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 15 毫秒
1.
Recent radar observations of Syrtis Major have shown it to be smooth and relatively homogeneous when sensed at centimeter wavelengths λ. There is a gradual decrease in surface roughness east to west across the basin, which correlates with an apparent decrease in small (< 1 km diameter) crater density. Root mean square surface slopes σ range from more than 1.5–2.0° in the east to less than 0.5° along the western margin at λ = 12.6 cm. The surface appears somewhat rougher at λ = 3.6 cm; a σ ∝ λ?0.3 dependence is inferred. Radar reflectivity increases from about 5% to about 12% across Syrtis Major, being greatest near the western margin. High-resolution (8 to 20 m/pixel) Viking images suggest that an increasing amount of resurfacing has occurred in western Syrtis Major compared with the eastern parts. The radar, infrared, and optical imaing evidence are consistent with resurfacing by geologically recent, low-viscosity lavas which were subsequently mantled by thin layers of aeolian material. Each data set may be taken as a unique source of scale-dependent information on surface materials and properties. From comparison of radar-derived surface roughness with image-derived crater density curves we conclude that processes other than cratering control the surface texture on 0.03- to 100-m surface scales.  相似文献   

2.
The relative radar reflectiveness of Saturn's classical ring sections were estimated from delay-Doppler observations made at 12.6-cm wavelength. The A and B rings are responsible for most, if not all, of the radar echo. The average radar reflectivity per unit projected area of the A ring is nearly (～90%) as large as that of the B ring. The outer half of the B ring contains the most reflective part of the ring system. There is no firm evidence for detection of radar backscattering from particles interior to the B ring, exterior to the A ring, or from the planet itself. The radar reflectivity of the C ring is certainly no more than one-half that of the B ring, and probably is much less. Unexpectedly large amounts of power at Doppler shifts near the center of the echo spectrum, previously reported at both λ3.5 and λ12.6 cm for ring-plane tilt angles δ ≥ 24.4°, are not apparent in λ3.5- or 12.6-cm spectra obtained at δ ≤ 21.4°.  相似文献   

3.
Ground-based observations of the CO2 distribution on Mars were made this past opposition from Cerro Tololo Interamerican Observatory. Almost complete coverage of the Martian surface from 40°N to 60°S was obtained. Agreement with previous Kitt Peak observations is good, and confirmation of a pressure anomaly in the Tharsis region has been obtained. The ridge whose eastern slope is Syrtis Major stops at about 15°S, in agreement with the 1971 radar data. The Noachis-Hellas region south of Syrtis Major appears at about average altitude, indicating that the dust storm of 1971 was already active in that region as early as the end of August.  相似文献   

4.
Bistatic radar observations of Mars' north polar region during 1977–1978 showed surface rms slope σβ ranging from 1 to 6°; these values apply to horizontal scales of 1–100 m. Values of roughness tend to decrease with increasing latitude (especially over 65–80°N), but there are many exceptions. The smoothest surfaces (σβ≤1°) appear to be inclusions within generally rougher (σβ～3°) terrain, rather than broad expanses of very smooth material. The permanent north polar cap is relatively uniform with 2.5?σβ?3.0°. Considerable structure has been found in echo spectra, indicating a heterogeneous and perhaps anisotropic scattering surface. Echo spectra obtained from the same region, but several months apart (1°<LS<62°), show no significant differences in inferred roughness. Estimates of reflectivity and dielectric constant are systematically low in the polar region. This may indicate that surface material north of 65°N is less dense than that near the equator, but more study of these data is needed. Estimates of surface roughness and dielectric constant in the equatorial region are consistent with results from Earth-based measurements to the accuracy of our analysis.  相似文献   

