| Piú viste - Mars Reconnaissance Orbiter (MRO) |
PSP_009039_1660_RED.jpgCraters in South-Eastern Syria Planum (natural colors; credits: Lunexit)58 visiteThis image shows two landforms that appear similar, but are the result of two very different geologic processes.
These two depressions are craters. The smaller, rounder crater formed when an asteroid collided with Mars. This impact blasted out the pre-existing rocks, forming this quasi-circular crater.
The larger, more irregular-shaped crater is a Pit Crater. These types of craters form through collapse of the ground surface into large underground voids. In this Region of Mars, these underground voids are likely caused by the movement of magma (molten rock) through the subsurface. As the magma moves underground, it forces the rock apart and forms large “caverns.” These voids are structurally unstable and can lead to collapse of the overlying rock, forming pit craters at the surface.
Impact Craters are distinguished from Pit Craters by the presence of a raised rim. Rock blasted out during the impact falls back to the ground and accumulates near the crater, forming this raised rim. Upward warping of the ground during the impact process also contributes to the raised appearance of the crater rim. Since Pit Craters form through collapse, their rims are at the same level, or perhaps slightly lower, than surrounding ground surface.
The Impact Crater has a bright streak extending South-East (toward the upper right). The bright material is dust, deposited downwind of the crater by prevailing winds. Zooming into the streak, small bedforms, presumably composed of dust or dust aggregates, are visible. Similar features are seen in other dusty regions of Mars.MareKromium
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PSP_005410_1115_RED_abrowse.jpgPolar Pit Gullies (MULTISPECTRUM; credits: Lunexit)58 visiteThis image shows Polar Pit Gullies in a depression. The gullies do not appear to have been active recently, as their channels and alcoves are covered with polygonal fractures and ripples that have formed over time. The alcoves contain boulders from eroding layers up-slope. Several of the alcoves extend to the slope rim, suggesting head-ward erosion.
The rest of the scene contains abundant polygonal ground, thought to have formed by processes involving ground ice. This image is at a High Latitude where polygonal terrain is common. This feature is not found in Equatorial Regions, which supports a relationship with ground ice because ground ice is not stable near the equator today.
There are several muted circles on the plains in the lower half of the image; these are possibly relaxed craters. If a crater forms in ice-rich ground, the ice enhances the degradation of the crater and gives the crater a “softened” appearance.MareKromium
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PSP_009114_2645_RED.jpgEroding Dunes in Chasma Boreale (natural colors; credits: Lunexit)58 visiteSand moves along a planetary surface by a process scientists call “saltation”, whereby the individual grains are driven by the wind and bounce forward in short hops. In a process that is not yet completely understood, sheets of saltating sand grains organize themselves into sand dunes, visible in this image as the dark features.
Sand dunes move by having the wind push sand grains up and over the top of the dune where they then slide down to the base. The steep side of the dune that the sand grains slide down is called "slip-face" and it is the constant transport of sand from the downwind side of the dune to the "slip-face" that makes the dune move forward in this direction. HiRISE data allow us to see which side of these dunes has the steeper slope (such as the aforementioned "slip-face"), telling us what direction the dune — and strong near surface winds — are moving.
Yet something else is also happening to these particular dunes. Dark streaks lead away from the dunes toward the lower left of the image. These streaks are caused by sand grains being blown off the dunes and saltating away. This is not ordinarily a cause for concern because in a stable dune, individual grains are constantly added and removed; however, there does not appear to be any influx of sand upwind of these dunes, so they are probably being eroded.
It is also interesting that these streaks do not point in the same direction as the "slip-face". One possible scenario is that the dunes migrated Westward when sand supply was more plentiful. Today, the wind direction has shifted, blowing more toward the South-West, and the influx of new sand has ceased, such that in the future, the dunes will completely erode away. Repeated HiRISE observations will be able to look for changes in the shape and size of these dunes.MareKromium
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PSP_009161_1450_RED-01.jpgLong Shadows over Ariadnes Colles (edm - natural colors; credits: Lunexit)58 visiteThis edm shows one of the hills in detail.
The hill appears criss-crossed by long fractures and most of them made apparent by the shadows they cast.
