When the Mars Exploration Rover, Opportunity, landed on Meridiani Planum in January 2004, it quickly found what it had been sent from Earth to find: evidence of liquid water in the Martian past.
Opportunity was targeted on Meridiani because remote sensing from orbit by the Thermal Emission Spectrometer (TES) on NASA's Mars Global Surveyor showed that portions of Meridiani contain up to 20 percent gray crystalline hematite at the surface. Hematite is an iron-oxide mineral, and on Earth the gray crystalline variety forms mostly in association with liquid water.
On the ground, Opportunity discovered the hematite lies within BB-sized spherules, dubbed "blueberries" by scientists. Blueberries that litter the surface at the landing site are embedded within outcrops of soft, layered sandstone rocks.
As geologists reconstruct it, the blueberries formed when strongly acidic groundwater drenched the basaltic sandstone, which was rich in goethite, another iron-bearing mineral. The water altered the goethite into hematite, forming spherules within the rocks. Then, over unknown ages, as the acid-rotted sandstones weathered away, the tougher spherules came free and collected on the surface.
Image Credit: NASA/JPL-Caltech/Arizona State University
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