July 1, 2012

Another Half Crater

LRO images processed by Maurice Collins, New Zealand

In looking at an LRO mosaic of the Sirsalis Rille that Maurice assembled I noticed another half crater (left image).
This one occurs just northeast of Cruger A, the largest crater in the scene. The rim of the half crater seems to sit on
top of the left side of the rille. but the right side of the rille terminates the half crater rim. This is peculiar, but there is
another place on the Moon where a half crater is cut by a fracture - and remember these rilles are troughs where the
land between two facing fractures is down-dropped. The right hand image is a closeup of an older 12 km wide crater
sheared in two by the Liebig Fault in western Mare Humorum. In this case the reason for the non-appearance of the
right side of the crater is that subsidence on the east side of the fault carried the east side of the crater down, and
later mare lavas covered it. But this can't be the explanation for the Sirsalis half crater because only the land between
the facing faults subsides. The missing piece of the crater rim should be immediately on the right side of the rille, but
its not. But wait. Another half crater rim is northeast of the rille-cut half crater. The two half craters are the came size
and fit together pretty well (middle image). This would be an exciting discovery - direct evidence of lateral movement,
almost plate tectonics, on the Moon. But that interpretation can't be true because the space between the two halves
would have to be new terrain created as faults spread apart the two rim halves, and it doesn't look new. And what would
fill the dark rectangle left behind where part of the image was cut and pasted to the east? Probably the two crater halves
are unrelated and the real question is what happened to the missing part of the rille-cut crater. I don't have an answer
for that, do you?

Chuck Wood

Related Links
Rükl plates 50 & 51

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