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What is this mystery rock that 'appeared' near the Opportunity rover on Mars?

Comparison image showing the before and after photos of the mystery rock "Pinnacle Island." The after image is the same patch of ground as in the inset box in the before image.
Comparison image showing the before and after photos of the mystery rock "Pinnacle Island." The after image is the same patch of ground as in the inset box in the before image.
NASA/JPL-Caltech/Jason Major

There is another little Martian mystery that has people talking this week - the odd appearance a few days ago of a small rock a few feet away from the Opportunity rover, it was announced yesterday during the Opportunity: 10 Years on Mars event at NASA.

The rock, nicknamed Pinnacle Island, wasn't in images taken on sol 3528, but was in images taken of the same spot later on sol 3540. How did it get there and where did it come from?

As principal investigator for the mission, Steve Squyres, explained, the rock is whitish in colour, about the size of a doughnut with a darker spot ("jelly") in the middle area, which has a concave or hollowed-out appearance. The finding sparked questions and theories ranging from a nugget either left there by a nearby meteor impact or deposited somehow by the rover's wheels. Squyres thinks the wheel idea is much more likely than the random chance of a meteor happening to hit that close to the rover's location. Are there any other possibilities?

Microscopic Imager photos have also been taken of the object and analysis so far of the darker "jelly" has shown it to be rich in sulfur, magnesium and manganese (with twice as much manganese as any other rock examined before by the rover). It is thought that the rock has been flipped over, exposing its underside.

A fascinating mystery that is sure to keep the mission scientists busy for a while.

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