Mars Exploration Rovers: Spirit and Opportunity (NASA, 2004)

Spirit and Opportunity landed on Mars in January 2004 for planned 90-day missions. Opportunity drove for 14 years and 45 km before a global dust storm ended contact in 2018 — the longest surface mission in the history of planetary exploration. The twin rovers found definitive mineralogical evidence that Mars once had liquid water on its surface: hematite spheres (nicknamed "blueberries") that only form in water, sulfate minerals deposited by standing water, and rock layers showing repeated wetting and drying cycles. Mars had once been wet, possibly habitable, and potentially host to microbial life. This single finding transformed the search for extraterrestrial life from speculation to scientific priority.