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