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.
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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