Behind The Black – Robert Zimmerman

Cool image time! The image on the right, reduced and cropped to post here, comes from the December image release from the high resolution camera of Mars Reconnaissance Orbiter (MRO). (If you click on the image you can see the full resolution uncropped photograph.) Released without a caption, the release itself is intriguingly entitled, “Crater with Preferential Ejecta Distribution on Possible Glacial Unit.

The uneven distribution of ejecta material around the crater is obvious. For some reason, the ground was preferentially disturbed to the north by the impact. Moreover, the entire crater and its surrounding terrain look like the impact occurred in a place that was saturated somewhat with liquid, making the ground soft like mud.

That there might have been liquid or damp material here when this impact occurred is reinforced by the fact that this crater is located in the middle of Amazonis Planitia, one of the larger regions of Mars’ vast northern lowland plains, where there is evidence of the past existence of an intermittent ocean.

This however really does not answer the question of why most of the impact’s ejecta fell to the north of the crater. From the release title is appears the planetary geologists think that this uneven distribution occurred because the impact occurred on a glacier. As the ground has a lighter appearance just to the south of the crater, I suspect their reasoning is that this light ground was hard bedrock while the darker material to the north was that glacial unit where the ground was more easily disturbed.

This is a guess however (a common requirement by anyone trying to explain the strange features so often found on the Martian surface). Other theories are welcome of course, and could easily be correct as well.

Cool image time! The image on the right, rotated, cropped, and reduced to post here, comes from the December image release of the high resolution camera of Mars Reconnaissance Orbiter (MRO. Uncaptioned, the release titles this image “Cracks in Crater Deposit in Acheron Fossae.” If you click on the image you can see the entire photograph at full resolution.

Clearly the cracks appear to be caused by a downward slumping to the north, almost like a glacier made of mud. We can also see places on the image’s right edge where the mud appears to have flowed off a north-south trending ridge, then flowed downhill to the north. All of this flow is away from the crater’s central peak, which is only partly seen in the photograph near the bottom. That section is the central peak’s southwestern end, with the whole peak a ridge curving to the northeast beyond the edge of the image.

At the north edge of this mud flow the cracks become wider canyons, as if long term erosion is slowing washing the mud away. The flow then stair steps downward in a series of parallel benches. Meanwhile, in the flat central area of the mud flow above can be seen oblong depressions suggesting sinks that also flow to the north.

You can get a better idea of the crater’s overall floor and central peak by the low resolution context image to the right. The white rectangular box indicates the area covered by the full image above. A close look at this part of the crater floor suggests to me a circular feature like a faint eroded smaller crater that includes as its eastern rim the larger crater’s central peak. This impression suggests that the flows seen in the full resolution image are heading downhill into the lowest point of this smaller crater, that upon impact had reshaped the larger crater’s floor.

This impression however is far from conclusive. The features in the large crater could simply be the random geology that often occurs in the floors of impact craters.

What makes this particular mud slide most interesting, as is usually the case for most Martian terrain, is its location.
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