THREE THINGS TO NOTICE

LPOD-Apr13-09.jpg
image by Robert Dyrda, Inowroclaw, Poland

An infrequently imaged crater is always a treat, and Robert's excellent view of Harpalus and the region of western Mare Frigoris provides new data for description and speculation. Harpalus is known to be a young crater and its ejecta and secondaries are well documented in this image. But immediately on looking at this it seemed to me that the ejecta is not scattered equally in all directions - it is most conspicuous to the north and the south of the crater and much weaker to the east and west. This is a sign of oblique impact, but the Clementine higher Sun image does not suggest that the rays are asymmetrically distributed. Hmm. Are secondaries and similar ejecta more sensitive to angle of impact than rays? It wouldn't seem so since rays are also ejecta... Perhaps the most dramatic feature in this image is the contrast between the relatively smooth mare surface and the very rough texture of the crater South at the upper left. South is full of Imbrium ejecta, which here is a mega-gravel of small mountains and hills. A much less conspicuous feature is the small sinuous rille near the upper right corner. At first I thought this was the rille north of Sharp, but then realized it is a different one. Both rilles are very similar, starting in collapse depressions. Sinuous rilles in non-mare material are uncommon, but these two are joined by ones both east and west of the rim of Plato. Since most sinuous rilles occur in mare lavas, I wildly speculate that the existence of four in the Plato to Sharp arc of Imbrium ejecta implies that the ejecta is underlain by older mare material.

Chuck Wood

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