July 10, 2012
Home Again Zeta
top left and right Kaguya HDTV south up images from Shirao & Wood (2011), bottom right telescopic image from LRO QuickMap (NASA/Arizona State University)
Humans have nearly 400 years experience at looking down on the Moon with telescopes and most lunar orbiting spacecraft. Looking down in the sense that unless a crater is near the limb we see it from above, nearly vertically looking down. We construct interpretations based of these flat views, and sometimes we are fooled, at least I am. I have always looked at telescopic images such as Wes' spectacular sunset view and assumed that the triangular block (TB) along the west wall of Plato was a majestic peak, separated from the main wall, but sticking loftily into space. The spire-like shadow on Wes' image shows that the TB is not nearly as high as the peak Plato Zeta, but that it is still impressive. In looking at a high oblique Kaguya HDTV image I realized that the TB is really almost flat, or rather tilts away from the center of the crater. I shouldn't be surprised by its flatness because it is just a piece of the rim that broke off and slide down a few hundred meters. The topmost Kaguya view is a low oblique image with Plato at the horizon. It beautifully illustrates how shallow large craters are compared to their diameters, and also how exceptional the peaks Plato Gamma and Zeta are, rising significantly above the average rim height. Yesterday I noticed that much of the excess height of Gamma was due to a mass of mottled material sitting on Plato's rim. I speculated that the Gamma peak could be an giant mass of fallback ejecta with the mottled layer being anorthositic blocks excavated from below. Is that also the explanation for the unusual height of Zeta? The lighting on the western rim (tall right image) doesn't show the albedo blobs visible on the east wall, but there are similar size rounded masses sticking out of the wall. These rounded blobs make up about half of the northern side of Zeta with a gray unit underneath, just as in the eastern wall. The top 40% of the thickness of the TB is similar rounded material. I propose this is the same type of fallback ejecta as seen in the east, so that Zeta's height is due to an unusual thickness of debris. An awkwardness of this interpretation is that the southwest wall at top is made up nearly entirely of the rounded blobs, in other words if that is all fallback material there was no elevated rim of gray material here. Also note that pure anorthosite has not been identified at Plato, yet.
Jim Mosher pointed out that the Greek letter Zeta was traditionally assigned to the tall peak here labelled Zeta, but in the System of Lunar Craters Zeta was transferred to the slump mass TB. I can't remember why we did that back in 1964, but I may have actually been the one who did it, perhaps thinking that geologically TB was worthy of a designation. I am happy to return the Zeta to the peak Mädler assigned it to.
Rükl plate 3
Yesterday's LPOD: Why Does Plato Gamma Exist?
Tomorrow's LPOD: Personal Best