9659005] Most of the tread on the rover's wheels is a simple but effective zig-zag (chevron) pattern designed to provide a long lasting tread with good purchasing power on the dry and loose Martian soil. The whole wheel does not have the same thread, however, a small segment of each wheel tread is devoted to a series of cutouts. The pattern of the cutouts ”.— / .–. / .- .. ”spells out the initials of NASA's Jet Propulsion Laboratory via Morse code — JPL. As the rover explores Mars, it imprints the letters, leaving a node to its birthplace wherever it goes.
While the markings might be part of a project, they also serve a useful function. When the onboard cameras examine the tread left by the wheels, the different pattern introduced by the Morse code offers a sense of scale researchers can use as a reference to show distance, even if the terrain in the photograph is featureless.