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 157 毫秒
1.
In springtime on HiRISE images of the Southern polar terrain of Mars flow-like or rheologic features were observed. Their dark color is interpreted as partly defrosted surface where the temperature is too high for CO2 but low enough for H2O ice to be present there. These branching streaks grow in size and can move by an average velocity of up to about 1 m/day and could terminate in pond-like accumulation features. The phenomenon may be the result of interfacial water driven rheologic processes. Liquid interfacial water can in the presence of water ice exist well below the melting point of bulk water, by melting in course of interfacial attractive pressure by intermolecular forces (van der Waals forces e.g.), curvature of water film surfaces, and e.g. by macroscopic weight, acting upon ice. This melting phenomenon can be described in terms of “premelting of ice”. It is a challenging consequence, that liquid interfacial water unavoidably must in form of nanometric layers be present in water ice containing soil in the subsurface of Mars. It is the aim of this paper to study possible rheologic consequences in relation to observations, which seem to happen at sites of dark polar dunes on Mars at present. The model in this work assumes that interfacial water accumulates at the bottom of a translucent water-ice layer above a dark and insolated ground. This is warmed up towards the melting point of water. The evolving layer of liquid interfacial water between the covering ice sheet and the heated ground is assumed to drive downward directed flow-like features on slopes, and it can, at least partially, infiltrate (seep) into a porous ground. There, in at least temporarily cooler subsurface layers, the infiltrated liquid water refreezes and forms ice. The related stress built-up is shown to be sufficient to cause destructive erosive processes. The above-mentioned processes may cause change in the structure and thickness of the covering ice and/or may cause the movement of dune grains. All these processes may explain the observed springtime growing and downward extension of the slope streaks analyzed here.  相似文献   

2.
Images of the sub-polar regions of Mars show existence of various seasonal-depending features, including mosaics of dark patches called Dalmatian spots. We interpret formation of dark patches on dune fields by means of the non-uniform sublimation of winter CO2 ice deposit.  相似文献   

3.
Springtime low albedo features, called Dark Dune Spots, on the seasonal frost covered dunes on Mars between 77°N and 84°N latitude have been analyzed. Two groups of these spots have been identified: “small” and “large” ones, where large spots have diameters above 4 m, and complex internal structure. From these “large” spots branching seepage-like features emanate and grow on the steep slopes. They show a characteristic sequence of changes: first only wind-blown features emanate from them, while later a bright circular and elevated ring forms, and dark seepage-features start from the spots. These streaks grow with a speed between 0.3 m/day and 7 m/day respectively, first only from the spots, later from all along the dune crest.During this “seepage period” the temperature is between 150 K and 180 K at a 3-9 km spatial resolution scale, indicating that CO2 ice-free parts must be present at the observed dark spots. Around the receding northern seasonal CO2 cap, an annulus of water ice lags behind, which is probably present in the spots too where the CO2 frost has sublimated. Our model estimates show in the present work and in Kereszturi et al. (Kereszturi, A., Möhlmann, D., Berczi, Sz., Ganti, T., Kuti, A., Sik, A., Horvath, A. [2009b]. Icarus 201, 492-503) that the warming driven by solar insolation may produce not only interfacial water, but also bulk brines around the dune grains. The brine can support the movement of liquids and dune grains, enhances the sublimation of CO2 frost, and produce the dark features, as well as liquid modifies the optical properties of the surface.Signs of movement of dune material after the total defrosting of the terrain is also visible but it is uncertain because of the limit of resolution. In our previous work (Kereszturi et al., 2009b) we showed that resembling seepage-like streaks at the southern hemisphere might have been formed by ephemeral interfacial water, as well as these northern features. Such wet environments may have astrobiological importance too.  相似文献   

