November 9, 2014

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Return View

image by Benoit Schillings

Benoit provided one of the earliest LPOD images - the best view of Mersenius I had ever seen - it is still good check it. Now, ten years later he has this new, broader view of Mersenius and a much wider area. I think the old LPOD caption is still appropriate and reuse it here:

Mersenius is in the second rank of lunar craters. Not a must-see like Copernicus, Plato or Gassendi, but a good crater with more interesting features than are obvious. Mersenius is 84 km wide and about 2.3 km deep. But as has been known for more than 125 years, Mersenius appears to have a domed floor so its depth may be more varied than for normal craters. In his 1876 book, The Moon, Neison quoted Schmidt as saying that Mersenius had a "strongly-convex" floor and estimated its center to be 450 m higher than near the walls. This remarkable low sun image by Benoit Schillings gives little evidence for a gradually decreasing elevation westward from the center of the floor, but does show a shadow/depression where the west wall meets the floor. It seems unlikely that this alone is 450 m of relief, but then all these shadow measurements are ancient - are there no modern measurements of this crater's geometry? The line of overlapping craters on the floor are aligned with Imbrium, and thus may be distant secondaries from that basin-forming impact. The image also reveals more delicate rilles on the crater's floor than I have seen on any other Earth-based image. These are very difficult to image or observe - even the Great Schmidt of Athens saw only two, and they were "very difficult." Small pyroclastic deposits have also been detected around the rilles. Mersenius must be another floor-fractured crater.

With the altimetry tool of LRO QuickMap we can now check on Schimdt's value for the updoming of Mersenius' floor. The entire floor is strongly domed, about 500 m in a north-south direction, and 600 to 700 m depending on exactly which east-west transect is measured. Schmidt's value wasn't bad, especially for 139 years ago!

Chuck Wood

Technical Details
C14 with Zwo 120mm. Red filter

Related Links
21st Century Atlas chart 26.

Yesterday's LPOD: Most Famous Crater of SE Quadrant?

Tomorrow's LPOD: Model Mania


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