5.
Mariners 6 and 7 photographs of the equatorial region of Mars document a three-stage evolution of that part of the Martian surface: (1) High- and intermediate-albedo cratered terrains in Meridiani Sinus, Margaritifer Sinus-Thymiamata, Deucalionis Regio-Sabaeus Sinus, and Hellespontus; (2) low-albedo moderately cratered terrain and dark crater fill in Meridiani Sinus, Thymiamata, and Deucalionis Regio-Sabaeus Sinus and possible volcanism in the Hellas-Hellespontus border; and (3) high-albedo surficial deposits, banked-up crater fill, a possible bright-ray crater in Meridiani Sinus, chaotic terrain on the edge of the Margaritifer Sinus mesa, featureless terrain in Hellas and Edom, sinuous channel-like reentrants on scarps at the Hellas-Hellespontus boundary. Regional faulting seems to have occurred following formation of the old cratered plains and prior to formation of low-albedo plains in Meridiani Sinus and also prior to formation of canyon-like reentrants and featureless terrain along the Hellas-Hellespontus boundary.Mars has had a complex history of dynamic evolution, possibly analogous to the more stable regions of Earth. Its geochemical differentiation and thermal regime should account for long-term postaccretional tectonic and volcano-tectonic processes as well as for fluid media on its surface sufficient to cause erosion, including the cutting of large canyons.  相似文献   

6.
The results of the investigation of two regions of Venus by bistatic radiolocation are presented. The experiments were carried out at wavelength λ0 = 32 cm. Maps of the distribution of reflectivity were obtained and characteristics of the relief, dielectric permittivity, soil density, and refraction attenuation in the atmosphere were measured. The value of the dispersion of small-scale slopes in the observed regions, γ, varies between 0.4 and 2.2°. There are some features on the reflectivity maps. Some of these features may correspond to mountain slopes with values in the range 2 to 8°. Corresponding changes of relief heights are contained in the interval 0.8 to 2.6 km. The features are found within the region (in the venerocentric IAU system): ?26.5 to 25.0° latitude and 220.0?239.2° longitude. One area was revealed with large values of permittivity in the range 6.5–7.5, and soil density between 2.7 and 2.9 g/cm3. The center of this area is located at ?23.5° latitude and 230.4° longitude. The extent of this region is 80 km. The results of measurements of the refraction angle and the refraction attenuation of radio waves are in good agreement with the parameters of the atmosphere of Venus received from the Soviet landers.  相似文献   

7.
  相似文献   

8.
In this work we analyze the spatial structure of Jupiter's cloud reflectivity field in order to determine brightness periodicities and power spectra characteristics together with their relationship with Jupiter's dynamics and turbulence. The research is based on images obtained in the near-infrared (∼950 nm), blue (∼430 nm) and near-ultraviolet (∼260 nm) wavelengths with the Hubble Space Telescope in 1995 and the Cassini spacecraft Imaging Science Subsystem in 2000. Zonal reflectivity scans were analyzed by means of spatial periodograms and power spectra. The periodograms have been used to search for waves as a function of latitude. We present the values of the dominant wavenumbers for latitude bands between 32° N and 42° S. The brightness power spectra analysis has been performed in the meridional and zonal directions. The meridional analysis of albedo profiles are close to a k−5 law similarly to the wind profiles at blue and infrared wavelengths, although results differ from that in the ultraviolet. The zonal albedo analysis results in two distributions characterized by different slopes. In the near infrared and blue wavelengths, average spectral slopes are n1=−1.3±0.4 for shorter wavenumbers (k<80), and n2=−2.5±0.7 for greater wavenumbers, whereas for the ultraviolet n1=−1.9±0.4 and n2=−0.7±0.4, possibly showing a different dynamical regime. We find a turning point in the spectra between both regimes at wavenumber k∼80 (corresponding to L∼1000 km) for all wavelengths.  相似文献   

