火星的细节

来源:http://hirise.lpl.arizona.edu/PSP_003609_1110

HiRISE at One Year: Student Image of the Week-Seasonal Changes of South Polar Dark Dune Field  (PSP_003609_1110)

Credit: NASA/JPL/University of Arizona

This image was suggested by Andras Sik's SUPERNOVA astronomy and space research class at the Alternative Secondary School of Economics in Budapest, Hungary.

They interpret the image: "During local springtime, a varying albedo pattern can be observed on the surface of dark intracrater dune fields in the southern polar region of Mars. The origin of dark dune spots and the dark slope streaks emanating from them is yet uncertain. There are several possible explanations for these phenomena, like "sublimation of carbon-dioxide frost cover", "dusty carbon-dioxide gas eruptions through the frost cover" and "transient liquid water-formation under layered water-ice/carbon dioxide-ice cover". In this HiRISE image the distribution of dark streaks are not chaotic and their shapes are not fan-like; rather they are composed of confined, dozen-hundred meter long branches which follow the local topography and have accumulation zones at their end."

We are learning a great deal from the enigmatic dark spots that are found throughout the south polar region. These dark spots may have resulted from cold gas jets that form by sublimation of the ice bringing entrained dust to the surface. Small dark streaks may have formed by avalanches of sand or they may be patches of coarse-grained ice that are clear enough so that the dark material below the ice is visible.

The color image also provides helpful clues to understand this process. Bright white frost can be seen covering the surface. This frost is a probably a combination of frozen water and carbon dioxide ice. These bright patches are particularly prevalent along dune slip faces and around dark spots.

                               
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来源:http://hirise.lpl.arizona.edu/PSP_005939_1720

Layering along West Ganges Chasma  (PSP_005939_1720)

Credit: NASA/JPL/University of Arizona

This observation shows a sequence of layered sedimentary rocks exposed along the wall of Ganges Chasma.

Ganges Chasma is in the northeast part of the Valles Marineris system and cuts through surrounding plains interpreted to have been resurfaced by flowing lava. The Chasma is believed to have formed due to the collapse of plateau rocks along fault systems.

The plateau above the chasma is at the left side of the image and the wall of the trough descends to the east.

The layered sequence consists of many beds that are generally horizontal and laterally continuous. Some more pronounced layers appear to be weathering to form large-scale boulders (see subimage). Many thinner layers are apparent near the top of the wall. Material has also formed spurs and ridges along the wall of the trough.

                               
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来源:http://hirise.lpl.arizona.edu/PSP_005720_1885

Cerberus Fossae Fissures  (PSP_005720_1885)

Credit: NASA/JPL/University of Arizona

This image shows several parallel segments of the Cerberus Fossae. This is a system of fissures formed by extension and stretching of the near-surface of Mars. It is associated with some of the most recent large-scale events on the planet.

The Athabasca Valles channel system has its source at one segment of the Cerberus Fossae. This channel was most likely carved by a massive flood of water, perhaps released by the same tectonic processes which formed the fossae. Cerberus Fossae then extruded a large lava flow that draped Athabasca Valles. HiRISE images were recently used to describe details of this history in a paper published in the journal Science (Jaeger et al., Science, volume 317).

In the RGB false-color, this image shows a striking contrast. The plains are a generally bland beige, while the floors of the fissures are bright blue. Since the fissure walls are cutting through the lava which makes up the plains, the blue color probably indicates relatively dust-free exposed rock, while the beige is due to dust coating the level plains. Although the Cerberus Fossae released lava in places, at this site there is no evidence for a vent and the fissures have simply cut through pre-existing lava.

                               
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来源:http://hirise.lpl.arizona.edu/PSP_003708_1335

Gullies and Concentric Fill in an Unnamed Rampart Crater in Noachis Terra  (PSP_003708_1335)

Credit: NASA/JPL/University of Arizona

This image shows part of a relatively small (14 kilometers, or 9 miles across) unnamed impact crater in Noachis Terra. The lobate-shaped crater's ejecta, gullies in its walls, and "concentric crater fill" in the crater's floor may be indicative of the presence of water and ice at some point during its history.

The subimage of a CTX context image of this same crater shows the typical lobate ejecta pattern characteristic of rampart craters, indicating that ice- or water-rich materials were excavated at the time of impact. According to this theory, the rock ejected from the crater mixed with the water and water vapor, and thus flowed more easily along the surface. Another possible interpretation is that fine-grained rocky material mixed with air and formed the rampart pattern in the absence of water.

The gullies shown in this image are similar to terrestrial gullies produced by flowing water. The current Martian climate would not allow for liquid water to be stable at the surface for extended periods of time, though. It is so cold (in the order of -70oC or -90oF) that the water would freeze, and then it would sublimate quickly, because the air is very dry and thin (the atmospheric pressure is on average less than 1/100 of that on Earth). These gullies could have formed under a different climate, or perhaps they were carved by short-lived streams; they could also have been produced by dry landslides.

