Starship Asterisk*

Nathan Bridges wrote:

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Topography of Moving Dunes in Nili Patera (ESP_017762_1890)

Many HiRISE targets are imaged twice, close together in time, with each image having a different viewing angle. From this, we are able to make digital elevation models (DEMs) with precisions of tens of centimeters. Doing this for dunes can be particularly challenging, as they commonly migrate.

However, if the images are close enough together in time, or are inactive, a good model can be made. This is the case here, where an elevation model was produced of the Nili Patera dune field.

These dunes are active and, by computing the height of the moving dunes using this DEM, researchers have determined the sand flux (sand volume per length per time) and find that it is similar to that seen for dunes in Antarctica.

The research also finds that the dunes are moving as coherent masses of mobile sand. This work has important implications, as it shows that wind processes on Mars can transport considerable volumes of sand, which not only contributes to dune migration, but also abrasion (sand blasting) of the surface.

This is a stereo pair with ESP_018039_1890.

Candy Hansen wrote:

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The First Day of Southern Spring (ESP_028956_0930)

This image was taken on the first day of spring in the Southern hemisphere. Like the Earth, Mars is tilted on its axis, and the sun crosses the equator twice each year. On Earth we call this the equinox.

In the winter a seasonal polar cap composed of dry ice covers the polar regions. Although this is just the first day of spring and the sun is very low in the sky, activity associated with the sublimation (going directly from solid to gas) of the polar cap is already underway. A layer of ice coats this entire scene, and fans of dark material are deposited on top of the ice. This dark material is carried to the top of the ice by escaping gas flowing through the spidery channels carved in the surface.

In the summer, when the ice is gone, the fans are no longer visible.

Alfred McEwen wrote:

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A New Impact Site (ESP_029015_1705)

This observation shows a cluster of impact craters that formed between August 2005 and November 2010, first discovered in a Context Camera (CTX) image G05_020035_1699_XN_10S064W_101104.

What's unusual about this site is that it isn't as dusty as most places where new impacts are discovered. Often the airblast disturbs the dust to create a dark spot much larger than the crater and its ejecta, so the new impacts are most easily discovered over dusty terrains.

The dark ejecta is obvious while the larger dark spot here is subtle, but detectable in the CTX image. There is a tight cluster of craters rather than a single crater because rocky bolides often break up in the Martian atmosphere.

Mike Mellon wrote:

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Compositionally Diverse Bedrock (ESP_029234_2015)

Regolith, particulate fragmented rock and fine grained soils, generally covers most of the surface of Mars. Mantles of rocky dust often mute the landscape, filling in topographic lows and burying boulders. Sorted sand-sized soil grains are often evident in images in the form of sand dunes and ripples.

Clean exposures of bedrock are relatively rare. Fractured basement rock is sometimes visible between dunes. Sedimentary layers can also become eroded showing alternating bands, these visible bands manifesting from changes in rock strength between layers.The edges of scarps may also exhibit significant strength against erosion or large blocks and boulders at the base of the scarps. All of these examples are visible within this geologically rich image.

The abundance of rock exposures and relative scarcity of a obscuring soil layer make this region a good location to examine the compositional diversity of the parent bedrock.