This is an even more likely reason. I'd forgotten that MER had a stereo camera. If you look at a photo of the rover (https://en.wikipedia.org/wiki/Mars_Explorat

This is an even more likely reason. I'd forgotten that MER had a stereo camera. If you look at a photo of the rover (https://en.wikipedia.org/wiki/Mars_Exploration_Rover https://en.wikipedia.org/wiki/Mars_Exploration_Rover), you can see the stereo camera on the end of the arm. I'm changing my theory to this.