December 5, 2017

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Originally published May 31, 2008 LPOD-May31-08.jpg
image by HDTV on Kaguya, NHK; JAXA

There have been earthquakes in China, a huge hurricane in Burma, and a severe drought for LPOD. A drought of new high quality images, especially of lunar places infrequently seen. Fortunately, there are still some Kaguya HDTV images to dip into. This is Oppenheimer, a 208 km diameter crater within the farside South Pole-Aitken Basin. For such a large crater Oppenheimer surprisingly doesn't have evidence for a large central peak or a peak ring. Its main claims to fame are the three large pyroclastic deposits, most visible here around the small crater near the bottom of the image. A Clementine image shows four other smaller deposits on the right side of the crater that are not easily seen in this low Sun view. Most of the deposits are from dark halo craters along rilles, and the largest crater (Oppenheimer U at upper left) is seen on the Clementine image as being surrounded by dark ash - this may be the largest dark halo crater on the Moon. The straight feature at the upper part of the crater floor looks like a scratch or other defect but it is simply a straight section of one of the many circumferential rilles near the edges of the floor. Oppenheimer is like Alphonsus in having dark halo craters along rilles near the crater floor edge, but these pyroclastic deposits are much bigger than those in Alphonsus.

Chuck Wood

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(1) Oppenheimer looks worn down. Is this a really old crater? The wall of the crater showing at the bottom left of the image looks sharp and "fresh" in comparison.

Before the maria appeared, it seems that almost every inch of the Moon was covered in craters of various sizes. Did the impactors come from all directions at once-- striking the Moon while moving inward toward the Sun, while moving outward away from the Sun, and raining down from above and below the Moon's north and south poles, etc.? Or would they have come from a consistent source and direction in the solar system? How did the Moon get hit everywhere and from all sides?


(2) Bill - Yes, it is an older crater, based, as you noticed, on its eroded and low rim. There are craters at all latitude zones on the Moon so projectiles must have come from all directions. Presently most asteroids and about half the comets travel within about 30° of the ecliptic - only long period comets can come from any direction. But when the Moon was formed it was in a ring of debris from the giant impact that ejected Earth materials into space so projectiles probably could hit anywhere.


(3) Are you saying that the dark halo material are pyroclastic deposits? - Andrew Martin SFO

(4) Andrew - There are two kinds of dark halo craters - little volcanoes typically formed on rilles and impact craters that eject buried dark mare material. Almost every dhc inside another crater is volcanic, and almost every dhc not in a crater is an impact.


(5) Chuck – It seems to me that Oppenheimer U may fall in neither of these DHC categories. Oppenheimer H, the 33-km diameter crater on the right (which looks vaguely similar in morphology to "U") appears to have a string of tiny dark dots on left side of its floor that might be the sources of the dark ash within it. I wonder if Kaguya flew over "U" and got a closer look into it, if it might see little vents, like those in "H", giving rise to its blanket of dark deposits? That is, "U" might have a dark halo only because, by chance, the vents within had enough range to throw ash outside its rim, rather than because it is an inherently volcanic structure -- just as the main structure of Alphonsus is not volcanic even though there are volcanic features on its floor.

Also, although the pattern of ash features inside the larger Oppenheimer structure is very reminiscent of the pattern seen in Alphonsus, its location just outside the Apollo impact basin seems quite similar to the placement of the mysterious ash ring (whose source vent is also difficult to pinpoint) to the south of Orientale. Although Oppenheimer U is larger than other dark halo craters, it is much smaller than the Orientale ash ring, so a single suitably (and perhaps accidentally) located vent within it might be sufficient to account for its dark halo. But does the "punch" from the basin impact somehow force ash out through existing cracks to settle on the impact debris? Or did these cracks, and perhaps the impacts that exposed them, and the volcanic sequence that spews ash, occur later?

-- Jim Mosher

Jim - Thanks for your clarifying questions. I did not mean to say that Opp U was a volcanic DHC - it appears to be an impact crater, or rather a floor-fractured one that has been modified by a rise of volcanism under it that raised and cracked the floor. It is most likely that the dark deposits erupted from the rilles, rather than being a buried unit distributed by a chance impact. I played with the brightness of U on the Clementine image and was not able to find much bright signal within the black - it must be a relatively deep pyroclastic deposit. So I couldn't tell is there were small DHC along the rilles inside U. Because most of the dark material is inside U it seems likely that it came from the rilles on U's floor. It doesn't seem like the S Orientale ash ring to me.


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