16 Jan 2008

 
Colliding Dunes in Meridiani Planum

This observation shows a variety of wind-related features near the equator of Mars in northern Meridiani Planum.

Two distinct types of dark sand dunes are visible in this picture. The isolated dunes near the edge of the dune field are called barchans, and their shapes can be used to determine the wind direction. These barchans are about 200 meters (656 feet) across and display steep edges on their southwest sides, indicating that they are driven by northeasterly winds. This agrees with the orientation of the bright wind streak issuing from the crater at the top right of the picture, formed from bright dust eroding from the crater interior.

Towards the middle of the dune field, the shape of the dunes alters to the second type of dunes, known as “transverse” dunes. These are long ridges of sand that are oriented perpendicular to the prevailing wind direction, and tend to form where there is an abundant supply of sand. A closeup of the transition zone suggests that the individual barchans first merge into groups of two or three before the groups merge to form ridges.

Several indications suggest that the sand here is on the move. Sand fills small pits and polygonal fractures in the bedrock, indicating recent sand movement between the dunes. The transitioning dune types and the presence of outliers (dunes that form away from the main dune field) both suggest possible dune migration.

HiRISE will continue to monitor areas such as this over time, in order to quantify the rate of sand movement on Mars.

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Dust-Devil Tracks in Southern Schiaparelli Basin

Dust-devils are vortices of wind that form when air rising from a warm surface encounters shear in the above atmosphere. Martian dust devils can attain gargantuan proportions, reaching the size of terrestrial tornadoes with plumes that tower up to 9 kilometers above the surface. Dust-devils play an important role in sustaining the aerosols that make up Mars’ red sky and in cleaning the Martian surface after a dust storm.

This observation shows a region near the Martian equator that is a perfect tablet for the scribblings of dust-devils. This region is made up of dark bedrock that is thinly blanketed by bright dust. Dark tracks form when dust-devils scour the surface, exposing the darker substrate. The tracks tend to cluster together, as dust-devils repeatedly form over terrain that has been previously scoured and is consequently darker and warmer than the surrounding surface.

Once lofted by a dust-devil, the fine dust can be transported great distances before it settles again onto the surface.

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Intersecting Graben in Utopia Planitia

This image shows a set of intersecting faults in northern Utopia Planitia that expose materials buried beneath the surface. The area between each pair of faults has dropped below the surrounding surface, indicating that the crust fractured as it was pulled apart. Different generations of faults are visible by the cross-cutting relationships between the intersecting faults, and may indicate changes over time in the direction of the stress that caused the extension.

Several of the faults in this image expose a shallow, horizontal layer made up of dark rocks and boulders that is possibly volcanic in origin. A dark stain (blue in this enhanced color image) surrounds the layer, made up of fine grained materials such as sand or dust.

Elsewhere in the image, faint dark streaks issue from the layer as this fine material is removed by the wind. Erosion of layers such as this presents a possible source of basaltic sand that may supply dunes and ergs far away.

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Intra-Crater Deposits in Nilosyrtis

Both ancient and modern deposits within craters in the northern lowlands area of Nilosyrtis are visible in this HiRISE observation. This crater and its neighbors are partially filled with sediments that display unusual morphologies, having patterned interiors and radial filaments.

The crater centers are occupied by heavily eroded mounds of material that probably once buried the craters in this region. Horizontal layering is visible in similar mounds elsewhere in this image, and close inspection shows that these mounds are covered by rocks, presumably ejecta from distant impacts.

The accumulation of ejecta on their surfaces indicates that the mounds are not recent deposits of dust or sand, but rather are ancient sediments perhaps deposited in a primordial sea. The radial filaments are much more recent deposits, as shown by the lack of ejecta on their surfaces, and are likely made up of dust and sand that is trapped between the older mounds and the crater walls.

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Sand Dunes in Nili Patera Caldera

A set of dark sand dunes within the northeastern edge of a dune field in Nili Patera, a volcanic caldera in the Syrtis Major region of Mars is the focus of this HiRISE observation.

The lighter surface beneath the dunes is an ancient lava flow. The cracks in the flow probably formed when the lava cooled. Many of the cracks are dark and probably contain sand; the dark color of the dunes suggests that the sand is basaltic in composition and therefore originally derived from volcanic rock.

However, whether the sand formed from Nili Patera is not known, as it is possible the dunes have blown in from a more distal location. With two horns on one end and a rounded edge on the other, the dunes have a distinctive shape. The side of the dunes with the horns has a steeper slope — called the slip face &mdash and the rounded side exhibits a more shallow slope. These types of dunes are called “barchans” and, in analogy with similar dunes on Earth, form in areas with limited sand supply.

The horns of the barchans point in the downwind direction, indicating that the predominant surface winds in this region blew from the east-northeast (slightly upper right in the image). Zooming into the image, one can see landslides on many dune slip faces, indicating fairy recent slope failure. Very small ripples are visible on the dunes” surfaces, showing that the winds in this region have not only blown the barchans across the lava plain, but also modified the dunes themselves. Another HiRISE image, PSP_004339_1890, shows more barchan dunes in Nili Patera, south of this area.

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