Meridiani Planum

Meridiani Planum was chosen as a landing site because it contains an ancient layer of hematite, an iron oxide that, on Earth, almost always forms in an environment containing liquid water.

References:

• Opportunity (Miscellaneous)
• This image captured by the Mars Exploration Rover Opportunity's rear hazard-avoidance camera shows the lander that carried the rover 283 million miles to Meridiani Planum on Mars. The lander rolled onto the martian soil at 3:01 a.m. PST, on January 31, 2004. (Picture)