re: why bright?
In the lecture last night, Dr. Richard Zurek stated that it would be a matter of days before MRO put its instruments (especially CRIS) on the light material. They should be able to tell if it is salt or ice or what.
In the lecture, he gave a short list of possible mechanisms to produce the phenomenon and he did include CO2 on the list. He does favor the water solution because of the shape of the gullies, but he knows that the theory is not proven.