Martian spherules

Martian spherules (also known as hematite spherules or blueberries) are small spherules (roughly spherical pebbles) that are rich in an iron oxide (grey hematite, α-Fe₂O₃) and are found in exceedingly large numbers at Meridiani Planum, a vast Martian plain.

These spherules were discovered on the Martian day (sol) that NASA's Opportunity rover, part of the Mars Exploration Rover program, landed at Meridiani Planum. They are grey but look bluish next to the ubiquitous rusty reds on Mars, and since the first spherules found in Eagle crater were 3–6 mm in diameter, the Opportunity team quickly called them "blueberries".

Martian blueberries are either embedded or loose. That is, Martian blueberries are either embedded in the large body of sediments of Meridiani Planum, or they are loose blueberries that lie directly on outcrops of the sediments or lie on top soils spread over the Meridiani sediments. The size of these spherules varies by location and elevation across the Meridiani Planum.

Martian blueberries are rich in the iron oxide hematite, but determining how rich they are in this iron oxide has proven difficult. (more below). The formation of blueberries required aqueous chemistry and involved flows of acidic, salty, liquid water over the Meridiani Planum and over two geological epochs.