The asymmetric tread pattern on the wheels of the Curiosity rover spell out the initials “JPL” in Morse code. (• - - -, • - - •, • - • •) The distinct imprint left in the soil by these holes, referred to as “visual odometry markers”, will be used by onboard systems to help verify distances traveled. If the rover detects that it is not covering the expected distance on a given rout (indicating a loss in traction) it will automatically stop and radio home for further instruction. This precaution is taken to avoid any possibility of the rover getting stuck in soft soil, as Spirit did permanently in May of 2009.