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The article compares small iron oxide nodules recently found on Mars by NASA's Opportunity rover to similar spherules within certain sandstones of the Southwest. Early this year, scientists announced the discovery of tiny, spherical grains of hematite, a type of iron oxide, in some Martian sediments. Those spherules were dubbed blueberries because their distribution within the sediments reminded one of the scientists of blueberries in a muffin. The same type of iron oxide concretions form within some Utah sandstones, says Marjorie A. Chan, a geologist at the University of Utah in Salt Lake City. The Utah nodules probably formed when a stream of groundwater rich in dissolved minerals, including iron, encountered chemical conditions that caused the iron to precipitate. Detailed analyses of the sediments encasing the Martian blueberries would probably be even more revealing, but Opportunity isn't equipped for such investigations. Therefore, according to David Catling of the University of Washington in Seattle, Opportunity's landing site would be a good candidate for a future mission that can return samples to Earth.