HiRISE Updates (2016 Jan 27)

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HiRISE Updates (2016 Jan 27)

Post by bystander » Fri Jan 29, 2016 6:28 pm

Henrik Hargitai and Ginny Gulick wrote:

Pit Crater near Elysium Mons (ESP_043222_2035) (HiClip)

This image was taken of an area on the lower southeastern flank of the volcano Elysium Mons. In the center is a small, dark pristine-appearing pit approximately 130 meters in diameter, which is clearly visible among the numerous small impact craters that are heavily covered by dust and sediment. These pit craters are usually appear in the younger Tharsis volcanic region, but this is one of the first seen in the older Elysium volcanic region.

Garden variety craters are excavated by impacts and are characterized by raised rims, sloped walls and surrounded by ejecta blankets, but pit craters are simply sink holes in the ground with near vertical walls and floors that are only visible when the sun is high in the sky. They are deep holes that may lead to underground caves in volcanic terrain. This isolated pit crater is located in a lava field that is crisscrossed by long, linear troughs.

A commonly invoked hypothesis to explain these troughs is that they are collapsed lava tubes, essentially tunnels formed by underground rivers of lava. Lava tube collapse may begin with the buckling of its roof at one location where the roof is thinnest. At this point, light enters into the permanent darkness of the cave from above, forming a skylight. Pits or skylights are distinct from impact craters: they appear as dark, roughly circular shadowed holes because of their steep walls, while the shadows cast by impact craters generally shade only a portion of the crater floor, because the slopes of the walls are shallower. When this picture was taken, sunlight directly lit part of the floor of this pit crater, illuminating the large boulders (see close up image) on its southeastern slope.

The first skylights were discovered in 2007 on Mars, and in 2009 on the Moon. The famous Devil’s Throat is Hawaii is a good Earth-example: it’s 50 meters wide and accessible by foot. Pit craters provide a window into the subsurface structure of Mars, and prior to collapse, might have also provided potential habitats where organisms could be protected from the harsh, sterilizing UV radiation experienced at the surface.
M. Ramy El-Maarry wrote:

A Tale of Two Flows (ESP_043609_2230) (HiClip)

This image was taken in one of the regions on Mars well-known for its viscous flow features (VFF), which are massive flowing deposits believed to be composed of a mixture of ice and dust similar to glaciers on Earth.

In this particular region, an impact event occurred creating ejecta deposits that also appear to flow (probably because of their similarly ice-rich composition), and interact with the flows from the VFF. Looking closer, we can see that the VFF deposits (on the right) appear to be rougher in appearance than those of the impact ejecta.

We will need to study this image in more detail to understand how these flows have interacted with each other and what they can tell us about their composition and their flowing behavior properties.
Kirby Runyon wrote:

The Changing Sands (ESP_043617_1885) (HiClip)

As of this date, MRO has been orbiting Mars for nine years. One benefit of MRO’s nearly three completed extended missions is the chance to see landscapes change over time.

These barchan sand dunes are a case in point: these have been imaged at least five times already, and each time, a bit more movement is revealed.

The steeply-dipping slip faces (where sand slides down after it gets piled up on the dunes’ crests) indicate the direction of the wind, although this particular dune field seems to show *two* converging wind directions.
Cathy Weitz wrote:

Jarosite in Noctis Labyrinthus (ESP_043719_1725) (HiClip)

This image shows the western side of an elongated pit depression in eastern Noctis Labyrinthus. Along the pit’s upper wall is a light-toned layered deposit.

CRISM spectra extracted from the light-toned deposit are consistent with the mineral jarosite, which is a potassium and iron hydrous sulfate. On Earth, jarosite can form in ore deposits or from alteration near volcanic vents, and indicates an oxidizing and acidic environment. The Opportunity rover discovered jarosite at the Meridiani Planum landing site, and jarosite has been found at several other locations on Mars, indicating that it is a common mineral on the Red Planet.

The jarosite-bearing deposit observed here could indicate acidic aqueous conditions within a volcanic system in Noctis Labyrinthus. Above the light-toned jarosite deposit is a mantle of finely layered darker-toned material. CRISM spectra do not indicate this upper darker-toned mantle is hydrated. The deposit appears to drape over the pre-existing topography, suggesting it represents an airfall deposit from either atmospheric dust or volcanic ash.

This is a stereo pair with ESP_043363_1725.

Credit: NASA/JPL-Caltech/University of Arizona

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