But hold on, these alien markings belong to Earthlings. They are the byproducts of the Rock Abrasion Tool. That hardware -- affectionately called the RAT -- is carried by both rovers as they wheel about performing scientific duties at Gusev Crater and Meridiani Planum.

The RAT has been busy at work, grinding out science data unique in the history of space exploration. It is also part of a mystery with scientists and the RAT builders scratching their collective heads a bit.

The RAT was built by Honeybee Robotics, based in New York City. The robotically-controlled abrasion tool weighs a modest 1.5 pounds (about 685 grams) and measures 2.7 inches (7 centimeters) around by 4 inches (10 centimeters) long.

The primary purpose of the RAT is to remove dust and crust from martian rocks to reveal fresh underlying features. All operations of the RAT are performed autonomously, with commands to position and run the hardware sent from the Jet Propulsion Laboratory (JPL) in Pasadena, California.

Both Spirit and Opportunity have used the RAT, leaving circle patterns in target rocks.

"Maybe we are leaving something like crop circles for Martians to find thousands of years from now," said Honeybee Chairman, Stephen Gorevan, and Payload Lead for the RAT on the Mars Exploration Rover mission.

For the moment, there have been eight successful uses of the RAT on targets, Gorevan told SPACE.com. "This is already beyond what we thought we would do for the entire nominal mission. We are very pleased the RAT has worked so well and been an important part of the science path to discovery."

While the RAT has proven invaluable to scientists, it may be part of a baffling turn of events on Mars.

Smack dab in one Microscopic Imager picture relayed from the now known to be water-soaked Opportunity site is an odd, Rotini pasta-shaped object. When first seen by rover scientists, it caused a bit of a stir given its mini-fossil-like looks.

It still remains unidentified. But there may be a non-extraterrestrial explanation as to the nature of this eye-catching feature.

"Our first assessment was that we did not recall any previous RAT test that revealed a similar structure," said Gorevan. "But we are far from concluding that the RAT could not have been responsible for this feature. As it is impossible to prove a negative we will always come up short on certainty."  

Gorevan said that perhaps the RAT target or portion of the target has a gummy texture - it just might produce segmented curl cuttings.  

"A gummy texture may not be as unlikely as it sounds. For engineers familiar with drilling in cold regions, permafrost is surprisingly notorious for its difficult 'gummy' texture. Subjects for RAT testing did not include permafrost so we have no comparison images available," Gorevan explained.

Gorevan said that there have been a number of 'features' in the RAT holes produced so far. Those features are being studied to determine if they came about through the physical action of the RAT.   

There was, for example, a horizontal structure present in the one of the spherules sliced in the first time use of the RAT on Opportunity, Gorevan said.

"At first the geometry tended to lead us to a conclusion that it was unlikely that the RAT produced this structure. This was important as a naturally produced horizontal structure in the spherules could have some bearing as to whether the spherules are concretions," Gorevan added.

Concretions form as water flows through a rock, carrying dissolved bits of sediment along with it.

In this case, further analysis of the RAT telemetry and the Microscopic Imager revealed that the RAT had indeed produced the horizontal structure in that spherule, Gorevan concluded.