image by Stefan Lammel, Uxbridge, England
Nectaris and Humorum have the best exposed concentric structures of any impact basins on the lunar nearside. Nectaris has a huge partial rim - the Altai Scarp - and arcs of two more inner rings. Humorum doesn’t have massive mountainous rings as much as concentric rilles and mare ridges, beautifully seen in Stefan’s recent image. The reasons for the differences in these basins is totally unknown. When a small impact event occurs, the resulting few kilometers wide craters are so similar they could have been turned out on a lathe. But surprisingly the prodigious impacts that produced impact basins create structures of surprisingly diverse character. Perhaps small craters look so similar because they each form in a fairly homogeneous patch of crustal rocks, but basins are each unique because they excavate into large areas that encompasses multiple rock types and history. I am speculating that the energy of impact is less important than crustal structure in determining the looks of craters and basins.
27-April-07 21:21UT. 7″ f8 Newtonian + 3x PowerMate + DMK 21AF04 + green filter; Registax v4, PSE 5, 1/34s (gain 650), 900/4000, MAP: 30-40x 64×64.
Rükl charts 52 & 53