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1.
Raymond E. Arvidson 《Icarus》1974,21(1):12-27
Crater morphology and size play a major role in determining whether wind-blown streaks emanating from craters or dark splotches within craters will form. Both bright and dark streaks emanate almost exclusively from bowl-shaped craters. Dark splotches are found mainly in flat-floored craters, especially those that are deep and have high rim relief. Trends of dark splotches in the northern to southern midlatitudes closely follow those of bright streaks, suggesting both were formed by similar winds. In the high southern latitudes, on the other hand, dark splotch trends closely follow those of dark streaks.Qualitative models of streak and splotch formation have been derived from these data and results of Sagan et al. (1972, 1973). Bright streaks probably form by trapping and simultaneous streaming of bright dust downwind. Dark splotched craters in regions with bright streaks usually have upwind bright patches, suggesting these features form by dumping of bright dust over crater rims with some minor redistribution of dark materials toward the downwind sides of craters. Data are consistent with dark streaks forming by erosion or nondeposition of bright material or by trapping of dark material. Dark splotches in these regions are probably mainly the result of trapping of dark sand in the downwind sides of crater floors. Craters with dark splotches and dark streaks are usually rimless and shallow. This is consistent with ponded dark sands easily washing over crater walls and extending downwind.Plots of streak length versus crater diameter suggest a complex history of streak formation for most regions.Bright streak trends and latitudinal distributions are consistent with return flow of dust to the southern hemisphere. Some dark streaks may be direct relics of passing sand and dust storms. Trends of dark streaks and splotches away from the south pole are consistent with the spreading of a debris mantle from the polar regions toward the equator. 相似文献
2.
As in seen from comparisons of Mariner 9 images obtained in 1972 and Viking Orbiter 1 images obtained in 1978, several changes have occurred in the Cerberus region of Mars. Changes in the boundary of the low albedo feature resulted in an increase of the total area of Cerberus by slightly more than 1%, although the southwestern boundary had shifted as much as 90 km. Relative darkening of Cerberus has resulted in a more uniform tone, and is accompanied by the disappearance of dark filamentary markings. Although several bright streaks within Cerberus changed in length, neither lengthening nor shortening of the streaks predominated. However, changes in streak direction indicate a clockwise rotation of mean streak azimuth between 1972 and 1978. These changes in the outline and appearance of Cerberus can best be explained by eolian redistribution and removal of bright material during major dust storms. Volcanic flow fronts which show through the albedo feature indicate that the contrast between Cerberusand the surrounding light plains is not due to a difference in lithology, but to the distribution of surficial deposits. Because of local topographic influences on the regional atmospheric circulation patterns, it is probable that Cerberus will retain a similar appearance and location. 相似文献
3.
R. O. Kuzmin has proposed that all crater-associated wind streaks on Mars are depositional and consist of unresolved barchan-like dunes. He claims that any streak can appear either bright or dark relative to its surroundings depending on the azimuth of the Sun relative to the streak axis and on the elevation of the Sun above the horizon. Our studies of the entire Mariner 9 picture collection as well as of available Viking data lend no support to these ideas. We find that the conditions for visibility of bright and dark streaks are identical. In Mariner 9 images both types of streaks are visible for viewing angles ? ? 60°, illumination angles of 15° ? i ? 75°, and over the whole range of phase angles covered (about 15 to 85°). There are numerous examples of dark and light streaks visible at the same azimuth angle of the Sun, contrary to Kuzmin's claim. There is much evidence to indicate that bright and dark streaks differ both in morphology and in character. The common ragged dark streaks are probably erosion scars, while most bright streaks probably represent accumulations of bright dust-storm fallout. There is no evidence at present that these accumulations have a barchan-like texture. 相似文献
4.
A map of “albedo” boundaries (light and dark markings) on Mars was prepared from Mariner 9 images. After special digital processing, these pictures provide detailed locations of albedo boundaries, which is significant in interpreting recent eolian activity. Derivation of absolute albedo values from the spacecraft data was not attempted. The map correlates well with telescopic observations of Mars after the 1971 dust storm. 相似文献
5.
