re: Wouldn't sub-surface water cause them to corrode?
Possibly. But the possibility was raised at the Thursday news conference it was small amounts of water involved which would give them a sheen though might not corrode them.
Another possibility is that the shine is due to rock varnish, also called desert varnish.
There have been observed shiny rocks at both the Viking and Pathfinder landing sites that were attributed to varnish.
Rock varnish is believed by some to be completely of biological origin. Others believe it can have a chemical origin.
Bob Clark