Ceres, Part 2

Circling Occator Crater on Ceres (Silent)
The crater is about 60 miles in diameter and about 2 miles deep.
Space probe Dawn has moved closer to Ceres and is now orbiting at an altitude of 900 miles. Dawn has been busy doing what space probes do, which is taking pictures, so we have a better picture of the two white spots that showed up when Dawn arrived here six months ago. Still don't know what they are, but they appear to be near the center of a large impact crater. So once upon a time Ceres got hit with a pretty big rock. A collision like that releases a lot of energy in the form of heat, possibly enough to liquefy rock and shoot a stream back out into space. But what is the white stuff? It could be ice, but water ice would sublimate (evaporate), so you would need a pretty good size deposit for it to survive how many millions (billions?) of years since the big crunch. It could be some other kind of ice, but you still have the same problem. If it is some kind of solid, how has it managed to not become mixed in with the surrounding rock? We find mineral deposits on the Earth that are relatively pure. The only one I can think of that would be white is salt, and for some reason that seems unlikely. I guess I've never heard anyone suggest that they had found salt anyplace outside of Earth.

Part 1 here.