Opportunity is the second rover in the NASA MER mission (Mars Exploration Rover) to have landed on Mars, after Spirit.
The Opportunity rover landed on 24 January 2004 on the volcanic desert plain of Meridiani Planum, suspected to contain large amounts of haematite, a mineral that generally forms when iron-rich rocks are exposed to hot water.
There, Opportunity discovered chlorine and bromine, together with some unusual foliated rocks with deposits of agglomerated sand. These features suggest that there was an ancient sea on Mars. On 12 December 2005, "Oppy" celebrated its first birthday on Mars.
Opportunity's twin, Spirit, landed on Mars twenty days before.