What's that red jelly doughnut-shaped thing doing on the red planet?

'It's like nothing we've ever seen before.'- Steve Squyres, lead Mars Exploration rover scientists

That's what befuddled NASA engineers are asking themselves, after the Mars rover Opportunity snapped photos of a mysterious rock they say materialized recently in front of the automated explorer.

"It looks white around the edge in the middle and there's a low spot in the centre that's dark red," lead Mars Exploration rover scientist Steve Squyres told scientists last week at a Jet Propulsion Laboratory anniversary event.

"It looks like a jelly doughnut."

It's also about the size of one of the ringed pastries.

Images captured 12 Martian days before the strange sighting show an empty outcrop. Then, according to Squyres, the rock "just plain appeared" in front of Opportunity's field of vision.

'Total surprise'

Squyres said the rovers have never rolled over that spot.

Of all the stunning images fed back to Earth so far from the Martian surface, the discovery of this rock — now named Pinnacle Island — was perhaps the most startling.

Opportunity rover

Launched in July 2004, the Opportunity rover's initial mission was planned to last only 90 solar days (a Martian sol is equivalent to about 24 hours and 40 minutes).

"It was a total surprise," Squyres said. "We were like, 'Wait a second, that wasn't there before, it can't be right. Oh my God! It wasn't there before!'"

When Opportunity's instruments took readings and measurements of the weird celestial object, the engineers were even more puzzled by data they received about the composition of what Squyres described as "the jelly" filling.

"It's like nothing we've ever seen before," he said. “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."

2 theories

Although Squyres said NASA scientists have been arguing about what this all means, he said two prevailing theories have emerged about how the doughnut ended up in Opportunity's path.

One possibility is the wheels of the rover may have flicked the object, flinging it a metre or two away. Another is that "a smoking hole" emerged somewhere on the planet's surface after a meteor struck the area.

He believes the likely explanation is the rock was displaced by Opportunity's movements.

One way or another, Squyres said the mysteries of Mars continue to fascinate researchers.

“That’s the beauty of this mission," he said. "What I’ve realized is that we will never be finished. There will always be something tantalizing, something wonderful just beyond our reach that we didn’t quite get to — and that’s the nature of exploration.”