A mysterious rock which appeared in front of the Opportunity rover is "like nothing we've ever seen before", according to Mars exploration scientists at Nasa.
Experts said they were "completely confused" by both the origins and makeup of the object, which is currently being investigated by Opportunity's various measuring instruments.
Astronomers noticed the new rock had "appeared" without any explanation on an outcrop which had been empty just days earlier. The rover has been stuck photographing the same region of Mars for more than a month due to bad weather, with scientists at the Jet Propulsion Laboratory (JPL) in California monitoring the images it sends.
Nasa issued a Mars status report entitled "encountering a surprise", and lead Mars Exploration rover scientist Steve Squyres told a JPL event it seems the planet literally "keeps throwing new things at us".
He said the images, from 12 Martian days apart, were from no more than a couple of weeks ago. "We saw this rock just sitting here. It looks white around the edge in the middle and there’s a low spot in the centre that's dark red - it looks like a jelly doughnut.
"And it appeared, just plain appeared at that spot - and we haven't ever driven over that spot."
Squyres said his team had two theories on how the rock got there - that there's "a smoking hole in the ground somewhere nearby" and it was caused by a meteor, or that it was "somehow flicked out of the ground by a wheel” as the rover went by.
"We had driven a metre or two away from here, and I think the idea that somehow we mysteriously flicked it with a wheel is the best explanation," Squyres said.
Yet the story got even stranger when Opportunity investigated further. Squyres explained: "We are as we speak situated with the rover's instruments deployed making measurements of this rock.
"We've taken pictures of both the doughnut and jelly parts, and the got the first data on the composition of the jelly yesterday.
"It's like nothing we've ever seen before," he said. "It's very high in sulphur, it's very high in magnesium, it's got twice as much manganese as we've ever seen in anything on Mars.
"I don't know what any of this means. We're completely confused, and everyone in the team is arguing and fighting (over what it means).
"That's the beauty of this mission... what I've realised is that we will never be finished. There will always be something tantalising, something wonderful just beyond our reach that we didn't quite get to - and that's the nature of exploration."
Squyres was speaking at an event marking the 10th anniversary of the arrival of Opportunity and Spirit on the surface of Mars.
While Spirit lost contact with Earth and was later declared "dead" in 2010, Opportunity has now roamed the planet far in excess of what was originally planned as a three-month mission. Nasa said that with its maximum speed of just 0.05mph, as of "Sol 3547" (15 January 2014) Opportunity had covered just over 24 miles (38km).
Take a look at the Mars Exploration Rover project here:
In pictures: Mars Exploration Rover
In pictures: Mars Exploration Rover
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Mars Rover, Curiosity. After traveling 8 1/2 months and 352 million miles, Curiosity attempted a landing on Mars in 2012
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A self-portrait of the Mars rover Curiosity combines dozens of exposures taken by the rover's Mars Hand Lens Imager (MAHLI) during the 177th Martian day, or sol, of Curiosity's work on the planet Mars
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Mars Science Laboratory Curiosity rover examines a rock on Mars with a set of tools at the end of its arm, which extends about 2 meters (7 feet)
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This image mosaic taken by the panoramic camera on board the Mars Exploration Rover Spirit shows the rover's landing site, the Columbia Memorial Station, at Gusev Crater, Mars
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Views of the Sojourner Mars Rover and surface of Mars Ares Vallis
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A view of Mars southwest of the rover's landing site in the Gusev Crater. The landscape shows little variation in local topography, though a narrow peak only seven to eight kilometers away is visible on the horizon. A circular depression, similar to the one dubbed Sleepy Hollow, can be seen in the foreground
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Mars planet pathfinder vehicle on planet Mars
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Tracks made by Curiosity's tires during its first test drive as seen by Navcam: on board NASA's Mars rover Curiosity on Sol 16
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A little more than two weeks after its arrival on Mars, the $2.5 billion rover, which landed on Mars has performed a battery of tests and appears ready to embark on its two-year mission to explore the Red Planet in the hunt for signs of life
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Tracks made by Curiosity's tires during its first test drive on a mission to explore the Red Planet
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Mars rover Opportunity's robotic arm as it stretched over the surface of Mars
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NASA's Mars Rover Spirit took the first picture from Spirit since problems with communications began a week earlier. The image shows the robotic arm extended to the rock called Adirondack