“space chimney” re-imaged

Dear citizen scientists!

About 2 months ago here and here you have discussed this weirdly looking structure:

Back then I promised to get HiRISE to image this scene again when the ice is gone. The image is now here, very fresh from the production pipeline:

I framed this image exactly the same as the original “topic-starter”. This way everybody can see that we are talking about the same feature, as 2 months ago. The ice cover is not completely gone yet: one can still see small persistent leftovers in some shadowed places, mostly in small channels. But ice has cleared the feature in question. It happened to be a crater with an unusual inner surface. As often it is hard to see if it is a cone or a depression, but we know that the sunlight comes from bottom right-ish and this tells us it is a crater. To see it better, here is the best zoom HiRISE has to offer (appr. 30 cm/pix resolution):

Without the ice the crater floor looks smooth and its sides show very pronounced polygons. The ice in the trenches between these polygons created the illusion of a bright branching “crown” in our original image. And smooth blanketing on one side of the crater had smooth ice cover that tricked people into seeing Dalek or octagonal-based chimney. So sorry that neither of those are real!

When I saw the polygons inside the crater, I decided to ask my more knowledgeable colleague about them. Mike Mellon (from Southwest Research Institute in Boulder, CO) did a lot of research about polygons on Mars and Earth. He had a look at both, old and new, HiRISE images for us:

“I can see that there are loads of eroded polygons in the region. There are large ones with clear evidence of subdivision into smaller forms. Smaller polygons cover nearly all the surface (roughly 3 meters in diameter). In some areas they are clear and in others they are so small and flat they are a little hard to see. They are completely invisible at the old image because of its poor resolution. The polygons inside the crater are on the order of 5 meters with more deeply incised troughs, so they are easier to see.  The crater floor looks featureless. I see these same larger polygons in the neighboring pits, the pits that appear arrayed N-S.  The larger polygon size in the crater may be caused by sun light on the crater slopes.
The presence of polygons and their sizes are consistent with ice cemented soils about 2-5 meters deep. But it is not easy to interpret the details and especially hard to eliminate the seasonal effects from this discussion, as this location is very close to the polar cap and spider-related processes happens here and there.”

With this we came back to where we have started from: fans and spiders. The image made it to this project because it had them all. Now plus polygons, minus the chimney.