Starship Asterisk*

HiRISE Science Team wrote:

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Layered Material Cut by a Valley Connected to East Jezero Crater (ESP_026359_1990)

This image shows layered bedrock composed of light- and intermediate-toned materials. There are also darker bed forms that fill in low-lying topography, such as impact craters.

In the center of the image is a valley with darker fill extending from left to right. The darker materials within the valley might be fluvial sediments. At HiRISE resolution, we might be able to decipher the properties of the bedrock as well as what deposited the sediments.

Nathan Bridges wrote:

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Active Sand Abrasion in the Northern Polar Region of Mars (ESP_026839_2550)

The large dune field which surrounds Mars' North Polar cap is actively being modified by the wind, with dunes moving at rates of a meter or more per year (PDF). This new HiRISE image shows that the blowing sand is also abrading the ice-rich ground over which the dunes migrate.

Clearly visible in the black and white and color HiRISE frames is a linear texture on the interdune surface that is oriented north-northeast to south-southwest. This orientation matches that of the horns and slipfaces of the barchan dunes, which together indicate migration from the north-northeast to the south-southwest. Visible here are four zoomed views that provide details of this texture. Zoom A/blue box shows a typical barchan dune. The linear texture is visible, albeit subtlety, on the surrounding ground surface.

The texture is more apparent in the next views: A zoom of an interdune surface (Zoom B/red box) shows the wind-etched topography as a series topographic high and lows, with the directional trend indicated by the white arrows. This is also clearly seen next to another dune (Zoom C/yellow box). Further zooming in shows that the topographic highs contain boulders, which may be ice rich (Zoom D/orange box). Most of the sand abrasion probably occurs within the topographic troughs, accentuating topography and abrading away boulders, leaving remnant rocks on the highs. This shows that sand abrasion is actively modifying the surface in Mars’ northern latitudes.

Ken Herkenhoff wrote:

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Streaks on the North Polar Layered Deposits (ESP_026897_2655)

This image shows an exposure of the north polar layered deposits with strange streaks superimposed on the layers.

These streaks may be formed by winds blowing bright water frost over the surface, removing frost from the surface, or blowing dark material over the frost. These streaks make it a bit more difficult to see the angular unconformity running from upper left to lower right. The unconformity can be traced by finding where the layers at the top of the image are truncated by the layers at the bottom of the image.

This relationship shows that the layered deposits were eroded in this area, probably thousands to millions of years ago, before younger layers were deposited over them. The streaks over them were formed during the current northern summer, and may not persist for long.

Alfred McEwen wrote:

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Pluvo Point (ESP_026956_2545)

This image near Mars' North Pole was a public image suggestion, with the following rationale: "My three year old son has dubbed this white smudge 'Pluvo Point', he thinks 'we should take a picture of it to learn about how snow and ice comes down on Mars.' "

Although we can clearly see the bright area in this image, it isn't due to frost or ice at this time of year (early summer), and doesn't have the relatively blue color expected for frost. So, what does create the "white smudge"?

The image shows a typical region of northern plains covered by polygons and boulders. There are dark areas in low spots that are probably due to windblown dark sand. Where the sand is most abundant, it forms the dunes seen near the top and bottom of this image. The "white smudge" appears to be a region with relatively little dark sand. It isn't actually white, but it is brighter than other nearby regions. These could be low hills where the sand doesn't collect.