Nasa thinks it might know what caused a 'mysterious rock' to suddenly appear in front of its Opportunity rover.

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.

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

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


But while the rock's appearance was a surprise, it now seems clear what caused it - NASA.

The space agency said it had narrowed down the time of the rock's appearance to a period of just four days - Sols 3536 and 3540 to be precise.

James Rice, science team member of the Mars Exploration Rover Project, said that the date range corresponds to a manoeuvre carried out by the rover known as a "turn in place and bump".

As the rover turned, it apparently popped out the rock. Though that's not quite the end of the mystery.

Rice told SpaceCoalition.com:

"We popped it out. But we are still trying to figure this out along with where it originally was located (I have a candidate divot) and then the bigger question: what is this rock?"

He added that the rock has a very unusual composition, and definitely warrants further study.

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.