"Concentric crater fill," crater-interior deposits that form concentric ridges, is found at high latitudes (approximately above 30 degrees from the equator). Theoretical calculations indicate that ice may exist under the surface at such high latitudes, mixed with rocks and soil. The roughly concentric ridges and troughs in this crater's floor may have been produced by compression caused by viscous flow of a thick mixture of rocks, soils, and ice inward from the crater's walls.

                               
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来源:http://hirise.lpl.arizona.edu/PSP_005710_1555

Rugged Crater Floor in Terra Tyrrhena  (PSP_005710_1555)

Credit: NASA/JPL/University of Arizona

This image shows part of the floor of a large crater in Terra Tyrrhena. Here, dark mesas contrast with light-colored, topographically lower outcrops.

The mesas have sharp cliffs, indicating the dark materials are not loose, but rather consolidated; layering is also visible along the cliffs. The underlying light materials are also layered and profusely fractured. Their superposition relationship indicates that the light materials are older than the dark, mesa-forming materials.

The image also shows a number of crest-like features criss-crossing the landscape. The fact that their relief is so prominent indicates they are more resistent to erosion (harder) than their surroundings. These crests could be indurated fractures (fractures cemented by fluids) or dikes. Dikes are tabular intrusions of rock formed when underground magma is injected along fractures.

The lowest regions in the image are covered by dark-colored dunes (see for instance the NW or upper left section of the image). These are possibly accumulations of fine-grained materials eroded-out from the mesas.

                               
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来源:http://hirise.lpl.arizona.edu/PSP_003565_1495

Student Image of the Week: Confluence of Nirgal and Uzboi Valles  (PSP_003565_1495)

Credit: NASA/JPL/University of Arizona

This target was suggested by Jonna Sotelo Douglas in Tucson, Arizona. She writes: "This image is of the confluence of the Nigral and Uzboi Valles and part of the southern rim of Luki crater. The confluence area is marked with dunes, boulders, and other types of sediment formations. As the elevation lowers from the crater wall, the area smoothes out. There are gullies on both sides of the crater wall."

Nirgal and Uzboi Valles are two valleys to the north of the Argyre impact basin in the southern hemisphere. At this location, the smaller channel, Nirgal Vallis, empties out into the larger Uzboi Vallis.

Most of the features in this image are covered by the dust that is ubiquitous on Mars, hiding the original fluvial features. However, a small patch of resistant outcrop can be observed on the southern side of the crater rim. These appear as white patches that are draped by the darker wind-blown rippled sand (see subimage 1). Even the resistant outcrop has been modified by wind, giving it a pockmarked or scalloped texture.

Layering can be seen in the resistant outcrops and in the gullied inner wall of the crater (see subimage 2). These layers may have been fluvial, volcanic, or aeolian (wind-blown). Given their presence within the valley floor, these may represent some of the sedimentary layers deposited by water when it flowed through the valley.

                               
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来源:http://hirise.lpl.arizona.edu/PSP_005588_1445

Proposed MSL Site in Ariadnes Colles  (PSP_005588_1445)

Credit: NASA/JPL/University of Arizona

HiRISE image of a proposed landing site for the Mars Science Laboratory (MSL) in Ariadnes Colles (updated landing ellipse location).

For more information about the mission and a timeline, visit the MSL Web site.

                               
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来源:http://hirise.lpl.arizona.edu/PSP_005703_1750

Layered Sediments in Tithonium Chasma  (PSP_005703_1750)

Credit: NASA/JPL/University of Arizona

This image shows a large outcrop of layered rock in Tithonium Chasma, a part of the Valles Marineris trough system.

Light, layered sediments are found at many sites on the floor of Valles Marineris. Observations drawn from this image will help understand the history of deposition at this location, and can be compared with nearby related rocks.

A light-toned unit (tan in the RGB color image) is found through most of the image. At high resolution, this unit often shows thin layers; it resembles light deposits found at many other sites. The light-toned materials are overlain by a dark unit (blue in the RBG color) that may have been deposited over the lighter rock much later. Variations in tone and color may indicate deviations in rock properties, such as the source material, cementation, or deposition process; in some cases they may simply be due to varying amounts of dust cover.

It is still unclear how the layered rocks in Valles Marineris formed. They might be sediments laid down in lakes or streams, wind-deposited, or volcanic materials. All of these processes can form layered rocks on Earth.

The dark material in the western part of the image appears to have once been a continuous layer over the current topography, now being eroded; if it was indeed draped, rather than laid down horizontally, it was most likely formed by dust or volcanic ash settling uniformly out of the atmosphere.