Multicolor imaging of Mercury has been performed with the 0.5 m Swedish Vacuum Solar Telescope (SVST) on La Palma at five elongations from 1995 to 1999, resulting in a global Minnaert normalized map of the surface at 200 km resolution. Short exposure CCD imaging has been performed in the optical and near-infrared with broad- and intermediate band filters at wavelengths from 550 to 940 nm. Positions for 86 and morphological parameters for 63 bright albedo features on the Hermean surface have been determined. The distribution of bright albedo features is shown to be spatially uniform on the well known (i.e., observed by Mariner 10) and poorly known hemispheres, as well as for the global surface. The number densities of bright albedo features on the two hemispheres are very similar. This indicates that the late evolutionary history of the Hermean regolith has not varied on regional to global scales in terms of impacts generating bright ray craters, constituting approximately 70% of the detected bright albedo features. The locations of bright albedo features correspond well to those determined from nominal resolution and smeared (to the approximate resolution of the SVST data) Mariner 10 maps. Feature parameters (radius, center intensity and intensity gradient) have been determined and correlated with the geologic nature of a subset of observed features imaged by the Mariner 10 Vidicon camera. No difference in feature properties is apparent between the poorly known and well known hemispheres. Based on a comparitive study of Mariner 10 image data, ray craters tend to have higher center intensities and smaller intensity gradients than bright albedo features which are not ray craters. It is however concluded that it is not possible to uniquely determine the geologic nature of features with a high statistical significance, based on their morphological parameters at 200 km resolution. We do not find any general correlation between the locations of radar-bright and optically bright or dark albedo features. The surface contrast decreases from 35% to 25% over the wavelength range 550–940 nm. The range of feature contrasts is similar for all surface regions, except for the ray crater Kuiper, whose contrast to the mean surrounding surface is 50% at a wavelength of 750 nm. Kuiper is an extreme albedo feature also in terms of its center intensity and slope. The mean value of the Minnaert slope parameter for the global surface is determined to 0.76±0.10. A measured constant value of the Minnaert slope with wavelength indicates that the spectral slope for typical Hermean regolith should be linear over the wavelength range 550–940 nm. 相似文献
6.
A comparison of crater-related wind markers in the north and south polar (40–90° latitude) regions of Mars has been made on the basis of comprehensive mapping from Viking Orbiter and Mariner 9 Images. Wind streaks show that present wind activity is most effective in both north and south in the southern spring and summer. This asymmetry is consistent with the present asymmetry of climate. The more massive intracrater dune fields are also oriented with the presently strongest winds. This alignment may reflect a long-term asymmetry in the effectiveness of northern and southern spring flow because reorientation times far exceed the period of cycles of hemispherical climate asymmetry, ≈51, 000 years. Streaks originating from dark crater splotches indicate that windflow away from the south pole is effective over a larger latitude range than it is in the north. This difference may be partly responsible for the contrasting distribution of dune sand in the north and south polar regions. 相似文献
7.
A statistical study of all crater-related wind streaks visible on Mariner 9 A-camera frames between latitudes 0 and 30°N has been completed. Of the 2325 streaks identified 1914 (82%) are light tone streaks, 189 (8%) are dark tone, and the remaining 222 (10%) are of mixed tone. Nine parameters characterizing each streak and its associated crater were measured and intercorrelated. Because of the large number of light streaks in our sample fir findings for this type of streak are most significant statistically: light tone streaks occur preferentially in Pc terrain (heavily cratered plains); they are preferentially associated with fresh craters; the surface density of light streaks is not a strong function of elevation; a significant latitude effect does emerge—the density of light tone streaks reaches a maximum between 10 and 15°N, and drops off appreciably both toward the equator and toward higher latitudes; the mean angular width of light streaks is about 25°—long light streaks are significantly narrower than short ones; about 50% of streaks have streak length/crater diameter ratios of ?4; light streak directions conform closely to the wind regime expected at the season of global dust storms (southern summer). Generally speaking, the results for dark and mixed tone streaks in the northern equatorial zone are similar, with the following possible exceptions: dark streaks may show a slight preference to form at higher elecations; dark streaks may be slightly wider on average than light or mixed tone streaks; mixed tone streaks do not share the preference for sharp craters exhibited by light and dark streaks; in general, the directions of dark streaks do not conform to the general circulation pattern expected at the season of global dust storms as well as do those of the light streaks. 相似文献
8.
Global mapping and photometry of selected areas on Mars are used to investigate the nature of bright and dark wind streaks that extend from topographic obstacles. Occurrence of both bright and dark streaks is strongly latitude dependent and is only weakly correlated with surface properties such as albedo and thermal inertia. Data on the colors, albedos, and phase behavior of streaks are consistent with models of bright streaks as mosaics of plains material and brighter, redder dust. Less than 20% of the ground need be covered by the optically thick dust in the brightest parts of the streaks; the amount of dust in optically thick layers could be as little as 10?3 g/cm2. Dark streaks can be interpreted as erosional windows in a patchy dust cover. Our model of dust deposition in optically thick patches is sedimentologically different from scenarios involving the deposition of ubiquitous, optically thin layers. It has the advantage that large amounts of dust can be deposited without affecting regional albedos. 相似文献
9.
The Lowellian canal network has been compared with the results of Mariner 9 photography of Mars. A small number of canals may correspond to rift valleys, ridge systems, crater chains, and linear surface albedo markings. But the vast bulk of classical canals correspond neither to topographic nor to albedo features, and appear to have no relation to the real Martian surface. 相似文献
10.
