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.
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.