This image of terrain inside Mars' Gale Crater and the inset of the calibration target for the Mast Camera (Mastcam) on NASA's Mars rover Curiosity illustrate how the calibration target aids researchers in adjusting images to estimate "natural" color, or approximately what the colors would look like if we were to view the scene ourselves on Mars, using the known colors of materials on the target.
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Credit: NASA/JPL-Caltech