July 10, 2020

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Micro Features

Originally published January 27, 2011 LPOD-Jan27-11.jpg
north to the left LRO-WAC image processed by Brendan Shaw

Brendan processed LRO WAC images showing this area of Mare Imbrium just north of Helicon and LeVerrier. There are a number of interesting small features to explore here, but first notice the broad scale difference in surface smoothness. In the annotated image below a horizontal line is the boundary between rough textured lava and a smoother surface below the line. Because this boundary is so straight it is probably a difference in processing of the LRO image, but there do seem to be fewer craters below the line. If it is real it is the edge between a younger lava unit and an older, rougher surface one. Nearly all of the interesting features here are above the line. For example, Brendan circled two objects. To the left is a crater that doesn't look like the normal impact craters. It has a very low rim and a shallow interior. Probably it is a normal impact crater that was overwhelmed by lava flows. The second circle on the right marks a more peculiar feature. It is circular with pit in its center - it looks like a squashed dome, or a Cheerio encrusted with lava. It also is similar to the oddity discussed recently. It must be some sort of volcanic structure. Next topic. Do you notice the rimless craters scattered across the image? There is one to the left of each "R" on the image below. These could be collapse pits, as occur over voids created by buried lava tubes that drain their lava. Or they could be normal impact craters that have been surrounded by later lavas that buried their bases and didn't quite overflow their rims. The latter interpretation makes sense for a few, but I see at least six here. But Brendan's circled crater could simply be the largest of such flooded craters.. The flooded craters labeled 3 are examples of lava flow interactions with existing craters. So there is evidence for both of the interpretations. Finally, number "1" on the image below indicates a short segment of a rille. Look closely at the area above the circle and you will notice a very low ridge that continues the direction of the rille. This could be a surface expression of the uncollapsed portion of the lava tube. I suppose that any piece of a mare looked at closely will show similar bits of its history.
Chuck Wood

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