image by Bob Pilz

Look carefully at the inner walls of Aristarchus. Notice anything unusual? Not the bright spot on the near rim which is where a ray was deposited. Look at the opposite rim where a terrace near the top of the rim bends sharply down and about halfway down the rim turns up and stops. The terrace directly below it repeats the Z shape. Terraces don't do this. They are places where a large, fairly coherent mass of rim material slides downslope as a unit. Often terraces can be traced halfway or sometimes nearly all the way around the rim, showing that the whole annular block slid down at the same time - what a sight! The Zs show up in Alan Friedman's image and a similar phase one by Wes Higgins. Strangely, I have not yet been able to find a spacecraft image with the right lighting to show the Zs. The famous Lunar Orbiter IV-150 shot has a high opposite lighting, clearly revealing the wall, but the Zs are not apparent. No better luck with Clementine, nor Apollo. Although it looks real I am beginning to think the Zs are a trick of lighting, rather than bending terraces. Who can come up with images to resolve this uncertainty?

Chuck Wood

Technical Details
2009/09/13, UT: ~10:47. 200mm f/6 Newtonian, Televue 4X Barlow, DMK 21BF04 camera, Blue filter, 15 fps, Exposure 1/30 sec, 800/18000 frames. Processed by Registax V5, PS CS4, FocusMagic. Taken from Lat: 35 degrees 36 minutes N, Long: 82 degrees 33 minutes W, Elev: ~850m.

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