The famed "blueberry" rocks discovered on Mars by NASA's Opportunity rover are not geological evidence of a history of ancient water on the red planet, a group of scientists now argue.

Instead, they propose that the tiny spherules are actually remnants of small meteorites that broke up in Mars's atmosphere. (See also "Mars Blueberries.")

In 2004, NASA’s Opportunity rover found countless gray-blue spherical rocks on Mars. Scientists proposed a number of theories—from volcanic eruptions to shockwaves caused by meteor impacts—to explain the tiny spheres (nicknamed blueberries by the discovery team), which contained large amounts of the iron-bearing mineral hematite.

The discovery team suggested that the blueberries formed when groundwater flowed through porous rocks, the way similar hematite-carrying spheres formed on Earth.