4.
Interfacial liquid water has been hypothesized to form during the seasonal evolution of the dark dune spots observed in the high latitudes of Mars. In this study we assess the presence, nature and properties of ices - in particular water ice - that occur within these spots using HIRISE and CRISM observations, as well as the LMD Global Climate Model. Our studies focus on Richardson crater (72°S, 179°E) and cover southern spring and summer (LS=175-341°). Three units have been identified of these spots: dark core, gray ring and bright halo. Each unit show characteristic changes as the season progress. In winter, the whole area is covered by CO2 ice with H2O ice contamination. Dark spots form during late winter and early spring. During spring, the dark spots are located in a 10 cm thick depression compared to the surrounding bright ice-rich layer. They are spectrally characterized by weak CO2 ice signatures that probably result from spatial mixing of CO2 ice-rich and ice-free regions within pixels, and from mixing of surface signatures due to aerosols scattering. The bright halo shaped by winds shows stronger CO2 absorptions than the average ice-covered terrain, which is consistent with a formation process involving CO2 re-condensation. According to spectral, morphological and modeling considerations, the gray ring is composed of a thin layer of a few tens of μm of water ice. Two sources/processes could participate to the enrichment of water ice in the gray ring unit: (i) water ice condensation at the surface in early fall (prior to the condensation of a CO2-rich winter layer) or during wintertime (due to cold trapping of the CO2 layer) and (ii) ejection of dust grains surrounded by water ice by the geyser activity responsible for the dark spot. In any case, water ice remains longer in the gray ring unit after the complete sublimation of the CO2. Finally, we also looked for liquid water in the near-IR CRISM spectra using linear unmixing modeling but found no conclusive evidence for it.  相似文献   

5.
If the recently observed young gullies on Mars represent seeps or outflows of aqueous fluid, that fluid must be highly concentrated brine to be even moderately stable under current martian surface conditions. Such brines are to be expected in the megaregolith if an initial saline hydrosphere underwent evapoconcentration via water escape from the atmosphere, evolved into subsurface CaCl2-enriched brines by chemical interaction with mafic rocks, and then froze until eutectic compositions were reached. Although most such brines would tend to sink deep into the megaregolith, they could be preserved locally at relatively high elevations as aquifers (or frozen paleoaquifers) perched between ground ice and crystalline salts.  相似文献   

6.
The dramatic hemispheric dichotomy in albedo displayed by Saturn's moon Iapetus has intrigued astronomers for centuries. Here we report on far-ultraviolet observations of Iapetus' bright and dark terrains from Cassini. We compare the reflectance spectra of Iapetus's dark terrain, Hyperion and Phoebe and find that both Phoebe and Hyperion are richer in water ice than Iapetus' dark terrain. Spectra of the lowest latitudes of the dark terrain display the diagnostic water ice absorption feature; water ice amounts increase within the dark material away from the apex (at 90° W longitude, the center of the dark leading hemisphere), consistent with thermal segregation of water ice. The water ice in the darkest, warmest low latitude regions is not expected to be stable and may be a sign of ongoing or recent emplacement of the dark material from an exogenic source.  相似文献   

7.
Iapetus, one of the saturnian moons, has an extreme albedo contrast between the leading and trailing hemispheres. The origin of this albedo dichotomy has led to several hypotheses, however it remains controversial. To clarify the origin of the dichotomy, the key approach is to investigate the detailed distribution of the dark material. Recent studies of impact craters and surface temperature from Cassini spacecraft data implied that sublimation of H2O ice can occur on Iapetus’ surface. This ice sublimation can change the albedo distribution on the moon with time.In this study, we evaluate the effect of ice sublimation and simulate the temporal change of surface albedo. We assume the dark material and the bright ice on the surface to be uniformly mixed with a certain volume fraction, and the initial albedo distribution to incorporate the dark material deposits on the surface. That is, the albedo at the apex is lowest and concentrically increases in a sinusoidal pattern. This situation simulates that dark materials existed around the Iapetus’ orbit billions of years ago, and the synchronously rotating Iapetus swept the material and then deposited it on its surface. The evolution of the surface albedo during 4.0 Gyr is simulated by estimating the surface temperature from the insolation energy on Iapetus including the effect of Saturn’s eccentricity and Iapetus’ obliquity precession, and evaluating the sublimation rate of H2O ice from the Iapetus’ surface.As a result, we found that the distribution of the surface albedo changed dramatically after 4.0 Gyr of evolution. The sublimation has three important effects on the resultant surface albedo. First, the albedo in the leading hemisphere has significantly decreased to approach the minimum value. Second, the albedo distribution has been elongated along the equator. Third, the edge of the low albedo region has become clear. Considering the effect of ice sublimation, the current albedo distribution can be reconstructed from the sinusoidal albedo distribution, suggesting the apex-antapex cratering asymmetry as a candidate for the origin of the albedo dichotomy. From the model analysis, we obtained an important aspect that the depth of the turn-over layer where the darkening process proceeded for 4 Gyr should be an order of 10 cm, which is consistent with evaluation from the Cassini radar observations.  相似文献   

