being approached or achieved by cratering processes. This hypothesis conflicts with some recent interpretations. However, it accounts for (1) a similarity in maximum relative crater density, below certain theoretically predicted values, on all heavily cratered surfaces; (2) a leveling off at this same relative density among 100-m scale (secondary?) craters in populations on lunar maria and other sparsely cratered lunar surfaces; (3) the approximate uniformity of maximum relative densities on Saturn satellites (in spite of dramatic variations predicted from nonsaturation models assuming heliocentric impactors). The lunar frontside upland crater population, sometimes described as a well-preserved production function useful for interpreting other planetary surfaces, is found not to be a production function. It was modified by intercrater plains formed (at least partly) by early upland basaltic lava flooding, recently confirmed spectrophotometrically. Consistent with this, counts in “pure uplands” (those lacking intercrater plains) match the proposed saturation equilibrium density. Variations among large (D > 64 km) crater populations are found, but these may involve several hypothesized mechanisms that rapidly obliterate large craters, especially on icy surfaces. Recent models, in which different populations of interplanetary bodies hit different planets, need further appraisal.  相似文献   

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