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Mars Exploration Rovers sundial A calibration target in the form of a sundial is mounted on the Mars Spirit and Opportunity rovers. Note the four colors in the corners. Pictures taken of the Martian landscape that have the sundial in the image are used to calibrate the color in images taken by the rover cameras. The shadow cast by the sundial also helps to measure the difference in lighting effects from the Sun and atmosphere, allowing the intrinsic color of objects to be determined. NASA / JPL-Caltech / Cornell