Explanation:

If a large new crater were to form on the Moon, the area north of Mare Frigoris would be a place that could use it. Most of the craters north of 60 degrees N latitude are old and battered by ejecta from the impact that formed the Imbrium basin. The neighborhood would benefit from a nice fresh 60 km wide crater to attract attention! But, if you look closely, you see that this area, like every area on the Moon, has features of interest. Based on its diameter of 95 km, Arnold must have originally looked like the 93 km wide Copernicus, with terraced walls, a broad flat floor and central peaks. But the terraces and peaks are long gone, along with the southern lip of the crater's rim. The ballistic pummeling from the Imbrium impact must have eroded the rim and filled in much of the crater. A fresh crater 95 km wide should have a depth of about 4.5 km, so Arnold (which has a rim of unmeasured height) must have a fill thickness of 3 km or more. If this fill were from Imbrium, the surrounding terrain should also have been plastered by a similarly thick cover of ejecta, but I would guess it is probably only a kilometer thick or less. From this, I wildly speculate that much of the light-hued, smooth plain material filling Arnold and elsewhere is volcanic, rather than ejecta. Certainly, some material covers Arnold's floor, for a number of craters are clearly filled with something, and two are completely buried and look like saucers in Ptolemaeus.

— Chuck Wood

Technical Details:
Sept 3, 2004 Stramaster 14.5 working at F/28, DMK-21F04 Firewire camera, 30FPS, stack of 264 frames from 2900.

Related Links:
Lunar Orbiter IV View
Rukl Atlas of the Moon, Sheet 52

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