                               
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来源:http://hirise.lpl.arizona.edu/PSP_005583_1700

Terraced Fan and Valley in Crater  (PSP_005583_1700)

Credit: NASA/JPL/University of Arizona

This image shows a terraced fan (center left) located adjacent to a short valley. Terraced fans are rare deposits on Mars: they have only been seen in a few dozen locations.

On Earth, terraced fans are found in alluvial environments and in deltas. When water flowing down steep slopes, such as mountains or hills, encounters a flatter surface, the sediments carried by the water are deposited along the shallow slopes to form alluvial fans. Deltas can also produce terraced fans when water from rivers and streams meets oceans or lakes; a delta deposit results at the contact between the confined, fast-moving water (river/stream) and the open body of water (ocean/lake).

The fact that this Martian terraced fan is found adjacent to a valley supports the hypothesis that water was the mechanism that transported and deposited the material that now forms the fan. It is unclear, however, if this fan formed in an alluvial or deltaic environment, or by some other process.

                               
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来源:http://hirise.lpl.arizona.edu/PSP_005381_0870

South Polar Layered Deposits with Surface Modification  (PSP_005381_0870)

Credit: NASA/JPL/University of Arizona

This image shows a scarp exposing the south polar layered deposits (SPLD). The polar layered deposits are thought to record recent climate variations on Mars, similar to ice ages on Earth.

Radar data indicate that the SPLD are ice-rich, with variations in dust contamination likely causing the layering visible here. HiRISE images of the SPLD will help to unravel Mars' climate history, but this image illustrates how this effort is complicated.

The development of surface features, by erosion, deposition, or modification of the surface, makes it difficult to determine the characteristics of the layers themselves. Some of the layers appear wavy, perhaps due to folding, flow, or uneven erosion since they were laid down. Short, branching, often radial channel systems are cut into the surface of the layers in places. These may be related to "spiders," thought to be formed as carbon dioxide gas flows along the surface when the seasonal polar cap sublimates in the spring. Pits and polygonal fractures are visible on the layers as well.

Apparently this exposure of SPLD is relatively old, as these features probably take many years to form. While these features are interesting in their own right, they disturb the SPLD outcrops and make it more difficult to measure the thickness of layers and compare them to other outcrops of SPLD.

                               
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来源:http://hirise.lpl.arizona.edu/PSP_005375_1675

Crater with Wind Streak  (PSP_005375_1675)

Credit: NASA/JPL/University of Arizona

This image shows a crater with a wind streak in the southwest Tharsis region.

The crater is a relatively pristine one that has not experienced extensive modification as seen by the existence of its steep slopes and raised rims. However, it has gone through some changes, such as the small crater on its south rim and the dunes on its floor.

The wind streak to the left of the crater indicates that the wind was coming from the northeast/east direction(the right of the image) when this crater formed. The wind caused the crater’s ejecta to be deposited in the downwind direction. The crater in the middle of the wind streak (see subimage, approximately 350 meters across), existed before the crater with the wind streak formed. There is a thin veneer of the larger crater’s ejecta on top of the smaller crater; also, there are ejecta blocks from the large crater’s impact visible on top of the smaller crater.

It is unknown what makes the wind streak two-toned with a darker halo surrounding the brighter interior. It is possible that the particles in the wind streak have different average sizes and that light scattering makes one set of particles appear brighter.

                               
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来源:http://hirise.lpl.arizona.edu/PSP_005680_1525

Possible Ancient Salt Deposits within Unnamed Crater in Terra Cimmeria  (PSP_005680_1525)

Credit: NASA/JPL/University of Arizona

The ancient cratered highlands of the southern hemisphere of Mars has an intriguing and complex history as it has been riddled with impact craters and modified by volcanic processes and by the wind.

Additionally, it is one of the most heavily dissected terrains on Mars exhibiting the densest population of valley networks: old dried up channels and valleys that may have been formed by surface runoff, the seepage of ground water, or both.

Recently, the Thermal Emission Imaging System (THEMIS) aboard Mars Odyssey (MO), in conjunction with spectral data from the Thermal Emission Spectrometer (TES) aboard the Mars Global Surveyor (MGS) have revealed the presence of a unique surface deposit that may be rich in chloride salts formed from the presence of liquid water. Three separate missions (MGS, MO and MRO) have come to reveal the composition and nature of these unique deposits, which, although they occur as relatively small deposits (less than 25 square kilometers) are widely distributed in Noachian (most ancient) terrains with fewer occurrences in the Hesperian (middle geologic time) terrains.

This HiRISE infrared color sub-image (approximately 900 meters wide) shown here is part of one such deposit in an ancient partially buried unnamed crater in Terra Cimmeria, that shows this deposit in a light-toned almost fleshy color. The deposit appears to be relatively thin and occurs in low-lying areas. It is also heavily pockmarked and discontinuous, possibly from removal of the material by erosion. Both of these aspects suggest that the deposit is indeed very old.

