What specific evidence did the Opportunity rover find on Meridiani Planum related to past water activity?

Answer

Small, spherical concretions of iron oxide nicknamed 'blueberries'

The Opportunity rover discovered 'blueberries,' which are small, spherical iron oxide concretions widely accepted as evidence that water once percolated through the sediment in that region.

What specific evidence did the Opportunity rover find on Meridiani Planum related to past water activity?

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