Starship Asterisk*

HIRISE Science Team wrote:

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Small Crater within Pollack Crater Containing Light-Toned Material (ESP_018212_1715) (HiClip)

This observation shows a small crater in within the much larger Pollack Crater containing light-toned material. Pollack Crater is a 90-kilometer diameter impact crater first imaged by the Mariner 9 spacecraft.

This material was first observed by the THEMIS team, and there are multiple Context Camera and Mars Orbiter Camera images showing that this light-toned material may be similar to the large “White Rock” outcrop to the northwest. HiRISE has also imaged White Rock several times. In these cases, we can see how different teams can work together to get an interesting image at high resolution for further study.

HIRISE Science Team wrote:

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Lava Against an Impact Crater in Elysium Planitia (ESP_018537_1860) (HiClip)

This image shows lava crumpled against the upstream side of an impact crater. In places where we see smaller ridges in the lava, they have steep faces that retain less dust and look rocky. Because of the lesser amount of dust, we might be able to see better details of the topography.

The crater itself is extremely old, having long been filled in with dust and its rim severely eroded. Note also the flat surrounding terrain.

This is a stereo pair with ESP_019170_1860.

Paul Geissler wrote:

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Looking for Changes in Dust Drifts West of Alba Mons (ESP_032709_2210) (HiClip)

This image was intended to search for surface changes after three Mars years in a dust-covered region west of the Alba Mons volcano. It was acquired at almost the exact same season as another image in 2007, and the illumination and viewing angles are very similar.

The earlier image showed thick dust deposits accumulated downwind of topographic obstacles, apparently formed by winds blowing across the surface. These deposits, nicknamed “whiskers,” are evidently rare on Mars, so far found in only a few places such as the summit of Tharsis and on the flanks of the giant volcanoes. Their ages and origins are still poorly known. We expected that wind-related changes could be taking place here in particular because of the location's low elevation in comparison to similar dust deposits elsewhere on Mars. The atmosphere is denser at this elevation, near Martian “sea-level,” than at the tops of the giant volcanoes, so the winds are more effective at eroding and transporting sediment.

A comparison of the new image with the image taken in 2007 shows no discernible differences in the dust deposits. Analysis of the full image is still ongoing, but it is already clear that no major surface changes took place in this location over the three Mars year interval between the two pictures. This is instructive because it tells us that the dust deposits are resistant to wind erosion (under normal winds at least; the last major global dust storm was in early 2007, before PSP_006271_2210 was taken) and that no new dust deposition has taken place.

This result suggests that either these deposits form and evolve very slowly, over time scales much longer than three years, or else they formed during a past period when the winds were much stronger than they are today.

Alfred McEwen wrote:

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Frosted Impact Crater in Late Northern Winter (ESP_032722_2405) (HiClip)

This image was planned to search for gully activity in the Northern Hemisphere. Changing gullies have so far been documented only in the Southern Hemisphere, where a greater thickness of carbon dioxide frost forms in the winter.

The gullies are active when this frost is present, especially in the late winter and spring as it sublimates. The well-preserved crater here has a bright gully deposit (visible in prior images acquired in late northern summer), which suggests recent activity. An animated GIF blinking between these two images (at reduced resolution) shows how it changes in appearance with the seasons.