Though the couple billion-dollar Curiosity rover seems to get all the attention on the red planet these days, it’s one of the veteran rovers on Mars that captured an event that is stunning scientists and space fans alike.

The crux of the issue surrounds these photos snapped by the rover Opportunity on two different missions.

sol3528 and sol3540

The image on the left was taken on the SOL3528 mission while the photo on the right, showing the new rock, was taken of the same area in the SOL3540 mission. (Images: NASA/JPL-CALTECH)

The rock perched on the ground in the right photo in the side-by-side comparison was not there before.

“It was a total surprise, we were like ‘wait a second, that wasn’t there before, it can’t be right. Oh my god! It wasn’t there before!’ We were absolutely startled,” Steve Squyres with Cornell University and working out of NASA’s Jet Propulsion Lab said, according to Discovery News, comparing the rock to the size of a jelly doughnut.

The news came at CalTech where scientists met this week for “10 years of roving Mars,” an event honoring the rovers Opportunity and Spirit, which is now defunct.

The two current options as to how the rock, dubbed Pinnacle Island, could have gotten there include: 1) being nearby all along and only becoming visible as the rover moved around disturbing it or, 2) it landed in this place as the result of a meteorite impact. Discovery News reported that the former theory is more likely.

“[…] I think it happened when the rover did a turn in place a meter or two from where this rock now lies,” Squyres said.

Squyres explained further why he thinks this is the more likely option. Discovery News reported:

Opportunity’s front right steering actuator has stopped working, so Squyres identified that as the possible culprit behind the whole mystery.

Each wheel on the rover has its own actuator. Should an actuator jam or otherwise fail, the robot’s mobility can suffer. In the case of this wheel, it can no longer turn left or right. “So if you do a turn in place on bedrock,” continued Squyres, “as you turn that wheel across the rock, it’s gonna kinda ‘chatter.’” This jittery motion across the bedrock may have propelled the rock out of place, “tiddlywinking” the object from its location and flipping it a few feet away from the rover.

An investigation to figure out how the rock truly got there is underway.