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Today’s cool image focuses on one of the weirdest flow features I have yet seen on Mars. The first photo to the right, rotated and cropped to post here, comes from a January 27, 2021 picture by the high resolution camera on Mars Reconnaissance Orbiter (MRO). This cropped section focuses on the middle of three such weird features, two close flows heading downhill on the interior rim of very eroded 28-mile-wide crater. For some reason the flows also have depressions on their crowns. The depressions almost look like someone carved them out with a spackling spatula. In fact, the MRO science team agrees, labeling this image as “Spatulate Depressions with and without Upslope Gullies.”
The second image to the right shows a wider crop of the same picture, and explains the reason for the last half of that label.
In this view you can see that the left two flows both have gullies uphill from them, while the right two do not. That of the middle two flows, so close together, only the left has an uphill gully is even more striking.
This crater is at 38 degrees south latitude in the southern cratered highlands, so it is possible that these lobate flows are signaling the existence of water ice below ground. Moreover, the uphill gullies for the two left flows suggests they are the source from which the material of those flows originated. That the right flows don’t have gullies however suggests otherwise, that the gullies and the lobate flows are not directly related.
But why do these flows have those interior depressions? I can make two wild guesses. First, maybe when these flows occurred they happened quickly, so that the flow front outpaced the material following, leaving more material at the head than behind it. Second, maybe the ice in the crowns of these flows had become exposed and began sublimating away, leaving the present depressions. The higher edge did not sublimate because it is made of the dust and debris, not ice, that the flow pushed ahead of it.
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The last image to the right, rotated, cropped, and reduced to post here, was taken by the wide view context camera on MRO. It shows the entire western half of this crater, with the white box marking the location of the second image above.
The impression left of the crater’s interior is that it has ice below the surface, and that this is sublimating away to leave the cracks and canyons, while also removing supporting material at the base of the rim and thus causing the type of avalanches geologists call mass wasting. The flows above are typical of that kind of avalanche.
The interior depressions in the flows however remain a puzzle. While I personally favor my second hypothesis, that the ice inside these flows has sublimated away, leaving a depression surrounded by the flow’s moraine, I have no enthusiasm for that explanation.
Above all, the most significant aspect of these features, despite their strangeness, is that they once again reveal the likelihood of underground ice in the mid-latitudes of Mars. There’s ice in them hills!