Microscopic "blueberries" on Mars: The first outcrop rock Opportunity examined up close was finely-layered, buff-colored and in the process of being eroded by windblown sand. Embedded in it and on top of it like blueberries in a muffin were little spherical grains. Black and white microscopic images like this one taken on Opportunity's 14th sol on Mars show the gray spheres which have weathered out of the rock and are resting in the darker soil. Scientists were able to determine that the blueberries are rich in the mineral hematite, which on Earth most often forms in the presence of liquid water.