Any explanation for the differences between these two pics?

http://home.comcast.net/~johang/ChangeBetweenDays.gif


The top image is from Sol 30
http://marsrovers.jpl.nasa.gov/galle...4P2949M1M1.JPG

The bottom image is from Sol 31 and at a differnt time of the day.
http://marsrovers.jpl.nasa.gov/galle...4P2953M2M1.JPG

The most obvious difference is at the edge of the sphere, between 7
and 8 o'clock, but there are also several other apparent movements.

Different light or electrostatic or wind or the rover nudged it or ?
Thanks for anything, just curious.

Stumped in Seattle

-j