The scientists behind NASA's Mars exploration mission are "completely confused" by the sudden appearance of a mysterious rock on the surface of the Red Planet.

During a briefing at the Jet Propulsion Laboratory (JPL) in California last week, Mars Exploration rover lead scientist Steve Squyres said the rock recently appeared, apparently out of nowhere, near the Opportunity rover.

"It's white around the outside, in the middle there's kind of a low spot that's dark red. It looks like a jelly doughnut," Squyres said of the rock photographed in a location where nothing had been 12 Martian days earlier.

"And it appeared, it just plain appeared at that spot and we haven't driven over that spot."

While Opportunity continues to investigate the composition of the mystery rock, Squyres said NASA's Mars scientists have two leading theories about where it may have come from.

"One is that we somehow flicked it with a wheel," he said, explaining that Opportunity had driven "a metre or two away."

While he suggested that's the more likely scenario, "The other is that there's a smoking hole in the ground somewhere nearby and this is a piece of crater reject."

Based on initial data retrieved by Opportunity's instruments, Squyres said the 'jelly' portion of the rock is, "like nothing we've ever seen before. It's very high in sulphur, it's very high in magnesium, it's got twice as much manganese as we've ever seen in anything on Mars."

The mystery is thrilling, he added.

"I don't know what any of this means. We're completely confused, we're having a wonderful time. Everybody is arguing and fighting... and that's the beauty of this mission."

The Opportunity rover, as well as the now "dead" Spirit rover, arrived on the surface of Mars ten years ago.

Opportunity continues to beam data back to Earth, despite being originally slated for a three-month mission.