Final answer:
Spirit was sent to Mars' Gusev Crater to look for signs of past water but found the area covered in lava flows, whereas Opportunity landed at Meridiani Planum and discovered evidence of ancient water in sedimentary rocks and minerals.
Step-by-step explanation:
The Mars Exploration Rover Spirit was sent to the Gusev Crater on Mars, which scientists believed to be an ancient lakebed. Spirit was tasked with investigating the geology and past environmental conditions at the site to help determine if water once existed there. Although Spirit discovered that the Gusev Crater's surface was covered with lava flows, it offered invaluable insights into the history of Martian landscapes and revealed silica deposits that suggested hot springs or steam vents once existed, indicating a potential for past life.
Meanwhile, Spirit's twin, the Opportunity Rover, had great success on the other side of Mars. Opportunity landed in Meridiani Planum, a flat plain chosen for its hematite content that typically forms in water. The big discovery made by Opportunity was the presence of layered sedimentary rocks and spherical formations called 'blueberries', which provided strong evidence for a history of liquid water and suggested a once habitable environment on Mars.