February 26, 2008
image by François Emond, Chorges (Hautes-Alpes) FRANCE
The simple description of down-dropped rings of wall rock does not do justice to the great variety of terraces. I often think of them as traceable all around a crater's walls and having nice stair steps down to the floor, but most craters seem to deviate a lot from that idealized model. François' magnificent image of Moretus shows how variable terraces are in one fresh crater. Most noticeable is the differences in the amount that the uppermost terrace has dropped. At the far (south) side of the crater the main scarp is twice as high as immediately to the right, and about four times as tall as further to the right. I count seven terraces on the wall to the left of where the central peak shadow points. But these terraces can't be traced very far. The top two stop at three mounds of slumped material. To the left of and partially in front of the tallest scarp it appears that a broad zone of chaotic collapse has erased all terraces. It is necessary to look at images with opposite illumination or a straight down views from polar orbiters to examine terraces all around Moretus. But it is less regular than I thought? Which crater has the most regular terraces?
15 February 2008, ~ 19 h 15 UT. Dobson T400 (~ 16″ Newton) + FFC Barlow lens + webcam CMOS PL1-M + red filter.
About 250 frames stacked in Registax.
Rükl plate 73
Yesterday's LPOD: Out On a Limb
Tomorrow's LPOD: Umbral Image
(1) There appears to be a sheer, straight-drop cliff on one side of Moretus. I don't recall seeing a feature like this in other craters. Is it unusual? If the Moon had an atmosphere, the Moretus cliff looks like it would be a great spot for hang-gliding.
(2) Bill - The steepest spots measured on the Moon are about 50° - the Moretus scarp looks steep - and its quite long. There are similar uneven collapse scarps at many craters, but this is a glorious example.
(3) Chuck - The article about steep spots measured on the Moon was very interesting. Are there any sheer cliffs on the Moon? I thought, for example, that perhaps the cliffs along the Alpine Valley might be a good candidate. (??)
(4) Not that we know of. The Straight Wall used to be depicted as vertical but measurements show it is about 20° - very steep to climb, or to fall down - but not vertical.