Oh, and it wasn't there one day, but it was the next.
Investigate Pinnacle rock
The easy explanation is that it was kicked up by one of the NASA rover's wheels. The more fun one is that's it's a crumpled piece of paper, a memo dropped by the director of John Carter, and it says, "This movie is really going to suck."
No rock (left): Pinnacle Island (right)
But the episode is a reminder that space missions, even in area we thought we new, and going to be filled with surprises.