9.
We present the results of polarimetric and photometric observations of dynamically new comet C/2002 T7 (LINEAR) at phase angles from 6° to 26°. During the observations, the comet was at a distance of 2.7–1.3 AU from the Sun and 1.7–2.0 AU from the Earth. The aperture polarimetry was made with the 2.6-m Shain telescope and the 1.25-m AZT-11 telescope of the Crimean Astrophysical Observatory and with the 0.7-m telescope of the Astronomical Institute of the Kharkiv National University during the period from November 21, 2003, to February 21, 2004. The wideband UBVRI and WRC (λ7228/1142 Å) filters and the narrowband GC (λ5260/56 Å) filter were used. The photometric observations of the comet were carried out on February 21, 2004, with narrowband filters isolated the BC (λ4845/65 Å) and RC (λ6840/90 Å) continuum and the C2 emission (λ5140/90 Å). The phase-angle dependence of linear polarization of the comet has been obtained, and its parameters, such as the minimal polarization P min = ?1.63%, the phase angle of the minimal polarization αmin = 10.6°, the inversion angle αinv = 22.7°, and the slope of the phase curve at the inversion angle h = 0.24% per degree, were found. From the photometric observations, the following quantities have been obtained: the column density of molecules C2 in the line of sight logN (C2) = ?9.15 mol/cm2 and their production rate log Q (C2) = 27.11 mol/s, the spectral gradient of reflectivity for the dust S′(BC, RC) ≈ 3%/1000 Å, and the dust production parameter Afρ equal to 371 and 273 cm for the blue and red continuum ranges, respectively. According to these results, the physical parameters of comet C/2002 T7 are close to the average characteristics of typical dusty comets.  相似文献   

10.
The paper presents estimates of the occurrence probability of slopes, whose steep surfaces could be dangerous for the landing of the Luna-Glob descent probe (Luna-25) given the baseline of the span between the landing pads (~3.5 m), for five potential landing ellipses. As a rule, digital terrain models built from stereo pairs of high-resolution images (here, the images taken by the Narrow Angle Camera onboard the Lunar Reconnaissance Orbiter (LROC NAC)) are used in such cases. However, the planned landing sites are at high latitudes (67°–74° S), which makes it impossible to build digital terrain models, since the difference in the observation angle of the overlapping images is insufficient at these latitudes. Because of this, to estimate the steepness of slopes, we considered the interrelation between the shaded area percentage in the image and the Sun angle over horizon at the moment of imaging. For five proposed landing ellipses, the LROC NAC images (175 images in total) with a resolution from 0.4 to 1.2 m/pixel were analyzed. From the results of the measurements in each of the ellipses, the dependence of the shaded area percentage on the solar angle were built, which was converted to the occurrence probability of slopes. For this, the data on the Apollo 16 landing region ware used, which is covered by both the LROC NAC images and the digital terrain model with high resolution. As a result, the occurrence probability of slopes with different steepness has been estimated on the baseline of 3.5 m for five landing ellipses according to the steepness categories of <7°, 7°–10°, 10°–15°, 15°–20°, and >20°.  相似文献   

11.
The observation of gullies on Mars raised questions about the presence of liquid water in the recent past. In some regions like Hale and Bond crater, gullies occur in one crater (Hale) but do not in another crater nearby (Bond). These regional differences have been interpreted as an argument for a formation of the gullies related to groundwater. The formation of gullies on Earth depends on rainfall and/or melting of snow as well as on several parameters such as the presence of steep slopes and sufficient amounts of fines and debris. We investigated the Hale/Bond region for differences in crater wall morphology and texture, slopes, and thermal properties to determine whether the gully formation is dependent on factors such as steep slope angles and availability of fine-grained material. Morphologically there exist two kinds of gullies in the Hale crater: Gullies on the south- and east-facing crater slopes have a pristine appearance with deep channels eroded into the talus material and well-preserved aprons. Gully-like features on the north- and west-facing slopes are degraded and superposed by craters, indicating that they are old in comparison to the pristine ones. However, their formation process is unclear and might be due to debris flows, surface runoff or dry mass wasting processes or a combination of these processes. The crater walls of Bond do not show gullies. Their morphology is most likely consistent with a degraded mantle deposit. Slope measurements reveal that the gullies in Hale crater occur on slopes between ～20° and ～30° in contrast to the slopes without gullies in Bond that are between ～10° and ～20° steep. Mean thermal inertia values on slopes with younger gullies are ～175 J m?2 K?1 s?1/2 corresponding to higher amounts of fine-grained material. At slopes with older gully-like features mean thermal inertia values are ～315 J m?2 K?1 s?1/2 corresponding to higher amounts of bedrock or possibly indurated grain sizes. Mean thermal inertia values of the Bond crater walls are ～230 J m?2 K?1 s?1/2 indicating more consolidated terrain possibly due to the cementation of the dissected mantle material. From our investigation we conclude that the occurrence of gullies in the Hale/Bond region most likely depends on the distribution of unconsolidated material and steep slopes. The regional and local gully distribution on Mars likely varies due to differences in topography and surface material properties. Their proposed clustered distribution on Mars is not an argument for a groundwater formation mechanism of the gullies.  相似文献   

