Behind The Black – Robert Zimmerman

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Every spring for the last seven Martian years scientists have eagerly aimed the high resolution camera on Mars Reconnaissance Orbiter (MRO) at the steep 1,500 to 3,000 foot high scarp at the edge of northern polar ice cap in order to capture images of what is Mars’s most spectacular annual event, the occurrence of tens of thousands springtime avalanches along that scarp.

Well, spring has returned to the northern hemisphere on Mars, and the scientists have begun another monitoring campaign. The photo to the right, cropped to post here, was taken on March 7, 2021 by MRO. It shows a particularly dramatic part of that scarp, a place where the scarp separates two curved alcoves and is thus narrowed down to a ridge about 1,000 feet high.

The nose of the ridge is sloping downward to the northwest, so the horizontal bands on its crest are actually evidence of older and older layers exposed as the elevation drops. The blue and black markings on the left slope are likely evidence of this season’s first avalanches, or might even be avalanches occurring as the picture was snapped! As explained to me by Shane Byrne of the Lunar and Planetary Lab University of Arizona during the last Martian avalanche season,

On Mars half of the images we take in the right season contain an avalanche. There’s one image that has four avalanches going off simultaneously at different parts of the scarp. There must be hundreds to thousands of these events each day.”

The overview map below shows the location of this picture, as well as all the other places the scientists have routinely monitored in the fourteen-plus Earth years since MRO reached Mars orbit.
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Today’s cool image to the right, rotated, cropped, and reduced to post here, was taken in January 2012 by the context camera of Mars Reconnaissance Orbiter (MRO). The location is a small section of the Tartarus mountain range that is cut by the Cerberus Fossae fissures, all located in Elysium Planitia, the large volcanic lava plain that lies between Mars’ big volcanoes. The white cross on the overview map below marks the location of the photo.

I picked this photo because it quickly shows us in one picture many of the typical features one finds in that lava plain.

For example, the distinct fissure that cuts across the mountains near the top of the picture is the northernmost large fissure of Cerberus Fossae. In my initial post on Cerberus I mistaken thought its large and many hundreds of miles-long fissures might be evidence of underground lava tubes. Since then I have learned while the depressions may signal underground voids, they are not a lava tubes but graben, cracks formed by the movement of the terrain on each side. The cracks opened when past volcanic activity caused the ground to swell upward, stretching and splitting it.

The dark splotch in the flat area just south of the fissure remains me of the maculae found in these lava plains to the west of Olympus Mons, splotches that for still undetermined reasons dust devils like to congregate, blowing off the red dust so that the dark basalt lava becomes visible. No high resolution image of this spot has yet been taken, so this is a pure guess on my part.

The mountains near the bottom of the photo illustrate the ancient lava flood that inundated these mountain peaks. The white box shows the area covered by the recent MRO high resolution image that I include below.
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Cool image time! The photo to the right, rotated, cropped, and reduced to post here, was taken on November 29, 2020 by the high resolution camera on Mars Reconnaissance Orbiter (MRO). Labeled simply as “periglacial survey,” it is one of almost two hundred such images taken by MRO over the years, almost all of which are in the high latitudes above 60 degrees, with most being in the southern hemisphere. Most appear to be close to or above Mars’s Arctic Circle, which means these are locations that will see little or no sunlight for a portion of the year.

I have been unable to contact the scientists doing this survey, so I will have to make an educated guess as to its purpose and goals. “Periglacial” refers to the outer fringes or margin of a glacier or large ice sheet. Thus, in the context of this survey, the scientists appear to be studying places where they think the Martian high latitude ice sheets are beginning to sublimate away. Today’s photo is a good example. It is located at 67 degrees south latitude, in the southern cratered highlands but in an area that appears to be relatively free of craters. Instead, the terrain appears somewhat flat with only periodic depressions and scarps. The MRO context camera photo below of the same area, rotated, cropped, and expanded to post here, illustrates this.
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