June 10, 2013

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Not Horrible Ejecta

image by Michael Wirths

Mike sent LPOD an image that covered J. Herschel, its swirl-ejecta floor and nearby area but what immediately caught my eye was the uneven distribution of the ejecta of Horrebow (Ho), so I cropped the full image down. Horrebow is a relatively fresh, 25 km wide crater formed on the boundary between highlands/ejecta and Mare Frigoris, cutting into the similar-sized Horrebow A. I suppose that ejecta covered the surrounding area in all directions, but there is little evidence for it on the highlands, nor on the mare between Horrebow A and J. Herschel F. The surfaces of the mare show why. The lavas where the rays are missing are smoother and contain fewer pits than the ejecta covered lavas and others to the west. The lavas are probably younger than Horrebow, covering its ejecta in that area. A QuickMap topographic traverse shows the ejecta-covered lava south of Horrebow is 60-80 m higher than the smooth lavas. So the smooth lavas filled a low spot along the edge of the highlands, covering the crater's ejecta. Mike's image also shows that the highland mare boundary is remarkably straight on both sides of Horrebow. This suggests that a fault downdropped the original Imbrium ejecta - well seen in this recent higher Sun view. Perhaps a post-Horrebow reactivation of the east side fault accounts for the lower surface to the east of the crater. Evidence for faulting along the northern edge of Mare Frigoris would support the interpretation that that is the northern rim of the Imbrium Basin.

Chuck Wood

Related Links
Rükl plate 2
21st Century Atlas chart 20.
Mike's bed & breakfast website

Yesterday's LPOD: Terminator Tales

Tomorrow's LPOD: Carboniferous Moon


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