NASA's Opportunity rover, older brother to the Curiosity rover that landed on Mars last month, has made a discovery that geologists find both puzzling and exciting, the US space agency says.

Opportunity, which has been on the Red Planet since 2004, has come across an outcrop of tiny spheres - up to around three millimetres in diameter - the likes of which scientists have never seen.

Mars rover Curiosity. Credit:NASA

"This is one of the most extraordinary pictures from the whole mission," said Opportunity's principal investigator, Steve Squyres of Cornell University in Ithaca.

"We never have seen such a dense accumulation of spherules in a rock outcrop on Mars."