A mysterious rock has appeared in front of a robotic rover on Mars - and Nasa has no idea how it got there.

The rock popped into existence in front of the six-wheeled rover Opportunity a few days ago, entirely out of the blue.

The "jelly doughnut-sized" rock, nicknamed "Pinnacle Island", is now resting in front of the rover, and scientists are trying to decide what to do.

"Mars keeps throwing new stuff at us!" said Mars Exploration Rover lead scientist Steve Squyres.

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Above: the rock (right) and the view taken by the rover just days before (left)


He told Discovery News:

"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."

So far Nasa has two theories for how it appeared - either the rover flipped the rock over as it rolled forward, or it just landed there after a meteorite impact nearby.

While the rock's appearance is a fluke, it could be useful. Nasa said the side now visible to the robot hasn't seen the surface for perhaps billions of years - meaning it could be important to study.

Opportunity is about to celebrate 10 years on the Red Planet - it was originally designed to last just three months, and has now been operating for more than nine years longer than expected.

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  • This Aug. 9, 2011 image provided by NASA shows a view from the Mars Rover Opportunity from the Western rim of the Endeavour Crater.

  • This undated image provided by NASA shows the Mars rover Opportunity looking back at an outcrop where it spent the Martian winter in 2012.

  • This image provided by NASA shows a rock that the NASA Mars rover Opportunity examined in 2012.

  • This image provided by NASA shows a shadow self-portrait taken by NASA’s Opportunity rover on the Martian surface.

  • This image provided by NASA shows a panoramic view from NASA's Mars Exploration rover Opportunity of "Solander Point."

  • This image provided by NASA shows the late-afternoon shadow cast by the Mars rover Opportunity at Endeavour Crater. The six-wheel rover landed on Mars in January 2004 and is still going strong. (AP Photo/NASA)

  • Handout photo issued by NASA Wednesday 21 January 2004 of a image mosaic taken by the panoramic camera onboard the Mars Exploration Rover Spirit.

  • Nasa undated computer generated image of what the it's Spirit rover would look like on the surface of Mars.

  • This magnified image taken by the Mars Rover Opportunity shows evidence of past flowing water.

  • This magnified image taken by the Mars Rover Opportunity shows evidence of past flowing water.