A mysterious rock which appeared in front of the Opportunity rover is “like nothing we’ve ever seen before”, according to Mars exploration scientists at Nasa.

Experts said they were “completely confused” by both the origins and makeup of the object, which is currently being investigated by Opportunity’s various measuring instruments.

Astronomers noticed the new rock had “appeared” without any explanation on an outcrop which had been empty just days earlier. The rover has been stuck photographing the same region of Mars for more than a month due to bad weather, with scientists at the Jet Propulsion Laboratory (JPL) in California monitoring the images it sends.