HiRISE Updates (2012 Jul 18)

[bystander]

Flows in Hellas Planitia (ESP_025925_1420)

Hellas Planitia is the interior of the Hellas impact basin, is one of the largest visible impact craters in the Solar System. Hellas is located in the Southern highlands and formed very early in the planet's history. The floor of Hellas includes the lowest elevations on Mars and some of the strangest landscapes.

The most striking feature of this observation are the incredible banded features, probably due to the flow of surface material. Although Martian flow features may have Earth analogs such as rock glaciers, it's uncertain as to what types of fluvial, glacial and mass-wasting processes are involved in their formation.

This is a stereo pair with ESP_017644_1420.

Gully Monitoring on Crater Slopes in Terra Sirenum (ESP_027343_1410)

These crater gullies lie on the northern wall of an unnamed 9-kilometer diameter southern hemisphere crater in Terra Sirenum. The image was acquired during early winter in the southern hemisphere, so the crater wall is in shadow.

These gullies were first imaged by HiRISE in 2006. Since that time the possible role of seasonal frost in gully formation along with the association of polygonal terrain with these and other gullies has garnered considerable interest. As a result, these gullies have become one of several locations being monitored by HiRISE throughout multiple Mars years. Over a dozen images of these gullies have been acquired to date throughout different Mars seasons.

In this image, frost (likely water-ice) is once again forming on these southern hemisphere mid-latitude crater slopes. The subimage shows gullies on the shadowed polar-facing slope. The large dynamic range of the HiRISE camera allows one to see into the shadows dimly lit by sunlight scattered by the surface and the atmosphere. These gullies are thinly veiled with frost and range in width from several meters to tens of meters and in length from a couple kilometers or so. Dark regions within the gullies are warmer areas where frost likely evaporated or melted exposing the darker underlying surface.

Frosted Gully Landforms (ESP_027647_1395)

This image shows a crater wall in the Southern hemisphere, with gully landforms.

Gullies like these are mostly found in the mid-latitudes, between 30-50 degrees north or south. In the Martian winter, frost (mostly carbon dioxide) can build up in the gullies, especially on the cold slopes that face the pole.

The bright, bluish (enhanced-color) frost can be clearly seen in the upper alcoves of gullies here. We now know that Martian gullies are active, and that most changes occur in the winter--it is likely that frost like this causes the activity in some way.

A Fading Impact Crater (ESP_027806_1700)

This cluster of craters formed quite recently from a weak impactor that broke apart in Mars' thin atmosphere before smashing into the surface. It was discovered by the MRO Context Camera (CTX) Team, who found a dark spot in a CTX image taken in August 2008 that was not present in earlier Mars Odyssey Mission THEMIS images from July 2005. HiRISE examined the feature in October 2008 and verified that the dark spot was impact ejecta excavated from beneath the bright surface.

On June 25 2012, HiRISE took another look at the young crater to see how it had fared after two Martian years. This image was timed to closely match the illumination and viewing conditions of the earlier HiRISE image. A comparison of the two images shows that the dark halo surrounding the crater cluster has nearly vanished. The delicate rays extending beyond the halo are also significantly faded. Only the individual craters remain distinctly dark in the new image.

This observation is important for two reasons. First, it raises questions about the Martian winds and sediments that produce such changes. Did the dark ejecta blow away, or was it buried by a layer of bright dust? Second, it tells us that the window for detection of these young craters can be very short. In this case, the dark spot that drew the attention of the Context Camera Team was the 200-meter diameter halo of ejecta that encircled the crater cluster. After two Martian years, the halo is gone and the impact cluster would not be easily detected.

Credit: NASA/JPL/University of Arizona

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