NASA's $820 million exploration rovers Spirit and Opportunity trawling Mars for signs of water and therefore, signs of life, appears to have left behind a rather unique of its own.

While the Jet Propulsion Lab's pair were driving around, they left criss-crossing tracks in the sand on the Red Planet, which appear to sketch out a crude drawing of an erect male penis and testicles.

The hilarious image, which is believed to be real, was first spotted by a Redditor on NASA's website, where mischievous engineers had even placed it on one of the mission's homepages - which promptly crashed the site. We note the page allows visitors to "enlarge" the image which surely generated a few laughs.

The picture was shared on Reddit with the caption: 'Mars Rover = $800m, Team to operate = $1bn. Drawing a penis on the surface of another planet = priceless.'

Although the rover's on Mars are able to drive themselves around, engineers of earth control their movements via 'satnav' maps which are periodically uploaded to the rovers.

Spirit and Opportunity were sent out to Mars in 2003 and safely landed in 2004. However, the agency later lost contact with the Spirit rover in 2010 after it got stuck in the sands, but Opportunity is still going strong, wandering around a crater named Endeavour and heading towards an area dubbed Botany Bay - near a spot called Nobbys Head. It is not clear which of the rovers, which are programmed by Nasa scientists on Earth, left the pattern behind, or when.