In the badastronomy.com forum Daniel Crotty presents an image (below) showing white colored material on *top* of rocks. This would argue against the interpretation suggested by Steve Squyres that the sulfates seen in Gusev might stem from water percolating up from the subsurface. Instead it would suggest sulfates formed from liquid water precipitation(rain) or liquid water droplets in a humid atmosphere.
I wondered whether white material on the rocks could be due to the dust kicked up by a RATTing but Crotty gives other images with no ratting nearby that also shows white rocks:

Dusty, bright material around Spirit, Sol 165
http://badastronomy.com/phpBB/viewtopic.php?t=14573&sid=7f6b82d247999d00a5c7c2db7756cee4

Bob Clark

Uncalibrated color image by Daniel Crotty.