re: It's a big mystery MORE
At a press conference I heard someone say that the 'dusting off' occurred episodically, especially while Opp'y was in the big crater -- the dust would slowly accumulate, then - swipe! -- in the morning it was reduced by some amount, then a few weeks of accumulation and another sudden partial cleaning. The presumption was episodic encounters with high-speed night-time winds -- perhaps dust devils.