The presence of such salts is intriguing, and strongly suggests that conditions were favorable for water near or at the surface in the geologic past. Polygonal cracks can be observed in this image and other images of these deposits elsewhere on Mars (PSP_003160_1410) and are similar to desiccation cracks (formed from the rapid evaporation and drying of a wet surface) and indicate that these may were more likely deposited at the surface. However, the volume and duration the water required for these deposits is still being investigated.

                               
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[ 本帖最后由 寻找拉玛 于 2008-1-31 21:21 编辑 ]

来源:http://hirise.lpl.arizona.edu/PSP_005346_1755

Low-Order Inverted Streams near Juventae Chasma  (PSP_005346_1755)

Credit: NASA/JPL/University of Arizona

This image shows plains northwest of Juventae Chasma, one of the Martian canyons that are part of the equatorial Valles Marineris system. The two most noticeable features in this scene are craters on mesas (plateaus) and raised, winding ridges.

The raised ridges are inverted channels. It is likely that liquid water, either pure or salt water, flowed through these channels. The channels are raised because streams transport sediment as they flow, deposit the heavier sediment on the stream floor, and, eventually fill in once their water supply dwindles. Over time, wind erosion modifies a landscape, and this has played an important role on these plains. It eroded the land around the channels leaving the remnant channels exposed and standing high. The channels did not erode as much since they were more resistant, possibly because the deposited sediment had cemented together.

The craters on mesas are also evidence of active wind erosion. When craters form, they eject material out onto the surrounding landscape. It appears that several of the craters’ ejecta visible here cemented, making the ejecta more resistant to erosion and leaving them standing high as craters on plateaus.

The subimage (approximately 1120 meters across) shows a juncture of two of the inverted channels. It is likely that the water flowed from the left to the right of the scene, because channels usually join rather than divert unless there is an obvious obstacle in the way. No such obstacle is seen here, but one might have been present when the stream originally flowed. However, there is no way of knowing this.

                               
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[ 本帖最后由 寻找拉玛 于 2008-2-1 01:16 编辑 ]

太强啦!! 要是中文的就更爽啦!!!

越来越强了，那43楼的可真象一条大河，火星上面真有这么漂亮吗？是真实的色彩吗？从机遇号和勇气号传回的图片看可没这么多色彩啊！

来源:http://hirise.lpl.arizona.edu/PSP_005442_1660

Graben in Memnonia Region  (PSP_005442_1660)

Credit: NASA/JPL/University of Arizona

This HiRISE image shows a graben (roughly 2.5 kilometers or 1.6 miles wide) cutting a ridge near Memnonia Sulci, a region of Mars west of the massive Tharsis volcanic province.

A graben is a trough created when two normal faults (faults dipping at an angle, with the overhanging wall sliding downwards) opposite each other lower the block of rock in between them. They are commonly associated with the tectonically-driven extension of a region and are also found in volcanic areas. Graben occur on many planets and moons, including Earth.

In this case, the graben (running roughly east-west, indicating extension in the north-south direction) is cutting a ridge, showing that the graben is more recent. The ridge may also be tectonic in origin, representing a different epoch and different type of tectonic activity.

Relationships like this can be used to determine the sequence of events in the history of the region. The graben walls can also expose the local rocks at depth; in this case there is no obvious indication of layering.

                               
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来源:http://hirise.lpl.arizona.edu/PSP_005514_1925

Radiating Dark Slope Streaks  (PSP_005514_1925)

Credit: NASA/JPL/University of Arizona

Centered in this image is a high-standing mound of material that sits within a large impact crater (the edges of which are not visible). Evident are numerous dark streaks that originate in the higher elevation areas near the center of the mound.

These streaks are possibly areas where the lighter surface dust cover has been disturbed and partially stripped away, revealing a darker layer beneath the dust. The dark streaks fade fairly rapidly as layers of bright dust settle over them.

                               
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来源:http://hirise.lpl.arizona.edu/PSP_005543_1725

Impact Crater Cut by Ganges Chasma  (PSP_005543_1725)

Credit: NASA/JPL/University of Arizona

The parial circular or ringed structure in the middle of this scene is an impact crater, approximately 3.25 kilometers (approx. 2 miles) in diameter.

Since its formation, the crater has had its southern half cut away by the formation of the gorge, called Ganges Chasma. The resulting exposure of rocks along the rim of the cliff allows planetary geologists to study a cut-away, side-view of layered rocks. This view is particularly interesting here because the rock layers of the upper plains are visible in their original form outside of the crater, and in modified form within/beneath the crater, along with structures imparted by the impact.

The floor of the crater may have been filled by lavas or other material that is more resistant to erosion than the surrounding layers, since the floor of the crater sticks out into the chasma.

                               
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