NASA's Curiosity rover may not look like an urban menace, but the robot explorer will in fact be steadily tagging the Martian surface as it trundles, leaving a name-check of the Jet Propulsion Laboratory back home. The clandestine graffiti is thanks to part of the rover's visual odometry system, John Graham-Cumming points out, which tracks the marks left by a series of asymmetrically arranged holes in the wheels. The position of those holes, however, isn't random: in fact, it's Morse Code.
In fact, there are three sets of notches, which progressively leave three rows of dots and dashes in the Mars grit. That pattern is ". - - -"; ". - - ."; ". - . ." or, translated into English, "JPL", the acronym for the Jet Propulsion Laboratory.
Unlike the footprints left on the moon by the Apollo astronauts, and which remain to this day thanks to the absence of atmosphere, Curiosity's tire marks won't stick around for long. Mars' atmosphere may be thinner than that of Earth, but that means thermal build-up is quick, causing sea-breeze style winds as gases rush from areas of different temperature.
However, there's also the potential for far more vigorous dust storms which could easily erase Curiosity's trail. Storms reaching in excess of 100mph have been observed on Mars, and indeed it was violent dust storms back in 2007 which left the Spirit and Opportunity Mars Exploration Rovers seriously short of power as layers of Martian detritus covered their solar panels and filled the air.