September 29, 2008
image by Kaguya Image Gallery (select the 2008/09/26 item)
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Gravity pulls matter downslope. On the Earth we sometimes see aprons of sediments and rock that have flowed down the sides of mountains, with gravity, often aided by the lubricant of flowing water, providing the motive force. As everyone knows, on the Moon gravity is only one sixth what it is on Earth, but it still causes material to move downhill. The most dramatic examples are the walls of complex craters, which collapsed into terraces or more ragged slumps into the giant holes excavated by impact. The very high resolution Kaguya Terrain Camera images of Mount Pico (featured obliquely on LPOD Sept 27) captured a number of examples of impact craters that happened to be formed near the edge of the mountain in the process of being devoured by it. A great example is near the center of the frame where a 350 m wide crater is half gone. Material is continuously (at a very low rate - otherwise 3.8 billion year old mountains like this would not still exist) being dislodged from the face of the peak and avalanching downslope. When this occurs, material that was previously on the surface of the mountain and darkened by cosmic and solar radiation slides down, revealing bright unradiated rock, which over hundreds of millions of years will itself become darkener. Sometimes, as can be glimpsed here, small impact craters dislodge bits of the peak's surface, starting the avalanche. Other times it is probably seismic shaking from nearby impacts or other Moonquakes. If you look around the edge of the peak using the full resolution image you will see more examples of a mountain eating a crater.
I am very pleased that my friend Sandi noticed this already!
Rükl plate 11
Yesterday's LPOD: Secondary Matters
Tomorrow's LPOD: Stages of Destruction