September 13, 2008

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Survival of the Fittest

image by Damian Peach. Darwin is the large crater on the left.

It's been four years since LPOD featured Darwin and its rilles, so here is an update from Damian. The crater is old, but it was also instantly aged by having the bad luck of pre-existing near where the Orientale Basin formed about 3.8 billion years ago. Orientale is only a short distance due west and its ejecta and seismic shaking must have profoundly modified Darwin - certainly its northern rim and most the floor are draped with debris. The lineated ridge-like material at the upper right on Darwin's floor is ejecta that apparently had been flowing across the surface and crumpled when it ran into the eastern wall. The rille that diagonally cuts Darwin is an offshoot of the long Sirsalis Rille on the right side of the image. Less well preserved rilles parallel the Darwin Rille to its north and south. I don't know what tectonic force cause these rilles to form, heading in this direction. But they can be placed in a geologic context. They must have formed after the Orientale onslaught or they would be more completely buried. The southern-most of the three Darwin rilles nearly disappears where it crosses a smooth patch on the southern floor of Darwin. That smooth stuff seems to be superimposed on Orientale ejecta so it must be younger, and it also must be younger than the faint rille. The smooth stuff is generally interpreted to be another type of ejecta from Orientale. If so, the rilles must also have formed about the time the basin did. That seems unlikely - are there other interpretations? And the famous dome in Darwin, is no where to be seen in this low Sun, high quality shot...

Chuck Wood

Technical Details
2006/04/12. C14 @ F41. LU075M.

Related Links
Rükl plate 50
Damian's website

Yesterday's LPOD: Tycho & Tokyo

Tomorrow's LPOD: Youmoon


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