8.
The Mars Express spacecraft has a highly inclined orbit around Mars and so has been able to observe the south pole of Mars in illuminated conditions at the end of the southern summer (Ls=330). Spectra from the planetary Fourier spectrometer (PFS) short wavelength (SW) channel were recorded over the permanent ice cap to study its composition in terms of CO2 ice and H2O ice. Models are fitted to the observed data, which include a spatial mixture of soil (not covered by ice) and CO2 frost (with a specific grain size and a small amount of included dust and H2O ice). Two different kinds of spectra were observed: those over the permanent polar cap with almost pure CO2 ice, negligible water ice, no soil fraction required, and bright; and those over mixed terrain (at the edge of the cap or near troughs) containing a significant soil spatial fraction, more water ice and smaller CO2 grain size. The amount of water ice given by fits to scaled albedo models is less than 10 ppm by weight. When using multi-stream reflectance models with the appropriate lighting geometry, the water amount must be 2-5 times greater than the albedo fit (less than 50 ppm). At the periphery of the residual polar cap, we found a region almost completely covered by water frost, modeled as a mixture of micron-sized and sub-mm sized grains. Our result using a granular mixture of micron-sized grains of water ice and dust with the CO2 grains is different from the modeling of OMEGA polar cap observations using molecular mixtures.  相似文献   

9.
Since their discovery in Voyager images, the origin of the bright polar caps of Ganymede has intrigued investigators. Some models attributed the polar cap formation to thermal migration of water vapor to higher latitudes, while other models implicated plasma bombardment in brightening ice. Only with the arrival of Galileo at Jupiter was it apparent that Ganymede possesses a strong internal magnetic field, which blocks most of the plasma from bombarding the satellite's equatorial region while funneling plasma onto the polar regions. This discovery provides a plausible explanation for the polar caps as related to differences in plasma-induced brightening in the polar and the equatorial regions. In this context, we analyze global color and high resolution images of Ganymede obtained by Galileo, finding a very close correspondence between the observed polar cap boundary and the open/closed field lines boundary obtained from new modeling of the magnetic field environment. This establishes a clear link between plasma bombardment and polar cap brightening. High resolution images show that bright polar terrain is segregated into bright and dark patches, suggesting sputter-induced redistribution and subsequent cold trapping of water molecules. Minor differences between the location of the open/closed field lines boundary and the observed polar cap boundary may be due to interaction of Ganymede with Jupiter's magnetosphere, and our neglect of higher-order terms in modeling Ganymede's internal field. We postulate that leading-trailing brightness differences in Ganymede's low-latitude surface are due to enhanced plasma flux onto the leading hemisphere, rather than darkening of the trailing hemisphere. In contrast to Ganymede, the entire surface of Europa is bombarded by jovian plasma, suggesting that sputter-induced redistribution of water molecules is a viable means of brightening that satellite's surface.  相似文献   

