White rings around the craters...duh!
I was looking at them as though the ring was on the outside of the crater rim. Without 3-D, it's easy to see an "innie" as an "outie" when looking at features.
The white rings around the rims of all the craters in this area are obviously the bedrock that is exposed at the rim when the craters are formed. Our own little crater would have such a "ring" if seen from overhead.