A rock that suddenly appeared next to the Mars Opportunity rover and resembles the shape and size of a jelly doughnut has completely baffled scientists.

The unusual find was announced at a NASA event on Thursday night to celebrate the robot's 10 years on Mars. The mystery rock was first reported by Discovery News.

According to the rover's principal investigator, Steve Squyres, the rock is white around the outside, and in the middle is a lower spot that's dark red.

"It looks like a jelly doughnut," Squyres excitedly told a packed room. "It just plain appeared at that spot," he added.

Squyres and colleagues have two theories on how the rock got there. The more likely scenario is that one of the rover's wheels "flicked it" as it was driving about a meter from where the rock now sits. Squyres also suggested that the rock could be a piece of material that was sprayed out from a nearby crater impact.

The composition of the "jelly" is like "nothing we've ever seen before," Squyres said. It's very high in sulfur and has twice as much manganese than anything scientists have previously analyzed on Mars.

Scientists will continue to study the rock, including how it got there and what it's made of.

“It obligingly turned upside down," Squyres told Discovery News, "so we’re seeing a side that hasn’t seen the Martian atmosphere in billions of years and there it is for us to investigate."

The Opportunity rover and its twin, Spirit, arrived on the Red Planet in 2004. The robots were only designed for a three-month mission, but operated much longer than that. Spirit stopped sending signals back to Earth in 2010, but Opportunity is still kicking after a decade on Mars.