The shadows indicate that the fractures “stick out” from their surroundings, and hence that they are more resistant to erosion. In terrestrial environments this occurs when fluids flow along the fractures, leaving behind cementing minerals or when fractures are filled by igneous materials.MareKromium
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PSP_009192_1890_RED-00.jpgRelatively Recent Slope Streak started from a Dust Devil (ctx frame - natural colors; credits: Lunexit)58 visiteThis Slope Streak occurred in the time between a Viking image of the bottom of this crater (713A57, which saw no streak) and a MOC image (R12/01917, as reported by Schorghofer et al. (2007).
That paper suggested that the Slope Streak may have been caused by a Dust Devil that had passed by (its track is visible in the MOC image). Our HiRISE image shows that there isn’t a small hill or anything at this Slope Streak’s apex, but that the dust devil track really does intersect with the apex. It is likely that the Dust Devil may have caused this Slope Streak.
Dark Slope Streaks are visible in many places on the Martian surface, often where the dust cover is thick.
One explanation for Dark Slope Streaks is that they are little avalanches in the dust. The apexes of Slope Streaks (such as the point where they start from) are often at little hills or ridges on a larger slope, with the thinking that the dust here is already close to being too steep, and then any little perturbation will start one of these tiny avalanches.
Small craters have also been seen at slope streak apexes.MareKromium
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PSP_008825_2040_red.jpgCharacterize Surface Hazards and Science of Possible MSL Rover Landing - Mawrth Vallis (natural colors; credits: Lunexit)58 visitenessun commentoMareKromium
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Psp_008719_1815_red.jpgCharacterize Surface Hazards and Science of MSL Rover Landing Site - Equatorial Regions/Meridiani Planum (natural colors; credits: Lunexit)58 visitenessun commentoMareKromium
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Psp_009716_1755_red.jpgSurface Features inside Gale Crater (natural colors; credits: Lunexit)58 visitenessun commentoMareKromium
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PSP_006672_1420_RED_abrowse~0.jpgFeatures of the Hellas Montes (MULTISPECTRUM; credits: Lunexit)58 visiteThe Hellas Montes are a group of mountains along the western rim of the giant Hellas Basin on Mars.
The Hellas Basin is the largest of the obvious impact craters on the Red Planet. It is very ancient and has been partially filled by sediments. The Hellas Montes are part of the eroded crater rim.
In the central part of this HiRISE image, we can see steep slopes where landslides have exposed a variety of rocks. The jumble of blocks, rather than stacks of layered sediments or lavas, is consistent with impact crater ejecta. On flatter slopes, the ground is covered with a mantling deposit that is generally considered to be ice-rich dust.
In the southern part of the image (Dx), a large circular depression — rimmed by a zone with many large boulders — is (barely) visible. This is an impact crater with a relatively thin mantling deposit on its rim.
MareKromium
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PSP_008234_2405_red-PCF-LXTT.jpgPeriglacial Landscape in Northern Tempe Terra (Enhanced Natural Colors; credits for the additional process. and color.: Dr Paolo C. Fienga - Lunexit Team)58 visitenessun commentoMareKromium
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Psp_009650_1755_red.jpgCrater Floor and Central Mound in Gale Crater (natural colors; credits: Lunexit)58 visitenessun commentoMareKromium
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Psp_009578_1715_red.jpgValleys Near Ganges Chasma (natural colors; credits: Lunexit)58 visiteSeveral valleys as well as light-toned material are visible in this HiRISE image of a portion of Allegheny Vallis along the Plains West of Ganges Chasma.
The main valley, which starts in a pit called Ophir Cavus and extends for 155 Km into Ganges Chasma, is visible in the center of the image from left to right. Smaller and shallower valleys can be seen mostly to the north (Sx) of the image.
The observation that there are several valleys here suggests that water flowed for some extended period of time or multiple times in order to change direction and produce different valleys.
Light-toned material is visible on the upper surfaces of the plains but not inside the valleys, perhaps because smaller amounts of water could interact with the lava plains at these higher elevations, while in the valleys larger amounts of flowing water eroded and removed the plains unit. The light-toned nature on the upper surfaces could have resulted from chemical alteration of the lava plains or deposition of evaporites as the water disappeared and left behind minerals once carried in the water.MareKromium
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