Mars rocks
Nasa baffled as rock 'appears' on MarsNasa

Scientists at Nasa are scratching their heads after the Martian rover Opportunity sent back images of a rock that has apparently materialised from nowhere – just a few feet from the robotic probe. So far scientists have been unable to explain how the rock appeared and where it comes from.

In one photograph, taken by the six-wheeled rover's panoramic camera or Pancam, all that can be seen is bare bedrock. Yet in a later photograph, taken by the same camera at the same location a bit later on, a doughnut-shaped rock suddenly appears.

The first image was taken on "sol 3528" of the mission and the second on "sol 3540". A sol is a Martian day, which is a few minutes longer than a day on Earth – on average 24 hours 39 minutes.

Originally scheduled to spend just three months on the Red Planet, Opportunity has now rolled across the bleak Martian surface for over 10 years, covering a distance of 23 miles and sending back photographs and data, including the identification of claylike rock particles that may indicate the presence of water. Opportunity has been in its current position on the rim of Opportunity Crater for over a month, stranded by dust storms.

"It's about the size of a jelly doughnut," said NASA Mars Exploration Rover lead scientist Steve Squyres. "It was a total surprise, we were like 'wait a second, that wasn't there before, it can't be right. ' We were absolutely startled."

So far scientists have two theories to explain the appearance of the mystery rock, which has been labelled "Pinnacle Island".

Either the rock was knocked into view by meteor impact, or "impact ejecta", or the rover itself caused the rock to shift as it manoeuvred, perhaps by "tiddly-winking" the stone with a rubber tyre. A more exciting possibility – that the rock was thrown at the rover by Martians – has yet to be ruled out.

However the rock came to be there, scientists are excited by the chance to study the underside of a rock that may have been hidden for billions of years.