10.
New narrow-band (100 Å) photoelectric slit scan photometry of Uranus has been obtained in the spectral region 6000 to 8500 Å. Coarse radial intensity profiles in seven wavebands are presented. Measurements of the point spread function have been used to partially remove the effects of atmospheric seeing. Restoration of the Uranus image, with a spatial resolution limit ～0″.5 arc, has been achieved by means of analytical Fourier-Bessel inversion. Results of the investigation confirm earlier studies of limb brightening on the Uranus disk. But not all strong CH4 absorption bands are found to exhibit limb brightening. Specifically, the CH4 bands at 8000 and 8500 Å show pronounced apparent limb darkening. Polar brightening may be responsible for the phenomenon. If so, an aerosol haze with a local optical thickness ～0.5 or greater would be required. Visibility of the dense cloud layer located deep in the atmosphere might also cause apparent limb darkening. If so, the maximum permitted [CH4/H2] mixing ratio in the visible atmosphere would correspond to ～3 times the solar value.  相似文献   

11.
A review of the available spectral geometric albedo measurements for Mars was presented earlier for the spectral region 0.3 to 1.1μ. A new observational study has greatly increased the store of data, especially for small Martian regions and for the infrared spectral region 1.0 to 2.5μ. Here we combine the new data with data both from the earlier review and, for the infrared spectral region, from the literature. We present a more complete picture of Martian spectral reflectivity properties than was available. This study should provide a more firm basis upon which models of Martian surface composition can be built. At visible wavelengths the Mars dark area Syrtis Major is red rather than green or grey in color; the bright area Arabia is even redder than Syrtis Major. Absorption bands, which differ between bright and dark areas, appear in the reflection curves. The 1μ absorption feature for dark areas is confirmed and more completely described. A previously unreported absorption band near 0.95μ for bright areas appears along with several absorption features in the infrared. The geometric albedo for Arabia reaches a maximum of about 0.43 at 1μ. The Bond albedo for this same area reaches a maximum of 60%. The bright area Arabia is occasionally three times brighter than the dark area Syrtis Major at red wavelengths. Published infrared reflection data available for Mars are not in complete agreement. Changes in brightness and color of Arabia are discussed which are not in agreement with traditional darkening wave theory.  相似文献   

12.
We present the first JHK near-infrared colorimetry giving some indication of the surface materials on the peculiar object 2060 Chiron. The colors appear to exclude relatively clean bright ice surfaces, such as those of Europa, Ganymede, Rhea, Dione, and theoretically studied micronscale clean ice particles. The colors are consistent with those of numerous outer solar system asteroids, which in all measured cases have: albedos of only a few percent, C?, RD?, or DM-type spectra, and no known H2O ice absorption features. The colors are also consistent with theoretical colors for certain size distributions of dirty ice grains. These and additional VJHK colorimetric data suggest that the spectrally dominant surface is C-type material, probably dark carbonaceous-like silicate dust, with a possible admixture of ice grains on a microscale. If Chiron has the low albedo common to such material on known interplanetary bodies, its diameter would be about 310 to 400 km, placing Chiron among the eight largest “asteroids”. Relationships to comet observations are discussed.  相似文献   

13.
Zachariadis  Th.G.  Dara  H.C.  Alissandrakis  C.E.  Koutchmy  S.  Gontikakis  C. 《Solar physics》2001,202(1):41-52
In this article we study chromospheric structures (spicules) crossing the solar limb in H images corrected for limb darkening. This correction enabled us to view structures both on the disk and beyond the limb in the same image. The observations were obtained at the Sacramento Peak Observatory at H±0.75 Å. The processed images reveal both bright and dark (relative to the local background) features crossing the limb. We also observed bushes (rosettes) crossing the limb, as well as structures indicating probably arch-shaped mottles beyond the limb.  相似文献   

