A strange rock that looks like a jelly doughnut has appeared on Mars, and scientists are closer to figuring out how it got there, a top NASA expert said Thursday.

The small, round object suddenly popped up in pictures taken 12 days apart by the US space agency's decade-old Opportunity rover.

On December 26, 2013, it was not there. On January 8, it was. But what is it?

"It looks like a jelly doughnut, white around the outside, red in the middle," said Steve Squyres, the principal investigator of the Mars Exploration Rovers.

He described the tint as a "weird deep red color, not a Martian kind of red," which is more of a rusty hue.

One thing is for sure. This is not a fried, sugary pastry.

"We have looked at it with our microscope. It is clearly a rock," he told reporters in a briefing to mark 10 years since NASA's solar-powered Opportunity landed on the Red Planet.

But it is a kind of rock no Earthling has seen before.

Squyres said scientists believe the rock, named "Pinnacle Island," got there when the aging rover did a pirouette turn in the dusty Martian soil and knocked loose a chunk of bedrock that rolled a short distance downhill.

"We think that in the process of that wheel moving across the ground, we kind of flicked it, kind of tiddly winked it out of the ground and it moved to the location where we see it," Squyres said.

Still, scientists have not found the divet the rock would have left behind. They think it is hidden beneath one of the rover's solar arrays.

The Opportunity team plans to manuever the robotic vehicle around a bit more to see if they can find the spot from which the rock emerged.

Exposing history

As to why it is such an unusual color, Squyres said it may be that humans are witnessing a surface that has not been exposed in a very, very long time.

"It appears that it may have flipped itself upside down," he said.

"If that is the case, what we are seeing is we are seeing the surface, the underside of a rock, that hasn't seen the Martian atmosphere for perhaps billions of years."

Already, an analysis of the rock with the Opportunity's spectrometer has shown a "strange composition, different from anything we have seen before," he told reporters.

The rock has a lot of sulphur, along with very high concentrations of manganese and magnesium.

"We are still working this out. We are making measurements right now. This is an ongoing story of discovery," he said.

Opportunity is one of two Mars Exploration Rovers. Its companion, Spirit, stopped communicating with Earth in 2010.

Both have lived long beyond their planned 90-day missions and have made important discoveries about water on Mars and environments that might have supported microbial life in the distant past.

John Callas, Mars Exploration Rovers project manager at NASA's Jet Propulsion Laboratory, said Opportunity is "still in very good health."

The rover has an arthritic arm, a hobbled front wheel and two broken science instruments, but has stayed relatively stable over the past two years, he said.

The Martian wind periodically clears dust off its solar arrays, allowing it to keep pressing on.

It is now exploring the rim of a 14-mile (22-kilometer) wide crater called Endeavour.

It younger and more robust cousin, a NASA rover called Curiosity, landed on the Red Planet in 2012.