Object moving on Mars (photos by Spirit)

Hello, can anyone explain me what is the moving object in this photo sequence?


For reference:
Photo 1: 1830 http://marsrovers.jpl.nasa.gov/gallery/all/2/n/1830/2N288828221EFFB000P0625L0M4.JPG

Photo 2: 1833 http://marsrovers.jpl.nasa.gov/gallery/all/2/n/1833/2N289093752EFFB074P1985L0M1.JPG

Photo 3: 1836 http://marsrovers.jpl.nasa.gov/gallery/all/2/n/1836/2N289363474EFFB0A1P1985R0M1.JPG

Photo 4: 1843 http://marsrovers.jpl.nasa.gov/gallery/all/2/n/1843/2N289976994EFFB0EOP0675L0M1.JPG

Photo 5: Sol 1830 left http://marsrovers.jpl.nasa.gov/gallery/all/2/n/1830/2N288828136EFFB000P0625R0M1.JPG


I have looked at a few of these raw images and it does look like something has moved. Not sure if it is just angle of view or something it would be nice to read some other views on the subject.