Neverfly,

Great question. The upper left is a rapid transition zone into the crater. This arch also correlates with a change in sheen. This sheen change in a negative tilt means wet to me. The short answer is this is a micro-topographic effect.

This link should give you a better understanding as you can see the image in context

http://flickr.com/photo_zoom.gne?id=...8997185&size=l

dfrank