14.
C.F. Capen 《Icarus》1976,28(2):213-230
This paper is a qualitative and quantitative investigation of recent seasonal and secular albedo feature variations on Mars using the improved red-filter photography of the International Planetary Patrol Program obtained during the 1971 and 1973 apparitons. The boundaries of telescopic albedo features showing significant changes and uncommonly bright areas were measured with an image projection reader and plotted on Mars maps in order to compare changes with Mariner 9 variable feature data obtained in 1972. The Daedalia-Claritas darkening (90–130°W, 25°S) of 1973 was the most significant change observed on Mars since the Laocoontis-Alcyonius darkening of the 1950 decade. The 1973 apparition presented six excellent examples of normal seasonal changes.Photographic photometry of 21 albedo features located in three longitude regions was systematically done on 120 original red-filter negatives, about half representing the Martian southern spring season and the other half the southern summer season. These two sets of photographs differed about 90° in Ls but were nearly equal in viewing geometry. They were identical in image scale, emulsion, color response, and quality. The data, consisting of 3960 density measurements, were analyzed in terms of albedo ratios and their fluctuations. Several types of diagnostic graphs are presented.Seasonal differences of the mean brightness ratios for most of the features were found to be consistent with the contrast changes shown on photographic Mars maps. Seasonal variations of brightnesses with latitude and region were indicated, but not in the strict classical sense of a wave-of-darkening. Most albedo features were more variable during southern spring than they were in summer. Most light albedo features were more active than dark ones during both seasons. There was a general loss of relative contrast in southern summer. No systematic dependence on solar phase angle was detected within the range (38° > i > 16°) spanned by both sets of photographs.  相似文献   

15.
Ice deposits of the permanent northern polar cap of Mars exhibit a clearly expressed layered structure connected with consecutive accumulation of a solid phase of H2O (ice) with different impurity fractions of dust. The observed structure of the northern polar cap deposits is direct evidence of the uneven accumulation of ice associated with climate variations on Mars over a long period of time. Outside the boundaries of the present northern polar cap, there are isolated remnant massifs of bright deposits with a clearly distinguishable layered structure. In this paper, we analyze the morphology, spatial distribution, and material composition of remnant massifs within the latitude belt of 60°–85°. The similarity in the structure and material composition found in the remnant massifs and the layered deposits of the northern polar cap of Mars apparently suggests that these formations were genetically connected in the past climatic epochs.  相似文献   

16.
S.J. Peale 《Icarus》1973,18(3):497-501
The diurnal brightening of the W cloud region of Mars during the flyby of Mariners 6 and 7 is likely due to the formation of water ice clouds. The water required in the cloud and as vapor in the atmosphere to produce the observed brightening is estimated to be roughly between 0.1 and 10μm. Either a qualified local source with this daily production over the W cloud area or a more probable daily uplift and saturation of existing atmosphere are compatible with observations.  相似文献   

17.
William K. Hartmann 《Icarus》1980,44(2):441-453
Observational and theoretical data converge on the conclusion that planetesimals in Jupiter's region of the solar nebula were initially composed predominantly of a mixture of roughly 39–70% H2O ice by volume, and 30–61% dark stony material resembling carbonaceous chondrites. Recent observations emphasize a division of most asteroid and satellite surfaces in this region into two distinct groups: bright icy material and dark stony material. The present model accounts for these by two main processes: an impact-induced buildup of a dark stony regolith in the absence of surface thermal disturbance, and thermal-disturbance-induced eruption of “water magmas” that create icy surfaces. “Thermal disturbances” include tidal and radiative effects caused by nearness of a planet. A correlation of crater density and albedo, Ganymede's dark-ray craters, and other observed phenomena (listed in the summary) appear consistent with the model discussed here.  相似文献   

