Spirit rolled up to a knobby rock just past where the "Columbia Hills"
start to rise from the surrounding plain. It touched the rock with a
mineral-identifying instrument at the tip of its robotic arm and
detected hematite. Hematite identified from orbit was NASA's key reason
for choosing Opportunity's landing site halfway around Mars from these
hills within Gusev Crater.
More: JPL -
Mars Rover Surprises Continue; Spirit, Too, Finds Hematite
Link: Cornell University -
Mars Rover Mission Journal