re: How about once liquid?
Hi, REB!
Ok, once liquid is good. :)
1. Too deep to see.
2. Filled with dark material, lava included.
3. Probably as Nick proposed in the middle of all this... too bright a material to be imaged. IF that's the case, then I'm thinking 'ice'... frozen liquid. Now, the original images show a brightness towards the right on the rim of the craters. It is possible that the sun had some reflective properties, but my mind tells me that the crater would have to be full to the rim with a frozen liquid in order to give off such an impression.
That's it for me on this one too. :}