OOPs!
This analysis only makes sense if the images were taken within a few minutes of each other, if they were taken hours or days apart, then all bets are off. I first thought these were views of the moon, where shadow positions move very slowly. If it is Mars, then non-sequential images would exhibit different illuminations.
UNLESS...images taken days apart were deliberately selected (or the synchronicity of the orbit chosen) so that sun angles would always be the same.