18.
《Planetary and Space Science》2007,55(13):2025-2036
Titan's vast equatorial fields of RADAR-dark longitudinal dunes seen in Cassini RADAR synthetic aperture images correlate with one of two dark surface units discriminated as “brown” and “blue” in Visible and Infrared Mapping Spectrometer (VIMS) color composites of short-wavelength infrared spectral cubes (RGB as 2.0, 1.6, 1.3 μm). In such composites bluer materials exhibit higher reflectance at 1.3 μm and lower at 1.6 and 2.0 μm. The dark brown unit is highly correlated with the RADAR-dark dunes. The dark brown unit shows less evidence of water ice suggesting that the saltating grains of the dunes are largely composed of hydrocarbons and/or nitriles. In general, the bright units also show less evidence of absorption due to water ice and are inferred to consist of deposits of bright fine precipitating tholin aerosol dust. Some set of chemical/mechanical processes may be converting the bright fine-grained aerosol deposits into the dark saltating hydrocarbon and/or nitrile grains. Alternatively the dark dune materials may be derived from a different type of air aerosol photochemical product than are the bright materials. In our model, both the bright aerosol and dark hydrocarbon dune deposits mantle the VIMS dark blue water ice-rich substrate. We postulate that the bright mantles are effectively invisible (transparent) in RADAR synthetic aperture radar (SAR) images leading to lack of correlation in the RADAR images with optically bright mantling units. RADAR images mostly show only dark dunes and the water ice substrate that varies in roughness, fracturing, and porosity. If the rate of deposition of bright aerosol is 0.001–0.01 μm/yr, the surface would be coated (to optical instruments) in hundreds-to-thousands of years unless cleansing processes are active. The dark dunes must be mobile on this very short timescale to prevent the accumulation of bright coatings. Huygens landed in a region of the VIMS bright and dark blue materials and about 30 km south of the nearest occurrence of dunes visible in the RADAR SAR images. Fluvial/pluvial processes, every few centuries or millennia, must be cleansing the dark floors of the incised channels and scouring the dark plains at the Huygens landing site both imaged by Descent Imager/Spectral Radiometer (DISR).  相似文献   

19.
UBV pinhole scans of the Saturn disk have been made with a photoelectric area-scanning photometer. Limb profiles, spaced parallel to the equator, were obtained over the entire southern hemisphere of the planet. Saturn was found to exhibit strong limb brightening in the ultraviolet, moderate limb brightening at blue wavelengths, and strong limb darkening in the visual region of the spectrum. Latitudinal variations in the disk profiles were found. In general, the degree of limb brightening decreases towards the polar region. Pronounced asymmetry is apparent in the disk profiles in each color. The sunward limb is significantly brighter than the opposite limb. This asymmetry depends on phase angle; approaching zero at opposition, it reaches a maximum near quadrature. Our observations are interpreted using an elementary radiative transfer model. The Saturn atmosphere is approximated by a finite homogeneous layer of isotropically scattering particles overlying a Lambert scattering haze or cloud layer. The reflectivity of the haze or clouds is a strongly dependent function of wavelength. Our best-fitting model consists of a clear H2 layer of column density ～31 km-am above the haze or clouds; the maximum permitted H2 column density is ～46 km-am. The H2 column density above the equatorial region appears to be less than at temperate latitudes. The phase-dependent asymmetry in the disk profiles is a natural consequence of the scattering geometry. Our results are consistent with current knowledge of the Saturn atmosphere.  相似文献   

20.
Fine-resolution (500 m/pixel) Cassini Visual and Infrared Mapping Spectrometer (VIMS) T20 observations of Titan resolve that moon's sand dunes. The spectral variability in some dune regions shows that there are sand-free interdune areas, wherein VIMS spectra reveal the exposed dune substrate. The interdunes from T20 are, variously, materials that correspond to the equatorial bright, 5-μm-bright, and dark blue spectral units. Our observations show that an enigmatic “dark red” spectral unit seen in T5 in fact represents a macroscopic mixture with 5-μm-bright material and dunes as its spectral endmembers. Looking more broadly, similar mixtures of varying amounts of dune and interdune units of varying composition can explain the spectral and albedo variability within the dark brown dune global spectral unit that is associated with dunes. The presence of interdunes indicates that Titan's dunefields are both mature and recently active. The spectrum of the dune endmember reveals the sand to be composed of less water ice than the rest of Titan; various organics are consistent with the dunes' measured reflectivity. We measure a mean dune spacing of 2.1 km, and find that the dunes are oriented on the average in an east-west direction, but angling up to 10° from parallel to the equator in specific cases. Where no interdunes are present, we determine the height of one set of dunes photoclinometrically to be between 30 and 70 m. These results pave the way for future exploration and interpretation of Titan's sand dunes.  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