Almost 10 years ago, Opportunity found what NASA nicknamed “blueberries” on Mars — small, iron-rich spheres that littered the planet’s surface and hinted at the presence of ancient water. Now the veteran rover is celebrating a decade of operation with its discovery of a mysterious “jelly doughnut rock.”

The pastry-sized rock, strangely appearing where there was nothing days earlier, is unlike anything scientists have ever seen before on the Red Planet.

“It’s white around the outside, in the middle there’s kind of a low spot that’s dark red: It looks like a jelly doughnut,” said Cornell University astronomer Steven Squyres, the principal investigator of Mars Exploration Rover Mission. He announced the finding last week at a celebratory event at the California Institute of Technology in Pasadena to honor the rover’s decade anniversary.

The rock turned up a few weeks ago on the rim of Endeavour Crater, a spot called Murray Ridge, where Opportunity is spending its sixth Martian winter. Before-and-after photos show the same patch of ground, riddled with pebbles and craggy stone. But the after photo, taken 12 days later, depicts the stark arrival of a white-and-maroon oddity.

“It appeared — it just plain appeared at that spot,” Squyres said. NASA has dubbed it “Pinnacle Island.”

He has two theories about how the rock got there, although neither is quite as exciting as some might hope. One possibility is that a nearby impact could have flung a piece of debris Opportunity’s way. Or, in what Squyres believes is the more likely explanation, one of the rover’s six wheels flicked it up out of the ground.

Scientists believe that Pinnacle Island landed upside down, giving them a serendipitous glimpse of the underside of a Martian rock that may not have been exposed to the atmosphere for billions of years. The NASA team is closely inspecting its composition, but so far, this rock is nothing like others they have seen.

“We’ve taken pictures of both the doughnut part and the jelly part,” he said at the event. “We got our first data on the composition of the jelly yesterday.”

The dark-red portion has lots of sulfur and magnesium, as well as twice as much manganese as anything previously measured on Mars. The results have deeply confused NASA scientists, Squyres said and have inspired heated debates about what this could mean.

The team has also released new Opportunity results, published in the journal Science on Thursday, of an ancient clay-forming, subsurface aqueous environment at the site of Endeavour Crater. Some of the exposed rocks are the oldest materials investigated to date by Opportunity, from a time when Mars would have been habitable.

The water present before the impact may have been nearly neutral to slightly acidic, harboring conditions that would have been favorable to life. The study complements the younger Curiosity rover’s recent findings of an early lake that once contained “drinkable” water.

Both rovers contribute to an emerging picture of Martian history: from a warmer, wetter environment with neutral waters several billion years ago, to diminished water activity that became more acidic. Then in the past 3 billion years, Mars turned into a very dry place.

Originally set for a mission of 90 days, Opportunity on Saturday will celebrate the 10-year anniversary of its arrival on the planet. In that decade, the rover has trekked almost 25 miles along the Martian soil. However, the trusty robot is starting to show its age.

“The front steering actuator is jammed, and the robotic arm has some arthritis to it,” said Mars Exploration Rovers project manager John Callas at a news conference Thursday. He also described Opportunity as “having a senior moment” due to problems with its flash memory. But all in all, the team has been pleasantly surprised at its long lifetime and numerous contributions to our understanding of Mars.

In its first year, near the landing site, Opportunity discovered what NASA scientists nicknamed “blueberries” for their size and spherical shape. They were scattered all over the surface and even embedded in rocks, like a blueberry muffin. The team thought the orbs were a possible sign of liquid water, because analysis revealed they were made of hematite, an iron oxide that typically forms in an aqueous environment.

“One of the things I like to say is that Mars keeps throwing new things at us,” Squyres said. “That’s the nature of exploration.”

Kim is a freelance science journalist based in Philadelphia.