Blueberries

Scientists thought these balls of hematite, or "spherules," taken by Opportunity's microscopic imager resembled blueberries on a muffin.

Spherules, which measure about 5 millimeters (one-fifth of an inch) in diameter, were spotted by Opportunity's microscopic imager. Researchers determined them to be iron-rich concretions that formed inside deposits soaked with groundwater.

The rover found them at its landing site, and again around the rim of Victoria Crater, where material had been thrown outward from the crater by the impact that likely created it.

Researchers figure there is a layer beneath the surface in the area that contained water long enough for the spherules to form, and they hope Opportunity will be able to spot that layer if it goes into the crater.