It's a question that's vexed casual observers, Mars lovers, even curious Redditors and computer engineers: What the heck are those circular markings all over the Curiosity rover? Tattoos?

It's actually less of a mystery than it seems, Chris Leger of JPL, one of the rover's drivers, told The Huffington Post. Leger said the markings are "fiducial marks," or "fiducials" for short.

"They're all so the rover can measure itself," he explained.

The circular markings are a staple of engineering that can serve two parallel purposes, Leger said: to calibrate cameras and to calibrate various mechanisms on the rover. They're a common feature in high-tech photography and robotics here on Earth, but the use of fiducials on extraterrestrial robots is relatively new. The technique started informally on the Mars Exploration Rover mission, which landed Spirit and Opportunity on the Red Planet's surface in 2004.

"We noticed that, over time, the accuracy of our robotic arm was degrading, and we weren't really sure why," Leger said of the previous mission. "So some of the engineers came up with an algorithm that would just recognize a circular feature on the end of the arm. And every day, it could track where that actually was versus where the robot thought it was."

The idea blossomed into more than just an engineering hack with Curiosity.

"I talked to the engineers who specialize in computer vision, and we came up with the design for the fiducials [on Curiosity]," Leger said. "We wanted something that would be easy for either a computer program or a person to accurately pick the center of. Having that intersection in the middle makes it easier for a person -- they can zoom in on the image and click exactly on that intersection -- and [the design] also makes it easy for a computer, because it can compute the center of a circle."

From there, basic trigonometry lets engineers piece together the positions and orientations of the various parts of the car-sized machine.

"We wanted to be able to measure all of the mechanisms on the rover," Leger said. So fiducials are plastered "all over the turret" on the end of Curiosity's arm, as well as on the tops of the steering actuators and on the top deck of the rover.

While there's no catchy nickname yet for the fiducials -- on a mission that's so far coined colorful terms like Bradbury Landing, Gale Crater and Yellowknife Quadrangle -- they do bear a slight resemblance to a certain terrestrial car marque.

"One of the engineers said they look a lot like the BMW logo," Leger said. "Maybe they'll be flattered that the most expensive vehicle in the world has that logo all over it."

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  • This 360-degree panorama provided by NASA Wednesday Aug. 22, 2012 shows evidence of a successful first test drive for NASA's Curiosity rover. The rover made its first move, Wednesday, going forward about 15 feet (4.5 meters), rotating 120 degrees and then reversing about 8 feet (2.5 meters). Curiosity is about 20 feet (6 meters) from its landing site, now named Bradbury Landing. (AP Photo/NASA)

  • MARS CURIOSITY

    This image provided Sunday, Aug. 19, 2012, by NASA shows a close-up view of a Martian rock that the NASA rover Curiosity zapped at using its laser instrument. Curiosity landed on in a giant crater near Mars' equator on Aug. 5, 2012 on a two-year mission to determine whether the environment was habitable. (AP Photo/NASA)

  • This image released on Friday Aug. 17,2012 shows bedrocks that was exposed after Curiosity's rocket stage fired its engines that blew away soil from the Martian surface. The Mars rover is preparing to aim its laser next week at a rock in the first test of the instrument. (AP Photo/NASA)

  • NASA's Jet Propulsion Laboratory logo is seen on a full-resolution image of the Martian surface from the Navigation cameras on NASA's Curiosity rover, which are located on the rover's "head" or mast, are downloaded at the Surface Mission Support Area, SMSA at NASA's JPL in Pasadena, Calif., Thursday, Aug. 9, 2012. The rim of Gale Crater can be seen in the distance beyond the pebbly ground. (AP Photo/Damian Dovarganes)

  • In this image released by NASA on Wednesday, Aug. 8, 2012, a self portrait of NASA's Curiosity rover was taken by its Navigation cameras, located on the now-upright mast. The camera snapped pictures 360-degrees around the rover. (AP Photo/NASA)

  • This image released on Wednesday Aug. 8, 2012 by NASA, shows a mosaic of the first two full-resolution images of the Martian surface from the Navigation cameras on NASA's Curiosity rover. The rim of Gale Crater can be seen in the distance beyond the pebbly ground. The foreground shows two distinct zones of excavation likely carved out by blasts from the rover's descent stage thrusters. (AP Photo/NASA)

  • This image released by NASA on Wednesday Aug. 8, 2012 taken by cameras aboard the Curiosity rover shows the Martian horizon. It's one of dozens of images that will be made into a panorama. Curiosity landed on August 5, 2012 on a two-year mission to study whether its landing site ever could have supported microbial life. (AP Photo/NASA)

  • This image released by NASA on Wednesday Aug. 29,2012 shows Curiosity's wheels after it made its third drive on Mars. The six-wheel rover landed on Aug. 5, 2012 on a mission to study the red planet's environment. (AP Photo/NASA)

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