left image by Stefan Lammel, Uxbridge, England; right image from Clementine
The irregular maria are typically small and seem to occur by happenstance. But few things on the Moon don’t have a story, so what is it for Mare Undarum? Stefan’s best-ever from Earth image shows that Undarum is a roughly circular (if you mentally correct for foreshortening or look at the Clementine image) patch of dark maria that fill low spots (ancient craters probably) and, like Australe, younger craters (Dubiago, Condorcet P & F, and Firmicus at left center). The circular shape suggests that the mare material fills a small lunar basin but there is no evidence, other than the lavas themselves, to suggest a prior depression. Indeed, topomaps show this to be a high area, and a crustal thickness map gives no evidence of thinning as under impact basins. To get a clue to why Undarum exists we need a more regional perspective. Undarum, like Spumans, Bonitatis and Anguis are all piddly little mare patches outside of the main mountainous rim of the Crisium impact basin. Although it is hard to see south of Crisium, a larger basin ring is clear to the north of the basin. All of these little maria are in the moat between two Crisium basin rings. Deep fractures associated with the basin allowed magma easy rise to the surface, and it ponded in the low spots.
7 Nov 2007, 00:27 UT. 10″ f4.8 Newtonian, DMK 21AF04, 4x PowerMate, red filter, Registax v4, PSE 5, 60fps, 1/60s, 600/5000, MAP: 29×64.
Rükl chart 38
Yesterday's LPOD: A Mystery Solved
Tomorrow's LPOD: A New Chu