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Impact crater formation is sometimes affected by preexisting target inhomogeneities like faults or joint sets in ways that cause the plan view of the crater to deviate from the idealized circular shape. The resulting polygonal impact craters have been known to exist on the Moon for over a century, and they have been subsequently identified on all types of solid surface bodies in the solar system, including all the terrestrial planets. Newly identified polygonal impact craters in the central southern near-side highlands of the Moon display a size distribution clearly different from the nonpolygonal craters, confirming earlier results: lunar polygonal impact craters “favor” the size range from ~20 km to 45 km. Similar results have been obtained also from Martian and Venusian impact craters. When polygonal impact crater diameter data from all three bodies are combined, it becomes apparent that something in the formation mechanics of impact craters apparently drives small complex craters to become polygonal more easily than simple craters or larger complex craters.
The interior of this floor has been flooded by lava, leaving only a low rim projecting up through the lunar mare. This rim is broken along the northwest face, leaving a gap into the interior. In the eastern half of the inner floor is a semi-circular feature that is concentric with the outer wall. There is also a low central ridge offset to the east of the interior midpoint.
Haldane is a somewhat extreme example of many fracture-floored craters in Mare Smythii.