This image shows a close-up of track marks left by NASA’s Curiosity rover. Holes in the rover’s wheels, seen here in this view, leave imprints in the tracks that can be used to help the rover drive more accurately. The imprint is in fact Morse code for JPL, which is short for NASA’s Jet Propulsion Laboratory in Pasadena, Calif., where the rover was built and the mission is managed.

Curiosity’s “visual odometry” software measures terrain features — such as rocks, rock shadows and patterns in the rover tracks — to determine the precise distance between drive steps. Knowing how far it has traveled is important for measuring any wheel slippage that may have occurred, for instance due to high slopes or sandy ground. Fine-grained terrains generally lack interesting features, so Curiosity can make its own features using its wheel tracks.

The Morse code, imprinted on all six wheels, is: .— (J), .–. (P), and .-.. (L).

from https://www.nasa.gov/mission_pages/msl/multimedia/pia16112.html