12.
Vesta’s crater sizes that are based on images returned from the Dawn probe orbiting the asteroid 4 Vesta are compared with sizes of spots on its surface derived from hydrosilicate equivalent widths and asteroid color indices B-V and V-R observed spectrophotometrically on earth using a spectral-frequency method. The sizes of craters and spots at the asteroid poles prove to correspond to the sizes of spots that are assumed to be located at latitudes of 40°...45° N and S. Comparative results for the crater sizes ranging from 10 to 100 km are tabulated. We conclude that crater sizes on asteroid surfaces can be determined using the spectral-frequency method.  相似文献   

13.
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   

14.
High-resolution radar observations of Mars at a wavelength of 3.8 cm have been carried out at the Haystack Observatory for a period of about 6 months surrounding the 1971 opposition. The relative surface height variation with longitude over a band of Martian latitudes between about ?14° and ?22° has been derived from these observations with an error of about 75m in the most favorable cases. At ?14° latitude, the dielectric constant was found to increase steadily from about 1.7 at 70°W to about 5.0 at 110°W. Over the same interval the rms surface slopes vary from 2° to 1°. The mean equatorial radius of Mars as determined from the combined radar data of 1967, 1969 and 1971 was found to be 3394 ± 2km.  相似文献   

15.
Meridiani Planum is the first officially recognized meteorite find on the surface of Mars. It was discovered at and named after the landing site of the Mars Exploration Rover Opportunity. Based on its composition, it was classified as a IAB complex iron meteorite. Mössbauer spectra obtained by Opportunity are dominated by kamacite (α‐Fe‐Ni) and exhibit a small contribution of ferric oxide. Several small features in the spectra have been neglected to date. To shed more light on these features, five iron meteorite specimens were investigated as analogs to Meridiani Planum with a laboratory Mössbauer setup. Measurements were performed on (1) their metallic bulk, (2) troilite (FeS) inclusions, (3) cohenite ((Fe,Ni,Co)3C) and schreibersite ((Fe,Ni)3P), and (4) corroded rims. In addition to these room‐temperature measurements, a specimen from the Mundrabilla IAB‐ungrouped meteorite was measured at Mars‐equivalent temperatures. Based on these measurements, the features in Meridiani Planum spectra can be explained with the presence of small amounts of schreibersite and/or cohenite and iron oxides. The iron oxides can be attributed to a previously reported coating on Meridiani Planum. Their presence indicates weathering through the interaction of the meteorite with small amounts of water.  相似文献   

16.
High-altitude spectra of Jupiter obtained from the Kuiper Airborne Observatory are analyzed for the presence of germane (GeH4) in Jupiter's atmosphere. Comparison with laboratory spectra shows that the strong Q branch of the ν3 band of germane at 2111 cm?1 is prominent in the Jovian spectra. The abundance of germane in Jupiter's atmosphere is 0.006 (±0.003) cm-am corresponding to a mixing ratio of 0.6 ppb. This trace amount of germane is consistent with chemical equilibrium calculations if the germane present at ～1000°K is carried up by convection to the spectroscopically observable region at ～300°K.  相似文献   