Richard E. D&#x;Alli 《Icarus》1977,31(1):146-156
Mariner 9 photographs of the southern hemisphere of Mars taken during the 1971 planet-wide dust storm display circular bright spots at a time when all near-surface features were totally obscured. Correlating the positions and diameters of these spots with topography shows that they correspond to craters. About half of all the large craters in thestudy area were brightened. The associated craters are large and flat-floored, have significant rim uplift, and contain dark splotches on their floors. The depth/diameter relationship of the bright spot craters is comparable to that of a planet-wide sample. Depth may not be important in selectively brightening certain craters. The visibility of bright spots in A-camera photographs is strongly dependent on the wavelength of the filter used during exposure. It is proposed that bright spots result from the multiple scattering of incident light in dust clouds entrained within craters during dust storms. The appearance of the dust clouds is a function of the availability of a dust supply and, perhaps, air turbulence generated by winds flowing over upraised rims and rough crater floors. Bright spots persist during the final stage of the planet-wide dust storm. If bright spots are dust clouds, this persistence demonstrates that crater interiors are the last regions of clearing. 相似文献
11.
David Pieri 《Icarus》1976,27(1):25-50
The distribution of small channels on Mars has been mapped from Mariner 9 images, at the 1:5 000 000 scale, by the author. The small channels referred to here are small valleys ranging in width from the resolution limit of the Mariner 9 wide-angle images (～1 km) to about 10 km. The greatest density of small band occurs in dark cratered terrain. This dark zone forms a broad subequatorial band around the planet. The observed distribution may be the result of decreased small-channel visibility in bright areas due to obscuration by a high albedo dust or sediment mantle. Crater densities within two small-channel segments show crater size-frequency distributions consistent with those of the oldest of the heavily cratered plains units. Such crater densities coupled with the almost exclusive occurrence of small channels in old cratered terrain and the generally degraded appearance of small channels in the high-resolution images (～100 m) imply a major episode of small-channel formation early in Martian geologic history. 相似文献
12.
A classification of Martian wind streaks has been developed to assist in investigations of eolian transport and related meteorological phenomena on Mars. Streaks can be grouped by their albedo contrast with their surroundings and by the presence of either topographic obstacles or sediment deposits at their points of origin. The vast majority of wind streaks can be included in three categories. (1) Bright streaks with no source deposit: interpreted to be formed by preferential deposition of dust from suspension. (2) Dark streaks with no source deposit: interpreted to be formed by preferential erosion of bright dust and its removal in suspension. (3) Dark streaks associated with deposits of sediment: interpreted to be formed by deposition of dark material moved by saltation. The orientations of the different streak types are distinctive and reflect both global flow patterns and slope-controlled winds. The wind directions derived from streaks and the geographical distribution of the features show a strong north-south asymmetry—consistent with the fact that perihelion (and hence maximum wind activity) occurs near southern summer solstice. 相似文献
13.
Mariner 9 was inserted into orbit about Mars on November 14, 1971, to study that planet for a period of 90 days. Observations and measurements made by Mariner 9 continued beyond the planned 90 days, providing data to meet all science objectives. The new knowledge of Mars gained from this mission has made obsolete all previous concepts of Mars.A general background of the Mariner Mars 1971 Project and the significant events of the Mariner 9 mission are presented. 相似文献
14.
Thomas E. Thorpe 《Icarus》1979,37(2):389-398
Low phase angle observations in the Chryse-Acidalia region have been obtained the Viking Orbiter 1 spacecraft under clearer atmospheric conditions than reported earlier. A variety of surface features were recorded, e.g., crater streaks, dark and bright patches. Several findings for this scene include: an abrupt brightness increase (10%) was found at phase angles less than 3°, an effect dependent on surface albedo and possibly particle distribution; a slight weakening of reflectance surge with decreasing wavelengths; a larger opposition effect for features of high albedo was recorded; and a greater reddening with increase phase angle took place for low albedo regions. Both reflectance and contrast values are provided at three wavelengths as a function of phase angle from 0.15 to 20°. 相似文献
15.
Positive isolated features or knobs have been observed on Mars since Mariner 9 first photographed the planet in 1972. More recently, the Viking Orbiters photographed the surface at increased resolution. With the use of Viking photomosaics, a systematic search for knobs was completed. The knobs were characterized by length, width, geographic location, proximity to streaks and geologic surroundings. Similar isolated features on Earth eroded by fluvial, glacial, and eolian processes were studied and measured. Comparison of length-to-width ratios of Martian knobs to isolated hills on Earth indicate that the Martian knobs are most similar to the isolated hills formed in a hyper-arid environment. The terrestrial features were probably formed initially when solid rock was fractured, then wind erosion, starting at the fractures, continued to sweep away sediments leaving isolated hills. Such hills in fluvial and glacial environments have length-to-width ratios significantly higher than those of the Martian knobs. Other diagnostic features associated with such environments are absent in the case of the Martian knobs. Moreover, streaks, splotches, dunes and pitted and fluted rocks, all indicative of a eolian regime, are associated with the Martian knobs. 相似文献
16.
