July 18, 2008

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And the Walls Came Tumbling Down

image by Kaguya Image Gallery, JAXA/SELENE

With this image you feel you can walk the rim of Tycho. The crestline is the upper edge of the crater rim scarp, with mounds of rock below. Some collapsed as a huge concentric slab creating the main terraces. The roundness of the terraces is unexpected - I think it is because they have largely been coated with impact melt that was ejected during the crater formation and then rained back to the surface. Just at the base of the rim scarp at lower left you can see two parallel ribbons of melt that flow parallel to the scarp. It also looks like some parts of the scarp are covered by melt - the smooth parts - and other places are uncovered with boulders jutting out of the wall. There are two quasi-terraces, the one in the middle is very rounded and seems to be covered by melt nearly everywhere. The lower terrace has more texture and seems to be cut by narrow radial gullies - did the melt carve them? On the bottom right along the lower terrace there is banding that parallels the edge of the floor. I wonder if this is a high water mark - if the impact melt pond on the floor was originally that high and then subsided. Lava flows in Hawaii have been observed to do this. When they are erupted they are full of gases that inflate them.As the gases escape the flow tops fall in on themselves. If that is what happened here it implies a lot of gas stayed in the melts as they were ejected and fell back to the surface - that seems unlikely! Note the trail of boulders near the bottom of the image. I thought at first that this was formed by a boulder that had been ejected and fell here, but you can see along the right side of the boulder trail that boulders are lined up suggesting they are part of a deeper rock unit. The fact that the impact melt, which is emplaced as part of the crater formation, is not disrupted (at least right here), shows that the modification of the crater to form terraces must have occurred during crater formation, not at some later time. Wow! I am anxious to walk along more of Tycho's rim!

Chuck Wood

Technical Details
Now that you have looked closely at this piece of the rim do the fly over again to interpret other parts too.

Related Links
Rükl plate 64

Yesterday's LPOD: Transformative Image

Tomorrow's LPOD: Dog of the Moon


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