17.
The Goldstone radar system was used at a wavelength of 12.6 cm to probe the Martian surface during the 1973 opposition. Measurements of range and reflected power were made at least weekly between July 12 and November 24. Surface cells isolated by the radar system were 8 km E-W × 110 km N-S. Altitudes were calculated from signal time delays measured relative to a triaxial ellipsoid and were combined with altitudes measured during the 1971 opposition. Contours of constant altitude were calculated at 200 m intervals between latitudes ?14° and ?22°. These contours are presented in conjunction with Mars charts derived from Mariner 9 television pictures. Reflected power was measured at angles of incidence between ?5° and +5°. These measurements were combined with those obtained during the 1971 opposition. Predictions of the reflected power versus the angle of incidence were calculated from the exponential surface model of Hagfors. The predictions were fit to the data in a least-squares sense, using a nonlinear iterative procedure, to yield estimates of surface roughness and reflectivity. The smoother regions exhibit a typical reflectivity of 8.2%. A tendency for the reflectivity to decrease with increasing roughness was observed.  相似文献   

18.
Mars surface characteristics at and near the Viking Chryse and Tritonis Lacus landing areas were determined by radio scatter using the new 12.6 cm radar at the Arecibo Observatory during 1975–1976. Interpretation of each power spectrum suggests rms surface tilts of 4° at the final A1WNW (47.9°W, 22.5°N) site, 5° near the original A1 site, and 6° between the two. At the back-up site (A2) surface roughness estimates were about 4°. Striking changes in surface texture have been found near the eastern bases of Tharsis Montes and Albor Tholus, each volcanic feature marking the western boundary of very smooth surface units. The roughness sensed at 1 to 100 m scales by radar appears to be relatively independent of the surface units defined at large scale lengths by photogeologists. Radar properties thus provide an additional means by which planetary surfaces may be characterized.  相似文献   

19.
We have measured the shape and absolute value of Venus' reflectivity spectrum in the 1.2-to 4.0-μm spectral region with a circular variable filter wheel spectrometer having a spectral resolution of 1.5%. The instrument package was mounted on the 91-cm telescope of NASA Ames Kuiper Airborne Observatory, and the measurements were obtained at an altitude of about 41,000 feet, when Venus had a phase angle of 86°. Comparing these spectra with synthetic spectra generated with a multiple-scattering computer code, we infer a number of properties of the Venus clouds. We obtain strong confirmatory evidence that the clouds are made of a water solution of sulfuric acid in their top unit optical depth and find that the clouds are made of this material down to an optical depth of at least 25. In addition, we determine that the acid concentration is 84 ± 2% H2SO4 by weight in the top unit optical depth, that the total optical depth of the clouds is 37.5 ± 12.5, and that the cross-sectional weighted mean particle radius lies between 0.5 and 1.4 μm in the top unit optical depth of the clouds. These results have been combined with a recent determination of the location of the clouds' bottom boundary [Marov et al., Cosmic Res.14, 637–642 (1976)] to infer additional properties about Venus' atmosphere. We find that the average volume mixing ratio of H2SO4 and H2O contained in the cloud material both equal approximately 2× 10?6. Employing vapor pressure arguments, we show that the acid concentration equals 84 ± 6% at the cloud bottom and that the water vapor mixing ratio beneath the clouds lies between 6 × 10?4 and 10?2.  相似文献   

20.
《New Astronomy》2002,7(4):171-183
We study the angular power spectra of polarized Galactic synchrotron in the range 10≲l≤800, at several frequencies between 0.4 and 2.7 GHz and at several Galactic latitudes up to near the North Galactic Pole. Electric- and magnetic-parity polarization spectra are found to have slopes around αE,B=1.4–1.5 in the Parkes and Effelsberg Galactic-Plane surveys, but strong local fluctuations of αE,B are found at |b|≃10° from the 1.4 GHz Effelsberg survey. The CPIl spectrum, which is insensitive to the polarization direction, is somewhat steeper, being αPI=1.6–1.8 for the same surveys. The low-resolution multifrequency survey of Brouw and Spoelstra [A&AS 26 (1976) 129] shows some flattening of the spectra below 1 GHz, more intense for CE,Bl than for CPIl. In no case we find evidence for really steep spectra. The extrapolation to the cosmological window shows that at 90 GHz the detection of E-mode harmonics in the cosmic background radiation should not be disturbed by synchrotron, even around l≃10 for a reionization optical depth τri≳0.05.  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