A time-sequential set of bolometric albedo maps for Mars has been constructed from Viking Infrared Thermal Mapper data. The maps provide global coverage in longitude for latitudes -60° to +60° at 1° by 1° spatial resolution. Individual maps are constructed under strict geometric constraints for a narrow range of Ls. The set of albedo maps spans a Martian year and includes maps before, during, and after the global dust storms of 1977. Transient brightenings associated with local dust storms or condensate clouds are apparent in some of the maps. During dust-free periods, bolometric albedo maps are generally similar to classical, visual albedo map of Mars. The distribution of bolometric surface albedos is bimodal with typical, clear-sky, Lambert albedos of 0.27 and 0.16 for bright and dark areas, respectively. Atmospheric effects strongly influence apparent surface albedos, especially for dark areas. Neither bright nor dark regions show measurable, long-term variations of bolometric albedos during clear periods. 相似文献
17.
The spreading angle of a number of light and dark Martian streaks is determined from selected Mariner 9 images. The resulting frequency distributions of spreading half-angles have maxima at ～5° for light, and ～7° for dark streaks; however the dark streaks have a secondary maximum spreading angle at ～14°. The smaller values, which include most streaks, are interpreted as crater-wake spreading phenomena. The larger value, found in only a few dark streaks or “tails,” may result from atmospheric diffusion and subsequent deposition of material from isolated sources such as vents or blowouts. An atmospheric diffusion-deposition analysis is presented, assuming this streak origin, from which it is possible to deduce the eddy diffusivity, K, in Mars' boudary layer. Calculated K values are found to agree with various theoretical estimates. They lie in the range 107 and 109 cm2 sec?1 and exhibit the proper scale dependence. Thus it appears that, in addition to streak-derived wind direction patterns and speed information, it is possible in a few cases to derive information on Mars' boundary-layer turbulence from streak-spreading measurements. 相似文献
18.
J. Veverka 《Icarus》1976,27(4):495-502
Some Mariner 9 B-frames show networks of criss-crossing rectilinear albedo markings typically 10 km long by 100 m wide. This paper discusses the location, variability and possible nature of these dark filamentary markings. Although not common on Mars, the markings are concentrated in at least two areas: Depressio Hellespontica and Cerberus/Trivium Charontis. In Depressio Hellespontica their emergence coincided with a general darkening of the region which occurred 2 months after the end of the 1971 global duststorm. The very definite criss-cross pattern of many of these markings suggests that they may be controlled by joints. It is unlikely that the markings are linear dunes. 相似文献
19.
《Icarus》1987,71(3):397-429
The results of a geological analysis of the Mariner 10 orange/UV color ratio man of Mercury (B. Hapke, C. Christman, B. Rava, and J. Mosher, Proc. Lunar Planet Sci. Conf. 11th 1980, pp. 817–822) are given. Certain errors that occured in reproducing the published version of the 1980 map are pointed out. The relationships between color and terrain are distinctly nonlunar. There is no correlation between color boundaries and the smooth plains on Mercury, in contrast to the strong correlation between color and maria-highlands contacts on the Moon. There are no large exposures of low-albedo, blue material that could be considered to be Mercurian analogs of high-FeTi lunar maria basalts on any part of Mercury imaged by Mariner 10. Three lines of evidence imply that the crust is low in Fe2+ and Ti4+: rays and ejecta blankets are bluer than most areas on Mercury; the Fe2+ band in Mercury's reflectance spectrum is very weak or nonexistent and the albedo contrasts are smaller than those on the Moon. There is no evidence in the spectral or albedo data that a lunar type of second wave of melting ever occured on Mercury; rather, the observations are most consistent with the hypothesis that the smooth plains are extrusive landforms derived from local material, possibly mobilized by the Caloris event. In several places correlations between color and topography can be explained if older, redder, higher-Fe materials underlie younger, bluer, lower-Fe surfaces. There is some evidence of late Fe-rich pyroclastic-like activity. 相似文献
20.
The Mariner 9 Ultraviolet Spectrometer has observed the 2550 Å ozone spectral absorption feature on Mars. This absorption was previously detected in the south polar region by Mariner 7 in 1969. Mariner 9 did not observe ozone at any time in the equatorial region, nor at the south polar cap during its summer season. However, ozone was found in the north polar region beginning at a latitude of 45°N and extending northward. Ozone later appeared in the southern hemisphere southward of 50°S as the Mars autumnal equinox approached. The presence of ozone on Mars seems to be coupled to the water vapor content of its atmosphere. 相似文献